Dysbaric osteonecrosis is a serious complication for those exposed to a hyperbaric environment, occurring in 17% amongst compressed air workers and 4.2% amongst divers. The prevalence of bone lesions increases with age, the number of bends, the number of hours worked in the present contact, obesity, experience greater pressures of air or at greater depths. Dysbaric osteonecrosis appears to be independant of decompression sickness.
Bones lesions are mainly symmetrical and multiple, occurring in (1) the shafts of the femora or tibiae and the heads and necks of the humeri or femora, (2) next to the joint surface, the so-called “juxta-articular lesions”. The juxta-articular lesions almost never occur near the articular surfaces of the knee or elbow joints. The sites in the skeleton at which the lesions occur are those at which fatty marrow is fond in the mature adult.
The pathogenesis of osteonecrosis may involve several factors: circulatory impairment by extravascular or intravascular bubbles, lipid aggregation, fibrin-platelet thrombi, release of vasoactive substances, bas-induced osmosis. It has been suggested that injured marrow adipocytes can release vaso-active substances that can play a systemic procoagulant role in triggering disseminated intra-vascular coagulation.
The basic radiologic abnormality is usually an increase in the density, less often it may be a decrease. There may be a long delay between the causal incident and the appearances of radiographic abnormalities. The radiographic abnormalities may be classified in three types: type A, juxta articular lesions leading to osteoarthritis; type B, head, neck and shaft lesions; type C, lesions looking like bone island. Early detection of the bone necrosis is important to monitor compressed air workers. Detection depends on good quality radiographs (modifications of the trabeculae in a susceptible area). Bone areas of bone damage may go completely unrecognised on a radiograph. Lesions, early or doubtful, can be confirmed by MRI or bone scintigraphy but only 18% of the positive bone scintigraphy will develop radiographic abnormalities.
Survey of caisson workers is necessary because they can develop lesions in previously normal areas in the absence of further exposure to hyperbaric pressures. It is not certain that a repair process always takes place and it is impossible to detect juxta articular lesions that do not progress to structural failure of the joint surface and those that are going to progress to a secondary degenative joint disease. So, there may be a long delay in making a diagnosis in an individual susceptible worker resulting in continous of the same type of work and thus exposure to the risk of further bone damage.
There is no good preventive treatment: prevention by the use of official decompression tables seems to be inadequate because decompression sickness tends to be underreported related to the delay of diagnosis and it has been found that the tables do not decrease the number of dysbaric osteonecrosis. Oxygen decompression seems to be one of the only viable method for the safely decompressing tunnel workers.
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