The relationship between anterior-posterior knee laxity and the structural properties of the patellar tendon graft. A study in canines.

We studied the relationship between anterior-posterior knee laxity and the structural properties of autogenous patellar tendon grafts used to replace the anterior cruciate ligaments in dogs 1 year after reconstruction. At 30 degrees (full extension for the dog), 60 degrees, and 90 degrees of flexion a significant inverse correlation was found between anterior-posterior knee laxity and the ultimate failure strength of the graft. Likewise, at 60 degrees and 90 degrees of flexion a significant inverse correlation was found between knee laxity and the linear stiffness of the graft. In all cases, as knee laxity increased, the ultimate failure load and linear stiffness values of the graft were found to decrease. There was also a significant increase in anterior-posterior knee laxity of the reconstructed knees (produced by an increase in anterior translation of the tibia relative to the femur) compared with the contralateral control knee. The group of dogs that underwent a sham operation without reconstruction of the anterior cruciate ligament had no change in anterior-posterior knee laxity or the structural properties of the anterior cruciate ligament. The clinical significance of this investigation is that knee laxity measurements demonstrating an increase in anterior translation of the tibia relative to the femur during graft healing may indicate that the graft has weakened and reduced structural properties.