Septic arthritis

Definitions & Key takeaways

Septic arthritis is a type of joint infection that occurs when a microorganism, typically a bacteria, invades the joint and causes inflammation and tissue damage. The invading bacteria may be carried by the bloodstream from an infectious focus elsewhere; introduced through a skin lesion and penetrates the joint; or arrive in the joint by extension from adjacent infected tissue. People with septic arthritis typically develop joint pain, impaired range of motion, and fever. The diagnosis involves getting a joint aspirate and imaging, and treatment includes antibiotics and surgical management. If untreated, septic arthritis may lead to joint destruction, or even cause the infection may also spread to other parts of the body.

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Introduction0:00–0:14

Septic arthritis, which is also called infectious arthritis, refers to any joint inflammation caused by a microbe - and usually it results from a bacterial infection of the joint.

Physiology0:14–0:55

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 that 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.

Routes of spread of infection0:55–1:20

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.

Pathology1:20–3:06

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 are powerful enzymes that are capable of digesting the collagen in the articular cartilage.
Also, bacteria present with pathogen-associated molecular patterns – or PAMPs, which is a group of bacterial surface antigens that are recognized by your innate immune system as foreign, and result 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. People with septic arthritis develop joint pain, impaired range of motion, and fever.

Symptoms3:06–4:12

But symptoms may differ according to the underlying pathogen. Gonococcal arthritis which is caused by Neisseria gonorrhoeae usually affects sexually active adolescents.
The bacteria typically spreads hematogenously and spreads from an initial infection of the cervix, urethra, or pharynx. Neisseria gonorrhoeae affects multiple joints, and also causes multiple skin lesions and tenosynovitis which is inflammation of the muscle tendons.
Now, the majority of septic arthritis is not due to Neisseria gonorrhoeae, and it’s called non-gonococcal arthritis. Non-gonococcal arthritis includes all of the other pathogens, but most often, a single joint will be affected - typically a knee.
Non-gonococcal arthritis is most often caused by Staphylococcus aureus which can completely destroy the joint within days unless it’s treated.
Other bacterial causes include multiple Streptococcus species, as well as Mycobacterium tuberculosis, and Borrelia species - which is the cause of lyme disease.

Diagnosis4:12–4:51

Diagnosis of septic arthritis starts with obtaining synovial fluid from the joint, which is done by doing a joint aspiration - where a needle is placed into the joint and fluid is aspirated or drawn out.
That synovial fluid can be sent for analysis - and typically there’s a high white blood cell count, positive gram stain, or a positive culture - which identifies the exact cause.
In addition, imaging in the form of an X-ray, ultrasound, CT scan, or MRI can also be very useful in demonstrating bone erosion and joint effusions especially for hip and sacroiliac joints, where synovial fluid aspiration is difficult.

Treatment4:51–5:11

Treatment of septic arthritis typically starts with antibiotic therapy and pain medication. In addition, it may require joint aspiration and washout, which can be done by a needle, a process called arthrocentesis.
Alternatively it can be done by opening the joint, a surgical operation known as arthrotomy. Alright, as a quick recap, in septic arthritis there’s joint inflammation, which can be caused by various bacterial infections which spread contiguously or hematogenously.

Review5:11–5:42

The minority of the time, it’s caused by Neisseria gonorrhoeae and it’s called gonococcol arthritis. But the majority of the time it’s caused by Staphylococcus aureus or some other bacteria and it’s called non-gonococcol arthritis.
Diagnosis requires getting a joint aspirate and imaging and treatment includes antibiotics and surgical management.