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<v ->The second mechanism for septic arthritis

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is spread from a contiguous contaminated source.

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And I could pick bone or soft tissue,

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let me start with bone.

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There are two mechanisms how septic arthritis can occur,

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following an infection within a bone.

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The first of these I show by this arrow,

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the infection within the bone may spread

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through the epiphysis maybe with fragmentation of bone,

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through the cartilage and reach the joint.

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The second way that this can occur is if the metastasis

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or epiphysis of the infected bone is intraarticular

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and then in fact what can occur,

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is the infection can spread directly into the joint.

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This is something we see regularly in the hip

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that a metaphyseal infection of the proximal femur,

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since that is intracapsular, that infection can spread

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into the joint without extending into the epiphysis.

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So there's a general rule on MR

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that if you have a metaphyseal infection,

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as we see here in a child,

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and there is a joint effusion nearby,

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that is strong evidence that a septic arthritis is present.

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When we have this sort of mechanism working,

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the joint fluid typically is contaminated

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before the synovial.

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So it's the opposite when compared to direct spread

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to the synovial membrane.

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Now, one of the interesting ways that bone infection spreads

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in the joint, is via what is called a Brodie's abscess.

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So a couple words about a Brodie's abscess.

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A Brodie's abscess is a sign

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of subacute or chronic osteomyelitis.

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It implies that at least at the cellular level,

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the infection is active, there are alive bacteria,

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although clinically, there may not be

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any active signs of infection.

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Typically seen in the lower extremity,

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the most common bone that is involved is the tibia

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and even further than that,

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it's the distal end of the tibia.

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Typical of a Brodie's abscess,

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is an elongated area of bone destruction,

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the axis oriented along the long axis of the tubular bone.

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The body recognizes this as an active infection

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and tries to get rid of it.

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And it does so by creating a tract.

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That tract can extend in a variety of directions.

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One of the directions in the immature skeleton,

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is inferiorly or downward to the level of the physis

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and no further.

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If the physis is closed or occasionally it can be open,

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that infection may pierce through the physis

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and reach the joint.

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The other direction is through the surface of the bone

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as shown by this red extension.

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So here's an example of a Brodie's abcess

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in a typical location involving the distal tibia,

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note the white arrow pointing to the tract,

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part of the track on it.

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That's the infection spreading down to the subchondral bone,

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destroying the cartilage, creating septic arthritis.

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So that is spread from a contiguous axial source.

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Clearly and well known to everybody,

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is we can have a septic arthritis related spread

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from a contiguous soft tissue source.

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We need to only look at the diabetic foot

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with the foot ulcer as shown here,

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the arrows indicating the tract at the site of ulcer

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and the contamination

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of the first metatarsophalangeal joint,

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septic arthritis and osteomyelitis.