The Influence of Fixation Stiffness on Bone Regeneration in a Rodent Bone Critical Size Defect Model

The interplay between mechanical stability and healing response in critical-size bone defects is poorly studied. The objective of this study was to investigate the relationship between Ratfix^TM constructs' stiffness in a rodent model of critical size defect (CSD) in femurs. We hypothesized that lower stiffness would result in improved bone regeneration. To test our hypothesis, we created CSD in male Fischer 344 rats and randomly assigned animals to rigid, intermediate, or flexible Ratfix^TM construct groups. In vivo radiographs were obtained every 2 weeks, and all animals were euthanized at 8 weeks for microCT analysis, histology, and biomechanical testing. At the end of the experiment, 73% of the flexible stiffness group, 100% of the intermediate stiffness group, and 63% of the rigid group demonstrated radiographic union. The intermediate group formed significantly more bone volume (BV) and callus volume (CV) than the rigid group, but was not significantly higher than the flexible group. Torsional stiffness and torque to failure of the intermediate group were over three-fold higher than the rigid group, but not significantly greater than the flexible group. Our study suggests that when treating CSD, mechanical stability is an important factor to facilitate bone regeneration, with optimal stiffness being critical, as both overly flexible and overly rigid constructs can impair healing.