VARIANT ANATOMY OF THE EXTERNAL SURFACE OF THE MASTOID BONE IN A KENYAN POPULATION

Abstract

Background: The external surface of the mastoid part of the temporal bone is important in localising the mastoid antrum and prevention of injury to the facial nerve in surgeries involving access to the middle ear cavity. Its morphology shows sex, ethnic and side differences. Population-specific data regarding this region among Kenyans, however, remains largely undescribed.Objective: To describe the variant anatomy of the external surface of the mastoid part of thetemporal bone in a Kenyan population.Materials and methods: 105 dry sexed skulls obtained from the National Museums of Kenyawere used to study the mastoid part of the temporal bone. Presence and types of the suprameatalspines and depressions, as well as the presence of mastoid grooves and canals on the externalsurface of the mastoid process were determined. The distance between the Suprameatal triangle(SMT) and the mastoid groove (MG) and canals (MC) were measured to approximate the relationbetween the two structures. Measurements of the lengths of the mastoid grooves and canals wererecorded by use of a digital Vernier calliper (precision of 0.01mm).Data wasinput into SPSS (Version 25.0) for analysis. Results: The most prevalent type of suprameatal spine was crest type (75.2%) whilethe triangular type was present in 15.8% of the skulls. Suprameatal depressions were shallow in67.1% sides and deep in 24.3% of total sides examined. Mastoid grooves were present in 30.5%of the sides examined and canals in 16.2%. Both mastoid canal and grooves were observed on thesame side in 24.3% sides. The mean distance from the suprameatal triangle to the superior end ofthe mastoid groove/canal on the right was 26.69 ± 5.89 mm while on the left it was 27.12 ±5.43mm. The mean distance on the right to its midpoint was 37.29 ± 6.10 mm while on the left itwas 24.64 ± 5.33mm. To the inferior end, the distance on the right was 38.93 ± 26.47 mm whileon the left it was 25.94 ± 5.02mm. Conclusion: The suprameatal spine and depression are presentin most Kenyan skulls. Surgeons can thus rely on these structures to localise the underlyingmastoid antrum. Similarly, surgeons operating around the mastoid area should be cognizant of thehigher-than-normal recorded prevalence of mastoid grooves in order to avoid iatrogenic vascularinjuries during cochlear implant surgeries.