Strain induced osteogenesis of the craniofacial suture upon controlled delivery of low-frequency cyclic forces.

Static forces have been used for more than a century to modulate osteogenesis of craniofacial sutures in not only laboratory research, but also clinical practice. Whether cyclic forces more effectively stimulate sutural osteogenesis than static forces is unknown. Here, the premaxillomaxillary sutures of growing rabbits received in vivo exogenous static forces with peak magnitude of 2 Newtons, or cyclic forces also at 2 Newtons but with frequencies of 0.2 Hz and 1 Hz. The static force and two cyclic forces did not evoke significant differences in the peak magnitude of static bone strain (506 microstrain 182; mean S.D.), 0.2-Hz cyclic strain (436 microstrain 191) or 1-Hz cyclic strain (461 microstrain 229). However, cyclic forces at 0.2 Hz delivered to the premaxillomaxillary suture for 10 min/d over 12 days (120 cyclic per day) induced significantly more craniofacial growth (p< 0.01), marked sutural separation, and islands of newly formed bone, in comparison with both sham controls and static force of matching peak magnitude. The bone strain threshold of approximately 500 microstrain for inducing sutural osteogenesis is lower than the minimum effective strain capable of inducing bone apposition in long bones. These data demonstrate, for the first time, that application of brief doses of cyclic forces induces sutural osteogenesis more effectively than static forces with matching peak magnitude.