Septic arthritis is an infection of a joint. It occurs most commonly in the large joints of the lower extremities, such as the knees and hips, however it can occur in other locations as well.
Septic arthritis most commonly occurs due to hematogenous seeding of the synovium from an infection elsewhere in the body such as pneumonia, impetigo, or other skin infections. It can also occur as a result of local spread of bacteria from an adjacent osteomyelitis. This tends to happen in the setting of metaphyseal osteomyelitis when the metaphysis is intracapsular (hip, shoulder, elbow, ankle). Although much less common, penetrating wounds into the joint can also cause septic arthritis.
When spread hematogenously, bacteria travel via the bloodstream to the synovial capillaries, at which point they form microabscesses that rupture into the joint. When spread locally, bacteria in the epiphysis perforate the articular cartilage to enter the joint. When this occurs, the patient typically presents with an acute episode of septic arthritis and the osteomyelitis does not tend to become apparent for several days.
Full Article - https://www.orthopaedia.com/pediatric-musculoskeletal-infections/#Septic-Arthritis
