The knee joint is a complex hinge that allows for two primary movements: flexion and extension. Flexion is the act of bending the knee, bringing the heel closer to the buttock, with a normal range reaching approximately 135 to 140 degrees. Extension is the straightening of the leg, returning it to a neutral position. Limited range of motion (ROM) describes a condition where the knee cannot fully achieve either of these movements, making everyday activities like walking, sitting, or climbing stairs significantly more difficult. This restriction is a common issue stemming from acute traumatic incidents, long-term degenerative processes, or the body’s healing response following surgery.
Sudden Onset Causes: Injuries and Mechanical Blocks
Limited knee movement that appears suddenly usually results from an acute injury causing an immediate physical or mechanical obstruction within the joint. Ligament tears, such as those affecting the anterior cruciate ligament (ACL) or meniscal cartilage, can cause instability and significant pain, leading to muscle guarding that severely limits motion. A displaced fragment of a torn meniscus, sometimes called a bucket-handle tear, can physically wedge itself between the femur and tibia, preventing the knee from fully straightening or bending, a phenomenon often described as the knee “locking.”
Acute trauma may also create mechanical blocks from tissue fragments within the joint capsule. Small pieces of bone or cartilage, known as loose bodies, can break away and float into the joint space. These foreign objects interfere with the smooth gliding action of the joint surfaces, momentarily catching in the joint mechanism and causing sudden, sharp restriction of movement.
Significant fluid accumulation, known as effusion (fluid) or hemarthrosis (blood), is another common cause of sudden ROM loss following injury. This rapid buildup increases internal pressure within the joint capsule, which mechanically restricts the joint’s ability to move through its full arc. The resulting swelling and pressure trigger pain receptors, causing the surrounding muscles to involuntarily tense and guard the joint against further movement.
Gradual Stiffness from Chronic Joint Conditions
Many conditions lead to a progressive loss of knee motion through long-term structural degradation and persistent inflammation. Osteoarthritis (OA) is the most common degenerative cause, where the smooth articular cartilage covering the ends of the bones gradually wears away. This loss of cushioning increases friction and causes the bones to react by forming painful bone spurs, called osteophytes, around the joint margins.
These developing osteophytes physically impede the joint surfaces from moving fully past one another, causing a gradual reduction in both flexion and extension over many months or years. The chronic inflammation and joint space narrowing associated with OA further contribute to stiffness, making the knee feel tight and difficult to move, especially after periods of rest.
Inflammatory arthritis, such as Rheumatoid Arthritis (RA), presents a different mechanism rooted in autoimmune processes. The immune system mistakenly attacks the synovium, the membrane lining the joint, causing chronic inflammation known as synovitis. This persistent inflammation leads to a thickening of the joint lining and damage to the joint capsule and surrounding soft tissues. Over time, this chronic thickening results in a restricted, painful joint that progressively loses its capacity for full movement. Less commonly, inflammation in tissues surrounding the joint, such as chronic tendinopathy or bursitis, can also cause guarding and secondary limitation of motion due to the intense pain associated with movement.
Restricted Movement After Surgery
A distinct cause of limited ROM is the body’s overzealous healing response following knee surgery, such as an ACL reconstruction or a total knee replacement. This condition is medically termed arthrofibrosis, defined as the abnormal or excessive formation of dense scar tissue, or adhesions, within the joint. Instead of healing with pliable tissue, the body produces thick, restrictive bands of collagen that tether the internal structures and physically block the normal movement of the knee.
Arthrofibrosis prevents the joint from achieving full extension or flexion. This excessive scar tissue formation is often related to an exaggerated inflammatory response to the surgical trauma. Factors that can increase the risk of developing this stiff knee syndrome include post-operative complications like infection, or prolonged periods of immobilization immediately following the procedure.
Recognizing When to Seek Medical Attention
Recognizing when to consult a healthcare provider is paramount for safety and effective recovery. Any sudden and severe inability to move the knee, particularly if it feels mechanically locked and cannot be straightened, warrants immediate medical evaluation. This symptom often indicates a displaced meniscal tear or a significant loose body requiring prompt attention.
Other serious signs, or “red flags,” suggest the need for immediate care:
- Inability to bear any weight on the leg after an injury.
- Symptoms suggestive of infection, including severe warmth and redness around the joint, accompanied by a fever or chills.
For chronic stiffness, persistent pain that interferes with daily activities, or a progressive loss of motion that worsens rapidly despite rest should prompt a visit to a specialist. Accurate diagnosis of the underlying cause is the necessary first step before establishing a treatment plan.