# Force of Intermaximillary Late Elastics from Different Suppliers: A Comparative In Vitro Study
## 4 Discussion
In the oral cavity, the characteristics of elastic materials are affected by physical, chemical, and biological factors, some of which are related to functional activities, salivary changes, and nutrition habits; therefore, the results of in vitro and in vivo studies need to be distinguished [7; 30; 31; 32]. [PERSON] et al. [22] compared in vivo/in vitro force degradation within 48 h for the sizes of 1/8\", 3/16\", \(\frac{1}{4}\)\", 5/16\", and 3/8\" and the forces of 2 oz and 3.5 oz. During the in vivo research, a group of patients wore clear aligners where the stretching length was precisely defined; this was followed by a comparison of forces in these elastics with those stretched equally in vitro under various laboratory conditions. Under forces of 2 oz and 3 oz, the mean force degradation for \(\frac{1}{4}\)\" elastics in vivo at 48 h was 33.83% and 36.94%, under in vitro conditions in artificial saliva was 24.60% and 26.89%, and under dry in vitro conditions was 16.44% and 16.21%, respectively. These results imply that force degradation in vivo is much greater than that in vitro. [PERSON] et al. [6] evaluated the 3/16\" Medium elastics from the 3M brand in vivo, stretched to 24.1 \(\pm\) 4.2 mm with prestretching, and found the following rates of force degradation: 13,4% at 1 h, 15.7% at 3 h, 18.5% at 6 h, 19.9% at 12 h, 22.2% at 18 h, 28.2% at 24 h, and 30.9% at 48 h. In our study, the rates of force degradation in the 3/16\" Medium elastics from the 3M brand stretched to three times the diameter (13.8 mm) were 10.05% at 2 h, 14.9% at 8 h, 20.43% at 24 h, and 24.79% at 48 h, which, in comparison with the above-mentioned in vivo study, are very similar rates of force degradation in vitro with a smaller stretching. The 3/16\" Medium elastics from 3M, stretched to 20 mm under various conditions, were evaluated by [PERSON] [7],who found that at 48 h, the residual elastic force was 61% in vivo, 86% in vitro in a dry environment, and 71% in vitro in artificial saliva, being close to the results regarding the residual force of the 3/16\" Medium elastics from the 3M brand in our study. In the study by [PERSON] et al. [18], who evaluated latex and nonlatex 3/16\" elastics from American Orthodontics in vivo and stretched to 25 mm, the force degradation of 3/16\" latex elastics at 24 h was 19.9%, which is in contrast with our results, where the force degradation in one day was greater than one-fourth of the initial force. Among the limitations of this study are the in vitro design and static stretching of the elastics, which make it impossible to fully reproduce clinical conditions, such as the simulation of the insertion and removal of elastics before and after oral hygiene, the influence of chemical factors from meals and drinks, etc., and which may have significantly influenced the results. There are different opinions concerning what initial force to choose for elastics in individual patients, as well as in terms of the timing of replacing elastic when worn continuously by patients. As early as 1970, [PERSON] and [PERSON] [21] suggested using 40% more force than optimal at the commencement of elastic application to compensate for the initial loss, as did some later studies [7; 14]. In order to maintain the high force of the elastics used, some authors recommend replacing them after 6 h because of force degradation [10], while others advise an interval of 12 h [5]. On the other hand, [PERSON] et al. [33] suggested that elastics did not need to be replaced so frequently by patients because, after initial degradation, the force may remain relatively constant for a few days [7; 20; 21; 33; 34]. [PERSON] et al. [12] also reported that the force value was remarkably stable after one day because the structural changes caused by repeated stretching were not cumulative. [PERSON] et al. [24] reported a force degradation of 15% in the first 3 h, followed by a drop of 3% across the next 8 weeks; therefore, they recommend replacing elastics three times a week.