Khosravi, Roozbeh R.KMichelaki, Eleftheria Iris2024-09-092024-09-092024-09-092024MICHELAKI_washington_0250O_27055.pdfhttps://hdl.handle.net/1773/51889Thesis (Master's)--University of Washington, 2024Clear aligner therapy has fundamentally changed with the integration of computer-aided design (CAD) and manufacturing (CAM) technology. The efficacy of aligner treatment is impacted by numerous factors among which the properties of aligner material play a significant role. A plastic sheet property is a function of its structure (multi vs monolayer sheet), composition and type of material, and thickness. All these factors collectively influence the levels of force induced by the aligner. Given the increasing popularity of clear aligners and the multitude of companies producing different aligner plastic trays, a need for a comparison study of various aligner plastics becomes evident. This study aimed to evaluate the force decay levels of an array of commonly used aligner plastic sheets to determine how structure, thickness, and material type influence the force decay signature of an aligner plastic sheet. The force decay signatures of 10 aligner plastics (five single-layered and five multilayer) were plotted using a stress-relaxation test. The test was conducted via a three-point bending apparatus (Chatillon Digital Force Measurement CS2-225, AMETEK unit), applying a constant and controlled pre-set load. Each sample was thermoformed and subjected to a deflection of 1.5mm (5% strain level) while being immersed in water at a controlled temperature (set at 37 Celsius degrees) for a total of 24 hours period. The same test was repeated three times on the different thermoformed samples. Mean force decay as a function of time was evaluated between different samples using repeated measures analysis of variance (ANOVA) followed by post-hoc testing using Holm’s method. The force decay of each plastic sheet was analyzed based on the force decay levels in Newtons (N) and the percentage of retained force (%). Within the same structure and thickness, significant variations were found among distinct brands. Monolayer 0.75mm plastic sheets exhibited significant differences in the percentage of retained force (p-value= 0.0160) and the absolute force loss in Newtons (p-value=0.045), while the differences in multi-layer 0.75mm sheets were marginally significant (p-value=0.061 and p-value=0.213, respectively). For 1mm plastic sheets, both the percentage of retained force (p-value= 0.0020) and the force decay levels in Newtons (p-value=0.0010) were significant. Regarding the structure of aligner sheets and the force decay, a significant difference between multi-layer and single-layer in the percentage of retained force was noticed (p-value = 0.025; 95% CI 1.4N to 21.3N), with the multi-layer plastic sheets exerting less initial force loads and demonstrating lower levels of force decay over 24 hours. Overall, plastic thickness significantly influenced the force decay levels (p value=0.0031). However, individual brand analysis revealed significant differences. Specifically, for Z Classic both the percentage of retained force (p-value=0.0040) and the force loss in Newtons (p-value=0.000153) were significant. Taglus aligner plastic exhibited a significant difference in the force loss in Newtons (p-value=0.0380), but not in the percentage of retained force. No statistical difference was found between the 0.75mm and 1mm replicates for the Atmos plastic. This study underscores aligner plastics' distinct force decay signatures, influenced by structure, brand variation, and thickness. It was concluded that the multi-layer plastic sheets exhibit lower initial force loads with a more controlled force decay for 24 hours. Aligner sheets with similar thickness and structure exhibit differing force decay behaviors, influenced by both material composition and manufacturing process. Plastic thickness is also a factor that affects the force decay signatures, especially evident in specific brands. The unique behavior of each plastic sheet could signal performance differences during active aligner treatment and retention post-treatment. Implementing a universal force level protocol for measuring the plastic force levels is crucial. This would standardize testing methods, allowing clinicians to accurately compare the commercially available plastic sheets.application/pdfen-USnoneDentistryDentistryThesis