the goal of the cervical surgery is to relieve the compression on the nerves and/or spinal cord , and anterior or posterior cervical approach is used to access the cervical spine depend on the type of surgery required6 ) .
the anterior approach can allow the surgeon access to almost the entire cervical spine , and often helps maintain the normal curvature of the spine4 ) .
the textbook of korean spinal neurosurgery society provide that general external landmarks : the hyoid bone corresponds to c3 ; the thyroid cartilage corresponds to c4 - 5 ; and the cricothyroid membrane identifies the c6 vertebral level3 ) .
cervical spine level estimation according to the external landmarks is useful , quick and less exposable to radiation , but , sometimes cervical level confusion to the target cervical level could be happen .
herein , the authors reviewed the corresponding between the neck external landmarks and cervical levels .
between january and december 2013 , the authors retrospectively reviewed cervical lateral radiographs which were taken in single university hospital by various cervical problems .
all radiographs and the patient 's information such as sex , age , and heights were collected using the retrograde medical chart review .
the neck external landmarks were defined as mandible angle , hyoid , thyroid , and cricothyroid in radiographs : mandible angle as the posterior border at the junction of the lower border of the ramus of the mandible ; hyoid as the superior surface of anterior margin of hyoid ; thyroid as the inferior margin of laryngeal incisures ; and crocithyroid as the lower border of the laryngeal prominence which connected to median cricothyroid ligament .
these landmarks and horizontally lined to cervical spine were corresponded in radiographs , and each reference level were measured as cervical vertebral body level or cervical disc level ( fig .
1 ) . for example , if the horizontal line were corresponded to c3 vertebral level , it was measured as " c3 " , and if the line were corresponded to the disc level between c5 and c6 , it was measured as " c5.5 " .
the authors used the imaging programs for horizontal line drawing between landmarks and cervical level .
the results are expressed as the meanstandard deviation . paired student t - test was used to assess the statistical differences using sas software for windows ( sas institute inc . ,
the average reference levels of cervical landmarks were c2.13 with mandible angle , c3.54 with hyoid , c5.12 with thyroid , and c6.01 with cricothyroid ( table 1 , fig . 2 and 3 ) .
the reference levels of cervical landmarks were differently observed by sex , age , and height accordingly mandible from disc level of c1 and c2 to c3 , hyoid from disc level of c2 and c3 to c5 , thyroid from disc level of c3 and c4 to c7 , and cricothyroid from c4 to disc level of c7 and t1 .
the reference levels were statistically differently observed according to the sex , age , and height ( p<0.05 ) , but the average difference were not meaningful to clinics because of its minimal actual differences ( within 0.5 cervical level ) .
the male group showed more wide variety of thyroid cervical reference level ( c4-c7 ) compared to female group ( c3.5-c6 ) .
indeed , taller subjects ( > 180 cm ) showed more narrowed cervical reference level of mandible , hyoid , thyroid , and cricothyroid compared to others ( < 180 cm ) .
the age also contributed the range of cervical reference level , and the range were slightly narrowed accordingly the ages of subjects ( for example , the cricothyroid reference level was narrower from c4-c7.5 in 1 decades to c5.5-c7 in 7 decades .
the mandible was comparatively observed in constant level ( c2 - 3 ) , but , the hyoid , thyroid , and cricothyroid showed relatively wider variation of reference cervical level according to the sexes with statistical differences ( p<0.05 ) .
cervical anterior approach is one of most common procedures to treat the cervical pathology . during the performing this technique ,
the general cervical spine estimation by external landmarks is useful , quick and less exposable to radiation techniques .
so , most textbooks advert about the external landmarks for cervical operation . yoomans neurosurgery5 ) commented that " the cricothyroid membrane identifies the c6 vertebral level .
the thyroid cartilage corresponds to c3 - 4 , and the space halfway between these two points corresponds to c4 - 5 .
a distance of two finger - breaths above the clavicle usually to c6 - 7 , and a distance of three fingerbreadths corresponds to c4 - 5 .
the c6 anterior tubercle can be palpated in thin patients . " , lu et al .
summarized that the external landmarks as hard palate to arch of atlas , lower border of mandible to c2 - 3 , hyoid to c3 , thyroid cartilage to c4 - 5 , cricoid cartilage to c6 , and carotid tubercle ( anterior transverse process ) to c65 ) .
the textbook of korean spinal neurosurgery society provide that general external landmarks : the hyoid bone corresonds to c3 ; the thyroid cartilage corresponds to c4 - 5 ; and the cricothyroid membrane identifies the c6 vertebral level3 ) .
these provided references were similar but slightly different references were observed according to different authors . in this study
also different reference levels as our result indicated : c2.13 with mandible , c3.54 with hyoid , c5.12 with thyroid , and c6.01 with cricohyoid .
but , the more important information is that the reference levels of cervical landmarks were differently observed by sex , age , and somatometric measurement ( height ) accordingly mandible from c1 to c3 , hyoid from disc level of c2 and c3 to c5 , thyroid from disc level of c3 and c4 to c7 , and cricohyoid from c4 to disc level of c7 and t1 .
this result were similar to previous other study which investigated the vertebral levels of the hard palate , hyoid bone , thyroid cartilage , cricoid cartilage , and bifurcation of the common carotid artery using computed tomography8 ) .
only the vertebral level of the hard palate ( c1 ) was consistent with contemporary descriptions .
other landmarks were located most frequently at the following vertebral levels : the center of the body of the hyoid bone at c4 ( 54% of cases ) ; the superior limit of the laminae of the thyroid cartilage at c4 in women ( 60% ) and c5 in men ( 52% ) ( p=0.02 ) ; the inferior border of the cricoid cartilage in the midline anteriorly at c6 in women ( 37% ) and c7 in men ( 47% ) ( p=0.008 ) ; and the bifurcation of the left and right common carotid arteries at c3 ( left 56% , right 62% ) .
vertebral levels of key bony / cartilaginous structures in the neck differ from standard descriptions but in the absence of a standardized cervical axial plane have limited value and clinical utility . while performing the anterior approach to the cervical vertebral bodies , injury to important anatomic structures in the vicinity of the dissection represents a serious risk5 ) .
the midportion of the recurrent laryngeal nerve and the external branch of the superior laryngeal nerve are encountered in the anterior approach to the lower cervical spine . avoiding injury to the recurrent laryngeal
nerve and superior laryngeal nerve is a major consideration in the anterior approach to the lower cervical spine .
the sympathetic trunk is situated in close proximity to the medial border of the longus colli at the c6 level .
indeed , the spinal accessory nerve ( embedded in fibroadipose tissue in the posterior triangle of the neck ) is prone to injury .
its damage will result in an obvious shoulder droop , loss of shoulder elevation , and pain .
prevention of inadvertant injury to the accessory nerve is critical in the neck dissection , so , the surgical level should be minimized eorros during the cervical anterior approach . in this study ,
the average reference levels of cervical landmarks were c2.13 ( c1.5 - 3 ) with mandible , c3.54 ( c2.5 - 5 ) with hyoid , c5.12 ( c3.5 - 7 ) with thyroid , and c6.01 ( c4-c7.5 ) with cricohyoid as it showed in table 1 and figures .
the reference levels were statistically differently observed according to the sex , age , and height , but these difference were not meaningful to clinics within 0.5 cervical level .
the male group showed more wide variety of thyroid cervical reference level ( c4-c7 ) compared to female group ( c3.5-c6 ) .
indeed , taller subjects ( > 180 cm ) showed more narrowed cervical reference level of mandible , hyoid , thyroid , and cricohyoid compared to others ( < 180 cm ) .
the age also contributed the range of cervical reference level , and the range were slightly narrowed accordingly the ages of subjects ( for example , the cricohyoid reference level was narrower from c4-c7.5 in 1st decades to c5.5-c7 in 7 decades .
the mandible was comparatively observed in constant level ( c2 - 3 ) , but , the hyoid , thyroid , and cricothyroid showed relatively wider variation of reference cervical level according to the sexes with statistical differences ( p<0.05 ) .
surface landmarks did not correspond to exact levels of the cervical spine , and only provide general reference points .
 objectivea general orientation along the cervical spine could be estimated by external landmarks , and it was useful , quick and less exposable to radiation , but , sometimes it gave reference confusion of target cervical level . 
 the authors reviewed the corresponding between the neck external landmarks and cervical levels.methodstotally 1,031 cervical lateral radiographs of different patients were reviewed in single university hospital . 
 its compositions were 534 of males and 497 females ; 86 of second decades ( 10 - 19 years - old ) , 169 of third decades , 159 of fourth decades , 209 of fifth decades , 275 of sixth decades , and 133 of more than seventh decades ( > 60 years - old ) . 
 reference external landmarks ( mandible , hyoid bone , thyroid cartilage , and cricothyroid membrane ) with compounding factors were reviewed.resultsthe reference levels of cervical landmarks were c2.13 with mandible angle , c3.54 with hyoid bone , c5.12 with thyroid cartilage , and c6.01 with cricothyroid membrane . 
 the reference levels of cervical landmarks were differently observed by sex , age , and somatometric measurement ( height ) accordingly mandible angle from c1 to c3 , hyoid bone from disc level of c2 and c3 to c5 , thyroid cartilage from disc level of c3 and c4 to c7 , and cricothyroid membrane from c4 to disc level of c7 and t1.conclusionsurface landmarks only provide general reference points , but not correspond to exact levels of the cervical spine . 
 intraoperative fluoroscopy ensures a more precise placement to the targeted cervical level . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSION