5. RPD Design - Retention Flashcards
Definition of retention
Term used to describe the resistance of a denture to lifting away from the tissues (vertical dislodging forces)
Methods of achieving retention (3)
Mechanical means - clasps
Muscular forces - shape of denture
Physical forces - coverage of mucosa
Types of mechanical retention (3)
Clasps
Guide surfaces
Precision attachments (used in conjunction with crowns)
Engage tooth undercuts (below bulbous part of a tooth)
Types of muscular retention (2)
The action of surrounding musculature on the shape of the non-impression surface (polished surface) of a denture
Patient’s muscular control
Types of physical retention (4)
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
Types of retention (2)
Direct retention
Indirect retention
Definition of direct retention
Resistance to vertical displacement of denture
Definition of indirect retention
Resistance to rotational displacement of denture
Can be used to provide stability, particularly for free-end saddles or very long bounded saddles
Features of direct retention
Should be close to the base, as close as possible to the saddle and adjacent to the abutment teeth
Definition of guide planes
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
Features of guide planes (2)
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
Definition of clasp (2)
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)
Types of clasps (2)
Occlusally approaching
Gingivally approaching
Definition of occlusally approaching clasps
Approach the undercut from occlusal surface
Single arm clasp
Circumferential clasp
Ring clasp
Definition of gingival approaching clasps
Approach the undercut from direction of the gingival margin
I-bar clasp
How is retention with clasps achieved (4)
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
Clasp efficiency depends on (3)
Clasp material
Cross-sectional thickness and shape
Length of the clasp
Features of clasp material (3)
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
Features of cross-sectional thickness and shape (2)
Round – flexes in all directions
Half-round – more resistant to vertical movement
Features of clasp length
A minimum length of 15mm is required for a metal clasp to engage a 0.25mm undercut (approx. length of a molar clasp arm)
Advantages of non-metal materials for clasps (2)
Appearance
Engage deep undercut
Disadvantages of non-metal materials for clasps (3)
Bulkier
Not adjustable
Cost (more expensive)
Information required to position a clasp (4)
Position of the survey line
Type of abutment tooth
Clasp length (15mm for cast CoCr)
Type of material - cast or wrought
Function of reciprocation
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
Features of reciprocation (3)
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
Types of frictional resistance (2)
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
Improving efficacy of guide planes/parallel surfaces
Make guide surfaces as large as possible
Features of altered path of insertion and removal (2)
Along the proximal surfaces of teeth
Different to the common path of insertion and will provide frictional resistance retention
Ideal pattern of retention (2)
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)
Influences of clasp placement (2)
Aesthetics
Saddle length
Definition of indirect retention
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)
Types of indirect retainers (4)
Major/minor connectors
Rests
Saddles
Denture base
Features of indirect retention (2)
Supporting elements (not clasps) Perpendicular to clasp arm
Retention issues
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)
Function of RPI
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
Components of an RPI (3)
Mesial rest
Proximal guide plate
Gingivally approaching I-bar clasp
Retention utilised in acrylic dentures (2)
Frictional
Muscular
Features of frictional retention
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
Features of muscular retention
Use of forces of associated muscles acting on the non-impression surface of the denture (shape)
Types of clasps and uses (3)
Ring clasp - lower molars (MB/ML undercut)
Single-arm clasp - upper molars (DB/DL undercut)
I-Bar clasp - lower teeth
Features of clasp design (4)
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)
How does an RPI provide indirect retention
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)
