Middle ear mechanics of cartilage tympanoplasty evaluated by laser holography and vibrometry

Abstract

Goals: To assess the effects of thickness and position of cartilage used to reconstruct the tympanic membrane (TM) using a novel technique, time-averaged laser holography.

Background: Cartilage is commonly used in TM reconstruction to prevent formation of retraction pockets. The thickness, position, and shape of the cartilage graft may adversely affect TM motion and hearing. We sought to systematically investigate these parameters in an experimental setting.

Methods: Computer-assisted optoelectronic laser holography was used in 4 human cadaveric temporal bones to study sound-induced TM motion for 500 Hz to 8 kHz. Stapes velocity was measured with a laser Doppler vibrometer. Baseline (control) measurements were made with the TM intact. Measurements were repeated after a 0.5- or 1.0-mm-thick oval piece of conchal cartilage was placed on the medial TM surface in the posterior-superior quadrant. The cartilage was rotated so that it was either in contact with the bony tympanic rim and manubrium or not.

Results: At frequencies less than 4 kHz, the cartilage graft had only minor effects on the overall TM fringe patterns. The different conditions had no effects on stapes velocity. Greater than 4 kHz, TM motion was reduced over the grafted TM, both with 0.5- and 1.0-mm-thick grafts. No significant differences in stapes velocity were seen with the 2 different thicknesses of cartilage compared with control.

Conclusion: Computer-assisted optoelectronic laser holography is a promising technique to investigate middle ear mechanics after tympanoplasty. Such positioning may prevent postoperative TM retraction. These findings and conclusions apply to cartilage placed in the posterior-superior TM quadrant.

FIG. 1

Photograph of a cadaver TM showing the position of the cartilage. A, The oval shape and size of the graft used. B, The graft is interposed so that it is in contact with the bony tympanic rim and manubrium (BC). C, The graft is not in contact with the bone (NBC).

FIG. 2

Time-averaged holograms at 500 Hz and 4 kHz. At 500 Hz, the fringes show a simple pattern. The grafted area also has fringes. At 4 kHz, the fringes show a complex pattern, and no fringes can be seen on the TM over the graft. BC indicates bony contact; NBC, no bony contact.

FIG. 3

Mean ± standard deviation of normalized ratios (in decibels) of the maximum mobility of TM after cartilage reconstruction. A, Small cartilage-induced changes in magnitude of maximum mobility of TM at 500 Hz. B, Cartilage-induced reductions in magnitude of maximum mobility of TM at 1 kHz. C, Cartilage-induced reductions in stapes velocity at 500 Hz. D, Small (8 dB or less on average) cartilage-induced changes in magnitude of stapes velocity at 1 kHz. In all, n = 4, 0.5-/1.0-mm NBC = 0.5-/1.0-mm cartilage without bone contact, 0.5-/1.0-mm BC = 0.5-/1.0-mm cartilage graft with bone contact. Asterisk marks those conditions that are significantly different (p < 0.05) from the control.

FIG. 4

Mean ± standard deviation normalized magnitude ratio of stapes velocity as a function of frequency from 200 Hz up to 20 kHz. The measurements made at frequencies between 2 and 3 and greater than 12 kHz are contaminated by measurement artifact, that is, at these frequencies, the entire bone was moving with significant velocity. The cartilage graft measurements (both thicknesses and positions) do not differ significantly from each other. n = 4, 0.5-/1.0-mm NBC = 0.5-/1.0-mm cartilage without bone contact, 0.5-/1.0-mm BC = 0.5-/1.0-mm cartilage with bone contact.