never goes slack in extension. compression locks them, and prevents all movements but flexion.
In extension, the opposite events occur and there is compression of During flexion of the lumbar spine there is an unfolding or straight-
posterior discal fibres. Extension is limited by tension of the anterior ening of the lumbar lordosis. Thus in full flexion the lumbar spine
longitudinal ligament, anterior discal fibres and impaction of spines or assumes a straight alignment or is curved slightly forwards. Normal
articular processes. It is marked in cervical and lumbar regions, and ranges of global lumbar flexion range from 58° to 72° in those under
much less at thoracic levels, partly because the discs are thinner, but 40 years old and 40° to 60° in the over-forties; females exhibit a
also because of the presence of the ribs and chest musculature. In full reduced range compared with males (Dvorak et al 1995, McGregor et al
extension, the axis of movement passes posterior to the disc, moves 1995). At an intersegmental level, the L3/4 junction and L4/5 junction
forwards as the column straightens and passes into flexion, and reaches exhibit the greatest mobility – approximately 12° and 13° respectively,
the centre of the intervertebral disc in full flexion. while at the lowest level (L5/S1) there is only 9°, and at the upper
In lateral flexion, which is always combined with ‘coupled’ axial lumbar levels only 8° and 10° respectively (Pearcey et al 1984). Move-
rotation, intervertebral discs are laterally compressed and contralater- ments into extension are the converse of those seen in flexion. Normal
ally tensed and lengthened, and motion is limited by tension of antago- ranges of global extension range from 25° to 30° in those under 40
nist muscles and ligaments. Lateral movements occur in all parts of the years old and 15° to 20° in those over forty. At an intersegmental level,
column but are greatest in cervical and lumbar regions. L5/S1 and L1/2 exhibit the greatest mobility at approximately 5°, while


BACk
746
5
NOITCES
the remaining levels exhibit less than 5° of extension. Ranges of lateral collection of pus within the vertebral canal, causing spinal cord com-
flexion and rotation in the lumbar spine are reduced compared to other pression (epidural abscess).
regions of the spine. Global lateral flexion ranges from 20° to 35°, and The integrity of the vertebrae may also be affected by malignant
rotation ranges from 25° to 40°; the ranges of both movements are disease, most commonly metastatic. Vertebrae have a copious blood
reduced with age. Assessment of intersegmental rotation and lateral supply throughout life, and many of the common cancers (e.g. breast,
flexion has proven difficult because of the limitations of measurement bronchus) spread via the arterial system. Cancers of the haemopoietic
techniques. system (e.g. multiple myeloma) also commonly affect the vertebrae.
Prostatic carcinoma has a predilection to metastasize to the vertebral
column, often using the venous (Batson’s plexus) rather than the arte-
ROLE OF MUSCULATURE rial route (Batson 1957). Metastatic deposits may occur within the
epidural space, compressing the contents of the dural sac at multiple
Although muscles will move the spinal column, the majority of mus- levels.
cular activity is involved in providing stability to maintain posture and Systemic inflammatory diseases may cause both deformity and insta-
to provide a firm platform for limb function. Hence the concept of ‘core bility of the vertebral column. Rheumatoid arthritis inflames facet joints
stability’ in modern rehabilitation programmes, especially in sports- and weakens ligaments, leading to instability, especially in the cervical
related problems. spine (see Fig. 43.57). Ankylosing spondylitis and other seronegative
It is important to recognize the way in which the muscles of the back arthritides affect joints and ligamentous attachments (entheses), leading
work in conjunction with those of the abdominal wall, particularly the to ectopic ossification of collagenous structures, fusion (ankylosis) of
oblique and transversus muscles, and with those of the lower limbs. interbody and facet joints, and loss of the normal spinal curvatures.


The erector spinae group and internal oblique and transversus Widespread new bone formation at and around the joints of the column
abdominis are anatomically and functionally connected by the thora- occurs in DISH (diffuse idiopathic skeletal hyperostosis). Such condi-
columbar fascia (which encloses the former, and into which the latter tions would seem to increase stability of the column, at the expense of
are inserted). This fascia, together with collagenous tissue within the its mobility and function, but an ankylosed spine is very liable to frac-
back muscles, plays an important role in resisting forward bending of ture, with an associated risk of neural damage.
the trunk, and during manual handling. The fascia is tensioned prima- Full stability and load-bearing capacity both require intact vertebral
rily by flexing the trunk, although this tension may be enhanced slightly bodies and intervertebral discs. Earlier views regarding the relative
by the lateral pull of the abdominal muscles. It is functionally advanta- importance of the disc-body complex and the posterior elements have
geous to generate tension in the fascia and muscle sheaths because the proved somewhat simplistic. Clinical observation led to the ‘three-
elastic strain energy stored in these stretched tissues can be used to help column concept’ of spinal stability, in which the column is divided into
bring the trunk to an upright position and so reduce the metabolic cost three longitudinal parts rather than two (Denis 1983). The anterior
of the movement (Adams et al 2006). The thoracolumbar fascia may column is formed by the anterior longitudinal ligament, the anterior
also have an important function in transferring load between the trunk half of the vertebral body and the anterior anulus fibrosus. The middle
and the lower limbs; tension in the fascia can be increased by the column is made up of the posterior longitudinal ligament, the posterior
actions of gluteus maximus and the hamstrings as well as by trunk half of the vertebral body and pedicles, and the posterior anulus fibro-
flexion. sus. The posterior column consists of the neural arch and facet joints
and the posterior ligamentous complex. The more columns that are
Musclesproducingvertebralmovements affected, the worse is the instability; an injury to two columns is usually
unstable.


The spinal column is moved both directly by muscles attached to it, Although regarded as the most influential system for classifying
and indirectly by muscles attached to other bones. Gravity always plays thoracolumbar spinal injuries, the concept was criticized for oversim-
a part: movements with gravity usually involve the eccentric contraction plifying the biomechanics of injury. A more recent classification system,
of antagonists. Flexion is effected by longus capitis and longus colli, aided by improved computed tomographic imaging, stresses the impor-
scaleni, sternocleidomastoid and rectus abdominis of both sides, aided tance of the posterior ligamentous complex in determining the stability
in the lumbar region by the abdominal obliques; extension by the of the injured spine in addition to the morphology of the injury (com-
erector spinae complex and the transversospinalis group, splenius, pression, translational/rotational or distraction injuries) and the neu-
semispinalis capitis and trapezius of both sides, together with the sub- rological status of the patient (intact, nerve root injury, complete or
occipital muscles; lateral flexion by ipsilateral longissimus, iliocostalis, incomplete spinal cord injury, cauda equina syndrome) (Lee et al
abdominal obliques and flexors, and quadratus lumborum; and rota- 2005).
tion by sternocleidomastoid, splenius cervicis, abdominal obliques, The elastic deformability of intervertebral discs permits tilting and
rotatores and multifidus. axial rotation between vertebral bodies, and also helps to reduce vertical
accelerations of the head. The main shock-absorbing mechanism of the
column stems from the spinal curves, which increase and decrease
FACTORS INVOLVED IN STABILITY slightly during locomotion against the restraining tension of the trunk
muscles. The elastic strain energy in the stretched tendons of the muscle
The vertebral column is remarkable in that it combines mobility, stabil- is responsible for shock absorption.
ity and load-bearing capacity and also protects its contained neural Both body height and spinal stability are subject to a marked diurnal
structures, irrespective of its position. Much of the stability of the ver- variation. Body height is affected by changes from recumbency to the


tebral column depends on dynamic muscular control but there are also upright posture. These diurnal variations appear to be due to changes
bony and ligamentous ‘static’ stabilizers. There is considerable varia- that occur within the cervical, thoracic and lumbar regions of the spine.
tion between segments of the column in terms of stability and mobil- Investigations using stereophotogrammetry have demonstrated that
ity; the most mobile levels are the least stable. The latter are those in 40% of diurnal changes occur in the thoracic spine and affect the degree
which the ratio of intervertebral disc height to vertebral body height is of kyphosis, and a further 40% in the lumbar spine, but do not affect
highest. Stability may be compromised by damage to any of these the lordosis (Wing et al 1992). The greatest change in vertebral column
structures. length is found in adolescents and young adults. The height loss occurs
Trauma may affect any vertebral region. Levels of specialized mobil- within 3 hours of rising in the morning, with an overall loss of about
ity (e.g. atlanto-axial joint) and the junctions of mobile and relatively 15 mm.
fixed regions (e.g. cervicothoracic, thoracolumbar) are particularly vul- Although the curvatures within the vertebral column contribute to
nerable to severe structural damage, often accompanied by spinal cord the changes in height, changes within the intervertebral disc contribute
and nerve injury. Injuries of the vertebral column may affect purely soft both to observed height loss and to variation in stability. Magnetic reso-
tissue (ligaments, joint capsules and muscles) or may affect bony struc- nance imaging investigations reveal a dynamic movement of fluid into
tures (see Fig. 43.58). Pure ligamentous/capsular injuries leading to and out of an intervertebral disc and adjacent vertebral body over a
instability may be particularly difficult to diagnose in the absence of 24-hour period. Body position affects the movement of fluid. In the
gross radiological signs. In the cervical spine, subluxation and disloca- early morning, the discs are swollen with water, the intervertebral liga-
tion of the facet joints commonly occur without bony injury because ments and the anulus fibrosus are taut, and the intrinsic bending stiff-


of the orientation of the articular facets. ness and stability of the osteoligamentous spine are relatively high.
Chronic infections of many types (e.g. tuberculosis) may involve the After several hours of normal activity, the discs lose approximately 20%
vertebrae and lead to their deformity and collapse, affect their mechani- of their water and height. This change makes the ligaments slack and
cal properties and compromise their neuroprotective function. Acute greatly reduces the bending stiffness of the spine, so that relatively more
infections, spreading locally or via the blood stream, may lead to the of the stability of the spine must then be provided by the musculature.


Surface anatomy
747
34
RETPAHC
The diurnal expulsion of water from intervertebral discs also affects the flattened (common in people with either acute or chronic low back
distribution of compressive loading in the spine. As the day progresses, pain) or reversed.
the hydrostatic pressure in the nucleus pulposus falls, and stress con- A common postural deviation seen throughout the spine is scoliosis
centrations arise in the anulus fibrosus and facet joints. or lateral curvature of the spine. It can be structural, compensatory or
All ligaments of the column, as well as the facet joint capsules, are protective. In structural scoliosis, the lateral curvature is associated with
important in the maintenance of stability. The anterior longitudinal vertebral rotation, and both the curve and the rotation become more
ligament is very strong, and resists translational displacement (shear) accentuated on forward flexion. Such a scoliosis is common in adoles-
of the vertebrae as well as extension. All the ligaments of the posterior cent girls and its cause is unknown. It may also be secondary to an
complex resist flexion and rotation, and their integrity determines the underlying disorder, e.g. muscular dystrophy, spinal muscular atrophy
range of movements allowed. These ligaments can support the whole or spina bifida. A compensatory scoliosis occurs when the pelvis is tilted
column when the muscles are inactive, e.g. in quiet standing. At the laterally, e.g. as a result of unequal leg length or of a fixed abduction or
limit of lumbar flexion, the column is supported mainly by the thora- adduction deformity at the hip joint. Usually, there is no intrinsic
columbar fascia and by collagenous tissue within the electrically silent abnormality of the spine itself and the scoliosis disappears when the
muscles of the back. pelvic tilt is corrected. A sciatic or antalgic scoliosis is a temporary
Movements are both determined and constrained by the shape and deformity produced by the protective action of muscles in certain
orientation of the facet joints, whose articular surfaces stabilize the painful conditions of the spine.
column primarily by resisting horizontal gliding (shear) movements Ergonomics has been defined as ‘the way humans work’, and it
and axial rotation. In the most mobile regions, the joint surfaces are permits an appreciation of the effects of tasks and the work environ-


flatter and more horizontally placed, as will become apparent if a ment on underlying postural biomechanics. Nachemson (1975)
typical cervical facet joint is compared with a typical lumbar joint. showed that discs were loaded maximally in sitting and in lifting in a
Certain regions of the vertebral column are further stabilized by forward-leaning position, so sitting posture and lifting have received
additional extraspinal factors. The thoracic spine is stabilized by its considerable ergonomic attention.
position as an integral part of the thoracic cage and by its strong liga- In sitting, the goal has been to determine the seat type and reclining
mentous linkages with the ribs. The sacrum is effectively a virtually fixed angle associated with lowest disc pressure and the least paraspinal
integral element of the bony pelvis. muscle activity. When sitting with the hips and knees flexed to 90°, the
The contribution to stability conferred by the musculature has been pelvis rotates posteriorly, flattening the lumbar lordosis and conse-
grossly underrated. The whole vertebral column is stabilized by the quently increasing the load on the intervertebral discs. Thus it is now
‘guy-rope’ or staying effect of the long muscles that attach it to the advised that, in sitting, the angle between trunk and thigh should be
girdles, the head and the appendicular skeleton. This effect is particu- between 105° and 135°, with the sacrum tilted at 16° and the fourth
larly marked for erector spinae, which controls global posture and and fifth lumbar vertebrae supported.
movement. The small and deep muscles of the back are best able to In lifting heavy weights, there is considerable initial compression of
resist shear movements between vertebrae because only they have lumbar intervertebral discs, and large increases in thoracic and intra-
sufficient angulation to the long axis of the vertebral column to abdominal pressure. The compressive force acting on the spine is shared
do this effectively. The deep muscles can also fine-tune intervertebral between the vertebral bodies and the neural arch. In the lumbar spine,
movements. the neural arch typically resists 20% of this force once the disc height
For most back problems in clinical practice, especially chronic low has been reduced by diurnal fluid expulsion, and when the spine is


back pain, enhancing muscle strength, stamina and coordination with positioned upright. However, age-related narrowing of the disc can
the many other muscle groups that contribute to stability, e.g. pelvic cause more than 50% of the compressive force to be resisted by the
girdle muscles, is the most appropriate and effective therapeutic avenue. neural arch, which may explain why osteoarthritis of the facet joints
Only a minority of cases benefit from surgery. Furthermore, neglecting commonly follows disc degeneration.
the musculature may explain the relatively high failure rates from When lifting, manual handling advisers emphasize the importance
surgery. of leg lifting as opposed to back lifting. Loads should also be kept close
Mechanical injury to the vertebral column may be produced by to the body to reduce the lever arm of the load. The use of deep inspira-
flexion, extension, distraction, rotation, shear or compression; move- tion to raise intra-abdominal pressure while lifting has also been
ments that cause damage are commonly combined, e.g. flexion, axial advised, as this is believed to offer further support to the lumbar spine.
rotation and compression. The spine is at risk when lifting is combined with twisting, lateral
bending and asymmetric postures. However, heavy lifting remains one
of the key work-related risk factors for the spine together with whole-
POSTURE AND ERGONOMICS
body vibration, prolonged sitting, twisting and bending.
Posture is a descriptive term for the relative position of the body seg-
ments during rest or activity. The maintenance of good posture is a SURFACE ANATOMY
compromise between minimizing the load on the spine and minimiz-
ing the muscle work required.
SKELETAL LANDMARKS
The well-balanced erect body has a line of gravity that extends from
the level of the external auditory meatus, through the dens of the axis
Vertebralspines
just anterior to the body of the second thoracic vertebra, through the
centre of the body of the twelfth thoracic vertebra, and through the rear
of the body of the fifth lumbar vertebra to lie anterior to the sacrum. In the midline, a median furrow runs from the external occipital pro-
The position of the line of gravity may move anteriorly with locomotion tuberance above to the natal cleft below (Figs 43.77, 43.78). The furrow


and may vary between individuals. is most shallow in the lower cervical region and is deepest in the mid-
The normal curvature of the cervical spine is a lordosis. However, as lumbar zone. Inferiorly, it widens out into a flattened, triangular area,
a result of pain, injury or poor ergonomics, this curve can become exag- the apex of which lies at the start of the natal cleft and corresponds
gerated to give a ‘protruding chin’ stance, i.e. hyperlordosis in the lower approximately to the spine of the third sacral vertebra. Palpation of the
cervical spine. median furrow reveals the sagittal curves of the spine (see Fig. 43.78):
The thoracic spine is held convex posteriorly, and this posture pri- the cervical curve is convex anteriorly (lordosis) and extends from the
marily results from the structure of the underlying vertebrae. However, first cervical to the second thoracic vertebra; the thoracic curvature is
this curve or kyphosis can become exaggerated to give the impression concave anteriorly (kyphosis) and extends from the second to the
of a rounded back. Poor posture and ergonomics can lead to this exag- twelfth thoracic vertebra; and the lumbar curvature is convex anter iorly
gerated curvature but other important causes include tuberculosis, a and extends from the twelfth thoracic vertebra to the lumbosacral
wedge or compression fracture of a vertebral body, Scheuermann’s prominence. The external occipital protuberance is subcutaneous and
osteochondritis, ankylosing spondylitis, osteoporosis and metastatic is palpable on the posteroinferior aspect of the cranium when it is
carcinoma. approached from below. The inion is the point situated on this protu-
The lumbar spine is held in a lordosis. The degree of this lordosis berance in the median plane. The tubercle on the posterior arch of the
is determined by the lumbosacral angle and is normally 30–45°. The atlas (C1) is impalpable, whereas the transverse process of C1 is palpa-
muscles responsible for this posture include erector spinae, rectus ble as an indistinct mass through sternocleidomastoid just inferior to


abdominus, the internal and external obliques, psoas major, iliacus, the the apex of the mastoid process. The spinous process of C2 is the first
gluteal and hamstring muscles. The lordosis can be increased (as a result palpable midline feature, located several centimetres inferior to the
of weak abdominal muscles and tight hamstring muscles), decreased, inion. The tips of the spines of most cervical vertebrae are obscured by


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748
5
NOITCES
C7
9
1 1
2 10
2
3 11
4 4
T7
1
B
5
T12 2
3 4
1
1
5
6 6
7
7
8
A C
Fig .43 .77A–C The surface anatomy of the back . Key: A, 1, median furrow; 2, spine of scapula; 3, trapezius (between solid black lines); 4, triangle of
auscultation; 5, latissimus dorsi (between dashed black lines); 6, lumbar (Petit’s) triangle; 7, posterior superior iliac spine; 8, superior part of natal cleft
(approximates S3) . B, 9, ligamentum nuchae; 10, spinous process of C7 (vertebra prominens); 11, spinous process of T1 . C, 1, erector spinae (black line
represents lateral border); 2, zone of spinal cord termination (white): ranges from the middle third of T11 to the middle third of L3; 3, mean level of spinal
cord termination (red): middle third of L1 vertebral body which corresponds with the lower part of the interspinous space between T12 and L1; 4, rib 12
(dashed black line); 5, zone of supracristal plane intersection with vertebral column (blue): ranges from the L2–L3 junction to the L4–L5 junction/
interspinous space; 6, highest point of iliac crest and supracristal plane (Tuffier’s line): almost always intersects the vertebral column from L4 to the L4/5
junction; 7, posterior superior iliac spine and interspinous plane, marked by a skin indentation (dimple of Venus) . Note the overlap between zone 5 and
the zone of spinal cord termination .
Fig .43 .78 The surface Table 43.3 Vertebral spines as landmarks for the viscera
anatomy of the back,
Spine/ Vertebral Viscera
oblique view . Key: 1,
C7 space body level
1 trapezius (upper fibres);


2, median furrow; 3, C5 C6 Cricoid cartilage; start of oesophagus
2
spine of scapula; 4, C7 C7-T1 Lung apex
3
trapezius (lower fibres T3 Upper T4 Aorta reaches vertebral column; medial part of scapular spine
overlying erector spinae);
T3-T4 space T4-5 disc Sternal plane/angle of Louis
4 5, triangle of auscultation;
T4 Upper T5 Tracheal bifurcation; concavity of aortic arch; azygos
6, inferior angle of
vein-superior vena cava junction; bifurcation of pulmonary
T7 scapula; 7, erector trunk; upper border of heart
5 6 spinae; 8, lumbar (Petit’s) T7 Upper T8 Inferior angle of scapula; lower border of heart; inferior vena
triangle; 9, posterior cava crosses diaphragm; central tendon of diaphragm
superior iliac spine; 10,
T10 Upper T11 Lower border of lung; cardia of stomach; upper border of
superior part of natal kidney
cleft: approximates the
T12-L1 Mid L1 Lowest level of pleura; pylorus; transpyloric plane; hilum of
spinous process of S3;
7 space left kidney; origin of renal arteries and superior mesenteric
C7, T7, spinous artery; pancreas (neck); spinal cord termination
processes of vertebrae
L1-L2 space Mid L2 Pancreas (head); duodenojejunal flexure
C7 and T7 .
L2-L3 space Mid L3 Lower border of kidney
L3-L4 space Mid L4 Bifurcation of aorta
8
L4-L5 space Mid L5 Formation of inferior vena cava
9
10 flexed. Each lies opposite the inferior part of its own vertebral body and
the inferiorly located intervertebral disc. The interspinous spaces are
represented by a palpable depression between adjacent spines.
Spinal levels of viscera
the overlying ligamentum nuchae, which forms a palpable raised
midline ridge when the neck is flexed. The ligamentum nuchae termi- In adults, the spinal cord terminates, on average, level with the middle


nates inferiorly at the spine of the seventh cervical vertebra (C7, vertebra third of the body of L1 (ranging from the middle third of T11 to the
prominens), which is the most superior visible projection in this region. middle third of L3): 25% of cords end below the level of the L1/L2
The spine of the first thoracic vertebra (T1) is palpable immediately intervertebral disc (Macdonald et al 1999). In children, the reported
inferior to it and is usually more prominent than the spine of C7. level of spinal cord termination varies according to the breadth of the
Identification of the remaining thoracic spines is not easy, even in a thin age range studied. On average, the cord terminates at the lower third of
subject when the trunk is fully flexed, because they overlap one another the body of L1 (ranging from the lower third of T12 to the middle of
in the mid-thoracic region. In the upper and lower thoracic regions, the L2) (Kesler et al 2007), but may terminate as low as L3 in younger
tips of the thoracic spines lie opposite the upper part of the vertebral children (Malas et al 2001). The palpable vertebral spines can be used
body below. In the mid-thoracic region, they lie opposite the lower part as landmarks for the levels of the viscera. Some of the more important
of the vertebral body below. The tip of the spine of each lumbar vertebra are shown in Table 43.3 (minor differences between the two sides are
can usually be palpated without difficulty, especially if the trunk is ignored).


Clinical examination
749
34
RETPAHC
Scapularlandmarks racic and lumbar regions, a broad elevation produced by the erector
spinae muscle group extends for about one hand’s breadth on either
side of the median furrow and is present between the iliac crest and the
The shape of the back in the upper thoracic region is determined largely
twelfth rib (see Fig. 43.78). The lateral border of this elevation then
by the scapula and the muscles attached to it. Bony scapular landmarks
crosses the ribs at their angles, passing medially as it ascends. The
are most evident when the upper limbs hang by the sides (see Figs
muscle group can be demonstrated by extending the back against
43.77A, 43.78).
resistance.
The scapula overlies the second to seventh ribs. Its superior angle is
Trapezius is a flat, triangular muscle that covers the back of the neck
palpable through trapezius, and its inferior angle through latissimus
and shoulder (see Figs 43.69 and 43.77A). The two trapezius muscles
dorsi; these angles are joined by the medial border of the scapula, which
resemble a trapezium in which two of the angles correspond to the
runs vertically. The scapular spine runs subcutaneously and is easily
shoulders, a third to the external occipital protuberance and the fourth
palpable from its root medially to the acromial angle and process later-
to the spine of the twelfth thoracic vertebra. On each side, the antero-
ally. When the upper limb is by the side, the spinous process of T3 is
superior border of the muscle forms the posterior boundary of the
level with the spine of the scapula, and the T7 spinous process is level
posterior triangle of the neck; it can be seen in muscular subjects, espe-
with the inferior angle.
cially during shoulder elevation against resistance or by extension and
lateral flexion of the neck against resistance. The lower fibres are best
Posteriorpelvicandsacrococcygeal
seen when the subject pushes both hands against a wall with the elbows
bonylandmarks extended.
Latissimus dorsi occupies the lower half of the back. It passes from


At the lower part of the back, the entire iliac crest can be palpated. It the spines of the vertebrae from T7 inferiorly, the thoracolumbar fascia
can be traced backwards and upwards from the anterior superior iliac and the posterior iliac crest, to the posterior axillary fold. In lean sub-
spine to its highest point, and then downwards and medially to the jects, with the arm adducted against resistance, the inferolateral border
posterior superior iliac spine, which is overlain by a dimple in the can easily be traced to the iliac crest. The upper border passes horizon-
skin. For landmarking during lumbar puncture (see Fig. 45.18), a line tally from the spinous process of T7 to the inferior angle of the scapula
(Tuffier’s) joining the highest points of the iliac crests is used. This and on to the posterior axillary fold.
line crosses the vertebral column at the level either of the body of L4 The triangle of auscultation (see Figs 43.69, 43.77A) lies between
or of the L4/L5 intervertebral disc in the majority of subjects (Render the upper border of latissimus dorsi, the lower inferolateral border of
1996, Walsh et al 2006, Chakraverty et al 2007) (Fig. 43.78). Greater trapezius and the inferomedial border of rhomboid major, although for
variability occurs in neonates, females and subjects with a higher simplicity the medial border of the scapula is often substituted for the
body mass index. It is important to remember that when a subject lies latter muscle.
on their side, the median soft tissue furrow may not coincide with the The lumbar (Petit’s) triangle (see Figs 43.69, 43.77A), one of the
median plane, especially in the lumbar region in higher body mass sites of the rare primary lumbar hernia, lies inferiorly just lateral to the
subjects. Careful palpation may be necessary to identify the vertebral highest point of the iliac crest, between the inferolateral border of latis-
spinous processes in this position. The spines of the sacral vertebrae simus dorsi, the posterior free border of external oblique and the iliac


are smaller and less well defined than those of the other vertebrae. crest.
Palpation places the posterior superior iliac spines along a line that
ranges from the L5–S1 vertebral junction to the S2 spinous process
CLINICAL EXAMINATION
(Kim et al 2007), whereas radiographic assessment shows the inferior
margin of the posterior superior iliac spines to be almost always coin-
cident with the second sacral spine (McGaugh et al 2007) (see Fig. Clinical examination of the back of the trunk and neck best follows the
78.16). The latter serves as a useful landmark for the inferior limit of order of inspection, palpation and movement. The examination will be
the adult dural sac. The tip of the coccyx can be felt deeply near the determined by the circumstances of presentation and by the history,
centre of the natal cleft. Moving the examining finger cranially enables and may include musculoskeletal, neurological and vascular observa-
palpation of the raised ridges of the sacral cornua on either side of tions. Information relevant to the neurological and vascular examina-
the midline; they demarcate the sacral hiatus and so form the land- tion of the skin and material relating to spinal movements and deeper
mark for performing a caudal anaesthetic block (see Figs 45.19, innervation are found above. Palpation of the region involves careful
45.20). assessment of the bony and musculotendinous landmarks described
above, looking in particular for asymmetry, deformity and tenderness.
Note that, apart from the spines, most of the bony elements of the
MUSCULOTENDINOUS LANDMARKS vertebrae and almost all of the intervertebral joints are not palpable
from behind. In regions of lordosis (sagittal plane curves of the spine
In the upper and middle cervical region, a median furrow lies between with anterior convexity, i.e. mid-cervical and mid- and lower lumbar),
the cylindrical prominences formed mainly by the semispinalis muscles, parts of the vertebral column can often be palpated anteriorly with care
which are accentuated by neck extension against resistance. In the tho- in well-relaxed, thin subjects.
Bonus e-book images


Fig .43 .11 A sagittal MR image of the lumbar Fig .43 .15 An anteroposterior arteriogram of Fig .43 .19 An arteriogram of the cervical
spine showing the L5–S1 intervertebral the lower thoracic and upper lumbar and upper thoracic regions of the 6-year-old
foramen (neuroforamen) and a pathologically vertebrae in a 6-year-old child . spine seen in Figures 42 .16 and 42 .18 .
narrowed L4–5 intervertebral foramen .
Fig .43 .16 A vertical radiograph of a section Fig .43 .24 The vertebral canal in section .
Fig .43 .12 A sagittal computed tomographic through a lumbar vertebra of a 6-year-old
(CT) image of the lumbar spine showing the child . Fig .43 .26 Markedly reduced L3–4 foraminal
left side of an isthmic spondylolisthesis volume secondary to advanced disc
resulting from bilateral fractures of the pars Fig .43 .17 A, The arterial supply to the degeneration resulting in bony neural
interarticularis . bodies of the upper cervical vertebrae and entrapment of the L3 nerve .
the odontoid process . Numerical
Fig .43 .13 (A) Sagittal and (B) axial MR designations apply to the same structures Fig .43 .31 An axial CT showing the carotid
images of a degenerative spondylolisthesis in B . tubercle of Chassaignac .
at L4–5 .
Fig .43 .18 A vertical radiograph of a section Fig .43 .33 An axial MR image showing an
Fig .43 .14 A ventral radiograph of a section through the fourth cervical vertebra of a intact transverse ligament .
through T6 of a specimen from a 6-year-old 6-year-old child, showing vascularity .
child injected with barium sulphate .
Continued


BACk
750
5
NOITCES
Bonus e-book images—cont’d
Fig .43 .35 A lateral cervical radiograph Fig .43 .39 A sagittal CT scan reconstructed at T10–11 sustained by a 54-year-old male
demonstrating a posterior ponticle that image of a type II odontoid fracture . after a major compression injury . B, The
forms the roof of a tunnel over the vertebral MR image shows gross disruption of the
artery . Fig .43 .57 Atlanto-axial instability in the supraspinous and interspinous ligaments .
setting of rheumatoid arthritis .
Fig .43 .38 A sagittal CT scan reconstructed Fig .43 .72 An axial MRI of the lumbar spine
image demonstrating an os odontoideum . Fig .43 .58 A, A sagittal CT reconstruction at the level of the L4/5 intervertebral disc .
showing an osseous and ligamentous injury
KEY REFERENCES
Adams MA, Bogduk N, Burton K et al 2006 The Biomechanics of Back Pain, MacNab I, McCulloch J 1990 Backache, 2nd ed. Baltimore: Williams &
2nd ed. Edinburgh: Elsevier, Churchill Livingstone. Wilkins.
A comprehensive and detailed source of information on the functional The functional anatomy of the lumbar spine, described as a basis for the
anatomy, tissue biology and biomechanics of the lumbar spine. clinical management of low back pain.
Batson OV 1957 The vertebral vein system. Caldwell Lecture, 1956. Am J Mercer S, Bogduk N 1999 The ligaments and anulus fibrosus of human adult
Roentgenol Radium Ther Nuclear Med 78:195–212. cervical intervertebral discs. Spine 24:619–26; discussion 27–8.
A pioneering study of the venous plexuses of the vertebral column, A human cadaveric microdissection study showing that the cervical anulus
which has become the standard of reference in its field. fibrosus is an anterior crescent rather than a uniformly circumferential
structure.
Bogduk N 2005 Clinical Anatomy of the Lumbar Spine and Sacrum, 4th ed.


Edinburgh: Elsevier, Churchill Livingstone. Newell RL 1999 The spinal epidural space. Clin Anat (New York, NY) 12:
The most thorough text currently available on the topographical and 375–9.
functional anatomy of the lumbosacral spine, with over 1000 references. The A review of the morphological, developmental and topographical aspects of
book incorporates biomechanical and physiological information that is the spinal epidural space.
related to the clinical problem of low back pain.
Viejo-Fuertes D, Liguoro D, Rivel J et al 1998 Morphologic and histologic
study of the ligamentum flavum in the thoraco-lumbar region. Surg
Denis F 1983 The three column spine and its significance in the classifica-
Radiol Anat: SRA 20:171–6.
tion of acute thoracolumbar spinal injuries. Spine 8:817–31.
A cadaver-based study detailing anatomical and histological findings with
A seminal paper for the understanding and classification of thoracolumbar
regard to the ligamentum flavum.
instability.
Willard FH, Vleeming A, Schuenke MD et al 2012 The thoracolumbar fascia:
Groen GJ, Baljet B, Drukker J 1990 Nerves and nerve plexuses of the human anatomy, function and clinical considerations. J Anat 221:507–36.
vertebral column. Am J Anat 188:282–96. A detailed review of the thoracolumbar fascia with regard to functional
An acetylcholinesterase whole-mount study of human fetal material giving anatomy and how it plays a role in spinal stability and as a potential pain
detail of the perivertebral nerve plexuses and of the sinuvertebral nerves. generator.


34
RETPAHC
Back
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The most thorough text currently available on the topographical and clinical management of low back pain.
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Dean NA, Mitchell BS 2002 Anatomic relation between the nuchal ligament
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Groen RJ, du Toit DF, Phillips FM et al 2004 Anatomical and pathological Pearcy M, Portek I, Shepherd J 1984 Three-dimensional x-ray analysis of
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Kim HW, Ko YJ, Rhee WI et al 2007 Interexaminer reliability and accuracy Salmon M 1994 Arteries of the muscles of the extremities and trunk.
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Klimo P Jr, Rao G, Brockmeyer D 2007 Congenital anomalies of the cervical Taylor JR, Twomey LT 1984 Sexual dimorphism in human vertebral body
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Twomey L, Taylor J, Furniss B 1983 Age changes in the bone density and A detailed review of the thoracolumbar fascia with regard to functional
structure of the lumbar vertebral column. J Anat 136:15–25. anatomy and how it plays a role in spinal stability and as a potential pain
Viejo-Fuertes D, Liguoro D, Rivel J et al 1998 Morphologic and histologic generator.
study of the ligamentum flavum in the thoraco-lumbar region. Surg Willems JM, Jull GA, J KF 1996 An in vivo study of the primary and coupled
Radiol Anat: SRA 20:171–6. rotations of the thoracic spine. Clin Biomech (Bristol, Avon) 11:
A cadaver-based study detailing anatomical and histological findings with 311–16.
regard to the ligamentum flavum.
Wing P, Tsang I, Gagnon F et al 1992 Diurnal changes in the profile shape
Walsh JC, Quinlan JF, Butt K et al 2006 Variation in position of the and range of motion of the back. Spine 17:761–6.
L4/5 disc inter-space from the anatomical landmark: Review of 450 Wood GW II 2103 Spinal anatomy and surgical approaches. In: Canale ST,
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White AA, Panjabi MM 1990 Clinical Biomechanics of the Spine, 2nd ed. Zumpano MP, Hartwell S, Jagos CS 2006 Soft tissue connection between
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Willard FH, Vleeming A, Schuenke MD et al 2012 The thoracolumbar fascia: brane: a cadaveric study. Clin Anat (New York, NY) 19:522–7.
anatomy, function and clinical considerations. J Anat 221:507–36.
750.e2


CHAPTER
44
Development of the back
Vertebrae and their alternating intervertebral discs are one of the main that supports the concept that somite number is controlled, in part, by
manifestations of body segmentation or metamerism. A chain of seg- species-specific cyclical properties of the presomitic mesoblast (Richard-
ments arranged in sequence allows the overall structure to bend when son et al 1998, Gomez et al 2008).
it is moved by the associated muscles. The original body segments, the The final determination of somitic boundary formation has not yet
somites, are formed by the epithelial paraxial mesoderm (Ch. 12) that been fully elucidated but seems to require a periodic repression of the
is found lateral to the neural tube and notochord in the early embryo. Notch pathway genes. The caudal presomitic mesoblastic cells are
The somites provide the embryonic cell populations for bone and thought to be maintained in an immature state by their production of
muscle. The vertebrae form between the early body segments by the
recombination of portions of the somites on the craniocaudal axis, and
the muscles attach to adjacent vertebrae. Each vertebra develops from
bilateral origins to form a midline centrum, two lateral arches bearing Rib, pedicle Dorsal part of Vertebral body
and lamina neural arch
transverse processes that develop lateral and dorsal to the spinal cord, Intervertebral disc
of neural arch spinous process
and a midline fused dorsal portion with a spinous process. Individual
vertebrae may be distinguished by modifications of these component
Pax-1 Msx1 Pax-1
parts. The intervertebral discs are of dual origin; the anulus fibrosus
Pax-9 Msx2 Pax-9
develops from the sclerotome and the nucleus pulposus from the
notochord. FGFs 4,6 Lateral sclerotome Dorsal sclerotome Ventral sclerotome
Scleraxis Scleraxis
BMP-4
SEGMENTATION OF PARAXIAL MESODERM Pax-1 Pax-1
Pax-9 Pax-9
Epiblast cells that ingress through the lateral aspect of the primitive Sclerotome


node and the rostral primitive streak (see Fig. 10.3) become committed von Ebner’s fissure
to a somitic lineage. After passing through the streak, these mesoblastic Pax-1 Epithelial/
cells retain contact with both the epiblast and hypoblast basal laminae Pax-9 mesenchymal Smooth muscle
as they migrate, and for some time after reaching their destination. Twist transition Fibroblasts
Their final destination is each side of the notochord, where the cell Hox genes Endothelial cells
population, initially presomitic or unsegmented mesenchyme, is
referred to as paraxial mesoderm when mesenchyme to epithelial trans- Wnts
formation occurs and somites form. Somites will form from cultured
Sonic hedgehog
presomitic mesoderm with or without the presence of neural tube tissue Somite Dermomyotome BMP-4
Noggin
or primitive node tissue. As well as specifying somitic lineage, the posi-
Pax-1
tion of ingression of the epiblast informs the specific destination of
Pax-9 Skeletal muscle
the cells. Thus, those cells that ingress through the lateral portion of
Twist Dermis
Hensen’s node form the medial halves of the somites, whereas those Hox genes Endothelial cells
ingressing through the primitive streak approximately 200 µm caudal Smooth muscle
to the node produce the lateral halves of the somites. The two somite Scapula
halves do not appear to intermingle.
Bilateral segmentation of the paraxial presomitic mesoblastic popu-
Ectodermal signals
lations, which divide into discrete epithelial spheres, occurs as a sequen- (Wnts) Cranial unsegmented mesenchyme
tial process along the craniocaudal axis. In avian embryos, a pair of Sonic hedgehog
somites is formed every 90 minutes until the full number is obtained. Paraxis Cranio-caudal polarization
The molecular pathway for this synchronous segmentation has been Presenilin-1 Regionalization
Notch Epithelialization
termed the segmentation clock. It has been identified as a conserved
Delta-1 Segmentation
process in vertebrates from fish to mammals, and is based on the rhyth-
Mesp1,2


mic production of messenger RNAs (mRNAs) for the transcription of EphA4
genes related to Notch, a large transmembrane receptor, and a number Uncx4.1
of other factors, including members of the Wnt and fibroblast growth Cadherins
factor (FGF) signalling pathways. Hox genes
Intrinsically coordinated pulses of mRNA expression appear as a
Ectodermal signals
wave within the presomitic mesoblast as each somite forms. As new Caudal unsegmented mesenchyme
(Wnts)
cells enter the paraxial mesoderm caudally, they begin phases of upregu-
lation of the cycling genes, followed by downregulation of these genes. FGF-8 Proliferation
Hairy-1 Oscillation of gene expression
During each cycle, the most cranial presomitic mesoblast will segment
Lunatic fringe Maturation
and undergo mesenchyme to epithelial transformation to form the next
somite.
Paraxial mesenchyme
Experimental evidence (from chick embryos) shows that newly
Origin from primitive streak or tail bud
formed paraxial mesoblast cells undergo 12 such cycles before they
during primary and secondary gastrulation
finally form a somite (Pourquié and Kusumi 2001). Thus, from ingres-
sion through the primitive streak to segmentation into a somite takes Fig. 44.1 Processes in the development of the paraxial mesenchyme in
approximately 18 hours. As the somite number varies between verte- the avian embryo. Signals are indicated by open arrows; genes are
brate species, it is likely that the rate of somite formation also varies italicized. Abbreviations: BMP, bone morphogenetic protein; FGF,
and may be longer in human embryos. Indeed, those vertebrates with fibroblast growth factor. (Redrawn with permission from Christ B, Huang
elongated bodies and many somites form somites more rapidly relative R, Wilting J 2000 The development of the avian vertebral column. Anat
to their developmental rate than those with shorter bodies, a finding Embryol 202: 179–94, Springer.) 751


Development of the back
752
5
noItceS
FGF8. The cells become competent for segmentation when FGF8 levels tin and cytotactin. Processes from the somite cells pass through this
drop below a certain threshold. They would then be in close apposition basal lamina to contact the basal laminae of the neural tube and noto-
to cells that have segmented (i.e. the next cranial somite). chord. Dorsoventral patterning of the putative vertebral column is
This area of research is moving rapidly. For an overview of verte- dependent on the activity of Sonic hedgehog (SHH) from the notochord/
brate segmentation and its clinical implications, see Pourquié (2011). floor plate, Wnt1 and Wnt3a from the roof plate, and Wnt6 from the
An overview of the processes involved in the development of the overlying ectoderm.
paraxial mesoblast, based on the work of Christ et al (2000), is shown The sclerotomal population has been subdivided in the chick, and
in Figure 44.1. the fates of the cells followed (Christ et al 2004) (Figs 44.4–44.6). The
SOMITE DEVELOPMENT
Fig. 44.2 A scanning
electron micrograph
Somitogenesis and somite development encompasses a process in of a lateral view of an
which five main stages can be identified (Figs 44.2–44.3). A local embryo, showing the
portion of the most cranial paraxial mesoblast undergoes compaction. somites. The cranial
The compacted cells undergo a mesenchymal/epithelial transforma- somites are at the
tion, resulting in an epithelial sphere of cells that surrounds free somi- upper border and the
tocoele cells. This stage marks segmentation of the paraxial mesoderm more caudal somites
and somite formation. Shortly after the somite boundaries have been are at the lower border.
defined, there is a region-specific epithelial/mesenchymal transition of A change in size of
the ventral and ventromedial walls of the somite to form the mesen- the cranially more
chymal sclerotome. As the embryo enlarges, the sclerotomal popula- advanced somites is


tions on each side become contiguous with the notochord and the apparent. (Photograph
neural tube. The rest of the dorsal lateral somitic epithelium remains by P Collins; printed
by S Cox, Electron
as the epithelial plate of the somite, also termed the dermomyotome,
Microscopy Unit,
a proliferative epithelium that will give rise to (nearly) all the striated
Southampton General
muscles of the body. Formation of the epaxial myotome begins. Seg-
Hospital.)
mentation of the paraxial mesoblast, mesenchymal/epithelial trans-
formation to form epithelial somites and the resultant somite
developmental processes (epithelial/mesenchymal transformation to
form the sclerotome) all occur in a craniocaudal progression caudal to
the otic vesicle from stage 9.
The Golgi apparatus, actin and α-actinin are all located in the apical
region of the epithelial somite cells. Cilia develop on the free surface.
The cells are joined by tight junctions (a variety of cell adhesion mol-
ecule has been demonstrated in epithelial somites). The basal surface,
corresponding to the outer circumference of the spherical epithelial
somite, rests on a basal lamina that contains collagen, laminin, fibronec-
Caudal
Neural tube
Mesenchyme
Somitogenesis Notochord
(1) Compaction
Mesenchymal/epithelial transition
to produce epithelial somite
Epithelial plate of the
(2) Epithelialization
somite/dermomyotome
Epithelial/mesenchymal transition
to produce sclerotome population
(3) Sclerotome formation
Cells proliferate from the dorsomedial
edge of the dermomyotome
(4) Sclerotome proliferation
and expansion Cells proliferate from all
edges of the dermomyotome
(5) Formation of the epaxial myotome
starts from the dorsomedial lip of
the epithelial plate of the somite,
which folds back on to its lateral Myotome cells form
surface the epaxial muscles
(6) Myoblast populations for the
limbs and tongue migrate from Sclerotome cells surround the
the ventrolateral edge of limb-level notochord and neural tube and


dermomyotomes become chondroblasts
Hypaxial muscles arise
from the ventrolateral edge Cranial
of the dermomyotome of
interlimb somites forming the
ventrolateral part of the myotome
Fig. 44.3 The stages of somite development. Development occurs in a craniocaudal progression. The more cranially placed somites (at the lower right of
the figure) are further developed than those caudally placed (at the upper left of the figure). The stages in somite development are given on the left of the
figure; more detailed information is given on the right.


Somite development
753
44
Retpahc
B
Surface ectoderm
Neural tube
Mesonephric duct
Dorsal half-somite
Ventral half-somite Somitocoele cells
DDSS
NNTT Notochord Aorta
SSMM
SSOO
WWDD C Surface ectoderm
NN VVSS
Dorsal half-somite
AO
Notochord
SSPP Ventral half-somite Somitocoele cells
A
Fig. 44.4 Development of the sclerotome. A, A scanning electron micrograph of a transversely fractured 2-day chick embryo. B–C, Diagrams of
transverse (B) and longitudinal (C) sections through the somites; the plane in C is indicated by the dotted line in B. The somites here are in the
epithelialization phase of somitogenesis. Abbreviations: AO, aorta; DS, dorsal somite; N, notochord; NT, neural tube; SM, somatopleure; SO,
somitocoele; SP, splanchnopleure; VS, ventral somite; WD, Wolffian duct (mesonephric duct). (A, Courtesy of Dr Heinz Jürgen Jacob, Bochum,
Germany.)
B Surface ectoderm
Neural tube Dermomyotome
Dorsal sclerotome
Myotome
Central sclerotome
Ventral sclerotome Lateral sclerotome
Notochord
Aorta
NNTT
DDMM
C
SSCC Cranial half of
central sclerotome
Surface ectoderm
NN
Caudal half of Dermomyotome
central sclerotome
Myotome
Ventral sclerotome
AO
WWDD Spinal nerve
SSMM
Notochord
von Ebner’s fissure
A
Somitocoele cells
Fig. 44.5 Development of the sclerotome. A, A scanning electron micrograph of a transversely fractured 3-day chick embryo. B–C, Diagrams of
transverse (B) and longitudinal (C) sections through the developing sclerotome; the plane in C is indicated by the dotted line in B. Abbreviations: AO,


aorta; DM, dermomyotome; N, notochord; NT, neural tube; SC, sclerotome; SM, somatopleure; WD, Wolffian duct (mesonephric duct). (A, Courtesy of
Dr Heinz Jürgen Jacob, Bochum, Germany.)
A B
Cranial half of the
Surface ectoderm Dermomyotome central sclerotome Surface
Myotome Spinal nerve ectoderm
Dorsal sclerotome
Dermis
Myotome
Somitocoele cells
Neural tube
Caudal half of the Dermis
Central sclerotome
central sclerotome
von Ebner’s
Ventral sclerotome
Notochord fissure
Lateral sclerotome
Notochord
Dermomyotome Ventral sclerotome
Aorta
Mesonephric duct
Fig. 44.6 Later development of the sclerotome. A, Transverse section. B, Longitudinal section through the dotted line indicated in A, showing the
sclerotomal subdivisions.


Development of the back
754
5
noItceS
A B C D
Dermomyotome
Notochord Mesonephric
duct
Dorsal aortae
Dorsomedial edge Ventrolateral edge Cranial and caudal edges
of the dermomyotome of the dermomyotome of the dermomyotome
Fig. 44.7 A–D, Epaxial and hypaxial muscle origin from the dermomyotome. A, Early dermomyotome proliferation. B, The dermomyotome at interlimb
level, producing epaxial myoblasts. C, The dermomyotome at limb level. Hypaxial dermomyotome cells de-epithelialize from the ventrolateral edge and
migrate into the limb bud. D, The local dermis is formed later by superficial de-epithelialization of dermomyotome cells.
Neural tube
Dermomyotome
Spinal nerve
The spinal nerve grows preferentially
through the cranial half of the sclerotome
The caudal half of a sclerotome fuses with
the cranial half of the sclerotome below
Ventral sclerotome
Notochord
The intervertebral disc forms
level with the spinal nerve
The notochord expands at the
Nucleus pulposus region of the intervertebral disc
Vertebral body
Intervertebral disc
Fig. 44.8 The formation of vertebrae and intervertebral discs from the mesenchymal sclerotomes. Each vertebra is formed from the cranial half of one
bilateral pair of sclerotomes and the caudal half of the next pair of sclerotomes.
main mass of the sclerotome is termed the central sclerotome, that cells, which remain mesenchymal throughout somite formation, give
portion close to the notochord is termed the ventral sclerotome, and rise to the vertebral joints, intervertebral discs and the proximal ribs
the portions adjacent to the dermomyotome are termed dorsal sclero- (Christ et al 2004).
tome and lateral sclerotome. The central sclerotome remains close to The dermomyotome is a proliferative epithelium that produces cells


the dermomyotome; it will give rise to the pedicles and ventral parts of from four borders (Fig. 44.7). Proliferation at the dorsomedial edge
the neural arches, and the proximal ribs. The ventral sclerotomal cells, produces cells that elongate from the cranial to the caudal edge of the
which were always laterally abutting the notochord, proliferate to form dermomyotome beneath its apical surface as they move laterally. Cells
an axial cell population within the extracellular matrix of the perinoto- similarly proliferate from the cranial and caudal edges of the dermo-
chordal space, now termed the perinotochordal sheath. The dorsal myotome and these cells also elongate across it. The cells that are
sclerotomal cells develop relatively late; they invade the space between produced from these three edges are termed the epaxial myotome, and
the surface ectoderm and growing neural tube, and form the dorsal part will give rise to skeletal muscle dorsal to the vertebrae, i.e. the epaxial
of the neural arches. The lateral sclerotomal cells give rise to distal ribs musculature. At limb levels, cells de-epithelialize and migrate from the
and endothelial cells of blood vessels. Generally, there is a dorsolateral ventrolateral edges of the dermomyotome into the limb bud. Cells
expansion of the whole sclerotome rather than the medial migration of produced from this portion of the occipital somites migrate anteriorly
a population of sclerotomal cells (Gasser 2006); this can be seen in to give rise to the intrinsic muscles of the tongue. At interlimb levels,
Figures 44.4–44.6. Sclerotomal cells also give rise to the meninges sur- the hypaxial myotome will give rise to intercostal and abdominal
rounding the spinal cord, local tendons and ligaments. The somitocoele muscles, and extend from the ventrolateral edge of the epithelial


Development of sclerotomes
755
44
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dermomyotome into the body wall as development proceeds (Scaal and Sclerotome Vertebral body Nerve
Christ 2004).
It was thought that the somite gave rise to segmental portions of the
dermis of the skin, as well as bone and muscle. However, it is now clear
04
that the somitic contribution to the skin from the epithelial plate is
limited to de-epithelialization of dermomyotomal cells over the epaxial Base of occipital bone
muscles alone, which is a much smaller distribution than the segmental
portion of skin usually implied by the term dermatome. The concept C1
that an embryological dermatome, derived from the somite, produces C1
all of the dermis of the skin is therefore outdated.
The regularity of somite formation provides criteria for staging C1
embryos. The staging scheme proposed by Ordahl (1993) will be used
in the following account of relative somite development. Ordahl noted C2
C2
that morphogenetic events occur in successive somites at approximately
the same rate. The somite most recently formed from the unsegmented C2
mesenchyme is designated as stage I, the next most recent as stage II,
and so on. When the embryo has formed an additional somite, the ages
C3
of the previously formed somites increase by one Roman numeral. C3
According to this scheme, compaction occurs at stage 0, epithelializa-
C3
tion at stage I, formation of mesenchymal sclerotome cells from stage
V, and myotome formation from stage VI; early migration of the ventro-
lateral lip of the dermomyotome and production of myotome cells are C4 C4
still occurring at stage X.
C4
DEVELOPMENT OF SCLEROTOMES
C5
C5
Sclerotomal populations form from the ventral half of the epithelial
C5
somite. An intrasegmental boundary (fissure or cleft, sometimes termed
von Ebner’s fissure), which is initially filled with extracellular matrix
C6
and a few cells, appears within the sclerotome and divides it into C6
loosely packed cranial and densely packed caudal halves. The epithelial


plate and, later, the dermomyotome span the two half-sclerotomes. The C6
bilateral sclerotomal cell populations migrate towards the notochord
and surround it to form the perinotochordal sheath. They undergo a C7
C7
matrix-mediated interaction with the notochord, differentiating chon-
drogenetically to form the cartilaginous precursor of the vertebral
C7
centrum. The perinotochordal sheath starts to express type II collagen,
and this is believed to mark a chondrogenic fate, in those mesenchyme C8
cells that contact it. Each vertebra is formed by the combination of C8
much of the caudal half of one bilateral pair of sclerotomes with much
of the cranial half of the next caudal pair of sclerotomes. Their fusion T1
around the notochord produces the blastemal centrum of the vertebra
(Figs 44.8–44.9). The mesenchyme adjoining the intrasegmental T1
T1
sclerotomic fissure now increases greatly in density to form a well-
defined perichordal disc, which intervenes between the centra of two T2
adjacent vertebrae and is the future anulus fibrosus of the intervertebral
symphysis (‘disc’) (see below). T2
The basic pattern of a typical vertebra is initiated by this recombina- T2
tion of caudal and cranial sclerotome halves (Fig. 44.10), followed by
T3
differential growth and sculpturing of the sclerotomal mesenchyme that
encases the notochord and neural tube. The centrum encloses the noto-
chord and lies ventral to the neural tube. Condensation of the sclero- T3
T3
tomal mesenchyme around the notochord and right and left neural
processes can be seen in stage 15 human embryos. The neural processes
curve to enclose the neural tube and extend to the dorsolateral angles
Fig. 44.9 The contribution of the somites to the vertebrae. Each somite
of the centrum. The neural arch consists of paired bilateral pedicles
induces a ventral root to grow out from the spinal cord. When the


(ventrolaterally) and laminae (dorsolaterally), which coalesce in the
sclerotomes recombine, the cranial half of the first cervical sclerotome
midline dorsal to the neural tube to form the spinous process. On each
fuses with the occipital sclerotome above contributing to the occipital
side, three further processes project cranially, caudally and laterally
bone of the skull. The cervical nerves, beginning with C1, exit above the
from the junction of the pedicle and laminae. The cranial and caudal
corresponding vertebra. Nerve C8 exits below the seventh cervical
projections are the blastemal articular processes (zygapophyses), which
vertebra (C7), and thereafter nerves arise below their numbered vertebrae.
become contiguous with reciprocal processes of adjacent vertebrae;
(With permission from Larsen WJ 1997 Human Embryology, 2nd edn.
their junctional zones mark the future zygapophysial or facet joints. The Edinburgh: Churchill Livingstone.)
lateral projections are the true vertebral transverse processes. Bilateral
costal processes (ribs) grow anterolaterally from the ventral part of the
pedicles (i.e. near the centrum), from the neighbouring perichordal
disc, and, at most thoracic levels, with accessions from the next adjacent tube. Dorsal segmentation is influenced by the spinal ganglia; experi-
caudal pedicles. The costal processes expand to meet the tips of the mental removal of the ganglia results in fusion of the neural arches
transverse processes. The definitive vertebral body is compound, and is and the formation of a uniform cartilaginous plate dorsal to the neural
formed from a median centrum (derived from the cells of the perinoto- tube.
chordal sheath), and bilaterally from the expanded pedicle ends
(derived from the migrating sclerotomal populations). These portions
of the vertebral body fuse at the neurocentral synchondroses. INTERVERTEBRAL DISCS
The segmental nature of the vertebrae is promoted by the notochord
and neural floor plate, which induce the ventral elements of the verte- Vertebral centra are derived from caudal and cranial sclerotomal halves.


brae and repress dorsal structures, e.g. the spinous processes (see Fig. An intervertebral disc is formed from the free somitocoele cells within
17.7). Excision of the notochord in early embryos results in fusion of the epithelial somite that migrate with the caudal sclerotomal cells and
the centra and formation of a cartilaginous plate ventral to the neural from notochordal cells. The sclerotomal mesenchyme that forms the


Development of the back
756
5
noItceS
Superior articular process
Somites/
Sclerotomes
Transverse process Pedicle Costal head
Intervertebral disc
Spinous process Centrum
Inferior articular process
Proximal rib Distal rib
Fig. 44.10 The contribution of two adjacent somites to one vertebra and rib. The intervertebral disc and the costal head are derived from somitocoele
cells from one somite, which migrate with the caudal half of the sclerotome. The proximal rib is formed from caudal and cranial somite halves with no
mixing of cells; in the distal rib, there is more mixing of cranial and caudal cells as segmentation diminishes in the ventral body wall. (Redrawn with
permission from Christ B, Huang R, Wilting J 2000 The development of the avian vertebral column. Anat Embryol 202: 179–94, Springer.)
centra of the vertebrae replaces the notochordal tissue that it surrounds. the lower thoracolumbar region and remaining centres then appeared,
In contrast, the notochord expands between the developing vertebrae spreading regularly and rapidly in craniocaudal directions.
as localized aggregates of cells and matrix that form the nucleus pulpo- The major part of the vertebral body, the centrum, ossifies from a
sus of the intervertebral disc (see Fig. 44.8; Fig. 44.11). The intermedi- primary centre dorsal to the notochord. Centra are occasionally ossified
ate part of each perichordal disc, which forms the anulus fibrosus, from bilateral centres that may fail to unite. Suppression of one of these
surrounds the nucleus pulposus and differentiates into an external produces a cuneiform vertebra (hemivertebra), which is one cause of
laminated fibrous zone and an internal cuff (which lies next to the lateral spinal curvature (scoliosis). At birth and during the early post-
nucleus pulposus). The inner zone contributes to the growth of the natal years, the centrum is connected to each half-neural arch by a
outer. Near the end of the second month of embryonic life, it begins to synchondrosis or neurocentral joint. In thoracic vertebrae, costal facets


merge with the notochordal tissue, and is ultimately converted into on the bodies are posterior to neurocentral joints.
fibrocartilage. Genetic labelling experiments in the mouse indicate that During the first year, the arches unite first in the lumbar region and
notochordal cells act as embryonic precursors to all cells found within then throughout the thoracic and cervical regions. In typical upper
the nucleus pulposus of the mature intervertebral disc (McCann et al cervical vertebrae, centra unite with arches in about the third year, but
2012). in lower lumbar vertebrae, union is not complete until the sixth year.
The original sclerotomes are coextensive with the individual meta- The upper and lower surfaces of the bodies and apices of the transverse
meric body segments; each sclerotomic fissure, perichordal disc and and spinous processes are cartilaginous until puberty, at which time five
maturing intervertebral disc lies opposite the centre of each fundamen- secondary centres appear: one in the apex of each transverse and
tal body segment. It therefore follows that the discs correspond in level spinous process, and two anular epiphyses (ring apophyses) for the
to (i.e. form the anterior boundary of) the intervertebral foramina and circumferential parts of the upper and lower surfaces of the vertebral
their associated mixed spinal nerves, ganglia, vessels and sheaths. Pos- body. Costal articular facets are extensions of these anular epiphyses;
teriorly, the foramina are bounded by the capsules of the synovial facet they fuse with the rest of the bone at about 25 years. There are two
joints; the rims of the vertebral notches of adjacent vertebrae lie crani- secondary centres in bifid cervical spinous processes. Exceptions to this
ally and caudally. Thus, all the structures listed (and other associated pattern of ossification are described in the appropriate subsections in
ones) are often designated segmental, whereas vertebral bodies are Chapter 43.
designated intersegmental because of their mode of development. Vertebrae are specified as to region very early in development. If a
(For further discussion of resegmentation theories, the reader is group of thoracic somites is transplanted to the cervical region, ribs will


directed to Müller and O’Rahilly 1986, Huang et al 2000, Stern and still develop. It is the sclerotome that is restricted; the myotome will
Vasiliauskas 2000.) produce muscle characteristic of the new location.
DEVELOPMENT OF VERTEBRAE Occipitocervical junction
The initial movements of sclerotomal cells round the neural tube and In humans, the junction between the head and neck (occipitocervi-
the expression of type II collagen signal the blastemal stage of vertebral cal, craniovertebral or spinomedullary junction) is placed at the
development (see Fig. 44.11). Chondrification begins at stage 17, initi- boundary between the fourth and fifth somites (Müller and O’Rahilly
ating the cartilaginous stage. Each centrum chondrifies from one carti- 1994, Muller and O’Rahilly 2003, Hita-Contreras et al 2014). In
lage anlage. Each half of a neural arch is chondrified from a centre, avian embryos, where all embryonic stages may be obtained experi-
starting in its base and extending dorsally into the laminae and ventrally mentally, the occipitocervical boundary has been determined within
into the pedicles, to meet, expand and blend with the centrum. By stage the fifth somite (Wilting et al 1995). The boundary can first be deter-
23, there are 33 or 34 cartilaginous vertebrae but the spinous processes mined in the human at stage 12 by the observation of hypoglossal
have not yet developed, so the overall appearance is of total spina bifida nerve rootlets (Fig. 44.12A). At stages 14 and 15, the occipitocervical
occulta. Fusion of the spines does not occur until the fourth month. junction is seen between the hypoglossal rootlets and the first spinal
The transverse and articular processes are chondrified in continuity with ganglion (Fig. 44.12B–C).
the neural arches. Intervening zones of mesenchyme that do not become The segmental pattern present in the development of the somites


cartilage mark the sites of the facet joints and the complex of costover- can be seen rostrally in the developing skull base, where mesenchymal
tebral joints, within which synovial cavities later appear. condensations equivalent to the centra of occipital somites 2, 3 and 4
A typical vertebra is ossified from three primary centres, one in each are apparent. The first occipital somite disintegrates early and, together
half-vertebral arch and one in the centrum (see Fig. 43.34). Centres in with somites 2–4, forms the basiocciput. Occipital sclerotomes 3 and
arches appear at the roots of the transverse processes, and ossification 4 are the most distinct at stage 14, by which time the first three sclero-
spreads backwards into laminae and spines, forwards into pedicles and tomes have fused. The hypoglossal rootlets pass through the less dense
posterolateral parts of the body, laterally into transverse processes, and portion of occipital sclerotome 4, accompanied by the hypoglossal
upwards and downwards into articular processes. Classically, centres in artery. Occipital sclerotome 4 forms an incomplete centrum axially and
vertebral arches are said to appear first in upper cervical vertebrae in the exoccipital elements laterally. The latter are regarded as corresponding
ninth to tenth weeks, and then in successively lower vertebrae, reaching to neural arches and form the rim of the foramen magnum. The occipi-
lower lumbar levels in the twelfth week. However, in a radiographic tal condyles develop from the cranial part of sclerotome 5, which is
study of unsexed human fetuses (Bagnall et al 1977), a pattern was derived from the first cervical somite.
noted that differed from such a simple craniocaudal sequence. A regular In the occipitocervical junctional region, the centra formed from
cervical progression was not observed. Centres first appeared in the sclerotomes 5, 6 and 7 have a different fate from those more caudally
lower cervical/upper thoracic region, quickly followed by others in the placed, whereas the lateral portions of these sclerotomes generally


upper cervical region. After a short interval, a third group appeared in develop similarly to those of lower ones. In a study of occipitocervical


Development of sclerotomes
757
44
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A Vertebral formation prenatal development. Axial transverse sections Costotransverse joint (synovial)
Costovertebral joint (synovial)
Endochondral ossification in
costal process and centrum
Notochordal remnant
For centrum
Neurocentral
For costal growth cartilage
process – disappears
Cartilaginous spine
Intersegmental Subperiosteal in infancy
(growth cartilage)
(vascular) ossification in
For neural arch anastomosis neural arch Fusion between – disappears in infancy
advancing cartilages
Mesenchymal blastema with centres of chondrification Cartilaginous vertebra with centres Bony vertebra with cartilaginous growth region
of primary ossification (third month)
B Vertebral growth, postnatal development
Coronal sections through vertebral bodies Superior view of a vertebra
Ring epiphysis
(apophysis)
Articular facet
Growth plates Vertebral
growth plate
Growth centres for
transverse processes
and spine
1 – 1 year postnatal 2 – Adolescence 3 – Adolescence
C Adult vertebrae: derivation
Cervical Thoracic Lumbar Sacral
Embryonic vertebral derivation: centra
neural arches of embryonic vertebrae
costal processes
Fig. 44.11 A–B, Vertebral development through blastemal, cartilaginous and pre- and postnatal ossificatory stages. C, The derivation of the principal
morphological parts of adult vertebrae.
segmentation in human embryos, Müller and O’Rahilly designated the Y. The posterior arch of the atlas arises from the dense area of scle-
three complete rostral centra that develop in the atlanto-axial region X, rotome 5 at the level of X. The XYZ complex belongs to the axis, which
Y and Z (Fig. 44.13; Müller and O’Rahilly 1986, Muller and O’Rahilly means that the atlas does not incorporate a part of the central column
2003). They noted that the height of the XYZ complex is equal to that (Müller and O’Rahilly 1994). The posterior arch of the axis arises from


of three centra elsewhere. X is on the level of sclerotome 5, and Y and the dense area of sclerotome 6 and is at the level of Y and Z, particularly
Z are in line with sclerotome 6 and with the less dense portion of scle- Z. XYZ correspond to the three parts of the median column of the axis,
rotome 7. During stage 17, a temporary intervertebral disc appears where X represents the tip of the dens, Y represents the base of the dens,
peripherally between Y and Z. It begins to disappear in stage 21, and Z represents the centrum of the axis. The latter differs from other
although remains may be found in the adult. No disc develops between cervical vertebrae in that it is thicker and square-shaped.
X and Y. The origin of the anterior arch of the atlas is unclear. It is Most defects of the atlas do not contribute to abnormal occipitocer-
evident at stages 21–23 at the level of X or, sometimes, between X and vical anomalies and are not associated with basilar invagination.


Development of the back
757.e1
44
Retpahc
The bony elements of the occipitocervical junction may be thought
of as a central pillar (basiocciput and dens of the axis) surrounded by
two rings (the rim of the foramen magnum and occipital condyles
superiorly, and the anterior and posterior arches of the atlas inferiorly).
Bony anomalies involving this embryologically complex region may
produce altered, unstable anatomical relationships that result in the
compression of underlying neural and/or vascular structures and may
also compromise cerebrospinal fluid dynamics (Menezes 2008, Pang
and Thompson 2011, Pang and Thompson 2014). The development of
the cervical spine, particularly of the upper cervical vertebrae, is closely
related to the development of the basiocciput and exocciput; anoma-
lous development will affect both regions. For example, abnormal pro-
lapse of the dens into the foramen magnum, a condition termed basilar
invagination, has been attributed variously to hypoplasia of the basioc-
ciput (clivus), occipital condyles or atlas; deficiencies in the bony arches
of the atlas with spreading of the lateral masses; and atlanto-occipital
assimilation (Smith et al 2010). Malfusions of the caudal portion of
occipital sclerotome 4 and the cranial portion of cervical sclerotome 1
may produce defects of the occipital condyles. The proatlas, a transient
bony structure derived from the fourth occipital sclerotome, usually
fuses with the three upper occipital sclerotomes to form the occipital
bone and the dorsal part of the foramen magnum. Very rarely, remnants
of the proatlas persist into adult life; several malformations and anoma-
lies of the most caudal of the occipital sclerotomes have been attributed
to proatlas segmentation failures (Menezes and Fenoy 2009, Muhleman
et al 2012).
Chiari malformation type I (CM-I) is defined radiographically as a
displacement of the cerebellar tonsils of 5 mm or more below the
foramen magnum in young adults. Classic CM-I is thought to be a


congenital malformation, in which the developing brain is compressed
within a hypoplastic posterior fossa, forcing the cerebellar tonsils down
through the foramen magnum. Patients with classic CM-I have a char-
acteristically small posterior cranial fossa, particularly below Twining’s
line (a line joining the anterior tuberculum sellae and internal occipital
protuberance), the foramen magnum is constricted transversely, and
superior and inferior outlet areas are reduced; the suggested pathogen-
esis is premature stenosis of the basiexoccipital and exosupraoccipital
synchondroses (Milhorat et al 2010). Tonsillar herniation is aetiologi-
cally heterogeneous and there are many causes that do not appear to
be related to skull-base hypoplasia; CM-I can also occur in association
with disorders that appear to be unrelated to skull-base hypoplasia and
may be acquired (Tubbs et al 2007).


Development of the back
758
5
noItceS
A Rhombomeres A B
Sclerotomes
Neural crest D 7 X IX V VI II II V 1 M2 M1 Mesencephalon 1 Basioccipital
Occipitocervical
junction Notochord D2
Diencephalon
Neural crest Hypoglossal roots D1 2
Somites T Telencephalon
= 0.1 mm
3
Hypoglossal Cranial nerve (XII)
canal
B Accessory crest Otic vesicle
Exoccipital
and nerve
4
7
D
Occipitocervical 1
junction 4 3 2 1 X IX VIV IIII V M2 1
Cervical ganglion 1 IX M1 5 X
Cervical ganglion 2 X D2 Optic vesicle
D1
Sclerotomes 2
Notochord T Y
and sheath 6
Hypoglossal nerve = 0.2 mm
Z
3
Cervical
7
C Occipitocervical junction nerves
Accessory nerve
3
Cervical ganglion 1 4
Hypoglossal artery
Cervical nerve 8
Cervical ganglion 2
4
5
9
5 4 3-1 5
6
7 Intervertebral disc
= 0.18 mm
8 Fig. 44.13 The relationship between the centra and neural arches
Notochord and of the vertebrae and the related spinal ganglia and nerves. A scheme
perinotochordal sheath Sclerotomes 1–8
of the details of the early development of the occipitocervical region.
Fig. 44.12 Reconstructions of the occipitocervical region of human A, The column of sclerotomes from occipital somite 1. B, A dorsal view of
embryos. A, Stage 12: occipital somites innervated by hypoglossal fibres the developing vertebrae, with the centra in the middle and the bilateral
(small yellow circles). Three cervical somites are shown. The crest-derived components of the neural processes laterally. X, Y and Z are three centra


ganglia of cranial nerves V, VII, VIII, IX and X are shown in green. Neural that will produce the tip of the dens of the axis (X), the base of the dens
crest associated with the occipital somites is hypoglossal and perhaps of the axis (Y) and the centrum of the axis (Z). An intervertebral disc
accessory (also shown in green). B, Stage 14: the somites have appears temporarily between Y and Z during stage 17. No disc develops
transformed into sclerotomes and moved ventrally. The less dense cranial between X and Y. The occipital condyles are derived from the first
and dense caudal parts of the sclerotomes and occipital sclerotomes 1–4 cervical sclerotome. (After Müller F, O’Rahilly R 1994 Occipitocervical
are indicated. Hypoglossal fibres and cervical ventral rami migrate segmentation in staged human embryos. J Anat 185: 251–8. Permission
through the less dense parts of the sclerotomes. The occipital neural from Blackwell Publishing.)
crest is now seen to be mostly accessory. The cervical crest is
subdivided into spinal ganglia. A perinotochordal sheath can be seen
extending rostrally to the termination of the notochord. C, Stage 15: the
dense parts of sclerotomes 1–8 are shown. Intersegmental arteries are
visible in the less dense areas of the sclerotomes, as are the spinal nerve Third to seventh cervical vertebrae
fibres. In all diagrams, the occipitocervical junction is indicated by the red
line. (After Müller F, O’Rahilly R 1994 Occipitocervical segmentation in
In cervical vertebrae 3–7 (see Fig. 44.11), the transverse process is dorso-
staged human embryos. J Anat 185: 251–8. Permission from Blackwell
medial to the foramen transversarium. The costal process, correspond-
Publishing.)
ing to the head, neck and tubercle of a rib, limits the foramen
ventrolaterally. The distal parts of these cervical costal processes do not


Abnormalities of the axis are usually concerned with fusion of the dens normally develop; they do so occasionally in the case of the seventh
with the centrum of the second cervical sclerotomes. Using the classifi- cervical vertebra, and may even develop costovertebral joints. These
cation of the three complete centra that develop in the atlanto-axial cervical ribs may reach the sternum.
region as X, Y and Z (Müller and O’Rahilly 1986), failure of fusion of The seventh cervical vertebra is transitional in shape between cervical
X with the YZ complex produces an ossiculum terminale, a dissociated and thoracic vertebrae. The laminae are longer than other cervical ver-
apical odontoid epiphysis. Failure of fusion of the XY complex with Z tebrae in the neonate and lie almost perpendicular to the basal plane;
at the dentocentral synchondrosis, or maintenance of the transitory the inferior articular facets are more upright, and resemble those of
intervertebral disc at this point, produces an os odontoideum, thought thoracic vertebrae; and, in the lateral view, the superior articular facets
to be induced by excessive movement at the time of ossification of the extend transversely to the top of the transverse processes.
dens (Crockard and Stevens 1995). Hypoplasia and aplasia of the X and Anomalies of the lower cervical vertebrae are generally caused by
Y centra, and aplasia of the Z centrum, will all lead to reduced size of inappropriate cervical vertebral fusion; collectively, this is termed
the dens. There are widely differing views about whether this will lead Klippel–Feil syndrome. This term includes all congenital fusions of the
to atlanto-axial instability. cervical spine, from two segments to the entire cervical spine. Affected
For further details of the development of the human craniovertebral individuals have a low posterior hairline, short neck and limitations of
joints and associated ligaments, see Hita-Contreras et al (2014). head and neck movement. Scoliosis and/or kyphosis is common.


Development of sclerotomes
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Thoracic vertebrae additional vertebra occurs over a period of 2–3 weeks; S2 is ossified by
22 weeks.
Very rarely, significant malformation of the sacral or lumbosacral
At stage 23, the neural processes of thoracic vertebrae are short, slightly
vertebrae may develop, often in association with a maternal history of
bifurcated and joined by collagenous fibres. The transverse process is
diabetes. When there is sacral agenesis, motor paralysis is profound
prominent. The three facets for articulation with the ribs at the costover-
below the affected vertebral level, whereas the sensory disturbance does
tebral and costotransverse joints are present. The thoracic neurocentral
not relate to the vertebral level so clearly and sensation may be present
and posterior synchondroses are not fused in the neonate; the posterior
down to the knees. Bladder involvement is a consistent feature.
synchondroses close within 2–3 months of postnatal development and
the neurocentral synchondroses are open until 5–6 years of age.
In general, the thoracic spine develops ahead of the cervical
SPINA BIFIDA
and lumbar spine. However, towards the end of the second month,
ossification begins in the cartilaginous vertebrae in a craniocaudal
progression. Spinal dysraphism is a generic term covering a wide range of develop-
mental abnormalities of the vertebral column and spinal cord, charac-
Ribs terized by a failure of fusion of midline structures (Sepulveda et al
Ribs usually develop in association with the thoracic vertebrae. Occa- 2012). The spectrum of defects includes non-fusion of the entire neural
sionally, they can arise from the seventh cervical and first lumbar tube with no vertebral arch development (craniorachischisis); non-
vertebrae. fusion of the rostral portion of the neural tube with no calvarial or
The costal processes attain their maximum length as the ribs in the occipital development (anencephaly); and non-fusion of caudal por-
thoracic region. Each rib originates from lateral sclerotomal popula- tions of the neural tube and local failure of vertebral arch development


tions, and forms from the caudal half of one sclerotome and the cranial (spina bifida) (see Fig. 17.8; Figs 44.14–44.16). Spina bifida may be
half of the next subjacent sclerotome (see Fig. 44.10). The head of the obvious at birth or before (e.g. spina bifida cystica and spina bifida
rib develops from somitocoele cells from one somite, which migrate aperta), or not obvious at birth (spina bifida occulta). Spina bifida
with the caudal half of the sclerotome. The proximal portion of the rib cystica occurs where the meninges have developed adjacent to or over
forms from both caudal and cranial sclerotomal halves; there is no the defective neural tissue. Local accumulation of cerebrospinal fluid
mixing of cells from these origins. The distal portion of the rib forms may push a defective neural plaque or spinal cord superficial to the level
from caudal and cranial sclerotomal halves; these cells mix as the rib of the vertebrae, so forming a meningomyelocele. Alternatively, if one
extends into the ventral body wall and segmentation diminishes. or two spinous processes are absent, a meningeal sac may protrude in
The ribs arise anterolaterally from the ventral part of the pedicles, the midline above a formed spinal cord, producing a meningocele. In
and form bilateral costal processes that expand to meet the tips of the spina bifida aperta, the exposed spinal cord has no cystic covering,
transverse processes. As they elongate laterally and ventrally, they come although it may be covered by thin membranes. Meningomyelocele
to lie between the myotomic muscle plates. In the thorax (see Fig. occurs in thoracolumbar, lumbar or lumbosacral regions; sacral lesions
44.11), the costal processes grow laterally to form a series of precarti- are less common. The vertebral lesion usually extends cranially further
laginous ribs. The transverse processes grow laterally behind the verte- than the neural lesion, showing deformities of the vertebral bodies and


bral ends of the costal processes, at first connected by mesenchyme that laminae. Vertebrae may be wedge-shaped or hemivertebrae, and ribs
later becomes differentiated into the ligaments and other tissues of the may be fused or absent. In spina bifida occulta, dysraphism affects
costotransverse joints. The capitular costovertebral joints are similarly primarily the vertebrae; the neural and meningeal elements are not
formed from mesenchyme between the proximal end of the costal proc- herniated to the surface and so the underlying defects are masked by
esses and the perichordal disc, and the adjacent parts of two (sometimes the skin. This closed form of spina bifida affects about 5% of the popu-
one) vertebral bodies, which are derived from the neural arch. Ribs 1–7 lation. It usually occurs in the lumbosacral or sacral segments; affected
(vertebrosternal) curve round the body wall to reach the developing individuals have bifid spinous processes often at the lumbosacral junc-
sternal plates. Ribs 8–10 (vertebrochondral) are progressively more tion and some have a pigmented naevus, angioma, hirsute patch,
oblique and shorter, only reaching the costal cartilage of the rib above, dimple or dermal sinus on the overlying skin.
and contributing to the costal margin. Ribs 11–12 are free (floating), Prior to antenatal diagnosis of spina bifida by ultrasonography, most
and have cone-shaped terminal cartilages to which muscles become live births with spina bifida cystica had a meningomyelocele. Prenatal
attached. diagnosis of meningomyelocele and termination of affected fetuses
have led to a significant decrease in the incidence of live births with
Lumbar vertebrae this condition.
Tethering of the cord and diastematomyelia
The ribs do not develop distally in lumbar vertebrae (see Fig. 44.11).
Their proximal parts become the ‘costal processes’, while the morpho-
logically true transverse processes may be represented by the accessory The greatest change of vertebral length occurs within the last 6 months


processes of the vertebrae. Occasionally, movable ribs may develop in of intrauterine life (Ch. 17). When there is any degree of spina bifida,
association with the first lumbar vertebra. the upward migration of the spinal cord, which normally occurs within
Lumbar intervertebral discs are thicker than thoracic discs. By stage the vertebral canal during growth, is limited, and the cord is said to be
23, the anulus fibrosus can be seen in the peripheral part, and internally, tethered. Neurological deficits caused by this condition in neonates
the notochordal cells are expanding to form the nucleus pulposus.
Sacrum A B C D
Sacral vertebrae have lower centra and are narrower overall from side
to side than their thoracic and lumbar counterparts. Each sacral vertebra
is composed of a centrum and bilateral neural processes. The contribu-
tion of the costal processes to sacral development was examined by
O’Rahilly et al (1990). These authors divided the neurocentral junc-
tional area into two parts: anterolateral or alar, and posterolateral. They
found the alar element in sacral vertebra 1 to be novel, since it was
absent in lumbar vertebra 5. There is support for this view if the course
of the dorsal rami of the spinal nerves is used to distinguish the costal
elements ventrally from the transverse elements dorsally. The alar ele-
ments of the sacral vertebrae are ventral to the sacral dorsal rami, and
both costal and transverse portions are posterolateral. The alar element
of S1 and S2 forms the auricular surface of the sacrum. At stage 23, the Normal Myelocele Meningomyelocele Meningocele
cartilaginous sacral vertebrae have joined and the outline of the future Fig. 44.14 The spectrum of spina bifida. A, Intact neural tube. B, Open
bone can be recognized. Individual pedicles and laminae are very small spina bifida – myelocele. C, Open spina bifida – myelomeningocele. D,
and can be detected in S3–5. Closed spina bifida with meningocele. (Redrawn with permission from


Ossification of the vertebral column proceeds in a craniocaudal Tulio G, et al, Neural Tube Defects: Craniorachischisis and Spina Bifida,
direction. After 16 weeks, it has progressed to L5. Ossification of each in Obstetric Imaging (Copel J (ed), Expert Radiology, Series, 2012).)


Development of the back
759.e1
44
Retpahc
A B C
A B C
Fig. 44.15 Fetal myelomeningocele. Upper panel: three-dimensional ultrasound maximum transparency mode shows the spinal defect in the coronal (A
and B) and oblique (C) views. Arrows in (A) depict the vertebral dysraphism. The level of the defect can be clearly established (numbers 1 to 4 in (B) show
the lumbar vertebrae). Lower panel: three-dimensional ultrasound surface rendering mode shows the spinal defect in the sagittal (A and B) and coronal (C)
views. (With permission from Sepulveda W, Ximenes R, Wong AE et al 2012 Fetal magnetic resonance imaging and three-dimensional ultrasound in clinical
practice: applications in prenatal diagnosis. Best Pract Res Clin Obstet Gynaecol 26:593–624.)
Fig. 44.16 Fetal magnetic resonance image shows a lumbar myelomenin-
gocele in the sagittal section. (With permission from Sepulveda W, Ximenes
R, Wong AE et al 2012 Fetal magnetic resonance imaging and three-
dimensional ultrasound in clinical practice: applications in prenatal diagno-
sis. Best Pract Res Clin Obstet Gynaecol 26:593–624.)