When the knee joint suddenly becomes unable to move past a certain point, the person is experiencing what is commonly known as knee locking. This sensation is a temporary, sometimes painful, inability to fully bend or straighten the leg. While the experience can be alarming, it serves as a distinct symptom that an underlying mechanical issue is interfering with the joint’s normal function. Identifying the specific cause is necessary because the treatment approach varies significantly depending on the nature of the blockage.
True Locking Versus Pseudolocking
Understanding the difference between a true mechanical lock and a pseudolock is the first step in determining the severity of the problem. True mechanical locking occurs when a physical obstruction within the joint physically prevents the knee’s movement. The knee gets completely stuck at a fixed angle, and no amount of effort will allow it to move past that point. This is often likened to a pebble jamming the hinge of a door.
Pseudolocking, on the other hand, creates the sensation of being stuck, but it lacks a physical obstruction. This feeling is typically a protective response where severe pain, inflammation, or muscle spasms prevent movement. Conditions like patellofemoral pain syndrome, severe arthritis, or swelling can trigger this automatic defense mechanism. The joint is still physically capable of movement, and gentle manipulation by a healthcare provider may reveal that the knee can move despite the discomfort.
A true mechanical lock is a red flag that usually indicates a structural problem requiring prompt attention. Pseudolocking, while painful, often responds to conservative treatment that addresses pain and inflammation.
Primary Mechanical Causes
True mechanical knee locking is almost always traced back to two primary structural issues that create a physical blockage. The most common cause is a tear in the meniscus, the C-shaped cartilage between the thigh and shin bones. A specific tear pattern, known as a “bucket-handle tear,” is particularly prone to causing this problem.
In a bucket-handle tear, a long, vertical fragment of the meniscal tissue remains attached at both ends but flips into the central joint space. This displaced flap physically wedges itself between the femur and tibia, immediately halting the knee’s normal gliding motion. The joint will remain fixed at a specific angle until the fragment either spontaneously repositions itself or is manually moved.
Another significant cause of true locking is the presence of loose bodies, sometimes called “joint mice.” These are fragments of bone or cartilage that have broken off due to trauma, degenerative conditions like osteoarthritis, or a disorder such as osteochondritis dissecans. The fragments float freely within the synovial fluid of the joint.
These loose bodies cause intermittent locking episodes when they migrate and become lodged between the articulating joint surfaces. The locking is often unpredictable, and the knee may unlock just as suddenly as it locks when the fragment shifts position.
Immediate Actions and Triage
When the knee locks, stop all activity and avoid putting weight on the affected leg. Attempting to force the knee to move risks causing further damage to the meniscus or joint surfaces. If the knee does not unlock quickly, rest in a safe seated or reclining position.
Applying the RICE protocol—Rest, Ice, Compression, and Elevation—can help manage associated pain and swelling. Ice should be applied for 15 to 20 minutes at a time to reduce inflammation. Gentle, non-forceful movement or slight rotation may encourage a loose fragment to shift and allow the knee to unlock, but this must be done with extreme caution.
Seek professional medical evaluation immediately if the knee remains persistently locked and cannot be straightened, if there is severe pain, or if the person is unable to bear any weight. These signs suggest a true mechanical obstruction that requires urgent diagnosis and likely intervention. Numbness or tingling below the knee is also a serious symptom that warrants emergency attention.
Professional Treatment Pathways
The medical approach to treating a locked knee depends entirely on the underlying cause identified through physical examination and diagnostic imaging, such as an X-ray or MRI. For cases of pseudolocking or minor mechanical issues, non-surgical management is often the first-line treatment. This includes targeted physical therapy to strengthen the muscles surrounding the knee, especially the quadriceps, which improves joint stability.
Anti-inflammatory medications, such as non-steroidal anti-inflammatory drugs (NSAIDs), help reduce the inflammation and pain contributing to pseudolocking symptoms. Activity modification is employed to avoid movements that irritate the joint and trigger spasms. Injections, such as corticosteroids, may also be used to provide localized relief from inflammation.
True mechanical locking caused by a displaced meniscal tear or loose bodies typically requires surgical intervention to resolve the obstruction. Arthroscopy, a minimally invasive keyhole surgery, is the standard procedure. During this surgery, miniature instruments are inserted to either remove the loose body or repair the torn meniscus.
For meniscal tears, the surgeon may perform a partial meniscectomy, trimming away the detached tissue, or a meniscal repair, where the torn cartilage is sutured back together. Removing the physical obstruction allows the knee to regain its full range of motion. If the knee cannot be fully extended, surgery is often recommended within one to two weeks to prevent long-term joint stiffness.