An Assessment of Hyperostosis Frontalis Interna Within a South African Skeletal Sample

Abstract

Hyperostosis frontalis interna (HFI) is a condition characterised by bony outgrowths located unilaterally or bilaterally on the intracranial surface of the anterior frontal bone. The prevalence and microarchitecture of this condition are poorly studied in South Africa. This study aimed to assess the prevalence, expression, and severity of HFI in a South African skeletal sample using a morphological approach and gain insight into the aetiology and pathogenesis of HFI by looking at the micro-architectural changes in affected cranial areas. A total of 2186 adult crania (20 - 90+ years) were sourced from the Raymond A. Dart Collection of Modern Human Skeletons to assess the prevalence and expression of HFI morphologically. HFI was observed in 17.15% (n = 375) of the cranial vaults assessed (n = 2186), with higher prevalence in females (n = 243/375, 64.8%) compared to males (n = 132/375, 35.2%), SA Black sample (n = 209/375; 55.73%) and middle- aged (50 - 69 years) individuals (n = 143/375; 38.13%). Intra-observer reliability for the morphological classification showed strong agreement (k = 0.891, p < .001), while inter-observer reliability was moderate (k = 0.713, p < .001). After the classification of HFI, 15 individuals with HFI and 15 without HFI were scanned using a Nikon XTH 225L Micro-CT X-ray unit to evaluate possible differences in the micro-architectural features of affected and unaffected crania at predetermined regions of interest. Poor agreement was observed for most intra- and inter-observer measurements of microarchitecture (CCC = -0.05 to 0.59), except for trabecular spacing (TbSp), which showed perfect intra-observer agreement (CCC = 1.00). An increased bone mineral density and bone-surface-bone-volume ratio were observed in individuals with HFI in both cortical and trabecular regions. Males with HFI displayed a significant decrease in the bone-volume-to-tissue- volume ratio, particularly in the endocranial cortical bone at the infraorbital foramen, while females with HFI exhibited decreased bone mineral density (BMD) in both the cortical and trabecular regions and decreased trabecular number (TBN) compared to females without HFI. Hershkovitz et al. (1999) scoring criteria can be effectively used to score HFI; however, modifications to the Type D classification are recommended to account for cases where HFI extends beyond the frontal bone. The prevalence and degree of HFI vary within populations, between sexes, and between age groups, highlighting the importance of demographic factors when studying the prevalence and aetiology of HFI. Based on microarchitectural results, we can 12 conclude that the microarchitectural structure of the frontal bone varies between individuals with and without HFI, across sex, age, and population affinity groups.