Effects of cortical bone thickness at different healing times on microscrew stability.

Objective: To compare the effects of different cortical bone thicknesses on the stability of microscrews at different healing times.

Materials: Sixty-four microscrews were inserted into the femurs of eight beagles, with four microscrews for one femur of one beagle dog. The dogs were sacrificed at 0, 3, 6, and 9 weeks after microscrew placement, respectively. All specimens were prepared for pullout testing. Cortical bone thickness was measured. Statistical analyses were conducted with analysis of variance (ANOVA) and Student-Neuman-Keuls (SNK) tests.

Results: Pullout forces in thick cortical bone sites are significantly higher than those in thin sites at 0 week and 3 weeks. For both thick and thin cortical bone thickness sites, the highest pullout forces were seen in the 0 week group and the lowest in the 3 week group. In the thin cortical bone thickness sites, the pullout force of the 3 week group was statistically different from those of the 6 week group and the 9 week group; however, no such differences were noted in thick cortical bone thickness sites.

Conclusion: Microscrews inserted into thick cortical bone thickness sites had better stability than those inserted into thin cortical bone thickness sites at early healing time. The difference diminished and became insignificant as healing time got longer. Longer healing time may be necessary if microscrews are inserted into thin cortical bone thickness sites.