Mechanics in Orthodontics part 2

1. 1 Mechanics in orthodontics Mechanics in orthodontics P Pa ar rt t 2 2 Prepared by: Prepared by: Dr Mohammed Alruby Dr Mohammed Alruby M Mechanics echanics in Orthodontics Part 2 in Orthodontics Part 2 Dr. Mohammed Alruby Dr. Mohammed Alruby

2. 2 Duration of force Orthodontic force may be: Continuous, interrupted, intermittent 1-Continuous force: = It maintains the same force value over an indefinite period of time as coil spring = When the force within suitable limit, it will initiate tissue reaction, direct resorption and reconstruction of supporting tissues. If the force is unduly reactivated ---------- hyalinization needs 2 – 4 weeks to be eliminated, if the force is reactivated during this period --------- permanent damaged 2-Interrupted dissipating force: =It is continuous force but its value is gradually decrease within a period of time as arch wire forces = This force is only effective over small amount of tooth movement after that the movement stop and the force need to be re-activated = In this type of force, the periodontal ligament (PL) has a chance to become re-constructed with increase in cell proliferation before re-activation of force. = Interrupted force is a continuous force acting over short duration (4week average) and causes small amount of tooth movement while continuous force acting over longer periods of time. Example: a-When ligating a rotated tooth to the arch wire by brass ligature b-When moving impacted canine into the occlusion using a spring incorporated into the arch wire and activating it 1:1.5mm or by ligating the eyelets on canine into the arch wire by ligature into moving the arch wire toward canine 1:1.5mm c-Using of closing loop opened to determined distance 1:1.5mm every month Advantage of interrupted force: 1-In cases where impacted canines to be brought into normal occlusion, the use of prolonged continuous force may disturb the pulp tissue due to the sudden movement of apical portion of the tooth following undermining resorption. While the use of interrupted force acting for short distance 1.5mm and for short period, steady movement with direct bone resorption and tissue re-construction will occur, the force is then re- activated to a new short distance 1.5mm and so on until complete eruption of canine takes place. 2-In cases of rotation, the use of interrupted force will provide steady movement and decrease the tendency for relapse 3-In cases of retraction, the closing loops made in arch wire when activated for short distance 1:1.5mm, it will allow steady movement and decrease the tendency for anchorage loss. 4-Interrupted force is recommended in treatment of adult open bite cases. Thus the advantage of interrupted force can be summarized as follow -Allow steady and controlled movement -Permit re-organization of supporting tissue before reactivation -Minimum discomfort to the patients and less damaged to periodontium -Enhance the post treatment stability, decrease relapse, decrease periods of retention 3-Intermittent force: = The force that acting upon the tooth periodically with a series of interruption, this force is delivered from removable appliances, the force acting by wearing the appliance and relieved by removable appliance Characteristic of this force: 1-Direct bone resorption 2-Increase cell proliferation 3-Slow rate of tooth movement 4-After the force is removed, the tooth tends to return to its original position ------ widening of periodontal membrane on the pressure side -------- increased cellular and vascular activity. But also slow rate of bone apposition occurs on the tension side ------- slow rate of tooth movement = If resistance to tooth movement occurs you have to increase duration of force application (time wearing) rather than increase the magnitude of force, thus the duration is more important than the magnitude, this is supported by Akerman 1966 M Mechanics echanics in Orthodontics Part 2 in Orthodontics Part 2 Dr. Mohammed Alruby Dr. Mohammed Alruby

3. 3 = Openhim (1942-1944) reported that the intermittent force has more biologic effect on tooth movement than continuous force, he observed that the application of light intermittent force will initiate direct resorption, the osteoclast does not disappear, immediately after removal of force but persist few days after force removal and thus the bone resorption continuous so that he suggested application of force for 2 -3 days then removal of force for similar periods = Reitan 1951 did not find a major difference between the results of continuous force and intermittent force. He reported that there is daily relapse on the use of removable appliances which ------- daily tissue construction and delayed tooth movement Advantages: 1-During the rest period the teeth moves slightly to tension side and remains in normal function for major part of treatment. 2-For the same reason the PL shows increase cell proliferation and increase vascularity which ----- tissue reconstruction during rest period 3-Smooth uniform tooth movement without damaged of periodontium or discomfort of patients can be obtained 4-Decrease relapse tendency and less retention periods Disadvantages: 1-Slow rate tooth movement and longer treatment time 2-This type of force producing tipping movement of the teeth which may causes unsatisfactory results, torque cannot obtain to upright the teeth 3-Demands patient’s cooperation Functional type of intermittent force: Can delivered to the teeth and jaws by loose functional removable appliance as activator Classification of orthodontic force according to its direction, magnitude, and point of application 1-Tipping force: The force that produced tipping movement, it is applied on a point on the crown away from the center of resistance, thus the tooth will tilt away from force The crown is moved while the root apex in place, the center of rotation lies near the apex, thus the force required to produce simple tipping is 50 – 70 gm The crown and roots moves in opposite direction; the center of rotation lies in the middle or above the middle of the root The root moved while crowns fixed, the center of rotation lies near the crown 2-Rotating force: Force couple application as by applying force at one point of the crown and other force at other point to prevent movement of other part of crown. It required 50 – 75 gm 3-Translating force: The force that produce bodily movement or translation of the tooth. The force must be applied over a wide area of crown as near as possible from center of resistance (more gingivally) with taken measures to decrease the moment created at the center of resistance The required force is about 2 to 3 times more than that of tipping as 150 – 200 gm is needed to move bodily 300 – 500 gm for molars 4-Torqueing force: Opposite to the tipping force as moving the apex of root instead of the crown. This accomplished by force couple, the center of rotation lies near to the crown The value of torqueing force lies between that of tipping and translation as 100 gm for canine torque 5-Vertical force: Intrusive: the force that produce intrusion of the teeth Extrusive: the force that produce extrusion of the teeth Intrusion require greater force than extrusion, very light force is required for this type of tooth movement (25 – 30 gm) M Mechanics echanics in Orthodontics Part 2 in Orthodontics Part 2 Dr. Mohammed Alruby Dr. Mohammed Alruby

4. 4 The force system to which the teeth and dentofacial structures are subjected Inherited natural forces: physiologic forces: a-Forces originating from masticatory muscles: It is the force of masticatory muscles to which the teeth and their supporting structures are subjected during function. In the fact the full force of mastication is exerted only for very short periods of time during day During mastication the force exerted upon the teeth range from 150 – 500 gm b-Force originating from circum-oral musculatures: The muscles of the tongue, lips and cheeks exert a variable force upon teeth and supporting structures, during normal activities (swallowing, mastication, respiration, speech, posture, facial expression) In all activities the force is variable and intermittent except in postural activities where the force is low value but continuous, so that it is considered the most important factor that when abnormal it will cause severe dentofacial problems The pressure of the lips, cheeks and tongue varies from few grams up to 100gm Pressure exerted during speech, respiration and swallowing has a variable magnitude c-Force originating within the teeth: Force of eruption Mesial drifting tendency Force of inclined plane which designated as the anterior component force II- Abnormal forces: Abnormal forces may affect the occlusion, tooth position and TMJ functions a-Tongue thrust: Result from abnormal tongue posture a forward displacement of the base of the tongue due to enlarged tonsils, adenoids, etc……….. Or abnormal tongue function (persistent infantile swallowing) Or vertical anterior dysplasia open bite ---- tongue thrust occurs as compensatory mechanism to seal the lip during swallowing b-Thumb sucking: It can cause changes in tooth position if the force is of appreciable magnitude and longer duration, it may cause: retroclination of mandibular incisors, Proclination of maxillary incisors, anterior open bite. c-Occlusal dysfunctions: Premature contact and occlusal interference may cause abnormal forces which causes traumatization, malposition of the teeth and severe damage to the periodontium and TMJ Crowding of mandibular incisors is common sequelae for occlusal dysfunction d-Traumatic occlusion: Has very harmful effects on periodontium which accelerate the loss of the tooth support and finally loss of the teeth e-Bruxism: May or may not alter the tooth position depending upon the presence of cuspal interference III- Mechanical forces: therapeutically induced forces: Force system in orthodontic and orthopedic appliances Mechanical forces are dispensed by storing them in elastic, arch wires, coil springs, finger springs, extra- oral forces, headgear and chin-cups The usual magnitude of force varies from 1 to 6 ounces for orthodontic forces and up to 5 pounds in some orthopedic forces. Classification of therapeutically induced forces: 1-Natural: Originating from oro-facial musculatures and transmitted to the jaws and teeth via functional appliances as: activator, Bionator, oral screen, Frankel Myofunctional therapy is also employ lip exercise to correct protrusion of incisors and increase tonicity of the lips, also to learn the tongue to assume a correct position during swallowing I- M Mechanics echanics in Orthodontics Part 2 in Orthodontics Part 2 Dr. Mohammed Alruby Dr. Mohammed Alruby

5. 5 2-Biomechanical: They are an artificial clinically induced forces dispensed to the teeth from mechanical appliances Classification of appliances according to functions: 1-Appliances for elimination of unwanted forces: habit breaking appliances 2-Appliances for redistribution of natural forces: all functional appliances 3-Appliances for stimulation of natural forces: lip pumber vestibular shield and Frankel 4-Appliances for introduction of mechanical artificial forces: -Fixed appliances: spring, arch wire, elastics -Removable appliances; spring, screw, elastics. -Extra-oral appliances: headgear, chin cup Orthodontic force system Laws and mechanics The moment as well as the forces generated by an orthodontic appliances system must be balanced There is more of problem in orthodontics than in most engineering application because the action of orthodontic appliance is affected by the biological response One couple systems: Depend on how wire engaged in the bracket, in orthodontic appliance, one couple systems are found when two condition are met: -Cantilever spring -Auxiliary intrusion or extrusion arches 1-Cantilever spring application; Used most frequently by bring severely displaced (impacted) teeth into the arch Advantages: -Minimal decrease in force with tooth movement -Excellent control of force magnitude Disadvantage: -Due to long duration action, spring do not fail safely as it distorted by patient, significant tooth movement in wrong direction is quite possible -The moment of force on unerupted tooth rotate lingually as the tooth is brought toward occlusal plane, so additional force direction added to overcome that 50 gm extrusion force on canine create 50gm intrusive Force on molars 1000gm to rotate the molar crown around the center of resistance as (the distance between molar tube and button on canine is 20mm) 2-Auxiliary intrusion or extrusion arches: The major use of one couple systems is for intrusion, typically of incisors that have erupted too much Intrusion arch is added with light force on anterior teeth and also there is light extrusion force on posterior teeth Usually premolar teeth also added to anchor unit or tying the molar teeth together with the rigid lingual arch to prevent buccal tipping M Mechanics echanics in Orthodontics Part 2 in Orthodontics Part 2 Dr. Mohammed Alruby Dr. Mohammed Alruby

6. 6 It is easy to activate auxiliary arch wire to produce extrusion of incisors rather than intrusion The force needed to extrusion is 4 to 5 times higher than intrusion and also the reaction against the anchor teeth is higher N: B: A-If the intrusion arch is tied behind the lateral incisor bracket: -The force applied is in the same line with the center of resistance -There is no moment to rotate the incisors faciolingually -The force on the anchor unit, the same as on the incisors. B- BUT if the intrusion arch is tied at midline and cinch back is done: There is a lingual root torque on incisors as it intrudes Moment in incisors is balanced by moment at anchor molars Two couple systems: ** Utility arch for intrusion: Utility arch produced by Ricketts and recommended for incisor intrusion rectangular wire by pass canine and premolars teeth, this long span provide excellent load deflection, the light force is necessary for intrusion When utility arch is activated for intrusion, the amount of intrusion, tips the crown facially and can prevent facial tipping by: -Apply retraction force on incisors, that create moment in the opposite direction, this occur by cinching or tying back the intrusion utility arch -Torque the wire of utility arch in anterior segment in lingual direction, can accentuate the torque by cinching the end of wire at molar tube A: intrusive force + tipping facially of anteriors B: intrusion + torque in incisor to prevent facial tipping M Mechanics echanics in Orthodontics Part 2 in Orthodontics Part 2 Dr. Mohammed Alruby Dr. Mohammed Alruby

7. 7 C: intrusion + retraction force on incisors by cinching back of wire at molar end, prevent facial tipping ** symmetric and a symmetric bends: When wire is placed into two brackets, the forces of equilibrium always act at both brackets Types of V bends: 1-Symmetric V bends: which create equal and opposite couples at the bracket, the associated force at each bracket also are equal and opposite, and therefore cancel each other A: equal and opposite moment Intrusive and extrusive forces cancel each other 2-A symmetric V bends: which create unequal and opposite couples that intrude one unit and extrude the other As the bends moves toward one tooth, the moment on it increase and the moment on the distant tooth decrease When the bend is one third of the way along the inter-bracket span, the distant tooth receive only force and no moment When the V bend moves closer than one third to one of the teeth, there is moment created in both teeth in the same direction M Mechanics echanics in Orthodontics Part 2 in Orthodontics Part 2 Dr. Mohammed Alruby Dr. Mohammed Alruby

8. 8 3-Step bend: which create two couples in same direction, produce intrusive force in both tooth and extrusive force on other one that produce couples in same direction In contrast two V bends, as there is little effect on either the force or the couple when the step is moves off- center N: B: In long span wire like utility arch, V bend at molar produce significantly Less moment and associated equilibrium forces than the same V bend Located at the same distance from incisors segment ** two couple arch wire to change incisors inclination: = if the wire (asymmetric V bend in rectangular wire) spanning from molars tube to incisors is located to rotate incisors that lead to tipping and extrusion of incisors (allow correction of cross bite in mixed dentition) = if the wire cinched behind the molars so it cannot slide, so the effect is lingual root torque and extrusion with mesial force on molars ** Posterior cross bite correction: Transverse movement of the posterior teeth: 2 X 6 appliance can use to produce transverse movement of 1st molars, premolars not tied to arch wires so long span produce desired forces and moment in this two- couple system M Mechanics echanics in Orthodontics Part 2 in Orthodontics Part 2 Dr. Mohammed Alruby Dr. Mohammed Alruby

9. 9 ** Lingual arch as two-couple system: = lingual arches are another example of two –couple, lingual arches prevent tooth movement rather than create it = bilateral toe- in bends at 1st molars create equal and opposite couples, the mesiodistal forces cancel and the teeth are rotating to bring mesio-buccal cusp facially = unilateral toe-in bend rotate the molar on the side of the bend to move mesially and create force to move the other molar distally =opening loop of transpalatal arch at middle allow bilateral expansion = twist in wire on one side can be used to create stationary anchorage to tip the opposite molar facially N: B: Mechanics for transverse discrepancy: = posterior cross bite is a common malocclusion and constitute an important part in daily orthodontic practice = correction of dental expansion can have done by: -Slow expansion of quad helix -Removable expansion plate = length of treatment is higher in quad helix than expansion plate = cross elastics is a convenient way to correct posterior cross bite and depend on the reciprocal anchorage = some cases during correction of posterior cross bite there is some extrusive force that can done in posterior segment, that may lead to premolar contact and clock wise rotation of mandible M Mechanics echanics in Orthodontics Part 2 in Orthodontics Part 2 Dr. Mohammed Alruby Dr. Mohammed Alruby