A biomechanical study on five unilateral external fracture fixation devices

Area of Science:

• Orthopedic biomechanics
• Biomaterials science
• Surgical device engineering

Background:

• External fixator frames are crucial for stabilizing diaphyseal fractures.
• Plastic, slip, and fatigue failures can compromise frame integrity and fracture healing.
• Understanding failure modes is essential for improving external fixator design and clinical outcomes.

Purpose of the Study:

• To investigate plastic, slip, and fatigue failures in commonly used unilateral external fixator frames.
• To evaluate the impact of simulated weight-bearing and cyclic loading on frame stability.
• To identify design factors contributing to premature failure and loss of fracture stability.

Main Methods:

• Experimental testing of five unilateral external fixators under simulated stable and unstable diaphyseal fracture conditions.
• Measurement of axial loads at which plastic or slip failure occurred.
• Assessment of interfragmentary motion changes after 10,000 load cycles simulating walking.

Main Results:

• Four fixators experienced plastic or slip failure at 50% of average adult weight-bearing (650 N), causing bone malalignment.
• A central screw in the clamp led to premature slip failure in three fixators.
• All fixators showed evidence of fatigue after 10,000 cycles, with significant increases in interfragmentary motion (up to 100% torsionally).
• Clamp slip occurred rapidly in one fixator under cyclic loading.

Conclusions:

• External fixator frames are susceptible to premature plastic or slip failure during routine weight-bearing on unstable fractures.
• Frame fatigue can significantly impair long-term interfragmentary stability.
• Clinical implications include potential for refracture, malunion, and compromised fracture healing due to device failure.