On the Statics of Malleus and Incus and on the Function of the Malleus-Incus Joint

Abstract

An electromagnetic probe was used for statistical measurements on human temporal bone preparations in the malleus—incus region. When forces of 5 and 10 mN (milli Newton) were applied to certain points, the displacement of the malleus was considerably greater close to the umbo than in the region of the short process. According to the lever laws, a pivot was determined which was situated approximately in the mass centre of malleus and incus if two measuring points were chosen which were situated close to the umbo. The pivot was far outside the mass centre when the measuring points were closer to the head of the malleus. The conclusion was that statistically both auditory ossicles have no fixed pivot; rather, they are in a position to adapt to each oscillation of the tympanum. When forces were applied alternately at the malleus and incus with the same lever arm, a different displacement was measured, varying with the force applied. Greater forces led to more pronounced differences in displacement than did smaller forces. When forces of 1 mN and below were applied, no differences in the displacement were ascertained. This shows that the malleus—incus joint was flexible if greater forces were applied. In dynamic terms, we can assume the existence of a protective mechanism about 120 dB.