Dear Editor
We read with great interest the review article written by Chin and Peh,1 entitled “Imaging update on musculoskeletal infections”, in which the authors reviewed the imaging features of musculoskeletal infection, including osteomyelitis, on radiograph, ultrasound, computed tomography, and magnetic resonance imaging (MRI). Although the authors have made an extensive list of characteristic MRI findings, we would like to add a discussion about an important imaging sign, that despite not being pathognomonic for the diagnosis of osteomyelitis, it is quite specific, and has a fundamental importance in the differential diagnosis between osteomyelitis and primary bone tumors.
In some cases, osteomyelitis may be confounded radiographically, and even on MRI, with a variety of benign and malignant bone tumors, especially in children. Then, MRI features that can help to differentiate infection from tumor are of utmost value.2 The extramedullary and/or intramedullary fat globules sign, seen as extraosseous or intramedullary fat, characterized by hyperintensity on T1-weighted imaging and conversely hypointensity on sequences with fat suppression, in the region of marrow signal abnormality, as well as in the adjacent soft tissue (Fig. 1), can improve the MRI specificity for the diagnosis of acute osteomyelitis.2 Sometimes, a cloaca can be seen, through which the fat globules, can be discharged to the soft tissue.3 The fat signal can be distributed diffusely throughout most of the infected bone, or focally, as one or more localized fat collections within the infected area.4
Fig. 1.
An 80-year-old man presented with fever and severe pain in the right thigh for one week. Complete blood counts disclosed leukocytosis and neutrophilia. MRI of the hips and thighs disclosed extensive bone marrow edema in the right femur, extensive periarticular and muscle edema, associated with small foci of focal fat globules, characterized by hyperintense signal on T1-weighted imaging, within the right femur (arrows in a) and adjacent soft tissues in the coronal (arrows in b) and axial planes (arrows in c), with signal suppression on fat-suppressed T1-weighted imaging (arrows in d). Pyogenic osteomyelitis was suspected, and a culture obtained at biopsy isolated Staphylococcus aureus.
Its pathophysiology is related with the increase in the intramedullary bone pressure, due to bone edema, secondary to the medullary bacterial proliferation, acute suppurative response, exudate formation and hyperemia, which lead to obliteration of the fat marrow MRI signal, as well as to septic necrosis, with lipocyte death and release of free fat globules. The increasing intramedullary pressure leads to a rupture of the osseous cortex and expulsion of medullary fat into the adjacent soft tissues.4,5 This sign is usually seen in aggressive acute osteomyelitis resulting in extrusion of medullary fat as opposed to a subacute, slowly progressive process.4
Although some authors have considered the fat globules sign to be pathognomonic for acute osteomyelitis,3 actually, it can also be seen in medullary infarction, trauma, bone sarcomas after treatment with chemotherapy and/or radiotherapy, and Gaucher disease. However, in the correct clinical context it can play an important role in diagnosis of acute osteomyelitis and in the differential diagnosis with bone tumors.4,5
Therefore, the visualization of the intramedullary and/or extramedullary fat globules sign on MRI is an important feature of acute osteomyelitis, which can be used in the differentiation from other bone diseases.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
References
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