Removable Partial Dentures Direct Retainers part I Basic

Removable Partial Dentures Direct Retainers- part I

Basic two types of direct retainers 1. Intra-coronal Within the contour of the crown e. g. precision attachment 2. Extra-coronal Outside the contour of the crown of the tooth e. g. clasp

Intracoronal retainer consists of a key and keyway

Extracoronal circumferential direct retainer

Intracoronal retainersthe exact parallelism of the retainer units results in a binding action when dislodging forces attempt to alter their parallel relationship �Opposing parallel walls serve to limit the movement & resist removal through frictional resistance.

Extracoronal retainers: �Come in three principal forms- Clasp- type retainer - Interlocking components or spring loaded devices - Attachments which use flexible rings or clips that engage rigid component on the external surface of abutment. -

Clasp-type retainers �it is the most common � 2 - it works through a flexible clasp arm which engages a depression (undercut) on the tooth cervical to the greatest convexity (the height of contour).

Inter locking devices

Spring loaded devices-

Retention by extracoronal retainers: �Mechanical resistance to displacement through the components placed on or attached to external surface of an abutment tooth.

Analysis of tooth contour for retentive clasps: �Before moving on to principles of clasp design its vitally imp to consider how tooth contour & RPD components must be related to allow a stable prosthetic function. �SURVEYING

Surveying �The vertical arm when brought in contact with a tooth surface, identifies the location on the clinical crown where the greatest convexity exists. �This is called the height of contour , a term 1 st used by Kennedy. * �The height of contour will change as the vertical position of the cast changes.

cone theory. �concept was 1 st advanced by Prothero in 1916�He described shape of crown of premolar & molar as that of 2 cones sharing a common base

�as the height of contour is traced , 2 basic regions are defined : � 1 - the occlusal or incisal region that is freely accessible to the prosthesis � 2 - the gingival region that can only be accessed by the flexible tip of the clasp.

�When the surveyor blade contacts a tooth on the cast at its greatest convexity, a triangle is formed. �The apex of this triangle is at the point of contact of surveyor blade with the tooth �And base is towards the gingival tissues. �This apical angle is called angle of cervical convergence

Classification: Those that approach the retentive undercut from an occlusal directioncircumferential clasp. 2. Those that approach the retentive undercut from a cervical direction- Bar clasp. 1.

Basic Requirements All clasps must be designed so that they satisfy the following six basic requirements: 1. Retention 2. Support 3. Stability 4. Reciprocation 5. Encirclement 6. Passivity

1. Retention is provided by the retentive arm which prevents the partial denture from displacement away from the tissues toward the occlusal. While the partial denture is seated the retentive tip must be passive. However, it should touch the tooth in the undercut area.

Amount of Retention: �Clasp retention is based on the resistance to deformation of metal. �Factors that effect retention can be divided into 1. Tooth factors 2. Prosthesis factors

Tooth factors: 1. Size of angle of cervical convergence 2. How far clasp is placed in ACG �

Size of angle of cervical convergence: �The location & depth of undercut available for retention are only relative to the path of placement & removal of RPD.

Prosthesis factors: � 1. 2. 3. 4. Clasp flexibility: Clasp length Clasp diameter Clasp cross-sectional form[ shape] Material used for making clasp[ alloy]

Clasp flexibility �Length of clasp arm�Longer the clasp arm the more flexible it will be.

Taper �The retentive circumferential clasp should taper uniformly from its point of origin to its tip.

Diameter of the clasp: �The greater the diameter of a clasp arm the less flexible it will be.

Cross-sectional form: �Flexibility may exist in any form, but is limited to only one direction in the case of the half-round form �The only universally flexible form is the round form.

Material used for construction: �Gold alloy have a greater flexibility than chrome alloys , but bulk of the prosthesis has to be increased to obtain the needed rigidity. �Chrome alloys have a higher modulus of elasticity & therefore less flexibility.

SECONDARY FACTORS: 1)frictional resistance 2)amount of tooth surface covered 3)intimacy of tooth clasp contact 4)direction of approach of retentive terminal

2. Support while chewing food the rest prevents tissue ward movement of the clasp assembly, plus directs the force along the long axis of the tooth, thus reduces periodontal tissue damage. PROVIDED BY: 1)occlusal rests 2)body and shoulders of clasp

3. Bracing/ Stability Bracing is the resistance to horizontal forces. This provided by the rest, minor connector and bracing arm. the bracing arm mainly provides reciprocation which is resistance to force applied by retentive tip. CAST CIRCUMFERENTIAL CLASP provides more stability than BAR TYPE CLASP

4. RECIPROCITY �RECIPROCITY it means by which one part of appliance is made to counteract the effect created by another part. RECIPROCAL ARM v/s RETENTIVE ARM

5. Encirclement There must be more than 180 degrees crown coverage otherwise, the clasp can slip-off the abutment tooth.

More than 180 degree �The engagement can be in the form of continuous contact, such as circumferential clasp, or discontinuous contact, such as Bar clasp.

5. Passivity The retentive clasp arm should be passive (no active force) �no pressure against tooth except when activated. �should permit slight degree of movement of base -without transmitting any appreciable amount of stress to abutment tooth.

factors about clasp design: 1. In a clasp only the tip of the retentive arm should be flexible, the other components are rigid 2. Selection of clasp will depend on the survey line (height of contour), retention area available, type of partial denture i. e. whether it is tooth-supported or distal extention removable partial denture. 3. Flexibility of clasp arm can be increased by curving and/or lengthening the arm. 4. The thickness of the retentive clasp arm tip should be ½ the thickness at its attachment to the body of the clasp arm 5. The greater the diameter of the clasp arm, the lesser will be the flexibility and vice versa. 6. Wrought metal is more flexible than cast metal. 7. If buccal retention is used on the left side of the arch, buccal retention must be used on the right side of the arch

Classification of extra-coronal retainers: 1. Supra bulge clasps (occlusally approaching, circumferential or encircling clasps). The retentive arm approaches the undercut area from the suprabulge direction. 2. Infrabulge clasps (gingivally approaching, projection or bar clasps) The retentive arm approaches the undercut from the infrabulge direction, e. g. Bar clasp arm (I-Bar etc. ) 3. Combination clasps a. ½ clasp is circlet and ½ clasp is Bar type or b. Combination of two types of metals-Chrome-cobalt an wrought wire

Types of circumferential clasps 1. Circlet clasp. 2. Half & half clasp. 3. Fish hook or hairpin clasp. 4. Reverse circlet clasp. 5. Embrasure clasp. 6. Ring clasp. 7. Back action and reverse back action clasp.