Dental movement physiology
Dental movement is produced as a result of a biological response to the physiologic reaction toward forces applied by our mechanical procedures. So when we design our appliances it is very important to evaluate the forces that they will generate during treatment.
Teeth are united Lo the maxillary bones by a peculiar type of joint, the dentoalveolar joint, different from any other present in the rest of the human body. This union is done by the periodontal insertion, represented by dental cement, periodontal ligament and alveolar bone. The periodontal ligament occupies a space of approximately 0.5 mm between the alveolar wall and the tooth cement and it is responsible for the dental joint. It is made out of collagen fibers that insert themselves in the root cement and in the alveolar bone intermingling with blood vessels, cellular elements, nerve endings and interstitial fluids.
Blood vessels are responsible for the periodontal ligament nourishment, and will provide a way of access for the cells responsible of bone remodeling. Existing nerve endings will transmit pressure sensation and proprioception. Periodontal fibers and interstitial fluids will provide together, daring brief time periods, an effective physiologic force dissipater and shock absorption system.
When a force is applied to a tooth, it dislocates itself in the interior of the alveolar space, stretching some periodontal fibers and compressing other periodontal fibers. Simultaneously, the liquid that fills the spaces between the fibers is compressed against the bone walls provoking a hydraulic resistance to tooth movement. At this moment, the load is transferred Lo the alveolar bone and due to its porosity, the interstitial liquid drains toward neighboring tissues, diminishing the hydraulic pressure. This way, the root gets even closer to the alveolar wall, compressing the periodontal ligament closer to the side the force was applied upon and stretching the ligaments on the opposing side, the vascular system, that occupies 50% оn the periodontal space, is compressed, and makes circulation difficult on the tension and compression areas.
At this moment, an inflammatory tissue response is produced increasing blood vasodilatation and promoting prostaglandin production in order to increase blood irrigation, stimulating the exit of monocytes that will fuse with one another, by doing this, multinuclear cells called osteoclasts will be originated, these are responsible for cortical alveolar resorption whenever periodontal ligament is compressed. On the side where distention of the periodontal ligament fibers is occurring, undifferentiated mesenchyma cells transform into osteoblasts and fibroblasts, which are responsible of forming bone tissue and collagen fibers respectively. When blood flow is limited, teeth do not move or they move slowly. Intense forces may limit the physiologic response and can affect the rate of dental movement.
Dental movement begins two days after the force is applied. This movement stimulates osteoclasts and osteoblasts to begin the bone remodeling process with apposition on the periodontal fiber tension side and resorption on the compressed periodontal ligament side. Slowly the alveolus dislocates in the direction of the applied force, with orthodontic movement as a consequence.
Brian Lee, following the works by Storey and Smith, evaluated the optimal force for canine retraction. In his study he proposed that 200 g/cm2 of exposed root surface to movement was the optimal pressure to apply In order to obtain an efficient dental movement.
Taking into consideration that force times surface unit is defined as pressure, the force applied must differ depending on the size of the root surface and the direction of the planned movement.
The mesiodistal size of the root surface is evaluated when the tooth is moving in an anteroposterior direction. The buccolingual size of the root surface is evaluated if the tooth is going to be moved in a transverse direction. When intrusion or extrusion of teeth is planned, the cross section of the root surface is evaluated.