Infectious arthritis or septic arthritis is the invasion of a joint by bacteria. It is a serious condition which if not treated properly and rapidly leads to the destruction of the joint. The cartilage lining of the joint is destroyed rapidly. The most common way in which bacteria enter a joint is via the circulation. Bacteria, which are in circulation from a septic focus, can seed a joint. They can also invade a joint from an adjacent area of sepsis like bony infection, infected implants and after an operation on a joint. They multiply in numbers rapidly and produce swelling, pain and inability to move the joint. A total knee replacement if not done under ideal theatre conditions can cause infection.

Symptoms
-Pain
-Redness
-Swelling
-Elevation of temperature wound problems after a knee replacement, pain.

Causes
-lowered immunity
-Diabetes
-Rheumatoid arthritis
-Septic foci like ingrowing toe nail, bunions, prostatic problems,dental sepsis.

Risks-
Any cause, which lowers the capacity of the joint to ward off an infection, leads to septic arthritis. These are diabetes mellitus, rheumatoid arthritis, H I V infection, immunosuppressive drugs, liver disease, malnutrition, poor health and old age. Total knee replacement done under non ideal theatre conditions like laminar airflow can result in infections. In India there are not many centres equipped with laminar air flow theatres and this invariably leads to early infections. Theatre discipline, preoperative antibiotics,antibiotic impregnated bone cement are mandatory and under these ideal conditions the incidence of infections is about 1%. In their absence the incidence of infection is significantly high.Infection can present with painful loosening of the knee joint

Treatment
Since the condition is a very serious one, an orthopedic surgeon should be consulted immediately. The surgeon may draw a sample of blood for culturing the organism and other tests. Then a sample of the pus from the knee joint is aspirated and sent for culture. The appropriate antibiotic should be given intravenously. This is best given in a hospital. The pus is let out, either by an open operation or arthroscopically. An infected total knee can lead to loosening and bone loss if not treated aggresively by debridment. This may need a two stage revision knee replacement.

Postoperative management
After arthroscopic debridement, postoperatively continuous passive mobilization exercises are started to regain the range of movement. If seen and trated early there is a good prognosis otherwise damage to the cartilage of the knee invariably results.