The knee joint connects the femur (thigh bone) to the tibia (shin bone) and is designed primarily for flexion (bending) and extension (straightening). When the ability to smoothly complete this range of motion is impaired, it signals that a mechanical or biological process is interfering with the joint’s function. Restricted knee movement is a common issue presenting as pain, stiffness, or a physical inability to bend the leg. Understanding the underlying causes is the first step toward effective treatment, and these causes generally fall into distinct categories of acute injury, chronic inflammation, or physical obstruction. Addressing the root cause is essential for restoring full mobility.
Sudden Restriction from Acute Injury
A sudden, forceful inability to bend the knee often follows an acute traumatic event, such as a sports injury or a fall. This immediate restriction results from structural damage combined with the body’s rapid inflammatory response.
Ligament tears, particularly to the anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL), compromise the joint’s stability, leading to an immediate sense of giving way. Instability prevents the coordinated rolling and gliding motion necessary for full flexion, as the muscles reflexively guard the joint against further displacement.
The trauma also frequently causes hemarthrosis, which is rapid bleeding into the joint space. This accumulation of blood physically occupies the limited space within the joint capsule. The sheer volume of fluid acts as a hydraulic buffer, making full knee flexion excruciating or physically impossible.
Certain meniscal injuries can cause immediate, painful restriction without a true mechanical lock. Intense pain and instability upon weight-bearing trigger muscle spasms, known as pseudo-locking, which inhibit movement as a protective mechanism. This acute pain overrides normal neuromuscular control, preventing the person from voluntarily moving the knee through its full range.
Restricted Movement Due to Inflammation and Pain
When the inability to bend the knee develops gradually, the cause is often chronic inflammation or degenerative changes within the joint.
Osteoarthritis (OA) is a common degenerative condition where the articular cartilage, the smooth tissue covering the ends of the bones, wears down over time. As this cartilage erodes, the joint space narrows, and bone spurs (osteophytes) can form around the joint margins. These structural changes physically impede the smooth articulation of the femur and tibia, leading to stiffness and a progressive loss of flexion range.
Chronic inflammatory conditions, such as Rheumatoid Arthritis (RA), restrict movement through an autoimmune mechanism. RA causes the immune system to attack the synovium, the lining of the joint capsule, leading to inflammation and thickening (synovitis). This chronic swelling and thickening of the soft tissues reduces the overall volume of the joint cavity, limiting the ability of the knee to bend fully. Over time, sustained inflammation can also damage the cartilage and bone, further contributing to permanent loss of mobility.
Localized Inflammation
Localized inflammation of fluid-filled sacs can also directly restrict movement. Bursitis is the inflammation of a bursa, such as the prepatellar or infrapatellar bursa, causing significant swelling over the kneecap. When the bursa becomes distended with fluid, it creates a physical mass that prevents the joint from achieving deep flexion. Conditions like gout, caused by the buildup of uric acid crystals, also lead to intense, acute inflammatory episodes that severely limit movement.
Physical Obstruction Inside the Joint
A specific cause of restricted movement is a true mechanical obstruction, often described as the knee “locking” or encountering a hard stop. This phenomenon occurs when a piece of material physically wedges itself between the articulating surfaces of the femur and tibia.
The most frequent culprit is a displaced meniscal fragment, often resulting from a serious injury such as a bucket-handle tear. In this injury, a large flap of cartilage breaks free but remains attached, flipping into the joint space. This displaced fragment acts like a doorstop, preventing the smooth rolling and gliding of the joint surfaces and causing a firm stop at a specific angle of flexion or extension.
Similarly, a loose body—a fragment of bone or cartilage that has broken off—can migrate through the joint and become entrapped. These loose bodies can originate from trauma or chronic conditions like osteoarthritis or osteochondritis dissecans. The presence of such an obstruction results in a sudden, reproducible inability to move past a certain point, often requiring a physical maneuver or surgical removal to resolve.
When to Seek Medical Help
The decision to seek medical attention depends primarily on the severity and nature of the symptoms experienced. Immediate emergency care is warranted for “red flag” symptoms that suggest a severe injury or acute infection. These include:
- A visible deformity of the knee joint.
- An inability to bear any weight on the leg.
- Numbness below the knee.
- Signs of infection, such as a high fever, extreme redness, or heat localized to the joint.
For less severe issues, schedule an appointment if the inability to bend the knee does not improve within a few days of at-home care, or if pain and stiffness interfere with daily activities or sleep. Initial self-care for minor restrictions can follow the RICE protocol: Rest, Ice, Compression, and Elevation.
If the restriction was caused by a traumatic event, or if you experience recurrent episodes of the knee catching or locking, consult an orthopedic specialist. Prompt diagnosis and treatment are crucial for preserving long-term joint function and preventing minor injuries from developing into chronic mobility issues. Early intervention often leads to better outcomes.