Knee pain when bending the knee to its fullest extent is a common complaint. Understanding the underlying reasons for this pain involves exploring the complex mechanics of the knee joint and recognizing various conditions that can affect its full range of motion.
The Mechanics of Knee Bending
The knee is a hinge joint, primarily allowing for bending (flexion) and straightening (extension) movements. This complex joint involves the interaction of three main bones: the femur (thigh bone), tibia (shin bone), and patella (kneecap). These bones are covered with articular cartilage, a smooth, slippery tissue that allows them to glide effortlessly against each other, reducing friction during movement.
Within the knee, C-shaped pads of cartilage known as menisci act as shock absorbers and help distribute weight evenly across the joint. There are two menisci in each knee: the medial meniscus on the inner side and the lateral meniscus on the outer side.
Ligaments, strong bands of tissue, connect the bones and provide stability; the major ones include the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL). Muscles like the quadriceps (front of the thigh) and hamstrings (back of the thigh) attach to the bones via tendons, facilitating movement and providing dynamic support.
Common Conditions Causing Pain with Full Knee Bend
Several conditions can lead to pain when the knee is bent all the way, each affecting different structures within the joint.
A frequent cause is a meniscus tear, often aggravated by deep knee flexion. It results from a twisting injury, especially with the foot planted, causing pain, swelling, stiffness, and a sensation of the knee catching or locking. Pain may be felt at the side or in the center of the knee, depending on the tear’s location.
Patellofemoral Pain Syndrome (PFPS), or “runner’s knee,” causes pain around or behind the kneecap. This pain often worsens with activities that repeatedly bend the knee, such as squatting, climbing stairs, or sitting for long periods with bent knees. PFPS can stem from overuse, kneecap misalignment, or muscle imbalances around the hip and thigh, irritating tissues beneath the kneecap. People with PFPS may also experience rubbing, grinding, or clicking sounds with knee movement.
Osteoarthritis, a degenerative joint disease, involves the gradual breakdown of articular cartilage within the knee. As cartilage wears away, bones can rub against each other, causing pain, stiffness, and reduced mobility, particularly with full flexion in later stages. This condition progresses slowly, with pain worsening over time, and can affect one or both knees. Swelling and inflammation may also be present as the joint degrades.
Plica syndrome occurs when a plica, a normal fold in the knee’s joint lining, becomes irritated or inflamed. This irritation results from trauma, overuse, or repetitive knee bending and straightening. Symptoms include pain and swelling, typically in the middle or inner part of the knee, and may involve clicking or popping sounds, or a catching sensation when bending the knee. Pain often worsens after prolonged bending or activities like squatting or climbing stairs.
Quadriceps or patellar tendinopathy, often called “jumper’s knee,” involves inflammation or degeneration of the tendons connecting the thigh muscles to the kneecap or the kneecap to the shin bone. While commonly associated with pain during activities like jumping or running, severe cases or specific movements can cause pain with full knee bend. Pain is usually localized to the front of the knee, just above or below the kneecap, and can be accompanied by stiffness or tenderness.
A Baker’s cyst, or popliteal cyst, is a fluid-filled sac that forms behind the knee. It usually arises from an underlying knee problem, such as arthritis or a meniscus tear, causing the knee to produce excess fluid. The cyst can cause a bulge, tightness, and pain behind the knee, often worsening with activity or when fully bending or straightening the knee. If large, it can mechanically limit full knee flexion.
When to Consult a Healthcare Professional
While minor knee pain may improve with rest, certain symptoms warrant medical evaluation. Consult a healthcare provider if your knee pain persists for more than a few days or significantly impacts your daily routine. Severe pain, inability to bear weight, or sudden, significant swelling signal a need for medical attention. Other “red flag” symptoms include the knee “giving way” or locking, a visible deformity, or pain after a traumatic injury with a “popping” sound. If you experience warmth, redness, tenderness around the knee, or a fever with knee pain, it could suggest an infection or another underlying illness.
Diagnosis and Management Approaches
When seeking medical attention for knee pain, a healthcare professional will begin with a detailed medical history and physical examination. During the exam, the doctor assesses the knee for swelling, tenderness, warmth, and visible bruising, evaluating its range of motion and stability. They may move your leg in different positions to identify specific pain points or restrictions.
To investigate the cause of pain, imaging tests may be ordered. X-rays are often the first step to check for bone fractures or signs of arthritis, though they do not show soft tissues. Magnetic Resonance Imaging (MRI) provides detailed images of soft tissues like ligaments, tendons, and cartilage, valuable for diagnosing meniscus tears or ligament injuries. In some cases, a CT scan or ultrasound might also be used.
Management approaches for knee pain often begin with non-surgical options. Rest, ice, compression, and elevation (RICE) help reduce pain and swelling, especially after an acute injury. Physical therapy is a common and effective treatment, focusing on exercises and stretches to strengthen knee-supporting muscles and improve flexibility. Medications, such as over-the-counter nonsteroidal anti-inflammatory drugs (NSAIDs) or prescribed pain relievers, manage pain and inflammation. Injections, including corticosteroids to reduce inflammation or hyaluronic acid for lubrication, may also be considered.
Surgical interventions, such as arthroscopy for meniscus repairs or joint replacement for severe osteoarthritis, are reserved for specific conditions when conservative treatments have not provided sufficient relief.