Septic arthritis is acute onset of monoarticular inflammation of a joint due to an infectious etiology. It is usually bacterial but can be viral or fungal. Septic arthritis causes significant morbidity and mortality and requires prompt diagnosis and treatment. Risk factors include age older than 80 years, smoking, comorbid conditions (eg, diabetes, rheumatoid arthritis, skin infection, HIV infection, osteoarthritis), and other factors (eg, recent joint surgery, joint prosthesis, previous intra-articular injection). The clinical presentation of septic arthritis can overlap with those of many other joint conditions, which can make diagnosis challenging. Poor functional outcomes, such as amputation, arthrodesis, prosthetic surgery, and severe functional deterioration, occur in approximately 24% to 33% of patients with septic arthritis. Due to the significant sequelae associated with septic arthritis, it is critical for physicians to maintain a high index of suspicion for this condition. Management involves a combination of medical and surgical treatments tailored to infection severity, causative pathogens, and overall patient condition. Medical treatment is not inferior to surgical treatment. However, 30% of patients with septic arthritis ultimately require surgical treatment. The 90-day mortality rate of knee septic arthritis is 7% in patients 79 years and younger and from 22% to 69% in patients older than 79 years.

Case 3. OL is an 80-year-old patient with a history of diet- and exercise-controlled type 2 diabetes and hypertension. She presents to the office with worsening right knee pain and swelling that started 4 days ago. OL says the pain is different from that of previous flare-ups of knee osteoarthritis. She has received four previous intra-articular corticosteroid injections, the last of which was 5 months ago. Since yesterday, OL has noticed redness in her knee, difficulty bearing weight, and no similar symptoms in other joints.