The effect of increased stiffness of the incudostapedial joint on the transmission of air-conducted sound by the human middle ear.

Objective: To study the possible effects of increased stiffness at the incudostapedial joint (ISJ) on sound transmission in the human middle ear.

Background: The physiologic role played by the IS joint in the mechanics of human middle ear function is unclear. It is also unclear how fixation of this joint might manifest itself and what the implications are of fixing this joint during surgical reconstruction.

Hypothesis: Increased stiffness of the ISJ will affect sound transmission through the middle ear.

Methods: Cyanoacrylate adhesive was instilled around the ISJ joint in 5 fresh human cadaveric temporal bones to increase ISJ stiffness. Laser Doppler vibrometry measurements of sound-induced peak-to-peak displacement of the umbo and stapes footplate were made before and after stiffening the ISJ.

Results: At baseline, the measurements at the stapes footplate followed those at the umbo but were approximately 12 dB lower in the speech frequencies. We found that stiffening the ISJ produced an almost equal decrease in peak-to-peak displacement at both the umbo and the stapes footplate, with little change in their relative motion, consistent with an increase in ossicular impedance. The decrease was mainly between 400 and 1,000 Hz with a statistically significant mean magnitude loss of 6 dB at 740 Hz.

Conclusion: Increased stiffness at the ISJ results in a small, probably clinically insignificant decrease of 3 to 6 dB in middle ear sound transmission in the lower frequencies between 400 and 1,000 Hz.