The radiological appearance of fibrous dysplasia of bone (DF) is varied, as the images reflect the histological polymorphism of the disease. Thus, the degree of ossification of the tissue, whether be it more or less extensive, will correspond to radiolucent or radiopaque lesions.
1.1 The different forms
Roughly three types of appearance can be observed: either one that is homogenously dense, a “cloud of smoke” like appearance or one of a slightly condensed homogenous appearance said to look like ground glass. A homogenous dense appearance is less meaningful, whereas a cloud of smoke like appearance or that of ground glass confirms the diagnosis, as the lesion is too opaque for the size of the gap.
The frequent presence of lesional calcifications is observed around the edge, as well as in the centre, of lesions. The calcification of cartilage fibres can look like a cartilage tumour. The edges of these radiolucent lesions are generally neat (clear-cut) and condensed. A dense peripheral border is frequently present, and is readily oriented towards a benign lesion.
1.2 Degree of bone expansion
The degree of expansion of a piece of bone varies, and has been roughly categorised into 3 types by Laredo . The lesions covering the outermost edge of the bone without changing its contours leading to a framed or window like appearance (type 1). Type 2 corresponds to enlarged lesions with a thick covering and type 3 are enlarged lesions that have a fine periosteal covering. Expansion progresses continuously, although sometimes restricted by a site, over many millimetres, which is identifiable by a CT scan. These three different types can be found within the same lesion, with the size, number and form of the lesions varying greatly. The existence of multiple lesions, separated by normal bone, with a very irregular shape, elongated, and with multiple lobes is more indicative of FD than of a bone tumour. Type 2 and 3 lesions often comprise of notches on the endosteal surface. They pose no real mechanical issues as the cortical thickness is maintained.
1.3 The use of Computerized Tomography (CT) and Magnetic Resonance Imaging (MRI)
Plain film x-rays often allow a diagnosis to be made, but CT scans sometimes aid this diagnosis or allow the assessment the extent of bone extension to be made. Specifically, CT scanning allows an assessment at different angles, meaning that cracks or cortical erosion can be identified that were invisible on plain film x-rays . CT scanning can also be useful in the assessment of the effect of bisphosphonate therapy in maxillofacial and cranial lesions. Assessment through this technique is easier and more accurate than with plain film x-rays . Overall, the main interest of CT scanning lies in its use for the exploration of cranial and maxillofacial lesions as a means of diagnosis and also to investigate complications, whereas the interest for long bones is limited, and in this case it is used to identify erosion and cracks normally invisible under plain film x-ray for patients who are in pain but do not have any visible changes to their x-ray.
The appearance of FD under MRI varies according to the degree of mineralisation and its histology. Thus, with T1 spin-echo, FD presents with a moderately homogenous histology, whereas the signal varies greatly with T2 . The lesion will thus have a hyper-intense signal in two thirds of cases. The intensity of the signal for T2 depends on the degree of intralesional mineralisation. Highly mineralised sites will show a hypo-intense signal for the 2 sequences. The diagnosis is assisted by evidence of a peripheral edge with a defined hypo-intense signal, which separates the fibrous tissue from the adjacent normal bone.
APPEARANCE ACCORDING TO SITE
1.4 In the long bones
FD is found in the metaphysis or the diaphysis, more often in the centre of the bone rather than the outer, and in general is elongated in appearance.
1.5 In the spine
Small sized lesions are found that are puffy (which poses diagnosis issues with a malignant tumour) or totally within the bone, sometimes touching the posterior arc. Vertebral fractures are possible and a blown-out appearance of the bone can be the origin of a medullary compression.
1.6 In the face and skull
In the face, a dense ground glass like appearance will be readily observed, with enlargement of the diploe, an overall hypertrophic bone in the frontal region, whereas the maxilla and mandible will have a more mixed appearance. The teeth are often displaced. These features are distinguishable from those of cherubism, which is a genetic disorder of dominant autosomal transmission that is discovered in childhood, which combines symmetric hypertrophy of the mandibles and serious dental issues, but does get slightly better with growth. In the cranium, roughly three appearances can be observed: osteolytic, condensed forms and a moderately condensed form that resembles bones with Paget’s disease. The condensed forms essentially are located at the base of the skull, in a diffuse arrangement, or more restricted affecting only the sphenoid area. An increase in density to spongy bone is observed, and bone hypertrophy affecting both sides of the face that decreases the volume of the sinus cavity. Osteolytic DF forms are observed, above all, in the cranial vault, affecting one or many bones. The external table is most commonly affected, but if both tables are affected, thinning will be extensive. Diagnosis is differentiated from that of Paget’s disease by the existence of the cortical layer. A doughnut like appearance, measuring 1 to 5 cm in diameter with a clear centre and a dense thin border with neat edges is characteristic of FD.
1.7 In the ribs
FD id frequently puffy, elongated along the long rib axis, and limited by a fine periosteal covering distinct from the normal bone.
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Created: 09 june 2010