DYSBARIC OSTEONECROSIS

History
In the late 1800’s and early 1900’s many people worked building tunnels and bridges using compressed air in caissons to keep the workplace dry. It is from this population that Bassoe(1911) in the United States and Bornstein and Plate (1911) in Germany were the first to report disabling hip and shoulder conditions which were associated with radiological evidence of joint degeneration. The first case in a diver was not reported for another 30 years. (Grutzmacher, 1941). This, too, presented as pain in a joint. Around 10 or more cases were reported in the literature during the next ten years and all were in divers who had sought treatment for persistent joint pain. It is not possible to draw valid conclusions from these cases because the x-rays are not always published and there was no agreed standard for radiological diagnosis. Alnor et al (1964) found 72 cases of necrosis in 131 divers and of the 65 who had been kept under observation for more than 10m years, only twenty-two of them remained free from radiological lesions. Of the 43 with lesions, 17 had symptoms and 7 were “totally unable to work”.

Types
Osteonecrosis in divers is of two basic types: juxta-articular (subchondral), and shaft, a description that includes the neck and a portion of the long bone. The shaft lesions are predominately saponified fat. The juxta-articular lesions are of greater clinical significance. These lesions show areas of dead bone surrounded by a layer of collagen which forms a fibrous band and the formation of new bone. Beyond the area that can be detected radiologically is seen an area of creeping substitution and healing trabeculae (McCallum and Harrison, 1993).

Standardization od Diagnostic Procedures
Perhaps the most important step towards proper assessment of the significance of bone necrosis to the diving population was the standardization of diagnostic techniques. With agreement on x-ray diagnosis in the late 1950s and 1960s it was possible to move away from purely clinical descriptions of the illness to surveys of the prevalence of pre-symptomatic lesionsin the apparently healthy population. The radiological diagnosis of bone necrosis depends on the quality of the x-rays that are taken and the experience of those who read them. The x-ray remains the gold standard of diagnosis and although the disabling complications affect only the shoulders and the hips, extensive views of the lower femur and upper tibia are included in knee x-rays in order to find as many shaft lesions as possible.

MRI and MDP Scans
MDP (99mTechnetium Methyl-dipolyphosphate) scans are very sensitive to local bone pathology. A ‘hot spot” indicates increased perfusion and metabolism and may be seen only hours after a dive. However,in a Royal Navy survey (Pearson et al, 1982) only four divers became radiologically positive some two to three years after 22 had demonstrated positive scans. Eleven still had abnormal scans and 7 had reverted to normalcy. Thus a positive scan is of little diagnostic significance, but may indicate a need for radiological follow-up. Magnetic resonance imaging (MRI) has a remarkable power to detect early lesions and is becoming more readily available for routine screening of large populations.

Incidence increase with age and depth It was shown by the Decompression Sickness Registry(1981) that the percentage of bone necrosis, both shaft and juxta-articular, increases in a sample of divers with age and experience. At least one definite lesion was found in 4.2% of a population of 4980 divers. No necrosis was found in those who had never dived deeper than 30 m, but 15.8% was found in the 190 men who had dived deeper than 200m. Thus screening of deep divers is essential for their health if persons with developing juxta-articular lesions are to be removed from hazardous exposures in time to avoid joint collapse.