A knee that causes discomfort when the leg is both fully extended (straight) and fully flexed (bent) suggests a widespread problem within the joint. This pattern of pain across the full range of motion differs from issues that might only hurt in one position, indicating that multiple structures are likely involved or the entire joint is experiencing irritation. The knee routinely bears loads several times the body’s weight during activities like running or climbing stairs. This constant, high-level stress explains why knee pain is extremely common and why symptoms that persist across all movements warrant attention.
Patellofemoral Joint Dysfunction
The most common cause of pain that spans the entire range of knee motion involves the kneecap, or patella, and the groove it tracks in on the thigh bone (femur). This area is known as the patellofemoral joint, and dysfunction here is often broadly termed Patellofemoral Pain Syndrome (PFPS). The patella acts like a pulley, and its movement relies on precise tracking within the trochlear groove of the femur as the knee bends and straightens.
When the knee is bent, the patella is subjected to significant compressive forces, which increase with deeper flexion, causing pain when sitting for long periods or squatting. Conversely, as the knee moves toward full extension, the patella engages the groove, and any misalignment or poor tracking creates friction and abrasion, resulting in pain even when the leg is nearly straight. This poor tracking can be caused by muscle imbalances, particularly weakness in the quadriceps or hip muscles, which fail to guide the patella properly.
If this poor tracking continues, the cartilage on the underside of the kneecap may soften and wear down, a condition called Chondromalacia Patellae. While the cartilage itself does not have nerve endings, its breakdown causes inflammation in the underlying bone and the joint lining. This leads to generalized discomfort that is aggravated by the continuous sliding motion in both bent and straight positions.
Internal Structural Damage
Pain that is present regardless of whether the knee is straight or bent can signal a mechanical failure within the joint capsule itself. The menisci are C-shaped pieces of cartilage that act as shock absorbers and stabilizers between the shinbone and thighbone. A displaced piece of a torn meniscus can become mechanically trapped, causing constant irritation, clicking, or a sensation of the knee locking up, which prevents smooth movement through extension and flexion.
Osteoarthritis (OA) occurs when the protective articular cartilage is significantly reduced or entirely lost, leading to bone-on-bone friction. This loss of cushioning means the joint is painful whenever it is loaded or moved, with discomfort often worsening over time and becoming persistent even at rest. The stiffness and swelling associated with OA make it difficult to achieve full range of motion, hurting both when trying to straighten the leg and when trying to bend it fully.
In cases of significant ligament instability, such as an anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL) injury, the joint loses its primary mechanical control. This chronic looseness allows the bones to shift abnormally during simple movements, leading to secondary damage and generalized joint irritation. The abnormal movement patterns create chronic inflammation and pain throughout the joint, causing discomfort in virtually any position as the body attempts to stabilize the unstable structure.
Soft Tissue Inflammation and Systemic Causes
Inflammation of the tissues surrounding the knee joint can lead to pain that is constant and worsens with any movement, whether bending or straightening. Tendinopathy, such as patellar or quadriceps tendinopathy, involves the tendons connecting the kneecap to the shinbone or thigh muscle, respectively. Tendinopathy can cause pain that is tender to the touch and aggravated by the tension created when the knee is moved in either direction.
Bursitis, particularly prepatellar bursitis, involves the inflammation of the small, fluid-filled sacs that cushion the front of the kneecap. An inflamed bursa will cause swelling and pain directly over the front of the knee. This pain is triggered when the joint is compressed, such as when kneeling or bending, and also when the skin is stretched over the swollen bursa during full extension.
Systemic conditions like Inflammatory Arthritis, which includes Rheumatoid Arthritis, affect the entire joint lining (synovium) and are not caused by mechanical wear. This autoimmune inflammation leads to generalized joint swelling, warmth, and stiffness that is symmetrical, often affecting both knees equally. The pain from this widespread inflammatory process is typically worse after periods of rest, such as in the morning, and affects all movements, making both bending and straightening the knee difficult.
Necessary Medical Evaluation
Knee pain persisting across the full range of motion requires a professional medical evaluation to determine the underlying cause and guide appropriate management. A healthcare provider will perform a thorough physical examination to test stability, check for mechanical symptoms like locking or catching, and assess the location of tenderness. This hands-on assessment helps distinguish between issues related to the kneecap, ligaments, or surrounding soft tissues.
If a serious injury or advanced disease is suspected, imaging studies will be utilized. X-rays can reveal joint space narrowing characteristic of osteoarthritis or show bone spurs, while an MRI provides detailed images of soft tissues, including the menisci, ligaments, and cartilage.
Seek immediate medical attention if the knee pain is accompanied by certain red flags:
- A visible deformity
- A sudden inability to bear any weight
- Intense swelling accompanied by fever or chills
- A mechanical symptom like the knee locking completely