RPD DESIGN 1: Saddles + Support Flashcards
List the main denture components (3)
- Saddle - an edentulous area
- Denture base - a connector (connects saddles)
- Flange - replacement tissue extending to the vestibular sulcus
What is the function of the Kennedy Classification?
Describes the number + distribution of saddle (edentulous) areas
Kennedy Class I
Bilateral free-end
Kennedy Class II
Unilateral free-end
Kennedy Class III
Unilateral bounded
Kennedy Class IV
Anterior bounded (crossing the midline)
What are options for resisting movements towards the tissue (2)
- Utility of hard tissue
2. Spread the load over large surface area
Function of
- Tooth support
- Mucosa support
- Tooth Support
- Resists movement
- Directs load through PDL of adjacent teeth - Mucosa support
- Needs large surface area = load distributed over a wide area
Craddock Class I
Tooth support
- Teeth provide a HARD tissue resistance to occlusal loading
Craddock Class II
Mucosa
- A LARGE coverage provides resistance to occlusal loading
Craddock Class III
Tooth + Mucosa
- A combination of HARD tissue and LARGE coverage when there are reduced numbers of teeth and large edentulous saddles
Tooth support/ tooth borne denture advantages (5)
- Transmits load via the PDL membrane
- Feels like the natural dentition
- Is more comfortable for the patient
- Protects the soft tissue from trauma
- Is likely to stay in close contact with its supporting structures over a period of time
Mucosal support disadvantage
Allows the denture base to move slightly
- Possible damage to adjacent gingival margins
What teeth are the best for providing support?
MOLARS 2.7
- as they have the largest root area they can carry the greatest load
How much load can a healthy tooth carry?
A healthy tooth can carry its own load plus 1 and a half similar teeth
How is tooth support provided on the surface of adjacent abutment teeth?
Through metal alloy rests
Define rests
Provide support for the denture from vertical opposing forces
Why is incorporating rests into acrylic resin a problem?
Due to low base strength, placing a metal component potentially weakens the acrylic surrounding it
What are the 3 types of tooth rests?
- Occlusal
- Cingulum
- Incisal
Where are incisal rests most commonly used?
Lower anterior teeth due to aesthetic issues
Disadvantage of a small occlusal rest
If theres a small rest = small focus on load, which could have an adverse effect on tooth and tip it anteriorly
Advantage of a large occlusal rest
Recommended as this can direct the force down the long axis
Where are cingulum rests most commonly used?
Used mostly on maxillary and mandibular canines, but can be used on maxillary incisors
Advantages of cingulum rests (3)
- Aesthetically superior to incisal rests
- Apply stress at lower level - less rotation forces
- Less likely to break
What are the 2 possible materials for the denture
- Metal framework base (Co Cr)
2. PMMA
Advantage of Metal framework base
Rigid and strong
Properties of PMMA
- Flexible (not rigid) and poor strength
2. Can be used when opposing forces are light e.g. complete denture
State advantages of rests (3)
- Prevent movement of an RPD towards the mucosa
- Assist in distribution of occlusal load
- Prevent over-eruption of unopposed teeth
What is the exception to the rest placement when it comes to a free-end saddle?
A rest is NOT placed immediately adjacent to the saddle
Placed either mesially or distally
Rest placement for tooth supported bases?
The support should be on the tooth surface nearest to the base
Rest placement for tooth and mucosa supported bases?
The support should be on the tooth surface which is NOT next to the base
Why should occlusal rests NOT be placed in an occlusal centric stop?
The denture will be uncomfortable and the rest will interfere with the occlusion
Disadvantages of rest seats (3)
- Loss of occlusal stop when denture is not worn
- Destruction of tooth surface
- Exposure of dentine
What is mucosal support dependent on?
As large an area possible of mucosa being covered
Why should the base not be placed within 3mm of the gingival margins?
As it will place pressure at the gingival margin
Function of a wire stop?
To ensure the most distal tooth in an every design does not drift posteriorly
What is the less harmful denture design called?
‘Every’ denture design
Less harmful mucosa borne denture
Why are mandibular mucosa supported dentures not recommended?
There is insufficient area to provide support
How does a hard tissue provide denture support?
Resists movement
How does large tissue coverage provide denture support?
Distributes load over a wide area
What Craddock classification is used for a free end saddle?
Class III
Tooth + Mucosa borne
What Craddock classification is generally used for bounded saddles?
Tooth borne
When would a mucosa borne denture be used
Only if there are insufficient teeth for a tooth borne denture
What Craddock classification is generally used for acrylic dentures?
Class II
What is used to make a denture tooth borne?
OCCLUSAL RESTS
So there isn’t loading onto the bone below
Root area provides wide distribution of load
How does tooth support transmit load in a denture?
Periodontal membrane
How does mucosa support transmit load in a denture?
Has to cover as large an area as possible- reduction of the occlusal table
Why should slight movement of the denture base in mucosa borne dentures be avoided?
Possible damage to adjacent gingival margins
Give the problem with tooth extraction with reference to tooth support?
Periodontal area lost in extraction:
Left with mucous membrane which has 4x less coverage than the periodontal area
NOT AS GOOD FOR SUPPORT
What material is used for rests?
Metal
What material is used for acrylic resin rests?
Wrought stainless steel
When would a ledge usually be prepared for a rest?
In crown replacement
When would a ring rest be recommended? (2)
- Single standing teeth
- If the occlusion prevents occlusal rest
Why are ring rests usually avoided? (3)
- The ring is above the survey line but below the occlusal surface
- Could impact on occlusal surface
- Unaesthetic
Where do incisal/ occlusal rests extended from?
Denture base framework
Disadvantages of incisal rests (3)
- Poor aesthetics
- May interfere with incisal occlusion
- Not recommended on wear facets
Disadvantages of a cingulum rest
If theres no prominent cingulum present tooth prep is required
Advantages of a CoCr metal base framework (2)
- Rigid and strong
2. Single solid piece
How is the metal framework base produced?
By casting via a ‘lost wax technique’
Disadvantages of a PMMA (acrylic resin) denture base? (2)
- Flexible and poor strength
2. Any load resisted by rests through the denture base place an internal stress on the base material
When could PMMA denture bases be used?
When opposing forces are light e.g. complete denture
Why are rests often placed on the occlusal surface of a tooth?
To direct occlusal forces along the long axis of the abutment teeth
How is the problem of compressibility of soft tissue with mucosa borne dentures overcome?
Base placed within 3mm of the gingival margins as this will place pressure at the tooth and tissue gingival attachment
Disadvantages of every partial design (2)
- Can produce narrow and weak denture base areas
2. Metal inserted into acrylic causes an inherent weakness in the denture base
Give the vertical displacement of tooth attachment within its socket (periodontium).
0.1mm
Give the compression of mucosal covering over alveolar bone (mucoperiosteum).
2mm
What is the problem with tooth and mucosa dentures?
Mucosa compresses more than the tooth within its periodontal attachment
How is the load on teeth involved in tooth and mucosa support reduced?
- RPI
2. Altered cast technique
When would bounded saddles not be tooth supported?
If the saddles are longer than 3 teeth
