Germs (mainly bacteria) in the joint, which is real bad. It can cause permanent joint destruction, and can spread to the blood, causing general Sepsis & possibly death. Patients almost always have fever. The joint is tender to even relaxed movement or jiggling, and it’s usually at least a little red or swollen (except the hip, which is too deep to notice).
Bacteria can get in the blood directly (from injuries or surgery). They can also be spread by the blood from elsewhere. This can happen to anyone, but is more likely in patients with joints damaged by other types of arthritis.
If there’s any chance of Septic Arthritis, we refer patients right to an ER. There, they can “tap” the joint, removing fluid to examine under a microscope. A very high white blood cell count in the joint fluid suggests the diagnosis. A culture is sent to identify the exact bacteria (takes 2-3 days), and antibiotics are begun in the meantime.
A special type of septic arthritis is caused by Gonorrhea. It usually affects the knee, most commonly in women, and often occurs during or right after a menstrual period, when it’s easier for the bacteria to enter the blood & spread. The germ may be hard to find in the joint, easier to find in the vagina. Frequently, the woman had no symptoms ahead of time. I knew a mother whose teenage daughter had a red, swollen knee; they began treatment without knowing for sure, & the mom got very upset when she found out the reason (& I could never understand why).