RPD DESIGN 2: Retention Flashcards
Colour code for saddles
Yellow
Colour code for support
Red
Colour code for retention
Green
Colour code for bracing + reciprocation
Blue
Colour code for connector
Black
Definition of retention
The dentures resistance to vertical dislodging forces away from the tissue
Stops denture falling out
What 3 things help achieve retention?
- Mechanical means
- Muscular means
- Physical forces
How can retention be achieved through
- Mechanical
- Muscular forces
- Physical forces
- Mechanical - clasps, guide planes
- Muscular forces - shape of the denture
- Physical forces - Coverage of the mucosa
Classification of DIRECT retention
Resistance to vertical displacement of denture
Classification of INDIRECT retention
Resistance to rotational displacement of denture
What is a form of direct retention that provides an extra form of retention?
Guide planes:
> Carve tooth to make it parallel with the fake teeth
> When a clasp tips is exhausted, the guide planes provide longer term retention
What is a clasp?
A metal arm that when in position contacts the tooth preventing removal of the denture base
On what part of the tooth is a clasp placed?
Below the bulbous part of the tooth (the undercut)
How are undercuts identified?
Surveying the casts
What material can clasps be made in?
Clasps can be made in wrought metal and incorporated into the denture base
Or be included as part of a cast denture base
Disadvantages of a gingival approaching clasp?
They cross the gingival margin (which may create a bridge for infection - food can accumulate which is a potential for breeding + infection)
What teeth use a gingival approach for the clasps?
Anteriors + canines as its the only way of guaranteeing you will achieve 50mm in length
What are the different types of clasp options? (2)
Clasps approach the undercut from either:
- Occlusally approaching
- Gingivally approaching
Define reciprocation
Each clasp unit will have an opposing element (reciprocal clasp arm) to the retentive clasp arm to prevent pressure from the clasp acting on the tooth
Function of survey lines (3)
Determine the position of:
- The clasps and the retentive areas
- The reciprocal arm
- The guide planes and proximal plates
What is the path of insertion?
What does it allow?
- The direction in which a denture is placed or removed from the supporting tissues or abutment teeth
- Path of I gives anterior retention without need of clasps
What is the path of displacement?
The direction in which the denture is dislodged from its position during function
Function of RPI
Used in free end saddle designs to prevent stress on the last abutment tooth
What are the components of an RPI? (3)
Mesial rest:
Proximal plate (adjacent to saddle):
- Guide surface of 2-3mm
- Undercut to permit movement
Gingival approaching I-bar clasp:
Name examples of indirect retainers (5)
- Major connectors
- Minor connectors
- Rest
- Saddle
- Denture base
Name an example of direct retention
Clasps
Factors affecting retention (2)
> Size of the angle of cervical convergence
> How far the clasp terminal is placed into the angle of the cervical convergence
How is clasp flexibility achieved?
- The longer the clasp arm the more flexible
- By increasing the length, horizontal stresses to the abutment are reduced
Disadvantages of a clasp (2)
- Increases tooth coverage
2. Compromised aesthetics
Advantages a rest (2)
- Maintains the position of the clasp in relation to the abutment
- Transmits forces along the long axis of the abutments
What Kennedy classes require an I bar? (RPI)
1 + 2
What Kennedy classes require an indirect retainer?
1, 2 + 4
What is the desired pattern of retention shape?
Triangular to provide stability
Abutment modifications
- Lower heights of contour (preparing guide planes = most effective way to do this)
- Create undercuts if necessary (raising heights of contour by adding composite)
Describe how physical retention of a denture is achieved
Using forces of adhesion, cohesion and surface tension
How do adhesion forces physically retain a denture?
Surface forces of saliva on denture and mucosa
How do cohesion forces physically retain a denture?
Forces within saliva, viscosity
Describe the position of direct retention
> Close to the base
> Adjacent to the abutment teeth
Describe the position of guide planes
> Close to the base
> Parallel to the path of insertion
Where is a clasp placed?
Below undercut
When do clasps work most efficiently?
When used along with a rest (supporting component)
Give the name for a gingivally approaching clasp
I bar clasp
List the types of occlusally approaching clasps (3)
- Single arm clasp
- Circumferential clasp
- Ring clasp
When can a single arm clasp be used?
Upper molar
> Distobuccal
> Distopalatal undercut
What is a circumferential clasp?
Single arm with a reciprocal arm
When can a ring clasp be used?
Lower molar
> Mesiobuccal
> Mesiolingual undercut
What does clasp efficiency depend on? (3)
- Clasp material
- Cross-sectional thickness and shape
- Length of the clasp
Give a non-metal material that can be used for clasps
Acetal resin
Advantages of acetal resin as a clasp material (2)
- Appearance
2. Engages deep undercuts
Disadvantages of acetal resin as a clasp material
- Bulkier
- Non-adjustable
- Cost
Metal clasp length required toto engage a 0.25mm undercut?
15mm
Exception for when an occlusally approaching clasp is used for a premolar?
When the tooth is larger than usual
List the information required to position a clasp (4)
- The position of the survey line
- The type of abutment tooth
- Clasp length (15mm for cast Co-Cr)
- Type of material- cast or wrought
How does reciprocation work to prevent the tooth moving? (2)
- The retentive element of the clasp flexes over the undercut
- The reciprocal element remains in contact preventing the tooth being moved horizontally by the clasp flexing.
How can the efficiency of guide planes be maximised?
By making the guide planes as large as possible
Benefits of an altered path of insertion (3)
- Avoids using anterior clasps
- Can provide frictional resistance retention
- Denture sitting over undercut is a food trap
State the minimum number of clasps that should be used for a denture?
2 - one on each side
List some factors that affects the placement of clasps
- Aesthetics
2. Saddle length
How can retention be created if there are no undercuts?
TOOTH PREP
Addition of composite to create an undercut
Is retention required on every adjacent tooth to the saddle?
No
When is RPI mostly used?
Mostly in posterior mandibular cases, but can be present in maxillary arch too
