4. RPD Design - Support

Question 1

Definition of saddle

Answer 1

Edentulous area

Question 2

Definition of flange

Answer 2

Replacement tissue, extending from the gingival margin to the vestibular sulcus

Question 3

Definition of denture base

Answer 3

Connector

Question 4

Definition of support (2)

Answer 4

Term used to describe the resistance of a denture to occlusally directed load
Resistance to vertical force directed towards the mucosa

Question 5

Options for resisting movement towards tissue (2)

Answer 5

Utility of hard tissue

Large surface coverage

Question 6

Function of utilising hard tissue

Answer 6

Prevents/resists movement

Question 7

Function of large surface coverage

Answer 7

Distributes load over a wide area

You may have to utilise soft tissues when spreading the load

Question 8

Definition of Kennedy classification

Answer 8

Anatomical classification that describes the number and distribution of edentulous areas present
It does not describe the type of support required

Question 9

Types and explanations of Kennedy classification (4)

Answer 9

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

Question 10

Definition of Craddock classification

Answer 10

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

Question 11

Types of Craddock classification (3)

Answer 11

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)

Question 12

Principles of hard surface and load distribution (3)

Answer 12

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

Question 13

Features of tooth support (5)

Answer 13

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

Question 14

Features of mucosal support (2)

Answer 14

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)

Question 15

Features of a healthy tooth load (2)

Answer 15

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)

Question 16

Definition of rests (2)

Answer 16

Components which provide support for the denture from vertical opposing forces
Described by the part of the tooth they contact and are metal

Question 17

Function of rests

Answer 17

Oppose movement of the base towards the tissue

Question 18

Features of rests (4)

Answer 18

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

Question 19

Features of rests in CoCr metal framework bases (3)

Answer 19

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

Question 20

Features of rests in PMMA acrylic resin bases (3)

Answer 20

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))

Question 21

Functions of rests (7)

Answer 21

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

Question 22

Purpose of a rest

Answer 22

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

Question 23

Default rest position for bounded saddle

Answer 23

Immediately adjacent to saddle

Question 24

Default position for free-end saddle

Answer 24

Not immediately adjacent to saddle (mesial of tooth adjacent to saddle)

Question 25

Position of rests depends on the type of support of the base (2)

Answer 25

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

Question 26

Considerations for occlusal rests (2)

Answer 26

Avoid placing a rest in an occlusal stop | Rest seats can be prepared

Question 27

Possible consequences of preparing rest seats (3)

Answer 27

Loss of occlusal stop when denture is not worn Destruction of tooth surface Exposure of dentine

Question 28

Why should occlusal rests not be placed in an occlusal stop (2)

Answer 28

The denture will be uncomfortable | The rest will interfere with the occlusion

Question 29

For mucosal support, the gingival margins should be kept clear by

Answer 29

Minimum of 3mm because soft tissue is compressible

Question 30

Features of Every partial denture design (6)

Answer 30

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

Question 31

Function of wire stop posterior to last standing tooth

Answer 31

Ensure the most distal tooth does not drift posteriorly

Question 32

Compressibility of periodontium and mucoperiosteum using a tooth and mucosa support based denture (2)

Answer 32

Periodontium - 0.1mm | Mucoperiosteum - 2mm

Question 33

Problem with tooth and mucosa support based denture

Answer 33

Mucosa compresses more than the tooth within its periodontal attachment

Question 34

Solutions to problems with tooth and mucosa support based denture (2)

Answer 34

Stress relieving clasp system (RPI) | Using altered cast technique

Question 35

Features of tooth support (5)

Answer 35

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

Question 36

First choice of support for removable partial dentures (2)

Answer 36

Tooth support | Bounded saddle cases are tooth supported unless saddles are longer than three teeth

Question 37

Rests and long axis of tooth

Answer 37

Rests must direct occlusal forces down through the long axis (root) of the abutment tooth

Question 38

Benefit of Every design over mucosa-borne denture

Answer 38

Less harmful

Question 39

Why are mandibular mucosa supported dentures not generally recommended

Answer 39

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