Sample preparation
A master scan of a maxillary arch was captured utilizing a Trios Scanner (3Shape, Copenhagen, Denmark), and exported into uDesign 6.0 software (uLab Systems Inc., San Mateo, CA, USA). Two digital master models were produced: one had no attachments (NA), just the trimmed maxillary model while the other had attachments (rectangular, gingivally beveled horizontal attachments with a depth of 2.7 mm, a height of 4.2 mm, and a width of 4.0 mm (Fig. 1)) on all the maxillary teeth (YA). Four master models (2 NA & 2 YA) were printed with Sprint Ray Pro DLP Printer (SprintRay, Los Angeles, CA, USA) at 100 µm-layer thickness. SprintRay Die and Model Gray II photo-initiated methacrylate resin with a flexural modulus of 2650 MPa and a Flexural strength of 91.5 MPA was used for master model 3D printing fabrication.
Thermoformed aligner (TFA) fabrication
Models were processed following the resin manufacturer recommendations. They were cured using the SprintRay Pro Cure (SprintRay, Los Angeles, CA, USA). ATMOS thermoforming plastic 125 mm round sheets with 0.030″ thickness (American Orthodontics, Sheboygan, WI, USA) were thermoformed over the master models utilizing a Biostar (Scheu-Dental GmbH, Iserlohn, Germany) pressurized thermoforming machine per manufacturer recommendations. A total of 20 thermoformed aligners were created, 10 of the TFA-NA and 10 of the TFA-YA.
Direct-printed aligner (DPA) fabrication
DPA sample was fabricated utilizing the same digital NA and YA master models with uDesign 6.0 beta software. Aligners were digitally trimmed to approximately 1 mm past the gingival margin. 0.50 mm thickness and 0.05 mm offset of aligner from model were utilized. Two master aligner files were created with this method: DPA with no attachments (DPA-NA) and DPA with attachments (DPA-YA) were fabricated and exported as STL Files. The DPA master files were then imported into Uniz Software (Uniz, San Diego, CA, USA), rotated to -110 degrees and supports generated. DPA Aligners were printed on Sprint Ray Pro95 printer at 100 µm-layer thickness. Graphy Tera Harz TC-85DAC resin was used for printing (Graphy Inc, Seoul, Korea). The properties of the printed resin are described by the company as Shore Hardness (D) > 85, Flexural strength > 65 MPa, Flexural Modulus > 1500 MPa.
DPA with intact supports were removed from the printer build plate and placed in a centrifuge for 3 min to remove uncured resin. The aligner was then removed from the supportive scaffolding with finger pressure. Aligners were cured in a Cure M machine (Graphy Inc, Seoul, Korea). Aligners were cured for 35 min with nitrogen gas, then submerged in glycerin and cured without nitrogen gas for an additional 35 min. A total of 20 DPA aligners were created, 10 of the DPA-NA and 10 of the DPA-YA.
Test model preparation and fabrication
The test model was created by importing the master digital NA file exported into MeshMixer (Autodesk, San Rafael, CA, USA) where the model was segmented to remove UR1. The model was supported vertically to provide strength and clearance for materials testing (Fig. 2). The test model was printed with a Uniz Slash-C LCD 3D printer (Uniz, San Diego, CA, USA) utilizing AnyCubic Clear 3D Resin (AnyCubic, Shenzhen, China). The manufacturer reported resin properties are a shore hardness (D) of 79, tensile strength of 23.4 MPa and elongation of 14.2%.
Measurement method
A hand wheel operated manual force test stand with integrated digital caliper with mm resolution to 0.01 mm was paired with a ZP-50 digital force gauge (Baoshishan, Shenzhen, China) with resolution to 0.01 N. Calibration of the ZP-50 dynamometer was verified with a handheld Correx dynamometer (Haag-Streit Diagnostics, Köniz, Switzerland). The ZP-50 dynamometer was secured to the test stand in compression test mode. The selected test model was secured to the baseplate of the test stand utilizing a standard mini c-clamp (Fig. 3).
Given the temperature-sensitive shape memory properties of DPA, it was necessary to simulate the oral environment. Aligners were heated to body temperature (97.5 F) for a minimum of 5 min prior to testing by placement of each aligner in an individual water-filled bag (30–60 ml) in a temperature-controlled water bath. To further maintain the intraoral simulated temperature environment, a ceramic positive thermal coefficient heater was used.
The newton meter was lowered incrementally until a force was read on the digital force meter after placing each aligner to the test model. The meter was then raised until the force equaled zero. This process was repeated three times for each sample. The digital caliper was then zeroed, and the aligner was compressed with vertical compression on external incisal edge of the missing UR1. Compression occurred until a displacement of 0.10 mm in the gingival direction and then peak N reading was recorded, a timer was then set and at 20 s, the N reading was recorded, compression then continued to 0.20 mm displacement with a subsequent peak N recording and a further N recording after 20 s of force stabilization. This process continued until 0.30 mm displacement. A total of 40 aligners were tested in this manner on the test model, 10 DPA-NA, 10 DPA-YA, 10 TFA-NA, and 10 TFA-YA. All recorded data indicated the tested aligner number for quality assurance and appropriate statistical analysis.
Statistical methodology
Dynamometer readings were captured at each respective displacement. Readings were captured for peak force (N) and stabilized force (N).
All analyses were conducted using SAS version 9.3 (SAS Inc, Cary, NC) and the level of significance (α) was set to 0.05. Wilcoxon rank-sum test (nonparametric) was performed to compare the peak force and stabilized force among DPA and TFA with and without attachments.