Septic Arthritis of the Knee

Introduction

Septic arthritis is an infection inside a joint, such as the knee. It happens when bacteria enter the joint space and multiply, causing inflammation. The most common bacteria that cause septic arthritis are Staphylococcus aureus, Streptococcus species, and Neisseria gonorrhoeae. When the knee joint becomes infected, the body’s immune system responds by sending white blood cells to fight the bacteria. This leads to swelling, pain, and sometimes fever. Septic arthritis of the knee is a serious condition and requires urgent medical attention.

Symptoms of Septic Arthritis Affecting the Knee Joint

Some people with septic arthritis of the knee develop symptoms gradually over many days. However, many people will experience sudden and severe symptoms in the knee. These include swelling, redness, warmth, and intense pain in the knee joint. It usually affects only one knee. Patients often have difficulty moving the joint or putting weight on the leg. Some people may also have fever, chills, or feel generally unwell. In severe cases, the infection can spread to the bloodstream, leading to sepsis, which is a life-threatening condition.

Risk Factors for Developing Septic Arthritis

Some people are at higher risk of developing septic arthritis. Risk factors include:

• A weakened immune system (from conditions like diabetes, HIV, or sickle cell disease)
• Pre-existing joint problems such as rheumatoid arthritis or osteoarthritis
• A recent joint injury or surgery
• Use of medications that suppress the immune system, such as corticosteroids
• Skin infections or poor skin health, which may allow bacteria to enter the bloodstream

Diagnosing Septic Arthritis of the Knee

To diagnose septic arthritis, a doctor will begin with questions about symptoms, medical history, and possible risk factors. A physical exam is performed to check for swelling, tenderness, warmth, and reduced movement of the knee. The most important test is arthrocentesis, where a needle is inserted into the knee joint to draw out synovial fluid (the fluid that lubricates the joint). This fluid is tested for white blood cells, bacteria, and sometimes crystals. Blood tests may also be ordered to check for signs of infection. Imaging studies, such as X-rays, can help rule out other causes of knee pain or show damage to the joint, while MRI or ultrasound may show swelling and fluid buildup.

Treatment of Septic Arthritis of the Knee

Treatment for septic arthritis must start quickly to prevent joint damage. The main goals are to eliminate the infection, relieve symptoms, and protect the knee from long-term injury. Key elements of treatment include:

• Antibiotics: These are started right away, often through an intravenous (IV) line, to directly fight the infection. Once the bacteria type is known, the antibiotic may be adjusted to target it more specifically. Treatment usually continues for several weeks, sometimes starting with IV antibiotics in the hospital and finishing with oral antibiotics at home.
• Joint drainage: Removing infected fluid from the knee helps reduce pressure, pain, and bacterial load. This may be done by repeated needle aspiration, by placing a drainage tube, or during surgery.
• Pain and swelling control: Rest, elevation, and medications like acetaminophen or NSAIDs (if safe) can help manage discomfort.
• Physical therapy: Once the infection is under control, gentle movement and strengthening exercises help restore mobility and prevent stiffness.

When Surgery Is Needed

Surgery may be required if the infection is severe, if the fluid cannot be adequately drained with a needle, or if the infection does not improve with antibiotics alone. Surgical options include arthroscopic (minimally invasive) washout of the knee joint or open surgery in more advanced cases. The goal of surgery is to clean out infected material, relieve pressure, and prevent permanent damage to the cartilage and joint structures.

Prognosis

With prompt treatment, most people with septic arthritis of the knee recover well. However, if treatment is delayed, the infection can destroy cartilage and lead to long-term arthritis, stiffness, or chronic pain. In rare cases, the infection spreads, leading to sepsis, which can be life-threatening. Early diagnosis and aggressive treatment are the keys to protecting knee function and preventing complications.

Edited by Amari Eubanks and Stephen Pinney MD, September 20th 2025.