Difficulty bending the knee, or limited range of motion, is a common concern following anterior cruciate ligament (ACL) reconstruction. This post-surgical stiffness can range from a temporary, expected reaction to a more serious structural complication, medically termed arthrofibrosis. Understanding the difference between expected stiffness and a pathological physical block is important for a successful recovery. Recognizing the cause of the stiffness determines the appropriate steps for regaining full mobility.
Normal Post-Surgical Stiffness
The initial difficulty in bending the knee is a temporary state resulting from the body’s natural response to surgical trauma. Significant inflammation and swelling occur because the procedure involves cutting tissue and drilling tunnels into bone to place the new graft. This accumulation of fluid inside the joint cavity, known as effusion, physically limits the amount the knee can bend.
The body also employs a protective mechanism called muscle guarding, where the surrounding musculature, especially the hamstrings and quadriceps, involuntarily tightens. This muscle spasm is a subconscious reaction to pain, which further restricts movement. These factors combine to create a generalized stiffness that is expected to peak in the first few days and gradually subside as the acute inflammation resolves.
Post-operative immobilization protocols, such as wearing a brace, also contribute to temporary stiffness. Braces protect the newly reconstructed graft from excessive strain, but keeping the joint in a fixed position promotes tightness in the surrounding soft tissues. This combination of swelling, protective tightening, and immobility is part of the standard healing process. This expected stiffness is manageable with rest, elevation, ice, and pain medication, and resolves as the patient adheres to a progressive rehabilitation program.
Mechanical Roadblocks to Flexion
When stiffness persists and does not improve with physical therapy, or if there is a sensation of a physical obstruction, the issue may be structural. The most serious complication is arthrofibrosis, defined as the abnormal proliferation of fibrous scar tissue within the joint. This exaggerated inflammatory response leads to a progressive loss of motion, affecting both the ability to straighten and the ability to bend the knee.
This widespread scarring can adhere to the joint capsule and surrounding tissues, causing them to contract and significantly reduce the total arc of motion. Failure to achieve full knee extension early in recovery increases the risk of developing this condition. If true arthrofibrosis develops, conservative measures are often insufficient, and medical intervention, such as manipulation under anesthesia or an arthroscopic procedure, may be required to remove the scar tissue.
A more focal mechanical barrier is a Cyclops lesion, a localized form of anterior arthrofibrosis. This distinct fibroproliferative nodule forms in the front of the knee, often near the tibial tunnel of the new ACL graft. While Cyclops lesions are known for blocking the ability to fully straighten the knee, they can also cause overall loss of motion and a painful click or clunk. The nodule physically impinges between the femur and tibia as the joint moves, directly preventing full range of motion.
Another physical roadblock can be graft impingement, where the placement of the new ligament or surgical hardware interferes with the knee’s normal movement. If a Cyclops lesion or graft impingement is confirmed, physical therapy alone will not resolve the problem because the limitation is a physical blockage. These structural complications usually require a second, minimally invasive arthroscopic procedure to remove scar tissue or address the hardware to restore mechanical function.
Role of Physical Therapy and Range of Motion Milestones
Adherence to a structured physical therapy plan is the primary determinant in preventing long-term stiffness and achieving optimal outcomes. Early, controlled motion is important, as delaying mobilization or failing to perform prescribed exercises allows scar tissue to solidify, increasing the risk of developing arthrofibrosis. The goal is to regain motion without compromising the healing graft.
Surgeons and physical therapists set specific range of motion milestones that patients are expected to meet within certain timeframes. A common initial benchmark is achieving full knee extension and at least 90 degrees of knee flexion within the first one to two weeks following surgery. Failure to achieve full extension early in the process can have a negative long-term impact.
The progression continues with the goal of achieving full flexion, equal to the non-surgical knee, by approximately six to eight weeks post-operation. Exercises such as passive range of motion and heel slides are foundational for these early gains. Missing physical therapy sessions or neglecting the home exercise program allows soft tissues to tighten, making subsequent gains in flexion much harder to achieve.
When to Contact Your Surgical Team
While some stiffness is expected, certain signs indicate a potential complication requiring immediate medical consultation. A sudden, sharp increase in pain that does not improve with rest, ice, and medication is concerning. Persistent or worsening swelling beyond the first few weeks, or swelling accompanied by intense warmth or discoloration, should also be reported.
Signs of a possible infection include:
- A persistent fever, typically above 100.4°F or 101°F.
- Increased redness around the surgical site.
- Any unusual, foul-smelling discharge.
A deep vein thrombosis (DVT), or blood clot, is another serious concern. This is signaled by new or excessive swelling in the calf or thigh that does not subside with elevation, combined with pain, tenderness, or a warm sensation in the lower leg.
A consultation is also warranted if the knee’s range of motion plateaus despite consistent effort and adherence to the physical therapy program. If the knee is stuck at a certain angle and no progress is made for several weeks, this suggests a mechanical blockage, such as a Cyclops lesion or developing arthrofibrosis. Further diagnostic imaging may be required to address the underlying cause. Seeking timely advice for these symptoms is important for maintaining recovery momentum and preventing long-term deficits.