The value of the plain x-ray examination in early diagnosis is being questioned, and other imaging techniques are being increasingly used. Imaging techniques:
- Plain radiography.
- Computed tomography (including single photon emission computed tomography).
- Magnetic resonance imaging.
Bone scintigraphy (bone scans)
This investigation had an established role in the early detection of the bony reaction to osteonecrosis, before there are any changes on plain x-ray studies. Use has declined with the greater availability and lower cost of MRI (see later). Any lesion that stimulates bone formation is shown as a ‘hot spot’ by the radioactive bone-seeking tracer, on the scintigram (Figure 14.10). Technetium-99m–labelled methylene diphosphonate (MDP) is the most widely used tracer, but several newer diphosphonate compounds have been introduced that appear to have relatively higher skeletal affinity and may be more sensitive. These agents are injected intravenously, and images taken over time with a gamma camera. Scintigraphy-positive ‘lesions’ can be produced in animals as early as 2 to 3 weeks after decompression and have been shown to involve necrotic bone and osteogenesis at autopsy at 3 months, even though x-ray changes still had not developed in most cases.
Similar findings in humans with biopsy or radiological follow-up indicate that the ‘hot spots’ occur far earlier and are more numerous than the radiological changes. However, these ‘hot spots’ may resolve with no apparent longer-term changes and are therefore insufficient to establish a firm diagnosis. Overall, then, scintigraphy in early lesions is much more sensitive than radiology but has low specificity because any bone reparative reaction will be detected, no matter what the cause is.
Single position emission computed tomography
Single photon emission CT (SPECT) is said to improve specificity, and ‘cold’ areas occurring immediately after occlusion of blood supply may also be detected. Despite its promise, SPECT has not found a routine place in the diagnosis of DON, probably because of the high cost and poor availability of this technology.
CT gives greater definition revealing both structural collapse and areas of new growth. CT scans may help in the diagnosis of early or doubtful changes on plain x-ray films. This imaging technique is essential if some of the surgical techniques, such as rotational osteotomy, are being contemplated.
Magnetic resonance imaging
MRI can detect necrosis of marrow fat within 2 to 4 days of the ischaemic episode and thus offers the best opportunity for early diagnosis. MRI studies may indicate far greater necrosis than conventional plain radiography and can also reveal bone lesions at other sites when a lesion has been detected on plain x-ray films. MRI of the shoulder joint has been suggested as the best surveillance technique for professional divers who are exposed deeper than 15 metres (Figure 14.11). At present, MRI is not routinely used as a screening tool because of the cost of examination.
Invasive investigations have been undertaken to aid in earlier diagnosis and therapeutic intervention. These techniques include arteriography, intraosseous phlebography, intramedullary pressure measurement and core biopsy. The latter three investigations are often combined in a technique described by Ficat in 198511 as functional exploration of bone, but this approach has not been widely adopted.