denture design Flashcards
where should position of rest be
Watt et al 1958:
Abutment tooth should be loaded on aspect away from the saddle and if possible, the load should be spread to the neighbouring tooth
for distal extension saddles, this means occlusal rest should be on mesial aspect of premolar
defn of rests
a rigid extension of a fixed or removable partial denture which contacts a tooth or teeth to dissipate vertical or horizontal forces
functions of rests
Li Chun Fai’s Man Boob Is Really Enormous
1) transmit Load through longitudinal axis of tooth
- forces transmitted to the abutment will be directed apically down the long axis of the tooth
2) prevent Cervical displacement of the denture
- act as a vertical stop
3) prevent Food packing
4) Maintain planned clasp tooth position
- prevent it from sinking in
5) Bracing
- some shapes eg cingulum rest will transfer some of the horizontal functional force
6) Indirect retention: rests are placed anterior or posterior to the axis of rotation, prevent extension bases from lifting away from underlying ridges
7) provide positive Reference seats in rebasing or impression procedures
8) prevent Extrusion of unopposed abutment teeth by resting on occlusal surface
purpose of rest seat preps
- produce favourable tooth surface for support
- prevent interference with occlusion
- reduce prominence of rest (which will have tendency for food trap, irritate tongue, interfere with occlusion)
- prevent tooth migration
- improve accuracy of fit
- aid self cleansing of tooth surface
what are the requirements for connectors
1) rigid
- so that stresses applied to one part of the RPD are effectively distributed over the entire supporting area
- flexibility will cause concentration of forces which will damage hard and soft tissues
2) must not impinge on movable tissues or gingiva during insertion, removal or function
3)) provide patient comfort by conforming to existing anatomic structures
- avoid bony protruberances
- eg anterior border of maxillary major connector should not end on the anterior slope of a prominent rugae. should end on posterior slope instead to interfere less with patient comfort and speech
4) provide indirect retention
- can resist rotational movement about fulcrum line
5) avoid food entrapment
what are the requirements for borders of MAXILLARY connectors
because there are no movable tissues on palate, borders of maxillary connector hould be farther away from gingival margins than mandibular major connectors
>6mm from gingival margin for maxilla and >3mm for mandible
should be beaded (0.5-1mm wide and deep)
- bead lines are minor elevations at the borders that contact palatal soft tissues to provide mechanical seal and prevent food particles from collectng under the connector
what are the dimensions for an AP palatal bar and strap
and what are their advantages and disadvantages
BAR
- should be 6-8mm wide
- advantage: minimise soft tissue coverage, yet good resistance to deeformation
- disadv:
1) limited tissue contact so little support from palate, not a first choice
2) bulk and contour of bar is uncomfortbale and hence interferes with tongue and phonetics
STRAP
- 8mm in width
-6mm away from gingiva
- open area in the middle should be at lesat 20 x 15mm
- adv: rigid due to encirclement by AP strap, resist flexure, plus it can be thinner than bar
- disadv: extensive length of borders may irritate tongue
advantages and disadvantages of ring design
adv:
- rigid (especially can avoid torus, and its better than the u shaped one)
- antero posterior lengthening of base provides added stabillity
disadv:
- palatal aspects of teeth are covered
- lacks the support of a full plate (but this might not be necessary either, plus it allows greater stimulation of palatal mucosa)
dimensions for palatal plate, adv and disadv
dimensions
- either 6mm away from marginal gingiva or cover cingulum of anterior teeth
- posterior border extended to junction of hard and soft palate
- bead line along posterior border to prevent collection of debri
adv
- provides added retention from atmospheric pressure and interfacial surface tension
- support from palate
- stabilisation when dissipating horizontal and lateral forces via intimate contact with underlying soft tissues
- enhance thermal conductivity and permits stimulation of underlying soft tissue, giving long term tissue heatlh
- metal is less porous surface than acrylic resin and hence more resistant to colonization by microorganisms such as Candida albicans
disadv:
- extensive coverage may diminish taste perception
- if poor OH, then extensive tissue coverage and prolonged denture wearing will cause soft tissue hyperplasia
- interfere with tongue function and phonetics
- uncomfy for patients
requirements of mandibular connector
- rigid but not too bulky
- must not impinge upon movable floor of mouth, associated frena or mandibular tori
indications for lingual plate
- when insufficient vertical lingual depth exists for a lingual bar
- presence of high lingual frenum/ FOM
- inopreable lingual tori
- to support periodontically weakened teeth that require splinting and so the plate stabilises teeth and distribute forces
- if many teeth are guarded prognosis and may need to exo and replace in the future then the plate can be repaired
adv and disadv of lingual plate
adv:
- good for cases where natural teeth are spaced, plate may be waxed up with splits present for better esthetics
- depth of lingual plate acts as indirect retainer, limiting rotation of KC I and II dentures
disadv:
- extensive coverage may contribute to decalcification of enamel surfaces and irritation of soft tissues in patients with poor OH
difference between lingual and sublingual bar
sublingual bar is when the height of the FOM does not allow placement of the superior border of the bar at least 4mm below the FGM
whereas the dimensions for lingual are:
- at least 8mm depth
3mm from gingival margin
3-4mm thickness of bar
1-2mm from FOM
adv and disadv of lingual bar
adv:
- simple design
- minimal contact with soft tissues and teeth and hence decreased plaque accumulation
- increased tissue stimulation
disadv:
- fabrication and finishing need to be done well so that it will not be too thin and flexi, if not there will be concentration of potentially destructive forces
indications for labial bar
1) malpositioned or extreme lingually inclined teeth
2) large mandibular tori that precludes the use of lingual bar, plate and surgical removal is contraindicated
adv and disadv of dental bar
adv:
- frees gingiva
- provides indirect retention, bracing and support
disadv:
- slightly flexible
what are the four categories of minor connectors
1) minor connectors that join clasp assemblies to major connectors
2) minor connectors that join indirect retainers or auxiliary rests to major connectors
3) minor connectors that join denture bases to major connectors
4) minor connectors that serve as approach arms for vertical projection or bar type clasps
requirements of minor connectors
general:
- join major connector at 90 degrees
- cover smallest area of tissue possible
- fill embrasure space so as to minimize food trap + smooth surface presented to tongue
- there should be at least 5mm between 2 vertical minor connectors (Krol et al)
in order to join indirect retainers or rests to major connector
- must be rigid
- not in area of undercut
- positioned such that its least obtrusive to the tongue, hence usually in embrasure
