Davidson, John. K.

2018-09-13T16:02:20Z

2018-09-13T16:02:20Z

1964

Avascular necrosis of bone amongst compressed air workers is more common than is generally realised.

This radiographic investigation of the shoulder, hip and knee joints shows, that in a group of two hundred and forty -one compressed air workers at the end of a four year contract, there is at least a 12.4 per cent incidence of bone lesions and probably a 16.5 per cent incidence when more indefinite evidence is included. Eight out of the two hundred and forty -one men or 3.3 per cent had symptoms, and this includes one man who admitted having experienced symptoms before coming to the Clyde. In addition, an analysis of the site of all bone lesions shows that 55 per cent of the lesions involve or are adjacent to a joint surface and so liable to produce symptoms. This group of two hundred and forty -one men only represents 17.6 per cent of the force, a total thirteen seven men having been exposed to compressed air at one time or another during the contract and all these men are liable to develop avascular necrosis of bone. In fact another eleven cases of bone necrosis have presented at Hospitals in Glasgow and the West of Scotland over the past three years. All these men had worked at the Clyde Tunnel and had left employment before the survey commenced. Seven of the men had worked in compressed air for the first time at this tunnel.

The radiographic diagnosis of avascular necrosis of bone is based primarily on an absolute increase in radiographic density of the affected area. This increase in density results from theprocess of revascularisation of necrotic bone, and it has been shown during this process that new bone is laid down on the trabeculae of dead bone. This process is usually seen at a bone end and takes some time to develop. It is six months at least, usually a year, before an increase in radiographic density becomes apparent and during this period necrotic bone is indistinguishable from living bone. In the shaft of the long bones, the increase in radiographic density results from calcification in the margins of the necrotic area. A similar period of time must also elapse before this calcification becomes evident on a radiograph. Consequently it is clear that the radiographic abnormality requires time to develop and it is suggested that the incidence of bone lesions is probably higher. This could be demonstrated if it was practical to examine the same group of men at yearly intervals over thenext three years.

In a review of the published reports of a hundred and fifty - three compressed air workers, the head and neck of the femur was most frequently involved. This review also shows that lesions are multiple and symmetrical. Other reports of a large number of cases of bone necrosis shows variation in the frequency of thefype of radiographic change and the site of the bone lesions. Some reports are confined to the examination of long term compressed air workers and others did not include a full radiographic study of all the joints. Only a few reports indicate the alteration that may occur in the radiographic appearances over a number of years. It appears that the lesions involving the articular cortex of the head of the humerus and femur are progressive. These lesions usually become more dense with increasing revascularisation of the necrotic area. Nearly all such reported cases develop evidence of secondary osteoarthritis after a number of years.

The published reports of divers show that the bone lesions are identical with those found in compressed air workers. The lesions are multiple, symmetrical, and the head of the humerus is most frequently involved. Only one report indicates that bone lesions may develop following exposure to a reduced pressure of air in a high altitude decompression chamber. Several reports, on the other hand, have shown no radiographic abnormality in the study of a large numberof men who operate low pressure chambers.

Relation of the occupational history with the bone lesions of all the men who were examined radiographically at the Clyde, shows that bone lesions are more common at the higher end of the normal working range - that is above 30 lbs. p.s.i. The risk of developing a bone lesion increases with the length of exposure to compressed air. The possibility that a single exposure to compressed air may produce severe bone necrosis has been established and two of the cases reported in this thesis had a very limited exposure. This raises the question if certain men are more susceptible to bone necrosis and if these men can be identified.

The cause of the bone lesions is not at all clear. Experimental work has been largely unsuccessful in producing bone lesions. Possibly the presence of intravascular gas bubbles developing on the venous side, may cause vascular occlusion. It is likely that those men with bone necrosis may have experienced several symptomless ischaemic episodes.

Treatment of the condition is lengthy and largely unrewarding. Only a few people have a limited experience in managing this type of problem. At the moment, removal of the necrotic bone and replacement by bone grafts appears to be of some value in promoting further revascularisation and strengthening the necrotic bone. Surgical intervention depends on the severity of symptoms. Management should include the avoidance of heavy manual labour.

With such a prevalence of bone lesions, some of which are liable to cause symptoms, and as the treatment is lengthy and so unrewarding, it is obviously important to investigate any possible way that these lesions can be prevented. An important point is to establish the relation, if any, between the existing decompression schedules, as judged by the "bends rate" and the presence of bone lesions. At the Clyde, this rate was one of the lowest on record and yet bone lesions are unexpectedly frequent. It is known that many men with a mild pain - "the niggles" - do not return for treatment, and consequently the "bends rate" may be artificially low.

It is advisable that all men should be recompressed immediately a type 1 lesion develops, and that the men and contractors should be made more aware of the problem of bone necrosis. It is important that men should have a radiographic examination of the hip, shoulder and knee joints before exposure to compressed air. Those with evidence of a lesion involving or adjacent to a joint should be prevented in working in compressed air. Others with a 'shaft' lesion should be told of the nature of the abnormality and that while the lesion will not give rise to symptoms, a further exposure to air may cause other lesions which could give rise to a disabling condition.

Radiology has an important role to play in demonstrating those cases with avascular necrosis of bone and on studying the natural history of this disorder over a number of years.

http://hdl.handle.net/1842/32464

The University of Edinburgh

Annexe Thesis Digitisation Project 2018 Block 20

Radiology in decompression sickness - observations on compressed air workers at the Clyde Tunnel

Thesis or Dissertation

Doctoral

MD Doctor of Medicine