Applied Radiology

Discussion

Melorheostosis is a rare form of mixed sclerosing bone dysplasia. The name is derived from the Greek term melos (limb) and rhein (to flow). In children, it is characterized by dense linear streaks, mostly in the inner cortex of the affected bones. In adults, irregular ossification is seen along the outer cortex, mimicking candle drippings. Patients with melorheostosis may develop secondary symptoms related to limitation of motion, joint fusion, or periosteal irritation.^1,2 The etiology is obscure but Murry and McCreide³ suggested that an infection associated with nerve roots may be responsible for this rare condition. They found that the disorder most often involves one or more segmental sclerotomes, areas of a bone innervated from an individual spinal sensory nerve. This may in part explain the monomelic and linear track involvement and distribution of melorheostoses. A well known feature of hyperostotic bone remodeling is its distinctive appearance, resembling wax melting and dripping down one side of a candle. Several cases of this sign have been reported from antiquity; the skeleton of a 50-year-old Alaskan Eskimo from the fifth century A.D. and a 25- to 30-year-old female dating back to 4000 to 5000 B.C. from Northern Chile both showed hyperostotic lesions resembling candle wax.⁴ Despite this characteristic feature, atypical presentations, particularly forme fruste lesions, raise uncertainties about the diagnosis. Forme fruste lesions are considered the mildest manifestation of this condition, which may only focally involve the bone (monostotic) and, as such, is difficult to differentiate from a focus of myositis ossifications or periosteal osteosarcoma.⁵ Melorheostosis can be found in all age groups but is most frequently diagnosed in young adults. Contracture of joints, fibrosis of soft tissue, and changes in the skin may be present at birth,⁶ but in these cases definitive diagnosis generally is delayed until osseous abnormalities appear. There is no sexual predilection in the disorder. In 70 to 80% of cases, melorheostosis is monomelic and affects one or more contiguous bones.⁷ It is more common in lower extremities and is usually appendicular. Other possible sites include the skull, spine, ribs, and pelvic bones.^8,9 The disorder usually is asymptomatic in children; in adults, it presents with osseous complaints, usually beginning in the second and third decades of life. Occasionally, it may present with one or more palpable masses. Most of the clinical signs of the osseous dysplasia are associated with fibrosis, which is usually the cause for joint stiffness, angular joint deviations, contractures (mostly flexion), and other orthopedic problems.^8,10 Skin pigmentation, scleroderma, and tuberous sclerosis-like thickening have been described.^11,12 In rare instances, melorheostosis has been reported in association with tumors such as spinal column lipomatous osteosarcoma, and soft-tissue lesions such as desmoid tumor. Routine laboratory studies usually are unremarkable. Histologically, sclerosis, soft-tissue fibrosis, and periarticular calcification may be seen, and though these may lead to bony ankylosis of the involved joint, none of these signs are pathognomonic of melorheostosis. Differential diagnosis for this condition includes osteomyelitis, reflex sympathetic dystrophy, neurofibromatosis, soft-tissue sarcoma, and metastatic lesions. This patient's plain radiographs are characteristic (figure 1), showing wavy cortical hyperostosis which extends along the length of the bone, resembling flowing candle wax. Magnetic resonance imaging (MRI) (figure 2) features of melorheostosis include hyperostosis, which appears as uniform hypointensity on all imaging sequences. The Tc-99m-MDP three-phase bone scintigraphy image (figure 3) demonstrates moderate to severe increase in tracer in the early and late images, including soft-tissue and blood pool stages of scintigraphy. The course of this condition is unrelenting; however, it does slow considerably in adulthood. Treatment usually is symptomatic, though surgical measures such as osteotomies, excision, capsulotomies, or tendon lengthening often are pursued.

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