Morphologie stages in lamellar bone formation stimulated by a potent mechanical stimulus

Abstract

The temporal stages of lamellar bone formation were studied using an animal model subject to up to 16 weeks of a controlled, externally applied load. The left ulnae of 15 adult male turkeys were functionally isolated via transverse metaphyseal osteotomies, while transcutaneous Steinmann 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 normal to the bone's longitudinal axis of 2000 microstrain (με). The contralateral limb was left surgically undisturbed and served as a baseline control. Following a loading period of 4, 8, or 16 weeks, ulnae were harvested and prepared for quantitative bone histomorphometry. Compared with each animal's contralateral ulna, the area of the experimental ulnae increased by 12.5% (±5.6%) at 16 weeks. Periosteal mineral apposition rates in the loaded ulnae were significantly increased compared with control values, with a maximum rate of 6.0 ± 3.4 μm/day at 5 weeks, slowing to 2.0 ± 0.3 μm/day by 15 weeks. At 16 weeks, new bone was composed of primary and secondary osteons as well as circumferential lamellae, with osteocyte density and organization indistinguishable from that of the original cortex. Remnants of the initial woven bone response seen at 4 weeks remained clearly visible at both 8 and 16 weeks as diffusely labeled interstitial elements within the newly formed lamellar construct. The presence of secondary osteons, circumferential lamellae, and an osteocyte density and organization similar to that seen in controls suggests that the presence of woven bone in the initial stages of the adaptive process is not necessarily a pathologic or transient reaction to injury, but instead may be a normal stage in response to a potent mechanical stimulus.