Contact pressure on ACL hamstring grafts in the bone tunnel with interference screw fixation--dynamic adaptation under load.

INTRODUCTION Interference screws used in fixation of anterior cruciate ligament (ACL) hamstring grafts create mechanical hold by forcing the graft into frictional contact with the bone tunnel. We analyzed the resultant graft-tunnel contact pressure using an in vitro model of human cadaver 8mm hamstring grafts. METHODS Contact characteristics were assessed using both pressure sensitive films and a force sensor. Two screw sizes were investigated (8 and 9 mm in an 8mm Sawbone tunnel), both with and without a bone wedge between graft and screw. Separately, time dependent relaxation of contact force was recorded over a one hour epoch and associated tendon water loss was measured. Pullout testing of 8mm tendon grafts from 8mm holes in Sawbone and porcine femora were performed after 1 min and 1h. RESULTS During screw insertion, measured peak pressures (>40 MPa) exceeded the compressive failure stress of metaphyseal bone by more than an order of magnitude. Using a bone wedge between tendon and screw reduced local peak pressure by 85% but produced also inferior average contact pressure. In all approaches, initially achieved graft contact pressure rapidly decreased to approximately 25% within 30 min. Pullout strength was significantly reduced after 1h in comparison to 1 min in porcine bone as well as Sawbone. CONCLUSION Viscoelastic adaptation of the tendon is severe and critically reduces effective graft-bone contact pressure. Consideration of this newly recognized effect may open new and improved approaches for tendon graft fixation.