There may be a history of DCS or repeated inadequate decompression leading to investigation for possible disease. However, a definite connection between the site of DCS and the site of bone lesions has been notoriously difficult to establish. Early lesions are usually completely asymptomatic and may currently be detected only by bone scintigraphy (radioactive isotope scan), magnetic resonance imaging (MRI) or radiological examination. However, there are reports of persistent limb pain, in some cases quite severe, before the development of x-ray changes. Occasional patients have pain in the area of subsequent necrosis dating from the DCS incident. Persistent limb pain may be indicative of a bone compartment syndrome, which may progress to typical DON.
Symptoms of pain and restricted joint movement, usually affecting the hip or shoulder joint, may develop insidiously over months or years and are caused by secondary degenerative osteoarthritic changes.
An increase of 50 per cent in the total mineral content of the bone is necessary before it can be recognized as an area of increased density on the x-ray film, and these changes may take 3 to 6 months from the time of initial insult. MRI and, to a lesser extent, scintigraphy have emerging roles in earlier diagnosis, and reports have even suggested that causative bubbles may be visualized in the fatty marrow7.
X-ray lesions are usually found in the large long bones of the upper and lower limbs. These may be subdivided into juxta-articular (A) or head, neck and shaft (B) lesions.
There are two major sites for the radiological lesions, classified by their prognostic implications. These lesions may be present alone or in combination and are classified as juxta-articular lesions (A lesions) and head, neck and shaft lesions (B lesions).
These are also referred to as joint lesions or A lesions and are potentially disabling. They may eventually result in collapse of the articular surface. The most common sites are the hips and shoulders. The lesions predominate in caisson workers and divers working in undisciplined or experimental conditions. Rare cases have been reported in other joints, e.g. the ankle. It is estimated that about one in five articular lesions will progress to articular surface collapse and up to one in five of these will be treated by arthroplasty or other surgical procedures.
Head, neck and shaft lesions
Lesions away from the articular surface are referred to as medullary or B lesions. They are usually asymptomatic and are seldom of orthopaedic significance. The most common sites are the shafts of the femur and humerus. These lesions do not extend beyond the metaphysis or involve the cortex of the bone. The shaft is not weakened, and pathological fracture is a rare complication. New bone replacement has been observed in these lesions. Their importance lies in that they may demonstrate that people with the lesions are at greater risk of further DON, although this has not been proven statistically.
In assessing the radiological diagnosis of these lesions, it is important to realize that the X-ray will show only a fraction of the total lesion, and that some bone necrosis areas revealed by scintigraphy never become apparent on the x-ray studies.
Symptoms referable to juxta-articular lesions depend on the position and severity of the bone damage. Usually there is pain over the joint. This may be aggravated by movement and may radiate down the limb. There is often some restriction of movement, although a useful range of flexion may remain. In the shoulder, the signs are similar to those of a rotator cuff lesion, i.e. a painful arc from 60 to 180 degrees of abduction with difficulty in maintaining abduction against resistance. Lifting heavy weights may precipitate the onset of pain. Secondary degenerative osteoarthritis follows collapse of the articular cartilage and further reduces joint movement. The site of these lesions is approximately in the ratio of femur to the humerus, 1:2 to 1:3.
Malignant tumours of bone (usually fibrous histiocytoma) have been reported in cases of aseptic necrosis, many of which were asymptomatic. The risk appears greatest with large medullary lesions.