4. RPD Design - Support Flashcards
Definition of saddle
Edentulous area
Definition of flange
Replacement tissue, extending from the gingival margin to the vestibular sulcus
Definition of denture base
Connector
Definition of support (2)
Term used to describe the resistance of a denture to occlusally directed load
Resistance to vertical force directed towards the mucosa
Options for resisting movement towards tissue (2)
Utility of hard tissue
Large surface coverage
Function of utilising hard tissue
Prevents/resists movement
Function of large surface coverage
Distributes load over a wide area
You may have to utilise soft tissues when spreading the load
Definition of Kennedy classification
Anatomical classification that describes the number and distribution of edentulous areas present
It does not describe the type of support required
Types and explanations of Kennedy classification (4)
Class I - bilateral free-end saddle
Class II - unilateral free-end saddle
Class III - unilateral bounded saddle
Class IV - anterior bounded saddle (crossing the midline)
A modification to each saddle can be made depending on the number of additional edentulous saddles (class IV cannot be modified)
Most posterior saddle defines the classification
Definition of Craddock classification
Provides a simple classification based on support for a removal partial denture
It only gives the type of support and does not tell anything about the number or distribution of teeth
Types of Craddock classification (3)
Class I - tooth (teeth provide a hard tissue resistance to occlusal loading)
Class II - mucosa (a large coverage provides resistance to occlusal loading)
Class III - tooth and mucosa (a combination of hard tissue and large coverage when there are reduced number of teeth and large edentulous saddles)
Principles of hard surface and load distribution (3)
Class 1 - teeth embedded within bone (periodontium) with root area providing a wide distribution of load (hard)
Class 2 - showing a small area for distributing load, therefore not the best choice for support
Class 3 - the edentulous area is known as a free-end saddle. Limited number of teeth mean that mucosal coverage with a wide distribution is required
Features of tooth support (5)
Transmits load via the periodontal membrane
Allows the supported denture base to fell 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
Features of mucosal support (2)
Must cover as large an area as possible (reduction of occlusal table)
Allows the denture base to move slightly (possible damage to adjacent gingival margins)
Features of a healthy tooth load (2)
Potentially its own load plus one and a half similar teeth
The best teeth for providing support are those with the largest root area (depending on the health of the periodontal attachment)
Definition of rests (2)
Components which provide support for the denture from vertical opposing forces
Described by the part of the tooth they contact and are metal
Function of rests
Oppose movement of the base towards the tissue
Features of rests (4)
Best used as part of a cast metal base alloy
Wrought stainless steel can also be used for acrylic dentures
Can be incorporated into acrylic bases (can present a problem due to low base strength)
Types of rests - occlusal, cingulum, incisor, only, overlay, ledge, ring
Features of rests in CoCr metal framework bases (3)
Rigid and strong
All rests are integral to the base (connected to the major connector directly or by minor connectors)
The base is produced by casting using a ‘lost wax technique’ – more extensive technique
Features of rests in PMMA acrylic resin bases (3)
Flexible (not rigid) and poor strength
Rests are incorporated mechanically into (within) the base
Any load resisted by rests through the denture base place an internal stress on the base material (rests for support within an acrylic base are the exception to the rule although the can be used when opposing forces are light (complete denture))
Functions of rests (7)
Prevent movement of an RPD towards the mucosa
Assist in distribution of occlusal load
Direct retentive elements to work in a planned manner
Prevent over-eruption of unopposed teeth
Provide bracing on anterior teeth
Determine the axis of rotation for free-end saddle RPDs
Involvement in reciprocation and indirect retention
Purpose of a rest
Support the denture against vertical load (directed towards the tissue)
Rests are often placed in the occlusal surface of a tooth since this will direct occlusal forces down the long axis (root) of the abutment teeth
Default rest position for bounded saddle
Immediately adjacent to saddle
Default position for free-end saddle
Not immediately adjacent to saddle (mesial of tooth adjacent to saddle)
Position of rests depends on the type of support of the base (2)
Tooth supported base - support should be on the tooth surface nearest to the base
Tooth and mucosa supported base - support should be on the tooth surface which is not next to the base
Considerations for occlusal rests (2)
Avoid placing a rest in an occlusal stop
Rest seats can be prepared
Possible consequences of preparing rest seats (3)
Loss of occlusal stop when denture is not worn
Destruction of tooth surface
Exposure of dentine
Why should occlusal rests not be placed in an occlusal stop (2)
The denture will be uncomfortable
The rest will interfere with the occlusion
For mucosal support, the gingival margins should be kept clear by
Minimum of 3mm because soft tissue is compressible
Features of Every partial denture design (6)
Mucosa borne denture
Restores dental arch
Contact points between denture and abutment teeth
Wire stop posterior to last standing tooth
Gingival magins not covered
Metal into acrylic causes inherent weakness in denture base
Function of wire stop posterior to last standing tooth
Ensure the most distal tooth does not drift posteriorly
Compressibility of periodontium and mucoperiosteum using a tooth and mucosa support based denture (2)
Periodontium - 0.1mm
Mucoperiosteum - 2mm
Problem with tooth and mucosa support based denture
Mucosa compresses more than the tooth within its periodontal attachment
Solutions to problems with tooth and mucosa support based denture (2)
Stress relieving clasp system (RPI)
Using altered cast technique
Features of tooth support (5)
Periodontal membrane for support is more than 4 times the mucosal coverage for a lost tooth
A healthy tooth can potentially carry its own load when thinking about whether a tooth is suitable for support
Think of the root to crown ratio when thinking about whether a tooth is suitable for support
Tooth support is provided by metal alloy rests on the surface of adjacent abutment teeth
The rest should transfer load through the long axis of the tooth
First choice of support for removable partial dentures (2)
Tooth support
Bounded saddle cases are tooth supported unless saddles are longer than three teeth
Rests and long axis of tooth
Rests must direct occlusal forces down through the long axis (root) of the abutment tooth
Benefit of Every design over mucosa-borne denture
Less harmful
Why are mandibular mucosa supported dentures not generally recommended
There is insufficient area to provide support unless this denture is being used as a training appliance in preparation for a complete denture or overdenture
