RPD p622 Flashcards
support
definition
resistance of a denture to an occlusally directed load
kennedy classification
is
describes number and distribution of edentulous areas
kennedy class I
bilateral free end saddle
kennedy class II
unilateral free end saddle
kennedy class III
unilateral bounded saddle
kennedy class IV
anterior bounded saddle (crosses midline)
modifications to kennedy classes
take most posterior saddle as main classification each edentulous area in addition to this takes one mode point
e.g. upper arch with 2 posterior bounded saddles and 1 anterior bouneded saddle would be kennedy class III mod2
craddock classification
is
based on the support for the removable denture
options for craddock classification
tooth borne
mucosa borne
tooth and mucosa borne
tooth borne support
provides hard tissue resistance to occlusal loading i.e. when there is abutment teeth with support on bounded saddles
mucosa borne support
large covering provides resistance to occlusal loading
i.e. when the vertical biting force of the denture is resisted entirely by soft tissue
tooth and mucosa borne support
reduced number of teeth and large edentulous saddles
i.e. large free end saddle with mesial rest and mucosal support
what type of support is best
Tooth support is better as it transmits load through PD membrane, protects soft tissue, comfortable, more natural also doesn’t suffer
from muco-compressibility
Mucosal support may damage gingival margins and is more likely to displace, however large area=reduced occ. table
The best teeth for support are those with large root areas depending on PDL attachment
“A healthy tooth can carry 1.5x its own load”
rests
role
provide support against vertically opposing forces and can also be used for reciprocation
Best used as part of a cast metal base alloy
➡i.e CoCr Base= CoCr rests
➡i.e PMMA Base= Stainless steel rest (however can fracture base if thin)
5 functions of rests
- Prevent movement of RPD towards mucosa
- Distribute occ. load
- Direct retention to work in correct way
- Prevent overeruption
- Determine axis of rotation for denture
factors for rest position
3
In tooth borne, rest should be nearest base
For tooth and mucosa should be on surface not near base
In free-end saddle rest is not placed immediately adjacent to saddle to prevent tilting
Every design
mucosal support
Contact points between Denture and abutment teeth
Wire stop at most posterior teeth to stop distal drift
Uncovered gingival margins
7 basic support guidelines
RPD
Tooth support first choice
Bounded saddles tooth supported unless longer than 3 teeth
Rests must direct forces down long axis
Occlusion must be taken into account incase tooth prep needed
Mucosal support must cover a large surface area
‘Every’ design is a less harmful mucosa borne dentue
Mandibular mucosa supported RPD not recommended unless as training for complete denture or overdenture
retention RPD
Resistance of a denture to lifting away from the tissues (vertical dislodging forces)
direct retention
RPD
resistance to vertical displacement of teh denture
clasps
modes of direct retention
mechanical - clasps
clasps in RPD
placed below bulbosity of the tooth in the undercut
occlusally or gingivally approaching
single arm, circumferential or ring clasps
RPD clasp design
section - round or half round
length - longer then length the greater the flexibility, must be at least 15mm
thickness - if reduced by 1/2 the flexibility inc by factor of 8
clasp position and survey line
2/3 of clasp arm should be above the survey line
terminal 1/3 below the survey line engaging in the undercut
➡If not enough of an undercut it is possible to tilt the cast or use a composite build up *ONLY IN SMALL UNDERCUTS, OTHERWISE CLASP ARM WILL DE-BOND THE COMPOSITE
➡Alternatively you can prep an undercut with a crown bur
RPD rest types
6
occlusal / incisal
ledge
onlay
inlay
hook
ring (if occlusion tight)
CoCr clasp thickness
0.25mm
Au clasp thickness
0.5mm
Stainless steel clasp thickness
0.75mm
length of clasp
longer the length the greater flexibility, therefore clasp arm on molar
must be 15mm at least
frictional retention by
guide planes
guide planes
provide frictional retention
boudned saddle - have 2 or more parallel axial surfaces on abutment teeth which limit the POI and improve stablity
can be natural or prepped
larger the guide surface = the more efficient it is
altered path of insertion effect
eliminates undercuts (in common path of insertion) and creates new ones for retentive elements
common PoI = 90 degrees
tilt anterior or posterior to create new ones
muscular retention
in resting position from orbicularis oris and buccinator
RPI is a
stress reliving clasp system
RPI used in
distal extension saddles
kennedy I and II
RPI components
Mesial Rest (Occlusal), Rounded on impression surface
Proximal Plate (adjacent to saddle) guide surface of 2-3mm, with an undercut at tooth bulbosity to permit movement
I-bar clasp (gingivally approaching) at the greatest prominence of tooth contour
how does RPI work
Mesial rest is loaded
Proximal plate rotates into the distal undercut
I-bar clasp rotates into mesial undercut and isn’t touching tooth (it lies on largest bulbosity of crown before loading)
5 methods of direct retention in RPD
mechanical retention - clasps
frictional retention - guide planes
altered PoI
muscular retention
RPI
indirect retention
definition
resistance to rotational displacement of teh denture
types of indirect retention
6
- Indirect Retainers
- Major Connectors
- Minor Connectors
- Rests
- Saddle
- Denture
indirect retainers principles
resists rotation around a ‘fulcrum axis’.
* (fulcrum axis = the line of rotation – it is an imaginary line between the most posterior rest seats on the end of each arch)
* ( clasp axis = imaginary line between clasps on opposite sides of the arch)
When there are multiple clasp axes, use the one closest to the saddle, this is the MAJOR one.
Rests should be 90 to their potential path of movement in order to behave as indirect retainers
reciprocation in RPD design
resistance to side to side movement
Reciprocation is placed opposite to retentive clasp to assist in preventing unwanted movements/stresses of abutment teeth:
* Reciprocating arms provide some resistance but also allow flexure so that the retentive arm does not get put under too much pressure and break.
* Reciprocating clasps still need to engage undercut
* Try and encircle the tooth by 180 degrees
Guide planes
* Place suitable rigid components of the denture to resist horizontal and vertical forces.
RPI system – minor connector on mesial rest of tooth prevents excessive distal movement.
Bracing = general resistance to lateral movements exerted on the denture by tissues:
* Maxilla – palate and alveolar ridge can be taken advantage of
* Flatter ridge = more movement
* Major connector and flanges can be useful to help with bracing – this should be incorporated into the design.
indirect retention principles
Principles
* Retentive clasps should always be between the saddle and indirect retainer
* Retentive clasps should be as close to saddle as possible, whilst the indirect retainer is as far away from saddle as possible.
* Try not to have movements that will force pressure on a tooth/ortho movement.
maxillary major connectors options
can be open (better OH) or closed (greater retention)
mandibular major connecotrs options
plate or bars
mandibular plates
Can be thin in cross section and maintain rigidity
* 0.5mm
only recommended in mandibular arch where there is no space for a bar
(because they will have to cover gingival margins)
When less tooth support plates will confer mucosal support
leave free gingival margin where possible (3mm from GM for > 1.5 teeth)
mandibular bar
Less coverage of gingival margins
Must be thick to maintain rigidity
* 2mm thickness
Default connector in mandibular arch
lingual bar space requirements
8mm total
➡3mm- Distance from gingival margin
➡4mm- Height of the bar
➡1mm- Above floor of the mouth
mandibular major connector options
lingual bar (needs 8mm, 2mm thickness)
sublingual bar (4mm thickness)
kennedy bar/continual lingual clasp - on 2/3 of teeth
lingual plate - 0.5mm thickness, but gingival coverage
minor connectors definition
join components such as rests to the major connector
tranfer functional stresses to and from teh abutment teeth
requirements for minor connectors
4
rigid
finish above the survey line
cross gingival margin at right angles (easily cleaned)
cover as little of gingival tissue as possible
checklist for RPD setting teeth
- Contact points
- Use existing teeth to guide in 3 dimensions
- ICP for posterior teeth, make sure they are as centric as possible
- Occlusal dimension maintained (pin on table w/ articulator)
- Artificial teeth set to conform with existing occlusion - Refinement must be done in occlusion intra-orally (at chairside)
components used to survey casts
Analysing rod
Undercut gauges
* 1 line- 0.25mm- CoCr
* 2 lines- 0.5mm- Au
* 3 lines- 0.75mm- Stainless Steel
Surveying table
Surveying arm
Graphite marker
curve of spee
curvature of the mandibular occlusal plane beginning at the tip of the lower
incisors and following the buccal cusps of the posterior teeth, continuing to the terminal molar
curve of wilson
The curve of Wilson is the across arch curvature or posterior occlusal plane. Arc of the curve, which is concave for mandibular teeth and convex for maxillary teeth are defined by a line drawn from left mandibular first molar to right mandibular first molar
sphere/curve of monson
The curve of occlusion of natural teeth in which each cusp and incisal edge touches or conforms to a segment of the surface of a sphere, with its centre in the region of the glabella.
