(lower thigh at the knee) and 15% in the upper tibia (knee below the knee cap). Worsening of the condition from continued decompression in an asymptomatic x-ray finding may occur. Prevention Prevention is a more successful strategy than treatment. By using the most conservative decompression schedule reasonably practicable, and by minimizing the number of major decompression exposures, the risk of DON may be reduced. Prompt treatment of any symptoms of decompression sickness (DCS) with recompression and hyperbaric oxygen also reduce the risk of subsequent DON. Treatment Treatment is difficult, often requiring a joint replacement. Spontaneous improvement occasionally happens and some juxta-articular lesions do not progress to collapse. Other treatments include immobilization and osteotomy of the femur. Cancellous bone grafts are of little help. Prognosis If the diver has not been exposed to excessive depth and decompression and presents as DON, there may be a predisposition for the condition. Diving should be restricted to shallow depths. Divers who have suffered from DON are at increased risk of future fracture of a juxta-articular lesion during a dive, and may face complications with future joint replacements. Because of the young age of the population normally affected, little data is available regarding joint replacement complications. There is the potential for worsening of DON for any diving where there might be a need for decompression, experimental or helium diving. Physically stressful diving should probably be restricted, both in sport diving and work diving due to the possibility of unnecessary stress to the joint. Any diving should be less than 40 feet/12 meters. These risks are affected by the degree of disability and by the type of lesion (juxta-articular or shaft). Prevalence Dysbaric osteonecrosis is a significant occupational hazard, occurring in 50% of commercial Japanese divers, 65% of Hawaiian fishermen and 16% of commercial and caisson divers in the UK. Its relationship to compressed air is strong in that it may follow a single exposure to compressed air, may occur with no history of DCS but is usually associated with significant compressed air exposure. The distribution of lesions differs with the type of exposure - the juxta-articular lesions being more common in caisson workers than in divers. There is a definite relationship between length of time exposed to extreme depths and the percentage of divers with bone lesions. Evidence does not suggest that dysbaric osteonecrosis is a significant risk in recreational scuba diving. References External links