Investigation of the mechanics of Type III stapes columella tympanoplasty using laser-Doppler vibrometry

Abstract

Objectives: To investigate the middle ear mechanics of Type III stapes columella tympanoplasty using laser-Doppler vibrometry (LDV) and to determine whether LDV was useful in the identification of structural factors responsible for poor hearing outcomes.

Background: The Type III stapes columella tympanoplasty procedure involves placing a tympanic membrane (TM) graft directly onto the stapes head. Postoperative hearing results vary widely, with air-bone gaps (ABGs) ranging from 10 to 60 dB.

Methods: Laser-Doppler vibrometry measurements were performed in 22 patients (23 ears) who underwent Type III stapes columella tympanoplasty. The measurements were made at three locations on the TM graft: over the stapes head, over the round window, and on the anterior TM. The LDV results were correlated with the clinical and audiologic findings.

Results: The 23 ears were divided into three groups: Nonaerated ears (n = 2). The ABGs were 30 to 60 dB. The TM velocities over all three locations were 20 to 40 dB lower than normal umbo velocity (in normally hearing subjects). Fixed stapes with aerated middle ear (n = 2). The ABGs were 40 to 60 dB, and TM velocities were equivalent to normal umbo velocity in one case and lower by 15 to 20 dB in another case. Mobile stapes and aerated middle ear (n = 19). There were two subgroups in this category: 1) small ABGs less than 25 dB (n = 7) and large gaps greater than or equal to 25 dB (n = 12). There were small differences in TM graft velocity at all three measurement locations between these two subgroups. However, these small differences did not explain the large difference in ABG between the two subgroups.

Conclusion: Nonaeration of the middle ear and stapes fixation lead to large residual conductive hearing losses after Type III tympanoplasty. Laser-Doppler vibrometry can aid in the diagnosis of nonaeration of the middle ear but does not readily diagnose stapes fixation. Postoperative results can vary even in cases of a mobile stapes and an aerated middle ear. We hypothesize that these variations may be the result of differences in the coupling between the TM graft and the stapes head. Measurements of TM velocities by means of LDV did not show clear differences between cases with good hearing and cases with poor hearing in ears with a mobile stapes and an aerated ear. Except for diagnosis of nonaeration of the middle ear, LDV seems to have limited clinical usefulness to identify causes of failure after Type III tympanoplasty.

FIG. 1

Schematic diagrams showing (A) Type III stapes columella tympanoplasty, illustrating the TM graft coupled to the head of stapes, and (B) LDV measurement locations on the TM graft: 1) over stapes head, 2) over RW, and 3) over anterior TM.

FIG. 2

Illustration showing LDV measurements over TM graft in the 2 cases with complete middle ear nonaeration. All measurements in Patient 11, except at 2,000 Hz, were within the noise and are not shown. The mean umbo velocity in 56 healthy subjects from Whittemore et al. (14) and its SD and range are shown for comparison.

FIG. 3

Illustration showing the laser-Doppler vibrometry measurements over TM graft in the 2 ears with stapes fixation. The mean umbo velocity in 56 healthy subjects from Whittemore et al. (14) and its SD and range are shown for comparison.

FIG. 4

Illustrations showing (A) mean ABGs in patients with middle ear aeration and mobile stapes. These patients were divided into 2 subgroups: patients with good hearing outcomes (<25 dB [n = 7; solid black line with open circles]) and patients with poor hearing outcomes (<25 dB [n = 12; solid black line with open squares]). The respective SDs are also shown as shading around the mean. (B), Mean TM graft velocities over stapes (square), RW (pointed-up triangle), and anterior TM (pointed-down triangle) in patients with small ABGs. The mean normal umbo velocity from Whittemore et al. (14) and its SD and range are shown for comparison. (C), Mean TM graft velocities over stapes (square), RW (pointed-up triangle), and anterior TM (pointed-down triangle) in patients with large ABGs. The mean normal umbo velocity from Whittemore et al. (14) and its SD and range are shown for comparison.

FIG. 5

Illustrations showing the (A) mean TM graft velocity over stapes in 12 patients with aerated middle ear and mobile stapes but large ABGs, with its 95% confidence intervals (solid black line with open circles + shaded areas), compared with the mean TM graft velocity over stapes in 7 patients with aerated middle ear and mobile stapes but small ABGs (solid black line with open squares). (B), Comparison of the TM graft velocities over RW between small and large ABG groups. (C), Comparison of the TM graft velocities over anterior TM between small and large ABG groups.