Superior efficacy in suppression of heterotopic bone formation using fractionated irradiation of 5 x 2 Gy compared to a single dose of 7 Gy. An experimental study in rats.

Postoperative irradiation of the operative field is an established method to prevent heterotopic ossification in total hip arthroplasty. In this study two theoretical dose-equivalent regimens of radiation therapy were compared. Allogenic bone matrix was implanted in both thighs of 50 adult male Wistar rats to induce heterotopic ossification. Immediately after operation the implants of 40 animals were irradiated using a single-dose of 7 Gy or 5 fractions of 2 Gy each. Ten rats served as a controlgroup and did not undergo irradiation. Radiation therapy with 5 x 2 Gy led to a highly significantly better suppression of heterotopic ossification than irradiation with 1 x 7 Gy (p < 0.001; paired-t-test). Single-dose irradiation reduced the mean calcium contents to 138.87 +/- 22.84 micrograms Ca2+/mg implanted bone matrix; fractionated irradiation obtained a reduction to 63.35 +/- 21.16 micrograms Ca2+/mg implanted bone matrix. In thigh implants not exposed to irradiation the mean calcium content was 191.50 +/- 11.46 micrograms Ca2+/mg implanted bone matrix. Radiographically better suppression of bone formation could be documented after irradiation with 5 x 2 Gy compared to 1 x 7 Gy and non-irradiated implants. The histological aspect of the explanted specimens showed quantitatively more new bone formation in the non-irradiated controls than in both irradiation groups. In view of experimentally demonstrated better effects, as well as the reduced side effects, fractionated irradiation appears preferable.