The inability to fully straighten the knee, medically known as an extension deficit or extension lag, prevents the knee joint from achieving full extension necessary for efficient standing and walking. When full extension is lost, it creates an abnormal gait pattern and puts excessive strain on other structures in the leg, which can lead to long-term joint damage. This persistent deficit signals an underlying mechanical obstruction or a biological process preventing the quadriceps muscle from performing its primary function.
Mechanical Causes of Locking
A true “locked” knee occurs when a physical structure blocks the smooth movement between the femur (thigh bone) and the tibia (shin bone). The most frequent cause involves a torn piece of cartilage becoming physically wedged within the joint space.
A specific type of meniscal tear known as a “bucket-handle” tear is a frequent culprit in mechanical locking. This tear pattern causes a large, C-shaped flap of cartilage to detach from the meniscus while remaining connected at both ends, allowing the loose fragment to flip into the center of the joint. When this displaced fragment jams between the bone ends, it creates an immediate and firm block to knee extension.
Fragments of loose bone or cartilage, often referred to as “loose bodies,” can also cause a mechanical lock. These fragments may break off from the joint surfaces due to trauma or degenerative conditions like osteoarthritis. As the knee attempts to straighten, the loose body is trapped between the two main bones, physically preventing the final degrees of extension. This type of locking is typically intermittent, as the fragment can shift position within the joint cavity.
Non-Mechanical Causes of Inhibition
The knee can also be unable to extend fully due to biological responses like pain and swelling, which is sometimes called a “pseudo-lock.” When excess fluid accumulates in the joint capsule, known as joint effusion, the sheer volume physically limits the available space for the bones to move into full extension. Even a small amount of swelling can create significant pressure, causing a reflexive restriction of motion.
A major non-mechanical cause is arthrogenic muscle inhibition (AMI), a protective reflex that causes the quadriceps muscle to “shut down” after an injury or surgery. The presence of pain and swelling alters nerve signals sent from the joint to the spinal cord, which in turn prevents the brain from being able to fully activate the quadriceps. This lack of muscle activation means the patient cannot voluntarily complete the final phase of extension.
Furthermore, severe injury to the extensor mechanism, such as a complete patellar tendon rupture, results in an inability to actively straighten the leg. The patellar tendon connects the kneecap to the shin bone, and its rupture destroys the connection needed for the quadriceps to transmit force to the lower leg. In this instance, the knee may be manually straightened, but the patient cannot hold the position or lift the leg against gravity.
Immediate First Steps and Medical Urgency
For an acute inability to extend the knee, immediate steps can provide temporary relief, often summarized by the R.I.C.E. protocol. Resting the leg by avoiding weight-bearing helps calm the joint. Applying ice for 15 to 20 minutes several times a day reduces pain and swelling. Compression with a soft wrap and elevating the leg above the heart level both help manage joint effusion.
A sudden inability to extend the knee requires prompt medical evaluation. Certain symptoms warrant immediate emergency attention. These include an obvious deformity of the knee joint, a cold or pale foot, foot numbness or tingling, or an inability to bear any weight. Signs of a possible joint infection, such as rapid swelling accompanied by fever, chills, or skin warmth and redness, also require immediate medical intervention.
Professional Treatment Pathways
Once a healthcare provider has diagnosed the cause, treatment pathways divide into conservative and interventional options. Conservative management, often the first approach for non-mechanical issues, centers on physical therapy to restore the extension range of motion. Therapists will prescribe exercises like prone heel hangs and quad sets. Bracing may be used to provide a prolonged, low-load stretch into extension, while nonsteroidal anti-inflammatory drugs (NSAIDs) can help manage pain and inflammation.
Injection therapies are frequently utilized to manage symptoms related to inflammation and degeneration. Corticosteroid injections deliver a potent anti-inflammatory medication directly into the joint to quickly reduce swelling and pain, which can temporarily relieve the inhibition reflex. Viscosupplementation, which involves injecting a hyaluronic acid gel into the joint, is primarily used for osteoarthritis to improve lubrication and shock absorption, helping to restore smoother motion.
Surgical intervention becomes necessary for mechanical obstructions and complete structural tears. Arthroscopy, a minimally invasive procedure, is used to repair or trim meniscal tears, particularly bucket-handle lesions, and to remove loose bodies that are blocking joint movement. For severe ligament or tendon damage, such as a complete patellar tendon rupture or an ACL tear causing chronic instability, reconstructive surgery is performed to restore the structural integrity of the joint.