Dr.Krishna Kanth ReddyTOOTH MOVEMENTINORTHODONTICS

Orthodontic tooth movement is achieved with the application of force on the teeth using certain appliances. Tooth movement in response to these forces is governed by certain laws of physics. Biomechanics deals with these laws.

A clear knowledge of these concepts is essential for the proper understanding of the tooth movement and how we can optimize orthodontic treatment to achieve best results

CENTRE OF GRAVITYEvery object or a free body has one point on which it can be perfectly balanced. This point is known as the Centre of gravity.The movement of a free body depends upon the relationship of the line of action of the force to the centre of gravity.

Center of Gravity:

The center of gravity of the tooth is located more towards the crown of the tooth as the mass of the tooth is concentrated more coronally

Center of ResistanceIt is a point at which resistance to tooth movement is concentrated. It is at the approximate midpoint of the embedded portion of the root.

Center of ResistanceCenter of Resistance

Since the kind of reaction to an applied force system is so strongly dependent on the site of the center of resistance, it is of paramount importance to estimate where its location will be.

FORCEIt is a load applied to an object that will move it to a different position in space. Though defined in units of Newtons, it is usually measured in units of grams or ounces

MOMENTIf the line of action of an applied force does not pass through the Centre of resistance, the force will produce some rotation. The potential for rotation is measured as MOMENT. Is defined as a tendency to rotate

COUPLETwo equal and opposite, non-collinear forces

PHYSIOLOGIC TOOTH MOVEMENT Means the slight tipping movement of the functioning tooth in its socket and the changes in tooth position which occurs in young persons during and after tooth eruption.

ORTHODONTIC TOOTH MOVEMENTBasically there is no difference between the tissue reactions observed in physiologic tooth movement and orthodontic tooth movement.

However, the more rapidly teeth are moved during treatment, the tissue changes elicited by orthodontic forces are consequently more marked and extensive.

TISSUE RESPONSE AND TOOTH MOVEMENT DEPENDS ONForce MagnitudeDirection of ForceDuration and Type of Force.

Effects of force magnitudeIdeal orthodontic force = 26gms/cm2Lighter the force, the better is the tissue response.

TYPES OF TOOTH MOVEMENTTIPPING CONTROLLEDUNCONTROLLEDTORQUING BODILYINTRUSIONEXTRUSIONROTATION

TIPPING MOVEMENT

Simplest form of orthodontic movementRequires force of very low magnitudeCrown movement is more than the root movement.

Tipping movements are produced when a single force (for instance, a spring extending from a removable appliance) is applied against the crown of a tooth. When this is done, the tooth rotates around its centre of resistance, a point located about halfway down the root

When the tooth moves in this fashion, the periodontal ligament is compressed near the root apex on the same side as the spring, and at the crest of the alveolar boneMaximum pressure in the periodontal ligament is created at the alveolar crest and at the root apex. Progressively less pressure is created as the Cres is approached.

UNCONTROLLED TIPPINGWhen the force is applied, the crown moves in one direction and the root moves in the opposite direction. Here the centre of rotation lies somewhere near the centre of resistance of the tooth. This is referred to as Uncontrolled tipping.

CONTROLLED TIPPINGThe centre of rotation lies near or close to the apex. Here the crown moves in one direction but the root position remains the same or gets minimally displaced.

TORQUING MOVEMENTRoot movement is more than the crown movement.Requires force of high magnitude (2-3 times greater than tipping).

BODILY MOVEMENTCrown movement = Root movement.If two forces are applied simultaneously to the crown of a tooth, the tooth can be moved bodily (translated) i.e., the root apex and crown move in the same direction the same amount.In this case, the total periodontal ligament area is loaded uniformly. The centre of rotation lies at infinity.

EXTRUSIONExtrusion is the bodily displacement of a tooth along its long axis in an occlusal direction.Extrusion movements ideally would produce no areas of compression within the periodontal ligament, only tension.

INTRUSIONIntrusion is the bodily displacement of a tooth along its long axis in an apical direction.Light force is required for intrusion because the force will be concentrated in a small area at the root apex.Only if the force is kept very light can intrusion be expected.

ROTATIONTWO FORCES OF EQUAL MAGNITUDE ACTING OPPOSITE IN DIRECTION PRODUCES ROTATION.

OPTIMUM ORTHODONTIC FORCEOptimum orthodontic force is one which moves teeth most rapidly in the desired direction, with the least possible damage to tissues and with minimum patient discomfort.OOF is equivalent to the capillary blood pressure which is 20-26 gms/sq cm. of root surface area.

OPTIMUM FORCES FOR ORTHODONTIC TOOTH MOVEMENTTIPPING50-75 gms forceTRANSLATION100-150 gms forceTORQUING75-125 gms forceROTATION50-75 gms forceEXTRUSION50-75 gms forceINTRUSION15-25 gms force

EFFECTS OF FORCE DURATION AND FORCE DECAYThe key to producing orthodontic tooth movement is the application of sustained force.A minimum of 4-6 hours of continuous force is necessary to initiate tooth movement.Continuous force, 24 hours/day, produces the most efficient tooth movement.

CLINICAL EXPERIENCE SUGGESTS THAT THERE IS A THRESHOLD FOR FORCE DURATION IN HUMANS, PROBABLY AT ABOUT 6 HOURS, AND THAT INCREASINGLY EFFECTIVE TOOTH MOVEMENT IS PRODUCED IF FORCE IS MAINTAINED FOR LONGER DURATION

ORTHODONTIC FORCESWith many orthodontic devices, the force may drop all the way to zero. From this perspective, orthodontic force duration is classified by the rate of decay as:

CONTINUOUS INTERRUPED INTERMITTENT OHP

CONTINUOUS FORCEForce maintained at some appreciable fraction of the original from one patient visit to the nextEx: An ideal spring would maintain the same amount of force regardless of the distance a tooth had moved. Forces that are maintained between the activations of an orthodontic appliance, even though the force declines.

INTERRUPTED FORCEForce levels drop to zero between activationBoth continuous and interrupted forces can be produced by fixed appliances that are constantly present.

INTERMITTENT FORCEForce levels decline abruptly to zero intermittently , when the orthodontic appliance is removed by the patient.Intermittent forces are produced by all patient activated appliances, such as removable plates, head gear and elastics.

RATE OF TOOTH MOVEMENTAbout 1mm/month may be regarded as an acceptable rate of tooth movement.VARIOUS FACTORS AFFECT THE RATE OF TOOTH MOVEMENT:Force applied: Both light and heavy force will result in orthodontic tooth movement. However, it is generally felt that if light forces are used, minimizing hyalinization of the periodontal ligament, the rate of tooth movement will be greater.

AGE: In the adult, the periodontal ligament is much cellular than in the child. In addition, the alveolar bone in children is less dense than in older patients. This means that, in general, tooth movement in the adult will be slower.INDIVIDUAL VARIATION: There is individual variation in the response to orthodontic forces. This is at least part dependent on the density of the alveolar bone.

THEORIES OF TOOTH MOVEMENTPRESSURE TENSION THEORYFLUID DYNAMICS THEORYPIEZO ELECTRIC THEORY

PRESSURE TENSION THEORYBriefly this indicates that when a tooth is pressed from one side the periodontal fibers on the side are tensed, or straightened. The fibers on the opposite side are compressed.Schwartz felt that pressures should be limited to capillay

FLUID DYNAMIC THEORYis a relatively recent oneThe contents of the periodontal ligament acts like a colloidal solution to mechanical pressure. Pressure on a tooth elicits several reactions which are typical of viscous fluids, according to Bein.

If a viscous fluids is compressed between two flat bearings, it tends to move to the periphery.Intrusive pressure on a tooth tends to move the interstitial fluid toward the apex and the gingival margin. This acts to slow down the intrusive movement. This is the squeeze film effect.

PIEZO ELECTRIC THEORY OR BONE BENDING THEORYPiezoelectricity is a phenomenon observed in many crystalline materials in which a deformation of the crystal structure produces a flow of current as electrons are displaced from one part of the crystal lattice to another .Stipulates that the bone is deformed in response to pressure on the tooth, and reforms to adjust to the new position of the tooth.

Mechanical stress initiated by orthodontic force or alveolar bone deflection induced an electric polarization referred to as PIEZOELECTRIC RESPONSE.In electronegative regions, bone formation occurs, whereas bone resorption predominates in electropositive areas.

HISTOLOGICAL ASPECT OF TOOTH MOVEMENTSwhartz felt that pressures should be limited to capillary blood pressure; otherwise he felt there would be tissue necrosis in the periodontal ligament.