RPD - Design

Sumamry
The fun bit- designing the RPD! This lesson will guide you through each stage so you are confident
when designing RPDs for your patients!

Kennedy Classifications

om
l.c
These simply help us to categorise different partially dentate scenarios:
ta
Class 1: bilateral free end saddle
en
Class 2: unilateral free end saddle
Class 3: unilateral bounded saddle, posteriorly
Class 4: anterior bounded saddle
Modifications: Work from the back of the mouth to identify which kennedy class the patient
eD

fulfils first. Once this is determined, any additional gaps = modifications

is
ev
R
R
ev
is
eD
en
ta
l.c
om
R
ev
is
eD
en
ta
l.c
om
R
ev
is
eD
en
ta
l.c
om
R
ev
is
eD
en
ta
l.c
om
 om
l.c
ta
en
eD
is
ev
R

Group of diagrams showing the different Kennedy Classifications

Stages of Design

om
l.c
ta
Use this denture design sheet to practice!
en

Saddles
eD

This is the teeth to be replaced.

Check- if the adjacent teeth have tipped/migrated, is there enough room to place an acrylic
denture tooth in the gap?
What does the ridge look like – does it look acceptable to hold a denture?
is
ev

Support
R

Resist movement TOWARDS the soft tissues, this can be tissue/tooth borne.
If tissue borne, extends to full functional sulcus depth.
Tooth borne: this can be in the form of rest seats- either occlusal, cingulum or incisal. These
have to be prepared in the teeth using an appropriate bur or preparation method (Keep
reading!)
Rests are usually placed on the tooth surface nearest the saddle on either side.
Mesially if part of an RPI system - which is the furthest side of the tooth from the
saddle.
They can be incorporated into crowns of abutment teeth if planned correctly.
Ideally we want tooth-borne support due to the additional benefits of proprioception from the
PDL of the abutment teeth and less damage to the gingivae.
Consideration on orthodontic movements - ensure the rest seat is prepared so that the forces
go in the axial direction!
 om
l.c
Diagram showing rest seat principles. ta
Retention
en

This resists movement AWAY from the soft tissues (eg. Preventing a denture from being dislodged
eD

from a sticky toffee).

is

Optimum = we want resistance along path of displacement + withdrawal.

Retention can be Muscular (held in by the function of cheeks and soft tissues)/Mechanical.
It can also be adhesive forces eg. a saliva seal.
ev
R

Mechanical Retention - Clasps

Terminal 1/3rd engages undercut. (1)

We need 1mm minimum clearance from gingivae from the tip due to flexion – we want to
avoid gingival trauma.
Consider the material properties: too must flex = distortion or fracture.
Force required to flex the clasps over the max. bulbosities of teeth needs to be >
force attempting to dislodge denture.
Tooth angulation will also influence these factors.
 om
l.c
ta
en
eD

Diagram showing how the slope of a tooth affects retention.

is

Cross - Section
round will flex equally in every direction
ev

half round flexes horizontally more than vertically

R
 om
l.c
ta
en
Diagram showing the differences in flexion of different cross sections of clasps
eD

Length.
longer=more flexible.
Thickness
thicker = less flexible.
is

Curvature must also be considered.

ev

Alloy
remember your ‘stiffness’ and proportional limits from ‘materials + restorative’!
Cast Co-Cr:
R

0.25mm diameter, 15mm min. length (2)

Wrought Wires (SS/Gold):
0.5mm diameter, minimum 7mm length (2)
Types:
Occlusally approaching- molars and premolars.
C clasp
Ring clasp – (when undercut is not in an easy area for C clasp to engage). A ring
clasp should have a rest distally and mesially.
Gingivally approaching - consider aesthetics
I bar
L bar
T bar – typically mandibular canines
Dependent on bony undercuts and sulcus depth
 Only contacts tooth at its tip
Arguments that this is worse for gingival health/root caries
Need min. 4mm sulcus depth (1)
Contraindicated in buccal undercut of 1mm depth/3mm from gingival margin. (1)

om
l.c
ta
en
eD
is

Diagram of a ring clasp.

ev
R
 om
l.c
ta
en
eD

Diagrams of gingivally approaching clasps

Considerations when choosing a clasp - Summary

is
ev

Position of undercut – dependent on your final survey line!

Remember lower molars may be more undercut on the lingual side due to their
inclination- this would be the retentive surface.
R

Health of PDL – consider recession

Shape of sulcus – check for bony undercuts/soft tissue interruptions like frena., 4mm depth
and ridge of gingiva!
Length of clasp
Occlusion

Consider Attachments – will these be required for retention?

One part located in/on abutment tooth + opposing part in denture.
Can incorporate these into crowns
Reciprocation/Bracing

These resists side-to-movement/lateral forces.

Reciprocation is placed opposite to retentive clasp to assist in preventing unwanted
movements/stresses of abutment teeth:
Reciprocating arms provide some resistance but also allow flexure so that the retentive
arm does not get put under too much pressure and break.
Reciprocating clasps still need to engage undercut
Try and encircle the tooth by 180 degrees
Guide planes
Place suitable rigid components of the denture to resist horizontal and vertical forces.

om
RPI system – minor connector on mesial rest of tooth prevents excessive distal movement.
Bracing = general resistance to lateral movements exerted on the denture by tissues:
Maxilla – palate and alveolar ridge can be taken advantage of
Flatter ridge = more movement
Major connector and flanges can be useful to help with bracing – this should be

l.c
incorporated into the design.

ta
en
eD
is
ev
R
R
ev
is
eD
en
ta
l.c
om
Diagram showing bracing.

Indirect Retention

This resists rotation around a ‘fulcrum axis’.

(*fulcrum axis = the line of rotation – it is an imaginary line between the most posterior rest
seats on the end of each arch)
(* clasp axis = imaginary line between clasps on opposite sides of the arch)
When there are multiple clasp axes, use the one closest to the saddle, this is the MAJOR
one.

om
Providing indirect retention moves the fulcrum of movement.
Principles
Retentive clasps should always be between the saddle and indirect retainer
Retentive clasps should be as close to saddle as possible, whilst the indirect retainer is

l.c
as far away from saddle as possible.
Try not to have movements that will force pressure on a tooth/ortho movement.
ta
en
eD
is
ev
R

Diagram showing indirect retention placement in RPD in Maxilla.

 om
l.c
ta
en

Diagram showing indirect retention placement in RPD in Mandible.

eD

Major Connectors
is
ev

The join the components of RPD - joins the saddles.

Want it rigid, but not bulky.
Thickness decreased by half, means the flexibility increases by 8 - this must be
considered.
R

Should not impinge on gingivae and should be OHI friendly.

Think about: function, anatomical constraints, hygiene, rigidity + patient preference.
Wide coverage if maxillary – to spread occlusal load
Keep it away from anterior teeth and incisive papilla for hygiene if possible
Can also provide support and bracing.
Major connector types:
MX:
General rule: 3mm minimum gingival clearance – 6mm ideal (3)
Palatal plate
Leaves gingivae free
Rigid
Simple design
Keep away from gingival margins
 Try and fit to gingivae as much as possible if needs be
Pts may not like ‘bulkiness’
Ring connector
Allows speech function and taste.
Used when saddles are widely distributed across the arch
Palatal tori
15mm between anterior and posterior bar indicated. (3)
MD:
Main problem is distance between lingual gingival margin and functional depth of
floor of mouth (f.o.m).
Sublingual bar
Total clearance required: 5mm (3)
3mm from gingival margin

om
2mm height for the actual bar
4mm thickness for rigidity. .
Dimensions are different- using functional depth of f.o.m- differs from
lingual bar.

l.c
Lingual bar
MUST be 3mm away from gingivae (3)
4mm in height for bar + 2mm thick for tongue
Problem if prominent lingual frenum/mandibular tori present
ta
3mm from gingivae + 4mm height of bar = 7mm total depth!!
Dental bar
When there is insufficient depth for lingual/sublingual bar.
en
Can be just on the teeth and then combined with a lingual bar =
Kennedy bar
Disadvantage: Shows between gaps in between incisors
Needs to be 4mm in height + 1 mm clear of incisal edge + 3mm of
eD

sulcus
= 8mm crown height required (3)
Lingual plate
Avoid if possible- encourages periodontal disease and caries.
is

Covers lingual aspects of teeth + gingivae.

When teeth are poor prognosis and not enough gingival clearance.
ev

Plaque trap
Labial bar
Lingually inclined teeth
R

Swinglock dentures: hinged labial bar – useful when very little

undercut present.
Acrylic
Advantages/Indications = low cost and ease of modification, immediates,
poor prognosis teeth, transitional, young patients in growth.
Careful design as commonly known as ‘gum strippers’.
Be aware of spoon dentures and modified spoon dentures. – These are not
commonly used anymore.
Keep 3mm borders from all gingival margins. (3)
Can add SS wire clasps posteriorly- typically used as stops to prevent distal
drift and for added retention.
Connector has to be bulky to avoid flexure etc- patients may complain about
function (temperatures of food) and speech (lisps).
Flanges can also brace the denture.
Minor Connectors

Connecting components such as rest seats to the main body- either to the saddles or major
connector.
Ensure you have made room for these in the occlusion.

Remember to keep the design as simple as possible.

om
l.c
ta
en

Denture Design Video:

eD

Designing an RPD - Step By Step Demonstration

is
ev
R

Indirect Retention Video:

Short Video Explaining Indirect Retention

RPI System

om
l.c
ta
= Occlusal Rest, distal Guide Plane, Gingivally approaching I bar.
Used to prevent distal movement/tipping of abutment tooth to a free end saddle.
Allows the free saddle to rotate slightly without damaging the soft tissues or abutment tooth.
en
As the denture is pushed into the tissues, it rotates around the mesial rest- the plane and
I bar disengage from the tooth and remove any harmful torque forces.
Need this flexibility and adaptability of this system so that the saddle does not apply
unwanted/ unfavourable force to the tooth.
eD
is
ev
R
 om
l.c
ta
en
eD
is
ev

Diagram of RPI System

R

Narrow occlusal table – reduces load on tissue – useful in free end saddles.
Consider teeth type: can be anatomical, non-anatomical, semi-anatomical, flat
 om
l.c
ta
en
eD
is
ev
R

Tooth Modifications required for Denture Design

Modifying unfavourable survey lines

High survey line- too close to occlusal load
Clasp visible
May interfere with mastication or get caught on the occlusal surface when being placed
in the mouth and engaging.
Create retentive areas
Can build an undercut- thus lowering the survey line- using composite, to avoid these
problems.
These should be planned and put on the denture design sheet.
 Always smooth and polish resulting enamel surface and F- varnish.

Guide Planes

Guide plane = parallel surfaces on abutment teeth which are used to control the POI + add
stability. – these can be prepared if required (artificial) or use any natural ones.
Preparation
3mm vertical height (2-4mm range) (4)
no more than 0.5mm enamel removed (4)
increase stability

om
resists displacement forces in any direction (except path of displacement)
reciprocate
continuous contact with tooth – helps clasp retention
prevent clasp breaking/stretching

l.c
ensure patient inserts and removes the denture along the planned path
improve appearance
can adapt the saddle to fit snug against guide planes- minimising gaps
ta
en
eD
is
ev
R

Diagram showing the dimensions for an artificial guide plane.

Rest Seats

Made for tooth borne support

Must not interfere with occlusion or cause unwanted forces on the tooth
Must be <90 degree slope- imagine hanging of a cliff, you would prefer a shallow slope
than a steep one. It is the same with a rest seat, it should not have any sharp angles for
it to slip off!
Always requires vertical loading.
Avoid using functional cusps in preparation- Buccal lower, Palatal upper
Has to be prepped to be flush with the tooth surface to avoid plaque trapping.
Avoid box cut rest seats as these can promote stress at the sharp angles in the prep

om
1mm thick, 1/3 width and 1/3 length of tooth. (4)
If a clasp is to be added buccally from a rest – the rest may need to be carried in a
channel manner to the buccal surface to allow room for this.
Anterior teeth:
Cingulum often adequate

l.c
A round ended cylindrical bur should be used- the round bur creates undercuts
Should be cut in the axial load of the tooth
Lower incisors: enamel too thin so incisal rests common
ta
Lower canines rest seats may be built with composite – without disturbing the
occlusion.
See image of rest seats in the 'Support' section to understand these principles.
en
eD

Conclusion
You should now be confident with designing RPDs, as well as understanding the fine prints of
measurements etc- these are important!
is

Third Party Links

ev

Davenport J. A clinical guide to removable partial dentures. London: British Dental

R

Association; 2000.

References

1. Davenport JC, Basker RM, Heath JR, Ralph JP, Glantz PO. Retention. British dental journal.
2000 Dec;189(12):646-57.
2. Davenport JC, Basker RM, Heath JR, Ralph JP, Glantz PO, Hammond P. Clasp design. British
dental journal. 2001 Jan;190(2):71-81
3. Davenport JC, Basker RM, Heath JR, Ralph JP, Glantz PO, Hammond P. Connectors. British
dental journal. 2001 Feb;190(4):184-91.
4. Davenport JC, Basker RM, Heath JR, Ralph JP, Glantz PO, Hammond P. Tooth preparation.
British dental journal. 2001 Mar;190(6):288-94.
 Davenport JC, Basker RM, Heath JR, Ralph JP, Glantz PO. A system of design. British dental
journal. 2000 Dec;189(11):586-90.

This content has been written by and uploaded to ReviseDental.com. It is the work of the author and
should not be reproduced without express prior permission from the author through
ReviseDental.com. © Revise Dental. All rights reserved.

om
l.c
ta
en
eD
is
ev
R