RPDs Flashcards
saddle
edentulous area
flange
replacement tissue extending to vestibular sulcus
two choices of saddle
flanged
- don’t see gaps under false teeth, replace some of missing tissue
gum-fitted/open face
- straight after ext can get better fit w gum-fitted but over time get resorption
sequence of design
prosthesis - fixed or removable saddles support retention connector
support
resistance of a denture to occlusally directed load
options
- use hard tissues
- large surface coverage
Kennedy classification
anatomical - describes number and distribution of edentulous areas
- doesn’t describe type of support required
Kennedy classification rules
3rd molars generally ignored unless have direct part in denture design
most posterior saddle defines classification
modifications of each class
- numerical count of number of additional edentulous saddle areas present
- can’t modify class 4
kennedy class 1
bilateral free end
kennedy class 2
unilateral free end
kennedy class 3
unilateral bounded
kennedy class 4
anterior bounded (crossing midline)
Craddock classification
gives type of support, doesn’t give info about number or distribution of teeth
craddock class 1
tooth supported
- teeth provide a hard tissue resistance to occlusal loading
craddock class 2
mucosa
- a large coverage provides resistance to occlusal loading
craddock class 3
tooth and mucosa
- a combination of hard tissue and large coverage when there are reduced number of teeth and large edentulous saddles
- FES always class 3
which are the best teeth for support?
ones with the largest root area
what load can a healthy tooth support?
its own load plus 1.5 similar teeth
compare PD membrane mucosal coverage of a lost tooth for support
> x4
what ratio is important in working out support?
crown to root ratio
where should a rest transfer load through?
the long axis of tooth
tooth support
bone and root area provides wide distribution of load
transmits load via PDL - feels more natural
more comfortable
protects ST from trauma
likely to stay in close contact with supporting structures over time
bounded saddle cases are tooth supported unless saddles are longer than 3 teeth
mucosal support
cover large area
allows denture base to move slightly
- possible damage to adjacent gingival margins
lose area periodontium
- quantitative difference of 75% supporting tissue
approx 33% of natural tooth load
avoid base within 3mm of gingival margins
L mucosa supported dentures generally not recommended - insufficient area to provide support
where should the base in mucosa borne dentures be avoided?
within 3mm of gingival margins
why are L mucosa supported dentures generally not recommended?
because insufficient area to provide support
maxilla - primary support areas
hard palate
maxilla - secondary support areas
residual alveolar ridge
buccal vestibule?
mandible - primary support areas
buccal shelf
RM pad
mandible - secondary support areas
residual alveolar ridge
what is the overall fct of a rest?
provide support for denture from vertical opposing forces
rests incorporated into acrylic
can weaken surrounding acrylic - get internal stresses
but can be used when opposing forces are light
why aren’t small rests recommended?
apply large forces per unit area
functions of rests
prevent movement of RPD towards mucosa assist in distribution of occlusal load direct retentive elements to work in planned manner prevent over-eruption of unopposed teeth provide bracing on anterior teeth determine axis of rotation for FES RPDs reciprocation and indirect retention
where should rests be placed on bounded saddles?
rest placed adjacent to saddle
additional rests e.g. on next tooth can be placed to help distribute load if abutment tooth has a smaller root area
where should rests be placed on FES?
have it furthest side of tooth away from saddle to avoid rotation
why should you avoid placing a rest in an occlusal centric stop?
because it will interfere with occlusion
how much prep should be done for an occ rest seat and how can you measure?
1mm
bite on soft wax
disadvantages of prepping occ rest seats
loss of occlusal stop when denture isn’t worn
destruction of tooth surface
exposure of D
advantages of prepping occ rest seats
doesn’t annoy pts tongue
direct forces down LA
every partial denture design
mucosa borne denture which restores dental arch
contact points between denture and abutment teeth
ensure most distal tooth doesn’t drift posteriorly - wire stop
gingival margins not covered by denture design
weak bases
- narrow
- metal inserted into acrylic
difference in compressibility between tooth and mucosal support and consequence
periodontium - vertical displacement of tooth attachment 0.1mm within its socket
mucoperiosteum - 2mm
denture will rock if surfaces are of unequal compressibility
some ways to reduce load on teeth
use stress relieving clasp system (RPI)
use an altered clasp technique
rest seat teeth
periodontal condition size and position of saddle and abutments condition of supporting mucosa occlusion aesthetics
types of rest
incisal onlay crown overlay ledge ring cingulum occlusal
incisal rest
L anteriors
poor aesthetics
ring rest
recommended to direct forces down LA
all of rest must be above survey line
used for single standing teeth or if occlusion prevents occ rest
cingulum rest
likely need prep
canines, can be used on U incisors
apply stress at lower level, less rotational forces
retention
resistance of denture to vertical dislodging forces away from tissues
types of retention
mechanical: clasps, guide surfaces, precision attachments
muscular forces: on polished surface
physical forces: coverage of mucosa, adaptation, forces on imp surface - cohesion, adhesion, atm pressure, surface tension
direct retention
resistance to vertical displacement
indirect retention
resistance to rotational displacement
guide planes
2 or more parallel axial surfaces on abutments which limit PofI
resists displacement
supplementary retention
close to base and parallel to PofI
where should guide planes be placed related to gingiva?
3mm
far from gingiva as possible
what type of retention do clasps provide?
mechanical - engages undercut
when are clasps most efficient?
when used with a rest
above survey line what do clasps provide?
support (except I-bar) - stops it pushing down
2 ways of making clasps
make in wrought metal and incorporate into denture base
include as part of cast denture base
gingivally approaching I-bar clasp
need on premolar/canine for desired length
only tip contacts tooth - terminal end engages UC
ideally originates from a saddle
doesn’t provide support
infrabulge
length of tooth doesn’t have to be ≥15mm to accommodate 15mm clasp arm
occlusally approaching/suprabulge clasp
single arm or circumferential
terminal 1/3 in UC - rest must be above survey line
say which UC it engages - best usually linguals lower molars
length of tooth has to be ≥15mm to accommodate a 15mm clasp arm
reciprocation
when clasp flexes over bulbosity it applies load on tooth
have something on other side of tooth so as clasp flexes over bulbosity it prevents movement
e.g. connector up onto lingual surface
clasps have 2 components
retentive arm
reciprocation component
- counteracts the force of the retentive component with an equal and opposite force
what does the flexibility of the retentive arm of the clasp depend on?
material
length - longer = more flexible
thickness - thinner = more flexible
how difficult it is to dislodge clasp depends on:
flexibility of retentive arm
placement of retentive arm
depth of UC
what space do you need between the terminal end of a clasp and the gingiva and why?
1-2mm to avoid irritation to gingiva
what is indirect retention provided by?
support elements of denture: connectors, rest, saddle, base
principles of indirect retention
provided by support elements of denture: connectors, rest, saddle, base
rest has to be on opp side of clasp axis to saddle
should happen at 90 degrees to clasp axis on opposite side to saddle
retention guidelines
not required on every adjacent tooth to saddle
ideally 3 clasps
- or one each side of arch
triangular pattern of retention ideal
altering PofI gives you retention at saddle but not indirect
what is the RPI stress relieving clasp system used for?
used in FES to prevent stress on last abutment tooth (mostly L)
components of RPI system
occlusal rest - mesial of tooth
proximal plate - adjacent to saddle
I-bar clasp - can disengage on load
RPI system occlusal rest
mesial of tooth
rounded on imp surface
RPI system proximal plate
adjacent to saddle
guide surface of 2-3mm
UC to permit movement
RPI system I-bar clasp
can disengage on load
why don’t you need the RPI system with a maxillary plate?
stress on abutment tooth is negligible
which connector can’t you use the RPI system with and why?
lingual plate
have contact on back of that tooth
- unless transition at canine
ways of minimising stress on abutment if you can’t use RPI system e.g. if lingual plate?
make supporting connector wider
consider less teeth at saddle area
altered clasp technique
major connectors
part of RPD that connects components on one side of arch to components on other side of arch
guidelines for major connectors
rigid (no mods)
avoid covering gingival margins
comfortable - few edges as possible
cover as little tissue as is consistent with rigidity
modifications to guidelines for major connectors
base distribution need for tissue support need for indirect retention anatomical limitations prognosis of dentition prev denture influence
minor connectors
join components to major connector
transfer fct stress to and from abutment teeth
guidelines for minor connectors
rigid
finish above survey line
cross gingival margin at right angle (easier to clean)
cover as little gingival marginal tissue as possible
modifications to guidelines for minor connectors
cover gingival margins instead of lots of small windows - OH
plates
mucosa borne dentures CS thickness only 0.5mm may cover gingival margins therefore only recommended in mandibular arch where no space for bar could add pin dam for rigidity cover more tissue
finishing lines
groove at end of polished metal
don’t have acrylic just overlapping chrome as fluid can ingress
mandibular connectors
dental bar lingual bar lingual plate sublingual bar labial bar
lingual bar contraindication
if you think incisors will be lost
space required for lingual bar
height of bar 4mm
space gingival margin 3mm
above raised fct depth of FOM 1mm
=8mm
lingual plate
covers gingival margins and cingulums
sublingual bar
hard to record fct depth at chairside
uncomfy
labial bar
only use when L anteriors lingually inclined so can’t do lingual bar
maxillary connectors
anterior and posterior ring bar palatal plate - anterior, midpalatal, posterior horseshoe bar/plate posterior bar - offers less support to FES
acrylic connectors
anterior/mid palatal plate, full coverage or horseshoe
not strong enough for bar
spoon - flaps up and down, quite small - choke hazard
modified spoon - wings come up onto palatal surfaces above survey line
every - wire stops
bars
more likely choice for tooth supported
less coverage of mucosa and gingival margins
need to be thick in CS to maintain rigidity
- lingual bar 2mm
less space available in L arch so default choice
why shouldn’t you alter the PofI to create and undercut for a clasp?
no undercut in common PofD so would just fall out
only utilise UCs
open
try if ≥2 teeth between saddle areas
no gingival coverage, greater clearance
possible reduced irritation to gingival tissues
ideal but not always possible
metal backing
when occlusion means limited space between incisors
provide an occlusal contact therefore prevent pressure on upper artificial tooth from debonding to denture base
combination syndrome
only some L anterior teeth remain functioning against a complete U denture
makes ST loose and flabby
dimensions of rests
0.5-1mm thick
dimensions of lingual bar
height 4mm, thickness 2mm, oval or half pear shape
dimensions of sublingual bar
thickness 4mm kidney shaped
clearance of U connectors from gingival margin
5mm
cast CoCr clasp
15mm to engage 0.25mm UC
beading
all the way round border but stops 3mm from gingival margin
facilitate intimate contact - prevent food impaction
closed
more contact, greater retention, guide planes
possible increased irritation to gingival tissues
don’t stop connectors at gingival margins - don’t strip gingiva e.g. bring up onto cingulum of canines to protect gingiva
denture history
why teeth lost how long worn dentures how many dentures fav denture/preferred design prefer metal/acrylic
special investigations for abutments
periapicals
sensibility testing
surveyed study models
clinical photos
pros of RPDs
simple, restore fct and appearance less £ min tooth prep can restore longer edentulous spans can replace missing alveolar ridge tissues remove for cleaning/adjustment/repairs
cons of RPDs
aesthetics denture stomatitis compromise abutments may be bulky and plaque retentive gagging retention and stability
RPD indications
multiple missing teeth no suitable bridge abutments implants contraindicated immediate after extraction provisional during implant tx transitional to complete denture
RPD contraindications
untreated dental disease chronic poor OH pt acceptance SDA pt can fct with mobile teeth (unless transitional to complete)
consequences of missing teeth
anatomical
- EO: change in facial appearance, TMJ problems
- IO: alv resorption, tooth movement, toothwear
aesthetics - lose hard and soft tissues which support face
fct - mastication, speech
psychological
SDA indications
missing posterior teeth with 3-5OU
sufficient occ contacts to provide a large enough occ table
favourable prognosis for remaining anterior and premolar teeth
pt not motivated to pursue a complex Rx plan
limited financial resources on dental care
occlusal stability
absence of tendency for teeth to migrate other than normal psychologic compensatory movements occurring over time
SDA
where most posterior teeth missing
satisfactory oral fct without RPD (compliance can be low)
priority - maintain anterior and premolar dentition in one/both jaws
sufficient adaptive capacity when 3-5OU
- pair of occluding premolars 1
- pair of occluding molars 2
only works long term if remaining natural dentition can be preserved for the life of pt
SDA contradindications
poor prognosis for remaining dentition untreated/advanced PDD pre-existing TMD signs of pathological toothwear significant malocclusion - class 2 or class 3
determining factors for occlusal stability
PD support number of teeth interdental spacing occlusal contacts tooth wear get distal tooth migration in SDAs
pouring up
pour alginate imps in 100% dental stone
25-30ml H2O to 100g stone
saturate - softens particles of stone and reduces spatulation time
agitation
base - 30ml H2O to 110g stone - thicker to support weight
trim periphery (5mm from sulcus depth)
articulators
mechanical elements corresponding to anatomic structures
reproduces recorded relationships of M to M (and movements)
facebow
locate maxilla on articulator to correspond with hinge axis
types of articulators
simple hinge semi-adjustable average value virtual/digital fully adjustable ARCON NON-ARCON
simple hinge articulator
can’t reproduce mandible movements
open and close
hinge axis has smaller radius path of closure - can get discrepancies in occlusion
semi-adjustable articulator
some dynamic movement but pre-set by manufacturer
average value articulator
condylar guidance track fixed at average value (30%)
25 degrees
ARCON
condylar representation on lower arm of articulator
mimics what happens naturally
condylar track on maxillary component
NON-ARCON
condylar representation on maxillary element
condylar track on mandibular component
not anatomical
can lead to inaccuracies during protrusive movements
stability
the resistance to horizontal/lateral movement of the denture
abutments
structurally sound
good alignment and position
prev Rxs and endo txs satisfactory
roots and supporting alveolar bone functionally adequate
alveolar bone of ridge between or distal to the abutment teeth is adequate in quantity and quality
ST of ridge adequate in quantity and quality
when to record occlusion
when designing denture
- if can’t hand articulate/unstable - after primary imp but before design stage
help technician set up teeth
- if can hand articulate and stable - after master imps
- if do before - risk occlusal record blocks won’t fit on master casts
split cast mounting
easy removal of mounting - sodium silicate
mount upper cast first
pin on table
incisal post at 0
UC
areas below max contour
stages of surveying
occlusal plane horizontal - use flat ruler, common PofD tripod - 3 lines analysing rod - 'eyeball' for UCs, choose PofI graphite marker - tip lined up with gingival margin - long side of chiselled edge used against cast - survey all abutments and relevant STs - U and L survey lines
rotate platform to view from every angle
mark position where UC gauge contacts tooth surface - where terminal head of clasp can be placed
instructions to pt
insertion/removal coping with new dentures pain denture cleansing speech eating refer to clinic info leaflet
common PofD
90 degrees to occlusal plane - horizontal
tripoding
3 lines
mark PI/W (red) and PD (black)
prepping the cast after surveying before duplication
parallel surfaces for the denture are provided where required to the PofI
unwanted UCs eliminated using wax and trimming knife
altering PofI
provide retention - using guide surfaces of teeth improve appearance - close unsightly gaps eliminate interference - tooth or ridge UCs preventing a satisfactory PofI
what does the survey line indicate and what must be done with this?
indicates extent of UC
must be used or eliminated by blocking out
conformist
maintaining same occlusion
reorganised approach
altering occlusion
e.g. toothwear, every complete denture
master imps
greenstick to get imp of sulci and FES
alginate for U
silicone med-body for L
how to block out casts - options
wax and chisel on surveyor
plaster and chisel
blocking out - wax and chisel on surveyor
will be duplicated in “refractory” material for a CoCr framework
duplicated again to give a stone “working” cast
+ master cast not destroyed
- time
blocking out - plaster and chisel
ONLY PMMA
trial and process on this master cast
+ quicker
- cast may be broken after
post dam
groove in posterior of HP of master cast
posterior periphery of U RPD, in front of palatine fovea
cut on compressible tissue on the HP close to the jct of the hard/soft palate
not cut on SP - moves in fct
polished surface of the denture in this region is prepared to enable the denture to merge with the tissue
less obtrusive
record blocks
identify index teeth
adjust one block to keep index teeth in occlusion
adjust second block with first one still in to keep index teeth in occlusion
record occlusion
mark centre line
correct occlusal plane
blocking out
prep master cast
prevent a part of RPD entering an area that it shouldn’t
- rigid connector can’t enter UCs
- clasp arms should only engage UCs to a depth suitable for the material they are made from
so the processed denture will fit - however there will always be a space
in relation to survey lines only block out:
in relation to PofI and PoR
between high and low survey lines where a connector is being placed
from UC gauge mark to lower survey line where a clasp is being placed
RPDs made on master cast
blocked out in plaster - can process in heat-cured acrylic
wax - self-cured acrylic
good to consider a duplicate of blocked out cast
pin dams
much shallower groove than post dam
anterior aspect of finished edge
aids in ensuring flush fitting and deflection of food material not slipping under the denture
scribed onto master cast approx 5mm from gingival margins
clinical stages
primary imps (primary jaw reg if required) (survery, mount, design) tooth prep and master imps jaw reg trial delivery review
mouth prep
initial prosthetic tx - repairs and additions - temporary relines - occ adjustment - tx denture stomatitis pre-prosthetic surgery PD tx ortho tx - optimise space and abutment alignment fixed pros and endo
rest seats for upper anteriors
well-developed cingulum - prep stays within enamel
rest seats for lower anteriors
lingual surface too vertical and cingulum too poorly developed to avoid penetrating E
incisal rest seats
duplicating casts
reversible hydrocolloid - agar
condensation cured silicones
tooth prep
provide rest seats
establish guide surfaces
modify unfavourable survey lines
create retentive areas
rest seat prep
produce favourable tooth surface for support
prevent interference with occlusion
reduce prominence of rest
rest seats for posteriors
reduce MR (rest at least 1mm) 'saucer' shaped - allow some horizontal movement and dissipation of occlusal forces if no space occlusally for a clasp to extend buccally from an occ rest, the prep should be extended as channel onto buccal
alternatives to tooth prep
produce a rest seat in composite applied to cingulum area
bond a cast metal cingulum to tooth
tooth modification
unfavourable survey lines
clasp would be positioned too close to occlusal surface - occlusal interference - annoyance
deformation of clasp
creating retentive areas
addition of composite
need broad area of attachment of the restorations to the enamel
use ultrafine or hybrid composites
plastic/acrylic teeth
chemical bond with base natural appearance silent in function soft - low abrasion resistance tough easily trimmed/polished/customised cold flow under pressure insoluble in mouth fluids - some dimensional change
modified acrylic/composite/polymers teeth
chemical bond with base
partial bonding - recommended mechanical and/or bonding agent (4-META)
higher abrasion resistance
guide planes
≥2 parallel axial surfaces on abutment teeth which limit the PofI of a denture
may occur naturally but often need to be prepared
surfaces parallel to each other and the PofI
what do guide planes provide?
increased stability
reciprocation
prevention of clasp deformation
improved appearance
preparing guide surfaces
should extend vertically 3mm but be kept as far away from the gingival margin as possible
≤0.5mm E removal
5 stages of setting teeth
choosing artificial teeth matching natural teeth customising artificial teeth setting to existing dentition trial dentures
what type of tooth is the most popular?
acrylic
compare posterior tooth moulds used for RPDs vs complete
RPDs generally wider/larger
info from tooth/mould chart
shape - square, ovoid, tapered length anterior teeth height and width of central incisor occlusal tooth form length C-C (remember 2D)
porcelain teeth
mechanical attachment with denture base (silane coupling agent)
- metal pins anterior teeth
- holes (diatorics) manufactured into posterior teeth. Fill with the denture base
natural appearance
possible noise in fct
brittle
friable - grinding removes the surface glaze
hard - high abrasion resistance
- sometimes not recommended for occlusion opposing natural teeth
inert in mouth fluids - no dimensional change
high heat distortion - no permanent deformation under masticatory forces
can’t easily be customised by trimming ridge lap area or polished surface
setting anterior teeth
symmetry
- set to LA of corresponding tooth
- contact points
setting posterior teeth
central fissures conform
teeth set according to available space
- can set a premolar in a space occupied by a molar
marginal ridges same level as existing teeth
palatal cusp in contact with the central fissure of its antagonist on the opposing arch
aesthetics
available space - may need to compromise
not advisable to have large areas blocked out ‘dead spaces’ - food impaction
artificial teeth must conform to existing tooth surface wear, and follow natural tooth guidance
adjusting tooth mould
remove from ridge lap - part that contacts residual ridge
because if you remove from the length of a tooth (cervical margin or incisal tip) it can affect the shade
adjusting the tooth can affect the denture base and tooth bond
trial base
rigid acrylic/shellac with wax saddle
modelling wax to attach teeth
lost wax technique
precision metal casting
wax denture made on cast, mould made to surround shape, wax removed by melting, shape filled with molten metal
involves refractory model
sticky wax
ensure fully adheres to light cured base
contains a gum resin
refractory cast
no survey lines
heat resistant duplicate of master cast
made in phosphate-bonded investment material
- stronger than gypsum ones on heating
subject to surface wear - must be treated by hardening the surface of the model
- beeswax, resin, aerosol - model hardner
tooth debonding
more the ridge lap is reduced the less bonding area is available
if softer bonding area is reduced in area it can affect the bond with the ‘enamel’ layer having a harder surface
at least 2 layers of material
- ridge lap made up of a more heavily cross-linked acrylic, specifically to bond with the denture base
preventing debonding - heat cured dentures
adhesive failure
grind teeth with diamond cutters - rougher abraded surface (microabrasion)
no wax/grease on tooth surface (use detergent when boiling out to remove)
ensure no residue of mould seal on the tooth surface
use a post for retention when space is limited for PMMA
preventing debonding - self cured dentures
cohesive failure
grind teeth with diamond cutters - rougher abraded surface (microabrasion)
no wax/grease on tooth surface (use detergent when boiling out to remove)
ensure no residue of mould seal on the tooth surface
use a post for retention when space is limited for PMMA
+ add a drop of monomer to the tooth surface and allow to soak into the tooth before adding the self-cure PMMA
+ try to avoid heavy contact on the denture teeth - cuspal interference
sprue attachment
conducts molten metal into the mould
attached to the thickest and closest to the ingress of metal part of the casting
casting should progressively cool from the exterior to the centre of the metal inflow
- should prevent cooling shrinkage producing voids in the casting
placement and number of sprues also important to prevent air turbulence or air being trapped which would cause voids or porosity within the casting
sprue attached to a plastic cone which will be aligned in the casting machine
other wax - stabilise cone
cone and sprue attachment must be higher than any part of the wax pattern
to avoid air pressure - use air gates (1mm diameter) - carry air away from the casting
packing
pack PMMA under pressure
can do trial
3 types of porosity
granular
contraction
gaseous
0.25mm clasp
CoCr
0.5mm clasp
wrought gold
0.75mm clasp
SS wire
constructing denture - retention
wax retention pattern for FES
external finishing line
can use posts for anterior/single teeth
fill clasp from thickest part not from mesh
flasking
waxed denture on cast embedded in plaster
set
evacuate wax
- heat
- detergent
hooded method
- easiest way to flask a partial denture to avoid the UC
- artificial teeth and clasps held in investing plaster following flasking
post-processing
place on definitive cast
check occlusion
finish - burs - remove excess and any oxides
polish
- non-imp surface
- electropolishing - place wax over any thin areas e.g. wax to protect it
- polish with silicone rubber bur
mechanism of action of RPI
Rest mesially acts as axis of rotation. As the proximal plate and I-bar rotates downwards and mesially (respectively) around the axis of rotation during occlusal load. The I-bar and proximal plate disengage from the tooth/undercuts. Thus, potential traumatic torque is avoided
prevents stress on abutment tooth in FES
