Forces Required for Isolated Malleus Shaft Fractures

Abstract

Background and Hypothesis: Isolated malleus shaft fractures are rare cases. A commonly reported cause is a finger pulled out from a wet outer ear canal after a shower or bath. The objective was to investigate experimentally the mechanism and forces needed to establish an isolated malleus shaft fracture. Methods: Ten fresh-frozen human temporal bones were adapted to allow visual inspection of the structures involved while negative pressure trauma was applied. Thirty malleus bones were broken and the required forces were measured. Measurements from 60 adult test subjects were used to create mathematical and physical models to calculate and measure the forces necessary for generating trauma. To calculate the maximum muscle force developed by the tensor tympani muscle, the muscle area and fiber type composition were determined. Results: The temporal bone experiments showed that applied negative pressure in a wet ear canal could not fracture the malleus shaft with only passive counterforce from supporting structures, although the forces exceeded what was required for a malleus shaft fracture. When adding calculated counteracting forces from the tensor tympani muscles, which consisted of 87% type II fibers, we estimate that a sufficient force is generated to cause a malleus fracture. Conclusion:The combination of a negative pressure created by a finger pulling outward in a wet ear canal and a simultaneous counteracting reflexive force by the tensor tympani muscle were found to be sufficient to cause an isolated malleus fracture with an intact tympanic membrane.

Unique fiber phenotype composition and metabolic properties of the stapedius and tensor tympani muscles in the human middle ear

“…The tensor tympani tendon has been suggested to protect against outward displacement of the ossicular chain (Hüttenbrink, 1989). However, we have previously reported that the ligament by itself has very little effect in limiting the outward movement of the tympanic membrane and malleus (Rönnblom et al, 2021). In the same study, we found the combination of a negative pressure created by a finger being extracted from a wet ear canal and a simultaneous counteracting reflexive force by the tensor tympani muscle could cause an isolated malleus fracture with an intact tympanic membrane.…”

Isolated Fracture of the Malleus: An Overlooked Cause of Conductive Hearing Loss?

“…Upon sudden withdrawal of the finger from the EAC, sudden hearing loss and otalgia occur. The fracture is a result of opposing forces applied at different areas of the malleus—negative pressure applied to the manubrium laterally by abrupt finger removal and concomitant medial pull to the malleus head created by the tensor tympani 30 . The TM generally does not perforate at the same time given that it is more malleable and tolerant of abrupt pressure changes.…”