Background: Double-bundle reconstruction of the posterior cruciate
ligament has been advocated to better replicate the anatomy of the native
ligament and restore normal knee biomechanics. The goal of this study was to
measure knee laxities and graft forces following single and double-bundle
reconstructions and to compare these values with those for the intact knee in
a cadaver model.
Methods: Forces in the posterior cruciate ligament were measured as
the knee was passively extended from 120° to 0° with applied tibial
loading. Anterior-posterior laxities were measured as well. An anterolateral
tunnel was located at the anterolateral margin of the native ligament
footprint, and a posteromedial tunnel was placed at one of two locations
within the footprint; one location resulted in a wide bridge separating the
tunnels and the other, a narrow bridge. Testing was repeated with a single
anterolateral graft tensioned to match, within ±1 mm, the laxity in the
intact knee at 90° of flexion. Double-bundle reconstructions were tested
with the addition of a posteromedial graft tensioned at 30° of flexion.
Two levels of posteromedial graft tension (10 and 30 N) were studied in both
the narrow and the wide-bridge posteromedial tunnels.
Results: Mean laxities with a single anterolateral graft were 1.1 to
2.0 mm greater than normal between 0° and 30° of flexion. With the
posteromedial graft tensioned to 10 N in the wide-bridge tunnel, the mean
laxity of the double grafts was not significantly different from that in the
intact knee at any flexion angle. With the posteromedial graft tensioned to 10
N in the narrow-bridge tunnel, the mean laxity at 0° was 0.9 mm greater
than that in the intact knee. With the posteromedial graft tensioned to 30 N,
the mean laxity at 10° was 1.7 mm less than the intact-knee value in the
wide-bridge tunnel and 1.3 mm less than the intact-knee value in the narrow
bridge-tunnel. Increasing posteromedial graft tension from 10 to 30 N
decreased the mean laxities by 0.5 to 1.1 mm between 0° and 30°. Mean
graft forces following a single anterolateral reconstruction were not
significantly different from the native posterior cruciate ligament forces
under any mode of loading except valgus moment. With the wide-bridge tunnel,
the mean forces with the posteromedial graft tensioned to 10 N were somewhat
higher than the native posterior cruciate ligament forces at full extension;
when the graft was tensioned to 30 N, the mean forces were substantially
higher.
Conclusions: A single anterolateral graft best reproduced the normal
posterior cruciate ligament force profiles, but laxities were greater than
normal between 0° and 30° of knee flexion. The addition of a second,
posteromedial graft reduced laxity in this flexion range but did so at the
expense of higher-than-normal forces in the posteromedial graft.
Clinical Relevance: The results of this study suggest that it is
reasonable to question the rationale for adding a second, posteromedial
graft.