Dysbaric osteonecrosis or DON is a form of avascular necrosis where there is death of a portion of the bone that is thought to be caused by nitrogen (N<sub>2</sub>) embolism (blockage of the blood vessels by a bubble of nitrogen coming out of solution) in divers. Although the definitive pathologic process is poorly understood, there are several hypotheses:

  • Intra- or extravascular nitrogen in bones, "nitrogen embolism".
  • Osmotic gas effects due to intramedullary pressure effects.
  • fat embolism
  • hemoconcentration and increased coagulability.

Cause

Dysbaric osteonecrosis is associated with exposure to large ambient pressure changes, but the underlying etiology remains uncertain. Dysbaric osteonecrosis is radiologally indistinguishable from similar lesions not associated with pressure changes.

Early on there is flattening of articular surfaces, thinning of cartilage with osteophyte (spur) formation. In juxta-articular lesions without symptoms, there is dead bone and marrow separated from living bone by a line of dense collagen. Microscopic cysts form, fill with necrotic material and there is massive necrosis with replacement by cancellous bone with collapse of the lesions.

The following staging system is sometimes useful when managing lesions.

Its relationship to compressed air is strong in that it may follow a single exposure to compressed air, may occur with no history of DCS but is usually associated with significant compressed air exposure. The distribution of lesions differs with the type of exposure - the juxta-articular lesions being more common in caisson workers than in divers.

There is a definite relationship between length of time exposed to extreme depths and the percentage of divers with bone lesions. Evidence does not suggest that dysbaric osteonecrosis is a significant risk in recreational scuba diving.

References

  • ScubaDoc's Dysbaric Osteonecrosis page