Motion of the Human Tympanic Membrane and Stapes Velocity after Placement of a Total Ossicular Replacement Prosthesis with Cartilage Cover.

Cagatay Han Ulku MD1,2,3, Jeffrey Tao Cheng PhD1,2, Saumil N.Merchant MD1,2, John J Rosowski PhD1,2, 1: Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston MA USA; 2: Department of Otology and Laryngolgy, Harvard Medical School,

Boston MA USA; 3: Department of Otolaryngology Head & Neck Surgery, Meram Medical Faculty, Necmettin Erbakan University, Konya, Turkey

Introduction

Reconstruction of the ossicular chain is required in 40-90% of all tympanoplasties, making ossiculoplasty a frequently performed operation; however, the results of such reconstructions vary greatly. Causes of the variable post-op hearing results (5-60 dB CHL) are only partially understood: Failures are often associated with poor coupling of the prosthesis to the TM or inner ear, or lack of aeration of the middle-ear spaces. A common form of ossicular chain reconstruction uses an ossicular replacement prosthesis in conjunction with a sheet of cartilage to reduce the chance of the prosthesis extruding through the tympanic membrane (TM). A major mechanical factor in ossicular reconstructions is the tension produced by the prosthesis, which affects (i) the stiffnesses of both the annular ligament of the footplate and the TM, and (ii) the coupling of TM motions to the stapes. This tension changes with the length of the ossicular replacement prosthesis. In this study, we investigate how prostheses of different lengths, and different tensions, affect both TM and stapes footplate motion in a temporal bone model of tympanoplasty. Laser-Doppler vibrometry (LDV) measurements of footplate motion together with opto-electronic holography (OEH) measurements of motion of the TM surface and umbo were made before and after removal of the incus and stapes superstructure and the placement of Total Ossicular Replacement Prostheses (TORPs) of different lengths (Fig 1). The effect of different sizes of a cartilage disk interposed between the TM and an ossicular prosthesis was also investigated. The results address the roles of tension and cartilage size, and should help refine current surgical techniques, and lead to improved post-surgical hearing results.

MethodologyTemporal Bone (TB) Preparation and ConfigurationFour human temporal bones without history of otologic disease were used. TBs were obtained at autopsy within 24 hours of death from donors, and were used fresh or after refrigeration in normal saline at 4°C. Bones were prepared using universal precautions. The preparation included removal of the bony external auditory canal to expose the majority (>80%) of the TM surface, and a canal wall up mastoidectomy with wide posterior tympanotomy, including removal of the second genu and mastoid segment of the facial nerve to access the ossicles. The stapedius tendon was severed by KTP laser to maximize the exposure of the stapes. The tympanic ring of the TB was positioned perpendicular to the holographic illumination beam. The lateral surface of the TM was painted with a 60mg/cc suspension of ZnO powder in saline to increase the light reflected from the TM surface. Retroreflective balls were placed on the posterior crus of the stapes, and an LDV laser beam aimed at the reflectors through the open facial recess. The middle ear and mastoid were kept open to the atmosphere and the bones were periodically moistened by soaking in saline.

ManipulationsControl measurements were made in bones with intact ossicular chain. A KTP laser or joint knife was used to disarticulate the incudo-stapedial joint, the posterior incudal and incudo-malleal ligaments were severed, the incus was removed, and the measurements were repeated. The KTP laser was used to remove the stapes superstructure. Titanium TORPs (Kurz Co, Germany) of varied lengths were placed between the footplate and the posterior-superior quadrant of the TM (Fig 1). Each reconstruction started with the placement of a 0.5 mm thick cartilage oval on the TM coupled to a prosthesis length that was judged to be the surgical ‘Best Fit’. Two cartilage ovals were used: a ‘Small’ cartilage oval approximated the head of the TORP, the ‘Large’ cartilage oval had an area that was 2X the ‘Small’ area. After measurements with this reconstruction a ‘Loose Fit’ reconstruction with a TORP shaft 0.25 mm shorter than ‘Best Fit’ was placed and measurements repeated, a third set of measurements were made with a ‘Tight Fit’ TORP shaft 0.5 mm longer than the ’Best Fit’. In total, six TORP measurements were made: three length (or ‘Fit’) conditions and two cartilage sizes.

Conclusions Preliminary results indicate:

Reconstruction length and the size of the cartilaginous disk differentially affect the mobility of the TM and the stapes. However, there is significant variation in the acoustic response of similarly reconstructed ears, even in this relatively well-controlled preparation.

The data suggest an advantage to ‘Best Fit’ TORPS with a ‘Small Cartilage’ cover that does not exceed the area of the pedestal that interfaces with the TM.

ACKNOWLEDGEMENTS: This work was supported by grants from the NIDCD to JJR & JTC, and TUBITAK to CHU

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Figure 2. LDV and Holographic measurement device.

Figure 1. Interposed TORP with the cartilage cover and re�ector are seen through the facial recess

References1. Sade J. In Cholesteatoma and Mastoid Surgery, Ed: Sade J,

Kugler Publ, Amsterdam, 1982, p 1-3.2. Nadol JB and McKenna MJ. Surgery of the Ear and Temporal

Bone, 2nd Edition, Lippincott, Williams and Wilkins, Philadel-phia, PA, 2004, pages 273-324.

3. Gulya AJ, Minor LB, Poe DS. Glasscock-Shambaugh Surgery of the Ear, 6th Edition, People's Medical Publishing House-USA, Shelton, Connecticut, 2010, pages 49-72, 465-500.

4. Ruben RJ. The disease in society-evaluation of chronic otitis media in general and cholesteatoma in particular. In Cholestea-

reconstructions did lead to a small reduction in summed displacement at 1 kHz and above (Fig 6). The reduction was larger for the ‘Large Cartilage’ reconstructions. These results suggest that motion of large regions of the TM can be reduced to near 0, with no effect on umbo displacement at 4 and 8 kHz, and that the reduced displacement of the cartilage covered areas is significant at higher frequencies.

The Laser Doppler measurements of footplate motion were also quite variable. Fig 7 illustrates measurements of the dB change in velocity from control from two bones (’Small Cartilage’ results are on the left, ‘Large Cartilage’ on the right). In general, the ‘Best Fit’ TORP was associated with the largest footplate velocities (but see the ‘Small Cartilage’ results in TB5, where the ‘Best Fit’ produced the smallest velocity response). The footplate velocities were usually larger with the ‘Small Cartilage’ Reconstructions.In summary: -Incus removal led to increases in the motion of the umbo and the average TM displacement as well as changes in the location of displacement maxima at low frequencies. However, there was little change in high frequency motion of the TM. -The application of cartilage to the medial surface of the TM led to a reduction in the motion of the TM in contact with the cartilage. At 0.5 and 1 kHz, we observed a shift in the location of displacement maxima, and the cartilage and TORP reconstruction tended to reduce the motion of the TM and stapes relative to the controls, and simultaneously increase the motion of the umbo. At 4 and 8 kHz the effects of the cartilage and TORP on umbo displacements were small, but they produced small but reguilar decreases in TM volume displacement. -The effects of cartilage TORP reconstructions on footplate velocity were more variable, but the ‘Best Fit’ and ‘Small Cartilage’ were generally associated with larger velocities.

toma and Mastoid Surgery, Ed: Sade J, Kugler Publ, Am-sterdam, 1982, p 111-116.

5. Vrabec JT, Coker NJ. Stapes surgery in the United States. Otol Neurotol. 2004;25:465-469.

6. Merchant SN, Ravicz ME, Voss SE, Peake WT, Rosowski JJ. Toynbee Memorial Lecture 1997. Middle-ear mechanics in normal, diseased and reconstructed ears. J Laryngol Otol. 1998;112:715-731.

7. Merchant SN, McKenna MJ, Rosowski JJ. Current status and future challenges of tympanoplasty. Eur Arch Oto-rhino-laryngol. 998;255:221-8.

1 kHz Control Incus removed Small Cartilage Large Cartilage

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Figure 3. Stroboscopic-Holography measurements in TB2 at 1 kHz

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Figure 4. Stroboscopic-Holograpghy measurements in TB2 at 8 kHz

Figure 6. Decibel Change in Sound-Induced TM Volume Displacement vs Control. The median and range of the measurements are shown for the Small (on the left) and

Large (on the right) Cartilage reconstructions.

Figure 5. Decibel Change in Sound-Induced Umbo Displacement vs Control. The median and range of the measurements are shown for the Small (on the left) and

Large (on the right) Cartilage reconstructions.

Figure 7. The dB change in stapes velocity (re normal) associated with incus removal and the di�erent reconstructions in two of the four bones. Similar trends and signi�cant di�erences are seen in the data.

Results Stroboscopic holography was used to quantify the magnitude and angle of the displacement of over 400,000 locations on the surface of the tympanic membrane at 0.5, 1, 4 and 8 kHz. At 1 kHz and below (Fig 3), the TM surface motion induced by sound is relatively simple, the entire TM surface moves nearly in-phase (shown in hologram results as slowly varying phase gradients of less than 1/4 cycle over the TM surface) with one or two local displacement magnitude maxima. At 4 & 8 kHz (Fig 4) the TM surface motions are more complex: Rings of multiple displacement magnitude maxima and minima are arranged around the manubrium. These multiple magnitude extrema are associated with small but consistent phase variations that are also arrayed across the TM surface. Removal of the incus produced an increase in the TM displacements at low frequency, consistent with removal of cochlear load. Replacing the interrupted ossicular chain with a cartilage covered TORP changes the displacement pattern on the TM surface. While these reconstructions still produce relatively in-phase motion of the TM at low frequency and complicated surface motion of the TM at high frequency, the motion of the region on the TM surface in contact with cartilage was greatly reduced (the black circles in Fig 3 & 4), and associated with these reductions were noisy phase estimates. The cartilage reconstructions also resulted in changes in the locations of maximum motion.

The Holography measurements were used to quantify the sound-induced motion of the umbo (Fig 5) and the summed volume displacement of the TM surface (Fig 6). These measurements are illustrated in terms of the dB change relative to the control values. While the results in the four bones are quite variable, incus removal and subsequent reconstruction generally led to an increase in umbo and summed TM displacement at the lowest frequencies (Fig 5 & 6). Such decreases are consistent with the removal of cochlear load and annular ligament stiffness from the mechanics of TM motion. The interruption and reconstructions had little effect on umbo displacement at 4 and 8 kHz (Fig 5). The cartilage

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