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Joints of the skeleton are cushioned and lubricated by specialized system that prevents the grinding of bone on bone, which would otherwise rapidly destroy the ability of the joint to bend with supple ease. This system consists of joint cartilage which pads the ends of the long bones, and sacs (synovia, sin-OH-vee-uh) of clear fluid (synovial fluid) which further cushion the joint and make movement of the surfaces over one another essentially frictionless.
Septic arthritis means infection of the joint space with bacterial germs. When this happens, the joint rapidly fills with pus cells. These cells release killing substances directed against the bacteria, substances which can cause damage to the tissues of the joint. Left untreated for too long, bacterial infection can literally destroy the joint.
The most common germs involved in such joint infections are Staphylococcus aureus, Streptococcus pneumoniae, and Group A streptococcus. Other organisms include E. coli, Salmonella, and other so-called "gram negative" organisms. Hemophilus influenzae type B (HiB) once caused joint infections, but this disease has disappeared with universal HiB vaccination.
The bacteria usually reach the joint space through the blood stream from elsewhere in the body - the gut, an infection, or wound (called "hematogenous spread"). Sometimes septic arthritis arises secondarily from infection of the end of a long bone next to the joint. Blood flow is apparently most sluggish in this part of the bone marrow, and germs are more likely to be deposited here. From this seeding of infection, pus can burrow through the end of the bone and cartilage pad, and erupt into the joint space.
The result is swelling of the joint with pus, which usually causes excruciating pain upon movement of the joint through bending (flexion) or straightening (extension). The joint is red and warm to the touch. There may be systemic signs such as fever or chills.
The child may either limp or simply refuse to bear any weight on the affected extremity. Any joint can be affected, but the knee, hip and elbow joints are most often involved.
After physical examination, diagnostic tests would generally include
Septic arthritis is treated with intravenous antibiotics. The infection responds more quickly than infection of the bone, so the duration of treatment need not be as long. Initial treatment is in the hospital; home intravenous therapy may sometimes be possible.