Evaluation of a Hypergravitational Load on Mandibular Condyle Growth and Osteoblast Differentiation

Abstract

Previous studies have shown that mandibular condyle growth and development are stimulated by mechanical loading. Gravity acts as a baseline mechanical load to which every living being is exposed. The purpose of this study was to evaluate the effects of a two-week constant hypergravity load (2g) on: (1) condylar cartilage and bone parameters, and (2) osteoblast differentiation in condyle primary spongiosa. Twenty 60-day-old rats were divided into centrifuged/experimental (Expt.) and stationary/control (Cont.) groups. Ten rats were subjected to two weeks of centrifugation at the NASA Ames Research Center to simulate an increased induced-gravitational field. Comparison of Expt. and Cont. animals by Student's t-test revealed increased cartilage volume and decreased bone volume when determined as a percent of total condylar tissue (data expressed as Volume percent , mean±SD, *p< 05). Expt.: Cartilage: 42.3±7.6%; Bone: 36.7±2.9% Cont.: Cartilage: 35.2±7.8%; Bone: 40.0±3.9% The same pattern occurred when cartilage and bone volumes were expressed as a percent of the hard tissue fraction of the condyle. While there was a slight increase in the nonosteogenic cells, there were no differences in osteogenic precursor cells. Similarly, there were no differences between groups in volumes of cortical and trabecular bone, condylar perimeter, and cartilage thickness. Evaluation of condylar head shape expressed as a percent of the condylar field showed no significant differences between groups. This is in agreement with the lack of significance observed when comparing the total perimeter surface of the condylar cartilage and bone. However, a subjective evaluation of condylar head shape showed that 90 percent of the condylar heads of the Experimental group had a somewhat round shape, while 70 percent of the condylar heads of the Control group were shaped in the resemblance of a "mushroom." These results suggest that the mandibular condyle adapted to two weeks of hypergravity by increasing cartilage volume at the expense of bone volume. The rise in cartilage volume appeared to result from an increase in the resting layer of cartilage cells. The subjective visual difference on condylar head shape and the wider range of condylar head sizes in the Experimental group may reflect that rather than growth modification, mandibular condyles adapted to hypergravity within a range of developmental responses. No differences in osteoblast precursor cell differentiation or condylar size were apparent at this time. The increase in the nonosteogenic cell compartments (L and B cells) needs further evaluation. The mandibular condyle appears sensitive to both positive and negative gravitational variation. However, it may not be as sensitive to hypergravity as it is to hypogravity. The fact that no significant correlations were found in the Experimental animals between their significant weight loss or lack of weight gain during the experimental period, and variables such as linear cartilage thickness, fractional volumes of cartilage and bone over the condylar hard tissue fraction, and condylar perimeter, points toward the fact that hypergravity probably did not have a significant effect on mandibular condyle growth. It is also very likely that functional loads are more important than postural loads on the growth and development of the mandibular condyle. In fact, the functional load generated by the masticatory muscles may actually not have allowed the load of the 2g hypergravitational to be fully expressed on the mandibular condyles. This leads to speculations whether the mandibular condyles of the rats centrifuged in the 2g hypergravitational field were or were not trully exposed to hypergravity.