Aims: The aims of this study were to evaluate the biomechanical properties and mode of failure of four methods of fixation of hamstring anterior cruciate ligament (ACL) grafts. The fixation methods investigated included titanium round headed cannulated interference (RCI) screws, bioabsorbable RCI screws, Endobuttons and Bollard fixation. A 2-strand equine extensor tendon graft model was used because a previous study has shown it to have equivalent biomechanical properties to that of 4-strand human semitendinosus and gracilis tendon grafts.

Method: Thirty-two stifle joints were obtained from skeletally mature pigs, the soft tissues were removed and the ACL and PCL were sacrificed. Tibial tunnel preparation was standardised using the Mayday rhino horn jig to accurately position a guide wire over which an 8mm tunnel was drilled. A 2-strand equine tendon graft was then introduced into the tibial tunnel and secured with either a titanium RCI screw, a bioabsorbable RCI screw, an Endobutton or an expansile Bollard. The proximal part of the graft was attached to the crosshead of a materials testing machine using the Soffix. Five of each method of fixation were tested mechanically to ultimate failure and under cyclical loading.

Results: The mean ultimate tensile loads (UTL) were: titanium RCI screw = 444 N, bioabsorbable RCI screw = 668 N, Endobutton = 999 N and Bollard = 1153 N. The mode of failure for all RCI screws involved progressive tendon slippage past the screw. Under cyclic loading conditions the titanium and bioabsorbable RCI screws rapidly failed after several hundred 5 to 150 N cycles due to tendon damage and slippage. Both the Bollards and Endobuttons survived 1500 cycles at 50–450N, with less tendon slippage.

Conclusion: Titanium and bioabsorbale RCI screws provide poor initial fixation of tendon grafts used for ACL reconstruction and fail rapidly under cyclic loading. Both Bollards and Endobuttons provide sufficiently high UTL’s and survive cyclic loading to allow early postoperative mobilisation and rehabilitation. Caution must be used in the early postoperative period when using interference screws to secure a hamstring tendon graft because early progressive tendon slippage may result in excessive graft elongation and early clinical failure.