The inability to fully bend the knee while walking, known medically as a locked knee, is a restrictive symptom that interferes with the natural gait cycle. This restriction is often felt during the swing phase of walking when the knee must flex to clear the ground, or when attempting to stand up or sit down. The sensation can range from a temporary, painful catch to a complete, fixed inability to move the joint past a certain point. This limitation is caused by either a physical obstruction within the joint or a protective reaction by the body.
True Locking Versus Pain Inhibition
The experience of a knee that will not bend falls into two distinct categories: true locking and pseudolocking, also known as pain inhibition. True locking occurs when an object physically obstructs the joint mechanism, much like a pebble jamming a door hinge. This mechanical block prevents any further movement, fixing the knee at a specific angle.
Pseudolocking is caused by the body’s protective response to pain, not a physical wedge. Severe pain, significant joint swelling (effusion), or muscle spasms cause the surrounding muscles to tense involuntarily. This inhibition mechanism makes the knee feel stuck and immobile. While the knee retains the anatomical capacity to move, intense pain or spasm prevents that movement.
A true lock often requires a specific maneuver or surgical intervention to resolve the physical obstruction. Pseudolocking, however, may temporarily ease with rest, cold therapy, or pain relief medication that dampens the pain signal and muscle spasm. Identifying the type of restriction is the first step in determining the necessary treatment path.
Structural Causes of Mechanical Impairment
True mechanical locking stems from material interfering with the smooth articulation between the femur and tibia. The most common cause is a Meniscus Tear, where a segment of the C-shaped cartilage is torn and displaced. A “bucket-handle” tear is particularly problematic, as the torn flap can lodge itself directly between the bones, physically blocking movement. This obstruction usually results in a sudden, fixed inability to straighten the leg fully.
Loose Bodies within the joint space are another frequent culprit. These fragments of bone or cartilage break off due to trauma, degenerative conditions like osteoarthritis, or osteochondritis dissecans. These fragments, sometimes called joint mice, float freely until they are caught between the joint surfaces during movement. The sudden lodging results in a sharp, intermittent locking sensation that may resolve as the fragment shifts position.
Severe ligamentous injuries, especially to the Anterior Cruciate Ligament (ACL), can indirectly lead to locking sensations. While the torn ligament does not physically block movement, the resulting joint instability can cause the body to “lock down” the surrounding musculature to prevent buckling. Alternatively, the force of the injury may displace a piece of the meniscus or cartilage, creating an acute mechanical block. These structural impairments necessitate advanced imaging, such as an MRI scan, for definitive diagnosis and often require surgical intervention.
Inflammation and Chronic Conditions
Restriction in knee movement not due to a physical obstruction is frequently caused by inflammatory and chronic conditions that induce pain inhibition and stiffness. Osteoarthritis (OA), a degenerative joint disease where the protective cartilage wears down, is a leading cause of this non-mechanical restriction. The chronic inflammation and resulting stiffness severely limit the range of motion, making the knee feel restricted, especially after periods of rest.
Conditions like Rheumatoid Arthritis (RA) or other inflammatory arthropathies cause the immune system to attack the joint lining, leading to chronic swelling and pain. This sustained inflammation increases fluid within the joint capsule (effusion), which reduces the space available for the bones to articulate fully. The constant pain and swelling trigger the body’s pseudolocking defense mechanism, making the knee resistant to bending.
Localized inflammation from Bursitis (swelling of fluid-filled sacs) or severe Tendinitis can also contribute to restriction. Swollen tendons or bursae press against surrounding structures, causing intense localized pain that inhibits muscle function. Furthermore, the development of dense, fibrous Scar Tissue or Adhesions following surgery or prolonged immobilization physically shortens the soft tissues. This results in a stiff knee that gradually loses its ability to flex, requiring dedicated physical therapy.
Immediate Action and Medical Guidance
When the knee suddenly locks, the first step is to stop activity and protect the joint from further stress. The RICE protocol (Rest, Ice, Compression, and Elevation) is the standard initial management strategy to reduce pain and swelling. Applying ice packs helps control acute inflammation, and gentle, non-weight-bearing movement may be attempted if the pain is tolerable and the lock is not fixed.
Certain “red flag” symptoms indicate the need for immediate professional medical evaluation. These include:
- A sudden onset of locking accompanied by a distinct popping sound at the time of injury.
- A complete inability to bear any weight on the affected leg.
- Visible knee deformity.
- Pain and swelling accompanied by a fever or spreading redness, suggesting infection.
A physician, typically an orthopedist, will perform a physical examination to assess stability, range of motion, and tenderness. Imaging tests confirm the cause. X-rays identify bone spurs or loose bony fragments. If soft tissue damage is suspected, such as a meniscal tear or ligament injury, a magnetic resonance imaging (MRI) scan is used. Treatment ranges from conservative measures like physical therapy and anti-inflammatory medication to arthroscopic surgery to remove mechanical obstructions.