A locked knee occurs when the joint suddenly refuses to fully straighten or bend, causing an immediate loss of mobility. This symptom ranges from a brief, painful catching sensation to a complete inability to move the limb. Medical professionals categorize the cause into True Mechanical Locking (a physical blockage) and Pseudo-Locking (a protective response to pain or inflammation).
Causes of True Mechanical Locking
True mechanical locking occurs when a physical object or tissue fragment becomes wedged between the femur and tibia. This obstruction creates a hard stop, preventing the knee from achieving its full range of motion. The blockage is typically a piece of damaged cartilage or bone displaced from its normal position inside the joint space.
The most frequent culprit is a specific type of meniscal tear. The meniscus is crescent-shaped cartilage that acts as a shock absorber. A “bucket-handle tear” involves a long, vertical flap of the meniscus ripping away but remaining attached. This displaced flap flips into the center of the joint, physically jamming the hinge mechanism and preventing full leg extension.
Loose bodies are another common cause of physical obstruction. These small fragments of bone or cartilage break off due to trauma, degenerative disease, or osteochondritis dissecans. They float freely in the synovial fluid until they migrate and get caught between the moving surfaces of the joint bones. Their intermittent lodging causes repeated episodes of painful catching or locking.
Causes of Pseudo-Locking
Pseudo-locking presents the feeling of being stuck but lacks a physical obstruction. This sensation is a reflex response triggered by severe pain, swelling, or muscle spasm, where the body locks the joint to prevent painful movement. The knee joint remains capable of full motion, but surrounding muscles involuntarily guard the area, mimicking a mechanical jam.
Significant swelling, known as joint effusion, is a common cause of pseudo-locking. When acute inflammatory arthritis or traumatic injury causes a buildup of fluid inside the joint capsule, the pressure and resulting pain reflexively limit movement. The joint becomes too full and painful to fully articulate, causing the sensation of being stuck.
Patellofemoral issues, such as patellar maltracking, can also produce a sensation of catching or locking. Patellar maltracking occurs when the kneecap glides incorrectly in the groove on the thigh bone. This abnormal movement or a momentary subluxation, where the kneecap partially shifts out of place, can feel like the joint is momentarily jammed.
Severe localized pain from conditions like tendonitis or bursitis can initiate protective muscle guarding. The intense pain causes surrounding muscles, particularly the quadriceps, to spasm. This involuntary tightening prevents the knee from moving through its full range, leading to the temporary sensation of being locked.
When to Seek Medical Attention
Managing a locked knee initially involves gentle techniques, but several symptoms require immediate medical attention. If the knee is completely locked and cannot be straightened or bent, or if the locking is accompanied by severe pain, an emergency evaluation is necessary. The inability to bear weight on the affected leg or the presence of numbness or tingling in the foot are also signs requiring urgent care.
For initial relief while awaiting medical evaluation, the RICE protocol (Rest, Ice, Compression, and Elevation) helps manage swelling and pain. Applying ice for 15 to 20 minutes reduces inflammation, which may alleviate muscle spasm associated with pseudo-locking. If the knee remains rigidly fixed after rest and ice, a physical obstruction is probable and requires prompt assessment by a physician.
A medical evaluation begins with a thorough physical exam, often testing the knee’s passive range of motion to determine if the lock is mechanical or pseudo. Diagnostic imaging confirms the underlying cause, starting with X-rays to check for bone fragments or calcified loose bodies. If soft tissue damage is suspected, Magnetic Resonance Imaging (MRI) visualizes the menisci, ligaments, and cartilage, distinguishing between true mechanical blocks and other causes.