Comparing 3D printed twin block appliance designs for class ii division 1 malocclusion on optimised fit, retention and compressive strength / Roselyn Mathew

Roselyn , Mathew (2024) Comparing 3D printed twin block appliance designs for class ii division 1 malocclusion on optimised fit, retention and compressive strength / Roselyn Mathew. Masters thesis, Universiti Malaya.

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Abstract

Introduction: The Twin Block appliance (TBA) is a removable functional appliance used for skeletal Class II malocclusion in growing individuals. Its design has evolved with various modifications to improve patient acceptance and compliance. The advent of 3D printing technology in dentistry brings a shift towards to digital workflow. Currently, 3D printing TBA is new in the market with limited literature on effective designs. Thus, there is a need to explore 3D-printed Twin Block (3D-TBA) designs to accommodate the materials used in 3D workflow to ensure adequate fit and retention of the appliance. Objective: This study aims to propose key features for 3D-TBA designs based on best fit, retention, and compressive strength. Methods: This laboratory-based study assessed three designs for each dental arch, varying the incisal and occlusal coverage on two Class II Division 1 models (Model 1 and Model 2). The upper designs were extended palatally either to the cingulum plateau (U-D1), up to the gingival margin (U-D2) or further to cover a third incisal crowns (U-D3). The lower designs were extended either to the first permanent molar without any incisal coverage (L-D1), up to the second premolar with half incisal coverage (L-D2) or up to the second premolar with full incisal coverage (L-D3). Three ISO178 standard blocks of TR07 resin (Graphy, South Korea) and polymethyl-methacrylate (PMMA) were compared for flexural modulus and experts were consulted before design refinement. Each appliance was evaluated for fit and retention on their respective 3D-printed models, and compressive strength was measured using a universal testing machine (AG-X Series, Shimadzu, Japan). Results: PMMA exhibited a higher flexural modulus (2688.76 MPa; SD 1276.09) compared to Graphy TR-07 (41.70 MPA; SD 0.315). The fit of the 3D-TBA (U-D1 Model 1 and U-D3 Model 2) was inferior to the conventional TBA with significant differences in fit score (p = 0.010 and 0.001), respectively. No significant differences were found between the lower appliance of the 3D-TBA designs of study models 1 and 2 (p > 0.05). For the upper TBA, significant differences in retention scores were observed for U-D1 and U-D3 on Model 1 and all designs on Model 2 (p = 0.001). For the lower 3D-TBA, the retention scores in Model 1 did not exhibit statistical significance across all designs. However, in lower study Model 2, significant differences were found in L-D2 and L-D3 (p = 0.001 and 0.010, respectively). The Chi-square test revealed a statistically significant difference in compressive strengths (p < 0.001) for all designs in the upper and lower models, with the 3D-TBA having a greater compressive strength than the conventional TBA, regardless of their design. Conclusion: Design 2 on the upper arch (U-D2) and design 1 on the lower arch (L-D1) offered the best fit and retention. However, it is imperative to consider specific features of malocclusion before employing these designs. The 3D-TBA made from Graphy TR-07 resin exhibits superior compressive strength compared to the conventional PMMA-based TBA, enabling it to resist higher applied compressive forces before failure.

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