Effect of different pre-stretching methods on the force degradation of elastomeric chains: an in vitro study

To investigate the effect of different pre-stretching on the force degradation of elastomeric chains methods and provide experimental evidence for its application in clinical orthodontic treatment.

One control group (no pre-stretched treatment) and 15 experimental groups (pre-stretched to 25%, 50%, and 100% of the original length in 1-5 cycles) were set up by taking five loop lengths of short elastomeric chain and long elastomeric chain samples, respectively. The elastomeric chains from both the control and experimental groups were mounted on the testing fixture at identical initial lengths. To simulate tooth movement, the fixation device was adjusted to reduce the distance by 0.35 mm weekly. The remaining force of each group was measured at different time points over a 4-week period. Further, the experiment was conducted in a simulated salivary environment with additional thermal cycling.

Pre-stretching length and cycles showed a significant interaction effect on the remaining force of elastomeric chains (P < 0.001). For short elastomeric chains, the groups pre-stretched to 25% (P = 0.001) and 50% (P = 0.005) of the original length for 3 cycles showed significantly higher remaining force values than the control group at 24 h. At 7, 14, 21, and 28 d, the groups pre-stretched to 25%, 50%, and 100% of the original length for 3 cycles consistently maintained higher remaining force values than the control group (P < 0.001). Within the groups of 3 pre-stretching cycles, no significant differences in remaining force were observed among the 50% group and the 25% (P = 0.617) or 100% (P = 0.229) groups at 7 d. However, at 14 d, the remaining force of the 50% group was significantly higher than that of both the 25% (P = 0.003) and 100% (P = 0.027) groups. At 21 d, the 50% group maintained a significantly higher remaining force compared to the 100% group (P = 0.020), but the difference compared to the 25% group did not reach statistical significance (P = 0.052). By 28 d, the remaining force of the 50% group remained significantly higher than that of both the 25% (P = 0.003) and 100% (P = 0.003) groups. For long elastomeric chains, the 50% pre-stretch (3 cycles) group showed significantly higher remaining force values than the control group at 8 h (P = 0.005) and 24 h (P < 0.001). Throughout the experimental period at 7, 14, 21, and 28 d, the remaining force values of the 25% (7, 14 d: P < 0.001; 21 d: P = 0.003; 28 d: P = 0.008), 50% (P < 0.001), and 100% (P < 0.001) original length pre-stretching for 3 cycles groups were consistently higher than those of the control group. Within the groups of 3 pre-stretching cycles, the remaining force of the 50% original length pre-stretching for 3 cycles group was significantly higher than that of the 25% original length pre-stretching for 3 cycles group at all of the time points from 7 to 28 d (P < 0.001). Although no statistically significant differences were observed between the 50% and 100% groups at 14 d (P = 0.068), the 50% group exhibited significantly higher remaining force than the 100% group at 7 d (P < 0.001), 21 days (P = 0.005), and 28 days (P < 0.001).

Appropriate pre-stretching methods can effectively delay the force degradation of the elastomeric chain. Pre-stretching to 50% original length for three cycles can maximize the delay of the force degradation of the elastomeric chain.