Surface area-volume relationships for the mastoid air cell system and tympanum in adult humans: Implications for mastoid function

The middle ear (ME) surface area/volume ratio (SA/V) is greater than that of the tympanum. The rate of ME pressure decrease is proportional to the ME SA/V. The mastoid air-cell system (MACS) will not function as an ME gas reserve unless the blood perfusion/surface area is much greater for the tympanum than the MACS and decreases as MACS volume increases.To measure the MACS and tympanum surface areas and volumes and ascertain whether the MACS could function as an ME gas reserve.Twenty adult subjects with a wide range of MACS volumes underwent CT of their MEs. The left and right surface areas and volumes of the tympanum and MACS were reconstructed and entered into a simple perfusion-limited model of transmucosal gas exchange. In this model the MACS would be a gas reserve if ME SA/V was less than the tympanum SA/V or equivalently, if their ratio was less than a critical value of 1.Both MACS and tympanum SA were linearly related to their volumes. MACS SA/V and the ME SA/V were significantly greater than those for the tympanum. Inputting the measured values into the model yielded a critical value of 1.4, which was significantly greater than 1.