A 35-year-old male patient, originally from India, presented with worsening pain of the left wrist over the course of two months. He was otherwise asymptomatic with no history of trauma. Following failed conservative treatment in primary care, a plain radiograph of the wrist was performed.
A well-defined lytic lesion in the distal ulna can be seen with no cortical break identified on plain film. An MRI scan was then performed. On the T2 image a soft tissue mass within the distal ulna, erupting through the cortex of the bone and extending into the adjacent adipose tissue on the palmar aspect of the wrist joint was seen. The lesion enhanced vividly on administration of intravenous gadolinium. The lesion was in contact with the flexor carpi ulnaris tendon but invasion was not seen. The triangular fibrocartilage complex was intact and there was no involvement of the radius or carpal bones. No surrounding synchronous lesions were identified. In terms of classification, as the anterior margin of the lesion is indistinct but there are no permeative or moth-eaten features, the Lodwick classification would be grade 1C. This gives a likelihood of malignancy of 36%.
Background: The differential diagnosis for a lytic bone lesion is wide-ranging and includes infectious, neoplastic and cystic aetiologies. Factors such as age, previous medical history, family history, exposures, and the characteristics of the lesion itself in terms of location and radiographic appearance can be used to narrow the differential. The Lodwick classification system (illustrated in figure 3) can be used to evaluate the appearance of a lytic bone lesion and assess the likelihood of malignancy based on appearance, and has been demonstrated to be accurate and reliable [1, 2].
Clinical Perspective: The MRI findings in our case were determined to be most consistent with neoplasia and due to uncertainty regarding the histological origin of the lesion; an image-guided biopsy was performed. This in fact demonstrated necrotising granulomatous inflammation with well-formed granulomas and some central necrosis, consistent with a diagnosis of a tuberculoma. On further questioning our patient had been exposed to tuberculosis (TB) as a child but had never developed any manifestations of infection. A tuberculoma is a well-defined area of granulomatous inflammation caused by localised infection with mycobacterium tuberculosis and represents an extra-pulmonary manifestation of TB. Musculoskeletal involvement by TB is rare in comparison to other forms of the disease, with extra spinal musculoskeletal TB specifically occurring in only 1-2% of cases . It most commonly involves the spine but can also involve large weight-bearing joints, long bones, the skull and soft tissues.
Imaging Perspective: Due to its imaging appearance it can often mimic other pathologies such as malignancy on imaging, and is therefore an important differential diagnosis to the considered in anyone with a potential exposure to TB. The typical appearance on plain films is that of a lytic and well-defined lesion with or without surrounding sclerosis. Magnetic resonance imaging (MRI) is the modality of choice for diagnosis and may also detect any associated soft tissue or bone marrow abnormalities. MRI may also demonstrate intraosseous involvement earlier than with other imaging modalities. Marrow changes are demonstrated as areas of low and high signal intensity on T1- and T2-weighted images respectively, and show enhancement after the intravenous administration of gadolinium. Areas of necrosis appear hyper-intense on T2- weighted images and show no enhancement .
Outcome: In most cases of musculoskeletal TB, treatment with multi-drug chemotherapy regimens used for other forms of TB is sufficient. In severe cases, however, surgical intervention can be required.
Differential Diagnosis List
Tuberculoma of distal ulna
Infection or abscess
Benign neoplasm such as brown tumour of hyperparathyroidism or enchondroma
Malignant primary neoplasm such as osteoblastoma
Malignant fibrous histiocytoma
Giant cell tumour
Simple or aneurismal bone cyst
Tuberculoma of distal ulna