Mechanical Properties Of Orthodontic Latex Elastics In Vitro

Objective:To test the tensile properties of four kinds of orthodontic latexelastics commonly used in clinical, and study the force-degradationcharacteristics of them in artificial saliva environment, providing areference for clinical application.Methods:Choose four kinds of orthodontic latex elastics with the sameelastic parameter (3.5oz),consisting of3/16inch (4.76mm),1/4inch (6.35mm),5/16inch (7.94mm)and3/8inch (9.52mm) lumensizes. Use electronic universal mechanical testing machine to testtheir specialized mechanical properties respectively. Analyze therelation between load and the deformation of the four elastics within0-60mm strain range and then establish the deformation-loadtables. Measure the remaining forces of specimens withfour elongation distances (20mm、30mm、40mm、50mm) weretested in37℃artificial saliva environment, respectively for1hour、6hours、12hours、24hours and48hours. Then calculated thepercentage of remaining force and analyzed the results.Results:1. Inner diameter and deformation value both influenced the load of the orthodontic latex elastics（P=0.000) significantly; the load ofdifferent elastics with the same deformation was substantiallydifferent; the deformation-load curve approximated linearrelationship in the range of5-40mm deformation and the slopeswere of no statistical significance.2.1/4and3/8groups showed no significant difference betweenthe inner diameters elastics load and the corresponding standardvalue(P>0.5) under triple stretch length,while3/16,5/16groups did(P>0.05).3. The force of all orthodontic rubber elastics degraded mostobviously at the initial one hour, then the force degraded slowly.The mean percentage of remaining force of most rubber elasticswere between20%～30%in the first hour, and at48hours werebetween30%～40%.But in the50mm elongation distances, theforce of3/16group degraded50.7%of the initial force after the firsthour, and remained only about45%at48hours.4. At the same time point, when the distance is greater than20mm, the mean percentage of remaining force of all orthodonticrubber elastics emerged the characteristics:3/8>5/16>1/4>3/16.But when the distance was equal to20mm, the percentage ofremaining force of3/8group did not comply with the above rule.5. At the same time point, the mean percentage of the remaining force of5/16,1/4,3/16groups declined with the length of the tensileelongation,,3/8group performanced drop after the first rise in20mm. The mean percentage of remaining force at the extension of20mm were higher than those at the extension of30mm or40mm,but Statistical analysis showed that there were no significantdifference between them(P<0.05). However, there were significantdifference between the extension of20mm and the extension of50mm (P>0.05). And there were no significant difference betweenthe extension of30mm and the extension of40mm（P>0.05）,whilethey all had significant difference with the extension of50mm.Conclusion:1.The load of the orthodontic latex elastics varied with the innerdiameters and deformation and that indicates that elastics shouldbe selected appropriately based on the value of the force requiredin clinical.2. Not all of the elastics matched the specified index. It issuggested that the clinician do not completely use the elasticsaccording to the specification.3. The force-degradation occurred rapidly in the first hour, aboutbetween20%-30%, while the clinical selection elastics may belarger than the value of20%-30%of the force to compensate forthis loss. Force degradation within1-48hours tend to be stable, patients did not need to replace them every day. But consideringthe oral health problems, it suggest that patients replace theelastics every day.4. With the increasing in the stretched length, the forcedegradation ratio was increased in sequence. Small range ofopen-closed bite had little influence, patients are advised avoidopen big mouth when wearing rubber elastics.5. Clinicians may refer the deformation-load tables when selectrubber elastics for patients.