Dysbaric Osteonecrosis

"Dysbaric means "bad pressure' osteo means 'bone' and necrosis means 'cell death'. Therefore dysbaric osteonecrosis is simply bone cell death as a result of pressure changes."

By Dr. Sawatzky

Periodically someone writes an article about the threat of divers suffering permanent diving related physical damage. These articles cause a great deal of concern and generate a lot of publicity for a while and then they usually fade into the background. We have had questions about the significance of a patent foramen ovale, abnormal findings on CT scans, PET scans, SPEC scans and MRI of the brains of divers, and the damage seen at autopsy in the brains and spinal cords of divers. All of these findings are cause for concern but so far, none have been shown to have significant practical effect. The one long-term negative effect of diving that has been well documented, and that we all need to be aware of, is dysbaric osteonecrosis.

Dysbaric osteonecrosis (DON) is easy to remember if you break it down. Dysbaric means 'bad pressure', osteo means 'bone' and necrosis means 'cell death'. Therefore dysbaric osteonecrosis is simply bone cell death as a result of pressure changes.

There are many different terms that have been used to refer to pressure induced bone death including caisson arthrosis, caisson disease of bone, hyperbaric osteonecrosis, barotraumatic osteoarthropathy, avascular necrosis of bone, ischemic necro sis of bone, aseptic necrosis of bone, diver's bone rot, and diver's crumbling bone disease. Dysbaric osteonecrosis is the most accurate description and we will use it in this column.

DON is possible in anyone who is exposed to changes in pressure and therefore it is of concern for caisson workers (pressurized work spaces used in building tunnels, bridge pilings, etc.), divers, aviators, and astronauts. Caisson workers have been around the longest and therefore our initial knowledge of DON is from them.

In 1888, Twynam suggested that the areas of dead bone often seen in caisson workers were caused by pressure. In 1913, Bassoe suggested that DON was somehow related to decompression sickness (DCS) as he noticed that the caisson workers who had been bent the most also had the most DON. In 1931 the HMS Poseidon, a Royal Navy submarine, had an accident and five men were trapped at 38 meters (120 ft) sea water pressure for two to three hours before they escaped to the surface. All five developed DCS soon after surfacing. Twelve years later, when three of the five had x-rays taken, all three had DON, one with partial collapse of both femoral heads. None of the five were divers or had other known pressure exposures. Therefore we know that DON can develop from a single exposure to pressure.

In 1941 we find the first reported case of DON in a diver and in 1943 Taylor noted that there is a delay of several months from the pressure exposure that caused the DON to the observance of DON on x-ray. About this time we also learned that DON can occur following exposure in caissons to as little as 17 psi (11.6 msw or 38 fsw) pressure.

How common is DON? Different studies have shown highly variable incidences. In caisson workers the average is around 19%. We know that up to 60% of caisson workers with more than 15 years experience have DON and that those caisson workers who have a history of DCS are also more likely to have DON. In addition, caisson workers who work at higher pressures are more likely to have DON. Aviators are frequently exposed to decompression stress. However, in aviators DON is extremely rare. There are only two well-documented cases in the literature and both followed severe DCS that was not treated.

In divers, the incidence of DON is even more confusing. The following table lists the findings of several studies. We will come back to the pathophysiology of DON later but it seems pretty clear from these studies that the risk of DON is highest in divers who have the least training, the least control, and (most likely) the least adequate decompression. From other studies we know that these same groups also have a very high incidence of DCS and that it is often inadequately treated.

Date     Exposure     			Number     % DON
1974     Japanese Shellfishermen     	301     50.5
1974     Gulf Coast Commercial Divers    330     27.0
1976     Royal Navy Divers     		350     4.0
1976     US Navy Divers     		611     2.5
1978     Hawaiian Fishermen     	20     65.0
1981     North Sea Commercial Divers    4422     4.4
1983     Japanese Shellfishemen     	747     25.0
1986     Australian Abalone Divers     	108     56.4

How is DON diagnosed? The most common way is through the use of x-rays. On x-ray, DON appears as a localized area of either increased or reduced bone density. It is important to realize that x-rays only show the concentration of calcium salts in the bone and that if you take an x-ray of a bone from a person who has been dead for several months, you will not be able to tell it from an x-ray of a live bone. In addition, the concentration of bone salts must increase or decrease by at least 50% before the change can be seen on x-ray. It takes at least three or four months and it may take as long as three to five years before DON lesions become visible on x-ray. This makes it very difficult to determine which pressure exposure caused the damage, a fact that is very important to the commercial diver trying to determine which of the several companies he has worked for over the past several years to sue! In addition, the area of change on x-ray is usually significantly smaller than the actual area of damage. Finally, if the DON lesion is near a joint, x-rays do not allows us to predict which cases of DON will progress to collapse of the cartilage and subsequent destruction of the joint.

The Medical Research Council in the United Kingdom established a radiological classification of DON that is used around the world. They divided the lesions into those near joints (juxta-articular or 'A' lesions) and those in the head, neck and shafts of the long bones ('B' lesions) as follows.

Juxta-articular     
A1 - Dense areas with intact articular cortex     
A2 - Spherical opacities     
A3 - Linear opacities     
A4 - Structural failures
	a. translucent subcortical band
	b. collapse of articular cortex
	c. sequestration of cortex   
A5 - Secondary degenerative osteoarthritis   

Head, Neck and Shaft
B1 - Dense areas
B2 - Irregular calcified areas
B3 - Translucent and cystic areas

We now have several more sophisticated ways to look at dysbaric osteonecrosis. Bone scans involve the injection into a peripheral vein of a radioactive material that is taken up by metabolically active bone. The body is then scanned using a camera that detects radioactivity and areas of increased or reduced activity are detected. In DON, the bone scan may be positive for increased activity (bone formation) within two to three weeks of the pressure exposure. Interestingly, animal studies have detected the ischemia (no blood flow, reduced activity) of DON with bone scans within 24 hours of the pressure exposure. The problem is that bone scans often detect lesions that never appear on x-rays (presumably because they heal normally, without a significant increase or decrease in calcium salts). Conversely, the bone scan may become negative for a lesion that is visible on x-ray for the rest of the diver's life. Therefore, bone scans detect lesions that may not be significant and fail to detect some lesions that are significant.

Magnetic Resonance Imaging (MRI) may be positive for fat necrosis in DON two to three days after the injury but then goes almost negative at two weeks before becoming positive again later. We have very little experience with MRI in DON. It is generally more sensitive than bone scans but again, many of the lesions detected are probably not significant. In addition, MRI is very expensive and not widely available.

Unfortunately, I am out of room in this column. Next time we will continue our discussion of where DON is found, the signs and symptoms, suggested pathophysiology and treatment. To prevent needless anxiety and concern, I will give you one of the conclusions now. DON is extremely unlikely to occur in a recreational diver who follows current decompression tables and is treated expeditiously for any case of DCS they might develop.