Friday, December 5, 2008

Osteomyelitis and Fly Larvae

Musculoskeletal infections can be a vexing, serious problem. For example, consider knee replacement surgery, a common procedure that is done over 400,000 times a year worldwide. If postoperative infection is avoided, the outcome is typically excellent. A post-operative infection, however, will often spell trouble, and a prolonged recovery.

Over time, we have gotten better at treating infections of bones and joints, particularly with the advent of antibiotics. In the pre-antibiotic era, some novel treatments were tried, including fly larvae:

In World War II, combat induced wounds to the extremities had a 20-25% rate of osteomyelitis. By the Vietnam War, this figure had dropped to about 8%, thanks to better treatment.

In the civilian world, infection is much rarer, but can still occur. Radiologists are often asked to assess for the possibility of bone, joint, or soft tissue infection, and the define its extent.

The question of whether or not osteomyelitis is present comes up most often in the foot. Collins el al. wrote an excellent article on the use of T1-weighted images when looking for pedal osteomyelitis (AJR 185:386-393, 2005). While signal abnormalities on T2-weighted and STIR images tend to be nonspecific, they noted that T1 signal abnormality that is medullary and confluent is highly suspicious for osteomyelitis.

Since then, I have used the principles outlined in their article when analyzing pedal osteomyelitis, and have had good success. I am not aware of any articles that address the MRI analysis of osteomyelitis in the hand and wrist. Given that the hand and foot are ontogenic homologs, I have transferred the principles of Collins et al. to analysis of osteomyelitis of the hand and wrist.

In this case, a 37 year-old man was sent for a finger MRI, due to pain and swelling. He suffered an injury at his work five weeks before the MRI.

A sagittal STIR image shows extensive marrow edema (red arrows) in the middle and distal phalanx, as well as a distal interphalangeal joint effusion (white arrow):

Marrow edema and excess joint fluid are worrisome findings, but tend to be nonspecific. Next, we examine the corresponding T1 weighted image:

The T1-weighted image depicts confluent medullary signal abnormality in the head of the middle phalanx (red arrow), and depicts extensive soft tissue edema. The presence of confluent medullary signal abnormality allows us to predict the presence of osteomyelitis with a high degree of confidence. Osteomyelitis and septic arthritis were confirmed at surgery.

Vic David MD

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