Observations on para-articular uptake of radiophosphate after hip replacement.

A correlation was made between the incidence and intensity of 99mTc-methylene diphosphonate (MDP) concentration and the radiographic incidence and volume of heterotopic calcium deposition in 44 patients with hip arthroplasties, as well as with the duration of the hip implants. The incidence of soft tissue para-articular MDP concentration was 95% compared to a 68% incidence of radiographically visible heterotopic calcium, bone or both. The concentration of MDP did not decrease on average with the age of the implant, which ranged from 6 months to 15 years, even though radiographs showed evidence of mature ossification in many of them. The cause of these sustained high uptakes is open to speculation, but the findings indicate that radiophosphate imaging is not a reliable means of assessing maturation of heterotopic bone associated with hip arthroplasty.