Click here for the PDF version

Contents

It has been argued whether the development of the mastoid air cell system is affected by inflammation-induced suppression or closely related to the hereditary constitution. Moreover, it has been difficult to estimate the volume of the mastoid air cell system, and to observe the three-dimensional image of the air cell system. In order to clarify these points, we have developed a technique for direct volume measurement based on high-resolution CT [1]. The present report will describe our findings on the measurement of the mastoid air cell system of infants, and its developmental changes with age.

The study covered 43 males and 37 females, aged 1–18 years. The temporal bone was imaged by slicing at 2 mm thickness through the high-resolution CT, so that the entire air cell system was included, and the resultant data were analyzed by the image processing technique.

It was confirmed that the area of the infant's air cell system tended to increase with age. By 9–10 years old, it has reached about 80% of the volume of an adult, and at 14–15 years old, it has reached the mean adult value. While the mean value of the mastoid air volume in adults, obtained from a technique for direct volume measurement from high-resolution computed tomography we developed, was 5.97 ml [1], in some cases of children of 11–12 years, the system was seen to have grown already to this size (Fig. 1).

We performed the three-dimensional display of the CT image of various morphological features of the system in order to elucidate where such differences in air volume had originated.

In this case, cartilaginous bone formation had thrived, while fibrous bone formation was still insufficient. The air cell system in this case markedly developed posterosuperiorly to the petrous pyramidal apex, and in the mastoid process due to cartilaginous bone formation, and pneumatization could be seen peripheral to the mastoid antrum. However, it was not that fibrous bone formation was insufficient, but that development of the cell system overall was insufficient. This case showed that development at each region of the air cell system, especially cartilaginous bone formation, thrived up to 18 months after birth (Fig. 2).

The air cell system in this case had developed anteriorly and posteriorly to the mastoid bone and in the direction of the mastoid process, and an arch had formed peripheral to the external auditory canal. Similarly, development was also seen medially and in the direction of the petrous part of the temporal bone. This case actually showed how at this age fibrous bone formation proceeds inferiorly, superiorly, posteriorly and also laterally in addition to cartilaginous bone formation posterosuperiorly and inferiorly. It is an interesting fact that even when air volume is low, if development proceeds smoothly, the formation at this age would already be virtually the same as that of an adult (Fig. 3).

The mastoid air volume in adults obtained from our technique was 2–18 ml (mean 5.97 ml) [1] or 1.26–20.05 ml (mean 5.97 ml) [2]. Air volume differs according to individual, and there is also considerable variation in the extent of air cell development. In cases of good development, air volume will reach the adult mean volume by about the age of 4–6 years. On the other hand, volume can also be extremely low even in adults.

Embryologically, the mastoid air cell system begins with the formation posteriorly of a lumen from the attic during embryonic development at around week 22 of pregnancy. It is virtually completed with the formation of the lumen of the mastoid cavity at week 34. The lumen formation and membrane epithelialization of the mastoid air cell continue until birth. There are various air cell formations. It is thought that differences occur in morphology because there are differences in developmental patterns according to the region of the air cell, and the differences in the extent of each region's development affects air volume overall. The differences in this developmental pattern are attributed to differences in the pattern of bone metabolism at each air cell region that occurs after birth. This bone metabolism pattern can be classified into cartilaginous and fibrous bone formation. The former is not easily affected by inflammation, while the latter is prone to inflammation. Fibrous bone formation occurs over a long period after birth, and differences in the extent of development are prone to occur. It has been reported that differences in air volume can occur as a result [3].

It was thus determined that differences in development of the cells that arose due to fibrous bone formation played an important role regarding the differences in air volume, and as with the cases presented, the differences in such development could occur at various times.