It may seem like the knee just bends and straightens, but there are actually six directions of movement in the knee. Each one is around a central point or axis. With six movements, the knee has motion around multiple axes. These include moving forward and back, side to side, and turning (rotating) in or out. Likewise, the muscles around the knee contract to create these six directions of movement.
Knowing about these motions is important when treating knee injuries. For example, it is well known that straightening or “extending” the knee is a problem after anterior cruciate ligament (ACL) injury. The muscle that controls knee extension (quadriceps) is weak after injury and takes a long time to recover. Much of rehabilitation is focused on this area.
Researchers wanted to know if other motions were also changed by ACL injury. How does the knee compensate for the loss of this ligament and the reduced muscle strength? Researchers looked at changes in knee motion at three different stages of injury or repair: (1) right after the injury, (2) when the knee injury was “chronic” and had not been repaired, and (3) after surgery to repair or reconstruct the ligament.
There were several interesting observations. First, the knee didn’t move in one direction without some movement or force in the other axes. This was also true of the muscles, so that one motion created a reaction in the other muscles around the knee. In the case of surgery, muscle strength gradually increased in all six directions from one month to one year after surgery. Knee extension was the slowest to recover.
In the case of a chronic ligament tear, the knee tried to take the load off the injured ligament. It also avoided knee positions that were unstable because of the damaged ligament. The knee does this by increasing the amount of outward rotation of the lower leg bone (tibia) as it joins the knee joint. This “unloads” the ligament and muscles. In the case of early injury, there hasn’t been enough time for the knee to make this change. After surgery, there is a small increase in outward rotation, but not as much as with long-term injury.
Knowing about these changes or adaptations helps with treatment. Added emphasis can be placed on strengthening the muscles that help protect and unload the ACL. These include the muscles around the knee, the muscles of the lower leg, and even the hip muscles. These exercises can be applied after surgery, too. In this way, the reconstructed ligament can be protected and the chances of reinjury reduced.