Failure strengths of concentric and eccentric implants for hamstring graft fixation

The purpose of this study is to compare the initial failure strengths of various interference screw devices used for tibial fixation of hamstring grafts in anterior cruciate ligament reconstruction and the effect of concentric or eccentric screw position. Quadrupled tendon grafts were harvested from freshly killed sheep. The grafts were then prepared and fixed in the distal femur using various devices (Intrafix (DePuy Mitek Raynham, MA, USA), RCI screw (Smith and Nephew Acufex, Mansfield, MA, USA), Wedge screw (Stryker Endoscopy, San Jose, CA, USA) in concentric position and Wedge screw in eccentric position with an interlock pin). The load required to cause mechanical failure of each construct was measured. The Intrafix device had a significantly greater mean strength to failure than all the other implants (mean 941 +/- 280 N) (P = 0.015 to P < 0.0001). The wedge screw inserted concentrically (737 +/- 134 N) had significantly greater initial failure strength than the wedge eccentric with interlock pin (458 +/- 266 N) (P = 0.03) and the RCI screw (464 +/- 107 N) (P = 0.00036). In this sheep model the Intrafix device with sheath inserted concentrically had significantly greater initial failure strength than the other interference screws. Concentric positioning of the wedge interference screw gave significantly greater initial failure strength of a quadruple tendon graft than eccentric positioning. The use of the Intrafix device or concentric positioning of an interference screw should result in increased initial fixation strength of hamstring grafts. This may allow more vigorous early rehabilitation and may result in less late graft laxity after anterior cruciate ligament reconstruction.