ACL Roof Impingement Revisited

OBJECTIVES: Anatomic femoral tunnel placement for single-bundle ACL reconstruction is now well accepted. The ideal location for the tibial tunnel, however, has not been studied extensively. A wide range of anterior to posterior (A-P) tibial tunnel locations are considered acceptable. Biomechanical data suggests that the anterior fibers of the native ACL are more functional. Similarly, ACL grafts placed more anteriorly in the footprint have resulted in improved clinical results in at least one study. However, the concern for intercondylar roof impingement has tempered enthusiasm for a more anterior tibial tunnel placement. Investigations by Howell and others on roof impingement have focused only on the transtibial technique. Our study seeks to characterize intercondylar roof impingement in a 3-D cadaveric model with both transtibial and independent femoral tunnel drilling techniques in the setting of an anteriorly positioned tibial tunnel. METHODS: Twelve fresh frozen cadaver knees (six matched pairs) were randomized to either a transtibial or an independent femoral (IF) drilling technique. Tibial guide pins were placed in the anterior half of the ACL tibial footprint following arthroscopic debridement of the native ACL. A fluoroscopic calculation of the tibial guide pin location using the technique described by Staubli was used to ensure a relatively anterior position of the tibial tunnel (Staubli < 35). All efforts were made to place the femoral tunnel anatomically in the center of the footprint. An 8 mm Gore-Tex smoother was passed into the knee to function as a radiopaque surrogate graft, and the knees then underwent computed tomography in maximal extension. Graft-visualized 3D-CT reformatting was used to evaluate for roof impingement by analyzing the Impingement Review Index (IRI) as described by Iriuchishima. Tunnel morphology, knee flexion, and intra-articular graft angles were also recorded. RESULTS: Two grafts (2/6, 33.3 %) in the TT group impinged upon the intercondylar roof and sustained an angular deformity (IRI Type 1) Figure 1. No grafts in the IF group impinged; 4/6 (66.7%) grafts in the IF group touched the roof but did not deflect (IRI Type 2). The average tibial tunnel fluoroscopic Staubli ratio was 31.9 (range 22 - 39). The impinging TT grafts had an average Staubli ratio of 29.4 while the non-impinging TT grafts had more posterior tibial tunnels (Staubli 38). This did not reach statistical significance. All other grafts avoided contact with the roof (IRI Type 3). Two tibial tunnels in the TT group migrated posteriorly more than 6 mm after femoral tunnel drilling, and this may have prevented graft impingement. No significant differences were found between the tibial tunnel locations in the IF group. CONCLUSION: The independent femoral drilling technique appears to have a low risk for roof impingement in the setting of anterior tibial tunnel positioning, likely because of more favorable graft trajectory afforded by an anatomic femoral tunnel that lies below Blumensaat's line. Roof impingement remains a concern after transtibial ACL reconstruction with a more anterior tibial tunnel. An unintended protective effect may occur during transtibial drilling that expands the tibial tunnel posteriorly. Future clinical studies are planned to develop better recommendations for ACL tibial tunnel placement.