Presnted by Dr K. M. Rukhs Aif Zaman KhanDepartment of Orthodontics and Dentofacial Orthopaedics

1. Pressure Tension Theory by Schwarz: or Tissue Reaction to Orthodontic Forces:When force is applied on a tooth to bring about orthodontic movement, it results in formation of areas of Pressure and Tensionaround the tooth. Areas of Pressure are formed in the Directionof the tooth movement, while areas of Tension form in the oppositeDirection.

Bone is a living tissue which reacts to pressure and tension in a certain defined manner. Bone surface subjected to Pressurereacts by bone resorption while bone subjected to Tension exhibitDeposition.

Biology of tooth movement

When a tooth is moved due to application of an orthodontic force,there is bone resorption on the pressure side and new boneformation on the tension side.Biology of tooth movement

Phases of Tooth MovementStudies have shown that tooth movement progresses through threeStages. Burstone categories the stages as:a. Initial Stage b. Lag Phase c. Post Lag PhaseInitial Stage: During this phase, very rapid tooth movement is observed over a short distance which then stops. This movementrepresents displacement of tooth in the PDL space and probablybending of alveolar bone to a certain extent. Both light and heavyforces displace the tooth to the same extent during this phase.Tooth movement in the initial phase is between 0.4mm to 0.9mm andusually occurs in a weeks time

b. Lag Phase: During this phase little or no tooth movement occurs.this phase is characterized by formation of hyalinized tissue in the PDL which has to be resorbed before further tooth movement canoccur. The duration of the lag phase depends on the amount of force used to move the tooth. If light force are used, the area of hyalinization is small and frontal resorption occurs. If heavy force areused, the area of Hyalinization is large, resorption in this case isrearward and longer lag period occurs to eliminate the hyalinizedtissue. The lag phase usually extends for 2-3 weeks. Duration of this phase depends upon the density of alveolar bone, age of the Pt.,and extent of the hyalinized tissue

Phases of Tooth Movement

c. Post Lag Phase: After the lag phase, tooth movement progressesrapidly as hyalinized zone is removed and bone undergoes resorption. During the post lag period, osteoclasts are found over alarge surface area resulting in direct resorption of bony surface facing the periodontal ligament.FORCE

Tissue Reaction to Orthodontic ForcesPressure Side: [ P ]PDL: Compressed to about 1/3 rd of its thicknessBone: Bone Resorption Occur

Tension Side: [ T ]PDL: StretchedBone: Bone FormationFORCETPTPT= OSTEOBLAST CELL [ Bone Formation ]P= OSTEOCLAST CELL [ Bone Resorption ]

Optimal Forces for Different Tooth MovementsMovements Optimal ForcesTipping-----------------------50-75g / toothRotation----------------------50-75g / toothExtrusion---------------------50-75g / toothBodily----------------------100-150g / toothUp-righting-----------------75-125g / toothIntrusion----------------------15-25g / tooth

Optimum Orthodontic Force Optimum orthodontic force is one which moves the teeth most rapidly in the desired direction, with the least possible damage totissue and with minimum patient discomfort. From a clinical point of view, optimum orthodontic force has the following characteristics:Produces rapid tooth movementMinimum patient discomfortThe Lag phase of tooth movement is minimalNo marked mobility of teeth being moved

Types of Forces based on duration of applicationBased on the duration of Application, force can be divided into:Continuous Force: it is an active orthodontic force that decreases little in magnitude between appointment periods. Eg. Light wire appliance2. Intermittent Force: it is an active orthodontic force the decays to zero magnitude or nearly prior to next appointment.Interrupted Force: it is an orthodontic or orthopeadic force that is inactive for intervals of time between appointments. Eg. Head gear.

Interrupted ForceContinuous ForceIntermittent ForceDuration of ApplicationForce

Types of Tooth MovementThe prime motive of orthodontic treatment is to move the teeth intomore favorable and corrected positions. To achieve this goal, theteeth undergo a variety of movements in three dimensions such assagittal, coronal and transverse. Tooth movements within the oral cavity can be listed as follows:TippingRotationExtrusionBodily movementUp-rightingIntrusion7. Torquing

Tipping: During tipping, crown of the tooth moves in the direction of the force and the root in the opposite direction. It can be of two types: a. Controlled tipping b. Uncontrolled tipping

2. RotationRotations are labial or lingual movements of a tooth around its longaxis. If line of force does not pass through the long axis of the toothIt results in rotation. Correction of rotation requires Couple force.

3. ExtrusionExtrusion is the bodily displacement of a tooth along its long axis in an occlusal direction

4. Bodily Movement (translation)

5. Up-rightingDuring RX. The crown of certain teethWill be tipped in a mesio-lingualDirection with the roots tippedIn the opposite way. Tipping theseRoots back to get parallel orientationIs termed up-righting.

FORCEOcclusal plane6. IntrusionIntrusion is the bodily displacement of a tooth along its long axis in an apical direction

7. TorquingIt is a movement of the root without significantmovement of the crown. It is reverse tipping used to correct the effects of uncontrolled tipping

References:Orthodontics; The Art and Science; Fifth Edition; S. I. BhalajhiTextbook of Orthodontics; M. S. Rani