The inability to fully extend the knee is a common and often painful symptom that restricts mobility. This limitation is generally referred to as a “locked knee,” meaning the joint stops short of its normal, straight position. The inability to straighten the leg can signal a physical obstruction or a protective response to pain and swelling. Understanding this requires examining how injury or disease disrupts the normal extension mechanism.
The Normal Knee Extension Mechanism
Achieving full, stable knee extension involves a coordinated sequence of movements, not just a simple hinge action. The quadriceps muscle group, located at the front of the thigh, provides the main force for straightening the leg. This powerful muscle group pulls the shinbone (tibia) forward relative to the thighbone (femur) and is responsible for the majority of the extension motion.
As the knee nears its final 20 degrees of extension, the “screw-home mechanism” occurs. This is a slight external rotation of the tibia relative to the femur, which increases the contact area between the bones. This rotation allows the joint surfaces to achieve their most stable position, effectively locking the knee for weight-bearing activities like standing.
Once the knee is fully extended and locked, the major ligaments, particularly the anterior cruciate ligament (ACL), are pulled taut, providing passive stability. This passive tension allows the leg muscles to relax, conserving energy while standing upright.
Acute Mechanical Obstructions
A key reason for the inability to lock the knee is a “true mechanical lock,” which occurs when a physical object blocks the joint’s movement. In this situation, the knee stops abruptly, feeling jammed and immovable, and cannot be forced into full extension. The most frequent cause of this true lock is a tear in the meniscus, the C-shaped cartilage that acts as a shock absorber between the femur and tibia.
A specific type of meniscal injury, known as a bucket-handle tear, is particularly likely to cause a mechanical lock. In this injury, a piece of cartilage tears and flips over, lodging itself between the joint surfaces and physically preventing full extension. Loose bodies within the joint space can also create this hard stop. These fragments of cartilage, bone, or torn ligament tissue may break off due to trauma or degenerative conditions and occasionally get caught between the moving bones.
Causes Related to Swelling and Instability
The knee may also fail to reach full extension due to factors other than a physical block, a condition sometimes called a “pseudo lock.” One common cause is joint effusion, the accumulation of excess fluid within the knee joint capsule. This increased pressure inside the joint mechanically restricts the range of motion and often causes the knee to settle into a slightly bent resting position.
The presence of fluid or pain triggers a neurological phenomenon called arthrogenic muscle inhibition (AMI), which causes the quadriceps muscle to weaken and function improperly. This reflexive shutdown prevents the muscle from executing the final extension needed to engage the screw-home mechanism.
Damage to the major stabilizing ligaments, such as the ACL, can also prevent full extension by creating instability. An injured ligament causes the joint to feel loose or “give way,” prompting the surrounding muscles to guard against movement. This muscle guarding is a protective spasm that limits the joint’s range of motion, stopping the leg from straightening completely. Chronic conditions like severe osteoarthritis can also contribute, as inflammation and bone spur formation interfere with the smooth articulation required for terminal extension.
When to Seek Professional Help
If the knee locks and remains stuck in a bent position, or if there is an audible popping sound followed by a sudden inability to straighten the leg, immediate medical attention is advisable. These symptoms, especially when accompanied by severe pain, rapid swelling, or the inability to bear weight, point toward a substantial internal derangement such as a meniscal tear or a fracture.
A healthcare provider will perform a physical examination and may order diagnostic imaging, such as X-rays for bone fragments or an MRI for soft tissue damage like meniscal or ligament tears. Prompt diagnosis of a true mechanical lock is important because an extended locked position can lead to long-term issues like joint contracture. Initial treatment often involves rest, ice, and anti-inflammatory medication. However, a physical obstruction usually requires a procedure, such as arthroscopic surgery, to remove the blockage and restore full mobility.