Partial denture system of design Flashcards
ILO 2.6a: be familiar with the design and choice of materials used in the production of partial dentures, along with the knowledge of laboratory procedures
what are the alternative treatments to getting dentures?
- no treatment
- fixed prosthesis - conventional, resin bonded
- implant retained prosthesis
what are the key principles of partial denture design?
- replace lost teeth and tissues, restoring function, speech and aesthetics
- minimise damage to adjacent teeth, restorations and tissues
- designed with periodontal health in mind
why do clinicians design the denture not the technicians?
clinicians have seen the patient so understand:
* patient story and expectations
* condition of teeth and periodontium/oral health
* assessment of current dentures
* assessment of occlusion
* more complete understanding of alternatives
* to give clear cost information
what is the order of designing partial dentures?
- case assessment
- determine saddles
- choose denture base material
- choose path of insertion and delineate undercuts (survey)
- design support
- design retention
- design bracing/reciprocation
- connect saddles together
- indirect retention
- simplification
what is a saddle?
an area of oral mucosa where teeth are being replaced
what is Craddock classification? what does each classification represent?
classifies how the saddle is loaded
* class I: tooth borne
* class II: mucosa borne
* class III: tooth and mucosa borne
* class IV: implant borne
* class V: implant and mucosa borne
what is Kennedy classification? what does each classification represent?
classifies the position of the saddles
* class I: bilateral free end saddle
* class II: unilateral free end saddle
* class III: single bounded saddle not crossing the midline
* class IV: single bounded saddle crossing the midline
if there is more than one saddle, modifications are used but the most posterior saddle is the main classification
what KEnnedy classification is this?
Kennedy class II modification 2
what is the definition of support?
the resistance of a denture to occlusally directed load
what component of a denture provides support?
rests
what kinds of rests are there?
- incisal rest
- cingulum rest
- occlusal rest
what is the difference between tooth support and mucosa support?
tooth support
* prevents/resists movement and directs load through PDL of abutment teeth then to alveolar bone
* allows the supported denture base feel like the natural dentition
* more comfortable for the patient
* protects soft tissue from trauma
* likely to stay in close contact with its supporting structures over time
mucosa support
* requires a large surface area to distribute load to the alveolar bone
* allows the denture base to move slightly as it is compressible
* can cause possible damage to adjacent gingival margins
what is the order of teeth with the highest to lowest crown to root ratio? how much can a healthy tooth carry?
- molars
- canines
- premolars
- upper central incisors
- upper lateral incisors
- lower incisors
- a healthy tooth can potentially carry its own weight plus one and a half of similar teeth (x1.5)
how far should mucosal support be away from the gingival margin?
3mm as it will place pressure at the gingival margin
where is the primary and secondary support of the maxillary and mandibular arch?
- primary support - hard palate (maxillary), residual ridge/buccal shelf to the buccal sulcus (mandibular)
- secondary support - anterior hard palate, residual ridge (maxillary), residual ridge to the lingual sulcus (mandibular)
why should occlusal rests be large?
- they apply large forces per unit area
- large rests direct force down the long axis of the tooth to the PDL and alveolar bone
what are the disadvantages of incisal rests?
- poor aesthetic appearance
- may interfere with incisal occlusion
- not recommended on wear facets
what are the advantages of cingulum rests?
- aesthetically superior to incisal rests
- apply stress at lower level so less rotational forces
- less likely to break
what are the advantages of occllusal rests?
- prevent movement of the RPD to the mucosa
- assists the distribution of occlusal load
- prevents over-eruption of unopposed teeth
- helps determine the axis of rotation for free-end saddle PRDs
- can provide indirect retention
what is the default position of rests with a bounded saddle?
immediately adjacent to the saddle - distal side of mesial tooth and mesial side of distal tooth
what is the default position of rests with a free end saddle?
not immediately adjacent to the saddle - mesially on the abutment tooth so the tooth does not rotate towards the tissue (torque)
what are rest seats? what are their disadvantages?
when occluding teeth are close together and a rest seat needs to be placed, a rest seat can be made
* loss of occlusal stop when denture is not worn
* destruction of tooth surface
* exposure of dentine
what is the definition of retention?
the resistance of a denture to lifting away from the tissues (vertical dislodging forces)
what are methods of retention?
- clasps
- soft tissue undercuts
- adhesion (maxillary plates)
- friction (guide planes)
- path of insertion
- precision attachments/implants
what are the three ways that retention can be achieved?
- mechanical - clasps
- muscular - shape of denture
- physical - coverage of mucosa (adhesion, cohesion, atmospheric pressure (saliva))
what are guide planes? what do they do?
- parallel proximal surfaces of teeth that are close to the base
- provides supplementary retention in the form of friction
- can adjust proximal surfaces of teeth to make them parallel
what is a common path of insertion?
a single, defined direction in which a removable partial denture is designed to be inserted and removed from the mouth
why may you need to alter a path of insertion?
- to get a closer fitting denture to the abutment teeth
- to get the acrylic closer to the vestibular sulcus
- better aesthetics
- better localised retention
describe the principal method of providing retention
clasps - metal arm that is placed below the bulbous part of a tooth in an undercut and contacts the tooth to prevent removal of hte denture base
what does surveying casts help you to do?
- helps choose retentive elements (clasps) by identifying undercuts
- helps find path of insertion and removal of denture
what are the two types of clasps?
- occlusally approaching - approaches from above the survey line into the undercut
- gingivally approaching - approaches from the gingival sulcus
what clasp is this?
I-bar clasp (gingivally approaching)
what clasp is this?
self reciprocating clasp / ring clasp / circumferential clasp (occlusally appraoching)
what clasp is this?
single arm clasp with a reciprocal arm / C-clasp (occlusally apporaching)
what is reciprocation?
- an element on an occlusally appraoching clasp that opposes a retentive arm
- the retentive arm applies a horizontal force onto the tooth when it comes into contact with it
- the reciprical arm also contacts the tooth, counteracting the horizontal force and preventing tooth movement
what is the optimal pattern of retention?
triangular (as big as posible)
describe why indirect retention is needed in dentures with free end saddles
- when a dislodging force (i.e. sticky foods) is applied to a free end saddle, it can lift up and dislodge the whole denture
- the rests on the denture act as pivot points/fulcrum and there is resistance from the clasp by the saddle
- when the distance between the fulcrum and resistance is greater, it is more effective at resisting the dislodging forces
what other components can provide indirect retention?
- major connectors
- minor connectors
- rests
- saddles
- denture base
what are some denture base materials?
- CoCr
- acrylic
- PEEK - poly ether ether ketone
- gold
what is the difference between a major and minor connector?
- major connector connects all the components together
- minor connector connects, rests, clasps and bracing arms to the major connector
what are some common mandibular major connectors?
- lingual bar
- lingual plate
- acrylic plate
- dental bar
- sub-lingual bar
- combination of plate and bar
what are some common maxillary major connectors?
- palatal plate
- ring (anterior/posterior bar)
- mid palatal plate
- anterior palatal bar (horseshoe bar)
- anterior palatal plate (horseshoe plate)
- palatal bar
what is bracing?
the resistance of lateral movement of a denture provided by the major connector
what is beading?
- a thin layer of wax is placed around the border of the denture base but stops 3mm from the gingival margin
- facilitates an intimate contact between the denture and the tissue so prevents food ingression
what is the difference between plates and bars?
plates
* thinner in cross section (0.5mm) and maintains rigidity
* for craddock class II designs (mucosa borne) - less tooth support = greater mucosal support
* may cover gingival margins
bars
* thicker in cross section (2mm)
* for craddock class I designs (tooth borne)
* less mucosal coverage
what is the default choice of connector for the mandibular arch? what are the requirements for this?
- lingual bar as there is less space in the mandibular arch
- needs 7mm of space minimum between the gingival sulcus and the floor of mouth
- 3mm from gingival margin + 4mm for height of bar
what are the advantages and disadvantages of a ring bar design?
advantages
* little coverage of gingival tissue
* rigid
* wide relief of gingival margins to prevent food impaction
disadvantages
* less support to free end saddles across the hard palate (narror coverage)
* lots of edges that some patients may find uncomfortable
* greater cross-sectional thickness
what are the advantages and disadvantages of a plate design?
advantages
* thinner cross section
* wide relief of gingival margins to prevent food impaction
* support for free end saddles across hard palate
* less edges that patient may find uncomfortabele
* rigid
disadvantages
* mucosal tissue is covered so less natural sensation
what are some examples of maxillary PMMA connectors?
always plates and not bars as acrylic is not as strong as metal so if it was narrower, it would have to be thicker and bulkier
what is the difference between open and closed saddle designs?
- closed design: more contact with greater retention due to guide planes - can irritate the gingival tissues (saliva cannot pass bucally to lingual/palatal)
- open design: no gingival coverage with greater clearance - less irritation to gingival tissues (saliva can pass from buccal to ligual/palatal)
what can be used with tooth position to ensure they don’t debond?
metal posts within acrylic teeth and a metal backing
* tooth trial required for correct tooth position and post and backing position
* when occlusal contact occurs when lower teeth contact the cingulum of the upper anterior teeth, it applies pressure which can debond the teeth so metal backing prevents this
what is the difference between tooth and mucosal support?
there is a difference in compressibilty between periodontium (tooth within the socket) and the mucoperiosteum (mucosa coverign alveolar bone)
* periodontium can compress 0.1mm
* mucoperiosteum can compress 2.0mm
what is the RPI system? where is it usually used? what does it consist of?
- the RPI system is a stress relieving clasp system commonly used on mandibular free end saddles to protect the abutment tooth when occlusal load is applied to the saddle
- mesial Rest
- Proximal guide Plate
- gingivally approaching I-bar clasp
describe how the RPI system works
- torque occurs when the occlusal forces are applied to the free end saddle and the rest places horizontal forces distally on the abutment tooth, leading to PDL breakdown and eventually extraction
- during occlusal load, the proximal plate and the I-bar moves down and forwards
- the I-bar is out of contact with the tooth and the proximal plate sinks down into the undercut so the only thing on the tooth is the mesial rest
- the proximal plate resists the horizontal forces distally on the abutment tooth
what are the advantages and disadvantages of an RPI system?
advantages
* no torque on the abutment tooth
disadvantages
* windows created by minor connector - pts. may not be able to tolerate and can be a plaque trap
* loading on the free end saddle may accelerate bone resorption
