Knee locking is a sensation where the joint feels physically stuck, preventing the ability to fully bend or straighten the leg. This common symptom suggests a problem within the complex mechanics of the knee. Understanding the underlying cause is the first step toward finding relief and determining the necessary treatment. The term “locked knee” describes two distinct mechanical issues: true locking, which involves a physical blockage, and pseudo-locking, which is a temporary malfunction caused by pain or instability.
True Mechanical Locking Versus Pseudo-Locking
The distinction between true mechanical locking and pseudo-locking is based on whether a physical object is jamming the joint. True mechanical locking occurs when a piece of tissue or bone physically wedges itself between the femur and the tibia. This severe mechanical blockage makes it impossible to fully extend or flex the knee. The inability to move the joint is absolute, similar to a pebble caught in a door hinge.
Pseudo-locking, sometimes called “catching,” is a sensation of the knee feeling stuck or giving way, often due to severe pain or muscle spasm. The joint is not physically obstructed in this scenario, and movement is technically possible, though pain inhibits the motion. This locking feeling is usually brief and resolves quickly. It often occurs as the body’s protective defense mechanism to prevent movement that would cause further damage.
Physical Obstructions Causing True Locking
True mechanical locking is most frequently caused by damage to the menisci, the C-shaped fibrocartilage cushions in the knee joint. When a meniscal tear occurs, especially a “bucket handle” tear, the torn fragment can displace and flip into the joint space. This displaced cartilage acts as a physical block, getting caught between the two main bones of the leg and preventing full extension.
Another structural cause of true locking involves loose bodies, sometimes referred to as “joint mice.” These are fragments of bone or cartilage that have broken off and float freely within the knee joint. They can result from trauma, severe osteoarthritis, or osteochondritis dissecans. When a fragment moves into the narrow space between the femur and tibia, it suddenly obstructs the joint’s movement, causing the knee to lock.
Functional Issues Causing Pseudo-Locking
Pseudo-locking results from issues related to joint stability, muscle function, or inflammation, rather than a physical object. A frequent cause is patellar instability or maltracking, where the kneecap does not glide smoothly within the groove on the thigh bone. This improper movement causes a catching sensation and temporary inhibition of motion, creating the feeling that the knee is locked.
Severe inflammation from conditions such as osteoarthritis can also lead to pseudo-locking through pain and muscle guarding. The inflammation triggers muscle spasms as a protective response, which limits the range of motion and mimics a mechanical lock. Plica syndrome involves an inflamed fold of tissue in the knee’s lining that becomes irritated or thickened. When this inflamed plica catches between the joint surfaces, it can produce a painful snapping or temporary locking sensation.
Diagnosis and Treatment Options
A complete inability to unlock the knee requires prompt medical attention, as true mechanical locking indicates significant structural damage. For any persistent locking, a healthcare provider begins with a physical examination to test the joint’s range of motion and stability. They determine if the locking is hard (true mechanical) or soft (pseudo-locking) based on the knee’s response to manipulation.
Diagnostic imaging is used to identify the underlying cause. X-rays check for bone-related issues, such as fractures, bone spurs, or loose bone fragments. A Magnetic Resonance Imaging (MRI) scan is the most effective tool for visualizing soft tissue damage, confirming a meniscal tear or cartilage damage.
Treatment varies widely based on the diagnosis. Pseudo-locking caused by patellar issues or mild inflammation is often managed conservatively with anti-inflammatory medications and physical therapy. Physical therapy focuses on strengthening the muscles around the knee to improve stability and correct patellar tracking. True mechanical locking caused by a displaced meniscal fragment or a loose body usually requires arthroscopic surgery to repair the torn meniscus or remove the obstructing fragment.