The transverse occipital ligament: An anatomic, histologic, and radiographic study

Background context: The craniocervical region is an osteoligamentous complex that provides structural stability and movement by means of numerous ligaments. Fundamental knowledge of these ligaments is important for physicians who treat patients with disorders of this region to reduce morbidity and mortality. There is a paucity of data in the literature regarding the morphology, function, and classification of the transverse occipital ligament (TOL). Purpose: The purpose of this study was to investigate the prevalence, morphology, and variations of the TOL in a large number of adult human cadavers using dissection, histology, and digital radiography. Study design: Cadaveric laboratory study. Sample: Thirty-two formalin-fixed human adult cadavers were dissected in the study. Fourteen cadavers were found to have a TOL. Outcome measures: Measurements using a digital caliper, high-resolution digital photography, histologic staining with bright-field microscopy, and digital radiography. Methods: The posterior musculature and related soft tissues were dissected and underlying bony elements removed. The TOL was identified, isolated, measured, and then removed for histologic preparation with hematoxylin and eosin staining. Anteroposterior open-mouth digital radiographs of the upper cervical spine with monofilament attached to the TOL were used to demonstrate its relations to the occiput, atlas, and axis. Results: The TOL was present in 14 of 32 (44%) of the dissected cadavers (six male and eight female). Three types of TOLs were identified. Type 1 had bilateral connections to the alar ligaments and had fibers inserting onto the dens. Type 2 also had bilateral connections to the alar ligaments but did not have fibers inserting onto the dens. Type 3 neither had any connections to the alar ligaments nor had fibers that connected to the dens. Male cadavers always had a Type 1 TOL compared with 3 of 8 (38%) female cadavers and this difference was significant (p=.031). The TOL consisted of dense regular connective tissue with parallel arrangements of collagen fibers and interposed fibroblasts. Conclusions: Our data suggest that the TOL is not an anatomic variant and can be classified into three types. Future biomechanical studies can be designed to investigate the function of the TOL, although we hypothesize that it may act as a fulcrum during flexion and extension of the head because it is located between the apical ligament and superior crus of the cruciform ligament. Anteroposterior open-mouth digital radiographs revealed the location of the TOL with respect to the upper two cervical vertebrae. Future research should investigate the radiologic characteristics of the TOL using magnetic resonance imaging.