4: Influence of intraoral factors on optical and mechanical aligner material properties
Fayez Elkholy, Silva Schmidt, Masoud Amirkhani, Bernd G. Lapatki
Introduction
The triad of success of orthodontic therapy comprises patient compliance, biomechanical knowledge, and, for the therapy with aligners, sufficient understanding of the thermoplastic material used. Chapter 3 addressed the basic chemical and mechanical properties of commonly used aligner materials. This chapter will focus on the influence of different intraoral factors on the mechanical and optical properties of aligner materials.
To achieve an efficient orthodontic tooth movement, single aligners are usually worn for a period of 7 to 10 days and approximately 22 hours per day.1 During their period of use, aligners are subjected to a prolonged exposure to different factors that are influencing their properties. They can be subdivided into two main categories. On the one hand, there are factors inducing optical material changes, either in the form of discoloration or increased opacity; such effects are related to the presence of salivary enzymes, plaque, and food and beverage coloring.2–5 On the other hand, there are factors affecting the mechanical properties of aligners, including the periodic loading and unloading of the material during its clinical handling, combined with uneven local stress and strain distribution. It must be noted, too, that excessive occlusal forces (e.g., during involuntary clenching or grinding) and intraoral temperature fluctuations may influence an aligner’s properties.6,7 As this appliance, however, is to be removed during food or liquid intake and worn for only a relatively short period, the clinical relevance of the latter two factors may not be overemphasized.
The following sections will discuss the mechanisms of how intraoral factors influence optical and mechanical aligner properties and describe the clinical implications. Particular attention will be given to describing the material-specific characteristics of the two aligner materials mainly used (i.e., thermoplastic polyurethane [TPU] and polyethylene terephthalate glycol [PET-G]). TPU is used, for instance, in the Invisalign system (Align Technology, Santa Clara, CA, United States) or F22 Aligner (Sweden & Martina, Due Carrare, Padova, Italy), whereas PET-G is used in the Clear Aligner system (Duran, Scheu Dental GmbH, Iserlohn, Germany) and the Essix system (Essix A+, Dentsply Raintree Essix, Sarasota, FL, United States).8
Water absorption
Aligners are constantly subjected to saliva, which consists of 99% water. Hence it is crucial to understand the mechanism and effects of water absorption as well as the influence of water absorption on the mechanical material properties. As stated in Chapter 3, amorphous polymers such as TPU and PET-G possess relatively low molecular density, which provides free volume for water intake. A previous study comparing these two materials showed that TPU shows higher water absorption characterized by a weight increase of 1.45% after a 1-week water storage than PET-G showing only a 0.84% increase.9 Besides this weight effect, penetration of thermoplastic materials by water molecules also leads to modification of their internal structure. As explained in Chapter 3, this will result in plasticization because links between polymer chains are weakened or even destroyed, which reduces the internal cohesion and increases the molecular mobility.10 The resulting loss of elasticity might explain the appearance of internal cracks observed in aligners after clinical usage.6 It is interesting to note in this context that own studies on PET-G material characteristics using three-point bending of thermoformed rectangular specimens revealed that the sole water storage without subjecting the material to any mechanical loads has only a minor impact on the mechanical material characteristics (Fig. 4.1). In contrast, if PET-G is subjected to both water and a continuous mechanical load, the effect on the mechanical properties is much more pronounced, as indicated by the reduction of the bending forces of up to 43% (see Fig. 4.1).
Principally, water absorption could also induce dimensional changes of the aligners, known as hygroscopic expansion. In theory, this factor—besides other factors such as the initial play between the aligner and the setup model11–13—might affect the fit of the aligners and, consequently, might also induce an alteration of the forces applied to the individual teeth.9,14 A previous study on water adsorption of thermoplastic materials, however, did not find significant and plausible correlations between the rate of water absorption and the amount of hygroscopic expansion.9 For instance, TPU showed a lower hygroscopic expansion, although it showed the highest water absorption rates.
Optical changes
One of the main reasons for the popularity of aligners with patients lies in the invisibility or (better) the transparency of this appliance.15–17 These characteristics should be maintained throughout the treatment period because a discolored or opaque aligner (Fig. 4.2) might jeopardize the patient’s motivation and compliance.
Aligner discoloration is primarily related to superficial absorption or penetration of pigmentations in food and beverages. Coffee (i.e., the highest chromogenic agent), black tea, and red wine play a prominent role.2,3,18 It is noticeable in this context that the rate and extent of discoloration is material-specific.2 It seems that TPU-based aligners might exhibit faster discoloration rates than PET-G aligners. A possible explanation for this difference is the higher water absorption capability of TPU facilitating the accumulation of the pigments.9 In addition, the higher surface roughness of TPU might also facilitate the adhesion of pigments on the polymer film’s surface.4
Aligners might also lose translucency by the development of internal microcracks, formation of calcific integuments, or the accumulation of plaque on the aligner surface.5–7,19 Obviously, the two latter changes do not have a significant impact on the treatment success due to the short application period of each single aligner of maximally 2 weeks. Moreover, the loss of translucency can be minimized by maintaining good aligner hygiene through regular brushing with neutral soap and the use of denture-cleaning effervescent tablets containing sodium bicarbonate or sodium sulfate.5
Short-term mechanical loading of aligner materials
Single short-term loadings
For viscoelastic materials, it is known that during very short loading periods the elastic component dominates. This thesis was also confirmed by (unpublished) investigation of PET-G specimens by our group, consisting of two short loading-measuring cycles with a duration of only ca. 0.1 second each and a 2-minute break in between. The comparison of the forces obtained by the first and second measurements did not show a significant difference (Fig. 4.3).
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