Use of transtibial aimer via the accessory anteromedial portal to identify the center of the ACL footprint

PURPOSE: To assess the ability of a transtibial aimer with a 7-mm off-set in a standardized position to reach the center of the ACL footprint on the femur through the AM portal. METHODS: Nineteen cadaveric knees were dissected, and the perimeter of the femoral ACL footprint was marked. The aimer was placed just superior to the medial joint line close to the medial condyle through the AM portal. The guide was rested upon the posterior cortex and placed in three different positions: (A) at zero degrees in frontal plane and 60° in axial plane, (B) at 45° in frontal and 45° in axial, and (C) at the center of the ACL insertion site under direct visualization. A digital camera was used to take pictures on the axial plane, and Image J software was used for angle measurement. Aluminum beads were used to mark the three positions indicated by the aimer, and CT scans were performed. The distances from the true center of the ACL to each point were determined. RESULTS: Position A resulted in femoral tunnel placement furthest from the center of the ACL footprint (8.6 mm). Position B was at a distance of 3.2 mm, and position C was the most accurate, with an average distance of 2.0 mm. The angles required by Position C varied with an average of 54° ± 11° in the frontal plane and an average of 44° ± 6° in the axial plane. CONCLUSION: The 7-mm transtibial aimer was unable to reach the center of ACL footprint at a fixed orientation.