Retention for a Removable Partial Denture Dean L.Johnson, LIDS, MEd* This article addresses the complex nature of retention in a removable partial denture. Retentive features range from magnets and springs to clips, clasps, and interfacial surface tension. The retentive quality of an extracoronal clasp varies with the alloy, physical form, location on the abutment, and positional relationship to other elements. Surveying to identify both occlusogingival and mesiodistal undercuts when the path of random dislodging forces are not definitely controlled is needed for effective retention. Augmentation of ineffective retention in existing clasps should concentrate on methods of deepening the undercut or increasing the suprabulge. Tightening of clasps already in contact with a tooth frequently produces adverse changes. J Prosthod 7: tl-77. Copyright 0 1992 by the American College of Prosthodontists.
INDEX WORDS: retention, removable, partial, denture
P
ractitioners and researchers have shown that when a removable partial denture is placed into a healthy oral environment, when the design is developed by a dentist to meet specific needs of the patient, and when good oral hygiene is maintained, long-term success with removable partial denture treatment can be expected. Of the numerous services that a removable partial denture provides, the one that builds a patient's confidence is for the denture to remain seated on the supporting tissues during function. This article discusses the complexity of removable partial denture retention.
Removable Partial Denture Retention Mechanical retention generated by a removable partial denture (RF'D) clasp is only one of many factors that help maintain its resting position. As functional forces act to unseat an RF'D, the individual retentive quality of clasps, clips, springs, magnets, indirect retainers, frictional resistance, gravity, adhesion, cohesion, interfacial surface tension, and atmospheric pressure will act to resist dislodgement. The ability to maintain the resting position depends on all
*Director, Graduate Prosthodontiw: Uniuersi& 4 Oklehona Hrcllth Scicnces Center, College ojDentiTtV, Oklahoma Ci& OK. Address reprint requests to: Dean L.Jnhmon, DDS,MEd, C:[email protected] Oklahoma Health Sciences Center, Collexe ojDentirty, I001 Stanton I-. YounEBlud, Oklahoma Cip,OK 73190. Copyright Q 1992 by theilmeican Collqe ofPosthodontitts I059-941xi 9210101-0002$5.00/0
of the retentive elements' collective resistance to dislodging forces which will have unknown magnitude, frequency, direction, duration, and location of application. If one considers the partially edentulous mouth whercin only the maxillary and mandibular anterior teeth remain, the retentive quality in the RPD will depend on the denture base's ability to generate adhesion, cohesion, and interfacial surface tension between the two closely adapted surfaces of mucosa and acrylic resin with a thin salivary film interposed. In the presence of air, pressure differences produce a resistance to separation based on capillarity phenomena. When the film of fluid is thin and the contact area of the two surfaces is large, the resistance to separation is great. In reference to denture base retention, adhesion pertains to the ability of dissimilar niolecules such as those in saliva, mucosa, and acrylic resin, to attract each other. Cohesion pertains primarily to the ability of like molecules in the salivary film to attract each other. By excluding air from the space between the denture base and mucosa, atmospheric pressure will assist in keeping the denture seated. Also, because each denture has mass, gravity will act to diminish the retentivc values in the maxillary arch and increase those in the mandible. As contacting metal framework and abutment surfaces slide w e r each other, there develops frictional resistance whose magnitude varies with the area and pressure (load) exerted between the surfaces. In addition, for dentures whose path of dislodgement is not totally controlled and may possess a rotational component,
Journal OfProsthodontics,Voll,No 1 (Stptcmber), 1.992:fip 11-I7
11
12
Removable Partial Denture
indirect retention can be developed. Finally, and most commonly considered, the mechanical retention generated by flexible metal clasps is designed. The most frequently used mechanical retainer continues to be the extracoronally located clasp. Aclasp is “the part of a removable partial denture that acts as a direct retainer and/or stabilizer for the prosthesis by partially encompassing or contacting an abutment tooth.”’ The clasps are described as small masses of metal which keep the denture seated in its resting position when dislodging forces act upon it. The details pertaining to the means by which clasp tips provide effective retention are a bit complex. In general, there exists a need to understand the (1) internal characteristics of the alloy that permit it to flex and return to form, (2) physical form of the clasp as it effects the capacity to flex, (3) location of the clasp tip on the abutment, and (4) position of the clasp in relation to other elements of the RPD. Internal characteristics that enable a clasp tip to flex can be found in the ability of atoms in the alloy to move and then return to their original position. When the alloy is at rest, there exists an equilibrium of interatomic distance. As an external force is applied, atoms are moved and the energy that is imparted to the dislocation is stored. When the external force is removed, atoms return to their original location and the equilibrium of interatomic distance is regained. This flexural capability is referred to as elastic deformation. In an RPD, elastic deformation occurs whenever an insertion or dislodging force acts on the denture teeth, denture base, and other elements of the RPD. The abutment contour serves as an obstacle to passage of the clasp tip and thereby causes flexure. Consider what happens to a retentive clasp when forces cause the metal alloy to flex beyond its elastic limit, as when the clinician purposefully bends a clasp or a wire into a new configuration. Upon application of external force, elastic deformation occurs. Increased force produces plastic deformation as sheering forces cause the dislocation plane of atoms to slip at the borders of the crystal and produce permanent deformatione2Clinically, the progression of events are as follows:
I . Application of a strong external force. 2. Disturbance of the equilibrium of the interatomic space. 3. Displacement of atoms and storage of energy. 4. Slipping of crystals.
Dean L.Johnyon
5. Removal of external force. 6.Release of stored energy. 7. Return of most atoms to original location within the mass which in itself has been moved. 8. Equilibrium of the interatomic space. At this point in time, the clasp tip rests in a new location due to plastic deformation, yet has retained its capacity for elastic deformation. Variations in production methods produce similar alloys having different flexibility. The crystalline form of cast alloy displays no organized pattern or orientation. However, wrought metal clasps are described in the literature as having a more fibrous crystalline composition which, when coupled with its rolled, rounded form, provides more flexibility than cast metal clasp^.^ The effect that external form and mass have on the flexibility of a clasp is highly variable in clinical settings. Even though preformed patterns are used in the laboratory construction of clasps, manipulation and finishing diminishes the predictability of their performances. Likewise, variations in length, thickness, width, taper, cross-sectional form, material, and curvature rapidly alter a clasp’s flexibility. Alterations in length and thickness dominate changes because they bear a cube ratio effect on clasp behavior. The longer the clasp, the more flexible it becomes in a cube ratio. The thicker the clasp, the less flexible it becomes in a cube ratio. In addition, as the mesiodistal curvature of a circumferential clasp increases, (premolar versus molar) it becomes ~ t i f f e r . ~ . ~ When a cast clasp (I-bar) becomes more acutely curved in the plane of the flat side of its half-round form, it becomes stiffer.GAs the length of a clasp becomes more involved in the curve itself (in the plane of the flat side of a half-round cast clasp, ie, I-bar), it becomes stiffer.6All things considered, one can conclude that because of the variations in styling and production methods for retentive clasps, a practitioner has little chance of specificallyquantifjring the amount of retentive force a clasp will generate. Retention from a clasp tip must also be evaluated in terms of the distance transited while being dislodged. Ifwe focus on both the depth ofundercut and the acuteness of the curvature of the bulging tooth surface being traversed, the clasp will produce a quantity of retention that begins at zero (resting) and increases to maximum value at the summit of the bulge. When the tooth bulge slopes gently, the clasp tip may unseat by several millimeters before appreciable resistance to dislodgement is produced. Immediate and early retention is minimal, thus
13
September 1992, Volume I , Numbm 1
permitting some functional motion which can be annoying to the patient. Conversely, when the bulge of a crown is abrupt, the retentive capacity of a clasp is achieved quickly and over a short distance. Carrying this reasoning to its extreme, clasps would be most effective when their tips are located in a ledged recess. Such an arrangement would also require an ability in the patient to manually disengage each clasp tip in order to remove the RPD. Because this may be an inconvenient or impossible task for some patients, a preferred form for an undercut may be a recess having a pronounced curvature. Such a form would be similar to the mirror image of a chamfer. It would begin on the survey line and be prepared with the rounded periphery of a diamond wheel (Brasseler USA, Savannah, GA, 040-1.O-909) moving laterally across the buccal or lingual enamel to create a curved recess. The location of a retentive clasp tip on the abutment must be such that tooth curvatures will force the tip to flex while dislodging forces are acting to unseat the denture. Ordinarily, the body of a circumferential clasp embraces a tooth mesiodistally so that the flexible tip will automatically be located beyond the greatest mesiodistal height of contour. However, because bar clasps do not embrace the abutment, it may be critically important that the tip be located below the survey line and on the opposite side of the mesiodistal height of contour from the denture base. Direct retention is defined as the "retention obtained in a removable partial denture by the use of clasps or attachments that resist removal from the abutment teeth".' In locating the recess into which a retentive tip is to be placed, all avenues of escape
must be analyzed, that is, all three planes of space and 360" of direction in those planes. At the beginning of a dental cast survey, the combined common plane of occlusion and crest of the edentulous ridges are oriented parallel to the base of the dental surveyor. From this 0" orientation, the cast may be tipped slightly to align the majority of the proximal walls of teeth bordering edentulous spaces. No severe tipping can occur because the guide surfaces must be prepared in the mouth at the same degree of tilt. From the flat or mildly tipped orientation of the cast, one can obsenle below the survey line the recesses available for retention. However, if the selection of an undercut is to function as planned, the RF'D must have sufficient guiding surfaces to limit dislodgement to a path that is straight and parallel to the vertical spindle of the dental surveyor. Random dislodging forces have numerous components including a mesiodistal vector. Ordinarily, only toothborne or extension RF'Ds having strategically located modification spaces possess the number and location of guiding surfaces to specifically limit dislodgement to a finite path. For effective retention in all other clasp retained RPDs, an effort must be made to ensure that retentive tips cannot escape vertically or horizontally from their seated position in either a straight line, translatory, or curvolinear mesiodistal direction. Therefore, retentive clasp tips on many extension RPDs (Fig 1) should be located both below the traditional survey line and beyond the mesiodistal height of contour of the abutment (in a direction away from the extension denture base). The Ney dental surveyor can be adapted to accomplish the task of surveying in a second plane (Fig 2). Attention to placement of clasp tips will assure
I
I
I I I
I
I I
I 1
Figure 1. An I-bar located in an undercut on the mesio-distal height of contour may escape distally or disto-occlusally when no distinct path of dislodgement exists. (B) An I-bar located in an undrrcut beyond the mesio-distal height ofcontour (from an extension denture base) provides retention when the clasp tip is required to flex over a more prominent contour to escape from its resting position,
14
Removable Partial Denture
Dean L.7ohlnson
Figure 2. (A) Traditional survey line a. tnesiodistal height of contour linc b. (B) Traditional vertical contour line a, linr marking depth ofvertical undercut b, horizontal depth gauge c, pencil about to scribe horizontal depth by crossing vertical depth line b. (C) Mesial and vertical undercut idcntifics location of clasp tip.
that escape without flexure cannot occur in either a vertical or anteroposterior direction. In addition, contralateral positioning of clasps must be such that escape in d lateral direction cannot occur either. A buccal clasp located on one side of the arch requires that the clasp on the opposite side of the arch be located on the buccal. Opposing lingual clasp tips are acceptable also. A mixture of locations are workable so long as clasps on one side of the arch oppose those on the other side of the arch. The effect can be described as potentiation, that is, one clasp ensures that the other must flex to escape. In the proccss, lateral force will be applied to the abutment and shonld be reciprocated by guideplates, plating, minor connectors, rigid stabilizing clasps, natural proximal tooth contacts, or combinations of these. To maintain equilibrium of the tooth’s attachment apparatus, reciprocation must occur at the same time and height on the opposite side of the abutmentwhen the retentive clasp tip is activated. Effective reciprocation cannot be obtained when the opposite wall is inclined occlusalward, as with the lingual surfaces of all anterior tceth and tipped posterior teeth (Fig 3). In such situations, either the alveolar bone and attachment apparatus will resist the forces and
maintain position or it will lack the capacity to do so and the abutment will move. The depth of an undercut that a retentive clasp tip engages may be no less than 0.01 in. Occasionally, a 0.02-in and seldom a 0.03-in undercut is selected. Bates’ wrote that 0.01 in is the least that can be used and still obtain effective retention.5 Coolidge’ and Orhan et aI8described the variability of the thickness of the periodontal membrane and concluded that on average, the thickness was 0.25 mm +- 0.1 mrn.’J
Figure 3. Upon insertion, a
------
retentive clasp stresses the abutment from the time of initial suprabulge contact to the fullyseated position. Without simultaneous contacts on the opposite side of the tooth, reciprocation does not occur.
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Sejitevnber 1992, Volume 1, Numbo 1
Conversion of 0.25 mm yiclds 0.01 in so that under the most adverse circumstances of ill-design, the abutment may be able to cope with the stress. The conclusion is that 0.01 in is the least that can be used and is the kindest to the abutment. However, no errors in design or production can be tolerated when 0.01-in retention is being used. In measuring the 0.01 in depth of undercut, the general area must first be located with the analyzing rod in the dental surveyor. Diagnostic dental casts are secured onto the tilt-top table at a 0" tilt. After the cast is fully analyzed, the tilt may have shifted to something less than 8" and the 0.01-in undercut gauge is used to Fpecifically locate the clasp tip. The undercut gauge is replaced with a carbon rod and thc survey line is placed on all teeth and prominences of contours in the soft tissues. The undercut gauge is again placed into the vertical spindle of the survryor to verify the accuracy of the location for retentive clasp tips. The essential factor to remember is that the undercut gauge and vertical spindle must become a finite path of dislodgement in the mouth or else the quality of functional retention will vary from that which was planned. It is unlikely that more than an 8" tilt can be prepared intraorally. If you cannot prepare adequate numbers of guiding surfaces that are dispersed sufficiently to specifically control the path ofdislodgement, increase the depth of undercut to 0.015 in or 0.02 in.
Guidance of Dislodgement Theoretically, a minimum of three guiding surfaces that are well dispersed around more than half of a circle would produce control over the path of dislodgement. However, the dental arch never quite extends far enough to produce a form that equates to half a circle. With this diminished segment of a circle, control over the path of dislodgement requires four effective guiding surfaces. Kor will it suffice to have only unilateral control, for if either side of the RPD is not retentive, all is lost. To be effective, guiding tooth surfaces must also have length. All long guideplates, minor connectors, plating, and other clasps contribute to effective delineation of the path of dislodgement. One must recognize that absolute control over the path of dislodgement probably can be achieved only in RPDs that are toothborne. To lack control over the path of dislodgement is often the case and should merely alert the practitioner to cvaluate the functional quality of retentive elements in a mouth where undirected dislodgement may proceed under the influence of a random force. Such an understanding will prevent the error of tipping a diagnostic cast to produce the illusion ofcreating an undercut. If the proposed tipped path cannot be produced in the mouth, no specific undercut exists and a retentive clasp placed there may slip from the tooth (Fig 4).
v
wercut
excessive undercut
n p-/ ......
...... ....... ......
fh ...... ...... ...... ...... ...... ...... ...... ........ ....... . .... ...
+,I,
.................../*=*
i l
Figure 4. Tilting the diagnostic rast may appear to change the path of insertion/dislodgement. However, unlew the angled path ofdislodgementcan be firmly established in the mouth, no undercut will exist.
16
Remobable Partial Denture
On abutments that do not require the benefits of I-bar retainers (cleanliness, adjustability, improved esthetics) the complex problems of designs based on a path of insertion/dislodgement are best managed by using circumferential clasp retainers that are located in 0.015 in or 0.02 in undercut. In considering the location of a retentive clasp in relation to other elements of an WD, the functional movements of dentures that are tooth supported at one terminus and tissue supported at the other (either distal, anterior, or cross-arch) must be analyzed. Occlusal loading of the denture produces a major rotational axis bctwecn the contralateral rests located nearest the denture basefs). As loading and depression of the denture bases progress, elements of the denture located on the opposite side of the axis rise. Any retentive clasp among the opposite side elements must have increased flexibility or be deleted so as not to constantly lift the tooth. Increased flexibility can be induced by using wrought wire, increasing the clasp length, or decreasing the thickness. Random dislodging forces which act to unseat the denture bases of extension RPDs are resisted by all retentive elements previously listed. This means that in addition to the retentive features associated with tissue contact, the retentive contralateral clasp tips located nearest the denture base(s) will he activated. The initial holding action by the clasps, when combined with progressive dislodgement of the denture base(s), produces a transitory rotational axis between the clasp tips. Elements on the opposite side of the axis rotate tissueward as the clasps resist the progressive dislodgement of the denture base(s). This combination of events has the potential to displace incisor teeth or produce trauma of the soft tissues underlying the downward thrusting anterior elements unless rests are placed on anterior abutments (locations may be reversed, or cross-arch, for anterior extension, maxillofacial, and unilateral RPDs). By preventing the tissueward thrust of anterior elements, auxiliary rests can prevent tissue trauma, stop the rotation induced by the clasp tips, and become the determinants for the major rotational axis of the dislodgement motion. By stopping the rotational action initially located at the clasp tips, the auxiliary rests enable the retentive clasps to act to their full capacity. They indirectly contribute to retention and are called “indirect retainers” (Fig 5). The positional relationships of a dislodging load, retentive forces at clasp tips and indirect retainers
Dean L.Johmorz
Figure 5. Dislodging force (load) a, retentive cffort b, fulcrum c; dislodging load as it activate retentive clasp tip b; anterior rest c resists tissueward rnovernent of anterior elements and enables clasp to function to full capacity; anterior rest indirectly contributes to effectiveness of clasp and becomes known as an indirect retainer.
serving as major fulcrums (located furthest from the clasps on the opposite side from the denture bases) constitutes a Class In lever system.”-” Unfortunately, in a Class III lever system with clasps located between the fulcrum and the dislodging distal extension base load, the retentive force is at a serious mechanical disadvantage. Although there are no stereotype rotational axes in removable partial dentures, there will surely he one located along the crest of a well-formed residual ridge. Without effective guide-plates, denture teeth located labial/bucwl to the crest of the residual ridge may produce cross-arch dislodgement of the denture under unilateral occlusal loading. For satisfactory function, retention should be providcd to compensate for functional rotations generated by the loading of articulating surfaces located to the labial/buccal of well-formed ridges. If 0.01-in retention is routinely used and depends on the path of dislodgement to be exact, it may become necessary to provide additional retention at the 24-hour appointment after delivery. One reason is that construction procedures are so complex that freedom in the path may be produced inadvertently. Methods whereby play in the path of dislodgement may inadvertently occur in the dental ofice are (1) using tapered rotary instruments to prepare guide surfaces; (2) inclining straight-walled rotary instruments when preparing guide surfaces; (3) using naturally occurring inclined guide surfaces; (4)excessive physiological relief of guideplates and other rigid guiding elements at the framework try-in appointment; and (5) no surveying performcd in the dental office. Diminished control over dislodgement also may inadvertently occur in the dental laboratory by (1) blocking out the master cast with a tapcred instrument; (2) using a surveying instrument that possesses an automatic taper to the blockout; and (3)
17
September 1992, P’olume 1.Number 1
using a surveying instrument that is worn and has play in the vertical spindle when blocking out the master cast. Other factors leading to loss of planned retention include no undercut on abutment tooth, miscast framework with shortened circumferential clasp tip, polishing the internal surface of a clasp tip, and accidental deformation of the clasp after the plastic is processed.
Augmentation of Clasp Retention Retention can be increased after delivery but one should always remember that, at rest, a clasp must be passive, that is, it touches a tooth but exerts no force. Therefore, retention must not be increasrd by “tightening the clasp” which is already in contact with a tooth. Such an error will either cause the denture to unseat if the tooth surface is distinctly tapered toward the incisal or occlusal surface, product compression in the c r o w if it is perfectly reciprocated (however, the clasp will fatigue and the need for retention returns), or orthodontically move the tooth. The correct methods for augmenting retention are based on efforts to deepen the undercut, This can be done by bonding restorative material to increase the ~uprabulge,”-’~ creating a rounded ledge on the natural tooth at the upper border of the clasp tip by flattening the surface beneath the clasp tip and then bending the tip to touch the flattened surface, or bending the clasp tip into a deeper naturally occurring undercut.
Summary Guiding metal that fails to produce a distinct path of dislodgement allows the RPD to escape its seated position in most any direction, be it straight-line, rotary, or translatory movements. Surveying to locate both the mesiodistal and occlusogingival height of contour helps to identify the location of recesses
capable of providing retention for both extension RPDs which show uncontrolled dislodgement and for rigid metal retention (in “dual path” RPDs) which must not be allowed to creep from undercuts in any direction. The clasp retained removable partial can be a successful and rewarding method of restoring the partially edentulous mouth. To achieve long-term success, the oral environment must be healthy, plaque control must be effective, and the design plus mouth preparations must be planned and performed by the dentist.
References I. The.4cademyof Denture Prosthctics: GlossatyofProsthodontic Terms (ed 5). St. Louis, MO, hfosby, 1987 2. Skinner EW, Phillips R W The Science ofDental Materials (ed 6). Philadelphia, PA, Saunders, 1969, pp 248-259 3. Craig R G Restorative Dental Materials (ed 6). St. Louis, MO, Mosby, 1980, pp 128,294 4. Rates JE’: Retention of cobalt-chromium partial dentures. Dcnt Practitioner 19ti3; 14:168-17I 5. Bates JF: The mechanical properties of the cobalt-chromium alloys and their relation to partial denture design. Br Dent J 19651119389-396 6. Johnson DL, Stratton RJ, Duncanson MG Jr: The effect of single plane curvature on half-round cast clasps. J Dent Res 1983;62:833-836 7. Coolidge ED: The thickness o1 the periodontal membrane. JADA 1937;24:1260-I270 8. Orban B, Wentz FM, Everett FG, et al: Periodontics. St. Louis, MO, Mosby, 1958, p 39 9. Burns DM? MacDonald SGG: Physics for Biology and PreMedical Students. London, England, AddiTon-Wesley, 1970 10. Stearns 110:Fundamentals OfPhysics and Applications (ed 5). New York, NY, Macmillan, 1956 1 I . Morris 1%‘:The American Heritage Dictionary of the English Language (illustrated).American Heritage Publishing Co, Inc and Houghton Mifflin Co, Boston, ,MA, 1973, p 75 1 12. Jenkins CBG, Berry DC: Modification of tooth contour by acid-etch retained resins for prosthetic purposes. Br Dent J 1976;141:89-90 13. Piirto M, Ecrikainen E, Siirla HS: Enamel bonding plastic materials in modifying the form of abutment teeth for the better functioning of partial prosthesis. J Oral Rehab 197714: 1-8 14. Quinn DM: Artificial undercuts for partial denture clasps. Br Dent J 1981;151:192-194
INDEX WORDS: retention, removable, partial, denture
P
ractitioners and researchers have shown that when a removable partial denture is placed into a healthy oral environment, when the design is developed by a dentist to meet specific needs of the patient, and when good oral hygiene is maintained, long-term success with removable partial denture treatment can be expected. Of the numerous services that a removable partial denture provides, the one that builds a patient's confidence is for the denture to remain seated on the supporting tissues during function. This article discusses the complexity of removable partial denture retention.
Removable Partial Denture Retention Mechanical retention generated by a removable partial denture (RF'D) clasp is only one of many factors that help maintain its resting position. As functional forces act to unseat an RF'D, the individual retentive quality of clasps, clips, springs, magnets, indirect retainers, frictional resistance, gravity, adhesion, cohesion, interfacial surface tension, and atmospheric pressure will act to resist dislodgement. The ability to maintain the resting position depends on all
*Director, Graduate Prosthodontiw: Uniuersi& 4 Oklehona Hrcllth Scicnces Center, College ojDentiTtV, Oklahoma Ci& OK. Address reprint requests to: Dean L.Jnhmon, DDS,MEd, C:[email protected] Oklahoma Health Sciences Center, Collexe ojDentirty, I001 Stanton I-. YounEBlud, Oklahoma Cip,OK 73190. Copyright Q 1992 by theilmeican Collqe ofPosthodontitts I059-941xi 9210101-0002$5.00/0
of the retentive elements' collective resistance to dislodging forces which will have unknown magnitude, frequency, direction, duration, and location of application. If one considers the partially edentulous mouth whercin only the maxillary and mandibular anterior teeth remain, the retentive quality in the RPD will depend on the denture base's ability to generate adhesion, cohesion, and interfacial surface tension between the two closely adapted surfaces of mucosa and acrylic resin with a thin salivary film interposed. In the presence of air, pressure differences produce a resistance to separation based on capillarity phenomena. When the film of fluid is thin and the contact area of the two surfaces is large, the resistance to separation is great. In reference to denture base retention, adhesion pertains to the ability of dissimilar niolecules such as those in saliva, mucosa, and acrylic resin, to attract each other. Cohesion pertains primarily to the ability of like molecules in the salivary film to attract each other. By excluding air from the space between the denture base and mucosa, atmospheric pressure will assist in keeping the denture seated. Also, because each denture has mass, gravity will act to diminish the retentivc values in the maxillary arch and increase those in the mandible. As contacting metal framework and abutment surfaces slide w e r each other, there develops frictional resistance whose magnitude varies with the area and pressure (load) exerted between the surfaces. In addition, for dentures whose path of dislodgement is not totally controlled and may possess a rotational component,
Journal OfProsthodontics,Voll,No 1 (Stptcmber), 1.992:fip 11-I7
11
12
Removable Partial Denture
indirect retention can be developed. Finally, and most commonly considered, the mechanical retention generated by flexible metal clasps is designed. The most frequently used mechanical retainer continues to be the extracoronally located clasp. Aclasp is “the part of a removable partial denture that acts as a direct retainer and/or stabilizer for the prosthesis by partially encompassing or contacting an abutment tooth.”’ The clasps are described as small masses of metal which keep the denture seated in its resting position when dislodging forces act upon it. The details pertaining to the means by which clasp tips provide effective retention are a bit complex. In general, there exists a need to understand the (1) internal characteristics of the alloy that permit it to flex and return to form, (2) physical form of the clasp as it effects the capacity to flex, (3) location of the clasp tip on the abutment, and (4) position of the clasp in relation to other elements of the RPD. Internal characteristics that enable a clasp tip to flex can be found in the ability of atoms in the alloy to move and then return to their original position. When the alloy is at rest, there exists an equilibrium of interatomic distance. As an external force is applied, atoms are moved and the energy that is imparted to the dislocation is stored. When the external force is removed, atoms return to their original location and the equilibrium of interatomic distance is regained. This flexural capability is referred to as elastic deformation. In an RPD, elastic deformation occurs whenever an insertion or dislodging force acts on the denture teeth, denture base, and other elements of the RPD. The abutment contour serves as an obstacle to passage of the clasp tip and thereby causes flexure. Consider what happens to a retentive clasp when forces cause the metal alloy to flex beyond its elastic limit, as when the clinician purposefully bends a clasp or a wire into a new configuration. Upon application of external force, elastic deformation occurs. Increased force produces plastic deformation as sheering forces cause the dislocation plane of atoms to slip at the borders of the crystal and produce permanent deformatione2Clinically, the progression of events are as follows:
I . Application of a strong external force. 2. Disturbance of the equilibrium of the interatomic space. 3. Displacement of atoms and storage of energy. 4. Slipping of crystals.
Dean L.Johnyon
5. Removal of external force. 6.Release of stored energy. 7. Return of most atoms to original location within the mass which in itself has been moved. 8. Equilibrium of the interatomic space. At this point in time, the clasp tip rests in a new location due to plastic deformation, yet has retained its capacity for elastic deformation. Variations in production methods produce similar alloys having different flexibility. The crystalline form of cast alloy displays no organized pattern or orientation. However, wrought metal clasps are described in the literature as having a more fibrous crystalline composition which, when coupled with its rolled, rounded form, provides more flexibility than cast metal clasp^.^ The effect that external form and mass have on the flexibility of a clasp is highly variable in clinical settings. Even though preformed patterns are used in the laboratory construction of clasps, manipulation and finishing diminishes the predictability of their performances. Likewise, variations in length, thickness, width, taper, cross-sectional form, material, and curvature rapidly alter a clasp’s flexibility. Alterations in length and thickness dominate changes because they bear a cube ratio effect on clasp behavior. The longer the clasp, the more flexible it becomes in a cube ratio. The thicker the clasp, the less flexible it becomes in a cube ratio. In addition, as the mesiodistal curvature of a circumferential clasp increases, (premolar versus molar) it becomes ~ t i f f e r . ~ . ~ When a cast clasp (I-bar) becomes more acutely curved in the plane of the flat side of its half-round form, it becomes stiffer.GAs the length of a clasp becomes more involved in the curve itself (in the plane of the flat side of a half-round cast clasp, ie, I-bar), it becomes stiffer.6All things considered, one can conclude that because of the variations in styling and production methods for retentive clasps, a practitioner has little chance of specificallyquantifjring the amount of retentive force a clasp will generate. Retention from a clasp tip must also be evaluated in terms of the distance transited while being dislodged. Ifwe focus on both the depth ofundercut and the acuteness of the curvature of the bulging tooth surface being traversed, the clasp will produce a quantity of retention that begins at zero (resting) and increases to maximum value at the summit of the bulge. When the tooth bulge slopes gently, the clasp tip may unseat by several millimeters before appreciable resistance to dislodgement is produced. Immediate and early retention is minimal, thus
13
September 1992, Volume I , Numbm 1
permitting some functional motion which can be annoying to the patient. Conversely, when the bulge of a crown is abrupt, the retentive capacity of a clasp is achieved quickly and over a short distance. Carrying this reasoning to its extreme, clasps would be most effective when their tips are located in a ledged recess. Such an arrangement would also require an ability in the patient to manually disengage each clasp tip in order to remove the RPD. Because this may be an inconvenient or impossible task for some patients, a preferred form for an undercut may be a recess having a pronounced curvature. Such a form would be similar to the mirror image of a chamfer. It would begin on the survey line and be prepared with the rounded periphery of a diamond wheel (Brasseler USA, Savannah, GA, 040-1.O-909) moving laterally across the buccal or lingual enamel to create a curved recess. The location of a retentive clasp tip on the abutment must be such that tooth curvatures will force the tip to flex while dislodging forces are acting to unseat the denture. Ordinarily, the body of a circumferential clasp embraces a tooth mesiodistally so that the flexible tip will automatically be located beyond the greatest mesiodistal height of contour. However, because bar clasps do not embrace the abutment, it may be critically important that the tip be located below the survey line and on the opposite side of the mesiodistal height of contour from the denture base. Direct retention is defined as the "retention obtained in a removable partial denture by the use of clasps or attachments that resist removal from the abutment teeth".' In locating the recess into which a retentive tip is to be placed, all avenues of escape
must be analyzed, that is, all three planes of space and 360" of direction in those planes. At the beginning of a dental cast survey, the combined common plane of occlusion and crest of the edentulous ridges are oriented parallel to the base of the dental surveyor. From this 0" orientation, the cast may be tipped slightly to align the majority of the proximal walls of teeth bordering edentulous spaces. No severe tipping can occur because the guide surfaces must be prepared in the mouth at the same degree of tilt. From the flat or mildly tipped orientation of the cast, one can obsenle below the survey line the recesses available for retention. However, if the selection of an undercut is to function as planned, the RF'D must have sufficient guiding surfaces to limit dislodgement to a path that is straight and parallel to the vertical spindle of the dental surveyor. Random dislodging forces have numerous components including a mesiodistal vector. Ordinarily, only toothborne or extension RF'Ds having strategically located modification spaces possess the number and location of guiding surfaces to specifically limit dislodgement to a finite path. For effective retention in all other clasp retained RPDs, an effort must be made to ensure that retentive tips cannot escape vertically or horizontally from their seated position in either a straight line, translatory, or curvolinear mesiodistal direction. Therefore, retentive clasp tips on many extension RPDs (Fig 1) should be located both below the traditional survey line and beyond the mesiodistal height of contour of the abutment (in a direction away from the extension denture base). The Ney dental surveyor can be adapted to accomplish the task of surveying in a second plane (Fig 2). Attention to placement of clasp tips will assure
I
I
I I I
I
I I
I 1
Figure 1. An I-bar located in an undercut on the mesio-distal height of contour may escape distally or disto-occlusally when no distinct path of dislodgement exists. (B) An I-bar located in an undrrcut beyond the mesio-distal height ofcontour (from an extension denture base) provides retention when the clasp tip is required to flex over a more prominent contour to escape from its resting position,
14
Removable Partial Denture
Dean L.7ohlnson
Figure 2. (A) Traditional survey line a. tnesiodistal height of contour linc b. (B) Traditional vertical contour line a, linr marking depth ofvertical undercut b, horizontal depth gauge c, pencil about to scribe horizontal depth by crossing vertical depth line b. (C) Mesial and vertical undercut idcntifics location of clasp tip.
that escape without flexure cannot occur in either a vertical or anteroposterior direction. In addition, contralateral positioning of clasps must be such that escape in d lateral direction cannot occur either. A buccal clasp located on one side of the arch requires that the clasp on the opposite side of the arch be located on the buccal. Opposing lingual clasp tips are acceptable also. A mixture of locations are workable so long as clasps on one side of the arch oppose those on the other side of the arch. The effect can be described as potentiation, that is, one clasp ensures that the other must flex to escape. In the proccss, lateral force will be applied to the abutment and shonld be reciprocated by guideplates, plating, minor connectors, rigid stabilizing clasps, natural proximal tooth contacts, or combinations of these. To maintain equilibrium of the tooth’s attachment apparatus, reciprocation must occur at the same time and height on the opposite side of the abutmentwhen the retentive clasp tip is activated. Effective reciprocation cannot be obtained when the opposite wall is inclined occlusalward, as with the lingual surfaces of all anterior tceth and tipped posterior teeth (Fig 3). In such situations, either the alveolar bone and attachment apparatus will resist the forces and
maintain position or it will lack the capacity to do so and the abutment will move. The depth of an undercut that a retentive clasp tip engages may be no less than 0.01 in. Occasionally, a 0.02-in and seldom a 0.03-in undercut is selected. Bates’ wrote that 0.01 in is the least that can be used and still obtain effective retention.5 Coolidge’ and Orhan et aI8described the variability of the thickness of the periodontal membrane and concluded that on average, the thickness was 0.25 mm +- 0.1 mrn.’J
Figure 3. Upon insertion, a
------
retentive clasp stresses the abutment from the time of initial suprabulge contact to the fullyseated position. Without simultaneous contacts on the opposite side of the tooth, reciprocation does not occur.
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Sejitevnber 1992, Volume 1, Numbo 1
Conversion of 0.25 mm yiclds 0.01 in so that under the most adverse circumstances of ill-design, the abutment may be able to cope with the stress. The conclusion is that 0.01 in is the least that can be used and is the kindest to the abutment. However, no errors in design or production can be tolerated when 0.01-in retention is being used. In measuring the 0.01 in depth of undercut, the general area must first be located with the analyzing rod in the dental surveyor. Diagnostic dental casts are secured onto the tilt-top table at a 0" tilt. After the cast is fully analyzed, the tilt may have shifted to something less than 8" and the 0.01-in undercut gauge is used to Fpecifically locate the clasp tip. The undercut gauge is replaced with a carbon rod and thc survey line is placed on all teeth and prominences of contours in the soft tissues. The undercut gauge is again placed into the vertical spindle of the survryor to verify the accuracy of the location for retentive clasp tips. The essential factor to remember is that the undercut gauge and vertical spindle must become a finite path of dislodgement in the mouth or else the quality of functional retention will vary from that which was planned. It is unlikely that more than an 8" tilt can be prepared intraorally. If you cannot prepare adequate numbers of guiding surfaces that are dispersed sufficiently to specifically control the path ofdislodgement, increase the depth of undercut to 0.015 in or 0.02 in.
Guidance of Dislodgement Theoretically, a minimum of three guiding surfaces that are well dispersed around more than half of a circle would produce control over the path of dislodgement. However, the dental arch never quite extends far enough to produce a form that equates to half a circle. With this diminished segment of a circle, control over the path of dislodgement requires four effective guiding surfaces. Kor will it suffice to have only unilateral control, for if either side of the RPD is not retentive, all is lost. To be effective, guiding tooth surfaces must also have length. All long guideplates, minor connectors, plating, and other clasps contribute to effective delineation of the path of dislodgement. One must recognize that absolute control over the path of dislodgement probably can be achieved only in RPDs that are toothborne. To lack control over the path of dislodgement is often the case and should merely alert the practitioner to cvaluate the functional quality of retentive elements in a mouth where undirected dislodgement may proceed under the influence of a random force. Such an understanding will prevent the error of tipping a diagnostic cast to produce the illusion ofcreating an undercut. If the proposed tipped path cannot be produced in the mouth, no specific undercut exists and a retentive clasp placed there may slip from the tooth (Fig 4).
v
wercut
excessive undercut
n p-/ ......
...... ....... ......
fh ...... ...... ...... ...... ...... ...... ...... ........ ....... . .... ...
+,I,
.................../*=*
i l
Figure 4. Tilting the diagnostic rast may appear to change the path of insertion/dislodgement. However, unlew the angled path ofdislodgementcan be firmly established in the mouth, no undercut will exist.
16
Remobable Partial Denture
On abutments that do not require the benefits of I-bar retainers (cleanliness, adjustability, improved esthetics) the complex problems of designs based on a path of insertion/dislodgement are best managed by using circumferential clasp retainers that are located in 0.015 in or 0.02 in undercut. In considering the location of a retentive clasp in relation to other elements of an WD, the functional movements of dentures that are tooth supported at one terminus and tissue supported at the other (either distal, anterior, or cross-arch) must be analyzed. Occlusal loading of the denture produces a major rotational axis bctwecn the contralateral rests located nearest the denture basefs). As loading and depression of the denture bases progress, elements of the denture located on the opposite side of the axis rise. Any retentive clasp among the opposite side elements must have increased flexibility or be deleted so as not to constantly lift the tooth. Increased flexibility can be induced by using wrought wire, increasing the clasp length, or decreasing the thickness. Random dislodging forces which act to unseat the denture bases of extension RPDs are resisted by all retentive elements previously listed. This means that in addition to the retentive features associated with tissue contact, the retentive contralateral clasp tips located nearest the denture base(s) will he activated. The initial holding action by the clasps, when combined with progressive dislodgement of the denture base(s), produces a transitory rotational axis between the clasp tips. Elements on the opposite side of the axis rotate tissueward as the clasps resist the progressive dislodgement of the denture base(s). This combination of events has the potential to displace incisor teeth or produce trauma of the soft tissues underlying the downward thrusting anterior elements unless rests are placed on anterior abutments (locations may be reversed, or cross-arch, for anterior extension, maxillofacial, and unilateral RPDs). By preventing the tissueward thrust of anterior elements, auxiliary rests can prevent tissue trauma, stop the rotation induced by the clasp tips, and become the determinants for the major rotational axis of the dislodgement motion. By stopping the rotational action initially located at the clasp tips, the auxiliary rests enable the retentive clasps to act to their full capacity. They indirectly contribute to retention and are called “indirect retainers” (Fig 5). The positional relationships of a dislodging load, retentive forces at clasp tips and indirect retainers
Dean L.Johmorz
Figure 5. Dislodging force (load) a, retentive cffort b, fulcrum c; dislodging load as it activate retentive clasp tip b; anterior rest c resists tissueward rnovernent of anterior elements and enables clasp to function to full capacity; anterior rest indirectly contributes to effectiveness of clasp and becomes known as an indirect retainer.
serving as major fulcrums (located furthest from the clasps on the opposite side from the denture bases) constitutes a Class In lever system.”-” Unfortunately, in a Class III lever system with clasps located between the fulcrum and the dislodging distal extension base load, the retentive force is at a serious mechanical disadvantage. Although there are no stereotype rotational axes in removable partial dentures, there will surely he one located along the crest of a well-formed residual ridge. Without effective guide-plates, denture teeth located labial/bucwl to the crest of the residual ridge may produce cross-arch dislodgement of the denture under unilateral occlusal loading. For satisfactory function, retention should be providcd to compensate for functional rotations generated by the loading of articulating surfaces located to the labial/buccal of well-formed ridges. If 0.01-in retention is routinely used and depends on the path of dislodgement to be exact, it may become necessary to provide additional retention at the 24-hour appointment after delivery. One reason is that construction procedures are so complex that freedom in the path may be produced inadvertently. Methods whereby play in the path of dislodgement may inadvertently occur in the dental ofice are (1) using tapered rotary instruments to prepare guide surfaces; (2) inclining straight-walled rotary instruments when preparing guide surfaces; (3) using naturally occurring inclined guide surfaces; (4)excessive physiological relief of guideplates and other rigid guiding elements at the framework try-in appointment; and (5) no surveying performcd in the dental office. Diminished control over dislodgement also may inadvertently occur in the dental laboratory by (1) blocking out the master cast with a tapcred instrument; (2) using a surveying instrument that possesses an automatic taper to the blockout; and (3)
17
September 1992, P’olume 1.Number 1
using a surveying instrument that is worn and has play in the vertical spindle when blocking out the master cast. Other factors leading to loss of planned retention include no undercut on abutment tooth, miscast framework with shortened circumferential clasp tip, polishing the internal surface of a clasp tip, and accidental deformation of the clasp after the plastic is processed.
Augmentation of Clasp Retention Retention can be increased after delivery but one should always remember that, at rest, a clasp must be passive, that is, it touches a tooth but exerts no force. Therefore, retention must not be increasrd by “tightening the clasp” which is already in contact with a tooth. Such an error will either cause the denture to unseat if the tooth surface is distinctly tapered toward the incisal or occlusal surface, product compression in the c r o w if it is perfectly reciprocated (however, the clasp will fatigue and the need for retention returns), or orthodontically move the tooth. The correct methods for augmenting retention are based on efforts to deepen the undercut, This can be done by bonding restorative material to increase the ~uprabulge,”-’~ creating a rounded ledge on the natural tooth at the upper border of the clasp tip by flattening the surface beneath the clasp tip and then bending the tip to touch the flattened surface, or bending the clasp tip into a deeper naturally occurring undercut.
Summary Guiding metal that fails to produce a distinct path of dislodgement allows the RPD to escape its seated position in most any direction, be it straight-line, rotary, or translatory movements. Surveying to locate both the mesiodistal and occlusogingival height of contour helps to identify the location of recesses
capable of providing retention for both extension RPDs which show uncontrolled dislodgement and for rigid metal retention (in “dual path” RPDs) which must not be allowed to creep from undercuts in any direction. The clasp retained removable partial can be a successful and rewarding method of restoring the partially edentulous mouth. To achieve long-term success, the oral environment must be healthy, plaque control must be effective, and the design plus mouth preparations must be planned and performed by the dentist.
References I. The.4cademyof Denture Prosthctics: GlossatyofProsthodontic Terms (ed 5). St. Louis, MO, hfosby, 1987 2. Skinner EW, Phillips R W The Science ofDental Materials (ed 6). Philadelphia, PA, Saunders, 1969, pp 248-259 3. Craig R G Restorative Dental Materials (ed 6). St. Louis, MO, Mosby, 1980, pp 128,294 4. Rates JE’: Retention of cobalt-chromium partial dentures. Dcnt Practitioner 19ti3; 14:168-17I 5. Bates JF: The mechanical properties of the cobalt-chromium alloys and their relation to partial denture design. Br Dent J 19651119389-396 6. Johnson DL, Stratton RJ, Duncanson MG Jr: The effect of single plane curvature on half-round cast clasps. J Dent Res 1983;62:833-836 7. Coolidge ED: The thickness o1 the periodontal membrane. JADA 1937;24:1260-I270 8. Orban B, Wentz FM, Everett FG, et al: Periodontics. St. Louis, MO, Mosby, 1958, p 39 9. Burns DM? MacDonald SGG: Physics for Biology and PreMedical Students. London, England, AddiTon-Wesley, 1970 10. Stearns 110:Fundamentals OfPhysics and Applications (ed 5). New York, NY, Macmillan, 1956 1 I . Morris 1%‘:The American Heritage Dictionary of the English Language (illustrated).American Heritage Publishing Co, Inc and Houghton Mifflin Co, Boston, ,MA, 1973, p 75 1 12. Jenkins CBG, Berry DC: Modification of tooth contour by acid-etch retained resins for prosthetic purposes. Br Dent J 1976;141:89-90 13. Piirto M, Ecrikainen E, Siirla HS: Enamel bonding plastic materials in modifying the form of abutment teeth for the better functioning of partial prosthesis. J Oral Rehab 197714: 1-8 14. Quinn DM: Artificial undercuts for partial denture clasps. Br Dent J 1981;151:192-194
