pedal platform must be able to spread the stresses of standing and accentuating the longitudinal arches. It has been suggested that tension
moving, and be pliable enough to accommodate walking or running diminishes in the deeper plantar ligaments in this phase.
over uneven and sloping surfaces. To fulfil the second function, the foot The sole is transversely concave, both in skeletal form and usually
must be transformable into a strong, adjustable lever in order to resist in external appearance, and serial transverse arches are most developed
inertia and powerful thrust; a segmented lever can best meet such inferior to the metatarsus and adjoining tarsus. Transmission of force
stresses if it is arched. occurs at the metatarsal heads, to some degree along the lateral border
In infants and young children, fatty connective tissue on the plantar of the foot, and through subjacent soft tissues.
aspect may give the foot a flat appearance and soft tissues modify its In standing, with only body weight to support, both the intrinsic
appearance to varying degrees at all ages. The thickness of the medial and extrinsic muscles appear to relax (Perry 2010). If the longitudinal
midfoot plantar fat pad ranges from 3.1 to 4.9 mm (RiddifordHarland arches are allowed to sink as a result of muscular relaxation, the plantar
et al 2007). However, the skeleton of the human foot is normally ligaments tie the bones into an arched form. The medial arch is more
arched, and the sole of the foot is usually visibly concave. These arches elevated when the feet are together than when they are apart, i.e. inver
vary individually in height, especially the longitudinal in its medial sion with supination increases as the feet are separated. This medial sag
part. Since they are dynamic, their heights also differ in different phases can be countered by voluntary contraction of muscles such as tibialis
of activity. anterior. Pronation and supination ensure that in standing, whatever
The medial longitudinal arch contains the calcaneus, talus, navicular, the position of the feet, a maximal weightbearing area is grounded,
cuneiform and medial three metatarsals. Its summit, at the superior from the metatarsal heads along the lateral border of the foot to the


talar articular surface, takes the full thrust from the tibia and passes it calcaneus. The twist of the ligamentous skeleton of the foot imparted
backwards to the calcaneus, and forwards through the navicular and by pronation (which is partly undone in supination) prompts the liken
cuneiforms to the metatarsals. When the foot is grounded, these forces ing of the foot to a twisted but resilient plate (see Fig. 84.29), where
are transmitted through the three metatarsal heads and calcaneus (espe adequate ground contact was ensured whatever the angle between the
cially its tuberosity). The medial arch is higher, and more mobile and foot and leg, and adaptable resilience was imparted in standing and
resilient than the lateral arch; its flattening progressively tightens the progression.
plantar calcaneonavicular ligament and plantar aponeurosis. The lateral
arch is adapted to transmit weight and thrust rather than to absorb such
Bonus e-book images and video
forces; the long plantar and plantar calcaneocuboid ligaments tighten
as it flattens.
The lateral arch makes contact with the ground more extensively
than the medial arch. As the foot flattens, an increasing fraction of load Fig. 84.19 Turbo spin-echo, T1-weighted magnetic resonance (MR)
traverses soft tissues, which are inferior to the entire arch. The whole images of the left ankle of a woman aged 26.
lateral border usually touches the ground, whereas the medial border
does not. However, the medial border is visibly concave, usually even Video 84.1 Ankle block: surface anatomy.
in standing, which explains the familiar outline of human footprints


1451
48
RETPAHC
key references
KEY REFERENCES
BarclaySmith EB 1896 Astragalocalcaneonavicular joint. J Anat 30: Eekhof JA, Van Wijk B, Knuistingh Neven A et al 2012 Interventions for
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the importance of the joint complex in terms of hindfoot motion.
Jones FW 1949 Structure and Function as Seen in the Foot, 2nd ed. London:
Chen TM, Rozen WM, Pan WR et al 2009 The arterial anatomy of the Achil Baillière, Tindall & Cox.
les tendon: anatomical study and clinical implications. Clin Anat 22: Remains one of the classic texts on the foot.
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Kim PJ, Richey JM, Wissman LR et al 2010 The variability of the Achilles
A demonstration that the calcaneal tendon has three main territories of
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417–20.
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und Entwicklungsgeschichte 115:32–6.
A large cadaveric study in which the authors found that the arcuate artery
The first comprehensive account of talar blood supply and the identification
was present in only 16.7% of their sample of feet. They established that the
of the important artery of the tarsal canal.
lateral tarsal artery supplied dorsal metatarsal arteries 2–4 in 47.2% of
their sample, an arrangement that was more frequently found than the


commonly described arcuate artery.


48
RETPAHC
Ankle and foot
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Alexander R McN 1992 The Human Machine. New York: Columbia Univer ingrowing toenails. Cochrane Database Syst Rev 4:CD001541.
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Coskun N, Yuksel M, Cevener M et al 2009 Incidence of accessory ossicles
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Michaud TC 2011 Human Locomotion: The Conservative Management of
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300–5. Micheli LJ, Ireland ML 1987 Prevention and management of calcaneal apo
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Myerson MS 1997 Adult acquired flatfoot deformity: treatment of dysfunc Scheyerer MJ, Helfet DL, Wirth S 2011 Diagnostics in suspicion of ankle
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Surg 17:197–200. sett’s ligament). Skeletal Radiol 37:27–33.
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A large cadaveric study in which the authors studied the morphology and dome. J Foot Surg 25:369–73.
variations of the bands that comprise the deltoid ligament. They reported


Varner KE, Michelson JD 2000 Tarsal coalition in adults. Foot Ankle Int
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Perry J 2010 Gait Analysis: Normal and Pathological Function, 2nd ed. of the important artery of the tarsal canal.
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COMMENTARY
9.1
Nerve biomechanics
Although assumed postures and movements place physical forces on tive tissue fibres contribute to stiffness (Borschel et al 2003). Endoneur
peripheral nerves, the biomechanical properties of these nerves permit ial fluid and the perineurial sheath that maintains the fluid compartment
continued electrical signalling in the face of reasonable physical also contribute to stiffness. When a nerve is elongated, it undergoes
demands. Consider the sciatic nerve in an individual moving from a transverse contraction (Walbeehm et al 2004): the reduction in cross
standing to a seated posture. Where it crosses the lateral rotators of the sectional area increases endoneurial compartment pressure contribut
femur, the nerve undergoes lengthening, its crosssectional shape ing to nerve stiffness (Millesi et al 1995). As one might predict, nerve
becomes elliptical and narrowed, and endoneurial fluid is forced proxi roots, which lack a perineurial sheath, demonstrate less stiffness than
mally and distally away from the site of shape change. The segment of a peripheral nerve (Beel et al 1986). External features, such as nerve
the sciatic nerve in the thigh glides proximally, converging towards the branching and entering/exiting blood vessels, resist elongation of a
flexing hip joint and diverging away from the flexing knee joint (Topp nerve in its nerve bed, and increase nerve stiffness measured in situ
and Boyd 2006). (Millesi et al 1995).
When a nerve bed is elongated across a moving joint, the nerve
undergoes longitudinal tension. The biomechanical response of that
nerve may be documented in a load–elongation curve, in which load
is measured in Newtons and length in millimetres (Haftek 1970). To
enable comparisons to be made between nerves, it is helpful to measure
their crosssectional areas and starting lengths, and translate the load–
elongation curves into stress–strain curves. Stress is defined as the inten
sity of force per unit crosssectional area, and may be reported in N/m2
or MPa (Pascal). Strain is defined as the ratio of change in length to the
o ler nig gi tn ha
.
l Tl hen reg et h d, isa tn ind
c
ti s
r


eo gf it oen
n
sr e ap reo r st ee ed
n
a s
in
a lop ae drc –e en lt oa ng ge
a
o tif
o
nth e
o
ro sr tig rein ssa –l
Stiffness
s tit ora ni n
o
fc u verv rye s
l
i( tF tli eg l.
o
9 a. d1 . o1 r) .
s
tI rn
e
st sh e
re
i sn ui lt ti sa l
i
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n
r ie fig cio ann
t
o ef
l
oth ne
g
ac tu iorv ne ,
o
a rp sp trl aic ina
.
Slope =
In the ‘linear behaviour’ region, there is a correlation between the
applied tensile load or stress and the elongation or strain. In the ‘plastic’
region of the curve, minimal tensile load or stress results in mechanical Toe region
failure and discontinuity of nerve structures.
Several structural features of peripheral nerves may be related to
segments of the stress–strain curve. Nerve in situ is under tension; the
transition between the ‘toe’ region and the ‘linear behaviour’ region of
the curve corresponds roughly with the in situ strain (Kwan et al 1992).
When severed, a peripheral nerve recoils approximately 10% of its
length. The elasticity observed in the stress–strain curve is due to struc
tures that make up visible, periodic light–dark bands that were initially
described by, and named after, Felix Fontana (Fontana 1781). These
bands vary in their angulation, width, spacing and periodicity; they
have long been assumed to be optical artefacts produced by the char
acteristic zigzag course of individual nerve fibres (i.e. axons, both
myelinated and unmyelinated) lying within the perineurial sheath. A
strong linear relationship between increasing nerve strain and decreas
ing band frequency and axonal undulations has been demonstrated
experimentally in rat sciatic nerve using Fourier analysis (Love et al
2013). In vivo microdissection studies coupled with computer model
ling suggest that the layers of perineurial cells are probably responsible


for the shortwavelength, largeamplitude bands and that wavy spiral
ling nerve fibres in the endoneurium are responsible for the long
wavelength, smallamplitude bands (Merolli et al 2012), the two
patterns merging to form the visible bands of Fontana. Interestingly, a
recent study of Trembler-J mice, a model of Charcot–Marie–Tooth
disease, suggested that the altered bands of Fontana in Trembler-J sciatic
nerve appear to be the result of unusually long, sinuous axons moving
Fig. 9.1.1 Typical load–elongation (A) and stress–strain curves (B) for a
out of phase (Power et al 2015). The zigzag, spiralling nerve fibres are
peripheral nerve. The transition between the toe region and the linear
thought to provide the elasticity evident in the ‘toe’ region of the stress–
region in the stress–strain curve (asterisk) has been shown to correspond
strain curve. Increasing strain in the ‘linear behaviour’ region of the
approximately with the strain in situ. The slope of the stress–strain curve
stress–strain curve causes the bands of Fontana to disappear (Pourmand
is called the modulus of elasticity and represents the stiffness of the
et al 1994).
nerve, as seen in the load–elongation curve. If the slope is steep, then the
Anatomical features have been studied to discern their contribution
nerve has more stiffness and is less compliant to elongation. If the slope
to the ‘linear behaviour’ region of the stress–strain curve. The slope of is less steep, then the nerve has less stiffness and is more compliant to
this region is termed Young’s modulus of elasticity, or in the case of the elongation. Once the nerve has reached ultimate strain, the structural
load–elongation curve, it indicates the stiffness of a nerve. A steep slope integrity of the nerve is overcome and the deformation is termed ‘plastic’
indicates that a nerve is stiff, and significant tensile force results in only or ‘permanent’. (From Topp KS, Boyd BS 2006 Structure and
modest change in length. The modulus of elasticity or stiffness of an biomechanics of peripheral nerves: nerve responses to physical stresses


e80 acellular nerve is similar to that of fresh nerve, indicating that connec and implications for physical therapist practice. Phys Ther 86:92–109.)
)N(
daoL
Plastic region
Ultimate load
12
1 680
Linear
behaviour
region
Ultimate
elongation
4
2
0
0 5 10 15 20
Elongation (mm)
Plastic region
Ultimate stress
12
1 680
Co inrr e ss itp uo sn td ras
i
nw *ith
Linear
behaviour
region
Modulus
of
elasticity
U slt ti rm aa inte
4 Slope
=
2 Toe region
0
0 5 10 15 20 25 30 35 40 45
Strain (%)
)aPM(
ssertS
Kimberly S Topp
A
B


Nerve biomechanics
e81
1.9
YRATNEMMOC
In the ‘plastic’ region of the stress–strain curve, increasing longitu increasing stress along the ‘linear behaviour’ region of the stress–strain
dinal stress causes nonrecoverable elongation and structural integrity curve, long before the ultimate strain is reached.
is lost. The force per unit crosssectional area at the point of mechanical Nerve is best described as a viscoelastic tissue because its biomech
failure is termed ultimate stress and occurs at ultimate strain. At this anical properties are timedependent. In response to fixed tension,
point, a nerve behaves like a viscous material (Haftek 1970). Along the nerve demonstrates creep or elongation over time (Grewal et al 1996).
stress–strain curve, structures within the nerve become impaired in a Slow elongation allows for higher ultimate strain (Haftek 1970, Rydevik
hierarchical fashion. With increasing tensile load, there is first sliding et al 1990). When stretched to a fixed length, nerve exhibits stress
and loss of tenuous connections in the interface between the layer of relaxation or a reduction in tension over time (Wall et al 1991, Driscoll
inner perineurial cells and the sheath of outer perineurial cells and et al 2002).
associated epineurial tissues (Tillett et al 2004, Georgeu et al 2005). Returning to the example of the sciatic nerve, during movement from
With continued load, the core of nerve fibres and inner perineurial cell standing to seated, the nerve bed is elongated posterior to the hip as
layers is disrupted and function is impaired or lost; ultimately, the forward trunk motion causes flexion at the hip joint. The sciatic nerve
sheath of outer perineurial and epineurial tissues ruptures. Given that is exposed to tensile load or stress with resultant elongation or strain.
acellular nerve demonstrates significantly lower tensile strength and Adjusting to these forces, segments of the nerve converge towards the
ultimate strain than fresh nerve (Borschel et al 2003), the perineurial flexing hip joint, with minimal motion in distant segments (Topp and
cell layer should be appreciated for its ability to withstand stress. Boyd 2006). Endoneurial pressure resists transverse contraction. When


Although definitive anatomical studies are lacking, it is likely that the seated posture is assumed and maintained, the nerve undergoes
microscopic disruptions of perineurial cell–cell connections occur with stressrelaxation – until the next postural adjustment.
REFERENCES
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root: biomechanics. Exp Neurol 91:30–40. explain the ‘Bands of Fontana’ in peripheral nerves. Muscle Nerve 46:
Borschel GH, Kia KF, Kuzon WM et al 2003 Mechanical properties of acel 540–7.
lular peripheral nerve. J Surg Res 114:133–9. Millesi H, Zoch G, Reihsner R 1995 Mechanical properties of peripheral
Driscoll PJ, Glasby MA, Lawson GM 2002 An in vivo study of peripheral nerves. Clin Orthop Relat Res 314:76–83.
nerves in continuity: biomechanical and physiological responses to Pourmand R, Oches S, Jersild RA 1994 The relation of the beading of myeli
elongation. J Orthop Res 20:370–5. nated nerve fibers to the bands of Fontana. Neuroscience 61:373–80.
Fontana F 1781 Traité sur le vénin de la vipere, sur les poisons américains, Power BJ, O’Reilly G, Murphy R et al 2015 Normal nerve striations are
sur le lauriercerise et sur quelques autres poisons végetaux. On y a joint altered in the TremblerJ mouse, a model of Charcot–Marie–Tooth
des observations sur la structure primitive du corps animal. Différentes disease. Muscle Nerve 51:246–52.
experiences sur la reproduction des nerfs et la description d’un nouveau Rydevik BL, Kwan MK, Myers RR et al 1990 An in vitro mechanical and
canal de l’œil, vol. 2. Florence: Nyon L’Ainè. histological study of acute stretching on rabbit tibial nerve. J Orthop Res
Georgeu GA, Walbeehm ET, Tillett R et al 2005 Investigating the mechanical 8:694–701.


shearplane between core and sheath elements. Cell Tissue Res 320: Tillett RL, Afoke A, Hall SM et al 2004 Investigating mechanical behavior at
229–34. a core–sheath interface in peripheral nerve. J Peripheral Nerv Sys 9:
Grewal R, Xu J, Sotereanos DG et al 1996 Biomechanical properties of 255–62.
peripheral nerves. Hand Clin 12:195–204. Topp KS, Boyd BS 2006 Structure and biomechanics of peripheral nerves:
Haftek J 1970 Stretch injury of peripheral nerve. Acute effects of stretching nerve responses to physical stresses and implications for physical thera
on rabbit nerve. J Bone Joint Surg 52:354–65. pist practice. Phys Ther 86:92–109.
Kwan MK, Wall EJ, Massie J et al 1992 Strain, stress and stretch of peripheral Walbeehm ET, Afoke A, de Wit T et al 2004 Mechanical functioning of
nerve. Rabbit experiments in vitro and in vivo. Acta Orthop Scand 63: peripheral nerves: linkage with the ‘mushrooming’ effect. Cell Tissue Res
267–72. 316:115–21.
Love JM, Chuang TH, Lieber RL et al 2013 Nerve strain correlates with Wall EJ, Kwan MK, Rydevik BL et al 1991 Stress relaxation of a peripheral
structural changes quantified by Fourier analysis. Muscle Nerve 48: nerve. J Hand Surg Am 16:859–63.
433–5.


COMMENTARY
Functional anatomy and biomechanics
9.2
of the pelvis
Andry Vleeming, Frank H Willard
This commentary focuses on the anatomy and biomechanics of the men and women (Vleeming et al 2012). The increase in mobility of the
pelvic girdle and, specifically, the sacroiliac joints. In bipeds, the pelvis pelvic ring seen in the post-pubescent female pelvis is functional in
serves as a basic platform with three large levers acting on it (the spine allowing passage for the child during labour. When the data are com-
and two lower limbs). Movement of the pelvic platform upon the hip bined from published studies employing RSA and appropriate placing
joints relative to the femur, such as flexion and extension (pelvic ante- of markers, the maximum sagittal rotation of the sacroiliac joint never
and retroversion), and rotation and abduction/adduction, strongly exceeds 3.6° and translation of the joint never exceeds 2 mm (Kibsgård
influences lumbar spinal movement. As well as this substantial exter- et al 2012). If these data are amalgamated with the observation that
nal pelvic motion, internal pelvic motion through the sacroiliac joint osteophytosis is rare in women, regardless of age (1.83% of females),
is essential for effectively transferring loads between the spine and and is not very common in men (12.27%; Dar et al 2008), it appears
lower limbs. It has been postulated that the sacroiliac joints act as that small sacroiliac joint movements are present, even at an advanced
important stress relievers in the ‘force–motion’ relationships between age (Vleeming et al 2012).
the trunk and lower limb (Vleeming et al 2007). These joints ensure
that the pelvic girdle is a flexible ring of bone that will not easily frac- Sacroiliac joint stability
ture under the great forces to which it might be subject, either from
trauma or from its many bipedal functions (Lovejoy 1988). Analysis of
To illustrate the importance of both myofascial and ligamentous stabil-
gait mechanics demonstrates that the sacroiliac joints provide suffi-
ity of the sacroiliac joint, the biomechanical principles of form and


cient flexibility for the intrapelvic forces to be transferred effectively to
force closure were introduced (Vleeming et al 1990a, Vleeming et al
and from the lumbar spine and lower extremities (Vleeming et al
1990b). Form closure refers to a theoretical stable situation in a joint
2007).
with closely fitting surfaces, where no extra forces are needed to main-
The sacroiliac joint is a highly specialized joint that lends stable (yet
tain the state of the system. With force closure (leading to joint com-
flexible) support to the upper body. Both the tightness of the well-
pression), both a lateral force and friction are needed to withstand
developed dorsal fibrous apparatus and the specific architecture of the
vertical load. The structural features that contribute to sacroiliac joint
sacroiliac joint result in limited mobility. Numerous researchers have
form closure are complementary ridges and grooves of the articular
tried to model sacroiliac joint function by studying its principal dis-
surfaces; dorsocranial ‘wedging’ of the sacrum into the ilia; a particular
placement characteristics. A common assumption of these studies is
high coefficient of friction in the sacroiliac joint (Vleeming et al 1990a,
that increased loading on the sacrum leads to tilting of the sacrum
Vleeming et al 1990b); and the integrity of the dorsal binding ligaments
ventrally (nutation), a process by which most dorsal sacroiliac joint
in particular, which are among the strongest in the body (Vleeming et al
ligaments are stretched and the dorsal aspects of the iliac bones are
1990a). Both form and force closure are necessary for balancing friction/
drawn together (Solonen 1957, Vleeming et al 1990a, Vleeming et al
compression in the sacroiliac joint. Force closure is the result of altered
1990b). Counternutation normally takes place in unloaded situations,
joint reaction forces by tensing ligaments, fasciae, muscles and ground
such as lying. Nutation implies a forward tilting of the sacrum relative
reaction forces (Vleeming et al 1990a, Vleeming et al 1990b). Force
to a posterior rotation of the ilia, and vice versa in counternutation.


closure ideally generates a perpendicular compressional reaction force
to the sacroiliac joint to overcome the forces of gravity (Vleeming et al
Sacroiliac joint movement
1990b).
Pelvic motion of males and females has been investigated by roentgen Biomechanical considerations and the active
stereophotogrammetric motion analysis (RSA). RSA is a technique for
straight leg raise (ASLR) test in pelvic girdle
measuring small movements and is regarded as the gold standard for
pain (PGP) patients
determining mobility in orthopaedics (Kibsgård et al 2012). Several
studies applied this technique to measure the mean sacroiliac joint
mobility, especially around the sagittal axis, in patients with pelvic PGP patients, who test positively on the functional ASLR test, show an
girdle pain (PGP; Sturesson 2007). The average mobility for men is inability to raise the leg while lying supine. This test can temporarily
about 40% less than for women. However, with age, there was no be normalized by manual anterior compression of the pelvis or the use
detectable decrease in total mobility in either gender (in patients up to of a pelvic belt. This suggests that increased unilateral motion of the
50 years old). In fact, there was a significant increase of mobility with sacroiliac joint could lead to impairment and failure to lift the leg
age for both moving from a ‘supine to sitting position’ and from ‘stand- (Mens et al 1999).
ing to lying prone with hyperextension’ position, particularly in women. Likewise, Sturesson (2007), using RSA, studied the effects of surgical
Likewise, gender differences of symphysial motion were analysed in a application of an anterior external fixation frame for severe PGP
group of 45 asymptomatic individuals. In men, the average frontal patients. This resulted in a mean reduction of movement of the sacro-
plane movement was 1.4 mm, and in nulliparous women 1.6 mm. iliac joint around the helical axis of 59% and around the X-axis of 74%.
However, in multiparous women, motion increased to 3.1 mm (Garras In addition, the ASLR test and other evidence-based PGP tests were


et al 2008). normalized (Vleeming et al 2008). When the frame was tightened, a
The increased sacroiliac joint mobility in females compared to males nutation movement of the sacrum was noticed (Sturesson 2007).
has possible anatomical correlates. The curvature of the sacroiliac joint This and other studies mentioned in this commentary imply that
surfaces is usually less pronounced in women to allow for greater ‘too much movement’ of the sacroiliac joint could be a significant factor
mobility (Vleeming et al 1990a). Also, the pubic angle differs between in the onset of PGP (Vleeming et al 2008).
REFERENCES
Dar G, Khamis S, Peleg S et al 2008 Sacroiliac joint fusion and the Garras DN, Carothers JT, Olson SA 2008 Single-leg-stance (flamingo) radio-
implications for manual therapy diagnosis and treatment. Man Ther 13: graphs to assess pelvic instability: how much motion is normal? J Bone
e82 155–8. Joint Surg Am 90:2114–18.


Functional anatomy and biomechanics of the pelvis
e83
2.9
YRATNEMMOC
Kibsgård TJ, Røise O, Stuge B et al 2012 Precision and accuracy measurement Vleeming A, Volkers AC, Snijders CJ et al 1990b Relation between form and
of radiostereometric analysis applied to movement of the sacroiliac function in the sacroiliac joint. Part II: Biomechanical aspects. Spine
joint. Clin Orthop Relat Res 470:3187–94. 15:133–6.
Lovejoy CO 1988 Evolution of human walking. Sci Am 25:118–25A. Vleeming A, Mooney V, Stoeckart R (eds) 2007 Movement, Stability and
Mens JM, Vleeming A, Snijders CJ et al 1999 The active straight leg raising Lumbopelvic Pain: Integration of Research and Therapy, 2nd ed. Edin-
test and mobility of the pelvic joints. Eur Spine J 8:468–73. burgh: Elsevier, Churchill Livingstone, p. 658.
Solonen KA 1957 The sacroiliac joint in the light of anatomical, roentgeno- Vleeming A, Albert HB, Ostgaard HC et al 2008 European guidelines for the
logical and clinical studies. Acta Orthop Scand 27:1–127. diagnosis and treatment of pelvic girdle pain. Eur Spine J 17:794–819.
Sturesson B 2007 Movement of the sacroiliac joint with special reference Vleeming A, Schuenke MD, Masi AT et al 2012 The sacroiliac joint: an over-
to the effect of load. In: Vleeming A, Mooney V, Stoeckart R (eds) view of its anatomy, function and potential clinical implications. J Anat
Movement, Stability and Lumbopelvic Pain: Integration of Research 221:537–67.
and Therapy, 2nd ed. Edinburgh: Elsevier, Churchill Livingstone,
pp. 343–52.
Vleeming A, Stoeckart R, Volkers AC et al 1990a Relation between form and
function in the sacroiliac joint. Part I: Clinical anatomical aspects. Spine
15:130–2.


COMMENTARY
9.3
Articularis genus
Stephanie J Woodley
Articularis genus, sometimes referred to as the ‘articular muscle of the capsule (DiDio et al 1967, Kimura and Takahashi 1987, Woodley et al
knee’ (Puig et al 1996), is a small muscle located deep to vastus inter 2012, Reider et al 1981). The muscle may be triangular, rectangular or
medius on the anterior aspect of the thigh (Fig. 9.3.1). Due to its distal trapezoid in shape (DiDio et al 1967, Toscano et al 2004, DiDio et al
capsular insertions, it is morphologically similar to the subanconeus 1969). There is considerable variation in the reported mean muscle
muscle (articularis cubiti) of the elbow joint, although subanconeus is length (between 6.2 and 14 cm in adults; Ahmad 1975, DiDio et al
considered to represent the deep fibres of the medial head of triceps 1967, Toscano et al 2004, Woodley et al 2012), as well as the number
brachii (rather than being a separate entity) and its function is likely to of constituent bundles, which typically ranges from 1 to 7 (Puig et al
differ from that of articularis genus (Tubbs et al 2006). From an evolu 1996, Ahmad 1975, DiDio et al 1967, Toscano et al 2004, Sakuma et al
tionary perspective, fibres corresponding to articularis genus have been 2014) but sometimes exceeds 10 (DiDio et al 1969, Reider et al 1981;
described in early tetrapods (Diogo and Molnar 2014) and it is compar see Table 9.3.1). The number of bundles may be underestimated when
able to the subcrureus muscle of primates including apes, gorillas and viewed using magnetic resonance imaging (Puig et al 1996, Woodley


lemurs (Murie and Mivart 1872, Hepburn 1892). A distinct, yet small, et al 2012), possibly because of the reasonably compact nature of this
articularis genus is also found in domestic animals such as the dog, cat muscle. Recent dissection evidence demonstrates that 6 of 11 different
and goat (Getty 1975, Kincaid et al 1996, Glenn and Samojla 2002, muscle bundles, organized in superficial, intermediate and deep layers,
Evans 2013). are consistently present (Woodley et al 2012; Fig. 9.3.4). The superficial
While articularis genus has been of interest to anatomists for over layer usually comprises a large central bundle bordered by lateral and
two centuries (refer to DiDio et al (1967) for a historical account), medial bundles; fascicles in some of these bundles may insert into the
controversy still surrounds many aspects of its architecture. Discrepan bursa through an intermediary thin areolar membrane (see Fig. 9.3.4).
cies in the literature are most likely to be the result of differences in A central bundle forms the intermediate layer, and two bundles (medial
study methodologies, particularly with respect to the type and scope of and lateral) make up the deepest part of articularis genus, and may be
data collected and/or in definitions of the muscle and its components. accompanied by a third, central bundle (Sakuma et al 2014). Bundle
To date, most studies of articularis genus have been dissectionbased;
there are only three studies in which modern imaging techniques have
been used to investigate morphology (Puig et al 1996, Roth et al 2004,
Woodley et al 2012) (Table 9.3.1). Ant
Articularis genus consists of staggered layers of bundles (Puig et al Sup Inf
1996, Kimura and Takahashi 1987, Woodley et al 2012) that arise from Post
the distal third of the anterior, medial and lateral surfaces of the femur
(Ahmad 1975, DiDio et al 1967, Woodley et al 2012, DiDio et al 1969)
(Fig. 9.3.2), with a broad attachment to the proximal and/or posterior


margins of the suprapatellar bursa (Puig et al 1996, Ahmad 1975,
Kimura and Takahashi 1987, Toscano et al 2004, Woodley et al 2012,
S VI I D
Sakuma et al 2014) (Fig. 9.3.3) and specific regions of the articular
Fig. 9.3.2 A medial view of the bundles of articularis genus arranged into
superficial (S), intermediate (I) and deep (D) layers, inserting into the
suprapatellar bursa. Note that some superficial fascicles insert into the
Fig. 9.3.1 An anterior
deep surface of the vastus intermedius (VI) tendon.
view of the thigh
showing articularis
genus (AG) deep to
the reflected vastus
intermedius (VI) muscle.
VI Other abbreviations: AG
N, nerve branch to
VI
articularis genus from
the femoral nerve;
AG P, proximal border *
N of bisected patella;
VM, vastus medialis;
* midpoint of
suprapatellar bursa.
*
VM
P
Fig. 9.3.3 A sagittal proton density MRI scan of the knee from a
Sup
Med Lat 23-year-old female. The distal portion of articularis genus (AG), deep to
e84 Inf vastus intermedius (VI), is seen inserting into the suprapatellar bursa (*).


Articularis genus
e85
3.9
YRATNEMMOC
Table 9.3.1 Summary of data relating to the morphology of articularis genus
Study Study characteristics (type, General muscle morphology; origin and insertion Architectural parameters
specimen number and sex)
DiDio et al Dissection following injection of Separated from VI by connective tissue and fat Muscle length: mean 8 cm (range 3–13 cm)
(1967) gelatine Shape: rectangular 56%; trapezoid 27%; inverted trapezoid 17% Most proximal point of insertion: mean 10.5 cm (range 8–16 cm)a
n = 156 adults O: distal femur, anterior, medial and lateral Bundles: n = 1–6, most commonly 1 or 2 (78%); same number bilaterally in
104 male, 52 female I: articular capsule at the level of the suprapatellar bursa, mostly 60% of individuals
proximal, centre. Also to medial and lateral aspects, Maximum bundle width: mean 0.9 cm (range 0.1–4.0 cm)
occasionally anterior and posterior
DiDio et al Dissection Shape: triangular 44%; lambdoidal 27%; rectangular 24%; Muscle length (fetal, n = 14): mean 4.4 cm; range 1.2–5.4 cm
(1969) n = 66 (fetuses, newborn, adults) unclassified 5% Maximum muscle width (fetal, n = 14): mean 0.9 cm; range 0.2–2.0 cm
Histology O: distal third of femur, anterior (92.4%) and lateral or medial Bundles: n = 1, 66.7%; n = 2 divergent but not separate, 27.3%; n = 2, 1.5%;
n = 16 fetuses aspects (30.3%) unaccounted, 4.5%. Same number bilaterally in 82% of individuals
29–34 weeks I: articular capsule at the level of the suprapatellar bursa
Ahmad Dissection Margins demarcated by fat and connective tissue Muscle length: mean 9 cm


(1975) n = 20 O: distal femur, usually anterior, occasionally medial and lateral Fibre direction: mostly vertical, some inferomedial or inferolateral
aspects Bundles: n = 1–2 most common, sometimes 3–4
I: suprapatellar bursa, proximal border. Central, medial or lateral, Innervation: branch of nerve to VI
occasionally posterior or anterior aspects
Reider et al Dissection Flat and wispy without distinct investing fascia Muscle width: range 1.5–3 cm
(1981) n = 24 from 48 adults O: anterior aspect supracondylar portion of femur
26 male, 22 femaleb I: joint capsule at suprapatellar bursa
40–90 years
Kimura and Dissection Superficial and deep layers, separated by fat Most proximal point of insertionc: range 14.9–16.5 cm (superficial layer);
Takahashi n = 44 adults Superficial layer: derived from lower muscle bundles of VI 10.2–11.9 cm (deep layer)
(1987) 36 male, 8 femaleb I: suprapatellar bursa, posterior aspect, spreading medially and Bundles: n = >10 bundles
laterally. Posterior aspect of capsule Deep layer: mean 3.4 bundles; range 1–7, usually 2
Superficial layer: 2 bundles
Bundle length: range 5.2–6.6 cm (superficial layer); 3.5–5.0 cm (deep layer)
Innervation: femoral nerve, one branch derived from the common branch to
VI and VL, the other from that to VM and VI (superficial layer). Deep layer
supplied by common branch to VM and VI
Puig et al MRI (prospective and retrospective) O: anterior surface of femur Maximal muscle transverse aread: mean 1.2 ± 0.3 cm2; range 0.8–2.0 cm2
(1996) n = 40 from 34 patients I: suprapatellar bursa, proximal surface, medial and lateral aspect Highest point of insertiona: mean 6.4 ± 0.5 cm; range 3.5–9.0 cm


19 male, 15 femaleb of posterior surface Bundles: n = mean 2.4 ± 0.7; range 1–4
16–56 years Most proximal bundle: mean length 4.7 ± 0.4 cm; range 3.1–6.6 cm; mean
anglee 11.4 ± 1.3°; range 6.0–17.5°; mean transverse diameter (at origin)
0.9 ± 0.08 cm; range 0.6–1.3 cm
Most distal bundle: mean length 2.8 ± 0.4 cm; range 0.9–3.5 cm; mean
anglee 12.4 ± 2.5°; range 5.0–20.0°
Roth et al MRI (retrospective) Anteroposterior muscle thickness: range 0.1–0.8 cm
(2004) 92 knees from 84 patients
42 male, 42 female
20–76 years
Toscano et al Dissection Separated from VI by fat of variable thickness Muscle length: mean 6.2 cm; range 3.8–8.5 cm
(2004) n = 65 from 44 adults Shape: trapezoid 40%; rectangular 33%; triangular and irregular Most distal point of insertionf: mean 3.0 cm; range 2.0–4.1 cm
36 male, 8 femaleb 27% Bundles: n = range 2–7, 4 most frequent (33%)
O: distal third of femur, usually anterior (43%) Bundle orientation: mostly vertical, some oblique
I: suprapatellar bursa, often anterior (57%)
Woodley et al MRI and dissection Distinct from VI Muscle lengthh: mean 13.9 ± 1.1 cm; 12.3–17.4 cm
(2012)g n = 18 adults Staggered arrangement comprised of three layers (superficial, Muscle PCSA: mean 1.5 ± 0.7 cm2; range 0.5–3.3 cm2
8 male, 10 female intermediate and deep)


71–97 years O: distal 32% of femur (range 29–42 cm), anterior, anterolateral Fascicle length: mean 5.9 ± 1.0 cm
Histology or anteromedial surfaces Bundles: n = mean 7 ± 1.8; range 4–10, same number bilaterally in 44% of
n = 4 adults I: proximal and/or posterior wall of suprapatellar bursa, directly or individuals; MRI mean 3.8 ± 0.8; range 2–5, significantly less than
4 male via a thin areolar membrane; deep surface of distal tendon of dissection (p <0.0001)
68–80 years VI (superficial bundles); knee joint capsule, posterior or Bundle orientation: mostly vertical (1–5°) except three bundles (two of
posteromedial surface (one intermediate and both deep which were in deep layer; mean range 11–15°)
bundles) Most proximal point of insertiona: mean 12.4 ± 2.9 cm; range 7.2–17.4 cm
Most distal point of insertiona: mean 3.3 ± 2.8 cm; range 0–10.4 cm
Fibre type: inconclusive due to variation
Sakuma et al Dissection Superficial and deep layers, separated by fat in some All measurements relate to deep layer
(2014) n = 40 from 22 adults Deep layer comprised of three bundles (medial, central, lateral) Bundles: n = 2.7 ± 0.5
13 male, 9 female O: anterior aspect of femur Bundle length: mean 5.4 ± 1.3 cm; medial bundle longer than lateral
77–98 years I: Junction of suprapatellar bursa and joint cavity proper (deep (p < 0.05)
fibres) Bundle areai: 5.5 ± 2.6 cm2; medial bundle larger than lateral (p < 0.05)
and central (p < 0.05) bundles
aProximal border or apex of patella used as landmark. bRepresents number of cadavers/participants, not specimens. cRelative to the distal end of the medial femoral condyle, unclear if data from one specimen or all specimens.


dIn this study ‘±’ refers to SEM. eAngle recorded between anterior surface of femur and posterior surface of muscle bundle. fRelative to superior edge of trochlea. gAll data relate to dissection measures unless stated. hIn this
study ‘±’ refers to standard deviation. iCalculated by multiplying bundle length by width.
Abbreviations: I, insertion; MRI, magnetic resonance imaging; n, number; O, origin; PCSA, physiological cross-sectional area; VI, vastus intermedius; VL, vastus lateralis; VM, vastus medialis.
orientation is predominantly longitudinal, although the deepest two it is possible that this arrangement enables the longer, superficial fasci
peripheral bundles may be oriented obliquely (Puig et al 1996, cles to accommodate changes in muscle length (Woodley et al 2012),
Ahmad 1975, Toscano et al 2004, Woodley et al 2012). Symmetry of particularly during knee flexion, where it is postulated that the length
bundle arrangement in the right and left limbs of individuals appears of each bundle of articularis genus elongates twofold at the endrange
variable (40–82%; DiDio et al 1967, Woodley et al 2012, DiDio et al of movement (Kimura and Takahashi 1987).
1969, Sakuma et al 2014). There has been longstanding debate as to whether articularis genus
Articularis genus is diminutive: its mean physiological crosssectional is a deep bundle of quadriceps femoris or an independent muscle
area of 1.5 ± 0.7 cm2 (cadaver specimens; Woodley et al 2012) indicates (DiDio et al 1967). While it is true that articularis genus and vastus
that it is capable of generating only a small amount of force. Fascicle intermedius share an innervation from branches of the femoral nerve
length (mean 5.9 ± 1.0 cm; Woodley et al 2012) is likely to be influ (Ahmad 1975, Kimura and Takahashi 1987; see Fig. 9.3.1), other ana


enced by bundle location (Puig et al 1996, Kimura and Takahashi 1987, tomical data tend to favour the view that articularis genus is an inde
Woodley et al 2012). The deepest, most distal layer of articularis genus pendent muscle. First, although a small percentage of superficial
contains the shortest fascicles, and the superficial, proximal bundles fascicles in the superficial bundles of articularis genus may insert into
contain the longest (Woodley et al 2012; see Fig. 9.3.4). Functionally, the deep layer of the distal tendon of vastus intermedius (see Fig. 9.3.2),


ARTiCulARis gENus
e86
9
NOiTCEs
the bulk of articularis genus appears distinct from the overlying quad
riceps (Kimura and Takahashi 1987, Woodley et al 2012). Second, the
two muscles may be delineated by a distinct fascial layer of variable
composition and thickness (Ahmad 1975, DiDio et al 1967, Toscano
et al 2004), although this feature is inconsistent (Kimura and Takahashi
1987, Woodley et al 2012, Toscano et al 2004). The fibre type profiles,
albeit limited to four cadaver specimens, appear to differ between artic
ularis genus and vastus intermedius (Woodley et al 2012). Perhaps the
most reliable method of defining articularis genus is by reference to its
A
discrete distal insertion sites into the suprapatellar bursa and/or joint
capsule (Kimura and Takahashi 1987, Woodley et al 2012). Articularis
P VI * AM S genus may hypertrophy (Puig et al 1996) or atrophy (Toscano et al
* S 2004) in parallel with quadriceps femoris, but the significance of these
adaptations is unknown.
When contemplating function, the architectural arrangement of
articularis genus is paramount because direct evidence regarding its
action is limited to a single study (Ahmad 1975). Ahmad observed
elevation of the knee joint capsule and synovial membrane on stimulat
ing a branch of the femoral nerve innervating articularis genus in three
patients undergoing surgical amputation. This finding is consistent with
previous hypotheses, suggesting that articularis genus is responsible for
retracting the suprapatellar bursa or knee joint capsule proximally
B (Ahmad 1975, DiDio et al 1967, Kimura and Takahashi 1987, Roth
et al 2004, Toscano et al 2004), potentially preventing interposition of
these structures between the patella and femur during extension of the
S I S knee joint (Ahmad 1975, DiDio et al 1967). Functional implications
may be elucidated from comparison with the dog stifle (knee) joint,
P * D


where articularis genus usually consists of two thin bundles (medial
and lateral) forming an ‘invertedV’ shaped muscle (Kincaid et al 1996),
approximately 2 mm wide (Evans 2013). This morphology, together
with the distribution of muscle spindles, which are located both in the
vicinity of the musclecapsule interface and isolated within the capsular
connective tissue, suggest that in addition to tensioning the suprapatel
lar bursa (Kincaid et al 1996, Evans 2013), articularis genus may have
an important proprioceptive role (Kincaid et al 1996).
A detailed knowledge of the morphology of articularis genus is rel
C evant to understanding normal function of the knee joint as well as its
Inf Sup
role in possible dysfunction; for example, the possible protective role
that articularis genus may have in counteracting impingement of cap
D sular tissues (Ahmad 1975, DiDio et al 1967) might be appropriate to
Fig. 9.3.4 The three layers of articularis genus. A, The superficial (S) layer, consider in some individuals who present with undifferentiated ant
where some fascicles insert into the suprapatellar bursa via an areolar erior knee pain (Woodley et al 2012). It is perhaps surprising that this
membrane (AM). Note the presence of a complete superior plica muscle has not been afforded more attention; given its complexity and
(arrowheads), positioned between the knee joint cavity and the variability, there is scope for further examination of its detailed anatomy
suprapatellar bursa. B, The intermediate (I) layer beneath the reflected and function, particularly in healthy volunteers. Techniques such as
superficial (S) bundles. C, Two deep (D) bundles, positioned laterally and dynamic magnetic resonance imaging and electromyography may be
medially. Other abbreviations: P, proximal pole of bisected patella; VI, useful adjuncts to morphological investigations, and assist in contribut
vastus intermedius tendon; * proximal border of the midpoint of the ing to a better understanding of the functional and clinical relevance
suprapatellar bursa.
of this interesting muscle.
REFERENCES


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DiDio LJ, Zappalá A, Carney WP 1967 Anatomicofunctional aspects of the Anat Histol Embryol 25:113–16 + 6 pl.
musculus articularis genus in man. Acta Anat (Basel) 67:1–23. Murie J, Mivart St G 1872 On the anatomy of the Lemuroidea. Trans Zool
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Anat Rec 297:1047–75. Roth C, Jacobson J, Jamadar D et al 2004 Quadriceps fat pad signal intensity
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5th ed. Philadelphia: WB Saunders Company. Volume 1 p. 852, Vol 2 the deep layer of articularis genus muscle. Folia Morphol 73:309–13.
p. 1531. Toscano AE, Moraes ASR, Almeida SKS 2004 The articular muscle of the
Glenn LL, Samojla BG 2002 A critical reexamination of the morphology, knee: morphology and disposition. Int J Morphol 22:303–6.
neurovasculature, and fiber architecture of knee extensor muscles in Tubbs RS, Oakes WJ, Salter EG 2006 The subanconeus muscle. Folia Morphol
animal models and humans. Biol Res Nurs 4:128–41 (Warsz) 65:22–5.
Hepburn D 1892 The comparative anatomy of the muscles and nerves of Woodley SJ, Latimer CP, Meikle GR et al 2012 Articularis genus: an anatomic
the superior and inferior extremities of the anthropoid apes. Myology and MRI study in cadavers. J Bone Joint Surg Am 94:59–67.
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