RPD lab session Flashcards
What is an RPD?
A removable appliance which replaces one or more missing teeth but not the entire arch
Distinct from a bridge which is fixed
Mucosa borne RPDs
Made primarily from acrylic Metal components may be included for strength or clasping Relatively cheap Can be easily modified or added to Transitional denture
Tooth borne RPDs
Metal Framework
Made from rigid metal (Cobalt Chromium)
Teeth attached with acrylic or composite
Can only be purely tooth borne for bounded saddles
Strong and transmit load well
Complex, expensive, provided where oral health is good and stable
Can not be modified*
Tooth and mucosa borne
Metal Framework (CoCr0
Acrylic forms the fitting surface and provides support in the saddle area
Complex, expensive, provided where OH is good and stable
Most challenging RPDs
Classification by support
Craddock and Beckett
- Mucosa borne RPD: support gained from occlusal, cingulum or incisal rests
- Tooth borne RPD
- Tooth/ mucosa borne RPD: support gained from occlusal, incisal or cingulum rests and from the mucosa
Requirements of an RPD
Aesthetics Mastication Comfort Distribute occlusal forces to appropriate structures Retentive Space maintenance OVD maintenance
Advantages/ clinical indications of an RPD
Aesthetics Function -speech and mastication (SDA) Tooth movement prevention Maintenance of OVD
Disadvantages of RPDs
Tooth loss greater when RPD is provided rather than other methods of tooth replacement
> plaque accumulation
Caries, gingivitis, periodontal
Tooth movement*
Design dependent?
Damage to tooth tissue
Forces on teeth may impact on supporting structures
Clinical need vs pt demand
Aesthetics
RPDs replacing anterior teeth are more likely to be used
Those replacing posterior teeth only are often discarded
Preparing to provide an RPD
Medical history: Can the patient handle a RPD?
Presenting complaint: Aesthetic or functional?
Dental history:
-is the patient suitable for an RPD?
-does pt want and need an RPD?
-is the RPD likely to be destructive?
-is there a clinical need to make provision for future treatment?
Study models:
-edentulous areas: Size and position
-undercut: for retention (POD, POI & Clasping)
-occlusion (may need to mount on an articulator, therefore may need registration rims producing)
Components when designing an RPD
Saddles: number and extent
Support
-tooth/ tissue borne
-extent of connectors and saddle for mucosa borne
-occlusal rests for tooth borne
Retention
-physical, muscular, mechanical retentive forces
-surveying model for undercut, may be used relative to path of displacement
-path of insertion different to path of displacement?
-design options for clasps
Reciprocation for each clasp
Bracing: resistance to lateral movements
Connectors: design criteria and options for connectors
Indirect retention: for free end saddles
Classification by pattern of tooth loss
Kennedy Class I-IV with modifications
Kennedy Class I
Bilateral edentulous areas located posterior to the remaining natural teeth
Kennedy Class II
Unilateral edentulous area located posterior to remaining natural teeth
Kennedy Class III
A unilateral edentulous area with natural teeth remaining both anterior and posterior to it
Kennedy Class IV
A single, but bilateral (crossing the midline), edentulous area located anterior to the remaining natural teeth
Designing saddles
Which teeth need to be replaced
Can I reduce the occlusal table by using fewer or narrower teeth
Designing support
Resistance to movement towards the mucosa
Mucosa, tooth, or mucosa/ tooth borne
Designing support: mucosa borne
Make the footprint of the denture large to spread the load over a wide area
The hard palate generally provides adequate support for mucosa borne dentures to be used
Where possible extend the denture base to the maximum denture bearing area even if only a few teeth are being replaced
Designing support: tooth borne
Bounded Saddles
Position occlusal rests both sides of the saddle areas
Keep them as close to the saddle area as possible.
This ensures the load is transmitted from the saddle area to the tooth efficiently
Metal framework is rigid
Occlusal,Cingulum and Incisal rests need rest seats.
Occlusion
Direct loading down the long axis of the tooth
Overdenture - prep tooth
Designing support: tooth and mucosa borne
Free-end Saddles
Position the occlusal rest distant to the saddle areas
-this ensures load is transmitted down long axis of tooth when denture rotates
-also ensures clasps disengage from undercut under load when denture moves
Designing retention
Physical Forces: same as complete dentures
Muscular Forces: Same as complete dentures
Mechanical Forces:
-path of insertion
-clasping
Designing mechanical retention: path of insertion
-question you must ask
Can the path of insertion be altered such that it differs to the path of displacement?
Designing mechanical retention: clasping
-questions you must ask
Are the abutment teeth sound?
Is there enough undercut for the clasp to engage?
Designing retention: model surveyor
Parallelometer Holds tools in one plane Adjustable table Selection of tools -analysing Rod -pencil lead -chisel -measuring gauges
Designing retention: model surveying function
Shows the undercut areas relative to the path of displacement
Allows the survey line to be recorded on the study model
Allows assessment for a path of insertion
Allows planning for the position clasp arms
Engaging too much undercut may cause trauma to the tooth or cause the clasp to fracture
Finding areas on the soft tissue that may be used for added retention
To create guide planes
-on wax patterns of crowns to coincide with the path of insertion
-improve fit of the appliance and aid reciprocation
Mechanical Retention: Creating a Path of Insertion
Undercut relative to the path of displacement
- block-out undercuts relative to path of insertion prior to denture construction
- path of insertion does not = path of displacement
Undercut depth for clasps
Cobalt Chromium: 0.25mm
Gold: 0.5mm
Stainless steel: 0.75mm
Designing reciprocation
Clasps will always put a sideways load on teeth during function. This is bad.
Provision should always be made to oppose this force with a reciprocating component
Make reciprocating arms effective by using in combination with guide planes
Designing bracing
Resistance to lateral movements
Usually provided by other components
e.g. cross arch bracing
Designing connectors
Major and minor
Connectors get blended into components such that there is no ‘junction’
Connectors also contribute to: support, bracing, direct retention, indirect retention
Should finish 3mm from gingival margin or above the survey line, not in between the twoNeed to be robust enough to withstand use
Can be challenging to provide lower lingual connectors of adequate cross sectional area
Major connectors
link saddle areas and create rigidity of denture
Minro connectors
connect small components to the major connector
Requirements of a connector
Comfort Strength Support Retention Hygiene
Maxillary connectors: ring, skeletal or open design
WEIGH UP Comfort Simplicity Strength Support Retention Hygiene
Types of mandibular connector
Lingual bar Lingual plate Sub-lingual bar Dental bar or continuous clasp Kennedy bar
Designing indirect retention
This applies to free-end saddles to aid retention by ensuring the clasps work effectively
Occlusal rests are used to ensure ‘axis of rotation’ on displacement keeps the clasps moving along the long axis of the abutment tooth
Prep work for an RPD
Rest Seats
Guide planes
Creating undercut
Direct and indirect restorations
Spoon denture
Used as a temporary denture
Can have a modified spoon
Acrylic denture to replace one or two anterior teeth, usually the lateral incisor
Every dentures
Borders 3mm form margins
Open design at saddle/tooth junction
Point contacts between abutment and artificial
Flanges included to provide bracing
Posterior wire ‘stops’ to prevent posterior drift and loss of contact
Lateral stresses reduced by as much balanced occlusion as possible
RPI system components
Combination of
- occlusal rest (R)
- distal guide plane (P)
- gingivally approaching I-bar (I)
How the RPI system works
Used for Free end saddle cases
Minor connector and distal plate together act to provide reciproaction to the I-bar.
The I-bar should be on or anterior to the midpoint of the buccal surface
Path of insertion
Mechanical retention relative to the path of displacement
-consider creating guide planes to restrict the path of insertion, particularly when providing mucosa borne RPDs
Recording a path of insertion
Mark on the model
- 3 marks with lead at fixed height
- scribe side of model
Shortened dental arch
A reduced dentition may be adequate for many patients
It is more important to maintain the teeth that remain rather than replacing the missing ones
However, in the context of having a functional occlusion it may be less important than the number of teeth that occlude with each other
Shortened dental arch definition WHO
20 natural teeth throughout adulthood
Why consider SDA?
RPDs that replace molar teeth may not provide the patient with any immediate masticatory benefit
RPDs often require the patient to learn new skills to effectively use the appliance
A high standard of oral hygiene is required by the patient if further tooth loss is to be avoided.
Patients may find this harder to achieve with an RPD
Approximately 50% of all free-end saddle RPDs are not worn
Disadvantages of SDA
Increased risk of anterior tooth wear RPD provision in future may be more challenging -fewer teeth -tongue space -controlling a larger denture
Flexible dentures
Valplast
CoCr RPD/ onlay
Where space for acrylic teeth is limited.
Where increase in OVD
Telescopic crowns
Primary crown anchored to tooth made from suitable gold alloy
Telescopic crown attached to denture made from the same alloy
RPDs: roles and responsibilities of dentist
Assess pt Treatment plan Prep work Tooth prep Design partial denture Prescribe RPD Monitor denture in Changing oral environment
RPDs: roles and responsibilities of technician
Treatment plan with dentist
Design RPD with dentist
Construct RPD to prescription
RPDs: roles and responsibilities of clinical dental technician
Assess pt
Tx plan with dentist
Design RPD with dentist
Construct RPD to prescription
Amend detail but not direction of tx plan/ prescription
Monitor denture in changing oral environment
Communication between dentist/ technician/ clinical dental technician
Study models Written description of design Drawn prescription on paper or models Photographs Use recognised terms and of partially dentate and component parts
Steps: provision of RPDs at CCDH
- Study Casts
- Clinical Summary Sheet
- Provisional Design with DP/DW
- Definitive Design with Clinical Tutor
- Book job in with Production Lab
Study casts: edentulous areas
Size and position
Which are going to be saddle areas?
Is there room for teeth?
Is tooth preparation necessary to enlarge the gap?
Study casts: undercut assessment
For POI & POD
For clasping
For reciprocation
Study casts: occlusion
Is there room for the occlusal rest?
What will happen in excursion?
Is the ICP and OVD acceptable/ workable?
Study casts: questions to ask
Edentulous areas
Undercut assessment
Occlusion
Should study casts be mounted on an articulator in order to tx plan?
Do you need registration rims to do this?
Combination work
Study casts: combination work
Are any direct or indirect restorations being placed as part of the prep work?
Is a diagnostic wax-up required to plan these?
Clinical summary sheet
Designed to ensure you consider all the factors relevant to the provision of an RPD
Complete prior to requesting a provisional design
Definitive design
Transfer your approved provisional design to the Lab Card
Get a Clinical Tutors signature
Book in with the lab
The whole job gets booked in:
Trays, blocks, framework, try-in etc
Writing your prescription: draw your design including
Extension of saddles and the number and size of teeth
Position of occlusal rests
Clasp types on which teeth and corresponding reciprocation
Type of connectors and their extension
Colour of the acrylic base where important for aesthetics
Metal framework RPDs
Most commonly cast Co-Cr
Historically Au
Ti alloy is available
SLM production is rapidly developing
Fitting an RPD
Check the undercut first.
Remember blocking-out undercut on the model is necessary to provide a useable denture
When do we work to centric relation?
Complete dentures
Vertical dimension
Unstable ICP
Primary imps
For very simple acrylic dentures replacing 1 or 2 teeth a good primary impression may be all that you require
Tooth prep and secondary imps
If rest seats are required make sure these are cut to appropriate dimensions
There needs to be enough space for the rest without it affecting the occlusion
Don’t forget to leave enough space for the minor connectors or clasp arms
Ensure that rest seat preparations are picked up well by the secondary impression
Framework try-in
First check the metal framework fits well on the cast and is fully seated
Transfer to the mouth and ensure that the framework fully seats. Pay particular attention to the rest seats
Check the occlusion:
-if conforming to current ICP make sure the occlusion is not altered
If required, adjust until the framework seats fully and doesn’t interfere with the occlusion
Wax reg stage
This will be on a wax, shellac or acrylic base for acrylic dentures
This should be on the metal framework for a cobalt-chrome denture
Confirm:
-occlusal relationship
-tooth position
-incisal plane
Wax try in stage
Ensure the occlusion is identical to the wax registration
If there are discrepancies make any adjustments at this stage
Fit stage
Check there have been no changes to the occlusion from the wax try-in stage
Make any adjustments as required
Check record
Metal backing
Very occasionally you may be providing a co/cr denture with metal backings
If so, you will need to do a tooth try-in at this stage to tell the technician where the denture teeth (and therefore the metal backings need to be)
If so, this adds a further visit to the denture construction process
