Clinical History
A 51-year-old patient was referred to our University Medical Centre with the diagnosis of an osteosarcoma of the left upper arm. He had a 10-week history of pain and swelling of his left upper arm, which increased in the evening. He had no fever in this period.
Imaging Findings
The conventional radiograph of the left upper arm showed a geographic to moth-eaten lobulated osteolytic lesion in the proximal meta-diaphysis of the humeral bone with a discrete periosteal reaction. No matrix was present (Fig. 1). Magnetic Resonance Imaging of the left upper arm was performed. These images showed an intramedullary lesion of at least 20 cm. The T2 weighted image with fat-suppression showed several well-demarcated collections with high signal intensity.
One of these collections showed the typical "penumbra sign" on the T1 weighted image without contrast, which consist of a lower signal intensity area in the centre and a high signal intensity rim (Fig. 2a). This high intensity rim was enhancing after gadolinium administration while the centre did not enhance, consistent with fluid/pus (Fig. 2b, c). Furthermore, multiple high signal intensity foci on the T1 weighted images without contrast were visualised, compatible with fat globules (Fig. 3a, b).
Discussion
Subacute osteomyelitis can be difficult to diagnose because the characteristic clinical signs and symptoms of acute infection can be absent. Patients are often not systemically ill, and laboratory results may be normal. Osteomyelitis can be secondary to sepsis, surgery or trauma. Its origin is typically located in the metaphysis. In direct trauma, as may be the cause in our patient, sinus thrombosis leads to stasis and infection.
The first step in imaging primary osteomyelitis is a conventional radiograph. Typically, it shows a mixture of soft tissue swelling, osseous destruction, multilamellar periosteal reaction and reactive sclerosis. Brodie’s abscess usually presents as a well-defined round or ovoid radiolucency [1]. These radiographic appearances of subacute osteomyelitis can be mistaken for malignant or benign bone tumours. Although a matrix is not formed in the presented lesion, an aggressive osteosarcoma cannot be excluded.
MRI is the most sensitive (approaching 100%) radiological modality to detect osteomyelitis, however, the specificity is much lower (approximately 80%) [2]. Several reports focus on specific MRI findings that may increase specificity. First the ‘penumbra sign’ is described as a characteristic MR feature for subacute osteomyelitis. This is thought to be helpful in differentiating subacute osteomyelitis from a neoplasm [3, 4]. Davies et al described the presence of fat globules as a specific sign for osteomyelitis. They presume that the presence of intra- and extramedullary fat globules is the result of septic necrosis and destruction of the normal lipocytes. The pattern of fatty signal may be diffuse or focal [5]. Although this sign is described in the acute form of osteomyelitis, we observed it in our patient in combination with the penumbra sign. This is probably due to an acute exacerbation of a subacute/chronic osteomyelitis. The diagnosis was confirmed with biopsy, which showed a chronic osteomyelitis with an acute component. The patient was referred for appropriate treatment of the infection; cleansing and antibiotic therapy.
Conclusion: Diagnosing osteomyelitis can be challenging on both conventional radiographs and MRI. However, knowledge of specific MR signs can help in differentiating a lesion from a malignant origin.
Differential Diagnosis List
Subacute osteomyelitis of the left humeral bone.
Subacute osteomyelitis
Osteosarcoma
Final Diagnosis
Subacute osteomyelitis of the left humeral bone.
