Sexual dimorphism of the posterior cervical spine muscle attachments

Abstract

Cervical spinal injury and neck pain are common disorders with wide physical implications. Neck pain and disability are reported to occur in females more often than in males, and chronic or persistent neck pain after whiplash is twice as common in females. Female athletes also sustain a higher percentage of concussions compared to male athletes. Still, while sexual differences in clinical presentation and outcome are well-established, the underlying etiology for the disparity remains less clear. It is well-established that the origin and insertion landmarks of posterior neck muscles are highly variable, but we do not know if these interindividual differences are associated with sex. Expanding our knowledge on sexual dimorphism in the anatomy of the cervical muscles is essential to our understanding of the possible biomechanical differences between the sexes and hence improves our understanding as to why females suffer from cervical pain more than males. It is also of paramount importance for accurate planning of posterior cervical spine surgery, which cuts through the posterior cervical musculature. Therefore, our main objective is to characterize the anatomy of posterior neck musculature and to explore possible sexual differences in the location of their attachment points. Meticulous posterior neck dissection was performed on 35 cadavers, 19 females, and 16 males. In each specimen, 8 muscle groups were examined bilaterally at 45 osseous anatomical landmarks. Muscles and their attachment sites were evaluated manually then photographed and recorded using Microscribe Digitizer technology built into 3D models. A comparison of attachment landmarks between males and females for each muscle was conducted. Out of the eight muscles that were measured, only two muscles demonstrated significant sex-related anatomical differences-Spinotranversales (splenius capitis and cervicis) and Multifidus. Male Spinotransversales muscle has more attachment points than female. It showed more cranial insertion points in the upper cervical attachments (superior nuchal line, C₁ posterior tubercle, and mastoid process) and more caudal insertion points in the spinous processes and transverse processes of the lower cervical and upper thoracic vertebrae. Thus, the male subjects in this study exhibited a greater coverage of the posterior neck both cranially and caudally. Female Multifidus has more attachment points on the spinous processes and articular processes at middle and lower cervical vertebrae and at the transverse processes of the upper thoracic vertebrae. All remaining muscles exhibited no sexual differences. Our findings highlight, for the first time, a sexual dimorphism in attachment points of posterior cervical musculature. It reinforces the notion that the female neck is not a scaled version of the male neck. These differences in muscle attachment could partially explain differences in muscle torque production and range of motion and thus biomechanical differences in cervical spine stabilization between sexes. It sheds a much-needed light on the reason for higher whiplash rates, concussion, and chronic cervical pain among females. Surgeons should take these sexual morphological differences into consideration when deliberating the best surgical approach for posterior cervical surgery.

Keywords: gender; multifidus; neck muscles; neck pain; sex; splenius; whiplash.

FIGURE 1

A 3D model showing Microscribe Digitizer outline of a male cadaver with separately marked Spinotransversales muscle defined borders. Splenius capitis is displayed as a continuous line and Splenius cervicis as a dotted line. Muscles are shown from three views: top, left, and right. Spinous process (SP) is marked as squares, transverse process (TP) is marked as circles, mastoid process (MP) is marked as triangles, and superior nuchal line (SNL) is marked as a dashed line

FIGURE 2

Percentage of presence of Spinotransversales muscles for male and female subjects at statistically significant cervical anatomical attachment points. Spinous process (SP), transverse process (TP), mastoid process (MP), superior nuchal line (SNL). Statistical significance between columns marked as *p < 0.05;**p < 0.01;***p < 0.001

FIGURE 3

A 3D model showing Microscribe Digitizer outline of Spinotransversales muscle borders shown from three views: top, left and right. (a) Representing the male muscle and (b) representing the female muscle. Both muscles are seen as a continuous linescontinuous line, anatomical landmarks; spinous process (SP) C₁–T₆ marked as squares, facet joint capsules (FJC) C_2‐3 ‐ C_6‐7, and transverse process (TP) C₇–T₆ marked as circles, mastoid process (MP) marked as triangles, and superior nuchal line (SNL) marked as a dashed line

FIGURE 4

Image portraying an example of the anatomical structure of both male and female Spinotransversales muscles seen in cadaver subjects

FIGURE 5

Percentage of presence of Multifidus muscles for male and female subjects at statistically significant cervical anatomical insertion points: spinous process (SP), transverse process (TP), facet joint capsules (FJC), Mastoid process (MP), comparing 70 male and female cadaver muscles. Statistical significance between columns marked as *p < 0.05;**p < 0.01

FIGURE 6

A 3D model Microscribe Digitizer model outlining of an example of Multifidus muscle borders shown from three views: top, left and right. (a) representing male and (b) representing female muscles. Both muscles seen as a continues lines, anatomical landmarks; spinous process (SP) C₁–T₆ marked as squares, facet joint capsules (FJC) C_2‐3–C_6‐7 and transverse process (TP) C₇‐T₆ marked as circles, mastoid process (MP) marked as triangles, superior nuchal line (SNL) marked as a dashed line, and multifidus slips seen in top view and marked as a continuous line

FIGURE 7

Image portraying an example of the anatomical structure of both male and female Multifidus muscles seen in cadaver subjects