Effect of Bracket ’ s Slot Size on Canine Position and Space Closure Rate ( A Typodont Study )

Aims: This study aimed to determine the effect of newly introduced 0.020” slot size brackets on dimensional positions and space closure ratio of mandibular canine. Material and Methods: The study includes six groups according to slot sizes (0.018”, 0.020” and 0.022”) and materials of canine retraction (elastomeric power chain and closed coil spring). A 200 gm of retraction force was used on 0.017” X 0.025” stainless steel wire ligated to brackets by elastomeric ligatures along 13 mm available space. After immersion of the typodont in water bath with (50-55) C for 5 minutes, the rate of space closure were measured in millimeter using vernier (from the distal wings of canine bracket and the mesial wings of the second premolar), In both vertical and horizontal directions, digital images were taken by camera and the angle between canine extension bar )C B) and bite plane extension bar (BPB) was measured by protractor to determine tipping and rotation. Results: The results showed that 0.022” slot bracket groups had highest amount of rate of space closure tipping and rotation, while 0.020” slot brackets groups had higher space closure rate than 0.018” slot brackets groups. Conclusions: bracket slot 0.022” groups showed higher tipping, rotation and space closure rate, while 0.020” slot brackets groups had higher rate of space closure with same or better rotation and tipping control in comparison with 0.018” slot bracket groups.


INTRODUCTION
Orthodontic tooth movement during space closure is achieved through two types of mechanics; segmental or sectional mechanics (non-frictional system) and sliding mechanics (frictional system) that involve either moving the brackets along an arch wire or sliding arch wire through brackets and tubes. P (1) P One of main differences between two mechanics is the fric- Size on Canine  Position and Space Closure Rate  (A Typodont Study) tion, since sectional mechanics do not involve friction; while friction plays a significant role in sliding space closure. (2) Friction is the resistance to motion when an object moves tangentially against another. (3,4 ) During orthodontic space closure with sliding mechanics, frictional force is generated at the bracket arch wire interface and has a tendency to inhibit the desired tooth movement. (5) Tooth movement ceases when there is balance of the retraction force with the resistance force, frictional force and wire resiliency this is called "frictional lock". (6) In orthodontic mechanics, friction is affected by the type of arch wire and bracket material, the design of the bracket and bracket slots, the ligation method, the inter bracket distance, and the oral environment. Reducing friction will result in less applied force than is needed for moving teeth during orthodontic treatment. (7) After extensive experimentation with the edgewise appliance, Angle adopted the dimensions of 0.022 inch as his slot size because it allowed better control of crown and root position in all three planes of space using the precious metal arch wires available to him at the time, In the 1930s, stiffer, less expensive stainless steel arch wires were introduced, replacing the softer precious metal arch wires. These advances in metallurgy allowed orthodontists to provide similar clinical forces with smaller arch wires. This change in materials made it feasible to downsize from the traditional 0.022 inch bracket slot to the smaller 0.018 inch slot. (8,9) Another factor in determining clinical outcome of an orthodontic case is treatment time. Although arch wires of the same size and material have been shown in studies to align the mandibular anterior teeth faster using the 0.022 inch brackets than the 0.018 inch brackets, total treatment time with 0.022 inch brackets is longer compared with 0.018 inch brackets. However, none of these studies measured the quality of the outcomes of any of the completed cases. (9) The pre adjusted appliance seems to perform best in the 0.022" slot form, the larger slot allow more freedom of movement and helps to keep forces light, with the 0.018" slot the main working wires are normally more flexible than those used in 0.022" slots and hence show greater deflection and binding during space closure. (10) The ideal in the use of sliding mechanics would find the best combination of arch wire size, slot size, and force which would translate a tooth along an arch wire with minimal friction without excessive tipping and without unduly disturbing anchorage. (11) Aims of the study:

Effect of Bracket's Slot
To evaluate the effect of different bracket's slot width and type of retraction force(elastics and springs) on the rate of space closure, tipping and rotation of canine after sliding.

MATERIALES AND METHODS
The samples of this study divided into six groups (10 samples for each group), with a different slot sizes brackets (either 0.022", 0.020" or 0.018" slot size, two sets of brackets of each slot size) and methods of force application (elastomeric power chain and nickel titanium closed coil spring, 30 pieces of each). The retraction of right mandibular canine was done by using either a short continuous elastomeric power chain (six rings) or nickel titanium closed coil spring (9 mm in length) which were attached posteriorly to the molar's band hook and anteriorly to the canine's bracket hook along (0.017" × 0.025" ) stainless steel arch wire(6 pieces) and across an 13mm extraction space, and arch wire was ligated to orthodontic bracket by using elastomeric ligature.
Each one of these groups will undergo three measurements which are : tipping, rotation and rate of space closure.
The force applied was adjusted to be about 200 gm (measured by using a tension gauge) according to manufactures instruction (Ormco). The typodont was immersed in a water bath (HAAKE -England) with (50-55) 0C for 5 minutes then removed and immersed directly in a tap water. (12) The typodont used in this study was prepared according to manufacturer's instructions (Ormco). The preadjusted Roth stainless steel brackets (either 0.022", 0.020" or 0.018" slot sizes brackets, Dentos, South Korea) were fixed on the metallic typodont teeth by using epoxy steel adhesive. (13) The vertical position of brackets were checked by using the bracket positioning gauge, (14) the Roth stainless steel molar bands were also fixed to the metallic first molar tooth by using Zinc polycarboxylate orthodontic cement. A class II typodont wax form was used, so aligment of the teeth was done by using arch wires started from 0.014"NiTi and gradually upgraded till we reach 0.017"X0.025" Stainless steel archwire. (15) The first premolar was removed from the typodont to allow a space for canine sliding. Acrylic bite plane was constructed by taking a primary alginate impression for the aligned teeth then pouring by plaster materials then a special tray were made and a final impression taken with alginate to construct a master cast, then wax materials was applied in a way that cover the occlusal, incisal, palatal, labial and lingual surface of the teeth (except the canine and the first premolar extraction space), then the wax replaced by hot cure acrylic resin. (11) A special two stainless steel bars (rods) were constructed, the first was a bite plane extension bar (BPB) and the other was canine extension bar (CB), as shown in (Figure1).

Figure (1): Canine extension bar (CB) and bite plane extension bar (BPB)
The BPB is a L-shape bar made from 0.018" X0.025" stainless steel rectangular wire, the short arm is inserted in the lingual midline of the acrylic bite plane, this bar emerges upward for 10 mm then it bends (90 degree) and extends facially 20 mm to make right angle with CB (11) , the CB were also constructed from a from 0.018" X 0.025" stainless steel rec-tangular wire in a form of L-shape, the short arm is welded to the distal aspect of the right canine (the tooth intented to be slides). This arm extended upward incisally 10 mm then it bends at right angle and extended anteriorly 20 mm and 5 mm over the tip of canine cusp and under the BPB by about 5 mm. such two bars are used as a guide to determine the position

Effect of Slot Size on Canine Position and Space Closure Rate After Sliding.
Al -Rafidain Dent J Vol. 14, No1, 2014 of canine after each sliding procedure regarding the degree of tipping and rotation, this method is a modification of Huffman and Way procedure (16) .
A wooden table was constructed with two metallic bases, one to fix the digital camera (vertically for rotational measurements and horizontally for tipping measurements), and the other to fix the typodont in a way to allow a standard position for taking images before and after each sliding.

Canine position measurements:
Before starting sliding of canine into first premolar space, the following were checked: First: all the teeth were situated in well aligned position. Second: the distance between the distal wings of canine bracket and the mesial wings of the second premolar bracket was measured in millimeter by using digital vernier before each sliding (initial space before retraction) which found be equal to 14 mm. Third: the angle between BPB and CB should be 90P 0 P from both horizontal and vertical direction. This angle is considered as CB original angle (zero tipping and rotation before retraction).
Sliding was performed by immersion of loaded typodont (power chain or coil spring attached between canine and molar) in water bath at temp. 50 -55 degree for 5 min. so wax will soften as a reaction to heating permitting canine retraction to occurs, subsequently, typodont will remove and immediately immersed in tap water for other 5 minute for wax hardening and enhance teeth stability to occur for measurement.
After sliding, the distance between the distal wings of canine bracket and the mesial wings of the second premolar were again measured by using digital vernier after each sliding (remaining space after retraction), then the rate of space closure was measured (by subtraction of the remaining space from the initial space).
The canine's tipping degree after retraction was measured by taking an image to the typodont using digital camera, with transvers projection from right side (directly toward the right canine) where the angle between BPB and CB is exposed and then can be measured directly on the image using Protractor. This angle is considered as CB inclination angle, then the canine's tipping was measured by subtraction of the CB inclination angle from the CB original angle.
The canine's rotation degree after movement was measured by taking a photograph to the typodont using digital camera, with vertical projection from occlusal side (directly toward the right canine) where the angle between BPB and CB is exposed and then can be measured directly on the photograph using Protractor, this angle is considered as CB rotation angle, then the canine's rotation was measured by subtraction of the CB rotation angle from the CB original angle.
Statistical methods used include: descriptive statistics, analysis of variance (ANOVA) and Duncan multiple range testes.

RESULTS
The descriptive statistics (mean, standard deviations, standard error, minimum and maximum values) for the degree of tipping, rotation and rate of space closure of the three groups are listed in the Table (1).

Tipping:
The ANOVA test for the tipping showed that there is a significant difference among the six groups at p≤ 0.001 as shown in the Table 2. The Duncan Multiple Analysis Range Test showed that (S22) group had the highest amount of tipping with a significant difference from other groups (except E22) at p ≤ 0.001, while (S20) group had the lowest level of tipping with a non significant difference from E18, S18 and E20 groups at p ≤ 0.001 as shown in the Table 3.  12 ± 0.516 A E20 6.6 ± 0.476 B S18 6.5 ± 0.428 B E18 6.3 ± 0.44 B S20 6.00 ± 0.365 B *The mean in degree measurement. **different letters mean significant different at p ≤ 0.05. E22 : 0.022" slot bracket and elastomeric chain for retraction. S22 : 0.022" slot bracket and nickel titanium coil spring for retraction. E20 : 0.020" slot bracket and elastomeric chain for retraction. S20 : 0.020" slot bracket and nickel titanium coil spring for retraction. E18 : 0.018" slot bracket and elastomeric chain for retraction. S18 : 0.018" slot bracket and nickel titanium coil spring for retraction.

Rotation:
The ANOVA test for the rotation showed that there is a significant difference between the six groups at p ≤ 0.001 as shown in the table 4. The Duncan Multiple Analysis Range Test showed that (S22) group had the highest amount of rota-tion with a significant difference from other groups except (E22) group at p ≤ 0.001, while (S20) group had the lowest amount of rotation with a significant difference from other groups except E20 and E18 at p ≤ 0.001 as shown in the Table 5.

Table (4):
One way ANOVA analysis for rotation measurement. *The mean in degree measurement. **different letters mean significant different at p ≤ 0.05. E22 : 0.022" slot bracket and elastomeric chain for retraction. S22 : 0.022" slot bracket and nickel titanium coil spring for retraction. E20 : 0.020" slot bracket and elastomeric chain for retraction. S20 : 0.020" slot bracket and nickel titanium coil spring for retraction. E18 : 0.018" slot bracket and elastomeric chain for retraction. S18 : 0.018" slot bracket and nickel titanium coil spring for retraction. 20.9 ± 0.752 C

Rate of space closure:
The ANOVA test for the rate of space closure showed that there is a significant difference between the six groups at p ≤ 0.001 as shown in the Table 6. The Duncan Multiple Analysis Range Test showed that (S22) group had the highest level of rate of space closure with a significant difference from other groups at p ≤ 0.001, while (S18) group had the lowest level with a significant differences from other group except (E18) group at p ≤ 0.001. the remaining groups are distributed between the highest and lowest groups with a significant differences at p ≤ 0.001 as shown in the table 7. 6.45 ± 0.162 B S20 5.00 ± 0.23 C E20 4.78 ± 0.287 C E18 3.81 ± 0.244 D S18 3.74 ± 0.223 D *The mean in millimeters measurement. **different letters mean significant different at p ≤ 0.05. E22 : 0.022" slot bracket and elastomeric chain for retraction. S22 : 0.022" slot bracket and nickel titanium coil spring for retraction. E20 : 0.020" slot bracket and elastomeric chain for retraction. S20 : 0.020" slot bracket and nickel titanium coil spring for retraction. E18 : 0.018" slot bracket and elastomeric chain for retraction. S18 : 0.018" slot bracket and nickel titanium coil spring for retraction.

DISCUSSION
Regarding tipping, (S22 and E22) groups reflect highest amount of tipping from other groups, this is may be due to larger space between bracket slot and archwire from other groups lead to poor wire control on tooth position during sliding, this agree with Ehsania et al P (17) P who mentioned that the tipping is a constant phenomenon during sliding and it always occurs when orthodontic force is applied to the tooth. The highest degree of tipping that associated with 0.022" brackets groups may be attributed to high rotational degree present in the same groups that makes part of the arch wire out of the slot at the distoocclusal and distogingival wings. Huff-man et al, P (11) P mentioned that as wires decrease in size relative to bracket slot size creating more clearance between the arch wires and bracket slots there will be considerably more tipping.
Elastomeric ligatures that used in ligation were polyurethane based polymers, and studies have verified that these materials undergo stress relaxation and slow hydrolytic decomposition overtime P (18) P may lead to weaker ligation force.
No significance difference was found in this study between the effect elastomeric power chain and nickel titanium close coil spring on tipping .
The remaining groups (E20, S18, E18 and S20) reflect lower tipping magnitude than (S22 and E22) with non significant difference between them, this may be due to more close contact between arch wire and slot walls lead to better control over tooth position, in same time no significant difference in tipping was found between 0.018" slot and 0.020" slot bracket that may mean there was a small difference in controlling force of arch wire on tooth position in both slot dimensions. Also no distinctive effect on tipping was found between elastic chain and closed coil spring.
Regarding rotation, (S22 and E22) showed highest amount of rotation after sliding, retraction of the canine with a force labial to its center of resistance will cause a tendency for the tooth to rotate distopalatally, (19) a moment is necessary to counteract tooth rotation, this moment is exerted by the ligature tying the arch wire to the bracket, but the ligature will probably yield during the control intervals, resulting in rotation of the canine during its distal movement, (20) arch wire tend to escape from wide bracket slot and partial failure of elastic ligature to keep arch wire inside slot lead to this remarkable rotation.
In the same time (S20) group reflect smaller amount of rotation between all groups, smaller slot size than 0.022" groups lead to more friction between slot walls and wire surfaces, this friction may help in keeping wire inside slot and decrease rotation. Rotational play depend on the length of the rotational lever arm and the quality of the ligation technique to keep the archwire seated in the bottom of the slot. (21) No significant difference was found between (E20 and E18) on the effect of rotation during sliding.
Regarding space closure, both (S22 and E22) groups have highest space closure rates among all groups with a significant difference between them, minimal friction between arch wire and bracket lead to easy tooth sliding along arch wire these small friction come from wider bracket slots and relatively weak ligation force exerted by elastic ligature . Anderson et al 1970, suggested that friction can be reduced by having more freedom of movement between wire and bracket slot but these condition allow more tipping.
This study found significant difference between elastomeric power chain and nick-el titanium closed coil spring on rate of space closure on 0.022" slot size bracket, coil spring obviously have larger space closure rate than elastomeric chain, this may be due to more constant continuous force of coil spring in comparison to rapid dropping of force in elastomeric chain, also heat have deteriorative effect on elastomeric chain lead to force relaxation phenomenon. (23) In the same time, (E18 and S18) had minimal space closure rate due to high friction between bracket and arch wire that try to prevent sliding of tooth along arch wire, these greater friction resulted actually from the closer the fit between bracket and arch wire (24), the reduction of the clearance between arch wire and the bracket slot frictional resistance increases. (11) Ligature elastic either placed around the bracket in a figure-O pattern or in a figure-8 pattern, this ligation method will increase the friction by pressing the arch wire against the bracket slot. (25) In other hand (S20 and E20) had significantly intermediate space closure rate between 0.022" slot bracket and 0.018" slot bracket with a non significant difference between coil spring and elastomeric chain.
The results of this study have clinical value that using 0.020" slot brackets will significantly reduce tipping and rotation more than 0.022" and 0.018" slot brackets and in same time produce rate of space closure significantly more than 0.018" slot brackets does.

CONCLUSIONS
• Brackets with a slot gauge 0.022" have highest degree of rotation and tipping and largest space closure rate.
• Brackets with a slot gauge 0.020" have similar or better rotation and tipping control from 0.018 slot brackets.
• Rate of space closure in 0.020 slot bracket are larger than 0.018 slot bracket.
• As regard to the variables tested in this study, using 0.020 brackets are more beneficial than using 0.018 ones.