A dynamic and natural frequency analysis of the Fisch II spandrel using the finite element method

The recent introduction of the Fisch II spandrel total ossicular replacement prosthesis (TORP), has provided impressive initial hearing benefits in patients. The design, based on a combination of soft plastic and a highly flexible metal shaft, has been analysed in terms of its sound collecting and conducting behaviour using the finite element method. The analysis has examined the mode shapes, displacements and natural frequencies of the prosthesis for a variety of material properties and geometries. The results indicate the optimum vibration characteristics are obtained with a Polycell elastic modulus of 100 N/mm2, with the natural frequency independent of modulus above this value. Furthermore, the diameter of the spandrel shaft is crucially important, with an increase in magnitude of the first natural frequency of 54% for a 25% increase in shaft diameter. At a Polycell membrane thickness of 0.07 mm and a shaft diameter of 0.12 mm, the tympanic membrane and Fisch II prosthesis have almost identical frequencies over the 1st to 6th frequency range. This fact, together with the flexibility of clinical procedures involved in its use, may be the reason for its success.