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Year : 2016  |  Volume : 5  |  Issue : 4  |  Page : 281-283

Extraosseous fat-fluid level on computed tomography and magnetic resonance imaging: A specific sign of hematogenous osteomyelitis

Department of Radiodiagnosis, Maharishi Markandeshwar Institute of Medical Sciences and Research, Mullana, Ambala, Haryana, India

Date of Web Publication15-Nov-2016

Correspondence Address:
Dr. Puneet Mittal
448, Opp. Singla Memorial Hospital, Prem Basti, Sangrur - 148 001, Punjab
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/2278-344X.194135

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Extraosseous fat-fluid level is a rare but specific sign of hematogenous osteomyelitis. It results due to marrow necrosis due to infection with the release of fat globules and pus which extends into periosseous tissues through bony erosions. To form a fat-fluid level, the marrow necrosis has to be extensive which is not always present. However, when present and in appropriate clinical settings, it is fairly specific for hematogenous osteomyelitis. We describe radiographic, computed tomography, and magnetic resonance imaging findings in a case of osteomyelitis with extraosseous fat globules and fat-fluid level.

Keywords: Computed tomography, extraosseous, fat-fluid, magnetic resonance imaging, osteomyelitis

How to cite this article:
Mittal P, Gupta R, Mittal A, Gupta S, Mittal K, Taneja A. Extraosseous fat-fluid level on computed tomography and magnetic resonance imaging: A specific sign of hematogenous osteomyelitis. Int J Health Allied Sci 2016;5:281-3

How to cite this URL:
Mittal P, Gupta R, Mittal A, Gupta S, Mittal K, Taneja A. Extraosseous fat-fluid level on computed tomography and magnetic resonance imaging: A specific sign of hematogenous osteomyelitis. Int J Health Allied Sci [serial online] 2016 [cited 2022 Jul 6];5:281-3. Available from: https://www.ijhas.in/text.asp?2016/5/4/281/194135

  Introduction Top

Extraosseous fat-fluid level on magnetic resonance imaging (MRI) has been reported very rarely in osteomyelitis with only four previous cases reported in the literature.[1],[2],[3],[4] Although rare, it is a relatively specific finding which is helpful in the diagnosis of osteomyelitis in appropriate clinical settings. Therefore, this finding is important to recognize, especially when other imaging findings are equivocal or definite cortical erosions are not identified on imaging. This case report highlights the importance of this important sign, so that when present, it can help to reach a specific diagnosis and aid in timely management.

  Case Report Top

An 18-year-old male patient presented with complaints of pain and swelling in the right lower thigh for past 1 month. It was associated with fever. There was no history of trauma. On physical examination, there was focal tenderness, swelling and increased temperature in lower thigh along medial aspect. No external sinus opening was identified. Plain radiograph of lower thigh revealed long segment medullary destructive lesion with associated periosteal reaction and soft tissue swelling. Fat globules were identified in the soft tissue swelling on retrospective review [Figure 1]. MRI revealed long segment medullary signal abnormality involving lower meta-diaphyseal region of the right femur with extension into the distal epiphysis. There was extensive inflammation of soft tissues, with associated extraosseous fat globules and collection with fat-fluid level on medial aspect [Figure 2]. Computed tomography (CT) scan revealed extraosseous fat-fluid level with associated cortical erosions [Figure 3]. Based on the clinical and radiological findings, diagnosis of osteomyelitis was made which was subsequently confirmed by aspiration of frank pus from the soft tissue collection. The culture was positive for Staphylococcus aureus. The patient improved symptomatically after antibiotic treatment and percutaneous aspiration.
Figure 1: Frontal and lateral radiograph of the right thigh show destructive meta-diaphyseal lesion with wide zone of transition with associated periosteal reaction. There is soft tissue swelling, more so on the medial aspect. Subtle lucencies (white arrows) are seen in the soft tissue swelling indicating extraosseous fat globules

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Figure 2: Coronal T1-weighted (a), axial T1-weighted (b) and axial short tau inversion recovery (c) magnetic resonance images show extensive metadiaphyseal fat necrosis with extension into the epiphysis (white arrow in a). Extraosseous collection with fat-fluid level is seen on medial aspect with floating fat appearing hyperintense on T1-weighted images (white arrow in b) and showing suppression on short tau inversion recovery (white arrow in c). Other extraosseous fat globules are also identified (dotted white arrows in a and b). Small cortical erosions are also seen (dotted white arrows in c)

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Figure 3: Axial computed tomography sections with soft tissue (a) and bone (b and c) windows show extraosseous fat-fluid level (white arrow in a) and extraosseous fat globules (dotted white arrows in a). Multiple cortical erosions are also seen (white arrows in b and c)

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  Discussion Top

Osteomyelitis can sometimes present with diagnostic difficulties on imaging and can be confused with tumors and other pathologies.[3] The presence of extraosseous fat globules and fat-fluid level in the absence of trauma can help to make the specific diagnosis of osteomyelitis. The fact that this finding is rare is likely because it requires extensive marrow necrosis to proceed rapidly for radiographically visible fat globules and fat-fluid level to accumulate in extraosseous tissues.[4]

Acute hematogenous osteomyelitis is not uncommon in children and is most commonly caused by S. aureus.[5] It can be associated with soft tissue infection. Osteomyelitis starts in the metaphysis owing to the slowing of circulation in this region which promotes bacterial growth.[1]

Radiographs are usually the initial mode of infection but are only positive after 1 week. The earliest sign is deep soft tissue swelling followed by periosteal reaction. Thereafter poorly marginated changes of medullary destruction become evident. These findings are frequently nonspecific and can be confused with rapidly growing tumors like Ewing's sarcoma.[6],[7]

CT is more sensitive for early changes of osteomyelitis. It can demonstrate soft tissue swelling, periosteal reaction and cortical erosions earlier and to better advantage that radiographs. CT can also demonstrate extraosseous fat globules and fat-fluid level when present.[3],[4]

MRI is the most sensitive modality for diagnosis of acute osteomyelitis. It can demonstrate early marrow signal abnormalities before the radiographic findings set in. However, these findings are also frequently nonspecific, and mere evidence of marrow edema does not mean osteomyelitis as differentiation from reactive marrow edema due to soft tissue infection is difficult. More specific findings of osteomyelitis are the presence of periosteal reaction, cortical erosions, and sequestrum formation.[1] MRI can also well demonstrate extraosseous fat globules and fat-fluid level. The differential diagnosis also includes fat containing soft tissues masses which can occasionally show fat-fluid level such as lymphangioma, liposarcoma, and dermoid.[4],[8] However, these can be readily excluded based on the history, location, and other associated findings. Fat-fluid level can also be seen in the setting of trauma with intra-articular fracture when shearing forces cause release and extraosseous extension of fat globules through cortical defect which, along with hemorrhage results in lipohemarthrosis.[4],[8] In case of osteomyelitis, rapid fat necrosis can cause the release of fat globules, which along with purulent material can form fat-fluid level, as seen in our case.

In conclusion, extraosseous fat globules and fat-fluid level are specific signs of osteomyelitis in the absence of trauma and appropriate clinical settings and can be helpful in correct diagnosis, especially when other findings are equivocal.

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Conflicts of interest

There are no conflicts of interest.

  References Top

Hui CL, Naidoo P. Extramedullary fat fluid level on MRI as a specific sign for osteomyelitis. Australas Radiol 2003;47:443-6.  Back to cited text no. 1
Davies AM, Hughes DE, Grimer RJ. Intramedullary and extramedullary fat globules on magnetic resonance imaging as a diagnostic sign for osteomyelitis. Eur Radiol 2005;15:2194-9.  Back to cited text no. 2
Kumar J, Bandhu S, Kumar A, Alam S. Extra-osseous fat fluid level: A specific sign for osteomyelitis. Skeletal Radiol 2007;36 Suppl 1:S101-4.  Back to cited text no. 3
Swain FR, Strongwater A, Milman E. Diagnosis and triage of a patient with an extra-osseous fat fluid level. Emerg Radiol 2011;18:503-5.  Back to cited text no. 4
Vazquez M. Osteomyelitis in children. Curr Opin Pediatr 2002;14:112-5.  Back to cited text no. 5
Perron AD, Brady WJ, Miller MD. Orthopedic pitfalls in the ED: Osteomyelitis. Am J Emerg Med 2003;21:61-7.  Back to cited text no. 6
McCarville MB. The child with bone pain: Malignancies and mimickers. Cancer Imaging 2009;9:S115-21.  Back to cited text no. 7
Davis DL, Vachhani P. Traumatic extra-capsular and intra-capsular floating fat: Fat-fluid levels of the knee revisited. J Clin Imaging Sci 2015;5:60.  Back to cited text no. 8
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  [Figure 1], [Figure 2], [Figure 3]

This article has been cited by
1 Extraosseous marrow fat: an MRI sign of acute aggressive osteomyelitis
Dorissa Lahner Gursahaney,MK Jesse,Jason Stoneback
BJR|case reports. 2018; : 20180050
[Pubmed] | [DOI]


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