The knee joint is designed for a remarkable range of motion, allowing the lower leg to move from a straight position to deep flexion. This ability to bend, known as knee flexion, is required for daily activities like climbing stairs, squatting, or sitting comfortably. When this range of motion is significantly limited, the symptom is called a flexion deficit, which can be both painful and highly restrictive to mobility. The underlying reasons for an inability to fully bend the knee are diverse, ranging from sudden mechanical blockages caused by injury to gradual structural changes over time.
Acute Structural Damage
Sudden, traumatic events often result in immediate physical barriers that prevent the knee from achieving full flexion. One common cause is a meniscal tear, particularly if a large fragment of the cartilage cushion becomes displaced. A specific type of tear, known as a “bucket-handle” tear, can cause the torn piece of cartilage to flip into the joint space, creating a mechanical block against the smooth movement of the bones.
When this displaced tissue wedges itself between the thigh bone (femur) and the shin bone (tibia), the knee is said to be “locked,” making further bending or straightening impossible. This physical obstruction often requires surgical intervention. Acute injuries to stabilizing ligaments, such as the anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL), also contribute to flexion deficits, though through a different mechanism.
A ligament tear can cause significant internal bleeding and swelling, resulting in a large joint effusion that limits the range of motion due to pain and pressure. The resulting joint instability may cause surrounding muscles to involuntarily tense or “guard” the joint, preventing movement past a certain point. Fractures near the joint surface or the presence of a loose body, such as a small piece of bone or cartilage, can also act as a physical impediment, catching the joint during movement.
Chronic Degenerative Conditions
When knee flexion is lost gradually over months or years, the cause is often related to long-term wear and tear within the joint structure. Osteoarthritis (OA) is the primary chronic condition leading to reduced mobility, affecting the smooth, protective layer of articular cartilage. As this cartilage breaks down, the underlying bone attempts to stabilize the joint by growing bony outgrowths called osteophytes, or bone spurs.
These osteophytes form along the edges of the femur and tibia, reducing the available space for the joint to articulate and glide through its full arc of motion. The formation of these spurs ultimately limits the degree of knee bending. Cartilage loss itself also contributes to restricted movement, as the smooth surface is replaced by roughened bone, creating friction and pain that discourages movement.
Another degenerative cause is inflammatory arthritis, such as Rheumatoid Arthritis (RA), which involves chronic inflammation of the joint lining (synovium). This persistent inflammation leads to joint damage, cartilage erosion, and a stiffening of the joint capsule and surrounding soft tissues. The sustained inflammation and resulting scar tissue formation create a stiff, contracted joint capsule, making the act of bending the knee increasingly difficult over time.
Space-Occupying Issues
A distinct set of conditions that limit knee flexion involves an increase in volume or the presence of abnormal tissue within the joint space. Joint effusion, commonly referred to as “water on the knee,” is the accumulation of excess fluid within the joint capsule. This fluid, which can be blood, inflammatory fluid, or excessive synovial fluid, increases the internal pressure of the knee.
The pressurized fluid results in stiffness and reduced range of motion. In cases of significant effusion, the knee will often rest in a partially flexed position, typically between 15 and 25 degrees, because this position minimizes intra-articular pressure. The presence of effusion is a symptom, and the underlying cause can range from acute injury to chronic arthritis or infection.
A more permanent space-occupying issue is arthrofibrosis, characterized by the excessive formation of scar tissue, or adhesions, within the joint. This condition often develops following major surgery, such as an ACL reconstruction or total knee replacement, or after significant trauma. The dense, non-elastic scar tissue physically tethers and contracts the joint capsule and surrounding soft tissues, preventing the knee from moving freely and severely restricting the ability to achieve full flexion.
When Medical Intervention Is Necessary
While minor stiffness may resolve with rest and conservative measures, certain symptoms signal the need for prompt medical evaluation. A sudden, complete inability to move the knee, known as true mechanical locking, suggests a displaced meniscal fragment or loose body that requires immediate attention. Similarly, any inability to bear weight on the leg following an injury is a significant warning sign that the integrity of the joint structure may be compromised.
Severe swelling that develops rapidly often indicates bleeding within the joint, which can be associated with a serious ligament tear or fracture. Other red flag symptoms include intense redness, warmth, and fever accompanying the limited motion, suggesting a joint infection that requires urgent treatment to prevent permanent damage. A physician will typically begin with a thorough physical examination and may order imaging studies, such as X-rays to check for fractures or osteophytes, or an MRI scan to visualize soft tissues like ligaments and menisci.