Microhardness, Strains and Microdamage in Bone Surrounding Endosseous Implants Subjected to Bending Fatigue Loads

Abstract

Intense remodeling occurs in lamellar bone adjacent to osseointegrated endosseous implants. The purpose of this study was to compare microdamage accumulation subsequent to fatigue loading of bone that surrounds an endosseous implant at (a) immediate placement (non-adapted bone) and (b) following a 12 week period of healing (adapted bone). It was hypothesized that there would be less microdamage in the compliant adapted bone than in the older non-adapted bone adjacent to endosseous implants after being subjected to a fatigue protocol. Preliminary studies to measure the microhardness of bone surrounding implants and three dimensional finite element models to quantify the strain environment were required. In addition, a new fluorescent method to quantify microdamage was developed. Titanium endosseous implants (4 mm diameter) were placed into dog mid-femoral diaphyses and allowed to heal for 12 weeks. Block sections of bone, each containing one implant, were cut antero-posteriorly to obtain an implant containing lateral cortex. The medial cortex was used to produce non-adapted specimens by placing a 4 mm diameter titanium rod. Specimens were divided into two groups. Control (n=14 each for adapted and non-adapted, 0 N) and experimental specimens (n=13 adapted and n=14 non-adapted) were loaded in bending for 150,000 cycles at 2 Hz. Specimens were bulk stained with basic fuschin. Histomorphometric methods were used to evaluate crack numerical density, crack surface density and percent damaged area. Microdamage in the non-adapted and adapted specimens was compared using repeated measures ANOVA and Tukey's method at a 95% confidence level. Statistically significant differences (p<0.0001) existed for all measurements between the non-adapted and adapted specimens on the compressive cortices with adapted bone accumulating less microdamage than non-adapted bone. This study suggests that the more compliant adapted bone adjacent to the endosseous implant is relatively resistant to the imposed fatigue loads. The rapidly turning over bone may prevent microdamage accumulation and propagation, thus allowing for the successful adaptation of these implants.