RPD design - retention Flashcards
1
Q
what is retention
A
- what prevents dentures being displaced
- term used to describe the resistance of a denture to lofting away from the tissues = VERTICAL DISLODGING FORCES
- any portion of the prosthesis that contacts the teeth and helps to prevent the removal of the prosthesis
2
Q
how can retention be achieved
A
- mechanical
- muscular
- physical
3
Q
how is mechanical retention achieved
A
- mainly clasps = engage tooth undercuts
- friction also plays a role
4
Q
how is muscular retention achieved
A
- shape of denture
- action of surrounding musculature on shape of the non impression surface of denture
- patients muscle control = muscles, tongue, cheeks, lips
- comes through time = clasps will eventually fail and by then muscular force will keep denture in place
5
Q
how is physical retention achieved
A
- using existing forces of adhesion, cohesion, surface tension and atmospheric pressure on impression surface of denture
- closeness of adaptation, extent of denture base, peripheral seal
- mainly on acrylic dentures = not really available with cobalt chromium as made skeletal so dont get the same effect with adhesion/cohesion
6
Q
what is adhesion
A
surface forces of saliva on denture and mucosa
7
Q
what is cohesion
A
forces within saliva, viscosity
8
Q
what is atmospheric pressure
A
resistance to displacing forces
9
Q
what are the 2 classifications of retention
A
- direct
- indirect
10
Q
what is direct retention
A
- resistance to vertical displacement of denture
- using clasps
11
Q
what is indirect retention
A
- resistance to rotational displacement of denture
- helps prevent denture rotation out, not falling out
- e.g. in a free end bilateral = with sticky foods, there is nothing at the back of denture to prevent denture rotating out so need indirect retention
12
Q
how is direct retention achieved
A
- close to the base, adjacent to the abutment teeth
- similar to support
- want clasp tip as close to saddle as possible = need to use undercuts
- guide planes
= supplementary retention, frictional retention (is good)
= close to the base and parallel to path of insertion
= no deform over insertion, loading and removal
= keeps the space, prevents drift occurring
13
Q
what are the mechanical components that provide retention
A
- main one is claps
14
Q
what are clasps
A
- metal arm that, when in position contacts the tooth preventing removal of denture base
- to be effective clasps are placed below the bulbous past of the tooth (undercut) = bulbous part identified by surveying
15
Q
what are clasps made of
A
- wrought metal
- they are incorporated into the denture base or included as part of a cast denture base
16
Q
what two ways do clasps approach the undercut
A
- occlusally approaching
- gingivally approaching
17
Q
how do clasps work best -
A
- in conjunction with a rest
- helps prevent claps slipping down o gingival margin and causing damage
18
Q
what are gingivally approaching clasps
A
- comes from sulcus and engages with undercuts
- below lip line so is aesthetically better
- crosses ginvgival margin which creates a bridge across gingival crevice which can cause infection
- use gingival clasps for incisors, canines, premolars = is the go-to clasp
19
Q
what are occlusally approaching clasps
A
- less subtle as on occlusal surface
- but mainly at back of the mouth on molars so don’t see too obviously
20
Q
what is bad about longer clasps
A
- longer ones go around 3 tooth surfaces but as they are so long they can easily become distorted by occlusal forces
21
Q
what is reciprocation
A
- as part of the claps assembly each clasp unit will have opposing element to the retentive clasp arm to prevent pressure from the clasp acting on the tooth
- clasps cause tooth to move, reciprocation stops this = reciprocating arm on opposite side of tooth of clasp
22
Q
why does the reciprocating element need to be there before clasp
A
- will forget otherwise
- always factor in at design stage
23
Q
how does path of insertion help retention
A
- changing path of insertion will give anterior retention without the use of clasps
- may need to change path of insertion to improve retention and aesthetics = change to use undercuts for retention
- but changing anterior will have a consequence at back of the mouth
24
Q
what is the pattern of retention
A
- how many retainers on each arch
- when deciding on retention, first need to decide on the patter of retention
25
what is the usual pattern of retention
- usually 3 = 1 helps resits longitudinal plane of the other 2
- angular pattern of retention = want as big a triangle as possible as more stable
26
what may you do in a small saddle area for retention
- may not need a retainer on each abutment tooth
- reduce/simplify if possible = only put retainer on most posterior tooth
- removing one retainer will then change pattern of retention into a straight line
27
what is the stress relieving clasp system (RPI)
- system used in free end saddle designs to prevent stress on last abutment tooth = want to minimise stress on the tooth as much as possible, ideally only want force of rest on it
- go to design for bilateral free end saddle on lower
commonly adopted for distal extension removable partial dentures, Kennedy Class I and II
28
what makes up the stress relieving clasp system
- Rest (occlusal) = on mesial of the tooth, rounded on impression surface
- Proximal plate (adjacent to saddle) = guide surface of 2-3mm undercut to permit movement
- I-bar clasp (gingivally approaching) = greatest prominence of tooth contour
29
when is the stress relieving clasp system (RPI) used
- in free end saddle designs to prevent stress on last abutment tooth
30
what is indirect retention used for
- principally supporting elements
| - resistance to rotational displacement of denture
31
how is indirect retention obtained
- retention obtained by the extension of a partial denture base to provide the fulcrum of a Class II lever
- retainers providing indirect retention lie between the fulcrum and that part of the denture subject to displacing forces
- move supporting element away from the saddle area
32
why do you move the supporting element away form the saddle area
- if the clasp and supporting element are close together then it will dislodge easily
33
where are indirect retainers placed
- located on opposite side of retentive fulcrum line for mucosa supported base
- indirect retainers are placed on canines or mesial fossa or first premolars
34
what are some indirect retention
- major connectors
- minor connectors
- rest
- saddle
- denture base
35
what are the retention guidelines
- not required on every adjacent (abutment) tooth to denture saddle
- one clasp on each side fo the arch should be sufficient depending on the saddle length
- a triangular pattern of retention is desirable to avoid instability
- combination of retention methods can be used
- indirect retainers prevent rotation and can be used to provide stability particularly for free end saddles or very long bounded saddles
- stress relieving clasp system (RPI) is predominantly used for a mandibular free end saddle, but can be used on maxillary
- minimum retention would be retainer at each side of arch = couldn't have on only one side
