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 angles.
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.
1 Bioengineering Laboratory, Massachusetts General Hospital, 55 Fruit Street, GRJ 1215, Boston, MA 02114. E-mail address for G. Li: firstname.lastname@example.org