RPD system of design: support Flashcards
What are the features of design that we have to consider when thinking about RPDs?
Saddles Support Retention Bracing and reciprocation Connector
What are the questions we have to ask ourselves when considering denture design
Will a prosthesis be made?
What type of prosthesis: fixed or removable?
What teeth will be replaced and why? (saddle)
How will occlusal loading be resisted by the denture? (support)
- tooth
- mucosa
- tooth and mucosa
How will the denture be retained? (retention)
How the denture will resist horizontal movement? (bracing and reciprocation)
How will the components be joined? (connector)
What is the kennedy classification
Kennedy classification is an anatomical classification that describes the number and distribution of saddle (edentulous) areas.
are 3rd molars included when considering kennedy classification
third molars are generally ignored unless they play a direct part in the denture design AND the most posterior saddle defines the classification
how many kennedy classifications are there
4
what is kenedy class 1
bilateral free end
what is kennedy class 2
unilateral free end
what is kennedy class 3
unilateral bounded
what is kennedy class 4
anterior bounded (crossing the midline)
How do you incorporate additional saddles to the one which determines the classification
Each additional saddle is referred to as a ‘modification’ e.g. if a case had two bounded saddles present it would be classified as a ‘kennedy class 3 modification 1
what kennedy classification cannot be modified and why
class 4 because it's the most posterior saddle which takes the classification name and class 4 is anterior teeth that are missing
which saddle area determines the kennedy classification
posterior
what is meant by support
a term used to describe the resistance of a denture to occlusally directed load
what are the options for resisting movement towards the tissue?
utility of hard tissue - teeth
spread the load over large surface area - mucosa (but need a wide area)
why do you want to spread forces over a wide area
to minimise the direct load onto the bone
what is the craddock classification
craddock provides a simple classification based on support for a removable partial denture. It only gives the type of support and does not tell anything about the number or distribution of teeth
what is craddock classification 1
tooth support only
what is craddock classification 2
mucosa support only
what is craddock classification 3
tooth and mucosa support
why is it really good to use tooth support if you can
- transmits load via the periodontal membrane
- allows the supported denture base to feel 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
why must mucosal cover as large an area as possible?
for reduction of the occlusal table
why isn’t it good that the denture base is able to move slightly with mucosal support
could result in possible damage to adjacent gingival margins
how much load can a healthy tooth carry
a healthy tooth can potentially carry its own load plus one and a half similar teeth
how strong are each of the teeth in order
molars, canines, premolars, anteriors
Periodontal membrane for support is more than x times the mucosal coverage for a lost tooth
4
what are rests
rests are the components which provide support for the denture from vertical opposing forces
how are rests described
by the part of the tooth they contact e.g. cingulum, incisal, occlusal
what material are rests made out of
cast or wrought metal
what denture frame work are rests best used in
as part of a cast metal denture framework
what movement do rests oppose
the movement of the base towards the tissues
how common is it for wrought stainless steel to use used for rests on acrylic dentures
very rare
how common is it for rests (whether cast or wrought metal) to be incorporated into acrylic bases
rare
why is it a problem to incorporate rests into acrylic resin
gives low base strength
placing a metal component potentially weakens the acrylic surrounding it
where are incisal rests usually used
lower arch (aesthetics)
where are cingulum rests usually used
canines and upper centrals (not all teeth have significant cingulums)
why are small occlusal rests not recommended
they apply large forces per unit area
why are big occlusal rests recommended
this can direct force down the long axis (make as close to the midline as possible without adversely affecting occlusal relationship)
where do incisal rests originate from
extended from the denture base framework
problem with incisal rests?
poor aesthetic appearance, may interfere with incisal occlusion, not reccommended on wear facets
where do cingulum rests originate from
extended from denture base framework
how would you fit a cingulum rest if a prominent cingulum is not present
a preparation would be required to provide a vertical stop
where are cingulum rests usually found
maxillary and mandibular canines
which are more aesthetically pleasing, incisal or cingulum rests
cingulum
where are cingulum rests usually found
maxillary and mandibular canines, maxillary incisors and laterals
why are cingulum rests good
apply stress at lower level- less rotational forces
less likely to breakage, aesthetically good
if possible how should you not place a cingulum rest
so that it protrudes from the tooth, should be in line with the rest of the natural tooth shape, because if not it provides an area for plaque build up and is normally not accepted by patient
Features of a CoCr metal framework base
- rigid and strong
- all rests are integral to the base connected to the major connect directly or by minor connectors
- the base is produced by casting using ‘lost wax technique’ - more extensive technique
Features of a PMMA (acrylic resin) framework base
- flexible and poor strength
- rest 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 rather than the rule
- although can be used when opposing forces are light e.g. complete denture
Why have rests?
- prevent movement of an RPD towards the mucosa
- assist in distribution 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
- reciprocation and indirect retention
how can you work out if it’s possible to go for a completely tooth borne rest
with a formula (to do with crown to root ratio)
what is the purpose of a rest
to support the denture against vertical load-that is directed towards the tissue
what surface are rests generally placed on and why
occlusal surface since this will direct occlusal forces down the long axis (root) of the abutment teeth
what is the default rest position for a bounded saddle
immediately adjacent to the saddle on either side of the saddle (the default position may be changed depending on the opposing dentition)
where are rests placed for free end saddles
NOT immediately adjacent to the saddle
- still abutment tooth but the mesial side to try and prevent torquing on the tooth
(caveat: occlusal relationship will need to be checked to ensure suitability for patient)
where should the support be for tooth supported bases
on the tooth surface nearest to the base
where should the support be for tooth and mucosa supported bases
on the tooth surface which is not next to the base
where should rests not be placed on an occlusal surface
in an occlusal centric stop (denture will be unconfortable and the rest will interfere with the occlusion)
what are possible consequneces for rest seats
- loss of occlusal stop when denture is not worn
- destruuction of tooth surface
- exposure of dentine
in mucosal support (craddock 2 and 3) where is the primary and secondary support on uppers and lowers
Uppers
primary: hard palate
secondary palatal side of alveolar ridge
Lowers
primary: buccal side of mucosa
secondary: lingual side of mucosa
what is mucosal support dependent on
as large an area as possible of mucosa being covered
because soft tissue is compressible where is it inadvisable to place the base
within 3mm of the gingival margins as this will place pressure at the gingival margin
What is the Every partial denture design
a mucosa borne denture which restores the dental arch; with contact points between the denture and abutment teeth
to ensure the most distal tooth edoes not drift posteriorly a wire stop is incorporated
the gingival margins are not covered by this denture design
some designs such as these can produce narrow and therefore weak denture base areas
metal inserted into acrylic causes an inherent weakness in the denture base
