Early effect of parathyroid hormone (1-34) on implant fixation

Intermittent systemic administration of parathyroid hormone increases bone formation by stimulating osteoblastic activity. The current study determine d how parathyroid hormone (1-34) administration influences the bony fixatio n of stainless steel screws with time. A screw was implanted in the left ti bia and a metal rod was implanted in the right tibia in 30 adult male rats that then were injected three times a week with human parathyroid hormone ( 1-34) at 60 mug/kg/injection (n = 15) or saline (n = 15). The animals were euthanized after 1, 2, or 4 weeks of treatment. Eight additional rats recei ved only the screw and were euthanized immediately after implantation. No s ignificant effects of parathyroid hormone on body weight change or ash weig ht of the femurs were seen. The degree of fixation was assessed by measurin g pullout strength of the screws. The mean pullout strength immediately aft er implantation was 12 N. The pullout strength of the group injected with s aline was 33 N after 1 week, 23 N after 2 weeks, and 41 N after 4 weeks. Th e pullout strength of the group injected with parathyroid hormone increased to 43 N after 1 week, 58 N after 2 weeks, and 100 N after 4 weeks. The inc rease at 2 and 4 weeks was statistically significant. Strength reflects the mechanical properties of the bone within the screw threads. The contralate ral tibia with its metal rod was used for blinded histologic assessment. Pa rathyroid hormone increased the fraction of the metal surface having contra ct with bone without an intervening soft tissue layer from 45% to 69% after 1 week. The current results suggest that intermittent parathyroid hormone treatment can enhance early implant fixation by enhancing the density of th e surrounding bone and by increasing the implant bone contact.