Which is an example of a mechanical complication of an external fixator?

• A. Deep infection
• B. Pain at soft tissue
• C. Vascular injury
• D. Pin-bone interface failure

Answer: (D)

Mechanical complications are issues that arise from the hardware and how it interacts with the bone under load. Pin-bone interface failure fits this category because it reflects a problem at the connection between the pin and the bone—looseness, micro-motion, or breakage at that interface that compromises the stability of the fixator. This is a concern about the mechanical integrity of the construct rather than a biological or soft-tissue problem.

Deep infection is a biological complication: it involves microbial invasion and inflammation around the hardware, which can occur with any external fixator and isn’t about the mechanical stability of the pins themselves. Pain at soft tissue is typically related to irritation or injury to surrounding tissues rather than a failure of the fixation hardware. Vascular injury is a vascular complication from placement or surrounding structures, not a mechanical failure of the frame.

So pin-bone interface failure is the example that directly demonstrates a mechanical issue with the external fixator.