The effect of magnitude and frequency of interfragmentary strain on the tissue response to distraction osteogenesis

Bone regeneration is believed to be partially controlled by the applied local mechanical strain. To test whether the magnitude or frequency of discontinuous traction regulates the tissue response, defined daily strains were applied on mandibular osteotomies using an implanted mechanical distractor.

Unilateral mandibular osteotomies were performed in skeletally immature rabbits (n = 36). and distraction was done by applying 2,000, 20,000, 200,000, or 300,000 microstrains once or 10 times (2,000, 20,000 microstrains) per day, respectively. Sham-operated animals (n = 6), serving as controls, underwent frame application and osteotomy without distraction. At the end of the distraction process, the newly formed tissue was evaluated histomorphometrically by the use of a well-defined scoring system of bone-forming indices.

The highest bone-forming indices were detected in the osteotomized, nondistracted group and in samples exposed to a physiologic strain (2,000 microstrains). Application of hyperphysiologic strains (200,000 and 300,000 microstrains) resulted in the formation of fibrous tissue and decreased bone-forming indices. Using Kruskal-Wallis tests, a statistically significant relationship was found between the bone-forming indices and the applied strain magnitudes. Scanning and transmission electron microscopic examinations showed osteoblastic differentiation and early mineral deposition in samples distracted up to 20,000 microstrains, whereas higher strain magnitudes led to the formation of fibroblast-like cells surrounded by collagen fibrils and only slight mineralization. Multiple strain applications (10 cycles/d vs 1 cycle/d) did not alter the histomorphometric indices or ultrastructural morphology significantly but increased the amount of newly formed tissue.

These results suggest that the magnitude and not the frequency of mechanical loading controls the differentiation of bone cells and the subsequent formation of bone tissue.