Sir,

With great interest, we read the recent publications about bone infections in European Radiology [1, 2], which were helpful in finding the correct diagnosis in the following unusual case.

A 66-year-old male immigrant from Somalia presented with a chronic ulcerous tibial skin infection. The ulcer was localized at the middle third of the anterior tibia and measured 3 cm in diameter. Magnetic resonance imaging (MRI) was performed to detect signs of osteomyelitis or tumor infiltration.

MRI demonstrated the ulcer as a hyperintense soft tissue lesion on T1- and T2-weighted turbo spin-echo (TSE) images. A thick hypointense rim underneath represented the sclerotic periosteum (Fig. 1a). Contrast-enhanced fat-suppressed T1-weighted TSE images revealed a marked enhancement of the ulcer tissue, but only a moderate enhancement of the bone marrow (Fig. 1b). The absent penumbra sign [1] and abscess cavity were not suggestive of chronic forms of post-traumatic osteomyelitis [2] (Fig. 1a). Irregular margins and invasion of bony structures as a hint of a soft tissue tumor were also missing.

Fig. 1a, b
figure 1

Tropical ulcer in the lower leg of a 66-year-old-male. a Sagittal T1-weighted turbo spin-echo (TSE) image. The hypointense margin (arrow) underneath the moderately hyperintense ulcer tissue corresponds to sclerosis on radiography (not shown). No abscess cavity or penumbra sign is visible. b Sagittal contrast-enhanced fat-suppressed T1-weighted TSE image. There is a marked enhancement of the soft tissue within the ulcer (asterisk). Note the pronounced hyperintense ulcer ground (arrow). There is also a moderate enhancement within the spongy bone, but no signs of ulcer invasion into the bone

Full size image

From these clinical and imaging findings, we concluded a tropical ulcer in a typical location. The ulcer was completely excised and the defect closed with a skin flap that healed without any complications. Diagnosis was substantiated by microbiological proof of a mixed infection with species of Enterococcus, Corynebacterium, and Prevotella. Histologic analysis revealed chronic infection without signs of osteomyelitis or malignancy.

Tropical ulcers are necrotic skin infections seen in patients of all ages from tropical and subtropical regions [3]. These painful lesions result from an initial trauma. They can erode muscles, tendons, and, occasionally, the underlying bone, and mainly affect the middle third of the tibia [3]. Smear from the ulcer often reveals an infection of various microorganisms [3]. Radiographs may demonstrate cortical sclerosis and, in chronic cases, bone deformity. The imaging appearance with broad-based excrescences may resemble osteomas and is referred to as the “ivory ulcer osteoma.” Chronic ulcers may undergo malignant transformation, leading to epidermoid carcinomas [3].

The MRI findings of tropical ulcers do not include the penumbra sign in contrast to low-grade osteomyelitis. Bone marrow edema and contrast-enhancement due to hyperemia and increased endothelial permeability are non-specific signs, and can be met both in osteomyelitis and in tropical ulcers.

Although tropical ulcers are a rare entity in the Western world, it is very common in tropical regions. Immigrants and tourists with tibial ulcers might be referred for diagnostic work-up to MRI, such as in our case. MRI might especially be helpful in the preoperative work-up of patients with tropical ulcers to evaluate the extent and involvement of surrounding tissue.