Forces Required for Isolated Malleus Shaft Fractures

Area of Science:

• Otolaryngology
• Biomechanics
• Forensic Medicine

Background:

• Isolated malleus shaft fractures are uncommon ear injuries.
• A frequent cause is the forceful removal of a finger from a wet ear canal.
• The precise biomechanical mechanism remains incompletely understood.

Purpose of the Study:

• To experimentally investigate the mechanism of isolated malleus shaft fractures.
• To determine the forces required to cause such fractures.
• To model the role of the tensor tympani muscle in this injury.

Main Methods:

• Utilized fresh-frozen human temporal bones for direct observation.
• Applied negative pressure to the ear canal and measured fracture forces.
• Developed mathematical and physical models based on measurements from 60 adult subjects.
• Analyzed tensor tympani muscle fiber type composition (87% type II fibers) to estimate muscle force.

Main Results:

• Negative pressure alone in a wet ear canal did not fracture the malleus.
• Applied forces exceeded the threshold for malleus fracture without sufficient counterforce.
• Incorporating estimated tensor tympani muscle forces demonstrated sufficient force generation for fracture.

Conclusions:

• A combination of outward finger pull (negative pressure) in a wet ear canal and a reflexive tensor tympani muscle contraction can cause isolated malleus shaft fractures.
• An intact tympanic membrane is possible with this injury mechanism.