The inability to fully straighten the knee is a common symptom that significantly impacts daily life. Achieving full extension is necessary for a normal walking pattern; a bent knee shortens the effective length of the leg, leading to a noticeable limp and requiring more energy to move. This restriction alters the mechanics of the entire lower body, potentially causing strain and discomfort in the hip and lower back over time. Understanding the underlying cause is the first step toward restoring complete mobility.
Physical Obstructions Inside the Joint
One of the most immediate reasons for restricted knee motion is a physical obstruction, often described as a mechanical “locking” sensation. This occurs when a piece of tissue or bone becomes lodged between the femur and tibia, preventing the joint surfaces from gliding into the final, straight position.
A common culprit is a specific type of meniscal tear known as a “bucket-handle” tear. In this injury, a large, U-shaped fragment of cartilage breaks free but remains attached, allowing the loose flap to flip into the center of the joint and physically block full extension. The sudden onset of this block is often associated with a significant injury, such as trauma or a rapid pivot during sports activity.
Loose bodies, which are small fragments of cartilage or bone broken off due to injury or degenerative changes, can also float within the joint space. These fragments can become wedged in the hinge mechanism, preventing smooth movement. When true mechanical obstructions are present, a surgical intervention is often necessary to remove or repair the trapped tissue and successfully restore full knee extension.
Restriction Caused by Swelling
Fluid accumulation within the joint, or swelling, restricts motion but is not a true mechanical block. The buildup of excess fluid inside the joint capsule increases internal pressure, physically limiting the space needed for the bones to achieve full extension. This swelling may develop gradually following an isolated meniscal tear or rapidly with more severe injuries like an anterior cruciate ligament (ACL) tear.
Significant inflammation and swelling trigger a protective response called arthrogenic muscle inhibition. In this phenomenon, pain and pressure signals cause the quadriceps muscle—responsible for straightening the knee—to involuntarily weaken or “shut down.”
This reflex inhibition makes it difficult to actively contract the quadriceps powerfully enough to achieve the final degrees of extension. This lack of muscle activation combined with the fluid bulk creates a “pseudo-locked” feeling. Addressing this restriction involves managing the underlying inflammation to reduce fluid volume and aggressively re-training the quadriceps muscle to fire correctly, often starting with simple isometric contractions.
Soft Tissue Tightness and Scarring
A persistent inability to straighten the knee can result from chronic changes in the surrounding soft tissues. The muscles at the back of the leg, particularly the hamstring and gastrocnemius (calf) muscles, can become tight and resist the final straightening motion. If the knee is kept bent for an extended period after an injury or surgery, these muscles can adaptively shorten and physically pull the joint into a flexed state.
Another common cause of chronic restriction, especially following a major knee surgery like an ACL reconstruction or total knee replacement, is arthrofibrosis. This condition involves the formation of excessive, dense scar tissue and fibrous adhesions within the joint capsule. The proliferative scar tissue physically shortens and stiffens the soft tissues, including the patellar tendon and joint lining.
Arthrofibrosis prevents the normal gliding and rolling motions necessary for the knee to achieve a full, straight position. Restoring full extension in these cases requires dedicated, sustained stretching and mobilization techniques to break down the restrictive scar tissue and lengthen the contracted soft tissues.
Determining When to See a Doctor
While minor stiffness may resolve with rest and ice, a persistent inability to fully straighten the knee should be evaluated by a healthcare professional. One of the clearest signals for seeking medical attention is a sudden onset of mechanical locking, where the knee unexpectedly catches and cannot be moved, which may indicate a displaced meniscal tear or a loose body. Any inability to bear weight on the affected leg, or a sense that the knee is unstable and might give way, also warrants prompt medical assessment.
The presence of systemic symptoms alongside the knee issue should be treated as an urgent concern. If the restricted knee extension is accompanied by a fever, chills, or noticeable redness and warmth around the joint, it could signal a joint infection, known as septic arthritis, which requires immediate treatment to prevent cartilage damage. Even without these acute red flags, a simple lack of full extension that does not improve after a few weeks of observation should lead to a consultation with a physician.