Dysbaric osteonecrosis is the death of a portion of the bone that is thought to be caused by nitrogen embolization (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 embolization".
• Osmotic gas effects due to intramedullary pressure effects.
• fat embolization
• hemoconcentration and increased coagulability.

Process

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The diagnosis is made by x-ray/MRI appearance and has five juxta-articular classifications and forehead, neck, and shaft classifications indicating early radiological signs.

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 lesion begins as a random finding on x-ray without symptoms. Symptomatic lesions usually involve joint surfaces and fracture with attempted healing occurs. This process takes place over months to years and eventually causes disabling arthritis, particularly of the femoral head (hip).

In a study of bone lesions in 281 compressed air workers done by Walder in 1969, 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).

Worsening of the condition from continued decompression in an asymptomatic x-ray finding may occur.

Prevalence

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Dysbaric osteonecrosis is a significant occupational hazard, occurring in 50% of commercial Japanese divers, 65% of Hawaiian fishermen and 16% of commercial and caisson divers in the UK 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.

Treatment

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The treatment is less than successful, often requiring a joint replacement. Spontaneous improvement occasionally happens and some juxta-articular lesions don't progress to collapse. Other treatments include immobilization and osteotomy of the femur. Cancellous bone grafts are of little help.

The best treatment is prevention by using the safest decompression table possible. Because of the high relationship with DCS, all DCS symptoms should be treated with Recompression and HBO.

Diver Related

If the diver has not been exposed to excessive depth and decompression and presents as DON, there may be a predisposition for the condition. Diving should be restricted to shallow depths.

Diving Risk Assessment

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Risk from the Condition

• Fracture of a juxta-articular lesion during a dive.

Risks from the Treatment

• Failure of various modalities
• Unknown long term outlook for joint replacements in the younger population usually affected by DON

Risks to the Diver

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• Worsening of the condition by continued diving

Advising the Diver

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Potential for injury from future diving There is the potential for worsening of dysbaric osteonecrosis for any diving where there might be a need for decompression, experimental or helium diving.

Modifiers

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• Degree of disability (Staging)
• Type of lesion; juxtaarticular or shaft
• Findings on studies and degree of benefit from treatment as determined by studies.
• Need to dive; recreational, work related.

Dive or not dive

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Physically stressful diving should probably be restricted, both in sport diving and work diving due to the possibility of unnecessary stress to the joint. Any diving should be less than 40 feet/12 meters.

Diving medicine