Histomorphology-Correlated Imaging of Osteonecrosis

Abstract

Early diagnosis and proper staging is of crucial clinical importance for osteonecrosis (ON). To better understand the different imaging patterns, the underlying pathophysiological sequence has to be known. The initial stage 0 is less understood and all routine imaging modalities are negative. In stage I, the bone marrow changes can be visualised by bone scintigraphy and MRI only, but a sufficiently effective repair mechanism can make the lesion reversible. In stage II an insufficient repair process demarcates the necrotic lesion from the viable bone. At this “point of no return” the lesion becomes irreversible and on MRI the demarcation border appears as a “reactive interface” with a “double-line sign”. In this early stage II plain radiographs and computed tomography (CT) are normal or show only non-specific changes. In the late stage II the insufficient repair process leads to typical patchy subchondral radiolucencies mixed with sclerotic changes in the necrotic area surrounded by a sclerotic rim on radiographs and CT. In stage III the repair mechanism and the loading stress produce the subchondral fracture which can be best visualised as a “crescent sign” by radiographs or CT. In stage IV mechanical instability leads to flattening of the femoral head followed by late secondary joint destruction. Nevertheless in ON plain radiographs remain the first diagnostic step and allow exclusion of most of the differential diagnoses for hip joint pain. However, CT is very helpful for early detection of a possible subchondral fracture. Although bone scintigraphy is very sensitive in the early onset of the disease, it remains non-specific in most of the cases. MRI is the modality of choice for a proper diagnosis and staging in early ON.