Digital Design of Mandibular Removable Partial Dentures

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 Mandibular
Removable Partial Dentures

Digital Design of Mandibular
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 (3 Shape).

Data Acquisition
✦ The master cast is obtained in the usual way.
✦ The proposed RPD design is carefully outlined on the master
cast.
✦ The cast is then scanned with a high resolution lab scanner.
Virtual castMaster cast

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.
Master cast Virtual cast

Surveying and Determining the Most
Advantageous Position (MAP)
✦ 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
Undercut
depth

Surveying and Determining the Most
Advantageous Position (MAP)
✦ 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

Surveying and Blocking Out  
Wax Trimming
✦ When the MAP has been determined, all
undercuts are blocked out parallel to the path
of insertion. The desired undercuts for the
tips of the retainers are exposed (“trimmed”).
✦ In this example, 0.25mm undercuts are desired
for the two “I” bar retainers. Enough wax is
removed to expose the junction of the yellow
with light orange, which corresponds to the
0.25mm undercut. This is performed on the
midfacial portion of the second premolars.
Make sure the appropriate undercut is
exposed.
✦ It is advisable to err on exposing a bit more of
the undercut since achieving more retention
once the frame is cast is more difficult than
reducing the retention.
✦ The clinician can control whether the block-out is to be
truly parallel (0˚) or with more divergence (1˚-10˚).
The parallel block out is shown completely eliminating
all undercuts.

Wax Relief
✦ Because mandibular major connectors require relief so as not to impinge on the
gingiva during function, during this step the wax addition tool is used to arbitrarily
add wax on the surface of the lingual mucosa where the lingual bar major
connector will be located.
✦ By holding the left click button on the mouse and moving the cursor over the
desired areas, wax is added like a spray can. The maximum thickness can be
adjusted to avoid over or under application of wax. In conventional methods,
26-30-gauge wax is used. For this software 0.25 - 0.4 mm is equivalent wax relief
for the mandibular lingual bar major connector is applied arbitrarily. Three

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)
✦ Begin by selecting the tool for designing the wax relief. Begin with a series of
points to outline the desired area.
✦ The last point of the spline must be superimposed on the first point, in order to
complete the spline (ie. the loop). When the loop is completed the area will fill in
with a digital representation of a layer of relief wax.
✦ Wax relief for the open lattices are placed 2-3mm from the tooth-tissue junctions.

Denture Base Connectors
(Retention Grids)
✦ The open lattice will be laid over the wax relief during the “Clasp” step.
✦ Repeat this on the contralateral extension base. The borders of the wax relief
define the internal finish lines. Make sure that this line extends 2-3 mm from the
tooth tissue junction so that this area will later be covered with the minor
connector.
✦ The cross-section measuring tool allows one to precisely measure this distance.
The same tool is illustrated below where the distance from major connector to the
tooth tissue junction is evaluated.

Major Connector
✦ Bar type connectors are
designed by a series of points
that define the length of the
bar.
✦ One should extend the bar
about a tooth length beyond
the last abutment adjacent a
the edentulous extension area.
This will allow proper
connection to the open lattices
(denture base connectors).
✦ Once in place each point along the spline
can be manipulated to control both the
thickness and the width of the connector.
The space between the points is averaged
to produce a continuously smooth edge of
the connector.

Major Connector
✦ The half-pear cross sectional
configuration of a typical
lingual bar is predetermined by
the software. Advanced users
are able to customize the cross
sectional configuration by
manipulating the program
settings. Wax relief for the open
lattices are placed 2-3mm from the
tooth-tissue junctions.
✦ Notice that the clinician can control the half-
pear shape cross-section of the lingual bar.
The height of the bar should be approximately
4 mm. The thickness of the inferior portion of
the bar should be approximately 2 mm.

✦ The cross section tool is utilized to accurately measure distances, and thicknesses
etc. The software reads a measurement between any two points along a straight
line. Since we desire a minimum 3mm of clearance from the major connector to
the free gingival margins this tool is utilized to accomplish this task.
✦ The cross section tool requires the operator to position a disc that sections the cast, wax and
framework at any angle.
✦ Two selected points are used to measure the distance between the free gingival margin the
superior margin of the major connector. Notice that wax is seen in yellow; parallel block out is
seen labially and relief wax is seen between the major connector and the soft tissue.
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 denture base
connectors. The software allows the clinician to design these components in any
sequence. The authors recommend designing rests first.
Minor Connectors, Retainers
and Rests (Clasps)

✦ When two rests are abutted as seen here on the premolars, a single spline is used.
✦ Note that the minor connector connects the rest to the major connector. The window
associated with the cingulum rest is created in the following manner. A cingulum rest
is created with the occlusal rest tool. The window is created by combining the rest with
a minor connector as a short lingual plate.
✦ The anterior rest is open in the center to visualize complete seating the rest and to
facilitate cleaning.
Rests (Clasps)

✦ The same minor connector tool is used to form the denture base connector over
the previously laid relief wax. Begin with a spline forming the outline of the grid
pattern making sure to overlap with the proximal plate and the major connector.
In the example shown two cross struts complete the open lattice.
✦ The cross strut closest to the abutment tooth should be aligned with the
approach arm of the “I” bar retainer which is designed in the next segment.
✦ The denture base connector outline is formed within the perimeter of the relief
wax.
✦ Cross-struts are designed to complete the open lattice.
Denture Base Connectors (Clasps)

Retainers (Clasps)
✦ 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 (Clasps)
✦ 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.

✦ Select a retainer tool with a half-round cross section and place the first point of the
spline overlapping with a cross strut of the open lattice denture base connector.
Continue down the crest of the ridge about 3mm and swing mesially towards the
abutment tooth. Curve into the vertical arm and cross the tooth-gingival junction.
✦ The vertical arm should be parallel to the long axis of the tooth. Modify the half round
base and height to properly taper the vertical arm from 2mm at the bend to 1mm at its
terminus.
✦ Note that the retainer arm is aligned with the strut of the open lattice denture base
connector.
Retainers (Clasps)

Retainers (Clasps)
✦ 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.
✦ 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). It
is a common mistake by novices to end the I-bar at the undercut and not extend to
the height of contour.
✦ The I-bar is designed to engage the
premolar. Note that the blockout
has been removed in the area of
the tooth to be engaged by the tip
of the retainer.

Sculpt
✦ When the clinician reaches this point in the design process all previously designed
components fuse into one contiguous structure. This step allows use of tools to
smooth and increase or decrease the thickness of the framework. Irregularities are
often noted at the junctions between components. These are smoothed with the
“sculpt tool”.
✦ The “sculp” step allows thickening, thinning and smoothing. (a) Notice the
irregularities caused by overlapping splines of multiple minor connectors. (b) A
smoothing tool was used to smooth the interfaces between components.
a b

Use of the Sculpting and smoothing tools
✦ The thickening tool is used to thicken the areas of concern and when they are
appropriately thickened the color turns to green.
✦ Based on the settings chosen for this framework, the red areas indicate portions
of the framework that are thinner than 0.5 mm (a).
a b

✦ The first point of the external finish line (EFL) spline is
placed on the distolingual line angle of the proximal
plate (a). The spline moves distogingivally at about a
45˚ angle until it reaches the inferior aspect of the
major connector. Each point along the spline may be
broadened, narrowed, shortened or made taller
depending on need and location (b).
✦ To blend the EFL with the occlusal aspect of the
proximal plate the height and width of the first point on
the spline is made as short and narrow as possible.
The portion over the major connector is broad to make
it less conspicuous to the tongue.
✦ The spline has a wave-like cross section (b) where the
concave face of the wave must face the denture base
connector. If facing in the wrong direction, right click
on the spline and scroll the option “reverse spline.”
This will allow the wave to flip to the correct orientation
External Finish Line
a
b

✦ These images show the external finish line (EFL).
The spline begins at the distolingual line angle of
the abutment on the proximal plate and ends at
the inferior border of the major connector (a).
External Finish Line
✦ A close up view (b) of a point
on the EFL spline being
manipulated. Notice the
cross-section of the spline
has a wave-form (arrow)
with the face of the wave
pointing to the denture base
connector. Like all spline
points, the width and height
may be controlled.
a
b

✦ This final section provides the clinician an additional opportunity to “sculpt,” add
retentive posts where indicated, choose a stipple pattern for palatal major connectors,
and add casting sprues. Casting sprues are delegated to the technician who will cast
the printed pattern. Mandibular lingual bar major connectors are never stippled. The
example shown does not require retention posts.
✦ These images show the completed mandibular framework. (a) The completed
framework on the virtual cast. (b) The cast has been made invisible to allow an
unobstructed view of all surfaces of the completed framework.
Finalize
a b

✦ At any point in the design process a sliding toggle may be used to render the virtual
cast, block out and relief wax, or the in progress framework, translucent or completely
invisible (b). By doing so, one may gain an unobstructed view of the intaglio surface of
the framework. This is particularly useful for evaluating the juxtaposition of the internal
finish line to the tooth tissue junctions.
Finalize
ba

✦ When evaluating the finished design you may find portions of the framework that
require recontouring. The operator may jump back to any prior step and make
adjustments such as repositioning a spline or broadening a point on a spline. One
may return as far back as surveying and change the MAP by a couple of degrees.
The computing power of this software will re-render the design of everything
downstream from the alteration made.
Finalize
ba

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 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). Moreover, 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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Digital Design of Mandibular Removable Partial Dentures

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Digital Design of Mandibular Removable Partial Dentures