Serum alkaline phosphatase as an indicator of heterotopic bone formation following total hip arthroplasty.

Serum alkaline phosphatase (SAP) was analyzed in 193 patients treated with total hip arthroplasty (THA) and correlated with the degree of heterotopic bone formation (HBF) one year after surgery. The influence of indomethacin on changes in SAP related to the development of heterotopic bone was studied. Ninety-eight patients received 25 mg of indomethacin three times daily for the first six postoperative weeks; the remaining 95 patients received a placebo treatment. No further anti-inflammatory drugs were allowed during the six weeks. SAP was measured preoperatively and six weeks and 12 weeks after surgery. No patients at risk for developing heterotopic bone after THA could be identified from the preoperative level of SAP. The level of SAP six weeks after THA gradually increased with the amount of HBF. A rise in SAP above 250 IU/liter 12 weeks after surgery was associated with the development of severe heterotopic bone in 13 of 17 patients. Indomethacin inhibited the development of heterotopic bone associated with a rise in SAP following THA. Future studies on HBF and SAP following THA should include information on patient use of anti-inflammatory drugs in the early postoperative period.