The inability to fully straighten the knee, known as a loss of knee extension, is a common symptom. Full knee extension typically means the leg can straighten to 0 degrees, aligning perfectly. This limitation can significantly affect daily activities, from walking and climbing stairs to simply standing comfortably. Various factors, ranging from acute injuries to chronic conditions, can contribute to this restricted movement.
Inflammation and Swelling
Inflammation and fluid accumulation within or around the knee joint can impede full knee extension. When the joint experiences injury or disease, the body’s response involves increased fluid production, leading to swelling. This excess fluid creates pressure inside the joint capsule, limiting the space for the bones to move into a fully straightened position.
Pain receptors within the joint can also be triggered by this pressure, making attempts to straighten the knee painful and contributing to a protective guarding response that restricts movement. Conditions such as osteoarthritis, where cartilage wears down and bone spurs may form, can cause chronic inflammation and swelling that limit extension. Other inflammatory conditions, including bursitis (inflammation of fluid-filled sacs) or tendinitis (inflammation of tendons), can also lead to swelling around the knee that restricts movement. Acute injuries like sprains or ligament tears often result in immediate swelling, making it difficult to straighten the leg.
Mechanical Obstructions
Sometimes, the inability to straighten the knee stems from a physical blockage within the joint itself. This is often referred to as a “true locked knee” because a structural component prevents full extension. A common cause is a torn meniscus, where a fragment of the C-shaped cartilage gets displaced and caught between the thigh bone (femur) and shin bone (tibia). This displaced fragment acts as a wedge, blocking the joint’s movement.
Loose bodies, which are small pieces of bone or cartilage floating freely within the joint fluid, can also cause this mechanical impediment. These fragments can get lodged in the joint space, preventing full extension and often causing a popping or catching sensation. Ligamentous injuries, such as a tear to the anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL), can lead to instability or altered joint mechanics that hinder the knee from straightening.
Stiffness and Tissue Tightness
Beyond fluid accumulation or internal blockages, the knee’s inability to straighten can also arise from tightness in the surrounding tissues. Muscles like the hamstrings and calf muscles, located at the back of the thigh and lower leg, can become tight, pulling against extension. Prolonged immobility, such as after an injury or surgery, can contribute to this muscle shortening and stiffness.
Scar tissue formation, a natural part of the healing process, can become excessive, leading to a condition called arthrofibrosis. This dense, fibrous scar tissue can form within and around the joint, restricting its range of motion, including extension. Capsular tightness, a thickening or contracture of the joint capsule surrounding the knee, similarly limits movement. The joint capsule can lose its elasticity due to inflammation or prolonged disuse, making it difficult for the knee to reach its full extension.
When to Consult a Doctor
Consult a doctor for an inability to straighten the knee, especially if certain symptoms are present. If the problem arises suddenly, is accompanied by severe pain, or prevents you from bearing weight on the leg, seek immediate evaluation. A visible deformity, a distinct popping sound at the time of injury, or a persistent “locking” sensation where the knee gets stuck, warrant prompt medical assessment.
Consultation is also recommended if the issue persists or worsens over time, even without acute symptoms. Swelling, redness, or warmth around the knee, particularly if accompanied by fever, could indicate an infection or other serious condition. A diagnosis by a healthcare professional is necessary to determine the underlying cause and guide management.