Prevalence and Linear Metric Analysis of Anatomical Variations of the Clivus: An Osteological Study

Numerous clival variations have been described; however, certain entities remain poorly characterised, particularly the median basilar canal (MBC) and the recently described basilar tubercles of the clivus (BTC). This study documented prevalence, morphology, and selected linear metric analysis in a historical skeletal collection housed in the United Kingdom. A total of 161 dry skulls with intact clival regions were examined; their geographic and population origin, chronology, ancestry, age, and sex were unknown. Variations assessed were the MBC, BTC and other variations of the clivus including the fossa navicularis magna (FNM), basilar processes (BP), condylus tertius (CT), and prebasioccipital arch (PA). The MBC and BTC were further classified into subtypes. Linear metric analysis was performed for selected variations, and inter-observer reliability was assessed. The MBC was the most common variation (24/161, 14.9%), followed by BTC (11/161, 6.8%). The MBC comprised superior recess ( n = 19, 11.8%), inferior recess ( n = 4, 2.5%), and superior ( n = 1, 0.6%) types, whereas BTC included centric ( n = 6, 3.7%) and unilateral right ( n = 5, 3.1%) types. Other variations included FNM ( n = 8, 5.0%), BP ( n = 3, 1.9%), TFM ( n = 3, 1.9%), and CT ( n = 2, 1.2%), while PA was not observed. The BTC demonstrated mean lateral and vertical widths of 3.97 ± 1.65 mm and 4.20 ± 1.57 mm, respectively. Inter-observer reliability tests showed good to excellent agreement. Recognition of these variants is important for radiological interpretation and skull base surgery.