Background: An analysis of contemporary total knee arthroplasty
reveals that, on the average, patients rarely flex the knee beyond 120°.
The biomechanical mechanisms that inhibit further flexion after total knee
arthroplasty are unknown. The objective of the present study was to
investigate the capability of a single design of a fixed-bearing, high-flexion
posterior stabilized total knee arthroplasty system (LPS-Flex) to restore the
range of flexion to that of the intact knee.
Methods: Thirteen cadaveric human knees were tested, with use of a
robotic testing system, before and after total knee arthroplasty with the
LPS-Flex prosthesis. The passive path and the kinematics under an isolated
quadriceps force of 400 N, under an isolated hamstring force of 200 N, and
with these forces combined were determined. Posterior femoral translation of
the lateral and medial femoral condyles and tibial rotation were recorded from
0° to 150° of flexion.
Results: The medial and lateral condyles of the intact knee
translated posteriorly from full extension to 150°, reaching a mean peak
(and standard deviation) of 22.9 ± 11.3 mm and 31.9 ± 12.5 mm,
respectively, under the combined muscle forces. Following total knee
arthroplasty, the amount of posterior femoral translation was lower than that
observed in the intact knee. At 150°, approximately 90% of the intact
posterior femoral translation was recovered by the total knee replacement.
Internal tibial rotation was observed for all knees throughout the range of
motion. The camspine mechanism engaged at approximately 80° and disengaged
at 135°. Despite the absence of cam-spine engagement, further posterior
femoral translation occurred from 135° to 150°.
Conclusions: The tibiofemoral articular geometry of the intact knee
and the knee after total knee arthroplasty with use of the LPS-Flex design
demonstrated similar kinematics at high flexion angles. The cam-spine
mechanism enhanced posterior femoral translation only at the mid-range of
flexion. The femoral component geometry of the LPS-Flex total knee prosthesis
may improve posterior tibiofemoral articulation contact in high flexion
Clinical Relevance: Although sufficient posterior femoral
translation and tibial rotation were restored following total knee
arthroplasty with the LPS-Flex prosthesis, these conditions alone may not be
sufficient to fully produce the amount of knee flexion that is observed in the
intact knee. The clinical outcome after total knee arthroplasty may be
affected by other factors, such as preoperative range of motion, flexion space
balancing, posterior tibiofemoral articular contact stability, quadriceps
contraction, and patient motivation.