Neuropathic arthropathy of the foot with and without superimposed osteomyelitis: MR imaging characteristics.

PURPOSE To determine retrospectively the magnetic resonance (MR) findings associated with pedal neuropathic arthropathy with and without superimposed osteomyelitis and to identify any useful discriminating features. MATERIALS AND METHODS Investigational review board approval was obtained and allowed review of records and images without informed consent. HIPAA compliance was observed. Contrast-enhanced MR images in patients with diabetic neuropathic arthropathy of the foot were examined by two reviewers in consensus. Affected joints were examined for marrow, articular, periarticular, and soft-tissue findings. Presence of superimposed osteomyelitis was documented. A subgroup that had undergone MR before infection was evaluated for comparison; chi(2) and t tests were used to evaluate the associations. RESULTS Of 128 neuropathic joints in 63 patients (24 female, 39 male; aged 31-78 years), 43 had superimposed osteomyelitis. Effusion was common in all neuropathic joints, but thin rim enhancement was more common in noninfected joints (62% vs 21%, P < .001) and diffuse joint fluid enhancement was more common with infection (47% vs 26%, P = .052). Subluxation, bone proliferation, fragmentation, and erosion were seen in both groups, but intraarticular bodies were more common in noninfected joints (53% vs 12%, P < .001). In the periarticular soft tissues, edema, enhancement, and ulceration were common in both groups. Fluid collections in the soft tissues were more commonly associated with infected joints (95% vs 48%, P < .001) and, when present next to an infected joint, were larger than those next to noninfected neuropathic joints (2.6 cm(2) [range, 0.3-8.6 cm(2)] vs 1.6 cm(2) [range, 1.0-2.4 cm(2)]). Soft-tissue fat replacement (68% vs 36%, P = .002) and sinus tracts (84% vs 0%, P < .001) were also more common with infection. In the marrow, periarticular signal intensity abnormality was common in both groups, but the extent was greater with infection. Subchondral cysts were seen almost exclusively in noninfected joints (76% vs 2%, P < .001). Similar results were obtained in the subgroup of 21 joints (15 patients) with both pre- and postinfection MR images. CONCLUSION Sinus tract, replacement of soft-tissue fat, fluid collection, and extensive marrow abnormality are MR imaging features indicating superimposed infection. Thin rim enhancement of effusion, presence of subchondral cysts, or intraarticular bodies indicate absence of infection.