Learning Objectives: Optimal prosthesis position at tympanic membrane and stapes; optimal prosthesis length; tension of annular ligament in middle ear reconstruction, tympanoplasty.
Introduction: For an optimal sound transfer function (STF) positioning and coupling are the most important factors in middle ear reconstruction with passive implants. Middle ear ventilation problems can change the position and coupling of prosthesis after surgery leading to dislocation or tension of the ligaments. Therefore mechanism and techniques in prosthesis design had to be established to prevent dislocation and tension.
Methods: STF between tympanic membrane and footplate was calculated in a Finite Element Model of the middle ear and measured with Laser-Doppler-Vibrometry in temporal bones. The coupling to the tympanic membrane, malleus handle, stapes head and footplate was compared. The influence of prosthesis’ length was measured in temporal bones using a memory-metal TORP. For the investigation of a new prosthesis concept a new flexible TORP was developed and measured in temporal bones during atmospheric pressure alternations.
Results: Malleus attachment near its neck improves the STF in higher frequencies in comparison to tympanic membrane coupling. The stapes head and the center of the footplate are both best suitable for prosthesis coupling. The elongation of prosthesis length between 50 and 200 µm leads to a frequency dependent STF reduction of 5 to 25 dB below 1.0 kHz. At frequencies >2.0 kHz the reduction was less prominent or the STF showed even an improvement of up to 10 dB (SPL).
Conclusion: At the tympanic membrane malleus handle is optimal for prosthesis coupling. At the footplate the center is the optimal position for TORPs. The correct length of implanted prostheses (functional length) should be measured before implantation to prevent tension at the annular ligament. In the future, prosthesis with pressure compensation elements might reduce the risk of dislocation and annular ligament tension. Tension has a significant impact on the STF in middle ear reconstruction.
