Retention Flashcards
(13 cards)
How is retention achieved
Retention in a denture can be achieved by mechanical methods, placing clasps in an undercut or utilising patient mucosa
•When using mechanical retention such as clasps the primary model requires surveying and sending to the clinician for them to design the RPD efficiently. It may be that the path of insertion may be altered to utilise undercuts or the clinician may make undercuts by placing composite on the tooth.
What should a clasp always be supported by
A clasp should always be supported with a rest- this can prevent the clasp from occlusal forces either becoming damaged or impinging on tissues and becoming damaged
•A cast clasp should engage 0.25mm of undercut. Clasps are only success if the force dislodging the denture is less than the force required to flex the clasp over the bulbosity- this can be dictated by the shape of the tooth. The larger the deflection (figure 3, tooth 1) the greater the resistance to displacement.
Flexibility is dependent on on the design
longer the clasp the more flexible it is, 15mm has been suggested as the appropriate length. The thinner the clasp the more flexible, and the design is important, a half round type clasp flexes more readily in the horizontal motion (figure 4), this is why most pre formed clasps come in this shape, tapered thinner at the end for flexibility
Clasp main component
minor connector, a principle rest, retentive arm, non retentive arm.
Gingival approaching claps
A gingivally approaching clasp contacts the tooth surface only at its tip. The remainder of the clasp arm is free of contact with the mucosa of the sulcus and the gingival margin.
The length of the gingivally approaching clasp, unlike the occlusally approaching clasp, is not restricted by the dimensions of the clasped tooth. The length of the gingivally approaching clasp arm can therefore be increased to give greater flexibility
Occlusally aproching
Only the terminal third of an occlusally-approaching clasp (should cross the survey line and enter the undercut area. If, in error, too much of the clasp arm engages the undercut, the high force required to move it over the maximum bulbosity will put a considerable strain on the fibers of the periodontal ligament and is likely to exceed the proportional limit of the alloy, thus distorting the clasp
Gingivally approaching shouldn’t
Should not touch soft tissues- this would cause trauma
•Can be placed slightly distally of the dentition to mask unsightly metal components. This is only if there is a suitable undercut.
•Due to this type of clasp passing gingival margins it has been criticized for oral hygiene. However, there isn’t any evidence to suggest one type of clasp is less hygienic than another.
•A shallow vestibule or severe tissue undercuts are contraindications. Both are likely to cause tissue damage and the latter is a hygiene issue as food can become trapped around the claps
RPI system
Is a system that combines an occlusal rest (R) distal guide plane (P) and gingivally approaching I bar clasp (I)
This system is used primarily in mandibular distal extension saddles.
It is designed to allow a vertical rotation of a distal extension saddle into the denture-bearing mucosa under occlusal loading without damaging the supporting structures of the abutment tooth. As the denture is pressed into the mucosa the denture rotates about a point close to the mesial rest and the I bar and plane disengage the tooth
Indirect retention
Classed as- part of a removable partial denture that assists direct retainers in the prevention of displacing forces.
Indirect retainer
F- fulcrum- indirect retainer- a component which obtains support
R-resistance- retention generated by clasp
E-effort displacing force (bolus of sticky food)
To obtain indirect retention the clasp must be placed between the saddle and the indirect retainer
Major connectors- maxilla
The indirect retainer in the first picture is ineffective due to the close proximity to the retentive clasp arm
Picture two is a modification of the design, the clasp axis is closer to the saddle and indirect retention is improved
Indirect retention
Tooth support is always preferable due to the compressive nature of mucosa……
If no alternative an indirect retainer on mucosa must covered a large area to spread load and limit potential tissue damage. Limited to the maxilla due to hard palate. Not very effective as an indirect retainer
Support for indirect retention
Support for indirect retention should sit at right angles to the path of movement.
If it rests on an inclined tooth surface there is the potential for teeth to move due to forces. Movement of the tooth will result in loss of support for indirect retainer.
What does this mean?
Clinicians should ‘cut’ rest seats into the dentition to provide this angle
