Lyme Disease Arthritis of the Knee

Introduction

Lyme disease is an infection caused by bacteria spread through the bite of an infected blacklegged tick. While it often begins with flu-like symptoms or a skin rash, it can also affect the joints, especially the knee. When Lyme disease causes joint problems, this is called Lyme arthritis. The knee may become swollen, painful, and stiff, sometimes making it hard to walk or bend the leg.

Symptoms of Lyme Disease Arthritis of the Knee

When Lyme disease affects the knee, symptoms can range from mild discomfort to significant swelling. The knee may look puffy and feel warm, and the swelling can come and go over weeks or months. Some people experience pain and stiffness without much swelling, while others notice large amounts of fluid building up in the joint. The condition may resemble other forms of knee arthritis, but the cause is different—it comes from the body’s immune reaction to the bacteria that triggered Lyme disease.

How is Lyme Disease Arthritis of the Knee Diagnosed?

To diagnose Lyme arthritis in the knee, a doctor will ask about recent tick bites, outdoor activity in areas where Lyme disease is common, and past symptoms such as rashes or fevers. A physical exam will focus on the swollen knee, and fluid may be drawn from the joint to test for signs of infection. Blood tests can help confirm the presence of antibodies to the Lyme bacteria, though results can sometimes be tricky to interpret. Imaging, like X-rays or MRI, is not always needed but may help rule out other causes of knee pain.

Treatment of Lyme Disease Arthritis of the Knee

Treatment for Lyme arthritis usually starts with antibiotics, taken for several weeks to clear the infection. Most patients improve significantly with this treatment. If swelling or pain persists after antibiotics, a second course of medication may be recommended. In rare cases, surgery is needed to remove inflamed tissue from the joint (synovectomy). The prognosis is generally good, especially when treatment begins early. However, some patients may continue to have mild joint symptoms even after the infection has cleared, due to lingering inflammation.

Edited by Stephen Pinney MD, September 19th 2025.