Hyperostosis frontalis interna (HFI) is manifested by the accretion of bone on the inner table of the frontal bone. Despite the vast literature on HFI, ambiguity exists as to its etiology, osteogenesis, demography, and history. This stimulated the present broad-scale study of HFI which included the evaluation of 1,706 early 20th century skulls (1,007 males and 699 females) from the Hamann-Todd and Terry human osteological collections, as well as 2,019 pre-19th century East-Mediterranean, Amerindian, and Central European skulls. In addition, 72 cadavers were dissected for gross inspection and histology. Special attention was paid to the relationship of the brain and meninges to endocranial lesions. HFI is an independent condition, not a symptom of a more generalized syndrome as suggested in the past. It can appear in a variety of forms but each is the result of the same process and probably of the same etiology. Investigators' previous failure to recognize the mild stages of HFI (types A and B) as an early form of the general HFI process led to erroneous statistics and interpretations of observations. HFI should also be considered a phenomenon separate from HCI, hyperostosis cranialis diffusa (HCD), and other endostoses, even when it appears in association with them. To avoid ambiguity and facilitate the description of cranial hyperostoses, uniform nomenclature (HFI, HCD) has been recommended. HFI is rarely seen in historic populations, regardless of geographical origin. It is most commonly found among females and is believed to be associated with prolonged estrogen stimulation. While its magnitude of manifestation and frequency are much higher in females, HFI is not a purely female phenomenon. Males with hormonal disturbances such as atrophic testis were found to manifest HFI type D. HFI is associated with age insofar as it is much less frequent in females under 40 years of age. Although advanced cases of HFI (types C and D) have been observed in individuals as young as 40 years of age, it is more frequently found after age 60. The frequency of HFI type D will not increase from age 60. Type-predicted analysis by cohort reveals significant ethnic differences. Changes in African American (AA) females appear earlier in life and progress more rapidly than in European American (EA) females. Analysis of radiographs shows a discrepancy between the anatomic prevalence of HFI and its radiological recognition, which is very poor for mild cases. This apparently resulted in the misconceptions that HFI is entirely an old-age phenomenon, and that it is exclusively female. Histological analysis shows that the inner table along with the closely attached dural layer play a major role in the osteogenesis of HFI. Contrary to previous models, no evidence for diploe or ectocranial plate involvement was found. Cadaver study suggests that the predilection for the frontal area may be related to an altered blood supply and/or vascular stretching.
