Cyclic loading of posterior cruciate ligament replacements fixed with tibial tunnel and tibial inlay methods

Background: The optimal method of replacement of the posterior cruciate ligament with a bone-patellar tendon-bone graft is not known. The purpose of this study was to compare the mechanical responses to cyclic loading tests of bone-patellar tendon-bone allograft replacements fixed to the tibia with one of two methods: a tibia] tunnel or a tibia] inlay technique. Methods: The proximal ends of sixty--two posterior cruciate graft replacements, thirty-one fixed with the tibia[ tunnel technique and thirty-one fixed with the tibial inlay technique in cadaver knees, were subjected to 2000 cycles of tensile force of 50 to 300 N with the angle of pull at 450 to the tibial plateau. The central 10 mm of the medial and lateral halves of previously fresh-frozen bone-patellar tendon-bone preparations from cadaver knees were used as the grafts. Two pairs of tibiae were used for testing; the two types of fixation and the medial and Jateral halves of the patellar tendons were distributed between the tibial pairs. Graft thickness was measured at the point of highest anticipated tissue deformation and at two additional locations at distances from these points. The total change in graft length after cyclic loading at an applied force level of 200 N was recorded. Elongation of the graft during loading cycles between 20 and 200 N of applied tensile force was also measured. A repeated-measures analysis of variance was used to compare all measurements between the inlay and tunnel techniques, and between the medial and lateral halves of the graft used for the inlay method. Results: Ten of the thirty-one grafts that had been passed through a tibial tunnel failed at the acute angle before 2000 cycles of testing could be completed; all thirty-one grafts that had been fixed to the tibia with use of the, inlay method survived-the testing intact. Evaluation of the twenty-one graft pairs that survived testing after both fixation techniques revealed that the grafts that had been fixed with the inlay method had significantly less thinning at all three measurement sites at the completion of testing; the mean reduction of thickness was 40.6% (at the acute angle) in the grafts fixed with the tunnel method and 12.5% (adjacent to the bone block) in those fixed with the inlay method. After 2000 cycles, the mean lengths of the grafts fixed with the inlay and tunnel methods increased 5-9 and 9.8 mm, respectively; 38% of this increase occurred during the first six loading cycles. After both methods of fixation, the mean graft elongation during a loading cycle decreased approximately 50% from cycle 1 to cycle 2000, resulting in an effectively stiffer graft construct. There was no significant difference in any measured parameter between medial and lateral graft halves. Conclusions: These tests showed that the inlay technique of posterior cruciate ligament replacement was superior to the tunnel technique with respect to graft failure, graft thinning, and permanent increase in graft length. Clinical Relevance: Grafts replacing the posterior cruciate ligament are subjected to repetitive mechanical loading, and our results demonstrated that, with either the tunnel or the inlay fixation technique, the graft undergoes thinning and permanent length changes-at the load levels used in.. these tests. These permanent length changes could be reduced substantially if the graft were cyclically preconditioned in situ before final pretensioning and fixation. The marked thinning of graft tissue at the acute angle and the permanent length changes of the tunnel grafts that did not fail may explain the increased posterior laxity observed in many patients who have undergone posterior cruciate replacement with use of the tunnel technique. The inlay technique of fixation significantly reduced these degradative effects. Regardless of the type of fixation to the tibia, there appears to be no advantage to using either the medial or the lateral half of a bone-patellar tendon-bone allograft. preparation.