Since retention is best when lifelong, factors affecting compliance, including comfort, speech, and hygiene are especially important to consider when it comes to.
Printing Orthodontic Retainers Using CAD/CAM Technology Jean-Marc Retrouvey, Erica Kader, Eric Caron, Faleh Tamimi, Nathan Light INTRODUCTION RETENTION Retention is necessary at the end of orthodontic treatment to avoid relapse of the teeth (1). Multiple forms of retention exist and present different advantages and disadvantages (Table 1). Since retention is best when lifelong, factors affecting compliance, including comfort, speech, and hygiene are especially important to consider when it comes to removable retention (7, 8). Table 1
CAD/CAM IN DENTISTRY Computer-aided design and computer-aided manufacturing (CAD/CAM) for dentistry has been evolving since the 1980’s. Its advantages include cost-effectiveness, quality control, speed, accuracy, and reduced labor (9, 10). Dental CAD/CAM applications involve: • Digitization of impression or model to produce STL file • Design of device using 3D software • Milling or printing of device Recently, frameworks for partial dentures have been successfully designed and fabricated using CAD/CAM (11). CAD/CAM has also permeated the field of orthodontics, with the customization of brackets, archwires, and clear aligners (12, 13). However, there is a paucity of research exploring the possibility of 3D printing of orthodontic removable retainers and appliances.
OBJECTIVE AND HYPOTHESIS The aim of this pilot study was to develop a prototype of a CAD/CAM-based removable retainer, and thus improve the manufacturing of retainers, as well as patient satisfaction and compliance. We hope to use the information gained through this pilot study to fuel future studies in CAD/CAM orthodontics.
MATERIALS AND METHODS FABRICATING A PROTOTYPE: Goals of First Design: • Durability • Minimal to no inter-occlusal metal • Use of a flexible, but non-deformable alloy • Improved undercut detection and utilization • Improved esthetics (less bulkiness and no large clasps) • Improved comfort • Wearability during eating Protocol: PVS impressions were taken in 8 volunteers and poured in dental stone. Models were digitized using Dental Wings TM model scanner to create an STL file (Figure 1). The STL file was opened using a proprietary, open-format software (DWOSTM) in order to design a virtual retainer. A virtual surveyor (DWOSTM) incorporated into the software was used to detect the appropriate path of insertion of the future appliance (Figure 2).
Figure 1. Digitization of stone model by Dental Wings (TM) Model Scanner
Figure 2. Virtual Surveyor used to detect blocked undercut (purple) and maximum undercut (yellow) areas.
The retainer was then designed using the software (Figure 3). Care was taken to avoid occlusal interferences and to engage appropriate undercut areas. Once the virtual design was complete, it was compiled by Dental Wings Compiling Software (DWOSTM) and sent to a 3D printing machine (Phenix Fusion Printer) and processed in chromium cobalt alloy using laser sintering (Figure 4). Laser sintering bypasses the potential for defects encountered with casting, as well as the thickness of metal required in order to withstand the casting process. We can print cleaner and thinner appliances in less time, with fewer processing steps. By compiling multiple 20 um layers of powder, lasersintered frameworks demonstrated a better fit with minimal distortion.
Figure 3. Retainer designed using 3D software
Retainer Delivery and Assessment: Maxillary and mandibular retainers were delivered. Volunteers were asked to wear the retainer for at least 1 hour, to speak while wearing it, and to try removing and inserting it.
Figure 4. Laser sintered chromium cobalt retainer tested intraorally
A questionnaire was filled out anonymously, consisting of a 7- point Likert scale that aims to assess the patients’ satisfaction with the comfort, speech, and fit associated with the appliance (Figure 5).
RESULTS
Figure 5. Mean responses to 7-point Likert scale questionnaire
All 8 volunteers scored the retainers favorably. The highest scores were given to the retentive features of the device, particularly, the ability of the retainer to resist forces of the mouth during speech (mean = 7 in both arches), forces of the tongue (mean = 6.86 in mandible, mean = 7 in maxilla), and resistance to rocking on opening and closing (mean = 6.71 in mandible, mean = 6.75 in maxilla). Lower scores were found in the domains of speech, particularly in the maxilla (mean = 5.75). Scores were also slightly lower in response to the ease of removal (mean =5.71 in mandible, mean = 5.50 in maxilla) and insertion of the appliance (mean = 6.14 in mandible, mean = 5.50 in maxilla).
CONCLUSIONS • The CAD/CAM retainers are comfortable, retentive and esthetic • They pose a more cleansable alternative to fixed retainers • These retainers can be adapted to accommodate different situations and amounts of undercut present • The design should be revised to improve ease of removal and insertion of appliance We have introduced a new design and method of fabrication for a retainer using CAD/CAM and laser sintering. Volunteers have validated the fit, comfort, and desirability of such a retainer, and we have already received ethics approval from the MUHC Review Board for a study to test the fit and comfort of printed appliances compared to traditional Hawley retainers. As well, further studies involving finite element analyses and metallurgical testing will be performed. We believe this is a promising avenue of research, and possibly a substantial improvement in removable retention.
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