Background: Anterior external fixator half pins are commonly used to secure a knee spanning external fixator. Knee joint penetration by the pins is a serious complication.

Objective: To determine the safe zone for the insertion of anterior half pins at the distal femur to avoid suprapatellar pouch penetration using fixed anatomical landmarks.

Materials and Methods: The distance from the medial and lateral epicondyle along the femoral axis to the perpendicular line of the superior reflection of suprapatellar pouch was measured in MRI images of 100 knees.

Results: The average distance from the superior pole of the patella to the superior reflection of the suprapatellar pouch was 26.5±7.2 mm (95% CI 25.1 to 27.9). The average distance from superior reflection of the suprapatellar pouch to the medial epicondyle was 47.5±6.3 mm (95% CI 46.2 to 48.7) and to lateral epicondyle was 53.0±6.6 mm (95% CI 51.7 to 54.3).

Conclusion: Insertion of anterior external fixator half pins into the distal femur should begin at least 5.0 cm above the medial epicondyle or 5.5 cm above the lateral epicondyle to avoid knee joint penetration.

Keywords: Safe zone, Anterior external fixation, Half pin, Suprapatellar pouch, Medial epicondyle, Lateral epicondyle