Removable Partial Dentures – Retainers, Clasp Assemblies and Indirect Retainers

Direct retainers are flexible parts of the casting that are designed to engage undercuts on the abutment teeth to resist removal of the prosthesis and to help prevent dislodgement.  There are two basic types of direct retainers – infrabulge and suprabulge rertainers. This program describes how each of these types of retainers are employed in the RPI and RPA systems. 


Retainers, Clasp Assemblies and Indirect Retainers — Course Transcript

  • 1. Retainers, Clasp Assemblies and Indirect Retainers Ting Ling Chang, Takahiro Ogawa and John Beumer III Division of Advanced Prosthodontics, Biomaterials and Hospital Dentistry This program of instruction is protected by copyright ©. No portion of this program of instruction may be reproduced, recorded or transferred by any means electronic, digital, photographic, mechanical etc., or by any information storage or retrieval system, without prior permission.
  • 2. Retainers, Clasp Assemblies and Indirect Retainers
  • 3. Retainers, Clasp Assemblies and Indirect Retainers Direct retainers That component of a removable partial denture used to retain and prevent dislodgment of a clasp assembly or attachment.
  • 4. Retainers, Clasp Assemblies and Indirect Retainers Clasp assembly The part of a removable partial denture that acts as a direct retainer (clasp) and/or stabilizer (minor connectors, proximal plates etc) for a prosthesis by partially encompassing an abutment tooth
  • 5. Types of Retainers (Clasps) Infra-bulge “ I”-Bar The approach of the direct retainer is from below the height of contour Supra-bulge clasp Circumferential clasp The approach of the direct retainer is from above the height of contour. Height of contour Height of contour
  • 6. “ I” Bar Retainers: Low retentive value compared to other types This is due to minimal tooth engagement and partly to purposeful design since support, stability and control of tooth position is established with positive rests, minor connectors and proximal plates
  • 7. “ I” Bar Retainers: Effectiveness of retention is dependent upon the stability of the RPD which in turn is dependent on the presence of properly designed minor connectors, guide-planes and proximal plates In particular, properly contoured guide planes-proximal plates are integral to successful use of “I” bar retainers
  • 8. “ I” Bar Retainers: Minimal tooth contact Allows for easier adjustment of the retainer Enables exact placement of retention contact This very important in extention base RPD’s Minimal Interference with natural tooth contour Benefits
  • 9. “ I” Bar Retainers : Maximum natural cleansing action Passive functional movement of an extension prosthesis (when the rest is properly positioned) Benefits
  • 10. Better Esthetics “ I” bars Only the tips are visible in most patients
  • 11. “ I” Bar Retainers Design Principles “ I” bar should cross tooth tissue junction at right angles and parallel to long axis of tooth. Horizontal portion of the “I” bar is placed on attached tissue whenever possible Connection of “I” bar to RPD casting should be in placed in the interproximal area between denture teeth (arrow).
  • 12. “ I” Bar Retainers Design Principles A space is created between “I” bar and mucosa (it is crossing over) to prevent tissue hypertrophy (arrows). Ideally “I” bar terminates in gingival 1/3 of the tooth. An active “I” Bar requires only 0.25 mm undercut.
  • 13. “ I” Bar Retainers Design Principles When properly designed the “I” bar engages the undercut down into the 0.25 mm region but the occlusal-incisal tip terminates at the height of contour When designed in this way the RPD will slide in out of place with a smooth frictional resistance Height of contour . .25 mm
  • 14. “ I” Bar Retainers Design Principles When properly designed the “I” bar engages the undercut down into the 0.25 mm region but the occlusal-incisal tip terminates at the height of contour When designed in this way the RPD will slide in out of place with a smooth frictional resistance Height of contour
  • 15. “ I” Bar Retainers – Design Principles The tip should be positioned at the point of greatest mesial distal curvature on the tooth
  • 16. “ I” Bar Retainers: Reciprocation An active “I” bar retainer can be reciprocated by one of the following: Opposing reciprocating clasp at 0.00 (“I” bar or “C” clasp) A combination of rest, minor connector and proximal plate. A lingual or palatal plate A combination of the above
  • 17. “ I” Bar Retainers Contraindications Teeth with short clinical crowns Guide planes-proximal plates will be short and in this situation the “I” bar may not provide acceptable retention High frenum attachments will preclude proper positioning of the horizontal component Severely tilted abutments The “I” bar will project into the cheek. This is particularly problem with regard to 2 nd molars in the maxilla Lack of attached gingiva Buccal of mandibular 2 nd molars
  • 18. Circumferential Clasp Suprabulge clasp The flexible retentive arm originates from a minor connector or a proximal plate and its terminal 1/3 crosses the height of contour into the undercut (.25mm or 0.1”) The rigid reciprocating arm stays at or above the height of contour Reciprocating arm Retentive arm Height of contour
  • 19. Circumferential Clasp Applications Posterior teeth Tipped teeth Maxillary molars Better bracing Unilateral defects Note: The portion the retentive clasp above the height of contour is relieved with rouge and chloroform
  • 20. Circumferential Clasp Applications Teeth with short clinical crowns in patients with long edentulous spans requiring additional retention Tipped teeth (maxillary molars) High frenum attachments Minimal levels of attached gingiva (mandibular 2 nd molars)
  • 21. Circumferential Clasp Maxillary 2 nd molars are frequently tilted to the buccal If you use an “I” bar to engage the buccal surface of the tooth it will project into the cheek and cause irritation A “C” clasp is therefore preferred in this situation Courtesy Dr.GE King Courtesy Dr.GE King
  • 22. Circumferential Clasp Maxillary 2 nd molars are frequently tilted to the buccal If you use an “I” bar to engage the buccal surface of the tooth it will project into the cheek and cause irritation A “C” clasp is therefore preferred in this situation Courtesy Dr.GE King Courtesy Dr.GE King
  • 23. Circumferential Clasp Maxillary 2 nd molars are frequently tilted to the buccal If you use an “I” bar to engage the buccal surface of the tooth it will project into the cheek and cause irritation A “C” clasp is therefore preferred in this situation Relieve clasp except at its terminius.
  • 24. Circumferential Clasp Maxillary 2 nd molars are frequently tilted to the buccal and the height of contour is high on the tooth surface If you use an “I” bar to engage the buccal surface of the tooth it will project into the cheek and cause irritation A “C” clasp is therefore preferred in this situation
  • 25. Circumferential Clasp In some patients the zone of attached gingival around the buccal surface of the mandibular 2 nd molar is minimal If so, if an “I” bar is used to engage this undercut surface, it will project into the cheek when the masseter muscle is contracted causing irritation to the mucosa. A “C” clasp is therefore preferred in this situation Courtesy Dr.GE King
  • 26. Circumferential Clasp In some patients the zone of attached gingival around the buccal surface of the mandibular 2 nd molar is minimal If so, if an “I” bar is used to engage this undercut surface, it will project into the cheek when the masseter muscle is contracted causing irritation to the mucosa. A “C” clasp is therefore preferred in this situation
  • 27. Embrasure clasps Used when restoring a patient with a unilateral extension area Biomechanically this is a disadvantage. However, when the teeth are sound and retentive areas are available, retention can made to be very effective with this design Usually employed in association with cast restorations Use in unprotected abutments is based on caries index, oral hygiene, opposing occlusion and tooth contours . Disadvantage: Insufficient space provided will predispose to a high rate of fracture
  • 28. Embrasure clasps Advantage Easy to approach the buccal undercut in the maxillary molar region Risks Fracture – It is difficult to obtain a nonporous casting throughout the clasp assembly
  • 29. Embrasure clasps Disadvantage when used in virgin teeth Frequently, too little tooth reduction is made in the proximal area As a result these clasps are susceptible to fracture because of lack of metal bulk and it is difficult to obtain a nonporous casting throughout the clasp assembly
  • 30. Embrasure clasps Disadvantage when used in virgin teeth This problem can be avoided by placing full veneer crowns on the abutments
  • 31. Indirect Retainers – Myth or Reality Definition: The component of a removable partial denture that assists the direct retainers in preventing displacement of the distal extension denture bases by functioning through lever action on the opposite side of the fulcrum line when the denture base moves away from the tissues in pure rotation around the fulcrum line.
  • 32. Indirect Retainers – Myth or Reality Frank RP & Nicholls JI. An investigation of the effectiveness of indirect retainers. JPD 1976;38:494-506. Methods: Tested the following combinations: 1) D rest 2nd PM/canine rest, 2) D rest 2nd PM/M rest 1st PM, 3) D rest 2nd PM/no indirect retainer, 4) M rest 2nd PM/no indirect retainer, 5) M rest 2nd PM/M rest 1st PM, 6) M rest 2nd PM/canine rest. Applied dislodging forces with Instron machine unilaterally, bilaterally, anteriorly and posteriorly. A wrought-wire clasp was substituted for the bar clasp on both 2nd premolars. Also rested with and without guide planes. Attached rests to framework with autopolymerizing acrylic resin. Conclusions: The type of clasp arm used has a much greater influence on the amount of denture base displacement than does the presence or location of an indirect retainer Usefulness of an indirect retainer in preventing occlusal displacement of a denture base appears to be very limited. It is probably more effective in distributing forces to teeth other than the direct abutments than in preventing denture base lifting Guiding planes are important in preventing denture base lifting Mesial rest placement decreases the distance from the fulcrum line to the indirect retainer, but this does not seem to increase denture base dislodgement.
  • 33. Indirect Retainers – Myth or Reality In this example the mesial rest on the right maxillary premolar acts as an indirect retainer. The forces of gravity tend to displace the obturator portion of the prosthesis, down and out of the defect. The indirect retainers resist this displacement
  • 34. Indirect Retainers – Myth or Reality The musculature of the lip tends to lift the prosthesis away from the tissue bearing surfaces in the anterior region. The indirect retainers resist this rotation. Indirect retention provided by: The rests on the 2 nd molars act as indirect retainers The longer the rests the more effective the indirect retention Axis of rotation (fulcrum line)
  • 35. Clasp Assemblies and Reciprocation and Encirclement
  • 36. Reciprocation Resistance to horizontal forces (primarily in the buccal and lingual direction) exerted on a tooth by an active retentive element Provided by the rigid reciprocating clasp arm opposite the retentive arm, minor connectors, proximal plates, lingual plates etc. Prevents tooth movement that may result from over adjustment of a retentive clasp arm (i.e. making the I-bar, or the retentive arm too tight)
  • 37. Clasp assembly Components Rest Clasp Direct retainer Reciprocating clasp Minor connector Proximal plate Major connector
  • 38. Retainer (Clasp) Assembly A Clasp (retainer) assembly is a combination of several RPD components that engage an abutment tooth extra-coronally for support, stability and retention of the partial denture. Direct retainer,”I”-Bar Rests Proximal plates Direct retainers Rests Minor connectors – proximal plates Reciprocating elements Direct retainer Circumferential type Minor connectors
  • 39. Encirclement The clasp assembly must engage more than 180 degrees of the circumference of the tooth Encirclement is achieved with a combination of retainers proximal plates, minor connectors and extended rests Otherwise the abutment tooth may away from the clasp assembly Examples of the methods used to accomplish this task are shown
  • 40. Reciprocation and Encirclement The clasp assembly must engage and encircle the tooth by more than 180 degrees of its circumference Otherwise, the tooth may move out from under the clasp assembly during function
  • 41. Reciprocation and Encirclement The clasp assembly must engage and encircle the tooth by more than 180 degrees of its circumference This is an example of insufficient encirclement
  • 42. Reciprocation In the classical RPI system (UCLA-Kratochvil) reciprocation is provided by the proximal plate and the minor connector
  • 43. Reciprocation Reciprocation of the “I” bar on the buccal surface of the molar is accomplished with the proximal plates
  • 44. Reciprocation Reciprocation of the “C” clasp on the lingual surface of the molar is accomplished with the proximal plate and the circumferential (“C”) clasp on the buccal side Requires rouge relief to make this design function properly
  • 45. Reciprocation Reciprocation of the “I” b bars on both premolars is accomplished with the proximal plates and the lingual plate
  • 46. Reciprocation In this example, reciprocation provided by the buccal “I” bar (the lingual “I” in this patient is the retentive retainer) and the proximal plate
  • 47. Reciprocation Reciprocation for the retainers on the cuspid and the molar in this unilateral case is accomplished with the proximal plates and lingual plate
  • 48. Bracing Courtesy Dr. T Berg
  • 49. Bracing (Stability) Definition: Resistance to horizontal lateral or torsional components of force generated during mastication or eccentric movements of the mandible It is provided primarily by the rigid portions of a clasp assembly such as the rigid reciprocating clasp elements, minor connectors, proximal plates, lingual plates and extended occlusal rests.
  • 50. Bracing (Stability) Guide planes of teeth engaged by proximal plates provide stress reduction in several areas including buttressing (a) a
  • 51. Bracing In this bilateral extension case, bracing is enhanced by the proximal plate, the minor connector and the “I” bar retainer
  • 52. Types of Bracing In this patient most of the bracing is provided by the distal proximal plates. However the lingual plate supplements bracing to significant degree
  • 53. Bracing During mastication significant lateral forces will be generated. In patients presenting with unilateral defects such as this, additional bracing is necessary in order distribute these forces equitably among the remaining teeth Here, in addition to the bracing effect of the proximal plates on the 2 nd molar and the 1 st premolar, additional bracing is provided by plating the lingual surfaces of the remaining dentition.
  • 54. Patients with unilateral dentition and large edentulous spaces such as in this case, require additional bracing. Here, in addition to the bracing effect of the proximal plates on the !st and 3 rd molars molar additional bracing is provided by plating the lingual surfaces of the remaining dentition. Bracing (Resistance to lateral forces)
  • 55. Bracing (Resistance to lateral forces) More bracing is required in maxillary resection defects because of the large edentulous extension area and long lever arms. Lingual plating is frequently employed In these types of defects.
  • 56. As the defect becomes larger and with the remaining dentition in a linear fashion, bracing becomes more aggressive. In this patient the lingual plate embraces all of the remaining teeth. Bracing
  • 57. Types of Bracing Reciprocation and encirclement of the molar in this rotational path RPD is accomplished with the proximal plate and the extended rest engaging its buccal slopes
  • 58. Passivity There should be no active force on any abutment teeth when the framework is seated and the retainers are engaged. Its retainers should only by activated when a dislodging force is applied
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