Transtibial Versus Anteromedial Portal Drilling for Anterior Cruciate Ligament Reconstruction: A Cadaveric Study of Femoral Tunnel Length and Obliquity

Purpose: To compare the obliquity and length of femoral tunnels prepared with transtibial versus anteromedial portal drilling for anterior cruciate ligament (ACL) reconstruction and identify potential risks associated with the anteromedial portal reaming technique. Methods: We used 18 human cadaveric knees (9 matched pairs) without ACL injury or pre-existing arthritis for the study. Femoral tunnels for ACL reconstruction were prepared by either a transtibial (n = 6) or anteromedial portal (n = 12) technique. For the anteromedial portal technique, a guidewire was advanced through the medial portal in varying degrees of knee flexion (100 degrees [n = 4], 110 degrees [n = 4], or 120 degrees [n = 4]) as measured with a goniometer. By use of a 6-mm femoral offset guide, two 6-mm femoral tunnels were reamed with the guide placed (1) as far posterior and lateral in the notch as possible and (2) as far medial and vertical in the notch as possible to define the range of maximal and minimal achievable coronal obliquity for each technique. All knees were imaged with high-resolution, 3-dimensional fluoroscopy to define (1) coronal tunnel obliquity relative to the lateral tibial plateau, (2) sagittal tunnel obliquity relative to the long axis of the femur, (3) intraosseous tunnel length, and (4) the presence of posterior cortical wall blowout. Data analysis was performed with a paired t-test and repeated-measures analysis of variance, with P<.05 defined as significant. Results: Preparation of a vertical tunnel was possible with both transtibial and anteromedial portal drilling. The maximal achievable coronal obliquity, however, was significantly better with an anteromedial portal compared with transtibial drilling. However, 7 of 36 tunnels (19.4%) showed violation of the posterior tunnel wall, and all of these cases occurred with the anteromedial portal drilling technique. In addition, 1 of 6 oblique femoral tunnels (16.7%) drilled with the transtibial technique and 5 of 12 oblique femoral tunnels (41.7%) drilled with the anteromedial portal had an intraosseous length less than 25 mm. Increasing knee flexion with anteromedial portal drilling was associated with a significant reduction in tunnel length, increase in coronal obliquity, increase in sagittal obliquity, and increased risk of posterior wall blowout (P<.05). Conclusions: The anteromedial portal technique allows for slightly greater femoral tunnel obliquity compared with transtibial drilling. However, there is a substantially increased risk of critically short tunnels (<25 mm) and posterior tunnel wall blowout when a conventional offset guide is used. Increasing knee flexion with anteromedial portal drilling allows for greater coronal obliquity of the femoral tunnel but is accompanied by a greater risk of critically short tunnels and posterior wall compromise. Clinical Relevance: Our findings provide insight into the potential risks and advantages of a transtibial versus an anteromedial femoral tunnel drilling technique in ACL reconstruction.