The sudden onset of severe knee pain without an apparent injury is a common, perplexing experience. This acute discomfort often leads people to suspect a torn ligament or a fracture, but the cause is frequently rooted in internal processes rather than external trauma. The knee is a complex structure involving bone, cartilage, fluid-filled sacs, and soft tissue, all susceptible to abrupt inflammatory, infectious, or mechanical disturbances. Understanding that internal disease, crystal formation, or structural failure can mimic the symptoms of an acute injury is the first step toward determining the appropriate course of action. These sudden flares represent a disruption of the joint’s internal equilibrium, causing pain, swelling, and loss of function.
Acute Inflammatory Joint Conditions
One intense form of sudden knee pain arises from the rapid deposit of microscopic crystals within the joint fluid, collectively known as crystalline arthropathies. The most well-known is gout, where a swift increase in uric acid levels leads to the formation of needle-shaped monosodium urate crystals. When these crystals precipitate into the joint space, they trigger a severe inflammatory response that causes intense pain, typically peaking within 12 hours.
A similar, though distinct, condition is pseudogout, formally known as calcium pyrophosphate deposition disease (CPPD). Instead of uric acid, this disorder involves the sudden release of calcium pyrophosphate dihydrate crystals into the joint fluid, often affecting the knee. Both gout and pseudogout attacks present with intense localized symptoms like redness, warmth, and pronounced swelling.
Other sudden, non-traumatic knee pain can be caused by acute flares of systemic inflammatory conditions, such as rheumatoid arthritis or seronegative arthropathies like psoriatic arthritis. These are autoimmune disorders where the immune system attacks the joint lining, or synovium. While these conditions are often chronic, a person may experience a sudden, isolated flare-up in a single knee joint that rapidly causes pain, stiffness, and noticeable swelling.
Infections and Localized Soft Tissue Issues
A serious cause of acute, non-traumatic knee pain is septic arthritis, an infection of the joint itself. This occurs when bacteria, most commonly Staphylococcus aureus, travel through the bloodstream from an infection elsewhere and settle in the joint space. The knee is the large joint most frequently affected because the joint lining has a limited ability to protect itself from infection.
Septic arthritis symptoms include rapid, intense pain, marked swelling, and warmth, often accompanied by a systemic fever and chills. The infection can quickly and permanently destroy the joint cartilage, sometimes within hours, making prompt medical intervention necessary.
A less severe, but acutely painful, issue is bursitis, the inflammation of a bursa, a small, fluid-filled sac that cushions tendons and bone. The prepatellar bursa, located directly over the kneecap, is a common site for acute bursitis, often caused by repetitive microtrauma, such as prolonged kneeling.
More concerning is septic bursitis, where the bursa becomes infected, usually through a small break in the overlying skin. This infectious form presents with significant localized redness and warmth, differentiating it from non-infectious inflammation. Another localized issue is the rupture of a popliteal, or Baker’s, cyst, a fluid collection behind the knee often secondary to underlying joint issues like arthritis. When this cyst ruptures, the synovial fluid leaks into the calf tissue, causing a sudden, sharp pain and swelling in the back of the knee and upper calf.
Sudden Internal Mechanical Failures
Mechanical issues within the knee can cause abrupt pain without trauma, often because pre-existing wear-and-tear reaches a breaking point. A common example is a degenerative meniscus tear, which occurs in older individuals due to the natural weakening and breakdown of the fibrocartilage over time. Unlike traumatic tears caused by a sudden twist, a degenerative tear may become symptomatic following a minor action, such as getting out of a chair or an awkward step.
Acute pain and mechanical symptoms occur when a small, pre-existing flap of the degenerative meniscus tears further or becomes suddenly displaced within the joint. This displacement can cause a sharp, catching sensation or a true locking of the joint, where the knee cannot be fully straightened. The sudden onset of symptoms is due to the mechanical obstruction of the joint surfaces rather than the initial, silent tearing process.
Another source of abrupt mechanical pain is the presence of loose bodies, which are fragments of bone or cartilage floating freely in the joint space. These fragments can originate from conditions like advanced osteoarthritis or osteochondritis dissecans, where a piece of bone separates from the main structure. While often asymptomatic, a loose body can migrate into the joint’s hinge mechanism, causing a sudden, sharp, and intense episode of pain and locking. The pain is caused by the fragment getting temporarily lodged between the moving surfaces of the femur and tibia.
When to Seek Immediate Medical Attention
Recognizing signs that indicate a potentially serious condition is important when experiencing sudden knee pain. Immediate medical attention is necessary if the pain is accompanied by a high fever or chills, as this combination suggests a systemic infection, such as septic arthritis. This condition requires urgent diagnosis and treatment to prevent rapid and irreversible joint damage.
A healthcare professional must be consulted promptly if you experience any of the following:
- Inability to bear weight on the affected leg, or if the knee collapses or buckles when attempting to stand.
- The knee is visibly deformed.
- Rapid and extreme swelling that feels hot and is accompanied by severe redness of the skin.
- The knee becomes truly locked, meaning you cannot straighten or bend the joint, which may indicate a displaced meniscal fragment or a loose body.