Crystal deposition in hypophosphatasia: a reappraisal.

Six subjects (three female, three male; age range 38-85 years) with adult onset hypophosphatasia are described. Three presented atypically with calcific periarthritis (due to apatite) in the absence of osteopenia; two had classical presentation with osteopenic fracture; and one was the asymptomatic father of one of the patients with calcific periarthritis. All three subjects over age 70 had isolated polyarticular chondrocalcinosis due to calcium pyrophosphate dihydrate crystal deposition; four of the six had spinal hyperostosis, extensive in two (Forestier's disease). The apparent paradoxical association of hypophosphatasia with calcific periarthritis and spinal hyperostosis is discussed in relation to the known effects of inorganic pyrophosphate on apatite crystal nucleation and growth.