Sudden knee pain often suggests an injury, but it can also arise from various non-traumatic conditions. These include inflammatory processes within the joint, mechanical issues, or even infections. Understanding these different origins is important for proper diagnosis and management.
Inflammatory Joint Conditions
Inflammatory conditions can cause sudden, severe knee pain without any preceding injury. Gout results from uric acid crystal accumulation within a joint, leading to intense pain, redness, swelling, and warmth. While commonly affecting the big toe, it can also present in the knee. Pseudogout, caused by calcium pyrophosphate crystals, presents similarly in its acute presentation.
Autoimmune conditions also contribute to sudden knee pain. A flare-up of rheumatoid arthritis (RA), an autoimmune disease, can cause acute, intense knee pain accompanied by swelling and stiffness. While RA typically affects multiple joints symmetrically, a sudden flare can localize in the knee. Psoriatic arthritis, an inflammatory arthritis linked to the skin condition psoriasis, can similarly cause sudden joint pain and swelling during an acute episode.
Non-Traumatic Mechanical Issues
Mechanical problems within the knee can also lead to sudden pain without direct injury. Bursitis is inflammation of bursae, small fluid-filled sacs that cushion the joints. When these bursae around the knee become irritated, often from repetitive motion or prolonged kneeling, they can cause sudden pain and swelling.
Tendinitis is inflammation of tendons, the strong fibrous cords that connect muscle to bone. Tendons around the knee, like the patellar or quadriceps tendons, can develop acute pain due to overuse or a sudden increase in physical activity. A Baker’s cyst, or popliteal cyst, is a fluid-filled swelling behind the knee. While often associated with underlying knee problems, it can rupture or become acutely symptomatic, causing sudden pain and swelling in the back of the knee.
Degenerative meniscal tears can also cause sudden knee pain without distinct injury. The menisci are C-shaped cartilage pieces that act as shock absorbers in the knee. Cumulative wear and tear can weaken this cartilage, making it susceptible to a sudden tear from a simple twisting motion or squatting, leading to acute pain, locking, or clicking sensations.
Infections Affecting the Joint
Septic arthritis, an infection within the knee joint, is a serious cause of sudden knee pain that requires immediate medical attention. This condition occurs when bacteria, viruses, or fungi infect the joint fluid and surrounding tissues, with Staphylococcus aureus being a common bacterial culprit. Onset is rapid, characterized by severe pain, significant swelling, redness, and warmth over the joint, often accompanied by systemic symptoms like fever and chills.
Infections can enter the joint through various pathways, including the bloodstream from another site of infection, direct entry from a penetrating injury or surgery, or even from an injection into the joint. Prompt diagnosis and treatment are essential to prevent rapid joint destruction and the spread of infection throughout the body.
Pain Originating Elsewhere
Sometimes, pain felt in the knee is referred pain from other parts of the body. This occurs when nerve pathways transmit pain signals from a different source to the knee. For example, issues in the hip joint, such as hip arthritis, or nerve compression in the lower back, like sciatica, can manifest as knee pain. An altered gait adopted to compensate for hip or back discomfort can also place additional stress on the knee.