Physical behavior of pre-strained thermoset and thermoplastic orthodontic chains.

The physical properties of pre-strained (50% and 100%) thermoset (TSU) and thermoplastic (TPU) elastic chains of four different manufacturers (n=120) were determined after storage in different environments. The in vitro force-decay was investigated after storage in the media air, distilled water and artificial saliva at 37 degrees C. The modified pull test was performed with a universal testing machine Model 4444 and load cell 2530-427. The initial and residual forces (after 1 h, 2 h, 4 h, 8 h, 24 h, 7 days, 21 days) were used to calculate the relative force. All chains show a force decay that is greatest in the first hour regardless of pre-strain or storage media. Over the entire incubation period, a significant difference between the providers was found. The paired comparisons showed no difference between the relative forces of chains made from TPU nor between the chains made from TSU. Yet, a highly significant difference between these two groups was found.