The knee joint is the largest and most complex joint in the human body, functioning primarily as a modified hinge. Its function is centered on two main actions: flexion, or bending, and extension, or straightening. The inability to fully perform either of these movements is known as restricted range of motion, a common symptom that interferes with daily activities like walking or climbing stairs. Understanding whether the restriction is a natural limit or a physical obstruction is the first step in addressing the issue. Restriction can manifest suddenly after an injury or develop gradually.
How the Knee Joint Naturally Stops
The human knee is designed with built-in mechanisms that prevent it from bending backward, a movement known as hyperextension. Normal knee movement involves straightening the leg to an alignment of zero degrees, with some individuals naturally capable of a few degrees of hyperextension. This protective restriction is managed by a combination of bony architecture and strong soft tissues. The primary limitation to hyperextension is provided by the posterior joint capsule. Deep to this, the oblique popliteal ligament acts as a static checkrein, tightening as the joint nears full extension. The cruciate ligaments, particularly the anterior cruciate ligament (ACL), also become taut in terminal extension, stabilizing the joint. These structures work together to stop the joint before the bones collide or the joint becomes unstable. This anatomical arrangement ensures stability when the leg is straight, allowing for efficient standing and walking.
Acute Injuries That Cause Sudden Restriction
When the knee suddenly refuses to move or gets stuck, it is often due to an acute injury causing a physical obstruction. This is referred to as “true mechanical locking,” signifying that a piece of tissue is jamming the joint mechanism. The most frequent cause of true locking is a meniscal tear, particularly a large flap tear where the torn cartilage flips into the joint space. The displaced cartilage acts like a doorstop, preventing the smooth gliding motion required for full extension or flexion. Another cause of mechanical locking is a loose body, a fragment of bone or cartilage floating freely within the joint fluid. These fragments can lodge between the joint surfaces, causing an immediate block to movement.
A different type of sudden restriction is “pseudo locking” or guarded restriction, which occurs after acute trauma like a severe ligament tear or fracture. In this scenario, nothing is physically blocking the joint, but intense pain and swelling cause the surrounding muscles to spasm and contract involuntarily. This muscle guarding is the body’s protective mechanism to immobilize the injured area. Acute ligamentous injuries, such as a tear of the ACL or posterior cruciate ligament (PCL), often lead to internal bleeding and swelling that triggers this guarded limitation of motion.
Chronic Conditions Leading to Lost Range of Motion
A gradual loss of the ability to fully bend or straighten the knee indicates a chronic condition resulting from long-term changes within the joint structure. One common cause is arthrofibrosis, characterized by the excessive formation of dense, internal scar tissue following surgery or injury. This scar tissue acts like a tough internal tether, shortening the soft tissues and preventing the joint from achieving its full range of motion, resulting in a flexion contracture. Progressive joint degeneration, such as arthritis, also leads to chronic restriction. As the protective cartilage wears away, the body may react by forming osteophytes, or bone spurs, along the edges of the bones. These bony growths can become large enough to create a physical block, impeding smooth joint movement and limiting both extension and flexion.
Prolonged immobilization, such as after a severe fracture or surgery, contributes to chronic stiffness by allowing the joint capsule and surrounding soft tissues to shorten. Even if the original injury heals, this shortened tissue can permanently limit the range of motion. Treating these chronic restrictions requires addressing the underlying tissue stiffness.
Steps for Diagnosis and Recovery
If you experience sudden locking or a persistent inability to fully move your knee, seeking medical attention is important to identify the cause. The diagnostic process begins with a physical examination to assess the quality of the restriction, determining if it is a hard, mechanical block or a soft, pain-limited restriction. A complete medical history, including the mechanism of injury and the timeline of symptoms, provides clues about the tissues involved.
Imaging techniques are used to confirm the diagnosis and visualize the internal joint structures. Standard X-rays are useful for identifying bony pathology, such as fractures, bone spurs, or calcified loose bodies. Magnetic resonance imaging (MRI) is the preferred method for evaluating soft tissues, providing detailed images of the menisci, ligaments, and scar tissue.
Treatment is tailored to the specific cause of the restricted motion. For true mechanical locking caused by a loose body or a displaced meniscal flap, arthroscopic surgery is often required to remove the obstruction and restore joint mobility. Conversely, a pain-induced guarded restriction or chronic stiffness is managed with conservative care, including rest, anti-inflammatory medication, and physical therapy. Physical therapy focuses on targeted stretching and strengthening exercises to regain the lost range of motion and improve muscle control.