Interference screw position and hamstring graft location for anterior cruciate ligament reconstruction

Anterior cruciate ligament reconstruction with hamstring tendon graft and interference screw fixation has recently been considered. Concerns for the use of interference screws with soft tissue grafts include damage to the graft during screw insertion, decreased fixation strength, and a decrease in the bone-tendon contact area for healing within the tunnel when the screw is placed in an eccentric position. This last concern could be addressed by placing the interference screw centrally between the four limbs of the hamstring graft. The purpose of this study was to determine the mode of failure, the pullout force, and graft slippage before graft fixation failure of hamstring tendons fixed with an interference screw positioned eccentrically in relation to the hamstring tendons verses an interference screw positioned centrally between the four graft limbs. The semitendinosus and gracilis tendons were harvested from six, fresh cadaveric specimens. Each tendon was divided into two segments of equal length. Both the semitendinosus and gracilis tendon segments were looped to form four strands. The specimens were then fixed with a bioabsorbable interference screw in the two different positions and pulled from a standardized polyurethane foam. All tendons in both groups failed by pulling out from between the interference screw and tunnel, regardless of the screw position. No tendon was cut by the screw in either group. There was no significant difference between the forces required to produce specific amounts of graft slippage between the two fixation techniques tested. There was no significant difference between the average total slippage at maximum pullout, 11.8 mm for the screw placed in the eccentric position and 13.7 mm for the screw placed in the central position. The maximum pullout force averaged 265.3 N for the screw placed in the eccentric position, and 244.7 N for the screw placed in the central position; these values were not significantly different. Placement of the interference screw in the central position did not compromise strength and it improves graft contact within the bone tunnel. Interference screw fixation, when applied against a bone plug, has been shown to consistently have a pullout force of more than 400 N.