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CURRENT CONCEPTS
Femoral Tunnel Placement in ACL Reconstruction Central Footprint vs. AM bundle (continued)
In the Duke study ten patients underwent a central femoral footprint ACLR using a Flip-cutter while twelve patients had a transtibial ACL reconstruction. High resolution 3D MRI analysis was performed 18 months post surgery. Patients in the non-anatomic graft placement group demonstrated a significant decrease in cartilage thickness along the medial intercondylar notch in the operative knee relative to the intact knee (8%) while in the anatomic graft placement group, no significant changes were observed. It is however important when evaluating the significance of this paper that no differences were observed elsewhere in the knee and the area where changed occur was a small and takes minimal load. They concluded that their findings suggest that “restoring normal knee motion after ACL injury may help to slow the progression of degeneration. Therefore, graft placement may have important implications on the development of osteoarthritis after ACL reconstruction.”
The Danish ACL Registry was the first group to show a significantly higher failure rate with an anteromedial portal technique. The two recently completed studies above by Clatworthy, who had a large series, and Williams, who had a very high demand patient population group, have shown a similar significant increase in the failure rate when the femoral tunnel position is moved from the ‘anatomical’ anteromedial position to the ‘anatomical’ centre of the femoral footprint.
In changing to the central femoral footprint position the authors of the review all noted that the ACL graft was less isometric. It was common to have a negative Lachman but a grade 1 anterior drawer at the completion of the procedure. This finding has been published by Lubowitz.
Forty years ago, Artmann and Wirth reported that a nearly isometric region for tunnel placement existed in the femur such that there is minimal elongation of the native anterior cruciate ligament during knee motion The biomechanical rationale for choosing an isometric region of an ACL graft is that it will maintain function throughout the range of flexion and extension. A non-isometric graft would be expected to slacken during a large portion of the flexion cycle and not restrain anterior translation of the tibia or, if fixed at the wrong flexion angle, could capture the knee and cause graft failure due to excessive tension. These two theoretical undesirable effects from non-isometric graft placement are supported by many, experimental and clinical studies that have shown that non-isometric femoral tunnel placement at the time of the operation can cause recurrent anterior laxity of the knee. The most isometric region of the femoral footprint has been consistently shown to be localized eccentrically within footprint in a relatively narrow band like region that is proximal (deep) and anterior along the lateral intercondylar ridge within the footprint. This region corresponds to the direct insertion histologic region of the femoral footprint that has been shown carry the most load during stability testing.
A large body of literature demonstrates that a femoral tunnel in the center of the femoral footprint is less isometric than one placed in the more anterior region of the footprint. Indeed, the anterior position (high in the footprint) identified by Noyes demonstrates minimal anisometric with 1 – 4 mm of length change through the range of motion. In contrast, a central femoral tunnel would be expected to demonstrate 5 – 7mm of length change while a lower graft (in the PL region of the footprint) demonstrates approximately 1 cm of length change thru the range of motion. As such, central grafts, or grafts placed in the PL portion of the femoral footprint would be expended to see high tension or graft forces as the knee is flexed or lose tension completely if graft is fixed at full extension.
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Multiple cadaveric studies investigated the relationship between femoral tunnel positioning and time zero stability. These studies often demonstrated superior time zero control of knee stability, particularly in regards to pivoting type maneuvers, when the femoral tunnel was placed more centrally in the femoral footprint as compared to a tunnels placed outside the footprint.
However, there is an emerging body of literature that demonstrates no significant difference in time zero stability examination between an anteriorly placed femoral tunnel WITHIN the anatomic footprint (eccentrically located in the footprint) as compared to a centrally placed graft. Returning to the more isometric tunnel position that is still located within the femoral footprint would be expected to confer the benefits of an anatomically based graft position with the advantageous profile of improved isometry as compared to a central or PL placed graft. Biomechanical studies have documented that ACL graft fibers placed posteriorly (low) in the footprint causes high graft forces in extension and in some cases graft rupture. Accordingly, the importance of reconstructing the posterior region of the footprint to better control time zero stability is questioned.
30 ISAKOS NEWSLETTER 2015: Volume II