Fracture Risk through Oblique External Fixator Pin Tracks

A known complication following removal of an external fixator is fracture through a pin track. Certain advantages are granted by inserting external fixator half-pins at oblique (not orthogonal) angles, such as increased pull-out strength and increased stability against torsional strain with divergent half-pins. We sought to determine if oblique pin tracks also had protective properties against secondary fracture through the pin track after half-pin removal. Acrylic plastic tubing was used to evaluate the effects of pin track obliquity on load to failure after drilling pin tracks. Tubing of 5/8″ outside diameter with 3/8″ inside diameter was chosen to simulate a clinical representation of an adult femur. Torsional, anterior-posterior (A-P) 4-point bending, and medial-lateral (M-L) 4-point bending loads were applied to 12″ specimens with a 6 mm hole drilled mid-shaft at either 0, 10, 20, or 30 degrees. In the A-P 4-point bending model, the drill hole was oriented in the direction of the applied force. In the M-L bending model, the drill hole was oriented perpendicular to the applied force. With increasing half-pin obliquity, less force was needed to produce a fracture through the drilled pin track with torsional stress (13.0 Nm for 0 degrees, 10.6 Nm for 30 degrees, P=0.004). There was no significant difference in the amount A-P and M-L force needed to cause failure. The M-L pin trajectory demonstrated higher load to failure compared with the A-P pin trajectory. Highest load to failure was demonstrated in the 0 degree M-L sample. A statistically significant reduction in force was required to cause fracture with increased pin obliquity. The M-L pin trajectory demonstrated higher load to failure than the A-P pin trajectory. Increased pin obliquity may lead to higher risk of fracture after half-pin removal. Placing a half-pin at 0 degree, orthogonal to the direction of highest physiological force, is likely to reduce the risk of fracture after half-pin removal.