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Knee scar tissue develops as a result of traumatic injury to the knee or as a complication of knee surgery. It might form after procedures such as anterior cruciate ligament (ACL) surgery or total knee replacement. Like other types of scar tissue, scar tissue in the knee is a bundling of collagen fibers that develops during wound repair. The formation of scar tissue is often problematic because of its association with knee stiffness. During the healing process, some individuals develop a condition known as arthrofibrosis, in which excessive internal scar tissue is formed, causing the ligaments surrounding the joint cavity to tighten and restricting or fully preventing the bending of the knee.
The formation of excess knee scar tissue depends on the severity of the trauma and on genetic factors. In arthrofibrosis, open spaces within the joint are filled with stiff fibrous tissue, and the patient experiences a lack of motion, or a "frozen" joint. The lack of mobility in the knee further exacerbates the problem and results in the formation of stiffer scar tissue than would otherwise be present, restricting motion even more.
In cases where excessive joint stiffness is observed because of knee scar tissue, further surgical intervention might be required to return the knee to an acceptable range of motion. Six weeks after a total knee replacement, the patient is expected to be able to bend the knee at least past 90 degrees. If this is not the case, and if physical therapy has failed to produce the desired results, the patient might be returned to the hospital to undergo a process in which the knee is forcibly bent in order to break the scar tissue and restore motion. This procedure typically is conducted under general anesthesia and is known as manipulation under anesthesia (MUA). After the scar tissue has been broken through MUA, the patient must undergo more physical therapy.
While this intervention is sometimes necessary, orthopedic surgeons and practitioners of sports medicine usually recommend pursuing a conservative course of treatment when possible. The use of a continuous passive motion (CPM) machine is prescribed following knee surgery to prevent the formation of stiffness and scar tissue in the knee. The CPM works by gently bending the knee in a series of repetitions, extending the knee's range of motion without the need for physical exertion that could damage the knee. Physical therapy and deep tissue massage are also popular therapeutic methods used to improve flexibility and break down knee scar tissue.