A morphometric analysis of the foramen magnum region as it relates to the transcondylar approach

Background. The transcondylar approach is being increasingly used to access lesions ventral to the brainstem and cervicomedullary junction. Understanding the bony anatomy of this region is important for this approach. The purpose of this study was to conduct a morphometric analysis of the hypoglossal canal (HC), occipital condyle (OC) and the foramen magnum (FM) as it pertains to the transcondylar approach. Methods. 50 dry skulls provided 100 hypoglossal canals, 100 occipital condyles and 50 foramina magna. Twenty one parameters were analyzed. They were: length of the HC, diameter of the intra- and extracranial ends of the HC, angle of the HC to the sagittal plane, distance of the HC from the posterior, anterior and inferior margins of the OC, antero-posterior and transverse diameter of the OC, presence of condylar foramen, distance of HC from the jugular foramen intra- and extracranially, distance of HC from basion, opisthion, carotid canal and jugular tubercle. The anteroposterior and transverse diameters of the FM were measured and a FM index was calculated by dividing the AP diameter of the FM by the transverse diameter. The angle of the long axis of the occipital condyles to the sagittal plane was measured. Protrusion of the occipital condyle into the foramen magnum was noted. Where applicable, the measurements were made separately for the right and left side. Findings. The average length of the hypoglossal canal in this study was 12.6 mms. The hypoglossal canal makes an angle of 49 degrees to the sagittal plane. In 30% of the dry skulls studied, the HC was divided into two by a bony septum. The distance of the intracranial end of the HC from the posterior margin of the OC was 12.2 mms. The average anteroposterior length of the occipital condyle was 23.6 mms and the transverse diameter was 14.72 mms. The occipital condyle made an angle of 60 degrees to the sagittal plane. In 20% of the skulls studied the occipital condyle protruded into the foramen magnum. The condylar foramen was absent on the right side in 4 skulls and on the left side in 16 skulls. The average anteroposterior length of the foramen magnum was 33.3 mms and the width was 27.9 mms. When the foramen magnum index was > 1.2, the foramen was found to be ovoid. Forty six percent of the skulls studied exhibited an ovoid foramen magnum. Conclusions. The occipital condyle is frequently being drilled to expose lesions ventral to the brainstem. From our study, it is evident that the occipital condyle can be safely drilled for a distance of 12 mms from the posterior margin before encountering the hypoglossal canal. In 20% of the skulls the occipital condyle protrudes significantly into the foramen magnum. Wide and sagittally inclined occipital condyles, medially protruberant occipital condyles along with a foramen magnum index of more than 1.2 will require much more extensive bony resection than otherwise. With the availability of recent imaging techniques, it is possible to anticipate the extent of bony resection required in an individual case by using the above mentioned morphometric features.