The knee is a hinge joint, designed to allow movement in a single plane: flexion (bending) and extension (straightening). For a knee to be considered fully functional, it must achieve full extension (zero degrees) and substantial flexion, typically reaching 135 to 150 degrees. When an individual cannot fully bend their knee, a structural or biological impediment is disrupting the normal joint mechanics. This inability to reach full range of motion can stem from acute fluid accumulation, chronic bony changes, or restrictive internal tissue.
Acute Swelling and Inflammation
One of the most immediate causes of restricted knee flexion is the rapid accumulation of fluid within the joint space, known as an effusion. This fluid buildup (synovial fluid, pus, or blood) significantly increases intracapsular pressure. Since the knee joint capsule is a fixed volume, excess fluid physically pushes against surrounding structures, limiting movement.
If the fluid is blood (hemarthrosis), it often follows a severe traumatic injury, such as an anterior cruciate ligament tear. The presence of blood exerts intense pressure, contributing to pain and triggering a reflex inhibition of the quadriceps muscle. This protective reflex causes the large thigh muscles to temporarily shut down, preventing any forceful attempt at bending the knee.
Mechanical Obstruction
A mechanical block physically jams the joint, preventing motion at a specific angle. This differs from generalized stiffness and manifests as a sudden, hard stop when trying to bend or straighten the knee. The most frequent culprit for this true locking sensation is a displaced meniscal tear.
When a tear, particularly a large “bucket-handle” tear, occurs, the torn flap of cartilage flips into the joint space between the femur and tibia. This mobile tissue acts like a wedge, physically obstructing the motion required for full flexion. Similarly, “loose bodies”—fragments of bone or articular cartilage broken off due to trauma or degeneration—can float within the synovial fluid. If a fragment lodges between the joint surfaces, it creates a palpable barrier that halts the knee’s bending motion.
Long-Term Structural Deterioration
Loss of the ability to bend the knee is commonly linked to degenerative conditions, most notably osteoarthritis. This disease involves the gradual breakdown and loss of the smooth articular cartilage covering the ends of the bones. As cartilage wears away, the space between the bones narrows.
In response to mechanical stress, the body forms bony outgrowths called osteophytes, or bone spurs, along the joint margins. These fixed masses physically impinge upon one another when the knee is flexed, causing bony surfaces to collide and prevent full range of motion. Chronic pain and inflammation associated with advanced arthritis also lead to protective muscle guarding, where surrounding musculature shortens and tightens, restricting bending.
Post-Surgical Stiffness and Scar Tissue
Stiffness following injury or surgery is often caused by the formation of internal scar tissue, termed arthrofibrosis. This process involves the excessive production of fibrous connective tissue (collagen), which forms dense adhesions inside and outside the joint capsule. A dysregulated healing response can lead to this restrictive tissue formation.
These adhesions effectively shorten the joint capsule, surrounding ligaments, and tendons, physically tethering the structures and preventing smooth motion. Arthrofibrosis is a common complication after significant knee trauma or major surgeries, such as ACL reconstruction or total knee replacement. The physical constraint of this dense, non-elastic scar tissue is a powerful mechanical restraint on mobility.