Comparative pull-out and cyclic-loading strength tests of anchorage of hamstring tendon grafts in anterior cruciate ligament reconstruction

This study examined four devices for anchorage of hamstring tendons used as anterior cruciate ligament grafts: a stirrup, a clawed washer and screw, and "soft" and round-headed interference screws. Ultimate strength tests were performed using bovine tendons and bones. The stirrup was significantly stronger than the other anchorage devices, failing at 898 N. The clawed washer failed at 502 N, the soft screw at 691 N, and the round-headed screw at 445 N. Cyclic loading to 150 N (to simulate walking) caused elongation of 2.1 mm with the stirrup by 1100 cycles, and 6.7 mm with the clawed washer by 300 cycles. Different hole and soft screw diameters and placements (inside-out versus outside-in) allowed 1- to 3-mm slippage (no significant differences) by 1100 cycles. The round-headed screw allowed 6.3-mm slippage by 1100 cycles, and a sharp edge below the screw head caused tendon damage. Cyclic loads to 450 N (to simulate jogging) were then imposed until failure, and all specimens failed rapidly; only stirrup fixation kept all specimens intact after 300 load cycles. We concluded that anterior cruciate ligament reconstructions using hamstring tendons wilt slacken if rehabilitation is too aggressive, so forces on the reconstructed ligament should be minimized until tendon-to-bone healing occurs.