Biology and Biomechanics of Tooth Movement PDF

- The biology of tooth movement - Biomechanics of tooth movement Dr.Muhannad KAZZAZ introduction The application of a continuous force to a tooth results in remodelling of alveolar bone, reorganisation of the periodontal ligament and tooth movement. Orthodontic tooth movement is a complex p...

- The biology of tooth movement - Biomechanics of tooth movement Dr.Muhannad KAZZAZ introduction The application of a continuous force to a tooth results in remodelling of alveolar bone, reorganisation of the periodontal ligament and tooth movement. Orthodontic tooth movement is a complex process that involves the co-ordinated activity of many cell types and numerous chemical mediators. 2 ‫ د مهند قزاز‬01/27/2025 05:03 AM Effects of force on the periodontal ligament The application of a continuous force to a tooth surface results in the development of areas of compression and tension within the periodontal ligament. 3 ‫ د مهند قزاز‬01/27/2025 05:03 AM The magnitude of the force The magnitude of the force delivered is important in determining the tissue response. Ideally, orthodontic forces should not exceed the capillary pressure within the periodontal ligament as this produces ischemia and tissue necrosis. 4 ‫ د مهند قزاز‬01/27/2025 05:03 AM The optimal force for tooth movement also depends on: 1. the root surface area of the teeth to be moved. 2. teeth withofsmaller the type roots require tooth movement less force planned. for movement than teeth with larger.roots 5 ‫ د مهند قزاز‬01/27/2025 05:03 AM Tipping forces produce compressive loads that are greatest at the alveolar crest and root apex on diagonally opposite sides. Bodily movement results from even compressive loads along one side of the periodontal ligament. 6 ‫ د مهند قزاز‬01/27/2025 05:03 AM ‫‪7‬‬ ‫‪ 01/27/2025 05:03 AM‬د مهند قزاز‬ The ideal force levels for various tooth movements. The forces required for bodily movement are double those required for tipping as double the root surface area is compressed on one side during bodily movement. The forces for intrusion are the lightest because all the force is concentrated at the root apex which has a small surface area. 8 ‫ د مهند قزاز‬01/27/2025 05:03 AM Light orthodontic forces that do not exceed capillary pressure result in frontal (direct) bone resorption in areas of compression and deposition in areas of tension. The term frontal is used to denote that bone resorption occurs from in front of the.root 9 ‫ د مهند قزاز‬01/27/2025 05:03 AM Heavy Forces that exceed capillary pressure result in sterile necrosis in the area of the periodontal ligament affected. This process is termed hyalinisation because the area appears translucent (ground – glass) when viewed under the light microscope. Orthodontic tooth movement is delayed and bone resorption commences from beneath the area of necrosis within a few days. 10 ‫ د مهند قزاز‬01/27/2025 05:03 AM This is termed undermining (indirect) resorption because the cellular response occurs from the narrow spaces on the undersurface of the area of necrosis. 11 ‫ د مهند قزاز‬01/27/2025 05:03 AM ‫‪12‬‬ ‫‪ 01/27/2025 05:03 AM‬د مهند قزاز‬ It takes approximately 7–14 days for the process of undermining resorption to remove the lamina dura next to the necrotic periodontal ligament which is the point at which tooth movement occurs. 13 ‫ د مهند قزاز‬01/27/2025 05:03 AM Cellular responses to orthodontic forces Cellular responses are evident within the periodontal ligament after a few hours from the initiation of an orthodontic force. Many chemical mediators have been detected within the tissues following force application including prostaglandin E, cytokines and nitric oxide. 14 ‫ د مهند قزاز‬01/27/2025 05:03 AM Osteoclasts, are formed by fusion of circulating monocytes in response to specific signals from the osteoblasts, appear on the alveolar bone surface adjacent to the areas of compression within approximately 48 hours of force application. 15 ‫ د مهند قزاز‬01/27/2025 05:03 AM Osteoblasts are recruited from stem cells within the periodontal ligament and appear in areas of tension and deposit bone matrix approximately 48 hours after force application. As bone resorption occurs more rapidly than deposition, there is slight widening of the periodontal space during tooth movement. The periodontal ligament also undergoes considerable remodelling, with resorption of existing fibres and deposition of new fibres, mediated by fibrobasts present within the ligament. 16 ‫ د مهند قزاز‬01/27/2025 05:03 AM When orthodontic forces exceed capillary pressure, tissue necrosis occurs and undermining resorption is carried out by cell populations derived from the marrow spaces of the alveolar bone.  Resorption of the necrotic tissue also leads to resorption of root surface cementum adjacent to these areas, by cells termed cementoclasts, and may be the mechanism causing orthodontically related root resorption. 17 ‫ د مهند قزاز‬01/27/2025 05:03 AM Areas of resorbed cementum may undergo repair if the defects are small. With prolonged heavy forces, root resorption may outstrip deposition and there may be significant root shortening. 18 ‫ د مهند قزاز‬01/27/2025 05:03 AM  It is difficult to avoid undermining resorption even when care is taken during force application. This is because a degree of dental tipping is likely to occur within the socket that will result in the concentration of forces at the root apex and alveolar crest.  A time period of at least 4–6 weeks is recommended between appliance reactivation appointments to allow adequate time for repair of damaged tissues whilst the active forces from the appliance have declined. 19 ‫ د مهند قزاز‬01/27/2025 05:03 AM The rate of tooth movement Ideally, orthodontic forces should be applied for a 24 hours/day in order to produce the most efficient rate of tooth movement. Clinical experience suggests that at least 6 hours/day of force must be delivered to produce minimal movement. 20 ‫ د مهند قزاز‬01/27/2025 05:03 AM Under ideal conditions, the rate of tooth movement is approximately 1 mm/month. There is individual variation as the rate depends on the efficiency and magnitude of the cellular response and the density of the alveolar bone. The initiation of tooth movement may be slower in adults because of the reduced cellularity and vascularity of the periodontal ligament and the greater density of the alveolar bone compared to children. 21 ‫ د مهند قزاز‬01/27/2025 05:03 AM Mechanisms linking force application to tooth movement Two main theories have been proposed to explain the link between orthodontic force application and the cellular responses that lead to tooth movement: Pressure–tension theory Bioelectric theory 22 ‫ د مهند قزاز‬01/27/2025 05:03 AM  The pressure–tension theory suggests that areas of pressure and tension generated within the periodontal ligament result in alterations in blood flow, which triggers the release of chemical messengers that further trigger the cellular reactions associated with tooth movement.  The bioelectric theory links bending of the alveolar bone, during force application, to the generation of electrical currents which trigger the desired cellular events. It is not clear which of these theories is more valid, and it is possible that both mechanisms may contribute to orthodontic tooth movement. 23 ‫ د مهند قزاز‬01/27/2025 05:03 AM - Biomechanics of tooth movement Introduction: Biomechanics is the science concerned with the effects of forces acting on the human body. A basic understanding of the biomechanics of tooth movement will help to understand the effects an applied force will have on the direction of tooth movement. 25 ‫ د مهند قزاز‬01/27/2025 Centre of resistance Single teeth, groups of teeth and the facial bones have a centre of resistance. This is the point in a body at which resistance to movement is concentrated. If a force is applied directly to the centre of resistance of an object, bodily movement will occur. For a restrained object, such as a tooth, the centre of resistance does not coincide with the centre of mass and is determined by the shape of the tooth and properties of the alveolar bone and periodontal ligament. 26 ‫ د مهند قزاز‬01/27/2025 The centre of resistance should be visualised in all planes of space. For a single rooted tooth, with normal periodontal attachment, the centre of resistance is approximately halfway down the root surface. For multi-rooted teeth the equivalent point is in the area of the root furcation. Alveolar bone loss results in apical movement of the centre of resistance. 27 ‫ د مهند قزاز‬01/27/2025 The centre of resistance of the maxilla is thought to lie in the area of the premolar roots. It is an important point to appreciate when applying headgear forces for maxillary restraint. 28 ‫ د مهند قزاز‬01/27/2025 Forces, moments and couples Orthodontic tooth movement is dependent on the application of forces. A force is a vector that can be described by its magnitude and direction and can be broken down into its individual components. This can help determine the effects a force will have on the direction of tooth movement. 29 ‫ د مهند قزاز‬01/27/2025 The moment of a force is the component of the force that tends to cause rotation. Mathematically, the size of a moment of a force is equal to the magnitude of the applied force multiplied by the perpendicular distance between the point of application and the centre of resistance. 30 ‫ د مهند قزاز‬01/27/2025 This is of relevance to orthodontics because a force cannot be applied directly to a tooth’s centre of resistance, which lies along the root, and must be applied to the crown. This will produce a moment which will cause the tooth to rotate. 31 ‫ د مهند قزاز‬01/27/2025 A couple is produced when two equal and opposite forces act to cause rotation. The size of a couple is equal to the magnitude of the forces applied multiplied by the distance between them. 32 ‫ د مهند قزاز‬01/27/2025 Couples are commonly generated during fixed appliance treatment between an archwire and bracket slot.They can be used to alter the inclination of teeth or to act in the opposite direction to the moment of an applied force to produce bodily tooth movement. The term torque is often used in orthodontics to describe a moment or couple. 33 ‫ د مهند قزاز‬01/27/2025 Torque The term ‘torque’ in orthodontics refers to the.„ differential movement of one part of a tooth, while physically restraining any movement of the.other parts The term is often applied to movement of root without.the movement of crown Root torque is usually achieved by applying a force couple to the crown of the tooth, at the same time mechanically restricting crown movement in the opposite 34 direction ‫ د مهند قزاز‬01/27/2025 05:03 AM Types of tooth movement Tipping occurs when the crown of a tooth moves more than its root in a given direction. This form of movement is common during orthodontic treatment as the moment created by the applied force will tend to cause tipping. It is important to remember that when a tooth tips, the crown and root move in opposite directions. 35 ‫ د مهند قزاز‬01/27/2025 Types of tooth movement Bodily movement occurs when the crown and root of a tooth move an equal distance in the same direction. This type of tooth movement can only be produced with fixed appliances where a couple can be created between an archwire and the wall of a bracket slot to control tipping that accompanies the application on an orthodontic force to the crown of a tooth. 36 ‫ د مهند قزاز‬01/27/2025 ‫‪37‬‬ ‫‪ 01/27/2025‬د مهند قزاز‬ When a mesial or distal force is applied to the labial surface of a tooth, the moment of the force will cause rotation. This is commonly encountered during fixed appliance treatment. It is good practice to tie teeth firmly to the archwire to prevent unwanted rotation, when such forces are applied. 38 ‫ د مهند قزاز‬01/27/2025