Changes in Roughness and Mechanical Properties of Invisalign® Appliances after One- and Two-Weeks Use

Abstract

The aim of this study was to estimate the possible changes of surface roughness and the mechanical properties of Invisalign^® appliances over one- and two-week of service. Forty appliances with attachments were retrieved after the end of orthodontic treatment from different patients. Half of them had been used for one week (1W), and the rest for two weeks (2W). Ten unused Invisalign^® appliances were used as the control (CON). An equal number of teeth possessing attachments were cut of aligners deriving from all groups (1W, 2W, and CON), and the Sa, Sq, Sz, Sc, and Sv roughness parameters of the internal surface of the aligner attachment area and the opposite lingual side (which was in contact to enamel) were determined by optical profilometry. Then, ten first molars originating from all groups were embedded in acrylic resin, and were ground and polished. Instrumented indentation testing (IIT) was performed in order to determine the Martens hardness (HM), indentation modulus (E_IT), and relaxation index (R_IT), according to ISO 14577-2002. The produced data were statistically processed by one- or two-way analysis of variance (ANOVA) and multiple comparison post-hoc tests (a = 0.05). Both the surface roughness and mechanical properties of the retrieved groups (1W and 2W) showed statistically significant differences compared with CON, but without statistically significant differences between each other. The roughness variables of the as-received material were shown to be reduced after intraoral service demonstrating a wear effect. Ageing has a detrimental effect on the surface roughness and mechanical properties of Invisalign^® appliances, although this effect is restricted to the first week of clinical usage.

Keywords: Invisalign®; instrumented indentation testing; mechanical properties; orthodontic appliances; relaxation.

Figure 1

Representative 3D optical profilometric images from the control (CON) (A), and one-week (1W) and two-week (2W) groups (B). Please note the differences between scales.

Figure 2

Mean values and standard deviations of all of the roughness parameters tested. Statistical differences are the same for all of the parameters. Statistical differences were significant only between the CON, and 1W and 2W groups for both the lingual and attachment surfaces. Horizontal bars of statistical differences have been omitted for the sake of clarity.

Figure 3

The tetragonal pulse applied for testing the mechanical properties of all of the groups tested, imposing a constant indentation depth for duration of 60 s (A). Representative force–time curves showing the force change over the loading period (B). Force indention depth curves. The vertical lines pointed by the brackets reflect the time of constant indentation depth (C).

Figure 4

Mean values and standard deviations of the Martens hardness (A) and indentation modulus (B) of groups tested. Medians, 25% and 75% percentiles, and outliers (demonstrated as circular points) for the relaxation index (C). Horizontal lines connect the mean values without statistically significant differences.