A sudden or progressive inability to bend the knee, known as limited flexion, is a common symptom that significantly impacts daily life. This restricted movement ranges from a mild sensation of stiffness to a complete, painful inability to move the joint. The knee requires a substantial range of motion to perform everyday actions like sitting, climbing stairs, or squatting. Understanding the difference between a physical obstruction and stiffness caused by inflammation or scar tissue is the first step toward effective treatment. Limited flexion is not a diagnosis itself, but rather a sign that something is disrupting the complex mechanics of the joint.
Immediate Mechanical Blocks
A physical blockage inside the joint, often referred to as a “true lock,” causes a sudden, acute inability to bend the knee. This mechanical obstruction occurs when a piece of tissue or bone gets wedged between the joint surfaces, preventing the normal rolling and gliding motion. The most frequent culprit is a type of meniscal tear known as a “bucket-handle tear.” In this injury, a large fragment of the C-shaped cartilage breaks free and flips into the center of the joint, physically jamming the knee.
Another cause of mechanical blocking is the presence of loose bodies within the joint space. These fragments of bone or cartilage, sometimes called “joint mice,” break away due to trauma or degenerative conditions like osteoarthritis. If a loose body migrates into the space between the femur and tibia, it can suddenly obstruct movement, leading to a sharp, intermittent locking sensation. Unlike a consistent stiffness, this locking is often unpredictable and may resolve on its own as the fragment shifts position.
Severe ligament injury, particularly a major tear of the anterior cruciate ligament (ACL), can also lead to limited motion. In the immediate aftermath of an acute injury, the body’s protective response is to generate massive swelling, or effusion, within the joint capsule. This rapid fluid buildup physically restricts the available space, making it difficult and painful to move the knee, a phenomenon sometimes called “pseudo-locking” or muscle guarding. The surrounding muscles, such as the quadriceps, automatically tighten to immobilize the unstable joint, further limiting the ability to bend the knee fully.
Stiffness Caused by Inflammation and Disease
Limited knee bending can also result from systemic or localized conditions where inflammation and pain are the primary limiting factors, rather than a physical barrier. Osteoarthritis, the most common form of arthritis, involves the gradual wearing away of the protective cartilage on the ends of bones. This leads to joint space narrowing and the formation of bone spurs, which cause pain and stiffness that worsen over time. The joint’s inflammatory response also produces excess fluid, which creates a tense feeling and limits movement.
In contrast, Rheumatoid Arthritis is an autoimmune disease where the body’s immune system mistakenly attacks the synovium, the lining of the joint capsule. This systemic inflammation causes the synovium to become thick and swollen, leading to persistent joint tenderness, warmth, and reduced range of motion in multiple joints. Similarly, a flare-up of gout, caused by a buildup of uric acid crystals, can cause a sudden onset of excruciating pain, redness, and swelling. The intense inflammation makes any attempt to bend the knee nearly impossible due to the pain and acute fluid accumulation.
Bursitis, the inflammation of the small fluid-filled sacs that cushion the joint, can also contribute to limited motion. The bursae, such as the prepatellar bursa located in front of the kneecap, can become inflamed from overuse, prolonged kneeling, or infection. When a bursa swells with fluid, it creates a localized bulge of tenderness and pain that mechanically impedes the smooth movement of the joint. These inflammatory conditions are characterized by a stiffness that feels more global and painful, rather than the sharp, sudden stop of a mechanical lock.
Chronic Stiffness Following Injury or Surgery
A persistent loss of knee flexion that develops slowly after a traumatic event or operation is often the result of an overzealous healing response. This chronic stiffness is most commonly attributed to arthrofibrosis, sometimes called stiff knee syndrome. Arthrofibrosis involves the excessive formation of dense, fibrous scar tissue within the knee joint and surrounding soft tissues. In this condition, specialized cells remain overly active, laying down excessive amounts of collagen.
This collagen forms thick, rope-like adhesions that tether the joint capsule and the structures around the kneecap. This pathological scarring physically restricts the joint’s ability to glide and flex, creating an unrelenting stiffness. If the soft tissues surrounding the joint remain shortened and inflexible, the joint develops a contracture. A flexion contracture specifically refers to the inability to fully straighten the knee, but the overall tethering also restricts bending.
The stiffness from arthrofibrosis represents a failure of the body’s healing process to remodel scar tissue effectively. This issue is particularly noted following procedures like ACL reconstruction or total knee replacement. Early and aggressive physical therapy is intended to prevent this outcome by mechanically breaking up the forming scar tissue and maintaining range of motion. Without proper mobilization, the dense scar tissue can severely and permanently compromise the knee’s function.
Diagnosis and Treatment Options
Determining the precise cause of limited knee bending requires a comprehensive evaluation, beginning with a detailed physical examination. A physician will assess the knee for signs of swelling, warmth, and tenderness, and manually test the range of motion. Specific maneuvers, such as the McMurray test for meniscal tears, can help pinpoint the exact location of the internal problem. The provider will also look for “red flags” that signal an immediate medical emergency.
Imaging studies are routinely used to confirm the diagnosis and visualize the underlying structures. Standard X-rays identify bony issues, such as calcified loose bodies, severe joint space narrowing from advanced osteoarthritis, or signs of fracture. A Magnetic Resonance Imaging (MRI) scan provides detailed images of soft tissues, including the menisci, ligaments, and cartilage, making it the preferred tool for diagnosing tears and non-bony loose bodies. In cases of acute swelling, a joint aspiration may be performed to draw fluid for analysis, differentiating between simple inflammation, infection, or gout.
Treatment is highly dependent on the diagnosis, ranging from conservative management to surgical intervention. For acute injuries and inflammatory swelling, the R.I.C.E. protocol—Rest, Ice, Compression, and Elevation—is the initial approach to reduce pain and inflammation. Chronic stiffness due to contractures or mild arthritis is often managed with a structured physical therapy program focused on progressive stretching and strengthening. If a true mechanical block is present, such as a meniscal tear or symptomatic loose bodies, arthroscopic surgery is often required. This minimally invasive procedure allows a surgeon to remove the obstructing tissue or scar tissue to restore the knee’s full bending capacity.
Any sudden, severe pain accompanied by a fever, an open wound, or the inability to bear any weight are signs that warrant an immediate visit to an emergency room.