to occlusal forces with a minimum of bone substance. When forces are increased, the bony trabeculae also increase in number and thickness and bone is added to the external surfaces.

The periodontal ligament depends on stimulation provided by function to preserve its structure. Within physiologic limits the PDL can accommodate increased function by increasing its width. Forces that exceed the adaptive capacity of the periodontium produce injury called trauma from occlusion. When occlusal forces are reduced the PDL atrophies, appearing