Digital the design and fabrication of maxillary removable partial denture (RPD) frameworks is now a reality. The framework is designed digitally and printed with selective laser melting (SLM). Clinicians must be mindful that these exciting new technologies, are tools and cannot substitute for proper diagnosis and a thorough knowledge of the principles of RPD biomechanics and design.
Digital Design of Maxillary of RPDs – Transcript
1. Jay Jayanetti DDS Daniela Orellana DDS, MS Ting Ling Chang DDS John Beumer DDS, MS Division of Advanced Prosthodontics UCLA School of Dentistry Digital Design of Maxillary Removable Partial Dentures
2. Digital Design of Maxillary Removable Partial Dentures ✦ There are three basic phases of the digital workflow when designing and/or fabricating removable partial denture frameworks; data acquisition, designing (computer aided design (CAD)), and computer-aided manufacturing (CAM). The bulk of this presentation is dedicated to the design steps used in this workflow utilizing sample maxillary and mandibular casts ✦ This presentation describes the use of one of the most commonly used digital design programs available commercially (3 Shape).
3. Data Acquisition ✦ The master cast is obtained in the usual way. ✦ The proposed RPD design is carefully outlined on the master cast ✦ Bead lines must be inscribed prior to scanning. ✦ This cannot be accomplished digitally. Also, if the master cast is designed as shown here, some scanners can image the surface texture including the pencil markings. This can aid in the digital design process. ✦ The cast is then scanned with a high resolution lab scanner
4. Data Acquisition ✦ Intraoral scanners have not proven to be sufficiently accurate for making the full arch impressions that are necessary for the fabrication of an RPD metal framework. ✦ Please note that unlike single unit restorations that are designed with a die spacer of 100-micron thickness, more precision is necessary when making RPD frameworks because the rests and proximal plates must be in intimate contact with multiple abutment teeth across the arch.
5. Digital Design – Surveying and Blocking Out ✦ Color-coded isodepth curves delineate the depths of the tooth and soft tissue undercuts. The operator may tilt the cast in any direction by increments of a degree and the software immediately renders the survey line and associated isodepth curves. ✦ By rotating the cast, one can decide on the most advantageous position (MAP). Once the MAP has been determined, parallel bock out is automatically applied. Undercut depth
6. Digital Design – Surveying and Blocking Out ✦ The operator can rotate and zoom in on the digital model with great control. ✦ The various depths of undercut areas on the dentition and the soft tissue of the digitized master cast at a given path of insertion can be noted with multiple simultaneous views. ✦ The undercuts to be engaged are exposed and the RPD framework is then designed Undercut depth
7. ✦ Automated parallel blockout based on the chosen path of insertion. The operator can control whether the block out is to be truly parallel (zero degree) or with more divergence (1˚, 2˚, or 3˚). Digital Design – Surveying and Blocking Out b a
8. Digital Design – Surveying and Blocking Out ✦ The yellow coloring in “b” denotes the retentive region of interest. The .25mm undercut begins at the junction of the yellow and light orange region. For example, if an “I” bar retainer is used the tip of the retainer should terminate at the occlusal side of the region of the tooth with yellow coloring and should engage the tooth cervically to the light orange-yellow junction. If the clinician wishes to engage a . 50mm undercut the wax block out cervically must be eliminated and the tip of the retainer extended to the junction between the light orange – dark orange. b a ✦ The operator can digitally remove the digital rendering of the block out wax from the desired retention areas for the RPD retainers.
9. Digital Design – Surveying and Blocking Out Wax Trimming ✦ When the MAP has been determined, all undercuts, including the measured undercuts required for retention, are blocked out parallel to the path of insertion. The next step is to quantitatively expose the desired undercuts for the tips of the retainers. All other unwanted undercut areas remain blocked out consistent with the path of insertion. ✦ Specific for this design, 0.25 mm undercuts are desired for all four retainers. Enough wax is removed to expose the junction of the yellow with the light orange. This is performed on the midfacial area of the canines and on the distal buccal line-angles of the molars. If a sharp angle is formed when removing the wax block out, it may be smoothed with the appropriate tool. Make certain that the appropriate amount of undercut is exposed. It is advisable to err on exposing a bit more of the undercut since increasing retention once the frame is cast is more difficult than reducing the retention. ✦ During the “Wax Trimming” section, arbitrary wax relief may be added where indicated such as underneath a mandibular bar type major connector (discussed in the mandibular example below) or underneath an infrabulge retainer.
10. RPD design steps ✦ This section of the design software is divided into 5 steps beginning with “Retention grids” where relief wax for open lattices or mesh type denture base connectors are designed. “Major connectors” is the second step in the design process followed by the design of “Clasps” assemblies. The “Clasps”, design tool is also used to design minor connectors and rests. The so called “Sculpt” tool is next, which allows the clinician to thicken, thin, smooth and blend specific areas of concern. The final step in the RPD Design section is referred to as “Finishing line”, a misleading term because only external finish lines are positioned with this tool. Internal finish lines are positioned during step one or when the outlines of the retention grids are established. ✦ The design software follows a different sequence usually used when drawing the design on a physical cast. While this sequence helps avoid mistakes that require erasing pencil marks on a stone cast, on a virtual cast this concern is eliminated. While the sequence differs from conventional techniques, the RPD design should be consistent with the principles of RPD design.
11. Denture Base Connectors (“Retention Grids”)✦ There are a variety of denture base connectors that can be designed: mesh type connectors, open lattice type and metal base connectors with retention features such as posts. ✦ If a mesh type denture base connector is used it should be designed first. If a metal denture base connector is chosen, it is designed as an extension of the major connector. If an open lattice design is used, the wax relief (described in the software as “resin gap”) is designed during this step, while the struts are laid out along with minor connectors later in the design process.
12. Denture Base Connectors (“Retention Grids”) ✦ Wax relief is placed in the #3-#5 edentulous span in anticipation of designing on open lattice. The mesh retention grid overlying #12-#14 edentulous span is automatically laid with its own wax relief. The anterior edentulous site will have a metal base, which is an extension of the major connector and does not require wax relief. Notice also the bead lines that will define the major connector outline. ✦ The placement of relief wax is dependent upon the type of denture base connector chosen. For example, if a mesh type is selected, the relief wax is configured automatically by the software program. If an open lattice design is selected, the wax relief is outlined but the struts are designed during a subsequent design step. The gauge of the relief wax can be modified as needed.
13. Denture Base Connectors (“Retention Grids”) ✦ The term “spline” is introduced here. In mathematics, a spline is a curve that approximates points on a chart or a graph. The same applies here. When points are placed to design a line or an area, the program curves the line as opposed to connecting each point with the shortest straight line. The program also automatically adapts to the surface contour of the virtual cast. If desired, each point on the spline can be repositioned to manipulate the line or area. ✦ The sample design for this maxillary RPD will have all three types of denture base connectors. We begin by selecting the tool for designing the wax relief. Remember that this is needed for an open lattice connector. Begin with a series of points to outline an area. The last point in the series must be superimposed on the first point, which will close the loop which completes the layer of wax relief.
14. Denture Base Connectors (“Retention Grids”) ✦ When a spline is completed the area will fill with the selected material, in this case a layer of relief wax. The open lattice will be laid over the wax relief during a later step. By selecting a mesh and repeating the previous steps, a completed spline will render a digital representation of the chosen denture base connector, along with the wax relief beneath it. ✦ The sample design for this maxillary RPD will have all three types of denture base connectors. We begin by selecting the tool for designing the wax relief. Remember that this is needed for an open lattice connector. Begin with a series of points to outline an area. The last point in the series must be superimposed on the first point, which will close the loop which completes the layer of wax relief.
15. ✦ The outline of “area” type major connectors is defined by a series of points that cover the desired denture bearing surface. The spline should extend to include your bead lines. It is critical to have an overlap with any previously designed denture base connector to avoid an error message or voids between components. Each point defining the perimeter can be moved to extend or shorten the palatal coverage. ✦ Note that the white spline laid for the major connector outline overlaps with the denture base connector. Major Connector
16. ✦ A window spline is laid down inside the bead line. ✦ The rendering of the pattern after the window is placed, separates the anterior and posterior palatal straps. ✦ A-P palatal strap major connector is designed. ✦ When designing an A-P strap major connector, the clinician should define the perimeter of the palatal plate first, making sure to overlap with the mesh connector. ✦ A complete palatal plate is rendered. ✦ Second, a window is designed within the plate to separate the anterior from the posterior straps Major Connector
17. ✦ This step provides tools for creating clasp assemblies including rests, proximal plates, bracing components, and retainers. Minor connector splines are also utilized to form a latticework over the previously laid relief wax for connecting a denture base. ✦ The software allows the clinician to design these components in any sequence. The authors recommend designing the rests first. Minor Connectors, Retainers and Rests (“Clasps”)
18. ✦ Rests are designed with a single spline as opposed to an area. As described before, at each point along the spline one can control the width and thickness. With rests, when increasing the width, the algorithm of the software fills the concave surface area of the prepared rest seat. Therefore it is best to place the first point of the spline at the deepest and most central point in the occlusal rest seat. Two or three subsequent points are sufficient to complete the spline; one point on the marginal ridge and one point on the guide plane as seen. Minor Connectors, Retainers and Rests (“Clasps”)
19. 4 1 ✦ Last the palatal plate spline is a horizontally placed minor connector spline, that also overlaps with the occlusal rest and proximal plate splines as shown by “3”. Notice the overlapping splines occupy the same space without doubling or tripling the thickness. Minor Connectors, Retainers and Rests (“Clasps”) ✦ Illustrates the occlusal rest spline that begins at the depth of the spoon shaped mesial occlusal rest and wraps over the mesial marginal ridge and onto the guide plane as shown by “1”. A proximal plate spline overlaps the occlusal rest at the marginal ridge, continues gingivally and onto the tooth-tissue junction and mesially towards the edentulous space where the wax relief has been laid for the open lattice denture base connector as shown by “2”. 2 3
20. ✦ A cingulum rest with a window is also designed with the same occlusal rest tool and in combination with the minor connector as a short palatal plate. ✦ Illustrates a rest spline used to create a cingulum rest that begins and ends on either marginal ridge illustrated by “1”. It overlaps with the distal proximal plate “2” and the palatal plate mesially “3”. The width of the rest and palatal plate are controlled to expose a cingulum window. Minor Connectors, Retainers and Rests (“Clasps”) 1 2 3 ✦ The anterior rest is open in the center to visualize complete seating the rest and to facilitate cleaning.
21. ✦ Like the previous components, minor connectors are also defined by single splines. Once rests have been positioned, the minor connectors serve to connect these components to the major connector. Additionally proximal plates are designed where needed and extended to cover the tooth- tissue junction and extending to a denture base connector. Minor connectors ✦ In situations where relief wax was designed for an open-lattice denture base connector a grid pattern of struts is arranged to form the connector. Particular to infrabulge retainers, one of the cross struts of the open lattice should align with the approach arm of the retainer to provide the necessary bulk and rigidity. This also allows for the cast metal to flow from the strut into the I-bar.
22. Minor connectors ✦ Open-lattice. This image illustrates the creation of an open lattice denture base connector. In a prior step the relief wax was laid defining the internal finish lines. During the minor connector step, struts are positioned to create a network for retaining the acrylic resin base. ✦ This example shows a buccal strut that connects the proximal plate minor connectors of the canine and molar. It is positioned to allow room for setting the cervical ridge lap of the prosthetic teeth. For this small edentulous space, one cross strut is sufficient and creates an open design. This one cross strut will lead into the approach arm of the I-bar infrabulge retainer.
23. Retainers ✦ All retainer options are designed with a single spline. Each point on the spline can be modified in thickness and height, hence defining the 3-dimensional taper. Half-round dimensions at the shoulder of circumferential clasps should have a base of 1.5 to 2mm and a height of 0.75 to 1 mm. The dimensions at the tip should be half that of the shoulder. This will produce the appropriate taper, and appropriate flexure without concentrating stresses. The retainer tip is placed in the undercut previously determined. ✦ When designing retainers, a drop- down menu includes software specific terminology that may confuse the novice. The choices define the cross sectional shape, all of which are variations of the half-round. The advanced user can customize a preferred cross section and taper.
24. Retainers ✦ This diagram illustrates a proper taper of a retainer. Half round cross sections are seen for the shoulder and terminus. The base and height of the circle is indicated. ✦ A bracing clasp must be thicker and contact more surface area of the abutment tooth. They should be designed with less taper to enhance rigidity. The entire length of the clasp should lie at or above the height of contour.
25. ✦ This circumferential retainer originates and overlaps with the proximal plate. A few points define the spline with the last point positioned at the designated undercut (a). ✦ Notice that the parallel block out was removed to expose the junction of the yellow and light orange isodepth curve, which indicates a 0.25 mm undercut. ✦ Notice that at each point on the spline, the width and height of the haft round cross section can be modified (b). This allows the clinician to taper the retainer. In this example the shoulder of the retainer (indicated by the yellow arrows) is designed to be about 2mm in diameter (1mm radius). Not illustrated here are the dimensions at the terminus which are 1 mm diameter with a .5 mm radius. Retainers a b
26. ✦ The “I” bar spline should begin at a point in the denture base connector that will least interfere with setting of the prosthetic tooth, especially when space is limited. This means designing it so it lies at an embrasure. When originating from an open lattice, the approach arm of the “I” bar should connect to a cross strut of the latticework. “I” bar retainers are also half round in cross section. Retainers
27. ✦ The approach arm should have a base dimension of 2mm. It will curve into the vertical arm approximately 3mm from the free gingival margin and cross the tooth- gingival junction at 90˚. It should taper from 2mm to 1mm at its terminus. Retainers
28. ✦ The tip will make contact with the tooth at the exposed undercut and continue occlusally to the height of contour (the occlusal limit of the yellow isodepth curve). It is a common mistake by novices to end the I-bar at the undercut and not extend to the height of contour Retainers
29. ✦ An I-bar is designed to engage the canine. Note that the blockout has been removed in the area of the tooth to be engaged by the tip of the retainer. ✦ Note that the retainer arm is aligned with the strut of the open lattice denture base connector. ✦ To repeat, the tip should make contact with the tooth at the exposed undercut and continue occlusally to the height of contour (the occlusal limit of the yellow isodepth curve). Retainers
30. Use of the Sculpting tool ✦ When the clinician reaches this point in the design process all previously designed components will be automatically fused into one contiguous structure. This step allows for increasing or decreasing the thickness of any area of the framework. There is a virtual toggle that determines a tolerance for thickness. ✦ When set to 0.5 millimeter, for example, anything thinner than 0.5mm will show in red providing a visual indicator of potentially weak areas. The thickening tool is used in strokes over areas of concern. With every stroke the area is thickened and red areas change to yellow and then green when thickness reaches above the set tolerance
31. Use of the Sculpting tool ✦ Another tool in this section allows the clinician to smooth the surface. Areas of concern are junctions between components (examples rests to minor connectors, retainers to minor connectors, and minor connectors to major connectors). This latter feature is much like using an alcohol torch to flame over rough areas of wax. The degree and area size that is to be smoothed is controlled by a click of the mouse.
32. Use of the Sculpting and smoothing tools ✦ Based on the settings chosen for this framework, the red areas indicate portions of the framework that are thinner than 0.5 mm (a). ✦ The thickening tool is used to thicken the areas of concern and when they are appropriately thickened the color turns to green. ✦ A smoothing tool can be used to smooth the interfaces between components (b). a b
33. External Finish line ✦ External finish lines (EFL) are also designed with a spline. Each point along the spline may be broadened, narrowed, shortened or made taller depending on need and location. ✦ The first point is placed on the palatal line angle of the proximal plate. It swings palatally to allow space for positioning the denture teeth and for festooning. ✦
34. External Finish line ✦ This image shows splines for the external finish lines (EFL) of the three edentulous segments. The spline begins at the distopalatal line angle of the anterior abutment on the proximal plate and ends at the mesiopalatal line angle of the distal abutment.
35. ✦ When metal bases are designed, posts are necessary to retain the denture tooth and encompassing acrylic resin. Areas of increased horizontal leverage such as anterior teeth or excessive ridge resorption should include a vertical post to enhance the retention provided for the acrylic resin. Once the post is placed it may be tilted as necessary. It can also be stretched or shortened as needed Posts and screws
36. ✦ A braided post has been added to the metal base. It will help to retain the lateral incisor. Posts and screws
37. ✦ There is a second opportunity to refine the thickness and smoothness of the pattern with this tool. The entire framework is carefully scrutinized and changes in thickness and smoothness are made as necessary. Final Sculpting and Stipple
38. ✦ There are several choices available for stipple patterns and control of pattern surface texture. ✦ Finished design. The last step is choosing between textures available for the palatal straps. The operator has control of the depth of the pattern. One should keep in mind that the ability to magnify can lead novice operators to overestimate the depth of the stipple pattern. Final Sculpting and Stipple
39. Computer-aided manufacturing of the RPD framework ✦ Computer aided manufacturing of metal RPD frameworks has been slow to be adopted because the techniques used in early CAM system were exclusively subtractive methods, such as milling from a solid block of material. Although this method is effective when milling materials such as ceramics, waxes and resins, milling a pattern as intricate as a RPD framework from a solid metal puck is neither practical nor cost effective. ✦ In recent years, rapid prototyping (RP), a general term used for several additive layer manufacturing techniques, has been refined. The most common used are stereolithography (SLA), selective laser melting (SLM), selective laser sintering (SLS), selective deposition modeling, and 3D printing (3DP). Rapid prototyping
40. Computer-aided manufacturing of the RPD framework ✦ The most common CAM technology used in the fabrication of RPD frameworks is the printing of a light cured resin framework pattern. The pattern is printed with supporting struts and sprues followed by investment and casting. ✦ It is critical that the operator check the fit of the printed pattern on a master cast prior to investment. If the printed pattern fails to seat as designed one must troubleshoot the situation, ie. decide whether the error was made during the scanning or the printing. Adding wax or trimming with a dental hand piece permits small contour changes (see next slide). Designed framework Printed pattern on master cast
41. ✦ Printed framework remounted on an articulator, occlusion adjusted (red arrow) and wax additions made (yellow arrows) to improve centric contact. Notice that the addition of wax to the printed framework produced the desired contour (yellow arrows) of the cast framework. Alteration of the printed framework pattern
42. ✦ When adaptation is confirmed the usual and customary process of fabrication is pursued including investment, burn-out elimination, and casting. Finishing and polishing is performed as usual in order to seat the RPD framework onto the master cast. ✦ Designed framework (a). Printed pattern confirmed to fit the master cast (b). Cast framework completed (c). Computer-aided manufacturing of the RPD framework a cb
43. Computer-aided manufacturing of the RPD framework ✦ A recent study on printed RPD frameworks indicated that printed patterns are subject to distortion if left exposed to light. Therefore, the laboratory must minimize such exposure before investing the pattern (Cagino et al, 2017). Pattern checks should be limited to the technician just prior to investment. ✦ When an adjustment of the occlusion on mounted casts is desired, the articulator and mounted opposing casts should be sent to the laboratory. The clinician should refrain from requesting a pattern check for intraoral trial because of the risk of distortion.
44. ✦ Although a fully digital work flow is convenient, the laboratory must be provided a master cast in order to try in and verify the fit of the finished framework (Cagino et al, 2017). Finally, a stone master cast is necessary when an altered cast impression is planned. ✦ RPD frameworks manufactured with the CAD/RP method, although considered clinically acceptable, show slightly larger gaps between the occlusal rests and the corresponding rest seats compared to that of the investment casting control group. The authors own experience with the SLM technique has been mixed. The precision of fit achieved with this method has not yet matched the consistency seen with the conventional methods ( Ye et al, 2017). Computer-aided manufacturing of the RPD framework
45. ✦ A framework fabricated via selective laser melting requires more finishing and polishing than cast frameworks (a). ✦ This can be a source of error as demonstrated in “b”. (b) Note that the rest is not in intimate contact with the rest seat. Computer-aided manufacturing of the RPD framework
46. ✦ Recent studies have shown that the RPD frameworks manufactured with the CAD/RP method, although considered clinically acceptable, showed slightly larger gaps between the occlusal rests and the corresponding rest seats compared to that of the investment casting control group (Ye et al, 2017). ✦ The authors own experience with the SLM technique has been mixed. The precision of fit achieved with this method has not yet matched the consistency seen with the conventional methods. Computer-aided manufacturing of the RPD framework
47. Computer-aided manufacturing of the RPD framework A RPD framework fabricated with selective laser melting (SLM). Some labs have mastered the technology and as one can see in this example, excellent outcomes with regard to fit and finish can be achieved.
48. ✦ The computer-aided design and manufacturing technologies (CAD-CAM) will continue to evolve and the authors believe that a complete digital workflow will be possible for the design and fabrication of RPD frameworks in the near future. ✦ Clinicians must be mindful that all these evolving technologies, although exciting, are tools and cannot substitute for proper diagnosis and a thorough knowledge of the principles of RPD biomechanics and design. ✦ Regardless of who performs the digital design, it is the responsibility of the clinician to approve the design, and this cannot be ethically delegated to other allied heath care personnel. Computer-aided manufacturing of RPD frameworks – The Future
49. References and suggested reading ✦ Bibb RJ. Eggbeer D, Williams RJ, et al. A trial of a removable partial denture framework made using computer-aided design and rapid protyping techniques. Proc Inst Mech Eng H. 2006;220:793-7. ✦ Cagino C, Jayanetti J, Moshaverinia A. Dimensional accuracy of removable partial denture frameworks fabricated by rapid prototyping. Oral presentation Pacific Coast Society of Prosthodontics Meeting, June 2017. ✦ Campbell SD, Cooper L, Craddock H, Hyde TP, Nattress B, Pavitt SH, Seymour DW. Removable Partial Dentures: The clinical need for innovation.. J Prosthet Dent. 2017; 118(3)273-280. ✦ Gratton DG. Evolving technologies in implant prosthodontics. In Evidence Based Treatment Planning and Clinical Protocols. ed by S. Sadowsky Wiley Blackwell, 2017. ✦ Han J, Wang Y, Lu P. A preliminary report of designing removable partial denture frameworks using specifically developed software package. Int J Prosthodont 2010;23:37-5. ✦ Lang LA, Tulunoglu I. A critically appraised topic review of computer-aided design/computer- aided machining of removable partial denture frameworks. Dent Clin North Am. 2014;58:247-55. ✦ Ye H, Ning J, Li M, et al. Preliminary clinical application of removable partial denture frameworks fabricated using computer aided design and rapid prototyping. Int J Prosthodont 2017;30:348-53.
50. This presentation is based upon “Kratochvil’s Fundamentals of Removable Partial Dentures”, Quintessence Pub Co. 2018
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