Teaching Case

A 74-year-old non-diabetic male patient had presented with a 4 week history of worsening right-foot pain. Clinical examination was unremarkable. Initial laboratory results showed a WCC of 16 and CRP of 140. Further investigations with nuclear imaging techniques and MRI yielded an unexpected diagnosis.

A plain ankle X-ray on initial presentation showed mild degenerative changes associated with some soft tissue swelling, without evidence of septic arthritis. As the cause of pain was not identified, a bone scan localised with SPECT/CT was performed. It demonstrated diffusely increased vascularity in the right foot, with associated intense focal uptake mostly in the right middle cuneiform.

An MRI performed two weeks later depicted widespread marrow oedema in the midfoot, again mostly in the region of the cuneonavicular joint. On post-contrast imaging, there was florid enhancement in the marrow as well as surrounding soft tissue. A gallium scan was then performed to assess the possibility of infection. Gallium scan with SPECT showed abnormal uptake in the navicular-medial cuneiform region, with the navicular bone only involved at the joint line. These findings were most suggestive of osteomyelitis of the navicular and medial cuneiform bones.

Osteomyelitis of the tarsal bones in non-diabetics is an uncommon clinical entity. The diagnosis of osteomyelitis is based on a combination of clinical findings, laboratory results and imaging [1]. Diabetic and vasculopathic patients are at more risk, especially in the context of trauma or local infection [2]. Localised pain, erythema, swelling or functional limitation of the affected areas can all be part of the presenting clinical picture, yet the absence of these can not rule out underlying osteomyelitis [1]. Whilst this makes the diagnosis difficult for clinicians, it is also important to note that it commits patients to a prolonged course of treatment. Multiple imaging modalities are required in the diagnostic process, with a combination of anatomical and functional imaging using magnetic resonance imaging (MRI) and nuclear techniques such as bone scintigraphy and gallium imaging used.

Secondary to the gold standard of a bone biopsy, MRI is considered to be the test of choice due to its ability to differentiate between soft tissue and bone abnormalities [3, 4]. It has high sensitivity (77-100%) and specificity (80-100%) in detecting osteomyelitis in the feet [5]. MRI will readily distinguish between periosteal and medullary involvement, and these findings assist in determining the stage of the disease process [1]. In acute osteomyelitis, T1-weighted images will show low-signal intensity and high-signal intensity on T2-weighted and short tau inversion recovery (STIR) images, reflecting the decreased fat and increased water within the medullary cavity [1, 4, 5]. In the chronic form, cortical abnormalities can be evident, a finding which can also be detected on plain radiography in late disease [1, 4]. Bone scan with technetium-99m can demonstrate early changes, and when positive in all 3 phases has a sensitivity of 73-100% in diagnosing osteomyelitis [4]. However, image interpretation can be difficult due to a spectrum of pathology providing a positive scan. In addition, Gallium-67, which binds to acute phase reactants, can be used and is more specific than bone scintigraphy, hence can rule out osteomyelitis in a negative scan [6].

In conclusion, this case highlights the importance of imaging in diagnosing osteomyelitis. In the absence of predisposing factors or clinical evidence of infection, the diagnosis can be delayed and the use of multiple imaging modalities is needed. Treatment requires a prolonged course of parenteral antibiotics in all cases, and operative intervention in certain instances [1, 7]. Overall, osteomyelitis still represents a challenging entity for both clinicians and patients.