the natural latex contains 25%40% of isoprene polymer with high molecular weight and small amount of proteins and fatty acids . the latex elastics are still in common use to correct sagittal discrepancies or to improve the interdigitation of the teeth due to their low cost and high flexibility . with the light force provided by these elastic bands , proper rate of tooth movement with the least patient discomfort can be provided during interact mechanotherapies . rule of  3  is applied for interarch elastics , indicating that the elastics would exert the repeated force when they were extended 3 times of their diameter . the elastic properties of these latex materials depend on the twisted long molecular chains arranged irregularly and linked together with covalent bond at certain points . force decay of rubber elastic bands can be explained by the fatigue and creep force relaxation which can be accentuated under adverse environmental conditions . as a result , although these elastic bands are used extensively , their mechanical properties are not well defined . particularly , these properties are influenced by several factors related to the material such as loss of elasticity , amount of force decay , and composition of the elastics , and also , the environmental factors such as composition of saliva , intraoral ph and temperature variations , food texture and pigments . a few studies have evaluated the effect of salivary ph and its changes on the force degradation of latex elastics . previous studies found no correlation between changes of salivary baseline ph and the force relaxation of elastic . the ph of the oral environment which is determined by ph of both saliva and dental plaque can affect these intraoral latex elastics . ferriter et al . reported that the ph of the normal resting saliva was in the range of 5.67.6 which can fluctuate with diet solutions . even when strong acidic solutions are ingested , the salivary ph quickly reverts to the baseline ph of oral cavity . although intermaxillary elastics might be removed easily by the patient , it is not possible to expect them to remove and reinsert them for every food intake in a 24 h duration . during ingesting of very low ph representing very sour beverages , the high concentration of hydrogen ion may affect the force  degradation rate of latex elastic bands even if it reverts to baseline ph over time . changes in salivary ph have been evaluated in previous studies ; however , synergic impact of these changes with low ph liquid intake was not evaluated . the purpose of the present study was to assess a group of latex elastics in a static environment to define the influence of synergic effect of the intermittent 's low ph and various baselines ph of saliva . this study was designed to evaluate the effect of baseline salivary ph and synergic impact of exposure to intermittent lower ph level on the force relaxation of elastic bands . this study included four groups of 3/16 inch ( 4.8 mm ) , 4.5 oz ( 128 g ) natural rubber latex elastic ( 3 m unitek , monrovia , ca , usa ) . four acrylic boards on which the distance between two rows of stainless steel pins was 15 mm were constructed to test , 15 latex elastics in each group [ figure 1 ] . the elastics were within their shelf life period and had been stored in the plastic packages in a cool and dark place before the test . all elastics were randomly selected from different packs of the same type / brand to decrease the bias might be induced by variations in form and diameter of these latex elastic in a pack . preferably , elastics with the same diameter and thickness were blindly selected by unmagnified eyes . the artificial saliva solutions were provided by the pharmacy school of mashhad university of medical sciences ( mashhad , iran ) . each group of elastics was immersed in a tank of artificial saliva with given ph level . groups 1 and 2 were in a solution set at ph level of 7 and 5 , respectively , representing variation of salivary baseline ph . groups 3 and 4 were immersed in the solutions that every 120 min , their ph dropped ( from ph 7 to 4 ) and ( from ph 5 to 4 ) for 10 min in the first and third quarters of the 48 h experiment , respectively . no ph changing occurred in the second and the fourth quarters representing the rest time . ph levels were measured every hour with a calibrated ph / ion meter manufactures related ( crison , spain ) and were adjusted accordingly with 1 m citric acid and 1 m sodium hydroxide . samples were incubated at approximately 37c in an incubator ( memmert , germany ) through the whole experiment . force measurement was carried out using a calibrated digital force gauge ( lutron fg5020 ; taiwan , accuracy  [ 0.5% + 2 digit ] ) [ figure 2 ] . the acrylic plates constructed with two rows of 15 mm separated stainless steel pins and the mounted elastic bands . the digital gauge with an elastic band staked on its hooks . these measurements were taken in 0 , 4 , 8 , 12 , 24 , 36 , and 48 h for each plate by one investigator blindly . the force was recorded using two fixed jig separated 25 mm on a stand ( lutron fs1001;taiwan ) . the lower jig is attached to the stand ( lutron fs1001;taiwan ) and the upper jig is a part of the force gauge ( lutron fg5020;taiwan , accuracy  [ 0.5% + 2 digit ] ) . a consistent reading was established usually within 45 s for each elastic , and the plates were immersed in the solution immediately after each measurement . repeated measures analysis of variance ( rmanova ) and the post - hoc tukey 's test were used to find statistically relevant differences between the groups . means and standard deviation of the force values generated by each group of elastics when stretched to 25 mm are shown in table 1 . rmanova showed a significant interaction effect between time and group ( p = 0.014 ) . therefore , the changes in the force relaxation were not the same for all groups . the interaction between ph and time showed no significant differences ( p > 0.05 ) except at 36 h ( p < 0.05 ) . we employed one - way anova / tukey 's tests for subgroup ( between group ) comparison . the further analysis showed that each comparison between any two groups did not indicate significant differences ( p > 0.05 ) except between groups 1 and 3 ( p < 0.05 ) and between groups 2 and 3 ( p < 0.01 ) [ table 1 ] . even though no significance was found , in the first 4 h , the force decay of the group 4 was 10% of their initial force [ figure 3 ] , exhibiting the changes in the percentage of the initial force during the 48 h of the experiment in four groups . it is a representative force - time curve of elastics immersed in different ph levels . it shows a curve with four distinct phases of force decay : a high initial rate and the second lower rate of force degradation and the third slight force increase from 24 to 36 h and the fourth high rate of force decay . the percentage of the initial force remaining after 48 h was around 89.36% in group 1 , 81.41% in group 2 , 86.45% in group 3 , and 79.53% in group 4 . meansstandard deviations of force magnitudes in four groups of latex elastics with different ph levels over time line graph of force decay over time for elastic . many recent studies have investigated the mechanical and environmental factors that impact the force decay of the interarch elastics . the effect of individual salivary ph variability on the force decay of latex elastics has been investigated . they concluded that ph levels of 5 and 6 in the 24 h investigation were not significant contributors to the force relaxation of interarch elastics in comparison to ph of 7 . this study evaluated the synergic effect of ph=4 presenting very sour liquids , with ph=5 which present a decline of the ph from the baseline during intakes . lowering the ph to 4 for 10 min twice during 4 h enhanced the force decay in the first 4 h in comparison to the salivary ph baseline even though it was not significant . this primary rapid decrease of force in the first 4 h is consistent with the result of other studies in the literature . a gradual reduction of the force occurred over time which was confirmed by the statistically significant difference existed between each time of testing , through the experiment . however , more than 80% of the initial force was preserved after 48 h in all tested groups . this amount of force is still close to those required for the orthodontic tooth movements . the use of 4 times greater force magnitude at the beginning as suggested by andreasen and bishara could increase the risk of damage to the teeth and periodontal tissue . also , in other studies investigating the influence of hydrogen ion concentration , the amount of forces of inter - arch elastic after 12 hours and 24 hours were still in the acceptable range for the tooth movements . this study was conducted in a static situation ; however , the studies which evaluated the impact of cyclic stretching on interarch elastic did not report a significant influence on the force degradation except in the 1 h of the experiment . this study , similar to the most recent studies investigated the effect of ph on the force decay of elastic bands , was conducted in a static situation . however , the combination of these environmental impacts , different salivary ph baseline , beverages with low ph , and cyclic stretching at the same time may enhance the force degradation significantly which needs to be evaluated in the future . in the study by leo filho et al . , they did not report significant differences in the force decay of the interarch elastics immersed in different beverages . the duration of the investigation was almost 15 min and the intermaxillary elastic acted the same in exposure to the tested beverages . in their study , beverages were not subjected to any ph analysis , also the duration of the experiment was very short . moreover , salivary ph recovery following acidic beverage in the orthodontic patients was reported to be slower than nonorthodontic cases . in our study , reduction of the force in the first 8 h can be explained by the  swelling phenomenon . the molecular chains with cross - linked points enable the absorption of liquid in the latex reticular structure . after the plateau phase of force decay between 8 and 24 h , even intraoral water can plasticize cross - linked polymers including latex rubber , which leads to the reduction in the creep compliance during the experiment that may lead to the reduction of flexibility and an increase in the force for a few hours from 24 to 36 h. the fluctuation of ph had a considerable effect on this matter . the high rate of force decay from 36 to 48 h is due to the removal of the plasticizers by dissolution from the polymers molecular chain and elongation and decrease of elasticity from physical aging and environmental degradations . however , the remaining force was more than 79.53% after 48 h. this is consistent with the study by wang et al . , who also reported the remaining force of 86% after 48 h. although the length of time of exposures to the chemical and thermal insults is an important factor in the force reduction , the amount of force remained after 48 h is acceptable , if not ruptured due to accumulation of insults . referring to the results of this study , less compliance from the patients in 24 h and even in 48 h for elastic removal during low ph beverages ingestion scan still exerts an acceptable amount of force for the tooth movements . so far , with the limitations of this study , the results can not be fully extrapolated to the clinical situations . no significant correlation was seen between the fluctuation of ph and the force degradation in the latex elastic bands . time may be the main contributors to the force decay ; however , after the 48 h of experiment , the amount of exerted force was still in an acceptable range for orthodontic movements . although size and thickness of the product were controlled with unmagnified eyes , a more accurate method should be used to unify the samples in conducting these types of studies which evaluate the behavior of interarch elastics . the financial support for the progress of this study was provided by shiraz university of medical sciences . the authors of this manuscript declared that they had no conflicts of interest , real or perceived , financial or nonfinancial in this article . the financial support for the progress of this study was provided by shiraz university of medical sciences . the authors of this manuscript declared that they had no conflicts of interest , real or perceived , financial or nonfinancial in this article .
background : latex elastics are still in common use due to their low cost and high flexibility to improve sagittal discrepancies or interdigitation of teeth . 
 mechanical properties of elastics are influenced by several environmental factors such as ph changes . 
 this study evaluated similar latex elastics to define the influence of synergic effect of intermittent low ph and various baselines ph of saliva.materials and methods : four groups of latex elastics ( 3-m unitek , 3/16 inch ) were tested ( n = 15 in each group ) . 
 two groups of elastics were immersed in two tanks of artificial saliva with different ph levels of 7 and 5 , and two groups were immersed in two tanks of artificial saliva with intermittent drop of ph to 4 . 
 the force was measured when the elastics were stretched to 25 mm . 
 these measurements were taken in 0 , 4 , 8 , 12 , 24 , 36 , and 48 h for each group . 
 repeated measures analysis of variance ( rmanova ) and post - hoc tukey 's test were used to assess the findings . 
 the level of significance was 0.05%.results : the interaction between ph and time analyzed with rmanova showed no significant differences ( p > 0.05 ) except in 36 h ( p = 0.014 ) . 
 the tukey 's analysis showed that each comparison between any two groups did not indicate significant differences ( p > 0.05 ) except between groups 1 and 3 and between groups 2 and 3 ( p < 0.05).conclusion : no significant correlation was seen between fluctuation of ph and force degradation in latex elastic band except in 36 h.