INTERFERENCE SCREW POSITION AND HAMSTRING GRAFT LOCATION FOR ANTERIORCRUCIATE 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 da mage 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 c oncern could be addressed by placing the interference screw centrally between the four limbs of the hamstring graft. The purpose of this stu dy 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 ham string tendons verses an interference screw positioned centrally betwe en 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 gracil is tendon segments were looped to form four strands. The specimens wer e then fixed with a bioabsorbable interference screw in the two differ ent positions and pulled from a standardized polyurethane foam. All te ndons in both groups failed by pulling out from between the interferen ce screw and tunnel, regardless of the screw position. No tendon was c ut by the screw in either group. There was no significant difference b etween the forces required to produce specific amounts of graft slippa ge between the two fixation techniques tested. There was no significan t 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 t he screw placed in the central position, The maximum pullout force ave raged 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 no t significantly different. Placement of the interference screw in the central position did not compromise strength and it improves graft con tact within the bone tunnel. Interference screw fixation, when applied against a bone plug, has been shown to consistently have a pullout fo rce of more than 300 N.