The Biomechanics of Stapes Replacement

In humans, the rigidity of the annular ligament represents almost 90% of the impedance of the middle ear, especially at low frequencies (Huttenbrink 2003). This annular ligament therefore dominates the transmission of sound at speech frequencies in the normal middle ear in humans. The collagen fibers of the annular ligament determine the amplitude with which the stapedial footplate will vibrate in response to low-frequency acoustic stimulation. The sound pressure at the entrance of the cochlea is directly proportional to the volume velocity of the stapes footplate, which in turn corresponds to the volume of the liquid that is displaced by the vibration of the stapes footplate. The output of the middle ear can be quantified by the “volume velocity” of the stapes footplate. Volume velocity can be defined as the product of stapes linear velocity and the area of the stapes footplate (Rosowski and Merchant 1995).

The stapes footplate has an area of approximately 3.2 mm². This exerts vibrations with amplitudes of only a few nanometers for displacement of a large amount of fluid to transmit sound pressure into the cochlea at physiologic sound pressures. Therefore, following stapedotomy, the effective vibrating area is reduced to the area where the prosthesis has been inserted, reducing in turn the volume velocity. A stapes prosthesis, with its smaller contact area (32 mm² area of footplate vs. 0.12 mm² area of fenestra in a 0.4 mm² stapes prosthesis), can vibrate with much larger amplitude at equivalent sound pressures. Compensation takes place because the increased linear velocity of the smaller piston compensates for the decrease in the surface area.

HOW THE BIOMECHANICAL ACTIVITY OF THE MIDDLE EAR IS STUDIED

The biomechanical activity of the middle ear is studied in animal models, human cadaveric temporal bones, and mathematical models. Modern tools like laser Doppler vibrometry have provided better insight into the acoustic effects of stapedectomy/stapedotomy in humans. Mathematical models like the finite element model (FEM) and the simple lumped element model have a disadvantage in that any single incorrect parameter can distort a result.

The interface between the piston and the edge of the fenestra is considered to simulate the stiffness of the annular ligament (Causse 1989). The seal, in addition to transmitting pressure changes in a uniform manner in all directions in the vestibule, stabilizes the piston against changes in atmospheric pressure (Huttenbrink 2003). The vibrating (and therefore the energy transferring) area of the stapes prosthesis, is not limited to the diameter of the prosthesis. It will also include the area of the surrounding connective seal, which, being larger than the diameter of the piston, gives good results in hearing (Smyth and Hassard 1978).

APPLICATION OF PRESERVATION OF THE STAPES TENDON IN STAPES REPLACEMENT

Silverstein and colleagues (1998) found that preservation of the stapes tendon reduced incus erosion because the blood supply to the lenticular process was kept intact. Furthermore, they postulated that there was a more physiologic transmission for high sound pressure levels and a reduction in the discomfort in a noisy environment because of the preservation of the acoustic reflex.

In a normal ear, the pull of the stapes muscle rotates the footplate around its transverse axis and thus stretches the annular ligament, causing an increase in the tension of the collagen fibers of the annular ligament. This in turn causes a shift of the resonance to the higher frequencies (Huttenbrink 1995). The acoustic reflex induces a decrease of larger movements of the footplate and low-frequency transmission but an improved transmission of the more harmful higher frequencies of noise. Thus, it would seem that the preservation of the stapes muscle in stapedotomy does not serve the purpose for which it was intended.

Following stapedotomy with preservation of the stapedius muscle, a contraction of the stapedius muscle could be registered as a change in impedance of the tympanic membrane. Impedance change of the fenestral seal between the piston and the vestibule, however, cannot be estimated. At this time it is unclear what the other factors are that contribute to the pleasant hearing experienced by patients who have undergone stapedotomy and have had the stapedius muscle preserved (Huttenbrink 2003).