The clinical significance of the mastoid air cell system has been much discussed. 11 is suggested, that
the mastoid plays a role in middle ear pressure regulation and that mastoid size is a factor in middle ear
pressure regulation. The aim of the present study was to measure the volume and surface area of the
mastoid air cell system in both control subjects and patients with retraction pockets using digital image
processing method and to study the relation between mastoid pneutnatization and degree of retraction
pocket The study was conducted on 120 subjects divided: control group including 20 healthy subjects
(forty ears) 10 males and 10 females. Group B included 100 patient (one hundred and thirty nine ears)
41 males and 59 females with retraction pocket either on one or both sides with clinically detected
retraction pockets of tympanic membrane. The site and degree of the retraction pocket were recorded.
Computerized tomographic scanning of the mastoid air cell system was done to assess the mastoid
pneumatization The surface area and the volume of the mastoid air cells were measured using digital CT
scan. By using digital image processing to measure the volume of the mastoid air cell system , only the
air cells with a gray scale level similar to air outside the skull were easily selected on the CT films. Then
after image processing called " thresholding" only the areas and partial volumes of these extracted
pneumatized parts were calculated. The sum of partial volumes was calculated as the total volumes of
the aerated cavity. In 20 controls the mean of the surface area of the mastoid was 159.56 (±52.85) mm
and that of the volume was 62.47('±16.12) num No statistically significant difference was found between
male and female figures. In 74 ears with postero-superior retraction pocket, 23 ears (31%,), 29
ears(39.2%), and 22 ears (29.7%) showed degree I, II and III respectively. The mean (± SD) of the
surface area of mastoid air cells was 66.3 (±14.94), 55.67(±12.67) and 32.79(±I3.15) respectively. The
mean of the volume (± SD) of the mastoid air cells was 6.3(*1.24), 5.18 (+0.99), and 2.79(± .85), for
retraction pocket degree I, II and 111 respectively. In 65 ears with attic retraction pocket 23 ears
(35.4%), 26 ears (40%), and 16 ears (24.6%) showed degree I, II and III respectively. The mean (± SD)
of the surface area of mastoid air cells was 63.17(±14.4), 53.88(±8.3) and 33.01(±7.1) respectively. The
mean of the volume of the mastoid air cells was 6.2(±1.12), 5.03 (±0.85), and 2.84 (±0.87), for retraction
pocket degree I, II and III respectively. These results indicate that both Posterosuperior and attic
retraction pockets have an equal chance of occurrence when the mastoid becomes hypopneunzatized,
depending upon the presence or absence of other factors affecting middle ear aeration or infection and
the site of affection. The comparison of the control mastoid air cell volume and surface area with those
of all degrees of retraction pocket showed a statistically significant difference. The results also indicated
that mastoid air cells volume and surface area are inversely proportional to the degree of both types of
retraction pocket and retraction pockets are not likely to occur with moderate or high mastoid
pneunzatization. Although the results indicated a positive correlation between volume and surface area
in the mastoid air cell system, this correlation should be lost in mastoidectomy operations as the surface
are of the mastoid cavity is extremely reduced after mastoidectomy because the fine air cell walls are
removed. Therefore it may be not desirable to remove all the air cells when the mastoid process contains
healthy cells lined with thin normal mucosa. |
