Fluoride and the Rabbit Mandibular Condyle: A Strength and Histomorphometric Study

Abstract

The specific aims of the project were to assess the histomorphometric and strength changes that occurred in the fluoride treated trabecular and subchondral bone of the mandibular condyle. This project represented a specific part of a larger experiment recently conducted by the Indiana University Oral Health Research Institute. We tested the hypothesis that fluoride treatment inhibits bone remodeling activity, enhances bone modeling activity, and decreases bone strength. The rabbit mandibular condyle, because it is composed of trabecular and subchondral bone and has a high rate of remodeling activity, was used as a model for human trabecular and subchondral bone. The six-month study involved 24 young adult (four months old) female, Dutch Belted rabbits. The rabbits were randomly divided into two groups based on body weight and plasma fluoride levels. The control group received no fluoride in their drinking water while the experimental group received 100-ppm fluoride. Both groups of rabbits received approximately 12-ppm fluoride in their food. A pair of tetracycline labels (10 mg/kg i.m.) was given two weeks before the initiation of the fluoride treatment. A pair of calcein green labels (5mg/kg i.m., bid), administered one week apart, was initiated two weeks before the end of the experiment. The animals were euthanized three days after the final label injection. Histomorphometric measurements were made at X125 magnification on a Nikon FXA epifluorescent microscope utilizing stereological point-point hit (volume-related parameters) and linear intercept (surface related parameters) methods. The fluoride treatment resulted in a significant increase in the percentage of osteoid surface, but no increase in osteoid thickness. The bone formation rate showed a trend toward an increased formation rate in the fluoride treatment group. The observation that the mineralized surface and osteoid thickness showed no change suggests a normal rate of mineralization. However, more surfaces were covered with osteoid. Mechanical testing showed no change in stiffness and a weak trend toward a decrease in ultimate strength in the fluoride group. The data suggest that the influence of fluoride treatment had a minimal affect on the trabecular bone of the rabbit mandibular condyle. The results from the vertebra study in the same animals showed a similar pattern of change following fluoride treatment. It is speculated the limited variability between the vertebra and the condyle could be due to altered functional mechanical loading or regional characteristics.