The inability to bend the knee joint fully, or at all, is medically known as a loss of knee flexion. This limitation can range from mild, temporary stiffness to a complete, mechanically blocked joint that prevents simple movements like walking or sitting. The knee’s full range of motion is necessary for most daily activities, making any restriction a serious concern. Understanding the cause is the first step toward regaining mobility, as reasons vary widely from acute physical obstructions to chronic tissue changes. This review explores the most common causes of restricted knee flexion.
Physical Blockages: The Locked Knee Phenomenon
When the knee cannot be moved past a certain point, it is often due to a mechanical obstruction referred to as “true locking.” This occurs when a physical structure within the joint space gets caught between the femur and the tibia, preventing the smooth motion required for flexion. This restriction is typically abrupt and absolute.
One of the most common causes of true locking is a specific type of meniscal tear called a “bucket-handle tear.” The meniscus is a C-shaped piece of cartilage that acts as a shock absorber. In this injury, a large segment tears away but remains attached, allowing the mobile fragment to flip into the center of the joint. This physically wedges itself in the space, blocking movement and often leaving the knee stuck in a slightly flexed position.
Another mechanical obstruction is a loose body, a fragment of bone or cartilage floating within the joint capsule. These fragments break off due to trauma, degenerative joint disease, or decreased blood supply. A loose body may cause intermittent locking, shifting out of the way temporarily before becoming lodged again. The sudden, sharp pain and inability to move that accompany true locking require immediate attention.
Restriction Due to Swelling and Pain
Beyond physical blockages, a buildup of fluid within the joint, known as joint effusion, can physically restrict the ability to bend the knee. The joint capsule has a finite volume, and excess fluid accumulation takes up the space needed for the bones to articulate and move into a flexed position. This increases pressure inside the joint, leading to tightness and swelling that limits the range of motion.
Acute injuries, such as an anterior cruciate ligament (ACL) tear, often cause hemarthrosis (rapid bleeding into the joint space). This sudden influx of blood quickly creates a large effusion that physically prevents flexion. Inflammatory conditions also lead to significant effusion; for example, a severe flare-up of gout or septic arthritis causes the body to produce excess synovial fluid and inflammatory cells.
Pain itself acts as a powerful inhibitor, causing the body to reflexively guard the joint against movement, a phenomenon called muscle guarding. Severe pain, even without a mechanical blockage, can inhibit the quadriceps muscles, making it impossible to voluntarily relax and bend the knee. The body protects the injured joint by locking the surrounding muscles, preventing movement through the full range.
Stiffness from Soft Tissue and Scarring
A gradual loss of the ability to bend the knee, often occurring after surgery, trauma, or immobilization, points toward chronic changes in the surrounding soft tissues. The most common cause is arthrofibrosis, which involves the excessive formation of scar tissue (fibrosis) within the joint capsule and surrounding structures. While scar tissue is a normal part of healing, arthrofibrosis is an exaggerated inflammatory response that lays down dense, disorganized collagen fibers.
This dense, fibrous tissue restricts the movement of the joint surfaces and severely limits the flexibility of the joint capsule. Factors contributing to arthrofibrosis include poor early post-injury or post-surgical mobility, prolonged immobilization, and excessive initial swelling. The resulting stiffness is fundamentally different from an acute mechanical lock, presenting as dense, resistant tightness rather than a sudden obstruction.
The excessive scarring can also lead to contractures, where the surrounding musculature, particularly the quadriceps, becomes shortened and inflexible. This stiffness makes it physically impossible for the knee to move into a deep bend. This chronic condition requires targeted physical therapy and sometimes surgical intervention to release the restrictive scar tissue and restore motion.
When Immediate Medical Evaluation is Necessary
While some knee stiffness can be managed with rest and ice, certain symptoms demand immediate medical evaluation to prevent permanent damage. A sudden, complete inability to move the knee following trauma, especially if accompanied by a noticeable “pop” or a visible deformity, requires emergency assessment. These signs suggest a severe structural injury, such as a major ligament rupture or a fracture, that may require urgent stabilization.
Signs of an active infection are also an emergency, including a knee that is swollen, painful, red, and hot to the touch, or if the patient develops a fever and chills. Infectious arthritis can rapidly destroy joint cartilage and needs immediate treatment with antibiotics and drainage of the infected fluid. Any indication of neurovascular compromise, such as numbness, tingling, or a cold foot on the affected side, signals a potential emergency involving blood vessel or nerve damage.
A medical professional typically begins the diagnostic process with a physical examination and imaging. X-rays check for fractures or loose bodies, while magnetic resonance imaging (MRI) visualizes soft tissues, including the menisci, ligaments, and cartilage. Seeking prompt care for severe symptoms ensures a timely and accurate diagnosis, which preserves the long-term function of the knee joint.