The inability to fully straighten the knee while still being able to bend it suggests that the normal motion required for full extension is being physically blocked or neurologically inhibited. The knee joint requires a clear path for the femur (thigh bone) to glide over the tibia (shin bone) to achieve a fully straight position. When this motion is compromised, it signals a significant underlying issue that warrants immediate medical attention.
Mechanical Obstructions Preventing Straightening
A true locked knee occurs when a physical structure within the joint space prevents the knee from reaching full extension, even when relaxed. This is distinct from a knee that is simply painful or difficult to straighten. The presence of an obstruction allows the knee to bend (flex) freely because the object typically moves out of the way, but it jams the joint when the leg attempts to straighten.
The most common cause of a true mechanical block is a specific type of meniscal tear called a “bucket-handle tear.” The meniscus is a C-shaped piece of cartilage. In this tear pattern, a large fragment rips away from its attachment but remains connected at both ends, allowing the flap to flip into the center of the joint space.
When this torn meniscal fragment displaces, it lodges itself between the femur and tibia, acting like a wedge that physically blocks the final degrees of extension. Because the fragment is typically only an obstruction in the path of straightening, the knee can often still be bent without issue. This condition is considered an orthopedic urgency because the displaced fragment is deprived of nutrients and can be further damaged by continued movement.
Another cause of a mechanical blockage is the presence of a loose body. These are small fragments of bone or articular cartilage that have broken off due to trauma, wear-and-tear, or conditions like osteochondritis dissecans. These fragments float freely within the synovial fluid of the joint.
A loose body can randomly migrate into the joint space and become temporarily trapped between the moving surfaces of the femur and tibia, causing intermittent locking or catching. While a bucket-handle tear typically causes a fixed inability to straighten, a loose body may cause the knee to lock and spontaneously unlock as the fragment shifts position.
Inhibition Due to Swelling and Pain
In contrast to a true mechanical lock, a pseudo-locked knee occurs when the inability to straighten is not caused by a physical obstruction but by the body’s protective neurological response. This is often driven by joint effusion (swelling) and pain, which trigger a reflex known as arthrogenic muscle inhibition (AMI).
Joint effusion (swelling) increases the internal pressure within the knee. This increased pressure activates sensory receptors in the joint lining, sending signals to the central nervous system. The brain interprets these signals as a threat, which leads to a protective “switching off” of the quadriceps muscle.
This quadriceps inhibition is a spinal reflex that prevents the muscle from fully contracting, making it impossible to generate the force needed for full extension. The quadriceps muscle, responsible for straightening the leg, is neurologically prevented from doing its job, even though the muscle itself is not damaged.
Common injuries causing significant joint effusion and subsequent quadriceps inhibition include severe ligament sprains (such as an ACL tear) or substantial inflammation from conditions like osteoarthritis. While the joint is physically capable of moving, the pain and involuntary muscle shut-down prevent the patient from actively straightening the leg. This protective mechanism is often more pronounced at the end range of motion, making the inability to achieve the final degrees of extension a common symptom.
Essential Next Steps and Medical Evaluation
An inability to fully straighten the knee, whether due to a mechanical block or severe inhibition, requires prompt medical evaluation. A locked knee due to a displaced meniscal tear is particularly urgent, as early intervention can significantly increase the chances of repairing the cartilage rather than removing it.
Immediately following the onset of symptoms, the initial care should focus on the RICE principle: Rest, Ice, Compression, and Elevation. Resting the joint prevents further damage, and applying ice for 15 to 20 minutes at a time helps to mitigate swelling and reduce pain. Compression with a bandage and elevating the leg above heart level both assist in controlling joint effusion.
During a medical evaluation, the physician will first conduct a thorough physical examination to assess the knee’s range of motion, stability, and tenderness. They will attempt to differentiate between a true mechanical lock and a pseudo-lock caused by pain and muscle guarding.
To determine the underlying cause, diagnostic imaging is almost always necessary. Initial X-rays are used to rule out a fracture or to identify loose bodies or bone spurs associated with advanced arthritis. However, X-rays cannot visualize soft tissues. The definitive diagnostic tool for soft tissue injuries like meniscal tears and ligament damage is a Magnetic Resonance Imaging (MRI) scan, which provides detailed cross-sectional images of the cartilage and ligaments.