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Coupling problems in middle ear reconstruction

Zahnert T.

Published: MEMRO 2006, 4th International Symposium on Middle Ear
Mechanics in Research and Otology Selected Abstracts.

The normal and reconstructed middle ear can be considered as a mechanical
vibrating system. After the implementation of tympanoplasty as a standardized
surgical technique various reconstruction techniques and implants
were suggested for the reconstruction of the tympanic membrane and the
ossicular chain. Laser–Doppler-vibrometry and model calculations have
given new insight into the vibration modes of the normal and reconstructed
middle ear during the recent years. Nowadays it can be concluded, that
not only material properties of implants but also coupling factors have an
important influence on good hearing results. We investigated coupling factors
between tympanic membrane and the surrounding bone, between the
tympanic membrane and middle ear implants and between the prosthesis
and the ossicular chain using model calculations and temporal bone experiments.
The quality of the tympanic membrane, which can be considered
as the “motor of the middle ear”, has the most important impact on the
sound transfer to the inner ear. Ventilation and mucosa problems can damp
the tympanic membrane vibrations as well as the reconstruction techniques
or the mechanical properties of transplants. The coupling of the tympanic
membrane to either the surrounding bone or the cartilage transplants has
an influence on the stiffness. The contact of the tympanic membrane to the
malleus handle is of importance in order to allow good sound conduction
to middle ear prostheses in the high frequency range. Furthermore the contact
of prostheses to the stapes head or the footplate may influence hearing
results. In our investigations the angle of prostheses towards the tympanic
membrane and the stiffness of coupling plays an important role. Concerning
the angle it is of importance to distinguish between the x and y – direction.
An absolutely stiff contact between malleus and stapes can reduce the sound
transfer and increase the risk of prosthesis dislocation or even damage of
the annular ligament. Even nowadays modern middle ear reconstructions
can only simulate the simple function of a columella. In future it may be
important to invent middle ear implants which will be able to fulfil both
required middle ear functions – the sound transfer and the compensation of
atmospheric pressure changes. It can be assumed that hearing results may
improve due to an unstressed coupling of middle ear prostheses by taking
the above mentioned techniques and findings into consideration.