The effect of growth hormone on fracture healing in old rats.

The healing of fractures is known to decrease with age. Several mechanisms have been identified that might explain this age-related decrease in capacity for fracture repair, one of them being a decrease in growth hormone secretion. In the present experiment, two-year-old male rats with a standardized tibial fracture were given biosynthetic human growth hormone (b-hGH, 2.7 mg/kg/day in two daily injections) during the first 40 days of fracture healing and the controls were injected with saline. After 40 or 80 days of healing, the mechanical properties of the healing fractures were evaluated by three-point bending. At day 40, no differences were found in mechanical properties of fractured and intact tibiae between b-hGH injected rats and saline injected controls. At day 80, ultimate load, stiffness, and ultimate stress of the fractures had increased by 78%, 63%, and 58%, respectively, compared with the controls. In the contralateral, intact tibiae, ultimate load and energy absorption had increased by 12% and 17%, respectively, compared with the controls.