Anatomical Variation in Morphometry and Morphology of the Foramen Magnum and Occipital Condyle in Dried Adult Skull

Abstract

The foramen magnum (FM) is the largest opening and is outlet through which medulla oblongata and spinal cord along with meninges, vertebral arteries, anterior and posterior spinal arteries, tectorial membrane, alar ligaments, and spinal branch of the accessory nerve. Occipital condyle (OC) is an important part of craniovertebral or craniocervical junction located anterolaterally on either side of the FM. The objective of the study was to assess variations of FM and OC in Ethiopian dried adult skulls. Methods: Observation-based descriptive study design was employed. The study was undertaken in 54 FM and 108 OC of 54 adult Ethiopian skulls. The morphometry of FM and OC were determined using a sliding vernier caliper. Results: The mean values of anteroposterior and transverse diameters of the FM were 35.19 and 30.17 mm, respectively, and the mean area of the FM was 853.36 mm2. The shapes of FM were determined as round in 22.2%, oval in 18.5%, egg like in 20.4%, triangular in 3.7%, pentagonal in 11.1%, hexagonal in 7.4%, irregular in 13%, and rectangular in 3.7% of the cases. The mean length of right and left OC was 25.69 and 26.96 mm, respectively, and the mean widths of the right and left OC were 12.76 and 13.04 mm, respectively. Conclusion: Our study identified various shapes of FM and diameter. The anteroposterior diameter is greater than the transverse diameter. The morphometric study of OC confirmed the length of left side is significantly greater than the right side. Key Words: Foramen magnum, morphology, morphometry, shape (J Craniofac Surg 2019;30: 256–259) T he skull bone is rounded in shape and forms the frame work of the head skeleton. Except the mandible, lower jaw forming bone, all bones of the skull are firmly attached by fibrous joint named as suture. Unlike the roof, the base of the skull contains numerous foramina. The foramen magnum (FM) is the largest opening and an important landmark of the base of the skull. It is an outlet through which the medulla oblongata and spinal cord along with meninges, vertebral arteries, the anterior and posterior spinal arteries, tectorial membrane, alar ligaments, and spinal branch of the accessory nerve.1 Morphometric studies reported the variability of the shape of FM mainly classified as oval, egg-shaped, round, tetragonal, pentagonal, hexagonal, and irregular.2,3 Variations of the shape of FM have profound role on the diagnostic, clinical, and radiologic investigations.1 This may be because of compression of vital structures passing through FM in cases of FM herniation, FM meningioma, and FM achondroplasia.4 For instance, in the achondroplasic patients, the FM stenosis probably causes respiratory depression and increased sudden infant death rate.5 The occipital condyles (OCs), an important part of the craniovertebral or craniocervical junction, are located anterolaterally on either side of the FM.6 The OCs represent the cranial portion of the craniocervical junction. Each OC is oval in outline and oriented obliquely so that its anterior end lies nearer the midline than its posterior end. It is markedly convex anteroposteriorly, less so transversely, and its medial aspect is roughened by ligamentous attachments. The hypoglossal canal, directed laterally and slightly forwards, traverses deep to eac