The inability to fully straighten the knee, known as a terminal knee extension deficit, is a common symptom that warrants medical evaluation. This restriction can range from slight limitation to a complete inability to move the joint. This symptom is divided into two categories: “true locking,” where a physical object mechanically blocks the joint, and “pseudo-locking,” which is a protective response to pain or swelling. Understanding this difference is key to finding a resolution.
Causes Involving Physical Obstruction (True Locking)
True locking occurs when a piece of tissue or a fragment physically jams the knee joint, preventing the tibia and femur from achieving full extension. This mechanical blockage is often sudden, complete, and painful. The most frequent cause is a specific type of meniscal tear.
A bucket-handle tear of the meniscus is a common offender. A large, C-shaped flap of cartilage tears but remains attached, allowing it to flip over and displace itself into the center of the joint. Wedged between the bones, this flap acts like a doorstop to extension. This palpable mechanical block physically prevents movement, distinct from a muscle spasm.
Loose bodies, often called “joint mice,” cause true locking when they float freely in the joint fluid and lodge in the joint space. These fragments are small pieces of bone or cartilage that break off due to trauma, degenerative joint disease, or osteochondritis dissecans. When caught between the femur and the tibia, they create a physical obstruction that immediately halts movement.
Displaced ligament fragments are another cause of a true mechanical block, often following a severe injury like an anterior cruciate ligament (ACL) rupture. The torn stump of the ligament can become trapped in the front of the joint, preventing the knee from straightening completely. This impingement of the torn tissue directly causes the extension deficit.
Causes Related to Pain, Swelling, or Muscle Spasm (Pseudo-Locking)
Pseudo-locking occurs when the knee feels restricted, but no physical object blocks movement. The inability to straighten the knee is a protective reflex mechanism triggered by pain or inflammation. This phenomenon is often referred to as arthrogenic muscle inhibition, or “quadricipital shutdown.”
Significant joint effusion, or swelling caused by fluid buildup, commonly contributes to pseudo-locking. Increased pressure within the joint capsule limits movement and activates a neural reflex that inhibits the quadriceps muscle. This quadriceps deactivation, combined with increased hamstring tension, prevents the knee from achieving full extension.
Acute flare-ups of inflammatory conditions, such as severe arthritis, cause intense pain that triggers muscle guarding. The body’s defense mechanism causes muscles around the knee, especially the hamstrings, to spasm and contract. This involuntary muscle spasm effectively locks the joint in a slightly bent position to minimize pain and protect the inflamed area.
Conditions like patellofemoral pain syndrome or severe patellar maltracking can also cause a sensation of locking or catching due to pain and misalignment. Although not a true mechanical block, the resulting pain and protective muscle response can make the knee feel functionally locked.
When This Symptom Requires Emergency Medical Attention
While many causes of a restricted knee require prompt medical care, certain red-flag symptoms necessitate an immediate trip to the emergency room. These symptoms suggest a potentially limb-threatening injury or a rapidly progressing infection.
Signs of neurovascular compromise are a serious concern. These include numbness, tingling, or an inability to feel a pulse below the knee. A cold, pale, or bluish (cyanotic) foot or toes indicate compromised blood flow, often caused by a complete knee dislocation or a severe fracture.
Acute signs of infection, known as septic arthritis, are an emergency that can cause permanent joint damage if not treated swiftly. Symptoms include fever, chills, and the knee joint becoming noticeably hot, red, and painful to touch.
Any sudden, complete inability to bear weight or a visible deformity accompanied by extreme swelling and pain after significant trauma should be treated as an emergency. These findings indicate a complex fracture or a complete knee dislocation, which carries a high risk of vascular injury.
Diagnostic Steps and General Treatment Pathways
Diagnostic Steps
Diagnosis begins with a detailed history and a comprehensive physical examination. This exam includes specific range-of-motion tests, palpation for tenderness, and stability tests for ligament integrity. The physical exam aims to differentiate between a physical obstruction (true locking) and a pain-related restriction (pseudo-locking).
Imaging studies visualize the internal structures of the knee to pinpoint the cause of the deficit. X-rays are typically ordered first to rule out fractures, bone chips, or large calcified loose bodies. A magnetic resonance imaging (MRI) scan is the most effective tool for visualizing soft tissues, such as the menisci, ligaments, and cartilage, and is often ordered urgently if a mechanical block is suspected.
Treatment Pathways
Treatment depends entirely on the specific diagnosis. For pseudo-locking due to swelling or muscle inhibition, conservative management is the first approach. This involves rest, ice, and non-steroidal anti-inflammatory drugs (NSAIDs) to manage pain and swelling.
Focused physical therapy follows, designed to overcome muscle inhibition by focusing on hamstring relaxation and quadriceps activation to restore full extension.
For a confirmed true mechanical block, surgical intervention is often required. Arthroscopy, a minimally invasive procedure, is commonly used to remove loose bodies or repair a displaced meniscal tear. If a displaced bucket-handle tear is caught early, it may be repaired with stitches; otherwise, the unstable fragment may need to be trimmed.