Dysbaric Osteonecrosis: Radiology and Differential Diagnosis

Two main questions may arise in early or atypical lesions: Is the radiological lesion under examination either a variant of normal bone structure or perhaps a minor dysplasia of bone? Does the osteonecrosis have a cause other than the dysbaric environment?

Early diagnosis is based on minor alterations in the trabecular pattern of bone that result in abnormal densities or lucencies. Early detection of asymptomatic lesions may be verified only by serial radiological examinations, showing the progression of the lesion. Considerable skill is required in these assessments, and this work became the province of highly specialized panels of independent observers. Members compare their independent written reports before coming to a consensus position. In the United Kingdom a central registry for cases and x-ray studies was sponsored by a government body (the Medical Research Council). Lesions are classified as in Table 14.4 (Figures 14.2 through 14.9). This classification is useful to compare results among different studies. For a good review of this area see Williams and associates.

Figure 14.2 A1 lesions. Dense areas with intact articular cortex. At the top of the humerus are two areas where the trabecular pattern is blurred (arrows). The edge of the cortex looks ‘woolly’.
Figure 14.2 A1 lesions. Dense areas with intact articular cortex. At the top of the humerus are two areas where the trabecular pattern is blurred (arrows). The edge of the cortex looks ‘woolly’.
Figure 14.3 A2 lesion. Spherical segmental opacity (arrows). Originally called a ‘snowcap lesion’, this may remain symptomless.
Figure 14.3 A2 lesion. Spherical segmental opacity (arrows). Originally called a ‘snowcap lesion’, this may remain symptomless.
Figure 14.4 A3 lesion. Linear opacity. The dense line marked with arrows represents the lesion. The extremities of such linear opacities characteristically extend to the cortical margin.
Figure 14.4 A3 lesion. Linear opacity. The dense line marked with arrows represents the lesion. The extremities of such linear opacities characteristically extend to the cortical margin.
Figure 14.5 A4 lesion. (a) Translucent subcortical band: this lesion (between arrows) is sometimes called a ‘crescent sign’. Situated just under the articular cortical surface, the translucent line indicates that a sliver of the cortical surface is about to detach.
Figure 14.5 A4 lesion. (a) Translucent subcortical band: this lesion (between arrows) is sometimes called a ‘crescent sign’. Situated just under the articular cortical surface, the translucent line indicates that a sliver of the cortical surface is about to detach.

Figure 14.5 (Continued) A4 lesion. (b) Collapse of the articular cortex or subchondral depression: this tomogram shows a fracture line (arrows) developing between the sclerotic part of the bone above (which is being depressed into the femoral head) and the surrounding bone cortex. (c) Sequestration of the cortex: a loose piece of dead articular cortex has been pushed into the body of the femoral head, thus causing the latter to appear flattened (arrows).

Figure 14.5 (Continued) A4 lesion. (b) Collapse of the articular cortex or subchondral depression: this tomogram shows a fracture line (arrows) developing between the sclerotic part of the bone above (which is being depressed into the femoral head) and the surrounding bone cortex. (c) Sequestration of the cortex: a loose piece of dead articular cortex has been pushed into the body of the femoral head, thus causing the latter to appear flattened (arrows).
Figure 14.5 (Continued) A4 lesion. (b) Collapse of the articular cortex or subchondral depression: this tomogram shows a fracture line (arrows) developing between the sclerotic part of the bone above (which is being depressed into the femoral head) and the surrounding bone cortex. (c) Sequestration of the cortex: a loose piece of dead articular cortex has been pushed into the body of the femoral head, thus causing the latter to appear flattened (arrows).
Figure 14.6 A5 lesion. Osteoarthritis. This condition can supervene on any lesion in which disruption of the articular surface has occurred. In osteonecrosis, the cartilage often remains viable so that a joint space of reasonable size continues to be radiologically visible despite severe osteoarthritis.
Figure 14.6 A5 lesion. Osteoarthritis. This condition can supervene on any lesion in which disruption of the articular surface has occurred. In osteonecrosis, the cartilage often remains viable so that a joint space of reasonable size continues to be radiologically visible despite severe osteoarthritis.
Figure 14.7 B1 lesion. Dense areas. These areas can be seen just at and below the junction of the humeral head and shaft (arrows). They are typical of the osteonecrotic lesions seen in such sites, and it is unlikely that they will ever cause disability.
Figure 14.7 B1 lesion. Dense areas. These areas can be seen just at and below the junction of the humeral head and shaft (arrows). They are typical of the osteonecrotic lesions seen in such sites, and it is unlikely that they will ever cause disability.
Figure 14.8 B2 lesion. Irregular calcified areas. This condition is commonly seen in divers. Sometimes the appearance is that of rather foamy areas in the medulla at the lower end of the femur, often with a calcified margin. Sometimes femoral lesions have a hard, scalloped edge around a translucent area. Endosteal thickening frequently accompanies these lesions.
Figure 14.8 B2 lesion. Irregular calcified areas. This condition is commonly seen in divers. Sometimes the appearance is that of rather foamy areas in the medulla at the lower end of the femur, often with a calcified margin. Sometimes femoral lesions have a hard, scalloped edge around a translucent area. Endosteal thickening frequently accompanies these lesions.
Figure 14.9 B3 lesions. Translucent areas and cysts. A single cyst (arrow) is usually seen in the femoral neck. Sometimes a line of small cysts appears at the point where the hip joint capsule attaches to the femoral neck. These irregularities may also be found at the junction of the shoulder joint capsule and humeral neck. Some experts believe these multiple lesions are not osteonecrotic, but rather relate to past damage at the point of a capsule’s insertion into the neck of a bone.
Figure 14.9 B3 lesions. Translucent areas and cysts. A single cyst (arrow) is usually seen in the femoral neck. Sometimes a line of small cysts appears at the point where the hip joint capsule attaches to the femoral neck. These irregularities may also be found at the junction of the shoulder joint capsule and humeral neck. Some experts believe these multiple lesions are not osteonecrotic, but rather relate to past damage at the point of a capsule’s insertion into the neck of a bone.

The first decision to make is whether the bone is normal9. Cysts and areas of sclerosis occur sporadically in otherwise normal persons but also in other diseases. Chance cortical bone defects must he eliminated, and the recognition of the normal bone island is essential.

These are dense areas of bone within the cancellous bone structure but that are sharply defined, round or oval, with the long axis running parallel to the long bone. Thought to develop early in life, they have a normal trabecular pattern around them and have no clinical significance.

In our enthusiasm to monitor divers at risk, we must also be aware of the dangers of irradiation – the promotion of malignancy being the most obvious. Even with good equipment and technique, a diver receives one third of the annual maximum recommended dose of body irradiation for one long bone series. Unnecessary irradiation is to be avoided, and readers are advised to seek up-to-date recommendations on the frequency of long bone series in occupational divers. Currently, a common recommendation is a series at employment and then on leaving employment if more than 5 years have elapsed – with the former only required if the latter was not done on leaving the most recent employment. This practice protects both the diver and the employer from misinterpretation of bony changes.

Causes of radiological anomaly that may produce confusion in diagnosis include the following:

  1. Bone islands (see earlier).
  2. Enchondroma and other innocent tumours: These may calcify, causing an osteoclastic appearance in the shaft of the long bone. Medullary osteochondroma may show foci of calcification, which are more circular, whorled and in closer apposition than the foci of calcification of DON.
  3. Normal variants: These include sesamoid bones, the shadow of the linea aspera and its endosteal crest.
  4. Osteoarthritis: Osteoarthritis, not associated with juxta-articular DON, usually causes a reduction of the joint space, with sclerosis of the underlying bone on both sides of the joint. In DON, the cartilage space is not narrowed unless secondary osteoarthritis has occurred.

Other causes of osteonecrosis must be excluded (see Table 14.3). Both the radiological features and medical history are important in establishing the diagnosis. These causes should be rare in a fit, active diving population who have undergone medical assessment (see Chapters 53 and 54). Among the more important are the following:

  1. Trauma: DON has been reported remote from the site of multiple fractures.
  2. Alcohol: A history of heavy consumption or other organ damage may be obtained.
  3. Steroid therapy: The likelihood of osteonecrosis increases with increased dose, the minimum being 10 mg prednisone or its equivalent per day for 30 days. Short courses of high-dose ‘pulse’ therapy have also been incriminated.
  4. Haemoglobinopathies: Sickle-cell anaemia, thalassaemia and other variants may also be causative. Diagnosis is made by haematological investigations and by demonstrating lesions in the spine and skull.
  5. Specific bone necrosis syndromes such as Kienbock’s disease (spontaneous avascular necrosis of the carpal lunate) and Freiberg’s disease (second metatarsal head): The osteochondroses of epiphyseal heads, such as Legg-Calvé-Perthe’s disease (hip) and Kohler’s disease (tarsal scaphoid), have specific age and clinical parameters.
  6. Collagen diseases: Systemic lupus erythematosus and rheumatoid arthritis are associated with a very high incidence of osteonecrosis of the hip, both with and without steroid therapy.

The initial diagnosis of DON must be reasonably certain because it has serious implications for the professional diver. Solitary lesions especially require careful assessment, whereas multiple lesions make the diagnosis easier.

Symptoms and plain x-ray lesions are both relatively late manifestations, with MRI and scintigraphy (the latter now seldom used) both capable of identifying earlier lesions if sought (see later). The first radiological signs may be noted within 3 to 6 months of MRI changes, but they may take much longer – even years. The experienced radiologist looks for an increase in bone density as a result of the reactive changes to the presence of dead tissue, with new bone laid down on the surface of the dead bone.

The pathological lesion may never produce radiological changes. A 10-year radiological follow-up of 15 caisson workers revealed lesions in previously normal areas and worsening of known lesions despite cessation of further hyperbaric exposure10. Autopsy often reveals the pathological areas are far more extensive than the radiological demarcation. Diagnostic radiological parameters include the following:

Juxta-articular lesions (A lesions)

  • Dense areas with intact cortex (usually humeral head).
  • Spherical opacities (often segmental in humeral head).
  • Linear opacities (usually humeral head).
  • Structural failure showing as translucent subcortical bands (especially in heads of femur and humerus) and often collapse of articular cortex with sequestration.
  • Secondary degenerative arthritis with osteophyte formation.

There is usually no narrowing of the joint spaces until later stages. These lesions appear to be quite different from those of other causes of avascular necrosis.

Head, neck and shaft lesions (B lesions)

  • Dense areas, usually multiple and often bilateral, commonly in the neck and proximal shaft of the femur and humerus. These must be distinguished from normal ‘bone islands’.
  •  Irregular calcified areas in the medulla. These are commonly seen in the distal femur, proximal tibia and the proximal humerus. They may be bilateral.
  • Translucent areas and cysts, best seen in tomograms of the head and neck of the humerus and femur.
  • Cortical thickening.

Emphasis is on minor variations of trabecular structure, and special radiographic techniques combined with skilled interpretation are required. Cylinder cone and tomography may be used. Computed tomography (CT) or bone scintigraphy may clarify a questionable area.