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Synopsis: Careful placement of femoral and tibial tunnels, careful performance of a notchplasty, and the use of consistent arthroscopic and radiographic landmarks prevent surgical error.
Source: Fineberg MS, et al. Practical considerations in anterior cruciate ligament replacement surgery. J Arthroscopy 2000;16(7):715-724.
In this technical review, fineberg and colleagues describe the ACL anatomy and explain the terminology associated with this ligament that has developed over the past 30 years. The ACL "footprint" is described as such because of the distinct anterior "toe" that lies in close approximation to the intercondylar roof of the femur when the knee is in full extension. Functionally, the ACL provides a restraint to anterior translation and internal rotation of the tibia, varus and valgus angulation, and hyperextension of the knee.
The technical requirements of an ACL reconstruction must be within the parameters of normal ACL isometry, which is 2.5 mm from flexion to extension. Although the attention to the femoral origin was most focused early in ACL surgery, guidelines for proper tibial tunnel placement have been well described. Incorrect tunnel placement at both the femoral and tibial sides can result in improper graft dynamics and eventual failure. Anterior femoral tunnel placement creates a graft that either limits flexion or produces abnormal graft strain with eventual graft stretch and failure. Anterior tibial tunnel placement creates graft impingement in knee extension and potential for failure. Posterior tibial tunnel placement creates a graft that is too vertical, which doesn’t function properly in slight knee flexion where stability for pivoting or jumping is required.
Proper femoral tunnel placement requires a position posterior enough to leave only a 1-2 mm cortical "back wall" of the endoscopic femoral tunnel. The medial lateral position is slightly lateral to the center of the knee with the pin placed at 11 o’clock for the right knee, and 1 o’clock for the left. Critical femoral tunnel placement is required to avoid an anterior position that in turn creates abnormal graft dynamics and frequent failure.
Tibial tunnel placement can be deceiving and again is slightly lateral to midline on plain radiographs. Arthroscopic anatomic landmarks are used to place the center of the tibial tunnel in the posterior aspect of the ACL tibial footprint. This site is ideally at the junction of the posterior and middle one-thirds of the footprint. Additionally, the surgeon can reference from a site 7 mm anterior to the PCL edge, the anterior horn of the lateral meniscus, and the anterolateral slope of the medial intercondylar eminence. Careful evaluation of pin placement prior to tunnel reaming should be carried out by viewing the pin with the knee in and near full extension. Intraoperative radiographs in extension can be used to document tunnel position so that the anterior edge of the tibial tunnel is posterior and parallel with Blumensaat’s line to avoid impingement.
Graft-tunnel length mismatch can create difficulties in fixation, especially when the tibial tunnel is too short. A low angle of approach is frequently the culprit and can be avoided by using a 55° angled approach when creating the distal tunnel entrance site. With a short tibial tunnel, the ACL graft bone plug is prominent so that interference screw fixation is suboptimal and may require substitution with a staple or screw and post. In my experience, twisting the graft will allow for graft shortening and the ability to use an interference screw for fixation. Finally, screw divergence should be avoided to allow optimal fixation but is less important on the femoral side where bone density is the greatest. Also, graft position in the femur produces a wedge effect with inherent stability. Tibial interference fixation is the weaker link and convergent screw placement in this tunnel is of greater importance but technically easier than on the femoral side.
This current concepts review in the Journal of Arthroscopy details the technical points and potential pitfalls in endoscopic ACL reconstruction. It is useful for the practicing sports medicine specialist as it clearly identifies the key points in the technical exercise of an endoscopic ACL reconstruction. Careful attention to tunnel placement, adhering to identifiable landmarks, will help ensure isometry and avoid impingement and graft failure.