Direct Retainers Contents Introduction Classification Types of clasps
- Slides: 95
Contents • Introduction • Classification • Types of clasps • Requirements of clasp • Comparison of Circumferential and Bar clasps • Other clasp system • Recent advances • Conclusion • Bibliography
Introduction • Retention of removable partial denture is a unique concern when compared with other prosthesis, when dealing with the crown of fixed partial denture the use of preparational geometry & a luting agent combines to retain a prosthesis, but • In case of removable partial denture, mechanical retention is achieved by means of direct retainer by frictional means, by engaging a depression in the abutment tooth or by engaging a tooth undercut lying cervically to its height of contour.
“That component of a partial removable dental prosthesis used to retain and prevent dislodgment, consisting of a clasp assembly or precision attachment” -GPT.
Classification of Direct Retainers It is broadly classified into 1 Intracoronal direct retainers a. Precision attachments b. Semiprecision attachments 2 Extracoronal direct retainers a. Retentive clasp assemblies • Suprabulge • infrabulge b. Attachments
INTRA-CORONAL RETAINERS / INTERNAL ATTACHMENTS / PRECISION ATTACHMENTS: • It is an interlocking device, one component of which is integrated into a removable prosthesis to stabilize and/or retain it. • This principle was first formulated by Dr. Herman E. S. Chayes in 1906. • It resides within the normal contours of an abutment & functions to retain & stabilize a RPD.
Advantages 1. 2. 3. 4. Elimination of visible rest and support component Better vertical support Horizontal stabilization Better stimulation of underlying tissues because of intermittent vertical massage
Disadvantages 1. Require prepared abutments and castings 2. They eventually wear with progressive loss of frictional resistance to denture removal. 3. Difficult to repair and replace 4. Complicated clinical and laboratory procedures 5. Difficult to place completely within the circumference of tooth 6. Expensive
Limitations 1. Size of the pulp depends on age 2. Short or abraded teeth 3. Not to be used in extensive tissue supported distal extension cases unless some form of stress breaker is used.
EXTRA-CORONAL DIRECT RETAINERS / CLASPS: • It consists of components that reside entirely outside the normal clinical contours of the abutment. • They serve to retain & stabilize RPD. • They are divided into 2 distinct categories – Extra coronal attachments – Retentive clasp assembly
EXTRA CORONAL ATTACHMENTS • It was first introduced by Henry. r. Boos in the early 1900’s, modified by F. Ewing Roach • It derives retention from the close fitting components such as matrices & patrices • It permits vertical movement of the prosthesis during occlusal loading.
RETENTIVE CLASP ASSEMBLY • It’s the most common method for Extra coronal direct retention. • It was first appeared in the dental literature with Dr W. G. A. Bonwill’s description. • It operates on the principle of resistance of metal to deformation.
• In 1916 – Prothero’s Advanced Cone Theory was introduced which is the basis of clasp retention. Shape of the crowns of PM and molar teeth can be considered as two cones sharing a common base. the line formed at the junction of these cones represents the greatest diameter of the tooth.
• • Kennedy called it as Height of contour Cummer as Guideline De Van as Supra & infra bulge regions Height of contour changes depending on position of teeth. Retentive undercut exists only in relation to given path of placement or removal.
TYPES OF CLASPS 1) Circumferential/ Supra bulge/ Akers clasp: – Has two clasp arms that encircle the abutment tooth. Clasp approaches undercut from above the height of contour 2) Bar Clasp/ Infra bulge clasp: – Has an approach arm ending up in two clasp arms one placed above and the other placed below the height of contour. Clasp approaches undercut from below the height of contour.
Component parts of a clasp • • Rest Body Shoulder Retentive clasp arm Reciprocal arm Minor connector Approach arm
Rest: § Part of the clasp assembly that lies on the occlusal or lingual or incisal edge or surfaces of the teeth § Resist’s the tissue ward movement of the clasp. § Transmit functional forces along the long axis of the tooth.
Body: • Part of the clasp that connects the rests and shoulders of the clasp of the minor connectors. • It is rigid and lies above the height of contour • It contacts the guide plane of the abutment during the insertion and removal
Shoulder • Part of the clasp that connects the body to the clasp terminal • Lies above the height of contour • Provides stabilization against horizontal displacement
Retentive Clasp “A flexible segment of a removable partial denture which engages a under cut on an abutment which is designed to retain the denture” – GPT § § § Terminal 3 rd / retentive terminal/ Terminus Middle 3 rd Proximal 3 rd
Terminal 3 rd / retentive terminal/ Terminus – It is flexible and should point towards occlusal surface – Only component to lie cervical to height of contour to provide direct retention. Middle 3 rd – Provides limited flexibility Placed above the height of contour Proximal 3 rd – Rigid – Placed above the height of contour
Reciprocal Arm: • It is rigid and placed above the height of contour. • It is not tapered like retentive clasp • It is designed to contact the tooth before the retainer clasp does and to remain in contact while the retentive terminal passes the height of contour.
Purpose: - • Resist tipping force generated by the retentive terminal as it passes over the height of contour when partial denture is inserted or removed. • Helps in stabilization and reciprocation against retentive arm • May act as indirect retainer if placed on the opposite side of fulcrum
Minor Connector: • Part of the clasp that joins the body to the remainder of the framework
Approach arm: • It projects from the framework, runs along the mucosa turns to cross-gingival margin of abutment tooth. • Body and retentive terminal attached to it. • It is slightly flexible.
Requirements of the clasp 1. 2. 3. 4. 5. 6. Retention Support Stability Reciprocation Encirclement Passivity
Retention “It is the quality of the clasp assembly that resists forces acting to dislodge components away from the supporting tissues”. • Provided by the retentive clasp,
Amount of retention depends 1. Flexibility of clasp arm. 2. The depth of the retentive terminal extending into the undercut. 3. The amount of clasp arm that extends below height of contour.
Flexibility depends on: • Length of clasp arm (Double the length 5 times is the flexibility) • Diameter of clasp arm is • Cross-sectional form of the clasp inversely proportional to the flexibility – – Round -increases flexibility and ability to flex in all planes Half round - decreases flexibility and flexes only in single plane
§ Curvature of the clasp arm More the curvature makes the clasp more rigid. Material • • Cr-Co alloys have higher Modulus of elasticity than gold alloys so reduced flexibility. To obtain equal retention a greater depth of undercut is required for a wrought wire clasp. Materials used and the undercut to be engaged Cast chrome - 0. 010” Cast gold - 0. 015” Wrought alloy - 0. 020”
2, The depth of the retentive terminal extending into the undercut. • Its described in 2 distinct dimensions – A medio-lateral or horizontal dimension. – A occluso-apical or vertical dimension.
The position of the terminus is determined by the dental surveyor, by using the undercut gauge to visualize the amount of undercut. Angle formed between the analyzing rod & the tooth surface apical to the height of contour is angle of gingival convergence If the angle of gingival convergence is great, either the tooth is recontoured or specific clasp is given.
Support “It is the property of the clasp that resist displacement of the clasp in the gingival direction”. • it is provided by occlusal, lingual and incisal rests • It should transmit forces parallel to the long axis of the abutment.
Stability “It is the resistance to horizontal displacement of the prosthesis”. • It is provided by all components except retentive terminal • Cast Circumferential clasp provides the greatest stability.
Reciprocation: “It is the quality of the clasp assembly that counteracts lateral displacement of an abutment when the retentive clasp terminal passes over the height of contour” • It must be rigid • It should be placed above the height of contour. Preferably at the junction of the middle & gingival 3 rd. • If the height of contour lies in the occlusal 3 rd of the tooth either the tooth has to be reshaped or a lingual plate is more effective.
Encirclement: “It is the characteristic of the clasp assembly that prevents movement of the abutment away from the associated clasp assembly”. • Clasp must encircle 1800 or half of the circumference of the tooth • It may be continuous (circumferential) or broken (bar clasp). If broken it must contact at least 3 different areas of tooth.
Passivity: A clasp in place should be completely passive. • The retentive function is activated only when dislodging force are applied to the partial denture. • A clasp must be completely seated on a tooth to be completely passive.
Location of Retentive Terminal: • Usually mesial or distal line angle preferably the facial surface. • Maxillary premolar rarely shows lingual inclination. So buccal retentive area. • Molar teeth exhibit undercut on either or both of facial or lingual surfaces so retention may be used on buccal or lingual.
CIRCUMFERENTIAL CLASP/ SUPRABULGE CLASP/ AKERS CLASP • • It was first introduced by Dr N. B. Nesbitt in 1916. DESIGN RULES
1. Clasp should originate from the portion of the frame work that lies above the height of contour. Only the terminus should engage the undercut. 2. Terminus should be directed occlusally. 3. It should terminate at mesial/distal line angle never at mid facial /mid lingual. 4. Retentive arm should be as far apical on the abutment. 5. For a distal extension base, clasp shouldn’t engage mesiofacial surface of the posterior edentulous space & distofacial surface of the anterior edentulous space.
Advantages: - • Most logical clasp of choice for tooth supported partial denture because of its excellent support, bracing and retentive qualities. • Easy to design, construct and repair • Fewer problems of food entrapment compared to bar clasp
Disadvantages: • Covers more tooth surface so decalcification / caries • Damage to gingival tissue because of lack of physiological stimulation of the tissue. • Not possible to adjust with pliers because of its half-round configuration. • Alters the normal bucco-lingual contour of tooth. • Can increase the width of food table if positioned high on the tooth.
Simple Circlet Clasp: • It is most versatile and widely used clasp. • Approaches undercut, which is remote from edentulous space. • It is the clasp of choice in tooth-supported partial denture.
Advantages • Easy to construct, repair. • It provides better support, stability, reciprocation, encirclement, and passivity Disadvantages • Same as that of cast circumferential clasp
Reverse Circlet Clasp: • Indicated when undercut is located adjacent to edentulous space and bar clasp is contraindicated.
Disadvantages • It requires sufficient occlusal clearance, failure to do so results in a thin clasp that are susceptible to fracture. • Occlusal rest is placed away from the edentulous space so does not protect marginal gingiva. • Poor clasp esthetically.
Multiple Circlet Clasp: • it involves 2 simple circlet clasps joined at their terminal end of their reciprocal elements. • Used for multiple clasping in instances in which the partial denture replaces an entire half of dental arch. • Form of splinting weakened teeth by a RPD.
Embrasure Clasp/ Modified crib clasp: • Two simple circlet clasps joined a their bodies. • Used on the side of the arch where there is no edentulous space, used only when space is gained from opposing occlusion.
Disadvantages: - • Requires removal of increased tooth structure and occlusal rest preparation • Breakage is more common • Requires abutment protection in most of the cases by using inlays or crown since vulnerable areas of tooth are involved.
Ring Clasp • Engages the undercut after encircling almost the entire tooth from its point of origin. Indicated in cases of tipped molars, mandibular teeth tilted mesio-lingually and maxillary teeth tilted mesio-buccally • In case of mandibular molars, ring clasp begins on mesio-buccal surface and terminates in infrabulge area on mesio-lingual surface, it requires additional support with an auxiliary bracing arm.
Ring Clasp • If required an additional occlusal rest on disto-occlusal surface is given which provides additional support for the prosthesis.
Disadvantage • It alters the contour of tooth and interferes with the normal stimulation of surrounding mucosa. • It is susceptible to distortion. Contraindications: 1) Limited vestibular depth. 2) When bracing arm must cross soft tissue undercut.
C, Fish Hook, Hairpin, Reverse Action: • A simple circlet clasp in which the retentive arm loops back to engage an undercut apical to the point of origin. • Upper part of retainer arm is rigid and lower is tapered and flexible. • Indicated when retainer clasp must engage an undercut adjacent to the occlusal rest or edentulous space.
Disadvantages • It should have a sufficient vertical height. • Adequate space is required between the occlusal & apical aspect of the retentive arm to provide finishing of metal & to avoid entrapment of food debris. • Esthetically unacceptable if used on premolar & canine. • There should be no interference with the opposing arch in maximum intercuspation.
Onlay Clasp: • It consists of a rest that covers the entire occlusal surface & serves as the origin for the buccal & lingual arms. • It is indicated when occlusal surface of abutment tooth is below the occlusal plane, as a result of tooth, rotated or tipped. Onlay is used to restore normal occlusal plane. • Should be used only in caries resistant mouth unless tooth is covered by acrylic or gold crown.
Half and Half Clasp: • Consists a circumferential retentive arm arising from one direction & a reciprocal arm arising from the other. • The design was originally intended to provide dual retention, a principle that should be applied only to unilateral partial denture design.
Combination Clasp: • Introduced by Dr O. C Applegate in 1965. • Consists of an occlusal rest, a cast reciprocal clasp arm, and a wrought wire retentive arm. • Indicated in distal extention cases with undercut on mesio-buccal surface. .
Advantages: - – Flexibility – Better esthetically because it can be placed in gingival 3 rd – It can be adjusted in all planes – Because of minimum contact it can be used in caries prone mouths
Disadvantages: - • Extra steps for fabrication • No bracing or stabilization qualities because of increased flexibility • Prone to breakage or damage when the patient mishandles it.
BAR CLASP/ VERTICAL PROJECTION CLASP / ROACH CLASP/ INFRABULGE CLASP • Introduced in early 1900, but received attention in 1930 by DR F. Ewing Roach. • It approaches the undercut on the tooth from gingival direction resulting in Push type of retention.
Rules for use: • • • The Approach arm must not impinge on the soft tissue, no relief should be given under the approach arm, tissue surface of the approach arm should be smooth & well polished The Approach arm should cross perpendicular to free gingival margin. The Approach arm should never be designed under the soft tissue undercut. The approach arm should be uniformly tapered from its point of origin to the terminus. The terminus should be positioned as apical as possible. The minor connector that connects the Rest should be rigid & stabilize the prosthesis.
Indications: - • Retentive undercut adjacent to edentulous area. • When small degree of (0. 01”) undercut exists in cervical 3 rd • If buccal sulcus is more than 4 mm in depth • Tooth supported cases in the anterior region of the mouth.
Contraindications: • Shallow vestibule • Severe tooth and/or tissue undercut • Excessive buccal or lingual tilt of abutment teeth
Advantages: • Better retention because increase length of retentive arm and trip action. • Better esthetics because of gingival approach
Disadvantages: - • Greater tendency to collect and hold food debris • Reduced bracing and stabilization.
T-Clasp • It derives its name from the shape created where the retentive clasp arm joins the vertical aspect of the approach arm. • It is indicated in distal extention cases with undercut on distobuccal surface and toothsupported cases adjacent to edentulous spaces by using natural undercuts called Clasping for convenience.
Contraindications • If soft tissue undercuts exists. • When the height o contour is located on the occlusal 3 rd.
Modified-T / L Clasp: • It’s a T clasp that lacks nonretentive horizontal projection. • It is used on canines or premolar for esthetic reason.
Y-Clasp • A ‘Y’ clasp is formed when the approach arm terminates in the cervical 3 rd of the abutment, while the mesial & distal projections are positioned near the occlusal/ incisal 3 rd.
I-Bar • Only contact of retentive clasp with abutment tooth is the tip of the clasp, which is circular or oval. • It rarely used on disto-buccal surface of maxillary canine for esthetic reason. • Advantage includes better esthetics, minimum interference with natural tooth contours. • Disadvantage being problems with stabilization.
RPI – Concept • krol in 1973 introduced it. It consists of Mesial Rest, Proximal plate and I- Bar. • Mesial rest extends only in the triangular fossa, even in a molar preparation, circular concave depression on mesial marginal ridge of the canine. • The proximal plate contacts only 1 mm of guide plane which is only 2 -3 mm high occlusogingivally. • I bar terminus is a pod shaped to allow more tooth contact.
Modifications of RPI System: R P A • Similar to RPI but instead of I bar, Akers or circumferential clasp retentive arm is fabricated.
Advantages • Easier to grasp for removal of prosthesis. • It can be used in case of large tissue undercuts, high frenum etc. • Indicated when bar clasp is contraindicated and desirable undercuts is located in gingival 3 rd of tooth away from distal extention area.
R L S – System by Aviv L. et al. It consist of • Mesio-occlusal rest • Disto-lingual L-bar It is the direct connector, located adjacent to edentulous ridge. • Disto-buccal – stabilizer, It disengages from the tooth as denture base moves tissue wards.
Comparison of Circumferential and Bar clasps: Bar clasp Retention Circumferential clasp Pull type Bracing Greater Average Caries susceptibility Gingival health More Less Better Poor Aesthetics Poor Excellent Tolerance Well Poor Compactness More Less Push type
Other Clasp Systems:
Clasps with Splinting Action: 1) Extended arm clasp: made of Cr-Co alloy It can be used for splinting maxillary teeth
Clasps utilizing proximal undercuts: Saddle lock hidden clasp partial dentures. • It uses only proximal walls of abutment teeth for retention and uses infrabulge clasp which approaches the retention area from gingival aspect.
Advantages: – Esthetics – Increased retention because of trip action – Can be used when buccal or lingual survey line unfavourable – Compact design prevents dislodgement
Swing-Lock System: • It provides flange or bar that swings horizontally round over hinge and locks on the other side of the arch. Flange may cover the labial gingivae of anterior teeth or spurs from a surveying bar may engage the teeth and provide retention for the appliance. It requires high level of cleanliness to prevent gingival damage.
Esthetic clasp The various methods used to mask the metallic direct retainer are as follows. Macromechanical retention: Retentive beads and meshwork have been used to retain facing of either acrylic or composite resin.
Disadvantages • Bulk that is created by adding the veneer will enlarge the total size of the clasp thus defeating the purpose of disguising the clasp, • Bonding is unreliable, • GAP formation and microleakage when used in combination with composites.
• Micromechanical retention: It involves air borne particle abrasion. This helps to improve retention between the alloy and the resin. Disadvantages • Bond strengths obtained after the use of micromechanical systems are insufficient especially after thermal conditioning.
• Silica coating: This technique is based on adhesion of resin to silane bonding agents. These silanes, however, failed to bond directly to metals. The reason for such a failure is the lack of preferred substrate and groups required for a good chemical bond of silane to metal. Such end groups maybe Si-OH and Al. OH, which are not readily supplied by the alloys used. This new technique involves coating the metal with silica intermediate layer (Si. Ox-C) that bonds to metal and also supplies the -OH group for silane bonding. The tribochemical effect of air borne particle coated with silicic acid on the alloy surface renders it amiable to silane bonding agents.
• This coating allows the development of superior bond strengths to electroetching or chemical etching. Hence, even in the presence of the flexing retentive clasps the bond strengths are significant to prevent debonding. Disadvantages • Lack of long-term controlled studies limits the use of this technique.
Tooth coloured occlusal approaching polymethylene clasps are alternative to metal clasps
The Twin-Flex clasp • This consists of a wire clasp soldered into a channel that is cast in the major connector. • This clasp is flexible, it does not generate as much as torque when the distal extension is depressed. • The ability to adjust this clasp and its conventional path of insertion provides an excellent design option for retention to an adjacent edentulous segment
Disadvantages • There is extra thickness of major connector over the wire clasp, • an extra laboratory step incurs extra cost, • Difficulty in repairing the clasp if breakage occurs
Bibliography • Mc Cracken’s –Removable partial prosthodontics. • Stewart’s – Clinical Removable partial prosthodontics- third edition • Osborne & Lammie’s – Removable prosthodontics • The Twin-flex Clasp: An esthetic alternative. J Prosthet Dent 1997; 77: 450 -2. • The use of chairside silica for different dental application: A clinical report. J Prosthet Dent 2002; 87: 467 -72.