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BDS2 CDS Prosthodontics > 5. RPD Design - Retention > Flashcards

5. RPD Design - Retention Flashcards

1
Q

Definition of retention

A

Term used to describe the resistance of a denture to lifting away from the tissues (vertical dislodging forces)

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2
Q

Methods of achieving retention (3)

A

Mechanical means - clasps
Muscular forces - shape of denture
Physical forces - coverage of mucosa

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3
Q

Types of mechanical retention (3)

A

Clasps
Guide surfaces
Precision attachments (used in conjunction with crowns)
Engage tooth undercuts (below bulbous part of a tooth)

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4
Q

Types of muscular retention (2)

A

The action of surrounding musculature on the shape of the non-impression surface (polished surface) of a denture
Patient’s muscular control

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5
Q

Types of physical retention (4)

A

Using existing forces of adhesion, cohesion, surface tension and atmospheric pressure on the impression surface of the denture
Adhesion - surface forces of saliva on denture and mucosa
Cohesion - forces within saliva, viscosity
Atmospheric pressure - resistance to displacing forces
Closeness of adaptation, extent (area covered) of the denture base, peripheral seal

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6
Q

Types of retention (2)

A

Direct retention

Indirect retention

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7
Q

Definition of direct retention

A

Resistance to vertical displacement of denture

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8
Q

Definition of indirect retention

A

Resistance to rotational displacement of denture

Can be used to provide stability, particularly for free-end saddles or very long bounded saddles

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9
Q

Features of direct retention

A

Should be close to the base, as close as possible to the saddle and adjacent to the abutment teeth

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10
Q

Definition of guide planes

A

Two or more parallel axial surfaces on abutment teeth which can be used to limit the path of insertion and improve the stability of a removable prosthesis

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11
Q

Features of guide planes (2)

A 
Guide surfaces may occur naturally on teeth but more commonly need to be prepared
Guide planes (supplementary retention) should be close to the base and parallel to the path of insertion
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12
Q

Definition of clasp (2)

A

A metal arm that, when in position, contacts the tooth preventing removal of the denture base
To be effective as a retainer, the clasp is placed below the bulbous part of the tooth (undercut). The bulbous part of the tooth requires identification (surveying)
(Clasps work most efficiently when used in conjunction with a rest)

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13
Q

Types of clasps (2)

A

Occlusally approaching

Gingivally approaching

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14
Q

Definition of occlusally approaching clasps

A

Approach the undercut from occlusal surface
Single arm clasp
Circumferential clasp
Ring clasp

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15
Q

Definition of gingival approaching clasps

A

Approach the undercut from direction of the gingival margin

I-bar clasp

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16
Q

How is retention with clasps achieved (4)

A

Achieved by engaging an undercut area of the tooth
A flexible clasp arm deflects over the bulbous portion of the tooth to passively engage the undercut
To enable this to happen, the metal used must be elastic enough to engage and disengage the undercut
A metal must be chosen that will have a suitable modulus of elasticity

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17
Q

Clasp efficiency depends on (3)

A

Clasp material
Cross-sectional thickness and shape
Length of the clasp

18
Q

Features of clasp material (3)

A

Cast CoCr, cast gold alloy, cast titanium, wrought stainless steel, wrought gold alloy, wrought CoCr
Modulus of elasticity: CoCr twice that of Au
Proportional limit: stainless steel higher than both CoCr and Au

19
Q

Features of cross-sectional thickness and shape (2)

A

Round – flexes in all directions

Half-round – more resistant to vertical movement

20
Q

Features of clasp length

A

A minimum length of 15mm is required for a metal clasp to engage a 0.25mm undercut (approx. length of a molar clasp arm)

21
Q

Advantages of non-metal materials for clasps (2)

A

Appearance

Engage deep undercut

22
Q

Disadvantages of non-metal materials for clasps (3)

A

Bulkier
Not adjustable
Cost (more expensive)

23
Q

Information required to position a clasp (4)

A

Position of the survey line
Type of abutment tooth
Clasp length (15mm for cast CoCr)
Type of material - cast or wrought

24
Q

Function of reciprocation

A

As part of the clasp assembly, each clasp unit will have an opposing element to the retentive clasp arm to prevent pressure from the clasp acting on the tooth

25
Q

Features of reciprocation (3)

A

Denture is inserted, and clasp makes contact with the teeth
Retentive element of the clasp flexes over the bulbosity (identified by the survey line), while the reciprocal element remains in contact preventing the tooth being moved horizontally by the action of the clasp flexing
Clasp comes to rest passively against the tooth in the undercut

26
Q

Types of frictional resistance (2)

A 
Guide planes (supplementary direct retention) close to the saddle
Alternative path of insertion (inserting the denture to a different path of insertion to the path of displacement) can also be used
27
Q

Improving efficacy of guide planes/parallel surfaces

A

Make guide surfaces as large as possible

28
Q

Features of altered path of insertion and removal (2)

A

Along the proximal surfaces of teeth

Different to the common path of insertion and will provide frictional resistance retention

29
Q

Ideal pattern of retention (2)

A 
Triangular pattern of retention is desirable to provide stability
Straight line (across arch) if necessary (one clasp on each side of the arch should be sufficient depending on the saddle length)
30
Q

Influences of clasp placement (2)

A

Aesthetics

Saddle length

31
Q

Definition of indirect retention

A 
Retention obtained by the extension of a partial denture base to provide the fulcrum of a class II lever
(The retainer(s) providing direct retention lie between the fulcrum and that part of the denture which is subject to the displacing force)
32
Q

Types of indirect retainers (4)

A

Major/minor connectors
Rests
Saddles
Denture base

33
Q

Features of indirect retention (2)

A 
Supporting elements (not clasps)
Perpendicular to clasp arm
34
Q

Retention issues

A

If no undercuts on the teeth for retention, teeth may be prepared by the addition of composite on the tooth to create an undercut
(The composite build-up must be carefully done to ensure the undercut is not too great (large), otherwise the clasp may de-bond the composite from the tooth while engaging the undercut)

35
Q

Function of RPI

A 
Used in free-end saddle designs to prevent stress on the last abutment tooth
Stress relieving class system that is commonly adopted for distal extension removable partial dentures (Kennedy Class I and Class II)
Predominantly utilised for a mandibular free-end saddles or a very long bounded saddles
36
Q

Components of an RPI (3)

A

Mesial rest
Proximal guide plate
Gingivally approaching I-bar clasp

37
Q

Retention utilised in acrylic dentures (2)

A

Frictional

Muscular

38
Q

Features of frictional retention

A

Contact points between replacement teeth and natural abutments are maintained to provide frictional resistance by the inclusion of a wire stop of the last standing teeth (prevents distal movement of natural teeth)
A wire stop does not engage an undercut and, therefore, is not a clasp

39
Q

Features of muscular retention

A

Use of forces of associated muscles acting on the non-impression surface of the denture (shape)

40
Q

Types of clasps and uses (3)

A

Ring clasp - lower molars (MB/ML undercut)
Single-arm clasp - upper molars (DB/DL undercut)
I-Bar clasp - lower teeth

41
Q

Features of clasp design (4)

A

CoCr clasp 15mm long will engage a 0.25mm undercut
If the clasp length is less, a gingival approaching clasp, wrought wire or a specialised retention system will need too be employed
If proportional limit is exceeded, clasp will be permanently deformed
Alternatively, another material with a lower modulus of elasticity but a smilier proportional limit can be used (Pt-Au-Pd wire) or a material with a higher proportional limit but similar modulus of elasticity (stainless steel or wrought CoCr)

42
Q

How does an RPI provide indirect retention

A

Retention via the proximal plate and minor connector. If the gap between these is less than the width of the tooth, this provides reciprocation (the tooth cannot move palatally/lingually)

BDS2 CDS Prosthodontics (12 decks)

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