Overview
Septic arthritis is an infection inside a joint space, most commonly involving the synovial lining of the knee. The infection is usually caused by bacteria that enter the joint through the bloodstream, a nearby wound, surgery, or an injury. Because the immune system's response inflames and damages joint tissues, septic arthritis can rapidly lead to severe pain, swelling, and loss of motion. Prompt diagnosis and antibiotics are essential to protect the joint and prevent long-term damage.
Symptoms
Symptoms often start suddenly with intense knee pain that makes it difficult to move the joint. The knee is typically swollen, warm, and tender, and the pain is usually much worse with any attempt to bear weight or range of motion. Many people also develop fever and chills, although not everyone will. As the infection progresses, stiffness and functional limitation increase quickly.
Causes
Septic arthritis occurs when infectious organisms enter the joint space and multiply within the synovial fluid. Bacteria such as Staphylococcus aureus are common, and the infection may also be due to other bacteria or, less commonly, fungi or organisms related to specific exposures. The inflammation triggered by the infection can damage cartilage and other joint structures if not treated promptly.
Risk Factors
Risk is higher if you have a weakened immune system, such as from diabetes, cancer, chronic kidney disease, or medications that suppress immunity. Recent joint procedures, injections, open wounds, skin infections, or penetrating trauma can allow germs to reach the joint. Having a pre-existing joint condition, using IV drugs, or having bloodstream infections also increases risk.
Prevention
You can reduce risk by promptly cleaning and covering wounds and by seeking early care for skin infections near a joint. Managing chronic conditions like diabetes and avoiding unnecessary delay in treating fevers or suspected infections can lower the chance that bacteria spread to a joint. If you have had recent joint surgery or a joint injection, follow medical instructions closely and report new severe joint pain, swelling, or fever right away.
How the Diagnosis Is Evaluated
Clinicians start with a careful history of onset, fever or chills, recent injuries, injections, surgeries, and any signs of infection elsewhere. A physical exam looks for marked joint tenderness, warmth, swelling, and very limited range of motion, often with pain out of proportion to mild injury. Testing typically includes bloodwork (such as inflammatory markers and white blood cell count) and imaging to assess the joint, but the key diagnostic step is analyzing synovial fluid obtained by joint aspiration for cell count, Gram stain, and culture. Not every test is required for every patient, but joint fluid analysis is central when septic arthritis is suspected.
Nonsurgical Treatment Options
Treatment usually begins with prompt antibiotics to control the infection and is adjusted based on culture results when available. Joint aspiration may be repeated to help reduce pressure and confirm the organism, and it can also relieve symptoms. Pain and inflammation are managed with appropriate medications and strict activity modification to protect the joint while it heals. After infection begins to improve, guided physical therapy or range-of-motion exercises may help restore mobility and prevent stiffness, typically once it is safe and infection is adequately controlled. Corticosteroid injections are generally avoided during active septic arthritis because they can worsen infection risk.
When to Seek Medical Attention
Seek urgent medical attention if you have sudden severe knee pain with swelling and warmth, especially if you also have fever, chills, or feel very unwell. Go to the emergency department promptly if you cannot bear weight, if the joint becomes rapidly more painful over hours, or if you have significant redness spreading around the joint. Immediate evaluation is also needed if you have immune suppression, a recent joint procedure or injection, or a known infection elsewhere. Delaying care can increase the risk of permanent joint damage.
Frequently Asked Questions
Septic arthritis usually causes sudden, intense pain with marked swelling, warmth, and often fever, and the knee movement becomes very painful quickly.
Doctors confirm it mainly by aspirating joint fluid and testing it for infection with cell counts and culture, along with blood tests and sometimes imaging.
Next Steps
If you suspect septic arthritis, arrange urgent medical evaluation today, especially with fever or a rapidly swollen, warm, painful knee. Bring details about symptom timing, any recent injury or injections, and any other current infections or immune-related conditions so clinicians can test promptly.