Study of morphologic variability of incudostapedial angle and its relation with temporal bone pneumatization

Optimal sound transmission across the ossicular chain is dependent on the appropriate alignment of the middle ear ossicles. Incudostapedial joint (ISJ) is conventionally considered to be at right angle.

We intended to study the ISJ anatomy and the impact of temporal bone pneumatization on the same.

In a cadaveric study comprising of 47 human temporal bones, canal wall down mastoidectomy was carried out under microscopic guidance keeping the ossicular chain intact. The morphology of ISJ was recorded and analysed with respect to the pneumatization status of the temporal bone. The data analysis was performed using statistical software Stata version 12.0.

The mean ISJ angle for the 47 bones was 90.5⁰ (SD-15⁰; range:54⁰-122⁰). The mean angle in well pneumatized bones was 93.7⁰ (SD-16.5; Range:54°-122⁰) and in sclerotic mastoids was 88.7⁰ (SD-14; Range:68°-118⁰). The difference in the ISJ angle in these two conditions was not statistically significant (p = 0.27). The mean angle was found to be significantly more obtuse in the cases with partially eroded ISJ (111.4⁰ {SD-8.8; range:100.3⁰-121.9⁰}; p = 0.0001) and in the cases with an 'adherent/tilted morphology' of the stapes suprastructure with the promontory (mean-95.8⁰{SD-13.8; range:70.7⁰- 120.4⁰); p- <0.00001).

The ISJ angle shows considerable variations. This variability needs to be taken into account when undertaking middle ear reconstructive procedures, specifically the ones involving the stapes footplate. The mastoid pneumatization does not appear to have an impact on the ISJ angle.