CAD-CAM design and 3-dimensional printing of mini-implant retained orthodontic appliances

The objective of this article was to illustrate the digital process in the custom fabrication of metallic mini-implant supported appliances. An implant-supported appliance was produced for a patient using a CAD-CAM procedure without a physical impression or a printed model. The work flow consisted of mini-implant insertion into the palate, recording an intraoral digital scan, digital design with incorporation of a scanned expansion mechanism, direct 3-dimensional metal printing via laser melting, laser welding of the hyrax mechanism, insertion, and activation of the appliance. The favorable clinical outcome demonstrated that this procedure is an efficient and viable method for constructing an implant-supported palatal metallic appliance.

Highlights

  • CAD/CAM procedures can be used to fabricate palatal mini-implant borne appliances.

  • This method is efficient and accurate.

  • Advantage for the patients is greater comfort during the scan recording process.

  • Advantage for the clinician is no need for impression trays.

The adjunctive use of mini-implants is considered a staple in contemporary orthodontic care due to their versatility, minimal invasiveness, and cost effectiveness. They have enabled astute clinicians to bypass the need for extraoral appliances, support the biomechanical basis for selective tooth movement, and possibly avoid the need for adjunctive surgical intervention. The orthodontic specialty continues to make significant advances with the development and incorporation of various digital technologies including 3-dimensional (3D) digital casts, individual bracket setups, aligners, and customized archwires. The evolution of this progress is expected to naturally flow to the adaption of 3D printing of traditionally laboratory custom-made appliances. Graf et al presented an innovative method of 3D metal printing (laser melting) for rapid palatal expanders.

From the time when orthodontists first began to use palatal mini-implants in their treatment approaches, the method of connecting the orthodontic appliance with the implant has garnered little review and focus. Prefabricated appliances have been most commonly used (eg, Benefit system; PSM Medical Solutions; Tuttlingen, Germany), which can be directly adapted to the implants intraorally, or indirectly modified after recording an impression of the surgically positioned implants with the adjunctive use of impression caps.

The introduction of intraoral scanning devices enables the recording of intraoral scans of the implants to be performed with a high degree of accuracy. A scan body (analog to the impression caps) on the mini-implant can be used to enhance the precision of the scanning outcome, or the mini-implant can be scanned directly depending on the accuracy of the intraoral scanning device. Once the scan has been successfully procured, the clinician and laboratory technician can collaboratively design a customized appliance based on the individual treatment objectives and required biomechanical plan for the patient. The objective of this article was to illustrate a novel method for the digital CAD-CAM design and 3D printing of a mini-implant retained orthodontic appliance.

Material and methods

The process commences with the surgical placement of 2 mini-implants (PSM Medical Solutions; diameter, 2 mm; length, 9 mm) in the anterior hard palate. A TRIOS intraoral scanner (3Shape, Copenhagen, Denmark) is used to record color images of the maxillary arch, including the 2 mini-implants without transmission caps. The 3D stereolithographic file is sent directly to the off-site dental laboratory, where the appliance is digitally designed with readily available appliance designer software from 3Shape. Additional proprietary components to the software include scanned stereolithographic files of the mini-implants and expansion-screw mechanism.

The molar bands of the designed appliance were substituted with a circumferential ring, consistent with the c-clasp design commonly used in removable prosthetic designs.

The circumferential ring is designed with a thickness of 0.7 mm and positioned 0.05 mm (bonding space) from the tooth surface, permitting application of the requisite bonding material between the appliance and the tooth. The molar bands were palatally extended with an arm to the second premolar and the second molar. The buccal surfaces of the maxillary posterior dentition were concomitantly bonded with multibracket edgewise appliances, while maintaining the implant-supported appliance to serve as anchorage and provide for stabilization. Small projection tips on the buccal and palatal extensions were incorporated to aid in the removal of the appliance, because the highly polished surfaces of the appliance are too smooth for the required frictional force with a debonding plier.

The connection on the neck of the implants was designed on the surgically positioned and digitally matched implants as a round flat ring with the same height and diameter as the neck of the implant. In patients with a high arched and narrow palate, it might be difficult to scan the implant head circumferentially because of the relatively large size of the scanner head. In such cases, it is prudent to use a virtual implant analog to achieve perfect fitting of the ring on the implant neck. Furthermore, some intraoral scanners cannot directly scan metal surfaces of mini-implants due to their highly reflective nature; this may necessitate the use of a digital implant analog. The analog is comparable with the classic cast implant analog for the laboratory process. When 3 points are clearly marked on the scanned implant head, superimposition with the digital implant analog is achievable, resulting in a precisely defined form to design the implant-neck surrounding ring. Another possible solution to improve the surface recording of the mini-implants is to use either a scan body ( Fig 1 ), scan powder (eg, from 3M Unitek, Monrovia, Calif), or a prosthodontic occlusion spray. The expansion screw (Forestadent, Pforzheim, Germany; 12 mm expansion; 0.9 mm/turn) was digitized and inserted with the largest possible welding area to the designed wires. These wires are designed with a diameter of 1.2 mm, providing the connection between bonding site, implant head, and expansion screw.

Fig 1
Scan bodies (analogous to the impression caps) on the mini-implants. They may be used to enhance the precision of the scanning outcome (only needed for some scanners).

The digital design goes to a laser-melting machine ( Fig 2 ) (Concept Laser, Lichtenfels, Germany) where the primary structure of the appliance is printed 3 dimensionally with remanium star metal alloy (Dentaurum, Ispringen, Germany), commonly used in the fabrication of removable dental prostheses. The 3D metal printing process consists of 2 phases. Initially, the remanium star powder is spread in a layer of 25 μm (depending on the grain size of the metal-alloy powder) and laser melted in the required spots to construct a solid structure. The layering procedure is repeated until the whole structure is completed ( Fig 3 ). The laser melting device from Concept Laser has the smallest melting volume of 9 × 9 × 8 cm with a 110-W laser and requires 11 hours for the fabrication of 4 appliances, each with a build volume of 6 × 3 × 2 cm. The time required for fabrication of the appliances could be further reduced with at least 1 of the following approaches: (1) a larger machine with a greater build volume, (2) use of 2 lasers instead of a single laser, and (3) a machine with higher power wattage.

Fig 2
Principle of selective laser melting: The metal is applied in powder form in a thin layer by a coater. A laser locally melts the fine metal in powder form. The contour of the component is produced by redirecting the laser beam using a mirror deflecting unit (scanner). The component is built up incrementally layer by layer, applying more powder and then melting again (courtesy of Concept Laser, Lichtenfels, Germany).

Dec 8, 2018 | Posted by in Orthodontics | Comments Off on CAD-CAM design and 3-dimensional printing of mini-implant retained orthodontic appliances
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