Mechanics of Extraction Space Closure

1. 1 Mechanics of extraction space closure Mechanics of extraction space closure Prepared by Prepared by Dr: Mohammed Alruby Dr: Mohammed Alruby M Mechanics echanics of Extraction Space Closure of Extraction Space Closure Dr. Mohammed Alruby Dr. Mohammed Alruby

2. 2 Space closure Degree of space closure Moment to force ratio Segmental mechanics for space closure -Canine retraction -Incisors retraction Sliding mechanics for space closure Role of friction in sliding mechanics Factor influencing the sliding mechanics Mini-implant and space closure M Mechanics echanics of Extraction Space Closure of Extraction Space Closure Dr. Mohammed Alruby Dr. Mohammed Alruby

3. 3 Space closure: Bring opposing teeth together or segment of teeth by applying a force between them, this force is occlusal and buccal to center of resistance Greater amount of space closure will greater degree of side effects Degree of space closure: -Minor: less than 2mm -Moderate; less than 4mm -Severe: greater than 4mm Large space closure need more anchorage control over the desired tooth movement Anchorage type; 1-Absolute: no movement at anchorage unit 2-Maximum: minimum movement at anchorage unit 3-Moderate: same movement at anchorage unit as active 4-Minimum: major movement at anchorage unit Moment to force ratio: M/F: M / F ratio is a good way to describe or predict the quality of tooth movement M / F ratios: 5: 1 ------------------ required for tipping 7: 1 ------------------ required for controlled tipping 10: 1 -----------------required for translation 12: 1------------------required for root correction Loop design and M / F ratio: Mechanical properties of closing loops depend on many factors, such as loop height, shape, design and position, wire material. Loop height: As loop height increase, M / F ratio increase 6mm loop height ----------2 M / F ratio 10mm loop height ---------4 M / F ratio Loop shape: M / F ratio by T loop is higher than vertical loops M / F ratio by helix loop is higher than that without helix Loop placement: Higher moment produced at bracket closure to the loop Types of space closure mechanics 1-Segmental mechanics for space closure a-Canine retraction: Definition: movement of canine in distal direction from position close to lateral incisor to position next to premolars along gentle curvature of the arch Set up: there is an anterior-posterior space approximately 8mm distal to canine before space closure (21 X 25 stst) wire in (22 slot) can used to hold the arch as a separate unit Pre-activation of the loop: before loop is inserted pre-activation bends is done known as alpha bend (anterior curvature) and beta bend (posterior curvature). This bend is necessary to create a moment (Mc) that will counter act the moment of the force (Mf) The greater the degree of bend the greater Mc generated Activation of the loop: the final activation of loop is done only when it is placed in the bracket slot and the wire is pulled distally through posterior tube and cinched. The amount of distal pull is guided by the amount of space to be closed as: for 8mm extraction ----- 8mm activation is good M Mechanics echanics of Extraction Space Closure of Extraction Space Closure Dr. Mohammed Alruby Dr. Mohammed Alruby

4. 4 phase I: tipping: in initial phase of retraction which the spring is fully activated the center of rotation close to apex of canine so there is crown movement with little movement of root anchorage demand on molars is minimal if pre-activation bend is not done in wire loop there is uncontrolled tipping of canine will occur Mc< Mf phase II: translation: as the canine is distalize and the distance between two attachment decrease, the force level drops need more anchorage demands, both molars and canines tend to translate Mc = Mf M Mechanics echanics of Extraction Space Closure of Extraction Space Closure Dr. Mohammed Alruby Dr. Mohammed Alruby

5. 5 Phase III: root movement: Force level continue to drop More root movement than crown, root correction, Mc > Mf Anchorage reinforcement needed because there is some anchorage loss B- incisors retraction: Will proceed in a similar way as for canine retraction, during retraction there is more tipping movement so high Mc in posterior teeth is needed and less Mc at incisors. Phase I is more sufficient to close the space when incisors retracted Mc > Mf --------in molars Mc < Mf ---------in incisors 2-Sliding mechanics for space closure: Definition; pull or pushing the teeth along continuous arch wire, with a force delivery system adequate to produce movement Advantages: shorten treatment period, prevent excessive force application, improve patient comfort, more control during space closure a-Canine retraction: Before start canine retraction, all teeth must be levelled and aligned to a degree that arch wire chosen for retraction sits passively through the bracket slot Phase I: unsteady state: uncontrolled tipping This the initiation of canine retraction, single force applied to canine to produce moment Mf acting on (C Res) center of resistance of canine causing tip There is some degree of play between arch wire and bracket slot Tipping occurs in uncontrolled manner creating C Rot apical to C Res (If CRot apical to CRes so no Mc) Phase II; controlled state; controlled tipping: No clearance or play between arch wire and bracket, two point of contact between bracket and arch wire Mc opposite to Mf so no tipping. Mc< Mf Mc continue to increase and C Rot moves toward apex in pre-angulated bracket slot, the initial clearance is zero Phase III: steady state: translation state: Contact between arch wire and bracket edge increase leading to increase in Mc with in force level so Mc = Mf Phase IV: restorative state: = Constant decayed in distal movement ------ decrease force level ------ decrease Mf so Mf < Mc (with correction of root angulation (2nd order bend)) = Tremendous amount of friction at bracket wire interface that lead to relocation of C Rot gingival to C Res anywhere between bracket and C Res = If the force of retraction not decayed, this phase is not reached and there are only 2 phases (tipping only) = it is the root movement stage: Mc > Mf N: B: canine movements phases: 1-Uncontrolled tipping: Mf present and no Mc 2-Controlled tipping: Mf > Mc 3-Translation: Mf = Mc M Mechanics echanics of Extraction Space Closure of Extraction Space Closure Dr. Mohammed Alruby Dr. Mohammed Alruby

6. 6 4-Root correction: Mf < Mc B- incisors retraction: During space closure the incisors also allow the same phases as during canine retraction but the more dominant is I, II phases (tipping) due to greater play angle When we need to do more control for incisors retraction from the phase I by introduce Mc from the first phase of retraction by: 1-Torqueing the arch wire (twist the arch) 2-Increase the stiffness and size of main arch 3-Decrease Mf produced without compromising the force magnitude and this can achieve by change the point of force application relative to C Res of the teeth by using (power arm) design as the power arm close to C Res of canine during retraction Play angle: it is the angle between the bracket and the arch wire (line tangent bracket and line tangent wire surface) In 16 X 22stst-------- angle measured 16 to 18 degrees In 17 X 25 stst ------angle measured 13 to 14 degrees In 19 X 25 stst ----- angle measured 6 to 8 degrees In 21 X 25 stst ----- angle measured 2 to 3 degrees M Mechanics echanics of Extraction Space Closure of Extraction Space Closure Dr. Mohammed Alruby Dr. Mohammed Alruby

7. 7 N: B: = in a continuous arch system, there is a limiting possibilities of unpredictable canine movement. = During phase I, II movement of canine retraction, there is elastic deflection on the wire that create an extrusive force on incisors that cause lingual tipping = we can prevent that by: using high stiffness wire or using light force retraction or using overlay intrusion arch N: B: According to the third law of motion: = The retraction force on canine exert the same force on molars, this force will create moment on molar, causing it to tip mesially and this depend on: Surrounding bone, tooth characteristics, additional teeth ligated to the molars = by using heavier force; lateral open bite will occur due to tipping of canine and molars crown with no root correction that prevent tooth to enter phase III and IV of sliding movement. Role of friction in sliding mechanics: = in sliding mechanics, some of the applied force is dissipated as friction and the remainder is transferred = maximum biological tissue response occurs only when the applied force is sufficient magnitude to overcome friction and lie within the optimum range of force necessary for tooth movement = factors may cause friction: 1-Bracket width 2-Wire size 3-Materials 4-Tightness of ligation 5-Saliva Factors influencing the efficiency of sliding mechanics 1-Arch wire properties: = Stiffness of the continuous arch wire support the teeth, keeping it from the uncontrolled tipping when force is placed on it as using stst then niti wires. = The tooth will tip the wire contacts, the bracket at opposite corner of the slot, stop the tipping motion, this contact at corner appear to produce a counter acting moment that pulls the root of the tooth in the same direction as the crown moved. 2-For delivery system: Niti coil spring produced significant faster rate of retraction than active ligation or intra-oral elastics (O elastics) (3/16) Niti coil spring produced similar rate of space closure as power chain elastics 3-For ligation methods: Self-ligating bracket SLB produce low friction compared with conventional bracket. Mini-implant and space closure mechanics == Mini-implant (M I) serve as anchorage unit in situation where there is need for absolute anchorage, and used to produce constant and predictable force system, so, accurate and precise movement of active units can be achieving without possibility of anchorage loss == Basic principle of mini-implant M I: When using conventional mechanics, force application is usually parallel to the occlusal plane, the force ideal only in one plane, however in M I usually placed apical to occlusal plane between the roots, so the force is applied as an angle. == this angulated force broken into two compartments: a-Horizontal retraction force (r) b-Vertical intrusive force (I) M Mechanics echanics of Extraction Space Closure of Extraction Space Closure Dr. Mohammed Alruby Dr. Mohammed Alruby

8. 8 = the force by M I is much closer to C Res of anterior unit, therefore the mf is significantly less tha that of conventional mechanics, this means, there is less tendency of teeth to tip = in conventional mechanics, active part is the moved part and passive unit is the anchorage unit, but by using M I which is third counterpart, the selective moment between posterior and anterior segment is possible. Total force > retraction force > intrusive force (F > r > I) N: B: during retraction of anterior segment by using M I, there is two distinct force component: 1-Large retractive force (r) 2-Smaller intrusive force (I) 3-Causing enmass retraction with some intrusion of anterior segment M Mechanics echanics of Extraction Space Closure of Extraction Space Closure Dr. Mohammed Alruby Dr. Mohammed Alruby