Medial Portal Drilling: Effects on the Femoral Tunnel Aperture Morphology During Anterior Cruciate Ligament Reconstruction

Background: A goal of anatomic anterior cruciate ligament (ACL) reconstruction should be to create a femoral tunnel aperture that resembles the native attachment site in terms of size and orientation. Aperture morphology varies as a function of the drill-bit diameter, the angle in the horizontal plane at which the drilled tunnel intersects the lateral notch wall (transverse drill angle), and the angle of knee flexion in the vertical plane during drilling. Methods: A literature search was conducted to determine population-based dimensions of the femoral ACL footprint. The tunnel aperture length, width, and area associated with the use of different drill-bit diameters and transverse drill angles were calculated. The effect of the knee flexion angle on the orientation (anteroposterior and proximodistal dimension) and size of the femoral tunnel aperture relative to the native femoral insertion of the ACL were calculated with use of geometric mathematical models. Results: The literature search revealed an average femoral insertion site size of 8.9 mm for width, 16.3 mm for length, and 136.0 mm(2) for area. The use of a 9-mm drill bit at a transverse drill angle of 40 degrees resulted in a tunnel aperture area of 99.0 mm(2) and a tunnel aperture length of 14.0 mm. Decreasing the transverse drill angle from 60 degrees to 20 degrees led to an increase of 152.9% in length and of 153.1% in tunnel aperture area. When a 9-mm drill bit and a transverse drill angle of 40 degrees were used, the aperture seemed to best match the native ACL footprint when drilling was performed at a knee flexion angle of 102 degrees; deviations from this angle in either direction resulted in increasing tunnel area mismatch compared with the baseline aperture. Increasing the knee flexion angle to 130 degrees decreased the proximodistal dimension of the aperture by 2.78 mm and increased the anteroposterior distance by 0.65 mm, creating a mismatched area of 13.5%. Conclusions: The drill-bit diameter, transverse drill angle, and knee flexion angle can all affect femoral tunnel aperture morphology in medial portal drilling during ACL reconstruction. The relationship between drilling orientation and aperture morphology is critical knowledge for surgeons performing ACL reconstruction.