SLR - July 2018 - Sergio A. Salazar

Osteomyelitis of the Lower Extremity: Pathophysiology, Imaging and Classification, with an Emphasis on Diabetic Foot Infection

Reference: Mandell J, Khurana B, Smith JT, Czuczman GJ, Ghazikhanian V, Smith SE. Osteomyelitis of the Lower Extremity: Pathophysiology, Imaging and Classification, with an Emphasis on Diabetic Foot Infection. Emerg Radiol (2018) 25:175–188.

Reviewed By: Sergio A. Salazar, DPM
Residency Program: HealthAlliance of the Hudson Valley, Kingston, NY

Podiatric Relevance: The incidence of osteomyelitis has nearly tripled among older adults in the past 30 years, driven predominantly by an increase in diabetes-related pedal osteomyelitis. The foot and ankle offer unique anatomic, imaging and treatment considerations. The authors discuss literature and offer a review and update on the most reliable radiology techniques used to recognize the signs and hallmarks of osteomyelitis.

Methods:
Plain radiographs and MRI are the mainstay in imaging when it comes to diagnosing osteomyelitis with MRI being the gold standard. Modalities, such as CT and radionuclide imaging, can play a role in problem-solving difficult extremity infections. Despite the choices available to image osteomyelitis, it is essential to recognize the characteristics and subtleties of underlying bone infection radiographically as well as to troubleshoot when a result is unclear. Indications for imaging are high index of suspicion, chronic/nonhealing ulcer, recurrent cellulitis or abscess and draining sinus, among others.

Results: In terms of diagnosing osteomyelitis radiographically, plain x-rays are a clinician’s go-to imaging modality; however, it is essential to recognize that at least 30 percent of the bone matrix must be destroyed for changes to be evident on conventional radiography equivalent to a minimum of 10 to 12 days. Typical radiographic findings on x-ray would include periosteal reaction, cortical erosions, focal osteopenia and diffuse trabecular lysis. In contrast, CT is the best modality to assess chronic osteomyelitis, which would typically demonstrate hallmarks, such as sequesterum, involucrum and cloaca. CT can also offer a solution to inconclusive imaging by performing a CT-guided bone biopsy, which is the definitive gold standard in diagnosing osteomyelitis with the added benefits of pathologic confirmation as well as culture and sensitivity. In the instances of suspected osteomyelitis, the setting of concomitant neuropathic arthropathy or in the instance of superimposed hardware, a three-phase bone scan is sensitive but not specific and thus rarely helpful in isolation. To make this differentiation, a radiolabeled leukocyte scan is indicated with a sensitivity of 91 percent and specificity of 92 percent when combined with sulfur colloid imaging in diagnosing osteomyelitis. However, MRI remains the preferred modality for comprehensive assessment of suspected osteomyelitis in most cases with a meta-analysis showing a sensitivity of 90 percent and specificity of 79 percent. Intravenous contrast is preferred for assessment of suspected osteomyelitis but not necessary to diagnose it. A fluid sensitive sequence (T2-weighted with fat suppression) is the most sensitive sequence to evaluate osteomyelitis. Collins et al. confirmed that confluent intramedullary pattern is the single most reliable sign indicative of osteomyelitis of the three discrete patterns of decreased signal intensity.

Conclusions: As the authors discussed, the vast majority of adult osteomyelitis is caused by spread of contiguous focus of infection. Radiographs are often negative in the early stage of osteomyelitis; however, MRI is the best noninvasive imaging modality to evaluate for osteomyelitis with confluent intramedullary pattern representing infection. Bone biopsy remains the gold standard to diagnose osteomyelitis with the benefits of guided antibiotic therapy, which can lead to greater therapeutic success and limb salvage.

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