Читать книгу Advances in Hearing Rehabilitation - Группа авторов - Страница 38

Reconstruct to Malleus or to TM? Clinically, there are so many variables in play that it is difficult to determine if there is a mechanical advantage in reconstructing to the malleus instead of to the TM, with some series showing an advantage, and others no effect.

The malleus may provide some pressure gain through the catenary lever effect [16, 17], though this is controversial there may be some advantage in its utilization. Reconstructing to the malleus, however, means more angulation, a worse force vector and more of a rotational vector (Fig. 1).

We compared [18] reconstruction from the malleus (SHOM) to that from the TM (SHOT), and found a slight (about 5dB) advantage in the malleus reconstruction below 2 kHz (Fig. 2). However, differing tension effects in the 2 types of reconstruction may have contaminated this result. Shimizu and Goode [19] tried to remove the tension effects by using a SHOM reconstruction and then taking out the malleus and placing the prosthesis to the same position on the TM. They found little difference in the results.

Another point of controversy is where to connect on the malleus. Below 1 kHz, the umbo has higher vibration amplitudes [20] but it requires greater angulation to reach and has an unstable geometry. Also, experiments have not shown any advantage in reconstructing to the umbo versus the mid-manubrium [14], whereas others have actually found worse results in connecting to the umbo at high frequencies. Overall, mid-manubrium seems a reasonable choice for stability and function. The malleus at high frequencies also exhibits translational movements, in addition to rotational ones, and the short process is not a low amplitude node for the translational movements as it is for rotational movements. Malleus rotation posteriorly may add to stabilization of the prosthesis, but its mechanical effects are largely unknown.

Where to Place on the Footplate? Again, the findings are not clear. Temporal bone studies by Vlaming and Feenstra [21] found little difference in displacement when reconstructing to the anterior versus central footplate. Other studies have clearly found increased volume velocity of displacement by reconstructing to the centre of the footplate. The posterior position seems to be the worst [22]. In general, reconstruction to the center is advocated.

Reconstruct to Stapes Head or to Footplate? Leaving aside other factors affecting function, especially stability issues, there is conflicting literature as to whether reconstruction to the stapes head (SHOM, FOM) results in better hearing than reconstruction to the footplate (SHOT, FOT). The majority of the literature favors reconstruction to the stapes head if such an arrangement is possible [1]. There is, however, no a-priori mechanical reason for this, except that it is easier to constrain rotation movements on the capitulum than on the footplate. Clinical results may also reflect increased pathology when the reconstruction has to be to the footplate.

In a study where we compared reconstruction from the TM to the stapes head (SHOT) with reconstruction from the TM to the footplate (FOT) in temporal bones [23] there was little difference in the resulting stapes vibrations. In this kind of reconstruction, because it is mostly perpendicular, the force vectors are similar for the FOT and the SHOT. If, however, reconstructing to the malleus, then a SHOM (i.e., stapes head to malleus) has a larger angulation than a FOM. In comparing these SHOM/FOM reconstructions, Murugasu et al. [24] found about a 6 dB advantage in going to the footplate versus the stapes head, although their SHOM prosthesis did not have any design to engage the stapes and may have exhibited excessive rotational movements. There is no convincing evidence to suggest that one type of reconstruction to stapes head or to the footplate is more efficient than the other.

What Tension Should the Prosthesis be Under? We have consistently found tension to have one of the largest effects of any factor tested in reducing low frequency results. Below 2 kHz, excessive tension causes drops in the order of 10–15 dB. This may be due to stiffening the TM, the annular ligament, or both. In this lower frequency region, the ear is dominated by stiffness, so this effect is not too surprising. We have shown this effect for reconstructions to the TM (SHOT) [25] and to the malleus (SHOM) [18]. Others have reported very similar findings [21, 26, 27], consistently showing drops below 1–2 kHz with increased tension.

Clinically, it is difficult to adjust the tension in SHOT or FOT reconstructions, as the TM or reconstruction is laid back down over the prosthesis, but it can be modulated in SHOM or FOM reconstructions if the malleus is still robust. Sometimes tension is also needed for stability. We would recommend the minimum tension needed for stability.

What Should be Placed in the Interface with the TM? Many otologists prefer to cover allografts with cartilage as this reduces the extrusion rate and prevents the TM from retracting around the prosthesis. This cartilage can be varied in its thickness and in its dimensions. Other materials could also be placed here if they gave some mechanical advantage, for example, bone, fascia, fat or other material that might allow for expansion and compression as the position of the TM varied with middle ear pressure. In fact, interestingly, Yamada and Goode [26] report that spongy material at the TM-prosthesis interface could be used as a tension regulating device mitigating the effects of longer prostheses.

We investigated this interface with different sized cartilage pieces and found that smaller cartilage grafts gave better low frequency responses, probably because they did not distort the conical shape of the TM and cause increased stiffness of the TM [28]. We also found little difference in putting glass, cartilage or soft spongy material at this interface, except at frequencies above 4 kHz, where spongy material tended to cause a drop in transmission. In unpublished results, we have also found that the thickness of the cartilage at this interface had little or no effect on vibration transmission to the stapes.

Shimizu and Goode [19] performed similar experiments and reported similar results in that larger cartilage pieces showed larger drops in the stapes vibrations, particularly in the lower frequencies. They report better results at high frequencies with the largest cartilage pieces, which differs from our results and is not in keeping with the previously reported mechanics of the middle ear.

In a more recent study [27], Ulku et al. [27] found that small and large cartilage covers over a SHOT-type prosthesis had little effect on low frequencies, but used a cartilage of 8 mm² as their small cartilage and 16 mm² as their large size, a difference that is unlikely to increase tension in the TM. They did, however, find that larger cartilage pieces reduced high frequency responses, which they ascribe to a mass effect.

We would recommend, if cartilage is needed to prevent extrusion, that it be approximately the same size as the head of the prosthesis. Larger pieces of cartilage are, however, often used to prevent retraction pockets in the posterior tympanic membrane and this should also be considered when deciding how large the cartilage graft should be.