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 tibial tunnel or a
tibial inlay technique.
Methods:
The proximal ends of sixty-two posterior cruciate graft replacements,
thirty-one fixed with the tibial 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 45° 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 lateral 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.