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Effect of the Angle of the Femoral and Tibial Tunnels in the Coronal Plane and Incremental Excision of the Posterior Cruciate Ligament on Tension of an Anterior Cruciate Ligament Graft: An in Vitro Study

Investigation performed at the University of California at Davis, Davis, California

Richard Simmons, MS
1153 Calle Emparrado, San Marcos, CA 92069

Stephen M. Howell, MD
8100 Timberlake, Suite F, Sacramento, CA 95823. E-mail address for S.M. Howell: sebhowell{at}aol.com

M.L. Hull, PhD
Department of Mechanical Engineering, Bainer Hall, 1 Shields Avenue, University of California at Davis, Davis, CA 95616

In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from the ACL Study Group, courtesy of AirCast Corporation. In addition, one or more of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (Arthrotek, Inc.). No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Background: High tension in an anterior cruciate ligament graft adversely affects both the graft and the knee; however, it is unknown why high graft tension in flexion occurs in association with a posterior femoral tunnel. The purpose of the present study was to determine the effect of the angle of the femoral and tibial tunnels in the coronal plane and incremental excision of the posterior cruciate ligament on the tension of an anterior cruciate ligament graft during passive flexion.

Methods: Eight cadaveric knees were tested. The angle of the tibial tunnel was varied to 60°, 70°, and 80° in the coronal plane with use of three interchangeable, low-friction bushings. The femoral tunnel, with a 1-mm-thick posterior wall, was drilled through the tibial tunnel bushing with use of the transtibial technique. After the graft had been tested in all three tibial bushings with one femoral tunnel, the femoral tunnel was filled with bone cement and the tunnel combinations were tested. Lastly, the graft was replaced in the 80° femoral and tibial tunnels, and the tests were repeated with excision of the lateral edge of the posterior cruciate ligament in 2-mm increments. Graft tension, the flexion angle, and anteroposterior laxity were recorded in a six-degrees-of-freedom load-application system that passively moved the knee from 0° to 120° of flexion.

Results: The graft tension at 120° of flexion was affected by the angle of the femoral tunnel and by incremental excision of the posterior cruciate ligament. The highest graft tension at 120° of flexion was 169 ± 9 N, which was detected with the graft in the 80° femoral and 80° tibial tunnels. The lowest graft tension at 120° of flexion was 76 ± 8 N, which was detected with the graft in the 60° femoral and 60° tibial tunnels. The graft tension of 76 N at 120° of flexion with the graft in the 60° femoral and 60° tibial tunnels was closer to the tension in the intact anterior cruciate ligament. Excision of the lateral edge of the posterior cruciate ligament in 2 and 4-mm increments significantly lowered the graft tension at 120° of flexion without changing the anteroposterior position of the tibia.

Conclusions: Placing the femoral tunnel at 60° in the coronal plane lowers graft tension in flexion. Our results suggest that high graft tension in flexion is caused by impingement of the graft against the posterior cruciate ligament, which results from placing the femoral tunnel medially at the apex of the notch in the coronal plane.

Clinical Relevance: For the surgeon who prefers the transtibial technique, the present study shows that controlling the angle of the tibial tunnel controls the angle of the femoral tunnel and the graft tension in flexion.

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