Quadriceps function after TKA--an in vitro study in a knee kinematic simulator.

OBJECTIVE: The purpose of this in vitro study was to investigate the amount of quadriceps force required to extend the knee during an isokinetic extension cycle before and after total knee arthroplasty.
DESIGN: The magnitude of the extension torque used in simulating a constant moment extension exercise was derived from tests made using a dynamometer on patients who had received a total knee arthroplasty. Quadriceps force needed to extend the knee was measured.
METHODS: Human knee specimens (n=8, mean age=65 SD 7 years, all male) were tested in a kinematic knee simulating machine untreated and after implantation of two different knee prosthesis. During simulation, a hydraulic cylinder applied sufficient force to the quadriceps tendon to produce an extension moment of 31 Nm about the knee.
RESULTS: The quadriceps forces needed to extend the knee during the physiologic simulation reached levels as high as 1238.9 N (SD 46.6). After implantation the Interax-prosthesis, quadriceps force increased up to 13.9% (P=0.003), in conjunction with resurfacing of the patella 11.9% (P=0.01). With the LCS-prosthesis implanted, quadriceps force showed a somewhat smaller increase of up to 6.6% (P=0.007). RELEVANCE: The following study helps to clarify postoperative problems related to the extensor mechanisms of the knee after implantation of total knee arthroplasty.