DIRECT RETAINERS Dr. Waseem Bahjat Mushtaha PhD in prosthodontics

The removable partial denture must have sufficient retention to resist reasonable dislodging forces. Means of retention: A-Mechanical means: 1-Direct retainers.2-The intimate relationship of partial denture components with the abutments.3- The denture base engages soft tissue undercuts.4-Indirect retainers. B-Physiological means: The same physical means of complete denture retention act on the removable partial denture bases and maxillary major connectors.

DIRECT RETAINERS Definition:The component that engages an abutment tooth and in so doing resists dislodging forces applied to a removable partial denture is called direct retainer.

Types of direct retainers:1-Intracoronal retainers (internal or precision attachment): It consists of two units ,the first one is a female receptacle in a crown or inlay in the abutment. The second part is a male insert attached to the partial denture. The retention is derived by the frictional fit and wedging action between the male and female components when a dislodging force alters their parallelism.Advantages:1-Elimination of visible retentive and support components.2-Better vertical support than extracoronal retainers. since its location directs the force along the long axis of the tooth. 3-Horizontal stabilization to the dentures is provided.

Intracoronal retainers

Intracoronal retainers

Disadvantages:1-It require complicated clinical and laboratory procedures.2-Require prepared abutment and casting.3-Frictional retention is progressively lost because of its wear.4-It is difficult to repair and replace.5-It is not used in short crowns and large pulp.6-Expensive.7-lt requires stress breaker in distal extension cases

II-Extracoronal retainers (clasps): The clasp is a retainer in which a flexible arm engages an external surface of an abutment tooth in an undercut area cervical to the greatest convexity of the tooth (height of contour) Requirements of clasps:All clasps must be designed so that they satisfy the following six basic requirements: I-RETENTION:The retentive clasp arm provides resistance to vertical dislodging forces (retention).

Factors determining the amount of clasp retention: A-Amount of undercuts; the greater the amount of tooth undercut engaged by the clasp, the greater the retention. B- The greater the vertical distance the retentive clasp terminal is placed in an undercut, the greater will be the retention. C-The flexibility of the clasp arm; as the flexibility of the arm is increased, the retention decreases. The arm flexibility depends on:1-The diameter of the arm; the smaller the diameter of the arm, the greater the flexibility.2- The length of the arm; the longer the arm, the greater the flexibility.3- The taper of the arm; the greater the taper of the arm, the greater the flexibility.4-The cross-section of the arm; the half round clasp arm is less flexible than rounded clasp arm, which provides flexibility in all directions.5-The material of the arm; the chromium cobalt alloy is less flexible than cast gold alloy, and the wrought wire is more flexible than cast alloys.

II-BRACING (STABILITY):Rigid portions of clasps (the proximal two thirds of the retentive clasp arm and the entire length of the bracing arm) provide resistance to horizontal, lateral or torsional components of force (stability). III- SUPPORT: The rest provides resistance to vertical seating forces (support). IV- RECIPROCATION: The bracing arm provides resistance to horizontal forces exerted on a tooth by the retentive clasp terminal during seating and unseating of the prosthesis (reciprocation).

Extracoronal circumferential direct retainer

V- ENCIRCLEMENT:Adequate encirclement prevents horizontal tooth movement within the confines of the clasp. This is achieved by engagement of a greater than 180 degrees (more than half of the tooth circumference), or a minimum of three widely separated points of contact .VI- PASSIVITY:There should be no active force on the tooth when the clasp is in place. Its retentive function should be activated only when a dislodging force is applied.

TYPES CLASPSI-Occlusally approaching (circumferential or subrabulge) clasps:The retentive clasp arm originates from a minor connector or proximal plate, usually near the occlusal surface and approaches the undercuts from an occlusal direction. II-Gingivallv approaching (bar, roach or infrabulge clasp: The retentive arm originates from a major connector or denture base, passing adjacent to the soft tissues and approaching the tooth from a gingival direction. III- Combination: Cast clasp arm and wrought wire clasp arm.

I-OCCLUSALLY APPROACHING (CIRCUMFERENTIAL) CLASPS Design: 1-The retentive arm flexible terminal engages a measured undercut in the gingival one third of an abutment crown.2- The retentive clasp arm terminus should not contact the free gingival margin and should end at the mesial or distal line angle pointing towards the occlusal surface.3-The stabilization (reciprocation) arm and the proximal two thirds of the retentive arm should be placed no higher than the middle third of the tooth, preferably at the junction of the gingival and middle third.4- The bracing clasp arm should be slightly thicker than the retentive clasp arm to promote rigidity.

Cast half-round circumferential clasp on a molarHorizontal arm on molar - it is rigid, non-flexible, and placed in a non-retentive area

Advantages:l-Provides excellent support, bracing and retention.2- It is the easiest clasp to construct and repair.3-Causes less food impaction than the bar clasps.Disadvantages:1 -Covers more tooth surface than the bar clasp, with more caries susceptibility.2-The occlusal approach tends to increase the occlusal table.3-lt is not esthetically acceptable in anterior region of the mouth 4-lt can be adjusted in a buccolingual but not in occIusogingival direction.

TYPES OF OCCLUALLY APPROACING CLASPS l-Circlet (Aker's) clasp: Indication:It is used for the tooth- supported partial dentures.Design:I-It consists of: a- A minor connector.b- Occlusal rest.C-Cast circumferential retentive clasp arm.d-Cast circumferential bracing clasp arm.2-Engages 0.01 inch undercut.3-Provides bilateral bracing.

Reciprocal ArmOcclusal RestRetentive Arm

l-Circlet (Aker's) clasp

2-The half and half claspThe half and half clasp is a modification of the circlet clasp with the reciprocal arm coming from one direction and the retentive arm from the other. Two rests are used for this clasp. It is used on molars and premolars. The retentive tip is placed into a 0.01 inch undercut.

3-C-clasp (hairpin clasp): Indication:When a distofacial undercut exists. It is rarely indicated, since its retentive arm covers a large amount of tooth structure.Design:1-It consists of:a-Rest.b-Minor connector.

`c-Cast circumferential C- retentive clasp arm; it crosses the facial surface of the tooth from its origin then turns back to engage proximal undercut below its point of origin. d-Cast circumferential bracing clasp arm.2-lt engage 0.01 inch undercut.3-Provides bilateral bracing.Disadvantages:1-It covers considerable tooth surface.2-Display too much metal3-Its flexibility is limited.

C-clasp (hairpin clasp- fish hook)

3-The reverse circlet claspThe reverse circlet clasp is used when the retentive undercut is located on the surface of the abutment tooth adjacent to the edentulous space.It is indicated in class I & class II distal extension partial dentures where deep tissue undercut precludes the use of infrabulge clasp.

4-Embrasure clasp :Indications:For unmodified class II and class III. in the side were no tooth missing.Design:1-It is essentially two circlet clasps originating from common minor connector, therefore it is composed of: a-Two occlusal rests: Rests are used to provide support and avoid interproximal wedging and food impaction. Enough preparation for the occlusal rests and extension of the preparation to the marginal ridge buccolingual to accommodate the clasp arms is necessary. Teeth interproximal contact should be maintained during this preparation.b-One minor connector.c- Two cast circumferential retentive clasp arms.d- Two circumferential bracing clasp arms.2- Provides bilateral bracing.3-Utilizes 0.01 inch undercut.

Embrasure clasp

4-Multiple Aker's clasp: Indications:1-In tooth borne RPD when additional retention and bracing are needed.2-May be used instead of embrasure clasp when the undercut areas are adjacent to each other.Design:It is two opposing circlet clasps joined at the terminal end of the reciprocal arms

5-RPA-Clasp: Indication:Used in tooth mucosa borne partial dentures where an RPIclasp cannot be used because of bar clasp arm contraindications

Design:I-It consists of: a-Mesial occlusal rest.b- Minor connector, placed into the mesiolingual embrasure, but not contacting the adjacent tooth.c- Proximal plate: A distal guiding plane extending from the marginal ridge to the junction of the middle and gingival third of the abutment is prepared to receive proximal plate. The proximal plate contacts only 1 mm of the gingival portion of the guiding plane.d- Cast circumferential retentive clasp arm; arises from the proximal plate and engages mesiobuccal undercut. The shoulder of the retentive arm contacts the tooth at the height of contour at the junction of the middle and gingival third of the tooth.2- Upon function the proximal plate and the retentive tip move mesiogingivally into a deeper undercut to decrease stresses falling upon the abutment.2-It engage 0.01 inch undercut.3-Provides bilateral bracing.

7-Ring clasp: Indication:In tilted single standing posterior molar.

Design;1-Its component parts are: a- Rest (s): The main occlusal rest is placed on the mesial marginal ridge and an auxiliary rest may be placed on the opposite marginal ridge to prevent further movement of the mesially inclined tooth.b Minor connector.c-A single cast circumferential retentive clasp arm which encircles nearly the entire circumference of the tooth. *On mandibular molar the clasp arm begins on the mesiobuccal surface and terminates in an undercut area on the mesiolingual surface. *On the maxillary molar the direction of clasp is reverse; it begins at the mesiolingual surface and terminates on the mesiobuccal surface.d- Reinforcing strut arm for retentive clasp arm; provides rigidity to the clasp arm on the non retentive side. This rigidity is necessary for reciprocation and stabilization.2-Engages 0.02-0.03 inch undercut.3-Provides unilateral bracing.4-It covers too much tooth surface.

Ring clasp

II- THE GINGIVALLY APPROACHING CLASPS Design:1-The approach arm of the bar clasp must never impinge on soft tissue.2- The tissue surface of the approach arm should be smooth and polished with no relief provided.3- The approach arm should taper uniformly.4- The vertical portion of the approach arm should cross the gingival margin at 90 degree angle. 5- The distance between the vertical projection arm and other vertical component should be at least 5 mm. Advantages:1-Greater retention than occlusally approaching clasps due to the trip action.2-It is more esthetic than the occlusally approaching clasps.3-The flexibility of the bar clasp arm can be controlled by its taper and length.4-Covers less tooth structure than the occlusally approaching clasp (less caries susceptibility). Disadvantages:1-Greater tendency to collect and hold food debris (more gingival irritation) than the occlusally approaching clasp.2- The retentive arm does not contribute to bracing and stability.

Contraindications:l-When a deep cervical tooth undercut exists. 2- When a severe tissue undercut exists.3- When there is a shallow vestibule.4- When there is excessive buccal or lingual tilt of the tooth.5- When the height of contour is close to the occlusal surface of the tooth.

TYPE OF THE GINGIVALLY APPROACHING CLASPS I-Bar clasp (Roach or vertical projection): Indications:I-On abutment for tooth supported or tooth-mucosa supported RPD when there is distobuccal undercut.2-In situations in which esthetics is important.

Design:1-Component parts: It is usually used as a combination clasp from : a-Rest.b-Minor connector.C A cast bar retentive clasp arm.d- A cast circumferential bracing clasp arm.2-Provides unilateral bracing.3 -Engages 0.0 I inch undercut.4-The shape of the retentive terminal as a T, modified T, Y, or I classify the bar clasp. a-The I and Y retentive clasp arms provide tripping action, which is called push type clasp.

b- The T and modified T clasps did not have tripping action, since the retentive terminal actually engage the undercut from an occlusal direction. One terminal of the T -bar clasp lies above the survey line for bracing and the other terminal below the survey line for retention. The modified T-bar clasp has only one terminal below the survey line.

2-RPI- CLASP: Indication:Commonly used for tooth mucosa borne partial dentures. Design:1-It consists of: a-Mesial rest.b-Minor connector, placed into the mesiolingual embrasure, but not contacting the adjacent tooth.c-Proximal plate contacts approximately 1 mm of the gingival portion of guiding plane.d-Cast I bar retentive clasp arm located at the mesiobuccal prominence of the tooth or mesial to it.2-Engages 0.01 inch undercut.3-Provides unilateral bracing.4- The proximal plate and the minor connector provide stabilization and reciprocation.5-During function, proximal plate and I-bar clasp arm move in a mesiogingival direction-disengaging tooth. This distributes more functional load to edentulous ridge.

III- Combination ClaspIndication: Commonly used for tooth mucosa borne RPD, when there is mesiobuccal undercut on the abutment tooth.Design:1-Cornponent parts are :a- Restb- Minor connectorc- Wrought wire circumferential retentive clasp arm.d-Cast circumferential bracing clasp arm.2-Engages 0.01- 0.02 inch undercut.3-Provides bilateral bracing, but less than a cast circumferential clasp.

Advantages:1 - Greater flexibility.2-It is more esthetically acceptable because it is placed too much gingival.3-lt flex in all directions (round in cross section), which allow it to dissipate forces exerted on the abutment tooth.4- The retentive arm can be adjusted in all directions.5-Makes little tooth contact (line contact. rather than the surface contact of the cast clasp arm). Disadvantages:1-It is easily distorted or fractured by careless handling by the patient.2-It does not possess the bracing and stabilizing qualities of cast clasp arm.3-It involves extra step in fabrication

Wrought wire (19 ga.) clasp soldered to framework on a premolarRemote soldered or laser welded

Criteria for clasp selection The selection of the clasp depends mainly on the type of support, the presence of undercut areas, and esthetics.1-For bounded saddle; the retentive undercut present is used with any acceptable clasp type (clasping for convenience), 2-For distal extension base; a stress releasing clasp that equitably distribute the force between the abutment and the ridge is used: a-If a mesiobuccal undercut is available on the terminal abutment, a combination clasp with the wrought wire, RPI, or RPA clasps are used.b-If the retentive undercut is located on the distobuccal surface, a bar clasp, reverse circlet (the minor connector and the occlusal rests are placed mesially) and the C-clasp are used.c-If mesiolingual undercut is present a wrought wire clasp is used.