An in vivo and in vitro study on the force degradation and surface morphology of the orthodontic elastic ligatures

The objective of this study was to investigate the characteristics of force degradation and surface morphology of 9 brands of elastic ligatures over 6 weeks, both in vivo and in vitro, to provide some reference for clinical selection of appropriate elastic ligatures and determination of appropriate follow-up visit time.Choosing 9 kinds of brands of elastic ligatures (American Orthodontics, Ormco, 3M Grey, TP Orthodontics, Xihu, Yahong, Creative, Protect, 3M dusty blue), which are common in the market. For the in vivo study, 9 volunteers (5 males, 4 females, aged 22-26 years) were chosen to wear elastic ligature fixators, which were made to fix the elastic ligature in the mouth without bonding brackets on the teeth. The residual force and percentage of force degradation of elastic ligatures were estimated after 0, 1 hour, 2 hours, 8 hours, 1 day, 2 days, 1 week, 2 weeks, 4 weeks, and 6 weeks processed in vivo and in vitro (25°C dry air condition, 37°C artificial saliva, pH = 6.7 and oral environment); A 1-way analysis of variance and t test were used to identify statistical significance (P <0.05). The surface morphology of the initial state and after a 6-week process in vivo and in vitro were observed separately.The characteristics of force degradation and surface morphology of 9 brands of elastic ligatures were different over 6 weeks; With the extension of time, the tensile force declined, and the percentage of force degradation of all groups decreased sharply by about 9.1%-32.3% in the first day, then the rate reduced steadily. In addition, during the first day, the percentage of force degradation declined most sharply in the first hour (4.4%-16.5%). The percentage of force degradation of 9 elastic ligatures was reduced by about 11.3%-37.3% over 6 weeks in all groups. At the same time, the characteristics of force degradation in the same brand are that in vivo > artificial saliva > air (P <0.05). After a 6-week process, the surface morphologic defects were aggravated under a scanning electron microscope, corresponding to the change of mechanical properties in each brand.The mechanical properties of elastic ligatures are reduced with time extension and decreased greater in vivo than in vitro; According to the characteristics of force degradation of different brands of elastic ligatures, clinicians can choose the proper elastic ligatures at different stages of treatment, and adjust the follow-up visit time appropriately. For example, in alignment and leveling, the elastic ligatures with slow force degradation, such as Ormco, TP, and so on, maybe more suitable to ensure close contact between the archwire and the bracket. In the space-closing stage, the elastic ligatures with fast force degradation may be more appropriate to reduce the friction between the archwire and the bracket, such as 3M Grey, Yahong, Protect, and so on.