Septic arthritis

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.

Also, bacteria present with pathogen-associated molecular patterns – PAMPs, a group of bacterial surface antigens recognizable by your innate immune system as foreign, resulting in the initiation of an immune response.

Macrophages can spot bacteria by their PAMPs, they phagocytose them and at the same time release cytokines such as interleukin-1 and tumor necrosis factor alpha -TNFα. These are signaling chemicals that recruit more immune cells to the site of infection.

Also, these PAMPs get recognized by mast cells and they release histamine, which causes vasodilation and increases the vascular permeability.

The final result is that more blood comes into the area creating an inflammatory response, which makes the joint look red, swollen, and warm.

As more fluids leak out of the capillaries and accumulate in the joint space, there’s an increase in the intra-articular pressure, compressing and interrupting blood vessels supplying the joint.

Eventually, this leads to necrosis of the affected bones and cartilage, leading to joint destruction.