Septic arthritis. X-ray of a human pelvis showing deterioration of the hip (left on image) due to septic arthritis. The head and shaft of the femur (thigh bone) have a rough and motley appearance showing the extent of the degeneration of the bone. Septic arthritis, also known as pyogenic arthrttis or infective arthritis, is caused by invasion of bacteria into a joint where they cause an infection. The infected joint becomes red, painful and swollen. Treatment includes painkillers and anti-inflammatory drugs, physiotherapy and joint replacement surgery.