Knee Position During ACL Graft Fixation—Does it Really Matter?
Abstract & Commentary
Synopsis: In a biomechanical model, Hoher and colleagues suggest that knee position and applied load really does matter. They suggest that grafts should be tensioned in 30 degrees of flexion with a 67-N posterior tibial load to reproduce normal knee kinematics.
Source: Hoher J, et al. The position of the tibia during graft fixation affects the knee kinematics and graft forces for ACL reconstruction. Am J Sports Med. 2001;29(6):771-776.
Although ACL reconstruction techniques continue to improve, there remains a failure rate for primary ACL reconstruction of approximately 10%. In an effort to improve this rate, Hoher et al suggest that knee flexion and applied load during ACL graft fixation may better reproduce the normal kinematics of the knee.
Ten fresh-frozen cadavers were tested with the robotic/universal force-moment sensor testing system before and after ACL sectioning and reconstruction with a patellar tendon graft. Standardized techniques and testing were accomplished. The femoral side of the graft was fixed with a 9 ´ 25 mm interference screw, and the tibial side was left unfixed for mechanical testing. A customized tibial fixation device was developed to allow the same graft to be fixed rigidly in the same knee to simulate each tibial fixation option. A 44-N load was applied to the graft prior to fixation. The following options were tested in varied order:
1. Full extension with manual posterior tibial load;
2. 30° of flexion with no posterior tibial load;
3. 30° of flexion with a 67-N posterior tibial load;
4. 30° of flexion with a 134-N posterior tibial load.
The results indicated that option 3 best reproduced the normal in situ forces of the ACL. Options 1 and 2 resulted in increased translation and less than normal in situ forces. Option 4 actually resulted in the least anterior translation of the tibia (even better than the intact ACL), but resulted in increased in situ forces from over-tensioning.
Comment by Mark D. Miller, MD
This is another outstanding paper from researchers from the University of Pittsburgh—home of the robot! This innovative research design allowed them to study the same cadaver with different testing parameters—a truly novel approach. Although, as Hoher et al point out, the clinical situation may affect the results that were obtained in the lab, the take-home message is clear. Tensioning ACL grafts in full extension without a posterior tibial load may lead to excessive laxity and diminished in situ forces. Diminished in situ forces may adversely affect graft function and maturation. Excessive laxity obviously leads to graft failure. The concern that is not addressed in this study is whether tensioning the graft in flexion will adversely affect the patient’s ability to achieve full extension postoperatively. This is a question that can only be addressed clinically, although other studies have suggested that this will not be a problem with correct tunnel placement. Hopefully, a massive shift by orthopaedic surgeons to fix the tibial end of the graft in 30° of flexion and apply a posterior tibial load will not result in a resurgence of knee flexion contractures!
Dr. Miller, Associate Professor, UVA Health System, Department of Orthopaedic Surgery, Charlottesville, VA, is Associate Editor of Sports Medicine Reports.
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