The inability to fully bend the knee, known as restricted knee flexion, signals an underlying mechanical, inflammatory, or structural problem within the joint. A healthy knee should bend approximately 135 to 150 degrees, allowing the heel to nearly reach the buttock. When this range of motion is compromised, daily activities like climbing stairs, squatting, or sitting comfortably become challenging. Understanding the cause of this limitation is the first step toward restoring full movement.
Acute Injuries That Block Movement
A sudden inability to bend the knee fully, often following trauma, suggests a physical obstruction inside the joint. This phenomenon is known as a “true mechanical block” or “locking” because a piece of tissue or bone is wedged between the femur and tibia. The most frequent cause of this true locking is a specific type of meniscal tear called a “bucket handle” tear.
In a bucket handle tear, a large fragment of the C-shaped cartilage (the meniscus) tears away but remains attached. This fragment can flip over and become lodged in the joint space, physically preventing the bones from gliding smoothly through full flexion. Similarly, loose bodies—small fragments of bone or cartilage—can break off and float freely within the synovial fluid. These fragments can migrate and get caught in the joint mechanism, causing intermittent locking or a sudden block to movement.
Other acute injuries, such as ligamentous tears, typically limit motion through instability and pain, sometimes called a “pseudo-lock.” A tear of the anterior or posterior cruciate ligament (ACL or PCL) creates instability, causing surrounding muscles to involuntarily tighten as a protective measure. This muscle guarding, combined with swelling and pain, prevents the joint from moving through its full arc. The body’s reflexive response to severe injury is to immobilize the area to prevent further damage.
Swelling and Inflammation Limiting Flexibility
Fluid accumulation and inflammation are common culprits for restricted knee motion, causing limitation through pressure and pain. The knee joint is enclosed by a capsule containing synovial fluid. When the joint is irritated or injured, the synovial membrane can overproduce this fluid, leading to joint effusion, or “water on the knee.”
This excess fluid takes up space within the joint capsule, increasing internal pressure. When attempting to bend the knee deeply, the incompressible fluid physically resists compression, making full flexion painful and impossible. The stiffness felt from effusion is often described as a generalized inability to move.
Inflammation of the soft tissues surrounding the joint can also impede bending. Synovitis is the inflammation of the synovial membrane, which causes pain and contributes to effusion. Bursitis involves the inflammation of the fluid-filled sacs (bursae) that cushion friction points around the joint. The swollen bursa can become compressed during deep flexion, generating pain that stops the movement before the full range is reached.
Chronic Conditions and Scar Tissue Restrictions
Long-term degenerative conditions and the body’s healing response to past trauma can lead to permanent structural changes that restrict knee movement. The most common chronic cause is osteoarthritis, where the protective articular cartilage wears away. This cartilage loss leads to the development of bone spurs (osteophytes), which are abnormal bony growths that form along the edges of the joint as the body attempts to stabilize the damaged area.
These osteophytes can grow large enough to physically impinge on one another, creating a bony block that prevents the femur and tibia from completing the full motion required for deep flexion. The progressive nature of the disease, combined with chronic pain and inflammation, causes the joint capsule to thicken and contract, further reducing flexibility. Similarly, inflammatory arthropathies, such as rheumatoid arthritis, cause chronic joint destruction and remodeling that permanently limit mobility.
A particularly challenging restriction is arthrofibrosis, characterized by the excessive formation of internal scar tissue within the joint. This condition frequently occurs following knee surgery, such as ACL reconstruction or total knee replacement, when the body’s healing response goes into overdrive. The fibrous tissue physically tethers the joint structures, creating a stiff, restrictive sheath that prevents normal motion.
This internal scarring can limit both the ability to straighten and to bend the knee because the soft tissues are no longer pliable. The buildup of fibrotic tissue acts like a thick, non-elastic rope, making it nearly impossible to regain the final degrees of flexion. The severity of arthrofibrosis can range widely, sometimes developing in the early post-operative phase.
When to Consult a Healthcare Professional
Deciding when restricted range of motion warrants professional medical attention depends on the nature and severity of the symptoms. Any sudden, acute loss of the ability to bend the knee after an injury, especially if accompanied by a pop or snap, should be evaluated promptly. A true mechanical locking, where the knee gets physically stuck and cannot be moved, is a serious symptom requiring immediate diagnosis.
Red flags suggesting the need for urgent care include the inability to bear weight on the affected leg, rapidly increasing swelling, or a visible deformity of the knee. The presence of systemic symptoms like fever, chills, or redness and warmth around the joint are concerning signs, as they may indicate an infection.
For symptoms that develop gradually, such as mild, persistent stiffness that limits deep squatting, a non-emergency appointment is appropriate. If the restriction is chronic and steadily worsening, or if it significantly interferes with daily activities like walking or sleeping, a medical consultation is necessary. A persistent restriction that does not improve with rest or over-the-counter medication should always be assessed to determine the underlying cause and prevent further joint damage.