Melorheostosis

Melorheostosis, a combination of the Greek words melos (limb), rhein (to flow), and ostos (bone), is a rare form of mixed sclerosing dysplasia affecting the skeleton and adjacent soft tissues. In fact, soft-tissue abnormalities such as osseous, chondroid, vascular, and fibrocartilaginous masses can be seen in about 75% of cases

Melorheostosis, also known as candle disease of the bone, Leri disease, and osteosis eburnisans monomelica, has an incidence of fewer than 1 case per one million. It begins in early childhood and is evident by 20 years of age in about half of the cases. It follows a chronic course punctuated with periods of exacerbations and arrest, slowing down as the patient gets older.

The cause is unknown, but various theories have been proposed. The observation that the lesions appear in a monomelic sclerotomal distribution (areas of bone innervated by an individual spinal sensory nerve) has linked melorheostosis to an early somatic mutation, infection, or injury to a segment or segments of the neural crest during embryogenesis. Co-occurrence of melorheostosis, osteopoikilosis, and osteopathia striata (overlap syndrome) has led some to suggest a genetic cause; however, definitive evidence is lacking.

Histologically, melorheostosis is characterized by thickened trabeculae containing irregularly arranged Haversian canals surrounded by cellular fibrous tissue. While a benign condition, skin and soft tissue involvement can cause fibrosis and joint contractures, leading to deformity and limb-length discrepancies. Heterotopic bone formation and soft-tissue calcification can be seen in association with joint ankylosis.

Commonly one or several adjacent bones are affected in a sclerotomal distribution. The long tubular bones of the lower extremity are more frequently affected, with a predilection for the diaphyseal and the epiphyseal regions. However, any bone or any region of bone can be affected, including (rarely), the spine, skull and facial bones.

Para-articular soft-tissue masses seen in melorheostosis are not necessarily contiguous with the bony abnormalities. The masses can be mineralized or non-mineralized and are more commonly found medial to the hip joint and in the popliteal fossa. The case above has an irregular mineralized soft tissue mass lateral to the left greater trochanter.

Characteristic imaging findings include flowing periosteal hyperostosis along the cortex of a long bone that has a linear, segmental distribution. As mentioned above, one or more bones can be involved. Endosteal hyperostosis may be seen in later stages of the disease, obliterating the medullary cavity.

Less characteristic imaging findings include hyperostosis on the outer or inner aspect of the affected bone resembling an osteoma, an osteopathia striata-like pattern with long dense hyperostotic intramedullary striations near the inner cortex (second case, shown below), and nodular soft tissue calcifications that may resemble myositis ossificans.

As seen in the case below, bone scintigraphy can be positive, reflecting the increased bone turnover that is characteristic of melorheostosis. Mineralized soft tissue masses can also have increased uptake, but nonmineralized or minimally mineralized soft tissue masses can be occult on bone scintigraphy.

References

• Judkiewicz AM, Murphey MD, Resnik CS, Newberg AH, Temple HT, Smith WS. Advanced imaging of melorheostosis with emphasis on MRI. Skeletal Radiol. 2001 Aug;30(8):447-53.
• Janousek J, Preston DF, Martin NL, Robinson RG. Bone scan in melorheostosis. J Nucl Med. 1976 Dec;17(12):1106-8.
• Suresh S, Muthukumar T, Saifuddin A. Classical and unusual imaging appearances of melorheostosis. Clin Radiol. 2010 Aug;65(8):593-600.