Geometric Parameters of the Ossicular Chain as a Function of Its Integrity

HYPOTHESIS: Determining the spatial distribution of the malleus handle in relationship to the stapes as a function of the condition of the ossicular chain. BACKGROUND: The position of replacement prosthesis and by association the direction of the force vectors exerted on the stapes head or footplate depends on the location of the malleus handle in relationship to the stapes and the contact point of the prosthesis at the malleus. Ideally, the force of prosthesis is conducted along an imaginary line through the center of the footplate and the stapes head. METHODS: Fifteen cadaveric human temporal bone specimens were investigated by means of micro-CT followed by 3D analysis. For different contact points, the angulation of a virtual rod prosthesis compared with the ideal reconstructive situation was determined. RESULTS: In case of missing incus, angulation of an imaginary line from stapes head to the most medially located point of the malleus handle (on average, 0.6 mm above the malleus tip) averages 85 ± 8 degrees (95% CI, 80-89 degrees) and to the upper end of the malleus handle 69 ± 7 degrees (95% CI, 65-73 degrees). Angulation from the center of the footplate to the malleus tip averages 37 ± 7 degrees (95% CI, 33-41 degrees) and to the upper end of the malleus 33 ± 5 degrees (95% CI, 30-36 degrees). The differences of the spatial distribution of the malleus handle in relationship to the stapes in different conditions of the ossicular chain are not relevant. CONCLUSION: In contrast to the ideal reconstructive situation, the present data show a malleus-stapes offset with particular unfavorable angles for a stapes head to malleus tip assembly.