Forty-four patients who had fibrodysplasia ossificans progressiva
responded by mail to a questionnaire regarding the age at the onset of
heterotopic ossification at fifteen commonly involved anatomical sites. The
average age of the patients when they responded to the questionnaire was
twenty-seven years (range, three to sixty-nine years). The average age at
the onset of ossification was five years (range, birth to twenty-five
years). The most common sites of early heterotopic ossification were the
neck, spine, and shoulder girdle. Thirty-five (80 per cent) of the patients
had had some restrictive heterotopic ossification by the age of seven
years. By the age of fifteen years, forty-two (more than 95 per cent) of
the patients had severely restricted mobility of the upper limbs. In these
patients, heterotopic ossification proceeded in a direction that was axial
to appendicular, cranial to caudad, and proximal to distal; this pattern
appeared typical for fibrodysplasia ossificans progressiva.