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Telescopic and conical double crown systems originally were developed as tooth-borne restorations that were intended to close interdental spaces,1 and such concepts have been well proven for many years. A telescopic or conical system consists of a primary abutment, a secondary close-fitting component such as a coping or framework, and a tertiary frame. This type of restorative system is meant to be removable by the patient for cleansability. The abutments or primary telescopes and secondary cones for these restorations were typically hand-milled in wax, cast, and remilled in type IV gold alloys. The secondary telescopes and cones were also built using pattern resins and invested and cast. In the 1980s and 1990s, electroforming of the secondary telescope or cone was introduced, as was CAD/CAM integration for design and fabrication of the primary abutment components.
More recently, additive and reductive technologies have made the fabrication of tertiary structures and temporary restorations more efficient. For example, selective laser melting (SLM) and milling of these structures in materials such as titanium and cobalt-chromium (CoCr) have been incorporated into the process, as well as digital design and 3D printing of temporaries that fit over the primary telescopes or cones.
Clinical protocols have also improved. Intraoral scanners, for example, have revolutionized the way clinical data is acquired and impressions are made,2 and this has opened up a plethora of digital protocols for telescopic and conical solution cases. Even more recently, the introduction of photogrammetry in dental applications has increased clinicians' and dental technicians' ability to capture the exact location, orientation, and timing positions of dental implants in the patient, leading to greater possibilities and eliminating the need for an analog master model.
This article examines how digital technologies impact telescopic and conical clinical workflows and technical protocols.
Intraoral and Desktop Scanning
Intraoral scanning can be especially useful for preliminary record-taking. For example, an intraoral scan with implant scan markers or flags can be used to 3D print a preliminary model with analogs (Figure 1), which can easily be used to fabricate a verification jig. In this workflow, the entire verification jig guide pin on the preliminary model can also be scanned with either an intraoral scanner or desktop scanner, and a digitally designed custom impression tray that fits over the verification jig can be digitally designed and 3D printed. This workflow eliminates the need for a preliminary physical impression, which, in turn, also eliminates the need to physically ship preliminary models to the dental laboratory and back.
An intraoral scan using implant scan markers with a scan of an existing prosthesis or diagnostic wax try-in can also be used to assess restorative space. This facilitates a thorough evaluation by the dental team using measuring tools within software programs so that successful treatment options can be selected and those that are not viable, depending on the particular restorative space, can be eliminated.
Scanning can also be used to capture the telescopic or conical abutments on the master model to be used along with a preoperative scan of the approved wax try-in for the design of a temporary prosthesis. A scan of the telescopic or conical abutments on the master model, along with secondary galvano copings seated in place on top of the telescopic or conical abutments, may also be used to digitally design a tertiary structure that incorporates an accurate and uniform cement gap for intraoral pick-up from the patient, making it possible to achieve a completely passive prosthesis.
Scanning of a telescopic or conical primary bar can also be performed to obtain a highly accurate surface calculation in square millimeters (mm2), which can be used for gold solution calculation in the electroforming process to attain optimal results in telescopic and conical bar sleeve-based prosthetics.
Lastly, scanning and design options for tertiary structure configuration allow for the design of both thimble preparation tertiary structures and an individually designed and milled dentition to fit the thimble preparation tertiary structure. The resulting dentate STL files can be either 3D printed in a castable resin and invested for burnout and pressing in ceramic, or milled in zirconia, millable glass ceramic, or hybrid ceramic. Most recently, the potential to print resins that utilize nanoceramic fillers and display high strength and optical properties has emerged.3
Photogrammetry can be combined with intraoral scanning to obtain a very precise, verified final digital impression without the need of a physical impression. This groundbreaking advancement allows the clinician to take precise records that are not subject to the dimensional expansion and/or contraction of dental impression materials or gypsum products used in the pouring of these materials.4 Using a workflow that incorporates photogrammetric scans enables a high level of communication between the clinician and dental technician5 and facilitates immediate smile design as well as design of primary telescopic or conical components with a large degree of cross-arch accuracy. Because of this high level of cross-arch accuracy, the primary abutments are able to be designed and milled, and it is possible to even design a tertiary structure at the time of primary abutment design using a specially developed algorithm that takes into account the amount of abutment surface removed in the hand-milling process as well as the amount of surface added to the abutment by the electroforming process. (This is described in detail later in this article under the subheading "Tertiary Structures.")
Photogrammetry allows for a highly efficient clinical and technical workflow, especially for screw-retained "all-on-X" restorations, but also for telescopic and conical implant solutions.
Primary Telescopes and Cones
Most telescopic and conical cases comprise three major components: a primary abutment or bar; a secondary component, typically a coping; and a tertiary structure. As mentioned earlier, primary telescopes and conical abutments traditionally were made using a UCLA abutment, to which wax was added by hand, and the abutment was then milled in wax. Abutments were then invested and cast in a type IV gold alloy or CoCr. This was a laborious process, and material and technique complications often led to flawed results, such as porosities and gaps in the primary components (Figure 2).6
Digital technology has improved not only the clinical and technical workflows for such components, but it has also broadened the variety of materials that can be used for primary telescopes and cones. For example, based on the digital design, primary abutments can be milled from CoCr, titanium, zirconia, and new materials such as PEEK and PEKK.7 While these materials still require hand-milling to achieve the desired result (Figure 3), the initial planning and designing of the primary telescopes and cones has been vastly improved and made more efficient in the digital age. Depending on the selected method and material, a variety of retention levels and longevity periods can be achieved.8
A typical digital workflow for the design of primary telescopes starts with working from either a scanned implant master model or an intraoral scan combined with a photogrammetric scan of the implant positions.9 Most dental design softwares have a specialized "telescopic module," which keeps all of the abutments parallel to each other and set at the user-defined insertion/removal path (Figure 4). Options are also available to the designer in terms of surface angle; for example, the telescopes can be made to be 0 degrees or in any conical configuration from 1 degree and up. It should be noted that only abutments with 0-degree abutment walls are technically called "telescopes," whereas anything more than 0 degrees is considered to be conical in design.
Almost every aspect of the primary telescope or cone can be edited and altered digitally, including the marginal areas, and even the "safety" and "minimum thickness" safeguards within the software can be set to desired specifications. Screw accesses can be edited, and angulated screw channels can be utilized with an offset angle typically of up to 25 degrees for zirconium hybrid abutments, which is a newer, significant advancement. Third-party milling centers also are able to incorporate angulated screw channels for titanium and CoCr primary telescopic abutments by milling the entire abutment using Swiss turn milling technology.10
Digital technologies have also boosted efficiency when creating a primary telescopic or conical bar for use with a sleeve-type secondary component. The primary bar can simply be designed and manufactured followed by hand-milling, or for a truly customizable and combined analog approach the model can be scanned and an overbulked bar can be designed in the software. The overbulked bar design can be easily printed in a variety of 3D resins direct to the implant or multi-unit platform. The printed bar can be screwed onto the master cast and then hand-milled to final dimensions using analog matrices of approved tooth positions (Figure 5). The marginal apron can be customized by hand, and, once finalized, the primary bar can be scanned and created as a copy mill along with the scan of the model with scan markers and sent to the milling center for reproduction (Figure 6).
Secondary Telescopes and Cones
Secondary telescopes and cones traditionally were created with the use of a pattern resin applied directly to a highly polished primary abutment, which was then invested and cast in either a type IV gold alloy or CoCr. This process was considerably technique sensitive and quite laborious.
In the late 1980s electroforming was introduced as a viable means to predictably create a uniform secondary telescope or cone in an additive process of electro-deposition onto the surface of a primary abutment or duplicate die of the primary abutment.11 These "galvano" gold secondary components had the potential to fit the primary component to within about 4 µm, and most of the electroforming units were able to produce these copings in either a 200- or
300-µm thickness (Figure 7).11
The benefits of electroforming a high-percentage gold were obvious: pure gold was and is highly biocompatible, and the fits achieved with electro-deposition were, and still are today, outstanding. Also, electroforming units can be used to make galvano "sleeves" on primary telescopic and conical bar sections.12 Some of these units require a surface calculation of the bar in order to determine the correct amount of current to be delivered to the bar surface, as well as to calculate the amount of gold solution used. This is another area where digital advancements have become extremely useful. Traditionally, a technician would have to measure the bar's dimensions and calculate the surface in mm2. This was a somewhat imprecise and unpredictable way to calculate this surface. However, in the digital age technicians can simply import a scan of the primary bar into software, select the surface to be calculated, and attain an instant and precise measurement of the surface area in mm2 (Figure 8).
A totally digital workflow can also be used for designing copings to be milled in a variety of materials using "offset coping" or "secondary telescope" selections. This may allow for secondary design files to be milled in PEEK or PEKK, or the entire secondary and tertiary structure to be made as one piece in PEEK, PEKK, titanium, or CoCr. (This particular technique still requires hand adjustment to achieve acceptable fitting to the primary telescopic or conical abutment.)
It is important to note that a tactile scanner should always be used if designing the secondary component or coping. This is because the tactile scan will produce a much more accurate and precise digital record of the surface of the primary abutments compared to optical scanning.13,14
Traditionally, tertiary structures were waxed to an investment model where a cement spacer was created by hand and then cast in CoCr. This method was laborious and introduced the possibility of porosities in the tertiary structure.
Digital design affords technicians the opportunity to increase accuracy and consistency, because they can use set parameters for cement spacers that work ideally with the adhesive resin cements used to intraorally lute secondary components into tertiary structures. Digitally designed tertiary structures also enable the use of an infinite variety of design options depending on the patient-specific parameters of any case. For example, if there is limited restorative space in specific areas of a particular case, polished metal lingual or palatal areas can be utilized in key locations if needed. Digital cut-backs from the proposed tooth position can be incorporated, and a wide range of geometries can be used for acrylic and composite retention. The design can then be printed using SLM or milled from titanium or CoCr (Figure 9).
Digitally designed structures also can enhance the work order and save large amounts of time usually required when using a traditional chronologic fabrication workflow. For example, for most tertiary structures that are used in concert with a secondary component, the technician must wait until secondary component fabrication is complete. However, in an internal analysis conducted in the author's laboratory of primary, secondary, and tertiary component designs made over a 12-month period using the cross-section tool in a design software, an algorithm was found between primary component design and tertiary offset design that was consistent and which provided an ideal cement spacer (Figure 10). This algorithm can also be used to design the tertiary structure on the same day as the primary telescopic or conical abutment; it takes into account the reductive process of hand-milling and the additive process of galvanic coping creation or milled secondary fabrication. Using this digitally generated algorithm technique can reduce fabrication time significantly as all components can begin to be produced on the same day.
Tertiary structures can also be designed to make a prosthesis as minimal as dentition only, or to support gingival replication in cases where there is moderate to severe alveolar resorption.
Digital design also enables the inclusion of spacers from an STL file for specialized attachments that are designed to work in concert with telescopic abutments and bars. These STL file attachments can be easily snapped onto the surface of the primary abutment or bar digitally, and the tertiary frame can be designed over the primary structure with snapped attachment spacers so that, once milled, the attachments can simply be inserted into the tertiary frame.
Telescopic and Conical Temporary Restorations
Vast changes and improvements utilizing digital workflows and technology have also impacted the fabrication of temporary restorations. For many years temporaries were fabricated by hand with PMMA materials and composites designed for this purpose. Along with the advent of CAD/CAM milling came milled temporaries from tooth-colored materials, and many options are currently available, including multilayered pucks.
More recently, 3D printing has had a major influence on many facets of dental technology, including the fabrication of temporary restorations. Initially, 3D resins used for this purpose were found to be somewhat brittle, but now there are numerous nano-ceramic-filled 3D resins that are showing considerable promise.3 Nano-ceramic hybrid 3D resins have improved optical properties and strength versus earlier 3D resins. Additionally, anecdotally, newer protocols, 3D printer settings, and third-party build plates combined with special settings can allow for in-office printing of full-arch temporaries in approximately 30 minutes (Figure 11). Figure 12 and Figure 13 depict a finished telescopic case.
Although traditional analog-produced telescopic restorations have a long history, as digital technologies become increasingly available and prevalent, a gradual shift is occurring to incorporate these new technologies and materials into the workflow for such cases and develop new digital protocols and workflows for these restorations. Emerging materials and technologies are making telescopic and conical removable restorations more predictable, efficient, and accessible to patients today.
About the Author
Arian Deutsch, CDT
Deutsch Dental Arts
Sun City West, AZ
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