The consideration is whether this is soft tissue calcification or ossification. The differential for these differs. Calcification typically may form rings or arcs although may also appear as amorphous high density material. In distinction, ossification requires differentiation by formation of trabeculated medullary bone and hence often also cortical bone. In this case it is clear, especially in the pelvis that the high density bridging material is due to soft tissue ossification containing clearly definable trabeculated medullary bone.
Some soft tissue calcifying conditions can occasionally co-exist with ossification especially “metastatic or tumoral calcinosis” (idiopathic or related to phosphate metabolism disorders), focal changes of “calcinosis circumscripta” (scleroderma) or the more diffuse, collagen vascular disease associated (especially dermatomyositis), “calcinosis universalis”. None would apply to this diffuse well formed new bone case.
Pure soft tissue ossification may related to osteosarcomatous tumors which of course by virtue of the multifocal distribution would be unlikely in this case. Synovial sarcomas can also ossify. Diffuse heterotopic ossification, as in this case, may be due to trauma. Post-traumatic heterotopic ossification tends to be localised to a specific area of injury arising quickly within 4-6 weeks of injury. Starting as a diffuse faint veil this becomes ossified with trabecular bone that aligns along muscle planes. A very similar appearance can also occur more diffusely in patients with prolonged immobilization (including paraplegia/quadriplegia) as well as burns victims. The cause for this uncertain and may be due to unperceived trauma. The distribution is usually around large joints in the hips, shoulders and knees. Compared to heterotopic bone formation osteosarcoma may demonstrate connection of the medullary cavity and wraps around bones. Osteosarcoma ossification is usually denser centrally, whereas heterotopic bone is usually denser peripherally.
The abnormality the fits best for this diffuse condition of soft tissue ossification, also affecting the neck, is fibrodysplasia ossificans progressive (FOP) also referred to as myositis ossificans congenita progressive. This is a genetic dysplasia, transmitted in an autosomal dominant fashion with variable expressivity but complete penetrance. As most patients cannot have children, or elect not to, most cases encountered clinically are sporadic new mutations. The disease results in diffuse ossification of connective tissues rather than muscle. The condition usually starts in the neck and shoulder girdle in the first decade with progressive palpable fibrotic nodules that progress to ossification. These result in marked restriction of motion. The disease progresses caudally to the back and then the pelvic girdle. Unusually these patients often demonstrate macrodactyly of the great toe (often uniphalangeal), thumb or middle phalanx of fifth finger. In addition to extensive new “bones” pseudoexostoses may be mimicked by the presence of ossification of tendinous insertions.
Life expectancy can be considerably reduced by the progressive restriction of the thoracic musculature. No approved therapies exist currently and surgery or biopsy are relatively contraindicated as they may result in greater reaccumulation of soft tissue bone.