RPD design Flashcards
What is a Kennedy class I?
- Bilateral free end saddle
What is a Kennedy class II?
- Unilateral free end saddle
What is a Kennedy class III?
- Unilateral bounded saddle posteriorly
What is a Kennedy class IV?
- Anterior bounded saddle
How do you include mod in Kennedy class? Use with an example of Kennedy Class III mod 1?
- Work form the most posterior part of mouth to identify which kennedy class fits the pt
- Then any additional (smaller) gaps = no of mods
Picture is K class III mod 1 as has unilateral posterior bounded saddle with 1 more saddle
What is the checklist of things you need to include when designing your RPD?
- Saddles (What teeth need to be replaced)
- Support (resisting movement towards soft tissues)
- Retention (resistance of movement away from soft tissues)
- Reciprocation/bracing (resistance of side to side movement / lateral forces)
- Indirect retention
- Major connectors
- Minor connectors
What is meant by support? What are the two types?
- Resistance to movement towards soft tissues
- Can be tissue borne or tooth borne
How can support be provided by mucosa born source?
- Extends acrylic to full functional sulcus depth
How can support be provided by tooth borne source?
- Occlusal rest seat
- Cingulum rest seat
- Incisal rest seat
Where are rest seats usually placed?
- Placed on the tooth surface nearest the saddle on either side
- Mesially if part of RPI system (furthest side of tooth from saddle)
- Can also be placed on crowns of abutment teeth if planned correctly and in advance
Why do we want tooth borne support?
- Additional benefits of proprioception from the PDL of the abutment teeth
- Less damage to the gingivae
What can occur if the rest seat is not placed so that it is flush with the tooth surface?
- Plaque trap
- Prop open the bite leading forces in wrong direction
What is the meaning of retention? How can is be achieved?
- Resistance of movement away from soft tissue (prevent denture being dislodged by sticky toffee)
- Mechanical or Muscular (cheeks)
- Can also be adhesive forces (saliva suction seal)
How is mechanical retention achieved? What requirements do we need?
- Clasps that engage a 0.25mm undercut on tooth
- Terminal 1/3rd engages undercut
- Need 1mm minimum clearance from gingiva from the tip of clasp due to flexion (want to avoid gingival trauma)
What can too much flexion of a clasp cause ? How can this be avoided?
- Distortion of fracture
- Choose correct material as the force required to flex the clasp over maximum bulbosities of teeth needs to be > force attempting to dislodge denture
What is the diamater and min length needed of Cast co-cr alloy for
a clasp?
- 0.25mm diameter
- 15mm min length
What types of clasps are there ans what are they used for?
Occlusally approaching clasp for molars and premolars
- C clasp
- Ring clasp (when undercut not in easy area for C clasp to engage) Needs rest distally and mesially
Gingivally approaching clasp (aesthetics)
- I bar
- L bar
- T bar (mandibular canines)
But need 4mm sulcus depth and only contacts tooth at tip
Contraindicated in buccal undercut of 1mm depth / 3mm from gingival margin
What is reciprocation?
- Resistance of side to side movement
How is reciprocation achieved?
- Placed opposite to retentive clasp to assist in preventing unwanted movements/ stresses of abutment teeth
Done by
- Reciprocating arms provide some resistance and allow flexion so retentive arm doesn’t get too much pressure and break
- Reciprocating clasp still need to engage undercuts
- try and circle tooth by 180degrees
What is bracing? What can you use to help with this?
- General resistance to lateral movements exerted on dentures by
- Maxilla
- Flatter ridge (more movement)
- Major connector and flanges
What is indirect retention?
- Resistance to rotation around a fulcrum axis
- Providing indirect retention moves the fulcrum of movement
What is a fulcrum axis?
- line of rotation
- Imaginary line between most post rest seats on the end of each arch
What is the clasp axis?
- Imaginary line between clasps on opposite sides of arch
- When multiple use one closest to saddle as major one
What are the principles of indirect retention?
- 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 as far away from saddle as possible.
- Try not to have movements that will force pressure on a tooth/ortho movement.
What do minor connectors do?
- Connect the components such as rest seats to main body either saddle or major connector
What do major connectors do? What do we want the connector to be? What do we not want the connector to be?
- Join the components of RPD
Want and don’t want
- Rigid not bulky
- OHI friendly
- Not impinge on gingivae
- Wide coverage if maxillary to spread occlusal load
- Keep away from ant teeth and incisive papilla
What are the maxillary major connector types?
Palatal plate
- Leaves gingiva free
- Rigid
- Simple design
- Keep away gingival margins
- but pts may not like bulkiness
Ring connector
- Allows speech function and taste
- Used when saddles widely distributed in arch
- Palatal tori
- 15mm between anterior and posterior
Mandibular major connectors ? pros and cons
Sublingual bar
- Need 3mm gingival margin clearance and 2mm actual hieght of bar (5mm total clearance)
- 4mm thickness for rigidity
Lingual bar
- Must be 3mm from gingivae
- 4mm height bar and 2mm thick for tongue (tot 7mm depth needed )
- Problem if prominent lingual frenum or mandibular tori
Lingual plate
- Avoid if possible- encourages periodontal disease and caries.
- Covers lingual aspects of teeth + gingivae.
- When teeth are poor prognosis and not enough gingival clearance.
- Plaque trap
Labial bar
- Lingually inclined teeth
- 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.
What is an RPI system and why is used?
- Occlusal Rest, distal Guide Plane, Gingivally approaching I bar
- Used to prevent distal movement/tipping of abutment tooth to a free end saddle.
How does an RPI work?
- Allows the free saddle to rotate slightly without damaging the soft tissues or abutment tooth.
- 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.
Why might you modify 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.
What are guide planes?
- Parallel surfaces cut by clinician on abutment teeth which are used to control the POI + add stability
What are the advanatges of guide planes?
- Increase stability as it resists displacement forces in any direction (except path of displacement)
reciprocate - Continuous contact with tooth – helps clasp retention
prevent clasp breaking/stretching
ensure patient inserts and removes the denture along the planned path - Improve appearance as can adapt the saddle to fit snug against guide planes- minimising gaps
What are the requirements for a rest seat?
- Not interfere with occlusion or cause unwanted forces on tooth
- <90 degrees slope
- Need vertical loading
- Flush with tooth to avoid plaque trap
- 1mm thick , 1/3width and 1/3 length of tooth
What is meant by path of insertion?
- A change to path of displacement the improves retention
