The effect of tubularization on the mechanical properties of patellar tendon grafts

To determine the effect of tubularization on the prefailure mechanical properties of bone-patellar tendon-bone autografts used for anterior cruciate ligament repair, 10 bovine bone-patellar tendon-bone grafts were tested in tension before and after tubularization with running suture. The testing protocol involved a 5-N preload, 10 preconditioning cycles to 200 N, and a final test cycle to 950 N at 1000 N/sec. Five of the grafts were tested first as harvested (flat) and then again following tubularization. The remaining five grafts were tubularized prior to the initial testing, and final testing was done with the suture removed. Raw testing data were reduced to determine the amount of stretching associated with preconditioning, as well as laxity and stiffness of the preconditioned grafts. Tubularized grafts stretched significantly more than flat grafts during preconditioning: 3.5 times as much after the first preconditioning cycle (3.8+/-1.9 mm versus 1.1+/-0.78 mm) and 3.1 times as much after 10 cycles (5.0+/-2.1 mm versus 1.6+/-0.9 mm). There was no statistically significant difference in the stiffnesses of the tubularized and flat grafts, nor did tubularization have an effect on graft laxity. Interestingly, there was a slight increase in laxity the second time each graft was tested, regardless of whether the graft was flat or tubularized when it was first tested. These results highlight the importance of preconditioning patellar tendon grafts before fixation, especially those that have been tubularized.