Dysbaric osteonecrosis. Etiological and pathogenetic concepts.

Dysbaric osteonecrosis appears to be independent of decompression sickness. The 2 conditions, however, may share etiologic and pathogenetic factors. The incidence of osteonecrosis is influenced by the number of hyperbaric exposures, extent of pressure, decompression profile and possibly by the rate of compression and degree of obesity. Though etiology and pathogenesis are unclear, osteonecrosis is probably due to ischemia, with gas bubbles causing direct or indirect circulatory impairment. In vitro experiments, as well as human and animal studies, suggest multiple pathogenetic mechanisms: intraosseous vessel compression by extravascular bubbles; vessel obstruction by bubbles, fibrin thrombi, platelet aggregates, clumped erythrocytes or coalesced lipids; and narrowing of arterial lumina by bubble-induced myointimal thickening. Obstructing materials, whether autochthonous or embolic, may result from blood-bubble interface reactions. Rheologic changes and blood flow redistribution could play contributing roles. It seems likely that multiple pathogenetic factors act in concert or sequentially. Proposed nonischemic changes, such as hyperoxic injury gas-induced osmosis, or autoimmunity, lack sufficient supporting evidence. The peculiar vulnerability of bone may be related to gas supersaturation of the fatty marrow; sensitivity to extravascular gas pressure because of tissue rigidity; poor vascularization; and the presence of uranium 238 which promotes nucleation and subsequent gas bubble formation.