Effect of reamer design on posteriorization of the tibial tunnel during endoscopic transtibial anterior cruciate ligament reconstruction.

BACKGROUND It is known that small alterations in tunnel positioning during anterior cruciate ligament (ACL) reconstruction significantly affect ACL length and tensioning patterns as well as alter force vectors and joint kinematics. PURPOSE To compare the amount of inadvertent posteriorization of the ACL tibial tunnel anatomy during transtibial ACL femoral reaming in the "over-the-top" position with a full femoral reamer versus a half femoral reamer, in comparison to the native tibial ACL footprint. It is hypothesized that the half reamer will result in less distortion of tibial tunnel anatomy and improved anatomic footprint coverage. It is also hypothesized that the true center of the tibial ACL footprint lies more anterior than previously described. STUDY DESIGN Controlled laboratory study. METHODS Eight cadaveric knee specimens were securely fixed to a stationary table at 90° of flexion and neutral rotation. After removal of the anterior capsule and patella, native joint anatomy was precisely recorded with a digitizer accurate to 0.05 mm. Tibial and femoral tunnels were then drilled in the manner of transtibial ACL reconstructions using the optimal tibial starting point of 15.9 mm below the medial plateau and 9.8 mm posteromedial to the medial margin of the tibial tubercle. After the 11-mm tibial tunnel was drilled, femoral tunnels were first drilled with a 10-mm half-fluted reamer, followed by a 10-mm full reamer. Each tibial tunnel's location and geometry relative to the native ligamentous insertion sites and joint anatomy were digitized. RESULTS Digitized measurements of ACL insertional anatomy demonstrated that the center of the native ACL tibial footprint was 2.0 ± 0.49 mm (range, 1.1-2.7 mm) anterior to the posterior aspect of the lateral meniscus' anterior horn. Use of the 10-mm full femoral reamer resulted in a tibial-articular aperture that had a posterior edge 4.35 mm more posterior (P = .049) and extra-anatomic (P = .006) than the footprint of the 10-mm half femoral reamer. CONCLUSION Half-fluted reamers may be more advantageous for femoral tunnel reaming with a more oblique transtibial approach, as they result in less posterior tibial tunnel expansion than full reamers, possibly leading to improved graft function. Based on the digitized anatomy, the center of the tibial attachment site is anterior to the posterior aspect of the lateral mensicus's anterior horn, which has been traditionally described as the anatomic center. CLINICAL RELEVANCE Use of half-fluted reamers for transtibial femoral tunnel reaming could lead to more anatomic tunnel placement and possibly improved graft mechanics after transtibial single-bundle ACL reconstruction.