[Movements of stapes-piston prostheses in changes in static air pressure].

The displacements of stapes piston prostheses, induced by variations of static air pressure from 0 to +/- 400 mm H2O, were examined in 9 temporal bone preparations by a microscopic measuring method. In the normal middle ear, the gliding incudo-malleal joint, serving as a protective mechanism, confined these movements to a mean of 232 microns. These in- and excursions could exceed 0.5 mm, however, with an ankylosed incudo-malleal joint. Arthrosis of this joint is not rare in otosclerosis. In view of these excessive displacements, compared to inner ear dimensions, piston prostheses should always dip into the vestibulum in the lower half of the footplate, far from endolymphatic membranes. A simple technique may intraoperatively reveal the gliding or ankylosed condition of incudo-malleal joint surfaces, prior to adapting the length of the prosthesis.