Biomechanics of stapesplasty: a review.

OBJECTIVE: The replacement of an otosclerotic fixed stapes by a prosthesis significantly modifies the function of the normal ossicular chain. Because the ear works as a pressure receptor, a piston prosthesis will both modify the sound pressure transmission and respond to the ambient air pressure changes in a different way than the normal stapes. Both aspects, the acoustic transmission and the displacements of the prosthesis with variations of atmospheric pressures, are reviewed.
CONCLUSION: The diameter of a piston should not be smaller than 0.4 mm and it should be inserted into a significantly larger perforation of the footplate. A low mass is advantageous for transmission of higher frequencies. Firm attachment at the processus lenticularis is necessary for effective energy transmission. This is of paramount importance in a malleovestibulopexy because of the oblique position of the piston. A piston can be displaced up to 0.5 mm in the vestibule at ambient air pressure changes, as it is not attached to the annular ligament. These large movements explain why a short piston can be lifted out of the footplate fenestration (i.e., after sneezing) and why a piston with excessive length can impale the structures of the membranous labyrinth, causing vertigo (i.e., in a retraction of the tympanic membrane). However, flying or diving should be allowed generally, provided that a test with tympanometry, applying variable pressures of +/- 400 mm H(2)O, is tolerated without evoking vertigo.