Impact of bacterial exposure on the mechanical behavior of nickel-titanium open-coil springs

This in vitro study aimed to evaluate the effect of Streptococcus mutans on the force alteration of nickel-titanium (NiTi) open-coil springs under constant compression.

Sixty NiTi open-coil springs from three commercial brands (3M, Ormco, and American Orthodontics [AO]) were standardized to a resting length of 15 mm and compressed with a 240-g load using an Instron^® universal testing machine. Each brand was divided into control (immersed in artificial saliva) and experimental (exposed to S. mutans in Brain Heart Infusion medium) conditions. Samples were incubated at 37°C for 30 days, after which force levels at 0.5, 3.25, and 6.5 mm displacements were remeasured. Data were analyzed with paired t-tests, one-way analysis of variance, and Tukey's post hoc tests (p < 0.05).

All brands exhibited force increases after 30 days, though with heterogeneous patterns. 3M springs maintained relatively stable behavior, with consistent force increments across all displacements in both conditions. AO springs showed irregular responses, with losses at some displacements and substantial gains at others, especially under bacterial exposure. Ormco springs displayed the highest variability and most pronounced increases, with force gains exceeding 80 gf, suggesting possible microstructural modifications.

Force stability in NiTi open-coil springs is brand-dependent and influenced by bacterial exposure. While 3M springs demonstrated predictable performance, AO and Ormco exhibited less consistent behavior, which may risk excessive force application during clinical use. These findings highlight the importance of considering biological and environmental factors in the selection and monitoring of NiTi coil springs.