Detailed studies of the incidence of DON were not undertaken until the 1960s. Because the incidence of DON has fallen dramatically since that time, presumably following the advent of strict workplace health and safety rules based on sound decompression practices, most of the clinical and epidemiological work that informs the following discussion dates from the 1970s and 1980s. Figures should be considered cautiously because the radiologists or physicians in each survey may have used different radiological techniques and diagnostic criteria. Other factors influencing the results include the difficulty in obtaining adequate follow-up and the different decompression regimens used.
For example, at the Clyde Tunnel in Glasgow only 241 compressed air workers were surveyed of a total of 1362; 19 per cent of the workers surveyed had lesions, half of which were juxta-articular (next to a joint surface). By 1972, the UK Medical Research Council Decompression Sickness Council Panel had x-ray studies of 1674 workers, of whom 19.7 per cent had positive lesions. Also in 1972, a study by Jones and Behnke on the Bay Area Rapid Transit tunnelling project in San Francisco revealed no clinical or x-ray evidence of necrosis. All prospective workers had pre-employment x-ray studies, and those workers with lesions were excluded. The pressure ranged from 9 to 36 lb/square inch (62 to 248 kPa) gauge, with only one decompression per day. However, the follow-up period was relatively short.
The reported incidence in divers is exceedingly variable, ranging from 2.5 per cent in the US Navy to a doubtful 80 per cent in Chinese commercial divers. Some representative surveys are listed in Table 14.2. The lower incidences are reported in military series and commercial diving operations where strict decompression schedules are adhered to, whereas the incidence is much higher in the self-employed diving fishers of Japan, Hawaii and Australia. The Australian diving fishers undertake relatively deep dives with long bottom times and often inadequate decompression. There is also a higher incidence among divers more than 30 years old, which may reflect increased exposure rather than age itself.
The Medical Research Council Decompression Sickness Central Registry has x-ray studies for nearly 7000 professional divers, and in 1989, Davidson reported there were only 12 cases of subchondral bone collapse, i.e. about 0.2 per cent2. Asymptomatic shaft lesions appeared in about 4 per cent. In 1989, Lowry reported that the prevalence of crippling osteoarthritis leading to total joint replacement had been conservatively estimated at more than 2 per cent in Australian abalone divers1. Most cases in most series involve shaft lesions, which have no long-term significance to health and well-being, except for the rare possibility of malignant change.
Earlier UK studies on professional divers indicated that lesions occurred significantly more commonly among the older men who had longer diving experience and also who had exposures to greater depths. Only 0.4 per cent of the compressed air divers who had never exceeded 50 metres had these lesions. The helium-breathing divers who did not exceed 150 metres had an incidence of 2.7 per cent, which rose to 7.6 per cent if they had been deeper. There was a definite increase in incidence among saturation divers and those with a history of DCS. Approximately one fourth of the lesions were potentially serious, closely associated with joints.
Another UK study of caisson workers, with 2200 subjects, showed an incidence of DON of 17 per cent. The lesions were more often in older men with more exposure to pressure and also correlated significantly with DCS. The incidence rose to 60 per cent for workers who had worked for 15 years in compressed air.
Although rare, several cases have been reported in aviators not exposed to hyperbaric conditions.
Dysbaric osteonecrosis is rare in recreational scuba divers who breathe compressed air at depths of less than 50 metres and who follow the customary decompression tables.
Whether the incidence of bone lesions is related more to the cumulative effects of hyperbaric exposures than to the statistical chance of a single event increasing with multiple exposures is unknown.
The incidence of avascular necrosis of bone, within the general population not exposed to hyperbaric environments, is also not clearly defined.
The disease is rare in sport divers. A few cases (nearly all shoulder disease) have been reported, although it is likely that there are many other unreported sufferers. Gorman and Sandow3 and Wilmhurst and Ross4 published two typical case reports.