Effects of cyclic loading on the tensile properties of human patellar tendon.

Bone-patellar tendon-bone (BPTB) graft is a popular choice for ACL reconstruction. These grafts are subjected to cyclic loading during the activities of daily living. Significant knee laxity is observed in reconstructed knee shortly after reconstruction. The source of this laxity is not clear. The change in the tensile properties of the graft due to cyclic loading can be one of the reasons for the change in knee laxity. Twenty patellar tendons from fresh frozen cadaver knees were cyclically loaded at a stress amplitude equivalent to 33% of the failure strength of the contralateral patellar tendon for 5000 cycles at 1.4Hz. They were then tested in tension to failure. Failure properties and the low load properties such as toe-region modulus were calculated. The results were compared with those of contralateral patellar tendons that were not subjected to cyclic loading before testing to failure. Fatigue loading did not alter the failure and low load properties with the exception of failure strain which decreased by about 10% (P<.05). Cyclically loaded patellar tendons with higher tissue mass density possess higher strength, modulus of elasticity, toughness, and transition stress (P<.05). The results indicate that there is no significant change in graft properties because of cyclic loading with the above load magnitude. The change in knee laxity observed after reconstruction, hence, is not because of change in graft properties due to moderate cyclic loading. Other factors, such as plastic deformation (yielding) of the graft, might play a role in increased knee laxity after reconstruction.