Digital design of maxillary of rpd's

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

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).

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

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.

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

✦ 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

✦ 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˚).
b
a

✦ 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.

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.

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.

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.

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.

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.

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.

✦ 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

✦ 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

✦ 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”)

✦ 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.

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.
✦ 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

✦ 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.
1
2 3
✦ The anterior rest is open in the center to visualize complete seating the
rest and to facilitate cleaning.

✦ 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.

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.

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.

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.

✦ 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

✦ 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

✦ 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

✦ 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

✦ 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

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

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.

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

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.
✦

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.

✦ 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

✦ A braided post has been added to the metal base.
It will help to retain the lateral incisor.
Posts and screws

✦ 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

✦ 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

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

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

✦ 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

✦ 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).
a cb

✦ 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.

✦ 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).

✦ 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.

✦ 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.

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.

✦ 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

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.

This presentation is based upon “Kratochvil’s
Fundamentals of Removable Partial Dentures”,
Quintessence Pub Co. 2018

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Removable Partial Dentures, Esthetic Dentistry
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