Septic arthritis, also called infectious arthritis, refers to any joint inflammation caused by a microbe - and usually it results from a bacterial infection of the joint.
All types of joints; fibrous, cartilaginous, and synovial joints can get infected. So let’s just use the synovial type as an example.
A synovial joint consists of a joint capsule which has an outer fibrous layer, and an inner synovial membrane filled with synovial fluid.
The synovial fluid is a clear viscous fluid that looks like the white of an egg, and it helps lubricate the joint and absorb shock.
The synovial membrane has blood vessels that supply the joint with nutrients and oxygen.
The tips of the bones that come together to form the joints are covered by an articular cartilage, which is a slippery smooth layer of cartilage that also absorbs shock and reduces friction during movement.
Now, there are various ways by which a bacterium can get into your joint.
First, it can be from a preexisting infection in adjacent tissue, usually the bone, from where a bacterial infection can spread to the articulating part of the bone and then makes its way right into the joint.
It can also develop from hematogenous spread which is where the bacteria is somewhere else in the body like the lungs, and then travels through the bloodstream and gets into the joint.
So, let’s say a child falls on some dirty planks of wood and a nail pierces through their knee, infecting the synovial membrane with bacteria.
That bacteria could either come from the nail and be living in the environment, like Clostridium tetani which causes tetanus, or it could be bacteria that lives on the skin surface and gets shoved deep into the joint at the moment that the skin is pierced, like Staphylococcus aureus.
Once bacteria get into the synovial cavity, they start destroying the articular cartilage with their toxins.
One example of a toxin is chondrocyte proteases, which is a powerful enzyme that’s capable of digesting the collagen in the articular cartilage.