Biomechanical properties of patellar and hamstring graft fixation for anterior cruciate ligament reconstruction using an interference screw. Experimental study with roentgen stereometric analysis (RSA)

This experimental roentgen stereometric analysis (RSA) study was performed to measure micromotions between the graft and tunnel under submaximal load in anterior cruciate ligament (ACL) reconstruction. The aim was to evaluate the maximum load at failure, linear stiffness, and slippage of bone-patell ar-tendon-bone (BPTB) grafts fixed with interference screws compared to qua drupled hamstring grafts fixed with interference screws. We used 15 porcine tibia specimens for the study. In the BPTB group, the 10 X25-mm bone plugs were fixed in a tunnel with 10 mm in diameter using a 7X2 5-mm titanium interference screw (n=5) or a biodegradable screw (n=5). The five hamstring transplants were folded to a four-stranded graft and armed w ith a baseball stitch suture. The sutured side was fixed with a 7X25-mm bio degradable polylactide screw in an 8-mm tunnel. The tibial bones, tendon gr afts,and interference screws were marked with tantalum beads. The grafts we re mounted to a custom made load frame and loaded parallel to the axis unde r RSA control increasing the force in steps of 50 N. Micromotions between b one plug, screw, and tibia were measured with RSA. Accuracy of RSA for the in vitro study was evaluated as 0.05 mm. Hamstring grafts failed at significantly lower maximum loads (492 +/- 30 N) than BPTB grafts (658 +/- 98 N). Linear stiffness of the hamstring graft fixations w as eight times lower compared to the BPTB grafts (403 +/- 141 N/mm vs 3500 +/- 1300 N/mm). There was no significant difference between the biodegradab le and titanium screws in the BPTB group. Slippage of the graft started at 82 +/- 35 N load in the hamstring group and at 428 +/- 135 N in the BPTB gr oup. Slippage of the graft at 75% of the maximum pull-out strength was meas ured as 0.36 +/-0.25 mm in the BPTB and 2.58 +/-1.08 mm in the hamstring gr oup. The interference screw fixation of a quadrupled hamstring graft showed a lo wer linear stiffness and an earlier slippage compared to a patellar tendon bone plug. Slippage of the hamstring grafts at submaximal loads may result in fixation failure during rehabilitation.