An assessment of the impact of adhesive coverage and wire type on fixed retainer failures and force propagation along two types of orthodontic retainer wires: an in vitro study.

Objectives: To evaluate the force required to promote the failure of fixed orthodontic retainers with different adhesive (composite) coverage and to assess the presence and extent of force propagation with two different orthodontic retainer wires.

Materials and methods: Ortho-FlexTech and Ortho-Care Perform (0.0175 inches), each of 15-cm length, were bonded on acrylic blocks with different adhesive surface diameters (2 mm, 3 mm, 4 mm, and 5 mm). The samples (n = 160) were subjected to a tensile pull-out test, and debonding force was recorded. Fixed retainers using two different wires and 4-mm adhesive diameter were bonded on acrylic bases resembling a maxillary dental arch (n = 72). The retainers were loaded occluso-apically until the first sign of failure while being video recorded. Individual frames of the recordings were extracted and compared. A force propagation scoring index was developed to quantify the extent of force transmission under load.

Results: A 4-mm adhesive surface diameter required the highest debonding force for both retainer wires with significant differences compared with 2 mm (P < .001; 95% confidence interval [CI]: 8.69, 21.69) and 3 mm (P = .026; 95% CI: 0.60, 13.59). Force propagation scores were significantly higher for Ortho-Care Perform.

Conclusions: Based on this laboratory-based assessment, consideration should be given to the fabrication of maxillary fixed retainers using a minimum of 4-mm diameter composite coverage on each tooth. Force appeared to propagate more readily with Ortho-Care Perform than with a flexible chain alternative. This may risk stress accumulation at the terminal ends with potential for associated unwanted tooth movement in the presence of intact fixed retainers.