Pathologic fracture due to severe osteomalacia following low-dose diphosphonate treatment of Paget's disease of bone.

Ethane-I-hydroxy-I,I-diphosphonate (EHDP) 5 mg/kg body weight/day was administered to a patient suffering from Paget's disease of bone. After 150 days of treatment, when the plasma alkaline phosphatase was 40% of initial, he suffered a pathologic fracture of his Pagetic right patella, which was found to have osteomalacia of extreme severity. An iliac crest bone biopsy, following double tetracycline labels, also showed severe osteomalacia of Pagetic bone. Osteoclast acid phosphatase activity was reduced, as occurs in diphosphonate-treated rats. The patient's history included two previous pathologic fractures of Pagetic bone and unusual sensitivity to EHDP. It might be prudent to perform needle bone biopsy in order to exclude osteomalacia in EHDP-treated patients with these clinical manifestations.