CHAPTER 7 Direct Retainers
Retention of a removable prosthesis is a unique concern when compared with other prostheses. When one is dealing with a crown or fixed partial denture, the combined use of preparation geometry (i.e., resistance and retention form) and a luting agent can fix the prosthesis to the tooth in a manner that resists all forces to which the teeth are subjected. As was mentioned in Chapter 4, the direction of forces can be toward, across, or away from the tissue. In general, the forces acting to move prostheses toward and across the supporting teeth and/or tissue are the greatest in intensity. This is because most often they are forces of occlusion.
Forces acting to displace the prosthesis from the tissue can consist of gravity acting against a maxillary prosthesis, the action of adherent foods acting to displace the prosthesis on opening of the mouth in chewing, or functional forces acting across a fulcrum to unseat the prosthesis. The first two of these forces are seldom at the magnitude of functional forces, and the latter is minimized through the use of adequate support. The component part applied to resist this movement away from the teeth and/or tissue provides retention for the prosthesis and is called the direct retainer. A direct retainer is any unit of a removable dental prosthesis that engages an abutment tooth or implant to resist displacement of the prosthesis away from basal seat tissue. The direct retainer’s ability to resist this movement is greatly influenced by the stability and support of the prosthesis provided by major and minor connectors, rests, and tissue bases. This relationship of the supportive and retentive components highlights the relative importance of these component parts. Although the forces working against a removable partial denture to move it away from the tissue generally are not as great as the functional forces causing stress toward the tissue, the removable partial denture must have retention appropriate to resist reasonable dislodging forces. Too often retention concerns are given greater importance than is appropriate, especially if such a focus detracts from more serious consideration of the resistance of typical functional forces.
Sufficient retention is provided by two means. Primary retention for the removable partial denture is accomplished mechanically by placing retaining elements (direct retainers) on the abutment teeth. Secondary retention is provided by the intimate relationship of the minor connector contact with the guiding planes and denture bases, and of the major connector (maxillary) with underlying tissue. The latter is similar to the retention of a complete denture. It is proportionate to the accuracy of the impression registration, the accuracy of the fit of the denture bases, and the total involved area of contact. Retention can also be provided through engagement of an attachment mechanism on a dental implant.
The clasp assembly serves a similar function for a removable partial denture that a retainer crown serves for a fixed partial denture. Both must encircle the prepared tooth in a manner that prevents movement of the tooth separate from the retainer. To borrow from a fixed prosthodontic term, limiting the freedom of displacement refers to the effect of one cylindrical surface (the framework encircling the tooth) on another cylindrical surface (the tooth). It implies that the curve that defines the framework is properly shaped if it prevents movement at right angles to the tooth axis. This basic principle of clasp design offers a two-way benefit. First, it ensures the stability of the tooth position because of the restraint from encirclement, and second, it ensures stability of the clasp assembly because of the controlled position of the clasp in three dimensions.
Therefore the basic principle of clasp design, referred to as the principle of encirclement, means that more than 180 degrees in the greatest circumference of the tooth, passing from diverging axial surfaces to converging axial surfaces, must be engaged by the clasp assembly (Figure 7-1). The engagement can occur in the form of continuous contact, such as in a circumferential clasp, or discontinuous contact, such as in the use of a bar clasp. Both provide tooth contact in at least three areas encircling the tooth: the occlusal rest area, the retentive clasp terminal area, and the reciprocal clasp terminal area.
Figure 7-1 A, Line drawn through the illustration represents 180 degrees of greatest circumference of abutment from the occlusal rest. Unless portions of the lingual reciprocal arm and the retentive buccal arm are extended beyond the line, the clasp would not accomplish its intended purpose. If respective arms of the retainer were not extended beyond the line, the abutment tooth could be forced away from the retainer through torquing action of the clasp, or the removable partial denture could move away from the abutment. B, Bar-type clasp assembly engagement of more than 180 degrees of circumference of the abutment is realized by the minor connector for the occlusal rest, the minor connector contacting the guiding plane on the distal proximal surface, and the retentive bar arm.
Figure 7-2 A, Flexing action of the retentive clasp arm initiates medially directed pressure on the abutment teeth as its retentive tip springs over the height of contour. B, Reciprocation to medially directed pressure is counteracted by rigid lingually placed clasp arms contacting the abutments simultaneously with the buccal arms, or by rigid stabilizing components of the framework contacting the lingual guiding planes when the buccal arms begin to flex.
Figure 7-3 Retentive clasps should be bilaterally opposed. This means bilateral buccal or bilateral lingual undercuts should be used, as shown on this Class III, modification 2 arch, in which the retention may be (a) bilaterally buccal or (b) bilaterally lingual.
Figure 7-4 Simple mechanical laws demonstrate that the nearer stabilizing-reciprocal and retentive elements of direct retainer assemblies are located horizontal to the axis of rotation of the abutment, the less likely it is that physiologic tolerance of the periodontal ligament will be exceeded. The horizontal axis of rotation of the abutment tooth is located somewhere in its root.
Figure 7-5 A bar-type clasp on the mandibular premolar. A, Support is provided by the occlusal rest. B, Stabilization is provided by the occlusal rest and the mesial and distal minor connectors. C, Retention is provided by the buccal I-bar. Reciprocation is obtained through the location of the minor connectors. Engagement of more than 180 degrees of circumference of the abutment is accomplished by proper location of components contacting the axial surfaces. (The minor connector supports the occlusal rest, the proximal plate minor connector, and the buccal I-bar.)
Figure 7-6 Circumferential clasp on the mandibular premolar. Support is provided by the occlusal rest; stabilization is provided by the occlusal rest, proximal minor connector, lingual clasp arm, and rigid portion of the buccal retentive clasp arm occlusal to the height of contour; retention is realized by the retentive terminal of the buccal clasp arm; reciprocation is provided by the nonflexible lingual clasp arm. Assembly engages more than 180 degrees of the abutment tooth’s circumference.
As was mentioned earlier, reciprocal arms are intended to resist tooth movement in response to deformation of the retainer arm as it engages a tooth height of contour. The opposing clasp arm reciprocates the effect of this deformation as it prevents tooth movement. For this to occur, the reciprocal arm must be in contact during the time of retainer arm deformation. Unless the abutment tooth has been specifically contoured, the reciprocal clasp arm will not come into contact with the tooth until the denture is fully seated and the retentive clasp arm has again become passive. When this happens, a momentary tipping force is applied to the abutment teeth during each placement and removal. This may not be a damaging force—because it is transient—so long as the force does not exceed the normal elasticity of the periodontal attachments. True reciprocation during placement and removal is possible only through the use of crown surfaces made parallel to the path of placement. The use of cast restorations permits the parallel surfaces to be contacted by the reciprocal arm in such a manner that true reciprocation is made possible. This is discussed in Chapter 14.
Reciprocal arms can have additional functions as well. The reciprocal clasp arm should be located so that the denture is stabilized against horizontal movement. Stabilization is possible only through the use of rigid clasp arms, rigid minor connectors, and a rigid major connector. Horizontal forces applied on one side of the dental arch are resisted by the stabilizing components on the opposite side, providing cross-arch stability. Obviously the greater the number of such components, within reason, the greater will be the distribution of horizontal stresses.
The reciprocal clasp arm also may act to a minor degree as an indirect retainer. This is true only when it rests on a suprabulge surface of an abutment tooth lying anterior to the fulcrum line (see Figure 8-8). Lifting of a distal extension base away from the tissue is thus resisted by a rigid arm, which is not easily displaced cervically. The effectiveness of such an indirect retainer is limited by its proximity to the fulcrum line, which gives it a relatively poor leverage advantage, and by the fact that slippage along tooth inclines is always possible. The latter may be prevented by the use of a ledge on a cast restoration; however, enamel surfaces are not ordinarily so prepared.
Mechanical retention of removable partial dentures is accomplished by means of direct retainers of one type or another. Retention is accomplished by using frictional means, by engaging a depression in the abutment tooth, or by engaging a tooth undercut lying cervically to its height of contour. Two basic types of direct retainers are available: the intracoronal retainer and the extracoronal retainer. The extracoronal (clasp-type) retainer is the most commonly used retainer for removable partial dentures.
The intracoronal retainer may be cast or may be attached totally within the restored natural contours of an abutment tooth. It is typically composed of a prefabricated machined key and keyway with opposing vertical parallel walls, which serve to limit movement and resist removal of the partial denture through frictional resistance (Figure 7-7). The intracoronal retainer is usually regarded as an internal, or precision, attachment. The principle of the internal attachment was first formulated by Dr. Herman E.S. Chayes in 1906.
Figure 7-7 A, The intracoronal retainer consists of a key and keyway with extremely small tolerance. Keyways are contained within abutment crowns, and (B) keys are attached to the removable partial denture framework. C, Frictional resistance to removal and placement and limitation of movement serve to retain and stabilize the prosthesis.
The extracoronal retainer uses mechanical resistance to displacement through components placed on or attached to the external surfaces of an abutment tooth. The extracoronal retainer is available in three principal forms. The clasp-type retainer (Figures 7-8 and 7-9), the form used most commonly, retains through a flexible clasp arm. This arm engages an external surface of an abutment tooth in an area cervical to the greatest convexity of the tooth, or it engages a depression prepared to receive the terminal tip of the arm. The other forms both involve manufactured attachments and include interlocking components or the use of a spring-loaded device that engages a tooth contour to resist occlusal displacement. Another type is a manufactured attachment, which uses flexible clips or rings that engage a rigid component that is cast or attached to the external surface of an abutment crown.
Figure 7-8 Extracoronal circumferential direct retainer. Assembly consists of (A) the buccal retentive arm; (B) the rigid lingual stabilizing (reciprocal) arm; and (C) the supporting occlusal rest. The terminal portion of the retentive arm is flexible and engages measured undercut. Assembly remains passive until activated by placement or removal of the restoration, or when subjected to masticatory forces that tend to dislodge the denture base.
Figure 7-9 Extracoronal bar-type direct retainer. Assembly consists of (A) the buccal retentive arm engaging the measured undercut (with slight occlusal extension for stabilization, see insert where HOC is height of contour and UC is undercut); (B) the stabilizing (reciprocal) elements, with the proximal plate minor connector on the distal; (C) the lingually placed mesial minor connector for the occlusal rest, which also serves as a stabilizing (reciprocal) component; and (D) the mesially placed supporting occlusal rest. Assembly remains passive until activated.
In situations where support requirements are adequately met by available teeth and/or oral tissues, dental implants can be used for retention and provide the advantage of elimination of a visible clasp.
When a particular clasp design is selected for a given situation, its function and limitations must be carefully evaluated. Extracoronal direct retainers, as part of the clasp assembly, should be considered as components of a removable partial denture framework. They should be designed and located to perform the specific functions of support, stabilization, reciprocation, and retention. It does not matter whether the direct retainer-clasp assembly components are physically attached to each other, or originate from major and minor connectors of the framework (see Figures 1-2 and 1-3, B-E). If attention is directed to the separate function of each component of the direct retainer-clasp assembly, then selection of a direct retainer is simplified.
Although some rather complex designs are used for clasp arms, they all may be classified into one of two basic categories. One is the circumferential clasp arm, which approaches the retentive undercut from an occlusal direction. The other is the bar clasp arm, which approaches the retentive undercut from a cervical direction. A clasp assembly may comprise various retentive arms (i.e., a cast circumferential, a bar clasp arm, or a wrought-wire retainer), depending on the specific requirement for retainer construction, given the necessary adjustability, clasp approach position, and survey line location.
A clasp assembly should consist of four component parts. First, one or more minor connectors should be present, from which the clasp components originate. Second, a principal rest should be designed to direct stress along the long axis of the tooth. Third, a retentive arm should engage a tooth undercut. For most clasps, the retentive region is only at its terminus. Fourth, a nonretentive arm (or other component) should be present on the opposite side of the tooth for stabilization and reciprocation against horizontal movement of the prosthesis (rigidity of this clasp arm is essential to its purpose).
No confusion should exist between the choice of clasp arm and the purpose for which it is used. Either type of cast clasp arm (bar or circumferential) may be made tapered and retentive, or nontapered (rigid) and nonretentive. The choice depends on whether it is used for retention, stabilization, or reciprocation. An occlusal rest, such as in the RPI (rest, proximal plate, and I-bar component parts of the clasp assembly) concept, may be used rather than a reciprocal clasp arm to satisfy the need for encirclement, provided it is located in such a way that it can accomplish the same purpose (Figure 7-10; see also Figure 7-9). The addition of a lingual apron to a cast reciprocal clasp arm alters neither its primary purpose nor the need for proper location to accomplish that purpose.
Figure 7-10 The auxiliary occlusal rest (mirror view) may be used rather than the reciprocal clasp arm without violating any principle of clasp design. Its greatest disadvantages are that the second rest seat must be prepared, and that enclosed tissue space at the gingival margin can result in a food trap. The auxiliary occlusal rest is also sometimes used to prevent slippage when the principal occlusal rest seat cannot be inclined apically from the marginal ridge. Minor connectors used to close the interproximal space most often require rests on adjacent teeth to avoid a wedging effect when force is placed on the denture.
A wide variety of clasp assemblies are available for clinicians to use. This variety exists largely because of the imagination of clinicians and technicians who provided prostheses when tooth modification was not or could not be provided. To simplify clasp design and to improve the functional predictability of prostheses, today’s clinician must realize the need for tooth modification.
Some clasp assemblies are designed to accommodate prosthesis functional movement (as mentioned in the basic principles above), and others do not incorporate such design features. Although it has been demonstrated by Kapur and others that adverse outcomes are not always associated with the use of rigid clasp assemblies in distal extension classifications, the following clasp assemblies will be described as clasps designed to accommodate distal extension functional movement and clasps designed without movement accommodation. The clinician should not interpret these categories as mutually exclusive because most any clasp assembly can be used to retain a well-supported and maintained prosthesis.
Clasp assemblies that accommodate functional prosthesis movement are designed to address the concern of a Class I lever. The concern is that the distal extension acts as a long “effort arm” across the distal rest “fulcrum” to cause the clasp tip “resistance arm” to engage the tooth undercut. This results in harmful tipping or torquing of the tooth, which is greater with stiff clasps and increased denture base movement. Two strategies may be adopted to change the fulcrum location and subsequently the “resistance arm” engaging effect (mesial rest concept clasp assemblies), or to minimize the effect of the lever through the use of a flexible arm (wrought-wire retentive arm).
Mesial rest concept clasps have been proposed to accomplish movement accommodation by changing the fulcrum location. This concept includes the RPI and RPA [rest, proximal plate, Akers] clasps. The RPI is a current concept of bar clasp design that refers to the rest, proximal plate, and I-bar component parts of the clasp assembly. Basically, this clasp assembly consists of a mesio-occlusal rest with the minor connector placed into the mesiolingual embrasure, but not contacting the adjacent tooth (Figure 7-11, A). A distal guiding plane, extending from the marginal ridge to the junction of the middle and gingival thirds of the abutment tooth, is prepared to receive a proximal plate (Figure 7-11, B). The buccolingual width of the guiding plane is determined by the proximal contour of the tooth (Figure 7-11, A and C). The proximal plate, in conjunction with the minor connector supporting the rest, provides the stabilizing and reciprocal aspects of the clasp assembly. The I-bar should be located in the gingival third of the buccal or labial surface of the abutment in a 0.01-inch undercut (Figure 7-11, D). The whole arm of the I-bar should be tapered to its terminus, with no more than 2 mm of its tip contacting the abutment. The retentive tip contacts the tooth from the undercut to the height of contour (Figure 7-11, E). This area of contact, along with the rest and proximal plate contact, provides stabilization through encirclement (see Figure 7-11, C). The horizontal portion of the approach arm must be located at least 4 mm from the gingival margin and even farther if possible.
Figure 7-11 Bar-type clasp assembly. A, Occlusal view. Component parts (proximal plate minor connector, rest with minor connector, and retentive arm) tripod the abutment to prevent its migration. B, The proximal plate minor connector extends just far enough lingually so that it combines with the mesial minor connector to prevent lingual migration of the abutment. C, On narrow or tapered abutments (mandibular first premolars), the proximal plate should be designed to be as narrow as possible but still sufficiently wide to prevent lingual migration. D, I-bar retainer located at greatest prominence of tooth in the gingival third. E, Mesial view of I-bar illustrating the retentive tip relationship to the undercut and a region superior to the height of contour, which serves a stabilization function in encirclement.
Three basic approaches to the application of the RPI system may be used. The location of the rest, the design of the minor connector (proximal plate) as it relates to the guiding plane, and the location of the retentive arm are factors that influence how this clasp system functions. Variations in these factors provide the basis for differences among these approaches. All advocate the use of a rest located mesially on the primary abutment tooth adjacent to the extension base area. One approach recommends that the guiding plane and the corresponding proximal plate minor connector should extend the entire length of the proximal tooth surface, with physiologic tissue relief eliminating impingement of the free gingival margin (Figure 7-12). A second approach suggests that the guiding plane and the corresponding proximal plate minor connector should extend from the marginal ridge to the junction of the middle and gingival thirds of the proximal tooth surface (Figure 7-13). Both approaches recommend that the retaining clasp arm should be located in the gingival third of the buccal or labial surface of the abutment in a 0.01-inch undercut. Placement of the retaining clasp arm generally occurs in an undercut located at the greatest mesiodistal prominence of the tooth or adjacent to the extension base area (Figure 7-14, A and B). The third approach favors a proximal plate minor connector that contacts approximately 1 mm of the gingival portion of the guiding plane (Figure 7-15, A) and a retentive clasp arm located in a 0.01-inch undercut in the gingival third of the tooth at the greatest prominence or toward the mesial away from the edentulous area (Figure 7-14, C). If the abutment teeth demonstrate contraindications for a bar-type clasp (i.e., exaggerated buccal or lingual tilts, severe tissue undercut, or a shallow buccal vestibule) and the desirable undercut is located in the gingival third of the tooth away from the extension base area, a modification should be considered for the RPI system (the RPA clasp) (Figure 7-15, B). Application of each approach is predicated on the distribution of load to be applied to the tooth and edentulous ridge.
Figure 7-12 Bar clasp assembly in which the guiding plane (GP) and the corresponding proximal plate (PP) extend the entire length of the proximal tooth surface. Physiologic relief is required to prevent impingement of the gingival tissues during function. Extending the proximal plate to contact a greater surface area of the guide plane directs functional forces in a horizontal direction, thus the teeth are loaded to a greater extent than the edentulous ridge.
Figure 7-13 Bar clasp assembly in which the guiding plane (GP) and the corresponding proximal plate (PP) extend from the marginal ridge to the junction of the middle and gingival thirds of the proximal tooth surface. This decrease (compared with Figure 7-23) in amount of surface area contact of the proximal plate on the guiding plane more evenly distributes functional force between the tooth and the edentulous ridge.
Figure 7-14 Occlusal view of the RPI (rest, proximal plate, and I-bar component parts) bar clasp assembly. Placement of the I-bar in a 0.01-inch undercut (A) on the distobuccal surface; (B) at the greatest mesiodistal prominence; and (C) on the mesiobuccal surface.
Figure 7-15 A, Bar clasp assembly in which the proximal plate (PP) contacts approximately 1 mm of the gingival portion of the guiding plane (GP). During function, the proximal plate and the I-bar clasp arm are designed to move in a mesiogingival direction, disengaging the tooth. Lack of sustained contact between the proximal plate and the guiding plane distributes increased functional force to the edentulous ridge. Asterisk (*) indicates center of rotation. B, Modification of the RPI (rest, proximal plate, and I-bar component parts) system (rest, proximal plate, Akers or RPA clasp) is indicated when a bar-type clasp is contraindicated and a desirable undercut is located in the gingival third of the tooth away from the extension base area.
The bar clasp, which gave rise to the RPI, is discussed here because of this association. It may not be configured to allow functional movement, but it can be. The term bar clasp is generally preferred over the less descriptive term Roach clasp arm. Reduced to its simplest term, the bar clasp arm arises from the denture framework or a metal base and approaches the retentive undercut from a gingival direction (see Figure 7-11). The bar clasp arm has been classified by the shape of the retentive terminal. Thus it has been identified as a T, modified T, I, or Y. The form the terminal takes is of little significance as long as it is mechanically and functionally effective, covers as little tooth surface as possible, and displays as little metal as possible.
In most situations, the bar clasp arm can be used with tooth-supported partial dentures, with tooth-supported modification areas, or when an undercut that can be logically approached with a bar clasp arm lies on the side of an abutment tooth adjacent to a distal extension base (Figure 7-16). If a tissue undercut prevents the use of a bar clasp arm, a mesially originating ring clasp, a cast, or a wrought-wire clasp or reverse-action clasp may be used. Preparation of adjacent abutments (natural teeth) to receive any type of interproximal direct retainer, traversing from lingual to buccal surfaces, is most difficult to adequately accomplish. Inevitably the relative size of the occlusal table is increased, contributing to undesirable and additional functional loading.
Figure 7-16 Bar clasp arm properly used on the terminal abutment. The combination of rest, proximal plate, and bar clasp contacting the abutment tooth provides more than 180 degrees encirclement. A uniform taper to the bar ensures proper flexibility and internal stress distribution. Taper can originate from the minor connector junction or at a finishing line indicating the anterior extent of the denture base. Encirclement does not require that retentive tip modification (in the form of a T) be provided, and such a modification adds little to the clasp assembly.
Specific indications for use of a bar clasp arm include (1) when a small degree of undercut (0.01 inch) exists in the cervical third of the abutment tooth, which may be approached from a gingival direction; (2) on abutment teeth for tooth-supported partial dentures or tooth-supported modification areas (Figure 7-17); (3) in distal extension base situations; and (4) in situations in which esthetic considerations must be accommodated and a cast clasp is indicated. Thus use of the bar clasp arm is contraindicated when a deep cervical undercut exists or when a severe tooth and/or tissue undercut exists, either of which must be bridged by excessive blockout. When severe tooth and ti/>