RPD design - retention

Question 1

what is retention

Answer 1

• 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

Question 2

how can retention be achieved

Answer 2

• mechanical
• muscular
• physical

Question 3

how is mechanical retention achieved

Answer 3

• mainly clasps = engage tooth undercuts

- friction also plays a role

Question 4

how is muscular retention achieved

Answer 4

• 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

Question 5

how is physical retention achieved

Answer 5

• 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

Question 6

what is adhesion

Answer 6

surface forces of saliva on denture and mucosa

Question 7

what is cohesion

Answer 7

forces within saliva, viscosity

Question 8

what is atmospheric pressure

Answer 8

resistance to displacing forces

Question 9

what are the 2 classifications of retention

Answer 9

• direct

- indirect

Question 10

what is direct retention

Answer 10

• resistance to vertical displacement of denture

- using clasps

Question 11

what is indirect retention

Answer 11

• 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

Question 12

how is direct retention achieved

Answer 12

• 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

Question 13

what are the mechanical components that provide retention

Answer 13

• main one is claps

Question 14

what are clasps

Answer 14

• 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

Question 15

what are clasps made of

Answer 15

• wrought metal

- they are incorporated into the denture base or included as part of a cast denture base

Question 16

what two ways do clasps approach the undercut

Answer 16

• occlusally approaching

- gingivally approaching

Question 17

how do clasps work best -

Answer 17

• in conjunction with a rest

- helps prevent claps slipping down o gingival margin and causing damage

Question 18

what are gingivally approaching clasps

Answer 18

• 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

Question 19

what are occlusally approaching clasps

Answer 19

• less subtle as on occlusal surface

- but mainly at back of the mouth on molars so don’t see too obviously

Question 20

what is bad about longer clasps

Answer 20

• longer ones go around 3 tooth surfaces but as they are so long they can easily become distorted by occlusal forces

Question 21

what is reciprocation

Answer 21

• 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

Question 22

why does the reciprocating element need to be there before clasp

Answer 22

• will forget otherwise

- always factor in at design stage

Question 23

how does path of insertion help retention

Answer 23

• 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

Question 24

what is the pattern of retention

Answer 24

• how many retainers on each arch

- when deciding on retention, first need to decide on the patter of retention

Question 25

what is the usual pattern of retention

Answer 25

• 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

Question 26

what may you do in a small saddle area for retention

Answer 26

• 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

Question 27

what is the stress relieving clasp system (RPI)

Answer 27

• 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

Question 28

what makes up the stress relieving clasp system

Answer 28

• 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

Question 29

when is the stress relieving clasp system (RPI) used

Answer 29

• in free end saddle designs to prevent stress on last abutment tooth

Question 30

what is indirect retention used for

Answer 30

• principally supporting elements

- resistance to rotational displacement of denture

Question 31

how is indirect retention obtained

Answer 31

• 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

Question 32

why do you move the supporting element away form the saddle area

Answer 32

• if the clasp and supporting element are close together then it will dislodge easily

Question 33

where are indirect retainers placed

Answer 33

• located on opposite side of retentive fulcrum line for mucosa supported base
• indirect retainers are placed on canines or mesial fossa or first premolars

Question 34

what are some indirect retention

Answer 34

• major connectors
• minor connectors
• rest
• saddle
• denture base

Question 35

what are the retention guidelines

Answer 35

• 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