The safe zone for external fixator pins in the femur.

Objective: To define the anatomic "safe zone" for placement of external fixator half pins into the anterior and lateral femur.

Methods: In 20 fresh-frozen hemipelvis specimens, the femoral nerve and all branches crossing the femur were dissected out to their final muscular locations. The location where the nerves crossed the anterior femur was measured from the anterior superior iliac spine and inferior margin of the lesser trochanter. The knee joint was then opened, and the distance from the superior reflection of the suprapatellar pouch to the last branch of the femoral nerve crossing the anterior femur was measured, defining the safe zone for anterior pin placement.

Results: The last branch of the femoral nerve crossed at an average distance from the anterior superior iliac spine of 174 ± 43 mm (range, 95-248 mm) and from the lesser trochanter at a distance of 58 ± 36 mm (range, 0-136 mm). The average distance from the proximal pole of the patella to the superior reflection of the suprapatellar pouch was 46.3 ± 13.1 mm (range, 20-74 mm). Using the linear distance between the last crossing femoral nerve branch and the superior reflection of the pouch, the average safe zone measured 199 ± 39.8 mm (range, 124-268 mm). The safe zone correlated with thigh length (r = 0.48, P = 0.03). All nerve branches terminated at their muscular origins without crossing lateral to a line from the anterior greater trochanter to the anterior aspect of the lateral femoral condyle.

Conclusions: The safe zone for anterior external fixator half pin placement into the femur is on average 20 cm in length and can be as narrow as 12 cm. Anterior pins should begin 7.5 cm above the superior pole of the patella to avoid inadvertent knee joint penetration. Because the entire lateral femur is safely available for half pin placement, including distally, we recommend the use of alternative frame constructs with either anterolateral or lateral pins given the limitations and risks of anterior pin placement.