The sudden inability to bend or straighten the knee signals a serious problem within the joint’s complex mechanics. This restriction is classified into two types: a true mechanical lock, where a physical obstruction prevents movement, or a functional restriction (pseudo-lock), where severe pain, swelling, or instability inhibits muscle movement. Understanding the difference between these categories is the first step toward finding the underlying cause. Any persistent inability to move the knee requires professional medical evaluation.
Mechanical Locking of the Joint
A true mechanical lock occurs when a fragment of tissue or bone physically wedges itself between the smooth surfaces of the femur (thigh bone) and tibia (shin bone). This results in a hard, fixed stop that prevents the knee from achieving full extension or flexion.
The most frequent cause is a meniscal tear, specifically a “bucket-handle tear.” In this injury, a large piece of the C-shaped cartilage tears away and flips into the joint space. This displaced tissue physically blocks the normal gliding motion required to straighten the leg fully.
Small, mobile fragments of cartilage or bone, often called “loose bodies,” can also cause this block. These fragments may break off due to trauma or degenerative conditions like osteoarthritis. When a loose body floats into the space, it physically impedes movement, leading to intermittent locking sensations.
Patellar dislocation, where the kneecap slips out of its groove, is another cause of mechanical blockage. The joint’s extensor mechanism is compromised, and the knee cannot be actively straightened until the kneecap is returned to its proper position.
Severe Swelling and Inflammation
Functional restriction, or pseudo-locking, is characterized by stiffness or a “boggy” resistance rather than a hard stop, as pain and fluid accumulation inhibit movement. Excess fluid accumulation (knee effusion) creates pressure within the joint capsule, physically limiting the range of motion. A large effusion often forces the knee to rest in a slightly bent position, which minimizes intra-articular pressure.
Various forms of arthritis are common culprits, especially those causing inflammation of the joint lining (synovitis). Osteoarthritis causes chronic synovitis and persistent stiffness due to cartilage loss and bone-on-bone friction. Rheumatoid arthritis, an autoimmune condition, attacks the synovial membrane, causing severe swelling and stiffness that functionally restricts movement.
Acute inflammatory conditions like gout can cause a sudden functional lock. Gout involves uric acid crystals triggering an intense inflammatory response, resulting in rapid swelling and pain that prevents any attempt to move the knee. Bursitis, the inflammation of the fluid-filled sacs cushioning the knee, also causes extreme pain and swelling, leading to a protective muscle spasm that restricts movement.
Ligament and Tendon Damage
The inability to move the knee can stem from structural failure of the soft tissues responsible for stability and power. A complete rupture of the quadriceps tendon above the kneecap or the patellar tendon below it severely compromises the knee’s extensor mechanism. Since the connection between the powerful thigh muscle and the shin bone is severed, the person is physically unable to actively straighten the leg.
Major ligament tears, such as the anterior cruciate ligament (ACL) or medial collateral ligament (MCL), cause profound joint instability. The body responds to this instability with an involuntary protective spasm, engaging surrounding muscles. This muscle guarding locks the joint in a semi-flexed position to prevent painful, uncontrolled movement and further injury.
Fractures involving the bones around the knee also restrict movement. A displaced patella fracture disrupts the extensor mechanism and may introduce bony fragments that physically block motion. Fractures extending into the joint surface cause significant bleeding and swelling, resulting in severe functional restriction.
Actionable Steps and Warning Signs
For immediate self-care following a sudden knee injury, the R.I.C.E. protocol is the recommended first step. This involves:
- Rest by avoiding weight-bearing activities.
- Ice applied for 15 to 20 minutes several times a day to reduce swelling.
- Compression with an elastic bandage to minimize fluid buildup.
- Elevation of the leg above heart level to promote fluid drainage.
Over-the-counter nonsteroidal anti-inflammatory drugs (NSAIDs) can help manage pain and reduce localized swelling. However, the inability to move the knee often indicates a serious underlying problem requiring professional attention.
Several “Red Flags” signal the need for immediate emergency medical care. These include any visible deformity, the presence of fever or chills accompanying joint pain, or the inability to bear any weight on the leg. Numbness or tingling in the foot below the knee, which suggests potential nerve or blood vessel compromise, also warrants an immediate emergency room visit.