JCO Interviews Dr. James A. McNamara, Jr., on the Frankel Appliance, Part 2: Clinical Management
3D Metal-Printed Retainers
This column is compiled by Associate Editor Björn Ludwig, DMD, MS, PhD. Every few months, Dr. Ludwig presents a technique for computer-aided design and manufacturing in the orthodontic office. Your suggestions for future topics are welcome.
Three-dimensional printing is becoming commonplace in both dentistry and orthodontics. Filament and resin printers are the most popular, but metal printing has also found its way into our practices.
A growing number of software solutions allow us to convert our creativity into reality. This article focuses on the potential of digital design software to produce customized, patient-specific metal-printed retainers. Two different removable designs exemplify the endless possibilities of this technology; the authors also discuss concepts for 3D-printed fixed retainers, including one reminiscent of Zachrisson’s two-bonding-pad mandibular retainer. Hence, this article illustrates the future of retention concepts—without forgetting the past.
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3D Metal-Printed Retainers
Rapid prototyping, introduced in the 1980s for the manufacturing of solid models with complex parts, allows visualization and testing before fabrication and thus reduces costs.1,2 Selective laser sintering (SLS) is among the most common rapid-prototyping technologies used in dentistry.
According to the American Society for Testing and Materials, SLS—also referred to as selective laser melting, direct metal laser sintering, or electron-beam melting—is classified under the powder bed fusion (PBF) category.3-6 A 3D object is formed when a high-powered laser beam is directed at a layer of metal powder, sequentially fusing the powder particles based on the design of the object.3,7 Cobalt chromium, titanium, and gold* are the alloys frequently used for PBF in dentistry. Cobalt chromium alloys are nonprecious metals that offer excellent biocompatibility, good mechanical properties, and corrosion resistance in dental applications.8-11
The disadvantages of PBF are its high initial costs and certain limitations in the application software and production processes.12 Other methods used to print dental appliances include direct-light processing, inkjet-based 3D printers, and fused deposition modeling.2,13-15 Materials such as wax, plastics, ceramics, and metals can be utilized with these technologies.16
Computer-aided design and manufacturing (CAD/CAM) technology was initially developed in the 1950s to enable the modeling, design, and fabrication of various objects. Advantages include cost-effectiveness, quality control, speed, accuracy, and reduced labor costs.17,18 CAD/CAM was first put to practical dental use in the 1980s. The basic steps involved in production of an appliance include digitization of the impression or model to produce a stereolithographic (STL) file, virtual design of the appliance using specialized software, optimization of the design for 3D printing, and milling or printing of the appliance.19,20
CAD/CAM frameworks for partial dentures have been successfully designed and fabricated,21 and the technology has now permeated the field of orthodontics, where it is used to produce customized brackets, archwires, and clear aligners.22-24 There has been a lack of research, however, exploring the potential of CAD/CAM removable orthodontic retainers.
Retention is essential to maintaining the outcome of orthodontic treatment and preventing relapse.25 It has generally been accepted that an ideal retention appliance should not harm the oral tissues, should be easy to clean, and should not interfere with the occlusion. An ideal retainer should be able to resist masticatory forces, should be firm yet lightweight, and should provide good anchorage. While fixed retainers seem to be more esthetic than removable retainers and do not depend on patient compliance, removable retainers allow better occlusal settling and make it easier for patients to maintain oral hygiene. Removable vacuformed retainers are more comfortable, less bulky, and less labor-intensive to make than metal-framed retainers, but they are vulnerable to wear and tear.26-30 Since retention is a lifelong process, any factors affecting compliance—including comfort, speech, and hygiene—are especially important when designing removable retainers.31,32
This article reports the results of a pilot study regarding patient use and tolerance of esthetic metal-printed retainers that can facilitate hygiene maintenance and gingival health without interfering with speech or functionality. The main objectives of our initial design were durability; use of a flexible yet nondeformable alloy; provision of undercuts with minimal interocclusal metal; an esthetic appearance, without bulkiness or large clasps; and improved comfort and wearability, even while eating.
Materials and Methods
This pilot survey was conducted in 2017. The survey was reviewed and approved under reference 12-123-SDR by the Institutional Review Board of the Montreal General Hospital.
Polyvinyl siloxane impressions** were obtained from the upper and lower arches of eight previously treated volunteers. The impressions were poured in FastStone 3.*** The resulting models were scanned with a tabletop scanner,† and the STL files were uploaded to the Dental Wings† open-format software for removable partial framework fabrication (Fig. 1A). A virtual surveyor in the program was used to design the best path of insertion for the appliance with the appropriate amount of undercut for optimum metal engagement, which was determined to be .2mm (Fig. 1B). The retainer was then designed using the Dental Wings software (Fig. 1C). Care was taken to avoid occlusal interferences and to engage the appropriate undercut areas.
Fig. 1 A. Digitization of dental model using Dental Wings† scanner. B. Virtual surveyor used to design undercut areas and determine appropriate path of insertion (purple = blocked undercuts; yellow = maximum undercut). C. Retainer designed in Dental Wings software.
The virtual design was sent to a 3D printer‡ and processed in chromium cobalt alloy by SLS (Fig. 2). Laser sintering reduces the defects encountered in conventional casting of thinner appliances. The appliances can be printed more accurately, in less time, and with fewer processing steps.
The retainers were polished mechanically and electrochemically using the same protocol as for removable partial dentures.33 Three successive steps are used in mechanical polishing: ceramic beads, rubber particles, and corn kernels. The electropolishing is performed by immersing the retainers in ESMA E272 cobalt chromium solution in a rotating barrel (4,000rpm for 15 minutes).
Fig. 2 Laser-sintered chromium cobalt retainer tested intraorally.
Maxillary and mandibular retainers were then delivered. The volunteers were asked to wear the retainers for at least an hour, to speak while wearing them, and to try removing and reinserting them. They then completed an anonymous 12-item questionnaire, on a seven-point Likert scale, to assess satisfaction with their speaking ability and with the comfort and fit of the appliances in each arch.
Results
All eight volunteers scored the retainers favorably. The highest scores were given to the retentive features of the device—particularly the ability of the retainers to resist dislodgment during speech (a mean score of 7 in each arch), forces of the tongue (a mean of 6.86 in the mandible and 7 in the maxilla), and resistance to rocking on opening and closing (a mean of 6.71 in the mandible and 6.75 in the maxilla). Lower scores were given to speaking ability, particularly in the maxilla (a mean of 5.50), and to ease of insertion (a mean of 6.14 in the mandible and 5.50 in the maxilla).
Two patients treated with an interdisciplinary approach are reported as case studies. Both cases involved orthodontic space opening and maintenance for future implant placement. Metal-printed retainers were prescribed because of their comfort, ease of wear, and longevity.
Case 1
A 40-year-old female was referred to our clinic with a history of failed endodontic treatment and a fractured upper left central incisor (Fig. 3). The patient was diagnosed with a mid-root fracture and a periodontal abscess. Due to the impossibility of retreating the incisor and the high risk of losing the buccal plate during extraction of the root, we decided to extrude the incisor orthodontically and to replace it with an implant-supported full crown.
Porcelain brackets†† were placed on the maxillary anterior teeth, and a slow extrusion of the upper left central incisor was initiated. After six months, the periodontist was satisfied with the amount of extrusion, and the incisor was extracted (Fig. 4). The brackets were debonded, an immediate implant was placed, and a provisional acrylic partial denture was provided by the prosthodontic-periodontic team.
Fig. 3 Case 1. 40-year-old female patient with history of failed endodontic treatment and fractured upper left central incisor before treatment.
Fig. 4 Case 1. After extraction of upper left central incisor.
After the patient had difficulty adjusting to the acrylic retainer, a metal-printed retainer was fabricated using Dental Wings software as described above (Fig. 5). Because the patient was a broadcasting personality who required good esthetics, the retainer was designed without any anterior clasps. A composite pontic on a post was attached with composite to act as an interim prosthesis.34 This transitional prosthesis was used by the prosthodontist to contour the gingival tissue without placing any pressure on the integrating implant.
Fig. 5 Case 1. Metal-printed retainer with composite pontic.
The patient was highly pleased with the retainer in regard to both fit and esthetics. In retrospect, we would have extended the framework palatally to make it even more stable and rigid.
Case 2
A 15-year-old male presented with a Class I malocclusion, anterior crossbite, deep bite, congenitally missing lower central incisors, and peg-shaped upper lateral incisors (Fig. 6). Because of the dentoalveolar limitations, we decided to create space for one lower-incisor implant and temporarily restore the upper lateral incisors with composite.
Fig. 6 Case 2. 15-year-old male patient with Class I malocclusion, anterior crossbite, deep bite, congenitally missing lower central incisors, and peg-shaped upper lateral incisors before treatment.
After seven months of orthodontic treatment, a metal-printed partial mandibular retainer was designed using Dental Wings software (Fig. 7).
Fig. 7 Case 2. A. Metal-printed mandibular retainer designed using Dental Wings software. B. Framework on digital working model. C. Pontic area of framework. D. Weak areas in need of reinforcement.
Again, a composite pontic was included as a temporary replacement for the lower incisor (Fig. 8). The patient’s saliva appeared to create a cohesive seal, which is not observed with acrylic-based retainers.
Fig. 8 Case 2. Metal-printed appliance with composite pontic.
No adjustments were needed in either case, and both patients reported that the prostheses were comfortable. The elimination of clasps resulted in a better esthetic appearance.
Discussion
Although there have been considerable advancements in digital orthodontic treatment planning and clinical management, 3D-printing technology has not been widely applied to retention protocols.35 We can foresee even more developments in both software and hardware in the near future,36 including metal-printed fixed lower retainers. Several designs can be envisioned to take advantage of the rigidity of cobalt chromium alloys, which can be printed as thin as .5mm. A single passively bonded bar can be built (Fig. 9A), or a scalloped bar can be designed with individual bonding pads to allow the use of dental floss (Fig. 9B). A triangular hollow-pad design would enable rebonding if one attachment became loose (Fig. 9C).
Fig. 9 Potential designs of metal-printed fixed lower retainers. A. Single passively bonded bar. B. Scalloped bar with individual pads. C. Triangular hollow pads to enable rebonding.
Further studies involving finite element analysis and metallurgical testing are needed to compare our laser-sintered removable appliance to the Hawley retainer, which is the current standard of care. Our eight volunteers validated the fit, comfort, and appearance of the metal-printed retainer. Although their scores for speech were slightly lower than for other categories, further testing will evaluate how patients can adapt over time, in comparison with conventional retainers. The design is being revised to incorporate a buccal component that attaches to a clear, esthetic labial bow. Along with adjustments to the undercut engagement, this may alleviate the difficulty some patients experienced with removal and insertion of the appliance.
FOOTNOTES
- *BEGO USA, Lincoln, RI; www.begousa.com.
- **Aquasil, registered trademark of Dentsply Sirona, York, PA; www.dentsplysirona.com.
- ***FastStone Corporation, Calgary, Alberta; www.faststone.org.
- †Dental Wings, Inc., Montreal, Quebec; www.dentalwings.com.
- ‡Phenix Fusion, registered trademark of 3D Systems, Inc., Rock Hill, SC; www.3Dsystems.com.
- ††In-Ovation C, registered trademark of Dentsply Sirona, York, PA; www.dentsplysirona.com.
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DR. RETROUVEY -
DR. SADHAK -
DR. CARON -
DR. KADER
Dr. Retrouvey is Chair, Department of Orthodontics, University of Missouri-Kansas City, 5000 Holmes St., Kansas City, MO 64110; e-mail: jeanmarcretrouvey@gmail.com. Dr. Sadhak is an Associate Dentist, DSO Practice, Belmont, NH. Dr. Caron is Director, Milident Dental Laboratory, Gatineau, Quebec. Dr. Kader is in the private practice of orthodontics in Beaconsfield, Quebec.
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