The effect of radiation therapy on the fixation strength of an experimental porous-coated implant in dogs.

Radiation therapy prevents heterotopic ossification after cemented total hip arthroplasty, but its effect on the fixation of porous-coated devices is not known. The purpose of this study was to determine the effect of irradiation on the pullout strength of a porous implant in the canine model. A custom-designed titanium fiber metal-coated cylinder was implanted into the distal femurs of 12 dogs. One femur of each dog was irradiated with 10 Gy, given in four 2.5-Gy daily fractions beginning one day after surgery. Serial bone scans were performed at one, two, four, and six weeks postoperatively. At six weeks, the femurs were harvested, and eight were randomly selected for mechanical testing. Scintigraphic evaluation showed a significant reduction in uptake at one and two weeks in the irradiated limb, but this had returned to normal at four and six weeks. The failure to pullout was significantly less in the irradiated limb compared with the control limb six weeks postoperatively. In this canine model, the pullout strength of the titanium fiber implant was significantly reduced by a dose of radiation commonly used in the prevention of heterotopic ossification. If irradiation is chosen for prophylaxis, either the porous implant should be adequately shielded or a cemented implant should be utilized.