According to our experimental studies, two bone metabolic layers are related to growth ofthe mastoid process and expansion of pneumatic cells. One is the outer bone metabolic layer of membranous ossification that extrudes the temporal bone toward the outside. The other is the inner bone metabolic layer, which expands the temporal bone inwards. This two bone metabolic layers work together and this period is restricted to the period of mastoid process development. In this developmental period, if the middle ear inflammatory stimulus is strong enough to inhibit the inner bone metabolic layer, it may cause suppression of pneumatic cell expansion. Furthermore, it may cause suppression of growth of the mastoid process. However, if the middle ear inflammatory stimulus occurs after the period of mastoid process development no suppression of the pneumatic cell and mastoid process will be observed. New bone formation that narrowed the pneumatic space was not observed at all.
Thirty-six human temporal bones were taken from subjects ranging in age from the 30th week of pregnancy to 16 years of age after birth and who had normal ear drum. Two types of bone metabolism, chondral ossification and membranous ossification, were found in human temporal bones. The membranous ossification that was observed in our porcine experiment was also observed in the portions of the tegmen epitympani, posterior wall of the mastoideum and tip of the mastoid process. It was clear thatsuppression of pneumatization and the mastoideum occurred in parts of these membranous ossification points due to the stimulus of inflammation of the midde ear. In our experimental studies, suppression was observed only at thestage of mastoideum and pneumatic cell development. In human temporal bone, in the portions of the tegmen epitympani and posterior wall ofthe mastoideum, these were the membranous ossification points and the development stage was until 4 years of age. The findings of this study suggested that the low development of the middle fossa dura and prominent sigmoid sinus are caused by a continuous inflammatory stimulus of the middle ear before 4 years of age. On the contrary, chondral ossification was observed in two points and the inner bone metabolic layer in these points was not as clear as that of membranous portions. This finding suggested that the expansion of pneumatic cell may not be caused by the inner bone metabolic layer. The middle ear inflmmatory stimulus may not suppress the pneumatization in these points.