Methodology for laser vibrometer studies of stapes footplate displacement

Most modern studies of stapes displacement use laser Doppler interferometry. However, there are some practical difficulties to be overcome if consistent results are to be obtained. The purpose of this study was to produce a reliable methodology for measuring stapes displacement in vitro, particularly for evaluation of ossicular prostheses and hearing implants Reflectivity of the stapes footplate: Consistent optical reflection of the laser beam to the sensor has been improved by using retro-reflective paint instead of retro-reflective foil.

Reflective foil was difficult to adhere to the footplate in a manner that gave stable results, so a retroreflective paint was used. This method is similar to that of using glass micro-beads.

Surgical approach to the stapes: The internal meatal approach allows alignment of the beam along or close to the direction of vibration of the footplate, so cosine errors are small. It is a rapid, standard approach that does not need to disturb the middle ear cavity.

Other approaches restrict access to the beam, such that the vibration is measured at a large angle to the motion. This means that the actual level of vibration can be difficult to determine and there are problems in standardizing measurements when attempting to realign the temporal bone during multiple measurements on the same bone after, for example, inserting different types of prosthesis.

The methodology proposed is straightforward and gives reproducible results. It allows effective use of the time available for testing in vitro temporal bone specimens, while they remain viable.