Download coated elastomeric modules (super slick) and non

An In-vitro evaluation of frictional resistance of polymer- coated
elastomeric modules (super slick) and non-coated conventional
elastomeric modules after use in the oral environment at 1 week
and 1 month intervals.
Dr Raghu H.Ha, Dr. H. L.Umab, Dr. V. Shashikala Kumaric
a
Dental Surgeon, Mysore
Associate Professor, Department of Orthodontics, Government Dental College & Research Institute, Bangalore.
c
Professor& Head, Department of Orthodontics, Government Dental College & Research Institute,
Bangalore.
b
ABSTRACT:
Background and objectives: The aim of this study was in-vitro evaluation of frictional resistance of
polymer coated elastomeric super slick modules and non-coated conventional elastomeric modules
after their use in true oral environment during sliding mechanics at intervals of 1 week and 1 month .
Method: Ten subjects undergoing orthodontic treatment with pre-adjusted edgewise 022”×028” prescription and 019”×025” stainless steel arch-wire in place were chosen. In the upper arch, the arch
wire was ligated to the brackets with polymer coated super slick elastomeric modules and in the lower arch with non-coated conventional elastomeric modules and vice-versa for other five individuals.
The modules were retrieved after 1week and 1month of use in true oral environment. The as-received
and retrieved modules were tested for frictional resistance using an experimental model mounted on
the crosshead of an instron machine and the co-efficient of dynamic friction was recorded.
Results: There were statistically significant differences in the co-efficient of dynamic friction between the non-coated conventional (0.08 at 1week and 0.05 at 1month) and polymer-coated super
slick modules (0.17 at 1week and 0.11 at 1month) retrieved from the oral environment at different
time intervals. No statistically significant differences were found between the upper and lower modules of both types.
Conclusion: The co-efficient of dynamic friction of polymer coated super slick modules was significantly greater when compared to non-coated conventional modules in the true oral environment.
Since low frictional values are desired during leveling and aligning and during sliding mechanics,
polymer coated super slick modules are not advised for use as they can add to the burden of anchorage.
Key words: Friction, ligature, super slick modules, conventional modules
Introduction
During orthodontic space closure with sliding mechanics resistance to sliding is produced at the
bracket-arch-wire interface which tends to counteract the applied force and in turn the desired tooth
movement.1 Resistance to sliding in other words is friction.Friction is a force that retards or resists
the relative motion of two objects in contact. During space closure with sliding mechanics some
amount of applied force is dissipated as friction and the remaining is transferred to the supporting
structures of the tooth to mediate tooth movement.2 Friction is a clinical challenge and must be dealt
with efficiently to provide optimum orthodontic results.
The variables affecting friction or resistance to sliding is multifactorial. These can be either mechanical or biological. Mechanical variables include bracket material, slot size, bracket width and
angulation, wire size, wire shape, wire material, ligature material, force of ligation, etc. Biological
factors include saliva, plaque, acquired pellicle, corrosion, biological resistance, etc.2
Various methods have been used to reduce the friction of ligation such as stainless steel ligatures,
elastomeric modules and self-ligating brackets. Stainless steel ligatures produce variable ligation
forces, are time consuming to place and cause discomfort to the patient.3 Recently introduced selfligating brackets reduce treatment time, are comfortable to the patient and have low frictional resistance but they are more costly.4
Elastomeric modules have replaced stainless steel ligatures since their introduction in the
1960’s.Their advantages are quick application and removal, enhanced patient comfort, fluoride release potential and availability in a variety of colors for better patient acceptance.5 But the dentition
and soft tissues may be adversely affected by microbial accumulation around the ligated bracket and
arch-wire may not seat completely during torquing and rotation correction. Binding may occur during sliding mechanics, creating friction and thus loss of applied force.5
To overcome these disadvantages, a polymer-coated ligature Super Slick was introduced with
claims that it significantly reduced friction compared with non-coated conventional elastomeric
modules. These are manufactured with a special polyurethane mix by injection moulding technique
and coated with covalently bonded metafasix. These modules have claimed to reduce friction by
60% compared to uncoated modules with similar elastic properties.3
Conflicting reports exist regarding lubrication and its effect on friction. Saliva or a saliva substitute
serves as an excellent lubricant in the sliding of the bracket along the wire.6 Comparison of clinical
and laboratory measurements has shown that less force was needed to initiate movement when experimental models were used to measure resistance to movement intra-orally than in the laboratory.
This was due to the occlusal forces on tooth movement in the mouth.3 Elastomeric materials in general exhibit force decay after stretching, more so in the oral environment because of the moisture
from saliva.7 Besides dimensional alterations, the exposure of elastomers to water leads to weakening
of the intermolecular forces and subsequently to matrix decomposition and chemical degradation of
these materials.8 Any improvement in the frictional properties would be of clinical benefit only if the
coating remained functional in the oral environment. Kusy et al9 stated that experiments conducted in
artificial saliva were invalid because it is no substitute for human saliva. The in-vivo environment
differs markedly from ex-vivo conditions because of variables such as masticatory force and temperature.10
Thus the aim of this study was an in-vitro evaluation of frictional resistance of polymer-coated
elastomeric modules Super slick and non-coated conventional elastomeric modules after their use in
the oral environment at intervals of 1 week and 1 month respectively.
Materials and methods
Ten subjects were chosen from patients undergoing orthodontic treatment at the Department Of Orthodontics and Dentofacial Orthopedics, Government Dental College And Research Institute, Bengaluru.
All selected cases were under treatment with pre-adjusted edgewise 022”x028”slot prescription (Gemini, 3M, USA) and 019”x025”SS arch wire in place.Two types of elastomeric modules were compared;SuperslickTMelastomeric modules (TP orthodontics,LaPorte, Ind)(Fig1) and Conventional elastomeric modules (3M Unitek USA) (Fig2). In this study, upper arch was ligated with polymer-coated
super slick elastomeric modules and lower arch with conventional modules (Fig3) for five individuals
and vice versa (Fig4) for other five individuals. The upper and lower arches were randomly selected for
placement of elastomeric modules. Patients were instructed to maintain oral hygiene with no special
instructions during this period. The first set of modules remained in the patient’s mouth for1 week. On
retrieval, these modules were replaced by fresh set of modules of the same type, in the same way by the
same operator which were also retrieved from patient’s mouth after one month. This process was repeated in 10 patients. Specimens retrieved were stored in normal saline.
Experimental groups were as follows:
Group 1 C - 10 non-coated conventional modules at 0 hrs (as received).
Group 2 CU- 10 upper non-coated conventional modules after 1 week of intra oral use.
Group 3 CU- 10 upper non-coated conventional modules after 1 month of intra oral use.
Group 4 CL - 10 lower non-coated conventional modules after 1 week of intra oral use.
Group 5 CL - 10 lower non-coated conventional modules after 1 month of intra oral use.
Group 1 S - 10 polymer- coated modules at 0 hrs (as received)
Group 2 SU - 10 upper polymer- coated modules after 1 week of intra oral use.
Group 3 SU -10 upper polymer- coated modules after 1 month of intra oral use.
Group 4 SL - 10 lower polymer- coated modules after 1 week of intra oral use.
Group 5 SL - 10 lower polymer- coated modules after 1 month of intra oral use.
PREPARATION OF TEST ASSEMBLIES:
In actual clinical situation when sliding mechanics are employed to retract the anterior teeth, the friction arising from archwire contact, results not from a single bracket but more relevantly due to contact in the buccal segment attachments. Unlike previous frictional studies where the predominant test
model was a model or assembly having archwires being pulled through a single bracket slot only, the
present study evaluated the frictional forces arising when the archwire slides through an entire buccal
segment.Two experimental models reproducing the buccal segments of maxillary (Fig5) and mandibular arch (Fig6) with respective upper and lower 022”x028” preadjusted edgewise canine and 2nd premolar brackets and molar tubes bonded to respective natural teeth were used to assess the frictional
forces produced by two types of elastomeric modules on conventional stainless steel brackets. Each
experimental model consisted of an acrylic bar which had a line scribed parallel to its long axis. A
section of 0215”x028” stainless steel wire was used to align the brackets before embedding them in the
acrylic bar along the scribed line. This was to aid in aligning the pull of the wire through thebrackets,
so that friction was not induced by adverse tipping or torsion moments and also allow the brackets to
move along the wire as an axial tensile force was applied by the instron machine. According to Pizzoni et al11,the experimental setup in this study falls under the second group where the archwires
slide through brackets parallel to the bracket slot.
Instron 4467 (Instron Corporation, Massachusetts, USA) with 0.005% accuracy for set speed and
0.05% accuracy for applied force was used for checking the force in the as- received and retrieved
samples of both types of modules.0.019”x0.025” stainless steel wire was inserted into the bracket assembly and the as- received and retrieved modules from oral cavity after 1 week and 1 month use were
applied separately on to the brackets. The experimental models were mounted in the instron machine.
For every traction test over a distance of 15 mm at a speed of 5 mm/min the frictional forces were
recorded. Coefficient of dynamic friction was studied and the values obtained were non-parametric
(Table). All measurements were performed at room temperature of 20 ± 2ºC.
The Statistical software namely SPSS 15.0, Stata 8.0, MedCalc 9.0.1 and Systat 11.0 was used for
the analysis of the data. Student t-test (two tailed, dependent) was used to find the significance of
study parameters on continuous scale within each group. Student t-test for paired comparisons was
used to investigate the significance of the difference between single population means. No assumption was made about the population variances. A comparative evaluation study Split Mouth Design
was used in this study.
RESULTS:
Friction can be described by the co-efficient of friction which is a constant and is related to the surface characteristics of the materials. The co-efficient of friction can be described mathematically as
the frictional force that resists motion divided by the normal force that acts perpendicular to the two
contacting surfaces. There are two co-efficient of friction for a material. One is the co-efficient of
static friction and another is the co-efficient of dynamic friction which reflects the force necessary to
perpetuate this motion.2 Cacciafestaet al12 found that even though static frictional forces were greater
than the dynamic ones, no significant differences were found between static and dynamic frictional
forces.
Mean values and standard deviation of co-efficient of dynamic friction for the two sets of elastomeric modules of the experimental groups are shown in the Table. It also shows comparison of friction between the experimental groups at different time points,at 1week and 1month of intra-oral
use.The co-efficient of dynamic friction of non-coated conventional and polymer coated super slick
elastomeric modules in the as-received state was 0.20 and 0.18 respectively. There were statistically
significant differences between the retrieved non-coated conventional and polymer coated super slick
elastomeric modules with conventional modules showing a mean co-efficient of dynamic friction of
0.08 at 1week and 0.05 at 1 month of intra-oral use and super slick modules showing a mean coefficient of dynamic friction of 0.17 at 1 week and 0.11 at 1 month of intra-oral use during sliding
mechanics. The amount of force exerted by non-coated conventional modules was less when compared with super slick modules after 1 week and 1month of intra-oral use. There was no statistically
significant difference in the co-efficient of dynamic friction between the upper and the lower elastomeric modules.
Discussion
The success of tooth movement during sliding mechanics with pre-adjusted appliances depends to a
large extent on the ability of orthodontic archwire to slide through brackets and tubes with as much
minimum friction as possible.Various studies have shown that friction is produced between brackets,
tubes and archwires that tend to counteract the applied force and in turn the desired tooth movement
many a time causing loss of anchorage.1,2,13,14 Studies have shown that the portion of applied force
lost due to resistance to sliding can range from 12% to 60%.15 Iwasaki et al16 confirmed that during
sliding mechanics 30% to 50% of total frictional force generated by a premolar bracket travelling
along a 019”x025” stainless steel archwire is lost due to the friction of ligation. Thus when orthodontic tooth movement is being planned, an understanding of forces required to overcome friction is important so that the appropriate magnitude of force can be used to produce optimal biologic tooth
movement.12
It has been reported that friction is largely determined by the nature of ligation.12 Various studies
have described the significant influence of different modes of ligation and their resulting effects on
friction.Traditional methods of ligation include the use of elastomeric modules and stainless steel
ties. Stainless steel ties produced less frictional forces compared to elastomeric modules.17 Iwasaki
R.Laura et al16 reported that the frictional forces generated by stainless steel ligation was found to
vary considerably between loose and tight ligation in the range of 618±578g to 1470±915g respectively and significant inter-operator variability was present. Additionally, it was also observed that
there was a wide variation in the amount of force generated by stainless steel ties even when a single
operator was employed. Stainless steel ties are time consuming to place and cause discomfort to the
patient.3
Researchers have recently focused on elastomeric ligatures and self-ligating brackets as alternatives to reduce or eliminate the shortcomings of stainless steel ligatures. Several studies have reported significantly lower frictional forces produced by self-ligating brackets than conventional ligatures.2,3,12,16,18 Thorstensen and Kusy19 stated that resistance to sliding of self-ligating brackets in the
open state tied with ligature wire was comparable to that of conventional brackets. Franchi et al20
found no significant difference in friction between self-ligating brackets and nonconventional elastomeric ligatures (Slide,Leoneorthodontic products) but significant reduction in friction was reported
between Slide and conventional elastomeric ligatures.
Since their introduction to orthodontics, elastomeric modules have replaced stainless steel ties. A
single module produces a ligation force of 50-150g.2 Elastomeric modules may not completely seat
the wire during torquing or rotational corrections and binding may occur during sliding mechanics. A
polymeric-coated ligature SUPER SLICK (TP Orthodontics, LaPorte Ind) was introduced to the orthodontic market with claims that it significantly reduced friction compared with conventional ties.10
Research on sliding mechanics using super slick on 019x025 inch archwires demonstrated an advantageous reduction in frictional force compared with rectangular cross-sectional modules but not to
round cross-sectional non-coated modules and self-ligating brackets.7 However other studies have
reported that coated modules did not produce less friction than uncoated modules.3,6,16,18 Whereas in
some studies super slick modules showed similar frictional forces compared to non-coated modules.
Contrary to the studies7,
10
which showed that Super Slick modules significantly reduce friction
with sliding mechanics, our results indicate that they do not confer any advantage over conventional
modules, and in fact, their resistance was significantly greater.Similar results were obtained by Helen
Sylvia Griffiths, Martyn Sherriff, and Anthony John Ireland6 in their study.The co-efficient of dynamic friction of both types of modules reduced with time of stretch. As observed from the Table,
the co-efficient of dynamic friction exerted by conventional modules and super slick modules in the
as-received state is almost the same, which is 0.20 and 0.18 respectively with no statistically significant difference.Over a period of 1 week of intra-oral use, the retrieved conventional modules showed
a reduction in the co-efficient of dynamic friction of 0.08, both in the upper and lower whereas the
super slick modules over the same period remained almost same (0.17) as that in the as-received
state. This difference was found to be statistically significant. After one month of intra-oral use, the
retrieved conventional modules both upper and lower showed 0.05 and 0.06 respectively, a 75% reduction in the co-efficient of dynamic friction whereas the super slick modules reduced marginally
0.11 and 0.14 respectively by only 39%.This difference in reduction of co-efficient of dynamic friction between the two types of modules was found to be strongly significant. The co-efficient of dynamic friction of conventional modules reduced drastically with time of stretch compared to that of
super slick. Tayler N.G. and Icon K21 found that the ligation with loosely placed ligatures or
stretched modules reduced frictional forces and showed a steady reduction over a 3 week period.
Chimenti C, Franchi L et al22 stated that dimensions of elastomeric ligatures and elastomeric materials lubricated with silicon influence the static frictional resistance in sliding mechanics. The lubricated elastomeric ligatures generated significantly smaller frictional forces than non-lubricated elastomeric ligatures. Kuster et al23 in their comparative study between two brands of elastic chains
showed that the decline in frictional force during intra-oral use was much greater than in the laboratory tests. Similarly in our study, it was expected that in the oral environment with saliva as a medium, the dynamic friction of super slick and conventional modules would reduce. But only the dynamic friction of conventional modules reduced drastically by 61% whereas that of super slick reduced by 43% after one month of intra-oral use. Hain M, Dhopatkar A and Rock P4 stated that under
simulated effects of human saliva the coated modules produced 50% less friction than all other conventional ligation methods. However Datana et al5 investigated the degradation of force and surface
characteristics of conventional and super slick modules in true oral environment and stated that super
slick modules introduced increased frictional forces than conventional modules similar to our study.
The super slick modules seem to be highly stable in the oral environment which may be because of
the special surface coating.
Clinically the reduction in friction of conventional modules might be considered an advantage in
orthodontic procedures such as space closure and overjet reduction with sliding mechanics, especially in critical posterior anchorage. But other functions of the ligatures are put at risk, such as adequate
torque expression and complete seating of the archwire in the bracket slots. So when aligning and
levelling are priorities, the conventional modules are preferable because of their reduced friction instead of the highly stable super slick elastomeric modules.
It is clear from the present study that coated Super Slick elastomeric modules produce higher levels
of co-efficient of dynamic friction than the non-coated conventional elastomeric modules. Most likely the higher frictional values were the result of the stable polymer coating which may have caused
archwire binding in the brackets in true oral environment. Further investigation needs to be done to
study the surface characteristics of super slick modules and its effect on friction when used in the
oral environment.
CONCLUSION:
The co-efficient of dynamic friction of polymer coated Super Slick modules was significantly greater
when compared to non-coated conventional modules in the true oral environment. Low frictional
forces are desired during leveling and aligning and during space closure. Higher frictional forces are
desired for expressing the torque in the bracket and during finishing and detailing. Their use during
sliding mechanics is not advised as the higher frictional forces of super slick may add to the burden
of anchorage. Instead they can be used as active tie- backs for space closure and for full torque expression.
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