The inability to fully bend the knee (flexion) is a common symptom that interferes with daily activities like walking, sitting, or climbing stairs. The knee is a complex hinge joint where the femur, tibia, and patella interact, supported by ligaments, cartilage, and muscles. When this system is compromised, the body restricts movement as a protective measure, signaling an underlying problem. Understanding the specific cause is the first step toward regaining full mobility.
Mechanical Obstruction: When Something Is Stuck
Restricted knee flexion can be caused by a mechanical obstruction, where a physical object prevents the joint surfaces from moving freely. This is often described as a “locked knee” because the joint suddenly gets stuck at a specific angle. This blockage usually originates from damage to the internal structures of the joint.
A common culprit is a “bucket handle” meniscal tear, named for its appearance. This occurs when a longitudinal tear in the C-shaped cartilage separates the inner rim, allowing a flap of tissue to displace into the joint space. The displaced portion physically wedges itself between the thigh bone and the shin bone, preventing the knee from achieving full flexion or extension.
Fragments of bone or cartilage floating within the joint fluid, referred to as “joint mice,” can also cause a mechanical lock. These loose bodies migrate throughout the joint capsule and randomly become lodged in the articulation during movement. When these fragments catch between the moving surfaces, they interrupt the smooth gliding action necessary for bending and can lead to sudden, sharp pain.
Ligament instability, particularly following a severe injury, can also contribute to a mechanical impediment. If stabilizing ligaments, such as the anterior cruciate ligament (ACL), are torn, the joint can become unstable and shift out of alignment. This misalignment, or slight subluxation of the joint surfaces, prevents the bones from tracking correctly, creating a structural block to the full range of motion.
The Role of Swelling and Inflammation
Fluid accumulation and inflammation represent a restrictive category of limited knee movement. Excess fluid within the joint capsule, known as an effusion or “water on the knee,” creates hydrostatic pressure that physically limits the available space for the joint to move. This pressure increase can prevent the knee from bending fully.
The presence of effusion triggers a neurological response called quadriceps inhibition or protective muscle guarding. The body senses the distension and automatically reduces the activity of the quadriceps muscle group. This protective mechanism prevents the muscle from contracting forcefully, which would increase pressure within the swollen joint, contributing to stiffness and limited movement.
Inflammatory conditions, such as gout, rheumatoid arthritis, or flares of osteoarthritis, increase fluid production in the synovium, the lining of the joint. In gout, uric acid crystals accumulate, causing intense inflammation, swelling, and pain, making movement intolerable. The resulting stiffness is a biological consequence of the inflammatory environment, distinct from a structural blockage.
Severe bursitis, the inflammation of the fluid-filled sacs outside the joint, can also impede movement if the inflamed bursa is large or positioned strategically. While outside the main joint capsule, a swollen bursa creates external pressure that restricts the surrounding soft tissue, compounding the difficulty of achieving full knee flexion.
Soft Tissue and Chronic Flexibility Issues
Beyond acute mechanical blocks and fluid dynamics, the inability to bend the knee can stem from issues related to surrounding soft tissues and long-term joint changes. One persistent chronic cause is arthrofibrosis, which involves the abnormal proliferation of scar tissue within the joint capsule. This condition often follows trauma or surgery, such as an anterior cruciate ligament reconstruction or total knee replacement.
The dense, restrictive scar tissue physically tightens the space around the joint, preventing the knee from achieving its full range of motion. Specific formations, like the “cyclops lesion,” are nodules of fibrotic tissue that can form near the ACL graft site and directly block movement. This restriction is a physical limitation caused by the body’s healing response.
Severe tightness or contracture in the primary muscle movers, the quadriceps and hamstrings, also directly limits knee flexion. When the quadriceps muscles are chronically shortened, they act as a tether, mechanically restricting the knee’s ability to fold. The quadriceps tendon connects the muscle to the kneecap, and when this complex is tight, it increases the compression forces on the patellofemoral joint.
Tight hamstrings, which run down the back of the thigh, can also resist the bending motion. The length of these muscles must accommodate the full range of knee flexion, and chronic shortening creates a flexibility deficit that prohibits the joint from completing its full arc. Furthermore, tendon injuries, such as severe tendinosis or a partial rupture of the patellar or quadriceps tendon, can cause protective guarding, making the muscle unable to relax and allow full bending.
Recognizing Serious Symptoms and Seeking Care
While many causes of restricted knee movement are manageable, certain accompanying symptoms signal a medical urgency requiring immediate professional evaluation. Concerning signs include a sudden, complete inability to bear weight on the leg or a visible deformity of the knee joint. These may suggest a fracture, an acute ligament rupture, or a joint dislocation.
Fever or chills accompanying knee pain and swelling are serious red flags, potentially indicating a septic joint—a bacterial infection within the joint space. This condition can rapidly destroy cartilage and requires prompt medical intervention to prevent permanent joint damage. A true mechanical lock that leaves the knee unable to move also warrants immediate attention.
A healthcare provider will begin with a physical examination to test the stability and range of motion of the knee. Diagnostic tools, such as X-rays, can identify fractures or loose bony fragments. Magnetic resonance imaging (MRI) provides detailed images of soft tissues, including cartilage, ligaments, and menisci. In cases of significant effusion, a doctor may perform an aspiration to analyze the joint fluid, aiding in the diagnosis of infection or inflammatory conditions like gout.