Studies have shown that muscle strength and function don’t return fully in the leg after a total knee replacement (TKR). Motion analysis shows weakness and compensation patterns persist in the involved leg. These differences are seen as much as one year after the surgery.
In this study, physical therapists from the University of Delaware use a specific test to measure strength, function, and movement patterns one year after TKR. Patients included in the study had the same kind of TKR procedure and implant. The incision used extended into the quadriceps femoris tendon.
The TKR patients were tested using a standard test called sit-to-stand. Using a backless, armless chair, everyone stood up from a sitting position. The arms were folded across the chest or held in the lap. A second (control) group without TKR was also tested in the same way.
A 3-D, 6-camera motion analysis system was used to capture and analyze the motion. Two forceplates were placed on the floor (one under each foot). The forceplates measured ground reaction forces under each leg during the sit-to-stand task. The time to complete the task was recorded.
Several other tests were also performed. Electromyographic (EMG) activity of the quadriceps muscle was used to measure strength. The Timed-Up-And-Go (TUG) test, the Six-Minute-Walk-Test (6MWT), and the Stair-Climbing-Test (SCT) were also given. These three tests measure functional strength of the quadriceps femoris muscle.
All tests were given three months and one year after the procedure. Joint angles, forces, and amount of muscle activity were compared from side to side and with the control group. The authors were expecting the TKR group to have differences in results (compared to the control group) at the 3-month follow-up check. They expected to see similar results by the end of the one-year time period.
What they found was that the TKR group continued using compensatory movement patterns for the sit-to-stand test. Instead of using the hip flexor muscles, they were still using hip extensor muscles. In fact, they were using the hip extensor muscles even more at the end of 12 months compared to the three-month measurements.
This altered movement strategy helps reduce the demand on the knee extensor muscles (quadriceps) and unloads the involved leg. The authors say that altered movement strategies at three months are to be expected. But by the end of one year, strength is equal from side to side. So this type of compensation movement pattern is no longer needed.
Physical therapists need to be aware of abnormal movement patterns after TKR. Training to restore normal movements is important. Equal weight-bearing, strength, and forces on the joint may help avoid uneven load. The goal is to prevent movement patterns that can lead to future arthritic changes in the joint.