A limited ability to fully bend the knee, known as restricted knee flexion, is a common orthopedic symptom signaling an underlying structural problem within the joint. This inability to move the knee through its full range can significantly impede daily activities like climbing stairs, squatting, or sitting comfortably. Understanding the specific cause of this restricted movement is the necessary first step toward restoring full mobility and preventing further joint damage. The restriction can stem from a sudden mechanical block, chronic disease progression, or inflammation.
Understanding Normal Knee Flexion and Range of Motion
The knee is a complex hinge joint formed by the thigh bone (femur), the shin bone (tibia), and the kneecap (patella). Smooth knee movement, particularly flexion, relies on the synchronized gliding of these bony surfaces, which are covered in articular cartilage. Ligaments and the joint capsule provide stability and contain the lubricating synovial fluid.
Normal knee flexion typically ranges from 135 to 150 degrees, allowing the heel to move close to the buttocks. Restriction occurs when this natural path is physically obstructed or when severe pain and swelling prevent the muscles from attempting full range of motion. Pain often triggers a protective reflex called muscle guarding, where surrounding muscles tighten to prevent movement. A true mechanical restriction means a physical object is blocking the joint from moving further.
Acute Injuries and Mechanical Blocks
Sudden inability to fully bend the knee often points to an acute injury involving a physical obstruction, resulting in the knee “locking.” A common cause of this mechanical block is a torn meniscus. The menisci are C-shaped pieces of cartilage that act as shock absorbers between the femur and tibia. A large meniscal tear, such as a “bucket-handle” tear, can displace a fragment of cartilage that gets wedged between the joint surfaces, preventing the knee from bending.
Loose bodies, which are fragments of bone or cartilage floating freely in the joint fluid, can also cause intermittent locking. These fragments can get caught between the femur and tibia as the knee attempts to flex, momentarily jamming the joint. Acute ligament injuries, such as a tear of the Anterior Cruciate Ligament (ACL) or Posterior Cruciate Ligament (PCL), typically restrict flexion due to severe pain and massive swelling (hemarthrosis) within the joint. This fluid buildup creates pressure that mechanically limits the space available for the bones to move.
Traumatic injuries resulting in patellar dislocation or a fracture can cause immediate and severe flexion restriction. When the kneecap is forced out of its groove, the joint mechanism is disrupted, making movement extremely painful and impossible until the patella is realigned. An acute fracture of the femur, tibia, or patella causes intense muscle spasm and instability, instantly stopping the patient from attempting to bend the knee. These acute mechanical issues require prompt evaluation to relieve the obstruction and prevent secondary damage.
Chronic Conditions and Progressive Stiffness
In contrast to acute injuries, some conditions cause a gradual, progressive loss of knee flexion over time, leading to stiffness. Osteoarthritis (OA) is a common chronic cause, characterized by the deterioration of the articular cartilage lining the joint surfaces. As this cartilage wears away, the bones rub together, and the body may respond by forming bone spurs (osteophytes) around the joint margins. These bony growths can physically impinge on the joint space, preventing the femur and tibia from achieving full flexion.
Arthrofibrosis refers to the excessive formation of scar tissue within the joint capsule, often developing after injury or surgery. This internal scarring causes the joint capsule and surrounding soft tissues to tighten and contract, tethering the joint and preventing a full range of motion. Inflammatory conditions like Rheumatoid Arthritis also cause chronic stiffness, involving inflammation that damages the joint lining and cartilage. Persistent inflammation and joint effusion (swelling) cause the tissues to thicken and contract, leading to a permanent reduction in flexibility.
Severe, chronic inflammation of the fluid-filled sacs (bursae) or tendons (tendinitis) surrounding the knee can also contribute to stiffness. While inflammation itself may not be a mechanical block, the persistent pain and tissue thickening around the joint can lead to muscle guarding. This reluctance to move causes the soft tissues to shorten, limiting the available range of motion. This sustained loss of movement creates a self-perpetuating cycle of pain and stiffness.
Diagnosis and Initial Management
Any sudden, severe, or persistent inability to fully bend the knee warrants a consultation with a healthcare provider, especially if accompanied by fever, severe swelling, or a feeling of complete locking. The diagnostic process begins with a detailed physical examination to assess the quality of the restriction and the overall range of motion. Imaging is typically required to visualize the internal structures of the knee.
Initial imaging often includes X-rays, which are useful for identifying bony causes such as fractures, bone spurs associated with osteoarthritis, or calcified loose bodies. If soft tissue damage is suspected, such as a meniscal tear, ligament injury, or non-calcified loose bodies, a Magnetic Resonance Imaging (MRI) scan provides a detailed view of the cartilage, ligaments, and tendons. In the acute phase, initial home management focuses on reducing pain and swelling using the R.I.C.E protocol: Rest, Ice, Compression, and Elevation.
Professional management typically begins with physical therapy to carefully and gradually restore the lost movement. Therapists may focus on regaining full extension first, followed by flexion exercises, to ensure a functional range of motion is achieved. If a definitive mechanical block is identified, such as a torn meniscus flap or a loose body, surgery (often minimally invasive arthroscopy) may be necessary to remove the obstruction and allow the joint to move freely again.