after the introduction of ni - ti alloys to dentistry in 1970 , it has been widely used . producing constant forces in a wide deformation range is the most favorable feature of ni - ti alloys , which is due to the two - phase structure of the alloy ; the martensite and the austenite phases . for the best use , the alloy must be in the austenite phase when placed in room temperature . various orthodontic appliances are produced from ni - ti alloys such as closed or open coil springs . ni - ti coil springs , unlike elastomeric chains , do not undergo rapid force degradation and unlike stainless steel coils do not generate extremely high forces . temperature and strain could lead to phase change in ni - ti alloys . therefore , the temperature changes , which occur in the oral cavity may affect the forces generated by the coils . several studies have showed that prolonged strain and heat could reduce the predictability of forces produced by coils . it has been reported that preactivation can prolong the continuity of the forces produced by elastomeric chains . the aim of this study was to evaluate the effects of the aging process ( temperature changes and prolonged strain ) on ni - ti closed coils and also to assess the effects of preactivation on the forces generated by these coils . in this experimental in vitro study , sixty 9 mm medium ni - ti closed coils ( g&h wire co. , indiana , usa ) were divided equally in to four groups ( n = 15 ) . in the 1 group , the samples were stretched to 12 mm ( 3 mm activation ) using a universal testing machine ( walter + bai , switzerland ) . after that , the samples were returned to their original length by the testing machine , with the crosshead speed of 0.5 mm / s . holding jigs were designed ; it was comprised of stainless steel rods , which were placed at 12 mm distances [ figure 1 ] . the samples were carefully placed on the holding jig after every test to maintain their 12 mm length during the study . these measurements were repeated in t1 = day 22 and t2 = day 45 . in the 2 group the process was similar to the 1 group except that after the measurement , the samples that were placed on the holding jig were inserted in normal saline ( samen , mashhad , iran ) then transferred to an incubator ( behdad , tehran , iran ) and kept in 37c . before repeating the tests in t1 and the 3 and 4 group samples were preactivated by the amount of 30% ( 3 mm ) using the gauge provided by the manufacturer ( g&h wire co. , indiana , usa ) . in the 3 group , the process was similar to the 1 group and after the measurements , the samples were carefully placed on the holding jig and kept in room temperature and then the measurements were repeated in t1 and t2 . in the 4 group , after the initial measurements the samples were placed on the holding jig and kept in normal saline inside the incubator . before repeating the measurements in t1 and t2 the samples underwent 1000 thermo cycling cycles between 5c and 55c . finally , the data was analyzed by the repeated measure two - way anova using the spss software version 13 ( spss , chicago , il , usa ) . statistical analysis showed that the mean forces produced in all groups were slightly higher than the target force stated by the manufacturer ( 120 gf ) . the mean forces are shown in table 1 and figure 2 . comparing the nonpreactivated groups showed that there was no significant difference between the forces in the dry and incubated groups in different time periods ( p = 0.6 ) . in the preactivated groups , there was no significant difference between the forces in the dry and incubated groups in different time periods ( p = 0.8 ) . regarding preactivation , the mean forces of preactivated coils ( dry and incubated ) had no significant difference with the mean forces produced by nonpreactivated coils ( dry and incubated ) ( p = 0.7 ) . in the incubated groups , the mean forces slightly decreased from t0 to t2 , but it was not statistically significant ( p = 0.6 ) , but in the dry groups , the mean forces slightly increased , but this also was not statistically significant ( p = 0.4 ) . the mean values of unloading forces in different groups ( gf ) the mean values of unloading forces in different groups . pi = prestretched and incubated , pd = prestretched in dry condition , npi = nonprestretched and incubated , npd = nonprestretched in dry condition ( the mean values of unloading forces are presented in gf . ) , t0 = tested on day 1 , t1 = tested on day 22 , t2 = tested on day 45 . in this study , we evaluated the effects of aging and preactivation on the forces produced by ni - ti closed coils . we measured the forces in the days 0 , 22 and 45 , while two groups were kept in room temperature and two groups were incubated in 37c . we found that the mean forces produced by the coils , in all time periods , were slightly higher than the target force stated by the manufacturer . also reported that the mean forces produced by ni - ti coils were higher than the target force of the manufacturer . this difference could be due to different method of measurement between these studies and the manufacturer . in this study and the studies mentioned , the force measurements were performed by the highly accurate universal testing machine . the results of this study showed that the forces produced by coils in room temperature had no significant difference with the forces produced by coils kept in the incubator , which means that prolonged moisture and heat did not affect the forces produced by ni - ti closed coils . wichelhaus et al . showed that the presence of moisture and heat does nt affect the forces produced by ni - ti coils . on the other hand , tripolt et al . stated that heat increases the forces produced by ni - ti coils , but in the oral environment the magnitude of this force change is not significant . bourke et al . showed that ni - ti closed coils could maintain their force levels during oral environment changes . it seems that in normal conditions , phase change does not occur in ni - ti alloys present in the oral cavity . have reported that some manufacturers preactivate the ni - ti coils , which results in a longer plateau in the stress - strain charts of the coils . our study showed that preactivation does nt affect the forces produced by ni - ti closed coil in either dry or incubated environments . regarding the change in forces produced by ni - ti closed coils , our study showed that the incubated coil ( either preactivated or nonpreactivated ) forces decreased over time , but the coils kept in room temperature ( preactivated or nonpreactivated ) showed increased forces over time . nattrass et al . have reported an increase in forces produced by ni - ti coils over time whilst , angolkar et al . showed a decrease in ni - ti coil forces over time . reported that nitinol coils had no significant force change overtime ( after 45 days ) but ni - ti coils had a slight force decrease after 45 days which was statistically significant but did nt have clinical significance . study , the coils were stretched 50% of their initial length , whilst in our study , the coils were stretched 30% and a different brand of coil was used in their study , which could be the cause of different results . aging process does not affect the forces produced by ni - ti closed coil spring.preactivation has no statistically significant effect on the forces generated by ni - ti closed coils . aging process does not affect the forces produced by ni - ti closed coil spring . preactivation has no statistically significant effect on the forces generated by ni - ti closed coils .
background : the most favorable feature of nickel - titanium ( ni - ti ) alloys in orthodontics is producing constant forces in a wide deformation range . 
 ni - ti closed coils produce compressive strength and can be used for several purposes such as space closure . 
 the aim of this study was to evaluate the effects of the aging process ( temperature changes and prolonged strain ) on ni - ti closed coils and to assess the effects of preactivation on forces generated by these coils.materials and methods : a total of 60 ni - ti closed coils ( g&h wire co. , indiana , usa ) were divided into four groups ( n = 15 ) . 
 two groups were kept in room temperature and two were incubated in 37c in normal saline for 45 days . 
 all the samples were extended to 30% of their original length . 
 one group of the incubated coils and one group of room temperature coils were preactivated the same amount of 30% . 
 the incubated groups also received 1000 thermocycles on days 22 and 45 . 
 the unloading forces were measured by a universal testing machine on days 0 , 22 and 45 . 
 the data were analyzed using spss . 
 significance was set at 0.05.results:the unloading forces of the ni - ti closed coils was not affected by prolonged heat and moisture either in preactivated groups ( p = 0.8 ) or the nonpreactivated groups ( p = 0.6 ) . 
 furthermore , preactivation had no significant effect on the unloading forces of ni - ti closed coils ( p = 0.7).conclusion : within the limitations of this study , both aging process and preactivation do not affect on mechanical properties of nickel - titanium closed coil springs . 
 preactivation had no effect on the forces generated by ni - ti closed coils .