Dysbaric osteonecrosis (DON) is caused by a reduction in blood flow in the shafts of long bones secondary to breathing compressed air.
This reduction may result in damage to such structures as the humeral head of the shoulder and the femoral head of the hip.
Osteonecrosis has been associated with a variety of causes other than scuba, including long-term steroid use, blood clotting disorders, high cholesterol, trauma, and heavy long-term smoking or alcohol abuse. Additional risk factors involving scuba include inadequate decompression, a history of musculoskeletal decompression sickness (DCS), long and deep profiles, high frequency diving, advancing age and being overweight.
In the diver, DON may be the result of hyperbaric exposure and rapid decompression that causes nitrogen bubbles to obstruct blood vessels, although the exact mechanism is open to continuing debate (e.g., gas versus fatty embolism). But whatever the cause, the restriction of circulation is of concern because it can result in damage to joint surfaces.
Symptomatic osteonecrosis is very rare in the general population, but prevalence rates soar in commercial divers, such as underwater tunnel workers, fishermen and sponge divers. This is not surprising, as DON is associated with rapid ascent rates, long durations of diving, and diving beyond no-decompression limits. Most cases of DON are silent at onset and remain asymptomatic. When a symptom is reported, in the vast majority of cases it’s a deep, throbbing joint pain. Interestingly, the first radiographic evidence typically doesn’t develop until several months after hyperbaric exposure, but symptoms may arise anywhere from days to decades following the event(s). X-ray, CT scanning and MRI, in that order, are increasingly more sensitive in detecting DON-related abnormalities of bone.
The incidence of DON has dropped significantly over the past three decades, due to the development of accurate decompression tables, diving computers and increased diver awareness of DCS and its prevention. However, it remains a potentially serious issue, as treatment often is not always simple nor is benefit assured.
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