The inability of the knee to bend fully or at all can be a sudden and alarming symptom, severely impacting daily function. This limitation signals a disruption in the smooth mechanics of the joint, which is designed for a large range of motion. The problem generally presents in two distinct ways: a true mechanical blockage where a physical object prevents movement, or a restriction where pain, swelling, or stiffness inhibits the full range of motion. Understanding this difference is the first step toward identifying the underlying condition.
Understanding True Knee Locking Versus Restricted Movement
A true locked knee is characterized by the physical inability to fully straighten the leg, often stuck at a fixed angle of flexion (e.g., 45 degrees). This is a hard stop where the joint cannot move past a certain point, even with external force. The sensation is often described as a foreign body jamming the hinge mechanism.
In contrast, restricted movement, sometimes called pseudo locking, is the inability to move the knee due to severe pain, muscle spasm, or significant swelling. While the knee is painfully inhibited, there is no physical debris preventing movement within the joint capsule. Swelling (joint effusion) restricts motion because increased pressure within the joint cavity physically limits the space for the bones to articulate. This pseudo lock may sometimes be overcome with rest, pain relief, or gentle manipulation, unlike a true mechanical lock.
The knee joint normally allows for flexion up to about 135 to 155 degrees and full extension to zero degrees. This movement depends on the femoral and tibial bones gliding smoothly, cushioned by cartilage. When a physical fragment interferes with this glide, it creates a mechanical block, the hallmark of true locking.
Causes of Mechanical Blockage
True mechanical locking occurs when a physical structure within the joint gets trapped between the femur (thigh bone) and the tibia (shin bone). The most frequent cause is a tear in the menisci, the two C-shaped wedges of fibrocartilage that act as shock absorbers. A specific type, known as a bucket-handle tear, is particularly prone to causing locking because the torn piece flips into the joint space.
This displaced fragment physically obstructs movement, preventing the final degrees of extension necessary to straighten the leg. The locking sensation is often sudden and may be accompanied by a popping sound. Such tears frequently result from forceful twisting or rotational trauma while the foot is planted.
Another common source of mechanical blockage is the presence of loose bodies within the joint. These are fragments of bone or cartilage that have broken away and are floating freely in the synovial fluid. Resulting from trauma or degenerative conditions, these fragments may intermittently lodge in the joint space, causing a locking sensation that comes and goes.
If the patella (kneecap) partially or completely slides out of its groove on the femur (patellar subluxation or dislocation), it can also cause the knee to lock. The patella’s displacement disrupts the extensor mechanism of the leg, physically preventing movement. This instability often presents with a feeling that the knee is giving way, followed by a hard block to motion until the kneecap is relocated.
Conditions That Limit Range of Motion
Conditions that restrict the knee’s range of motion without a physical block are characterized by inflammation, pain, and stiffness. Osteoarthritis (OA) is a primary cause, where the protective cartilage on the ends of the bones wears down over time. This degeneration leads to bone-on-bone friction and the formation of osteophytes (bone spurs), causing pain and stiffness that severely limit the ability to bend the knee.
Inflammatory conditions, such as Rheumatoid Arthritis (RA), lead to chronic inflammation of the synovium (the joint lining), causing pain and swelling that restrict movement. The immune system mistakenly attacks the joint tissues, leading to joint destruction and chronic stiffness. The resulting joint effusion contributes to the sensation of pseudo locking by creating internal pressure.
Bursitis, the inflammation of the small, fluid-filled sacs called bursae around the knee, can also limit motion. For example, prepatellar bursitis causes swelling over the kneecap that makes full flexion painful and difficult. Similarly, inflammation of tendons, such as patellar tendonitis, produces pain that causes a protective muscle spasm to prevent further injury.
Acute injuries, such as a severe ligament tear like an Anterior Cruciate Ligament (ACL) rupture, often cause massive, rapid joint swelling. The resulting joint effusion and intense pain trigger a reflexive muscle guarding, which prevents the patient from moving the knee through its full range. Although the injury itself is structural, the immediate limitation of motion is due to the pain and swelling rather than a physical piece of tissue trapped in the joint.
Immediate Action and Medical Assessment
If your knee suddenly locks or refuses to bend, initial management should focus on resting the joint and reducing inflammation. The R.I.C.E. protocol—Rest, Ice, Compression, and Elevation—is the standard immediate care. Applying ice for 15 to 20 minutes several times a day can help reduce swelling and pain, potentially easing a pseudo lock.
However, you should seek immediate medical attention if you experience severe, unrelenting pain, a visible deformity, or a complete inability to bear weight. A true mechanical lock that does not resolve quickly is also a medical priority because prolonged blockage can sometimes lead to further damage to the joint cartilage.
During a medical assessment, a physician will perform a physical examination to test the stability and range of motion. To confirm the underlying cause, imaging tests are typically ordered. An X-ray can reveal bone spurs or loose bone fragments, while Magnetic Resonance Imaging (MRI) is the preferred method for visualizing soft tissues (menisci, ligaments, and cartilage).
The diagnosis will determine the necessary treatment, which may range from conservative measures like physical therapy and anti-inflammatory medications for restricted movement, to surgical intervention for a true mechanical lock. For a true lock caused by a displaced bucket-handle meniscal tear, arthroscopic surgery may be required to repair or remove the fragment to restore full range of motion.