When a force is applied to a tooth,areas of pressure and tension are produced around the tooth. Areas of pressure are formed in the direction of the tooth movement ,while areas of tension form in the opposite direction. There is bone resorption on the pressure side and new bone formation on the side of tension.
Histologic changes vary according to the amount and duration of force applied .it can be studied under 2 headings as:
- Changes following application of mild force
- Changes following application of extreme force .
Changes following application of mild force :
On pressure side : PDL in the direction of tooth movement gets compressed to almost 1/3 of its original thickness.increase in capillary blood supply causes a marketed increase in vascularity of PDL,which helps in mobilization of cells such as fibroblasts and osteoclasts. Osteoclasts are bone resorbing cells that lie within shallow depressions in bone called Howship`s lacunae. when orthodontic force is applied ,the trabeculae which are usually parallel to the long axis of the teeth become horizontally oriented and osteoclast cells start resorption of alveolar plate immediately adjacent to the PDL. This type of resorption is called frontal resorption.
On the tension side :when force is applied the PDL gets stressed on the tension side .so distance between bone and tooth is widened .Here also increased vascularity causes mobilization of cells like fibroblasts and osteoblasts. Immediately adjacent to lamina dura osteoid is laid down by osteoblasts in PDL,which in later matures to form woven bone.
Changes following application of extreme force
On pressure side: when extreme force is applied to tooth ,the root closely approximates the lamina dura,compresses the PDL and leads toocclusion of the blood vessels .the PDL is deprived of its nutritional supply leading to hyalinization.bone resorption occurs in adjacent marrow spaces and above ,below and behind the hyalinized area. . This type of resorption is called undermining resorption.
On the tension side: PDL gets over stretched leading to tearing of the blood vessels and ischemia. Here osteoclastic activity is more than formation ,so that tooth becomes loosened in its socket and pain and hyperemia of the gingiva may also occur.