Benign spotted bones: a diagnostic dilemma.

Spotted bones associated with benign conditions can have a very typical radiographic appearance and distribution; however, ambiguity in this appearance or a history of malignant disease can lead to diagnostic uncertainty and the need for further investigation. This clinical primer will focus on one characteristic benign condition, osteopoikilosis, its relevant differential diagnosis and workup. Osteopoikilosis was first described in 1915 by Albers-Schönberg as a sclerosing bone dysplasia of unknown cause. It is also referred to as spotted bones or osteopathia condensans disseminata. A diagnosis of exclusion, cases may be under reported. Prevalence in the general population is unknown, but an older retrospective review reported an estimated prevalence of 1 in 50 000. The lesions have been described in all age groups, and although prevalence studies have shown a higher frequency among men, the apparently unequal sex distribution may be a result of referral bias in the literature (men are more likely than women to present to hospital with traumatic injuries requiring radiologic investigation). Osteopoikilosis exists in hereditary (auto somal dominant transmission) and sporadic forms and is one of several bone dysplasias characterized by defective endochondral bone formation. Endochondral ossification refers to the formation of the long and flat bones, which begins from a primitive hyaline cartilaginous model. This process is in contrast to intramembranous ossification, which refers to direct transformation of condensed mesenchymal cells into cortical bone without a cartilaginous phase, as is typically seen in the formation of the skull bones. Most reported cases of osteopoikilosis are identified during the investigation of unrelated problems in which there is no clinical history suggestive of either malignant or systemic disease. In such situations, no further workup is necessary. The characteristic radiologic feature is multiple, punctate, sclerotic, round or oval foci symmetrically distributed in a predominantly periarticular fashion within the epiphyseal and metaphyseal regions. In the three patients described in this issue, most of the sclerotic foci are 1–2 mm, although some lesions measure up to 10 mm. Although further investigation is unnecessary in typical osteopoikilosis, when radionuclide bone scans are performed, their results are negative. In clinical and radiologic follow-up of osteopoikilosis, the lesions remain stable. Osteopoikilosis is typically an asymptomatic incidental finding, but it can be associated with other diseases. Most importantly, it must be differentiated from sclerotic metastases and other sclerosing dyplasias.