What is the best time of day to schedule a bone graft operation?

The circadian control of bone and cartilage induction in calcified, surface-decalcified and cryolytic allografts implanted heterotopically (im) or in fibular osteotomy sites was assessed in rats entrained in a 12/12 hour light cycle (photoperiod = 0800-2000 hr): heterotopic grafting was performed at intervals of 4 hours within a 24-hour period, and the animals were sacrificed 3-4 weeks after grafting. Interstitial chondrification (induction) in the vascular spaces of heterotopic decalcified bone allograft matrix was significantly greater in implants performed during the day (0800-1600 hr). This response (85Sr-retention, histiometry) was heightened in animals treated with cortisone at 1600-2000 hours, suggesting that the inductive rhythms may be associated with circadian fluctuations in the immune responsivity of the host animals to antigenic challenge. Bony union was more prevalent in osteotomy implants made at 0400-0600 hour (night) than at 1600-1800 hr (day), irrespective of graft type. The incidence of clinical non-union was 50-60 per cent greater during the day when autologous or surface decalcified allograft bone was used, and about 25 per cent greater during the day with fresh allograft bone. The differences in implant success may relate to the circadian shifts in the relative size of the pool of progenitor cells in the connective tissue bed surrounding the graft at the time of transplant surgery, and to the ability of trauma to act as an osteogenic stimulus.