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Septic arthritis



Musculoskeletal system


Pediatric musculoskeletal conditions
Musculoskeletal injuries and trauma
Bone disorders
Joint disorders
Muscular disorders
Neuromuscular junction disorders
Other autoimmune disorders
Musculoskeletal system pathology review

Septic arthritis


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High Yield Notes
4 pages

Septic arthritis

9 flashcards

USMLE® Step 1 style questions USMLE

3 questions

A 35-year-old man presents to his primary care physician’s office for evaluation of right knee and ankle pain for the past 2 days. He had similar pain over the left wrist and shoulder joints in the past year that self resolved. The patient has a painless rash on his hands which appeared around the same time. Past medical history is significant for a urinary tract infection 2 months ago which was treated with antibiotics. He has had 7 sexual partners over the past year and is currently sexually active. He uses condoms inconsistently. He denies any recent travel. His temperature is 38.2 °C (100.8°F), pulse is 80/minute, respirations are 16/minute, and blood pressure is 130/80 mmHg. Physical examination shows multiple nontender papulovesicular lesions on his hands. Passive flexion of wrists elicits a sharp pain. Which best describes the route of transmission regarding the causative organism resulting in this patient’s clinical condition?  

External References

Content Reviewers:

Rishi Desai, MD, MPH

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.


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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