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 embolism(blockage of the blood vessels by a bubble of nitrogen coming out of solution) in divers.[1] Although the definitive pathologic process is poorly understood, there are several hypotheses:
- Intra- or extravascular nitrogen in bones, "nitrogen embolism".[citation needed]
- Osmotic gas effects due to intramedullary pressure effects.[citation needed]
- fat embolism[citation needed]
- hemoconcentration and increased coagulability.[citation needed]
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.[clarification needed]
The following staging system is sometimes useful when managing lesions.[1]
- Stage 0 - Intravascular coagulation
- Stage 1 - Dead Bone without repair
- Stage 2 - Dead Bone with repair but without collapse
- Stage 3 - Dead Bone with repair and with collapse
- Stage 4 - Secondary degenerative arthritis
In a study of bone lesions in 281 compressed air workers done by Walder in 1969,[clarification needed] 29% of the lesions were in the humeral head (shoulder), 16% in the femoral head (hip), 40% in the lower end of the femur (lower thigh at the knee) and 15% in the upper tibia (knee below the knee cap).
Treatment is difficult, often requiring a joint replacement.[citation needed] Spontaneous improvement occasionally happens and some juxta-articular lesions do not progress to collapse.[citation needed] Other treatments include immobilization and osteotomy of the femur.[citation needed] Cancellous bone grafts are of little help.[citation needed]
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