Acoustic mechanisms: canal wall-up versus canal wall-down mastoidectomy.
The contribution of the middle ear air spaces to sound transmission through the middle ear in canal wall-up and canal wall-down mastoidectomy was studied in human temporal bones by measurements of middle ear input impedance and sound pressure difference across the tympanic membrane for the frequency range 50 Hz to 5 kHz. These measurements indicate that, relative to canal wall-up procedures, canal wall-down mastoidectomy results in a 1 to 5 dB decrease in middle ear sound transmission below 1 kHz, a 0 to 10 dB increase between 1 and 3 kHz, and no change above 3 kHz. These results are consistent with those reported by Gyo et al. (Arch Otolaryngol Head Neck Surg 1986;112:1262-8), in which umbo displacement was used as a measure of sound transmission. A model analysis suggests that the reduction in sound transmission below 1 kHz can be explained by the smaller middle ear air space volume associated with the canal wall-down procedure. We conclude that as long as the middle ear air space is aerated and has a volume greater than 0.7 ml, canal wall-down mastoidectomy should generally cause less than 10 dB changes in middle ear sound transmission relative to the canal wall-up procedure.
Whittemore
K R
KR
Department of Otolaryngology, Eaton-Peabody Laboratory of Auditory Physiology, Massachusetts Eye and Ear Infirmary, Boston 02114, USA.
Merchant
S N
SN
Rosowski
J J
JJ
D000158:Acoustic Impedance Tests; D004432:Ear, Middle / Q000502:physiology* / Q000601:surgery; D006801:Humans; D008416:Mastoid / Q000601:surgery*; D008954:Models, Biological; D013016:Sound; D013701:Temporal Bone / Q000601:surgery; D014432:Tympanic Membrane