Biomechanics of Orthodontic Tooth Movement _2 - Dr. Nabil Al-Zubair
Biomechanics of Orthodontic Tooth Movement Dr. Nabil Al-Zubair
Objectives• Define the term “biomechanical principles” with respect to various methods of orthodontic therapy.• Evaluate various force systems used for orthodontic tooth movement and orthopedic application.• Describe the biology of orthodontic tooth movement and discuss the histologic changes that occur with various force systems.
Physiologic tooth movement • Include :• Definition: naturally occurring tooth 1. Tooth eruption movement that take 2. Migration or drift of teeth place during & after 3. Changes in tooth position tooth eruption. during mastication
Tooth eruption• Definition: the axial movement of the tooth from its developmental position in the jaw to its final position.
Theories of tooth eruption 1 The root elongation theory2 The hydrostatic theory (vascular theory) 3 The alveolar bone growth theory 4 The pulp theory 5 The periodontal ligament theory
Migration or drift of teeth• Refers to the minor changes in tooth position observed after eruption of teeth The human dentition Shows a natural tendency to move in a mesial & occlusal direction • • howevertrue lower jaw show certain variation) largely the in case of maxillary dentition
Migration or drift of teethTooth migration is usually as a result of Proximal & occlusal wear of teeth Teeth undergo proximal & occlusal wear Move in a mesial & occlusal direction to maintain inter-proximal & occlusal contact.
Tooth movement during mastication Slight movement within its socketForce absorbed by bone bending Return to its original position as soon as the load is removed The teeth & periodontal structures 1-50 Kg based on type of food Intermittent heavy forcesShort duration Cycles of one second or less
Tooth movement during mastication 1-2 secs – PDL fluid displaced Pain2-3 secs – PDL tissues compressed The teeth & periodontal structures Long duration 1-50Kg / continuous
Histological basis of tooth movement• Single point force application to crown of tooth Tilting Center of Rotation
Histological basis of tooth movement Pressure side + --Tension side + - + - + + + + - + Tension side - + - + - + - - Pressure side
Histological basis of tooth movement Continuous optimal force- Direct bone resorption Pressure side - - Cell proliferation - - - Osteoclasts - - - -Pressure side -
Histological basis of tooth movement Continuous optimal force- Osteoid mineralised + Tension side + + PL fibers arebone Woven stretched + + + + + Tension side +Fibroblasts and osteoblasts + + Mature lamella bone.
Histological basis of tooth movement•If force exceeds capillary blood pressure Reduces blood flow Pressure side Will not produce optimal tooth movement. + -- Tension side + - + - + + + + - + Tension side - + - + - + - - Pressure sideTooth movement occurs with a force duration for only 6 hrs/day
Excessive force 1. No direct resorption of bone 2. Compression of blood vesselsThis results in sterile necrosis of cells (known as hyalinization because of the structureless appearance under the microscope.No tooth movement initially. Pressure sideDelay of 2-3 weeks then there is undermining resorption outwards from the marrow spaces of the adjacent alveolar bone. Pressure side
• The optimum force for tooth movement is 25 g/cm2 of root surface area.• The size of force is determined by surface area of the root and the type of movement i.e tipping < force than bodily movement.
Force – 50-75gForce –100-150g Force –50-100g Tooth movement Force –15-25g Force –50g
Excessive forces results in:-– delay in tooth movement– discomfort to patient– tooth mobility– root resorption– excessive force dispersed over anchor teeth, threshold exceeded
Timing of treatmentUsually the early permanent dentition 10-14 yrs. Reasons • Bone remodelling in response to orthodontic forces are more rapid in children. • Patient co-operation best around early teens. • Active tooth movement cannot begin until after eruption. • Growth can be used for overbite reduction, anteroposteior arch correction and spontaneous tooth movement.
BiomechanicsFree bodies :their mass is concentrated at a single point. This point is called the mass center• If, e.g., a free body is found in space and a force is applied at a specific point on it, then all body points will follow the same course.
The tooth not a free body The periodontal ligament Restrict its movement The alveolar bone.The point that corresponds to the mass center of a free body is the center of resistance of the tooth.
The center of resistance The point where tooth resistance to a movement may be concentrated; Center of Rotation (CR) M CR in other words, when a force is applied through this point, tooth D translation ensues.Forces applied at a distance from the CR also F create a moment that tends to rotate the tooth. Moment = Force x Distance
Moment = Force x Distance MMoment magnitude depends on1. force magnitude2. distance of the force vector from the CR D F
Moment / force ratio• When the orthodontist knows the location of a tooth’s CR,Moment / force ratio for every tooth movement may be calculated; thus, tooth movement may be controlled M D Distance = Moment / Force F
Moment / Force = Distance• Translation • Torque• Controlled tipping Moment to force ratio is between 1 and 7 Moment to force ratio is between higher 10 10 Moment to force ratio is 8 and than
Moment / force ratio • If, e.g., tooth translation is to be achieved M• An opposite moment counteracting the first one D • a force applied through the center F of resistance • Should be incorporated into the system.
M D• If, e.g., tooth translation is to be achieved F• An opposite moment counteracting the first one