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Introduction to a More Anatomic Approach to ACL Reconstruciton

As anterior cruciate ligament anatomy and kinematics become better understood, a more anatomic approach to ACL reconstruction is increasingly being pursued. ACL deficiency has consistently been demonstrated to give rise to increased anterior translation, medial translation, and internal rotation of the tibia during various loading maneuvers. Knee kinematics and stability have been shown to markedly improve following ACL reconstruction with restoration of native ACL insertion sites and ligament orientation. ACL reconstructive procedures that fail to alleviate knee instability post-operatively often lead to a predictively higher risk for osteoarthritis of the knee, failure to return to previous level of play, and poor subjective and objective knee outcome scores.

Although an understanding of ACL biomechanics and knee kinematics has significantly improved in recent decades, optimal surgical technique in ACL reconstruction remains uncertain; to date, no ACL reconstruction procedure has been shown to exactly replicate the biomechanics of the native ACL. Small variations in femoral tunnel positioning can drastically change ACL length, tensioning patterns as well as alter ACL force vectors and joint kinematics. Grafts placed higher on the femoral wall in ACL reconstruction—a less coronally oblique orientation—less effectively opposes rotatory loads as compared with grafts placed lower on the femoral wall. Decreased sagittal plane obliquity has also been implicated, largely because such an orientation incompletely resists anterior translational loads as compared with the native ACL. Regardless of which ACL reconstruction technique is utilized, a growing body of literature supports the notion that a more anatomic reconstruction better restores knee kinematics than non-anatomic reconstructions.

The modified transtibial endoscopic single bundle ACL reconstruction has been demonstrated to have equal efficacy in improving knee joint biomechanical stability as ACL reconstructions performed via an anteromedial portal technique and an outside-in technique. The most limiting aspect of this technique is the reliance of femoral tunnel positioning on tibial tunnel orientation and position; because the femoral tunnel is drilled through the tibial tunnel, the tibial tunnel represents a potentially unforgiving linear constraint to instrumenting the femur. The ideal scenario for transtibial reconstruction is one where the tibial tunnel is collinear with a line connecting the centers of both femoral and tibial ACL insertions. Such geometry has been shown to be impractical, however. As noted by Heming et al, a guide pin drilled through the center of both insertions will consistently exit the tibia within millimeters of the joint line. A tibial tunnel created with this proximal of a starting point likely would compromise tibial graft fixation and create significant graft-tunnel mismatch problems if a bone-tendon-bone graft was employed. If a more distal, traditional tibial starting position is employed instead, the resultant tunnel will be less aligned with the native ligament and will result in less-than-anatomic femoral tunnel positioning.

In a previous cadaveric study, we noted that tibial and femoral tunnels can be created in a highly anatomic manner using a transtibial technique but requires a fairly proximal, carefully chosen tibial starting position.20 In that study, however, an 11mm tibial reamer was utilized in all specimens which afforded great flexibility in placing the “over-the-top” femoral guide through the 11mm wide tibial tunnel onto an anatomic position on the femoral ACL footprint. At the present time, a large proportion of bone-tendon-bone and soft tissue grafts used are smaller than 11mm. It is possible that smaller tibial reamers would not allow for such precise anatomic femoral tunnel placement using a transtibial technique because the resultant smaller tibial tunnel would have too small a diameter. If this notion were proved true, a femoral independent drilling technique may need to be pursued for select cases in which a narrow tibial tunnel is anticipated.

The purpose of this study is to identify the impact of tibial reamer size on the ability to place anatomic femoral tunnels via a transtibial approach. It is hypothesized that there is a threshold for tibial tunnel size, under which, the surgeon will be unable to obtain anatomic femoral tunnel placement using a transtibial technique.

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