Effects of initial graft tension on knee stability after anterior cruciate ligament reconstruction using hamstring tendons: a cadaver study.

PURPOSE: Tension degradation within hamstring grafts and anterior knee laxity were analyzed in a cadaveric anterior cruciate ligament (ACL) reconstruction model undergoing cyclic motion. It was hypothesized that suture fixation of a hamstring graft would lose tension during cycling initially and then stabilize, and that anterior knee laxity would increase as tension was lost. Hamstring grafts fixed under 3 different loads were evaluated to determine how initial graft tension affected knee laxity after cyclic motion. TYPE OF STUDY: Cadaveric biomechanical analysis.
METHODS: Eighteen pairs of fresh-frozen hamstring tendons were tested on 2 cadaveric knees undergoing ACL reconstruction. The hamstring pairs were separated equally and randomly into one of 3 tension groups: 68 N (15 lb), 45 N (10 lb), and 23 N (5 lb). The loads were applied to the graft at 30 degrees of flexion, and the grafts were secured to the tibia with a suture and post technique. The knee was then cycled 1,000 times using an Instron machine (Instron, Canton, MA) through a range of motion between 0 degrees to 90 degrees. Constant monitoring and recording of graft tension was performed. A KT-1000 (Medmetrics, San Diego, CA) was performed (1) on the intact knee, (2) after ACL excision, (3) after ACL reconstruction and initial graft fixation, and (4) at the completion of the 1,000 cycles. An analysis of variance test was used to evaluate data.
RESULTS: The tension within the grafts after 1,000 cycles decreased to 34.5 N (7.6 lb), 16.8 N (3.7 lb), and 15.4 N (3.4 lb) from the preloads of 68, 45, and 23 N, respectively (P <.05 in all cases). This represented an average decrease of 50.2% of the initial tension after 1,000 cycles. Manual-maximum KT testing of the intact knees was 5.8 +/- 0.3 mm, and after ACL excision was 13.2 +/- 0.9 mm. KT testing revealed 6.0 +/- 0.9 mm, 8.1 +/- 1.9 mm, and 8.9 +/- 1.1 mm of anterior translation after fixation in the tension groups of 68, 45, and 23 N, respectively. After 1,000 cycles, the translation increased to 7.8 +/- 1.0 mm, 10.5 +/- 1.9 mm, and 10.3 +/- 1.5 mm, respectively.
CONCLUSIONS: This study showed that initial graft tension decreases with cyclic loading, resulting in increased knee laxity. To restore anterior translation to within 3 mm of the native ACL condition after cyclic loading, approximately 68 N of initial tension is required using this fixation technique.