The stimulating effect of growth hormone on fracture healing is dependent on onset and duration of administration.

The effect of onset and duration of growth hormone administration on the biomechanical properties of healing rat tibial fractures was investigated after 40 days of healing. Biosynthetic human growth hormone, 2.7 mg/kg body weight/day, was given in two daily injections to three groups of rats: (1) for the entire healing period; (2) for the first 20 days; and (3) for the last 20 days of healing. Three corresponding groups of control rats were injected with saline. In Group 1, maximum load and stiffness of the healing fractures increased to 165% and 175%, respectively, compared to the control group. In Group 2, maximum load, stiffness, maximum stress, and energy absorption at maximum load increased to 222%, 175%, 171%, and 247%, respectively, compared to the control group. In Group 3, no statistically detectable effects were found. The results show that growth hormone stimulates fracture healing both when given during the first part of the healing period and when given during the entire healing period.