MORPHOLOGIC STAGES IN LAMELLAR BONE-FORMATION STIMULATED BY A POTENT MECHANICAL STIMULUS

The temporal stages of lamellar bone formation were studied using an a nimal model subject to up to 16 weeks of a controlled, externally appl ied load, The left ulnae of 15 adult male turkeys were functionally is olated via transverse metaphyseal osteotomies, while transcutaneous St einmann pins permitted in vivo loading of the preparation via a servo- hydraulic actuator, For 5 days per week, the ulnae were exposed to 100 cycles per day of an applied load sufficient to cause a peak strain n ormal to the bone's longitudinal axis of 2000 microstrain (ys). The co ntralateral limb was left surgically undisturbed and served as a basel ine control, Following a loading period of 4, 8, or 16 weeks, ulnae we re harvested and prepared for quantitative bone histomorphometry, Comp ared with each animal's contralateral ulna, the area of the experiment al ulnae increased by 12.5% (+/-5.6%) at 16 weeks, Periosteal mineral apposition rates in the loaded ulnae were significantly increased comp ared with control values, with a maximum rate of 6.0 +/- 3.4 mu m/day at 5 weeks, slowing to 2.0 +/- 0.3 mu m/day by 15 weeks, At 16 weeks, new bone was composed of primary and secondary osteons as well as circ umferential lamellae, with osteocyte density and organization indistin guishable from that of the original cortex Remnants of the initial wov en bone response seen at 4 weeks remained clearly visible at both 8 an d 16 weeks as diffusely labeled interstitial elements within the newly formed lamellar construct, The presence of secondary osteons, circumf erential lamellae, and an osteocyte density and organization similar t o that seen in controls suggests that the presence of woven bone in th e initial stages of the adaptive process is not necessarily a patholog ic or transient reaction to injury, but instead may be a normal stage in response to a potent mechanical stimulus.