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Clasp partial dentures
Victor L. Steffel* D.D.S., Columbus, Ohio
CLASP PARTIAL DENTURES
In most instances, the clasp partial den ture is the restoration of choice in oc clusal reconstruction. The slight denture movement
allowed
by
passive
clasps
avoids overworking of abutment teeth and tends to stimulate and preserve ridges. The controlled, permitted move ment of a removable partial denture dur ing the chewing cycle distributes the masticatory loads to teeth and other sup porting tissues without trauma to any specific region. The bar clasp is much to be preferred to the cast circumferential clasp from the standpoint of the patient’s welfare.
“ More than forty years of careful obser vation and experience has led me to be lieve that when partial restorations are to be made, the modern clasp affords the most conservative and the most nearly universally applicable means of anchor age.” The foregoing quotation is from a July, 1934, treatise by the late F. Ewing Roach. Now, 26 years later, with im provements in methods and materials, this statement rings even more true than when it was first given consideration. This is possible because “A Clasp Partial is an appliance which derives its fixation from Extracoronal Attachments or Clasps, can be removed at will by the patient, and which supplies one or more teeth and adjacent structures in a partially edentulous arch.” Its external attach ments often require minimal tooth prep arations. It may rest entirely on remain ing teeth or obtain part of its support from the mucosa and ridge underlying
a base with a positively sustaining abut ment at only one end. When properly designed and when constructed of the highest grade of materials, the clasp par tial denture constitutes a long range ve hicle of function, an appliance the fabri cation of which falls within the sphere of abilities of the average dentist and the cost of which is quite apt to be within the scope of the patient’s economic status. From the standpoint of its providing a satisfactory service to the greatest pos sible segment of society, the clasp partial denture may be said to be to partial prosthodontics what the amalgam restoration is to operative dentistry. In most in stances, therefore, after all factors have been meticulously explored, the clasp par tial denture becomes the restoration of choice over other possibilities. DIRECT RETA IN ERS
The functions of clasps and precision at tachments, both direct retainers, are the same. However, these functions— support, stabilization and retention— are not at tained in the same manner, nor to the same degree of effectiveness by both types of retainers. With slotted attachments, functional stresses directed parallel to the long axes of the abutment teeth as well as those aimed laterally and anteroposteriorly are all efficiently supported and stabilized by the total fibers of the perio dontal structures as there results slight tooth movement, bodily. With clasps, on the contrary, stresses pointing parallel to long axes may use some or all of the in vesting fibers (depending on the shape of the rest seat), but only part of them
72/804 • THE JOURNAL OF THE AMERICAN DENTAL ASSOCIATION
when resisting horizontal shifts of an ap pliance. This is because clasps tend to tip abutment teeth rather than to move them en masse. Therefore, it is sometimes necessary to employ additional clasps to provide necessary stabilization without overworking abutment teeth and ridges. The secret of success in rendering this service seems to rest with the exercising of good judgment: namely, to utilize the precision attachment partial denture where it must be used and to construct a partial restoration with clasp fixation where it is thought feasible or where it is indicated for best results. Surely, both types assume an important role in oral rehabilitation. FU N C T IO N S
At this point, the expectations of direct retainers will be explored in order to de cide how well clasps “measure up.” The functions of a removable partial denture are: 1. The restoration of an ideal cosmetic effect harmonized with a desirable facial expression. 2. The preservation of the remaining teeth and of all other supporting regions and structures. 3. The restoration and preservation of occlusion and masticating efficiency, im paired by either malposition of remaining teeth or by loss of teeth. These functions seem adequately to en compass the problems. Generally speak ing, the problems can all be well solved by the use of the clasp retainer. It is possible to design clasps so as to avoid a display of metal. Also, the slight denture movement allowed by passive clasps avoids overworking of abutment teeth and tends to stimulate and preserve ridges. Finally, excessive cutting away of tooth structure is avoided by a clasping procedure. The “ diagnostic tripod”— the patient present, the roentgenograms illuminated, and mounted study casts at hand— merits emphasizing. By this thorough means of
diagnosing, it is possible to determine what qualities will be most desirable and possible of attainment in the partial den ture, and knowing the basic functions of clasps, the dentist can then design the clasps and the entire appliance intelli gently. FU N D A M E N TA L PRIN C IP LE S
Certain fundamental principles1 apply to all removable partial dentures, whether clasp retained or otherwise. Several gen eralizations seem in order. There are cer tain requirements for all removable par tial dentures if they are to be successful. Among these are the necessity for cross arch stability of the framework; typical design of saddles; selection of teeth which will enhance esthetics, but which are smaller posteriorly in the occlusal aspect than natural teeth; a generally preferred combination overall structure of metal and plastic bases, since such saddles lend themselves best to maximum ridge cover age, and to relining2 and servicing needs; and the use of plastic teeth adjacent to direct retainers, in order to minimize breakage. Regardless of the type of direct retain ers used, partial denture framework must be rigid. T o attain this quality, the bars should be sufficiently stiff to resist spring ing or bending under unilateral and bi lateral masticatory stresses. Stress loads, then, applied on one side of the arch will be transmitted to the opposite side with a complete and unmitigated bi lateral distribution of these forces. Only the indirect retainer, which is considered so necessary with clasp partial dentures, is generally omitted from the internal at tachment framework design. Further, it is never conducive to in ferior or sublevel partial denture service to splint teeth together rigidly by means of crowns or M O D gold inlays. Doing so insures a better overall sharing of the work load by consecutive teeth. Also, the splinted abutment teeth efficiently resist tipping and are prevented from being
STEFFEL . . . VOLUME 66, JU N E 1963 • 73/805
drawn out of contact with the adjacent teeth. The employment of large saddles for intimate tissue contact with as much sur face as will be tolerated by the patient contributes to maximum stress load dis tribution. This principle applies to all removable partial dentures that involve ridge tissues for part of the support. In selected instances (rarely necessary for the maxillary arch), already large and efficient bases or extension saddles can have their function additionally enhanced by a relining2 procedure. A more inti mate adaptation of a basal seat to its sup porting tissues always can be obtained by a meticulously developed relining im pression. Wherever it seems feasible, crowns or inlays should be applied to abutment teeth to provide perfectly contoured and more positive rest seats, to align the crown portion of leaning teeth into a long axis harmony with other abutments, to estab lish coronal shapes and undercuts ideal for engagement of clasp components, and to cut down the incidence of possible subsequent caries. It would be extremely radical daily practice, however, to insist that such operative procedures be carried out routinely, that is, in every instance involving clasp partial service. More often than not, teeth can be utilized as clasped abutments just as nature has provided them or as they have been previously filled and crowned, without additional extensive precious metal restorations. Fillings, crowns, and so forth, can then be placed later during the years of service of the removable partial denture if and when caries does occur. When to do what will be decided by the educated and ex perienced dentist. S T R E SS -B R E A K E R S
The use of elastic spring stress-breakers or stress equalizers and the introduction of hinges between clasps and distal exten sion bases as a matter of regular policy is to be emphatically condemned. Such
gadgets generally defeat the purposes for which they are intended, and are not suc cessful in shielding abutment teeth against trauma or against a failure to preserve the alveolar ridges. They are subject to metallic fractures that require frequent repairs and they often detract from the bilateral inflexibility of the partial den ture. The result is bilateral instability. Spring stress-breakers do not positively limit with a stop the movement of a saddle tissueward under a load. As a re sult, the ridges usually are overworked and traumatized with excessive resorption sequelae; the teeth do not do their full share of providing support and resisting excessive shifting of a restoration; soft tis sues distally adjacent to abutments are pinched and strangulated; the appliance feels bulky to the tongue and presents ad ditional areas for the trapping of food, and the abutment teeth often are sub jected to a tipping strain as the opposite ends of the stress-breaking wires spring in an arc. A hinged stress-breaker allows move ment of the distal end of a saddle from side to side as well as up and down. So the efficient distribution of stress loads to all possible areas in an arch is nullified. With such a design and construction, ridges undergo rapid shrinkage, while the patient suffers daily from a sense of inse curity due to the looseness and lateral wobbling of the extension bases. Remov able partial dentures are complex enough at their simplest; they are subject to breakage even at their best. Partial den tures, therefore, should always be con structed as simply as possible consistent with function. As Elliott® stated: “ . . . the majority of partial dentures can be planned in a simple manner and still ren der as satisfactory service as many of the more complex designs requiring the use of precision type retainers and stressbreakers.” A tooth which is so weak and infirm as to indicate the need for some type of stress-breaking linkage to pamper it should, perhaps, be sacrificed. With the tooth gone, there could then be no pos-
SUPRA BULGE
INFRA BULGE Fig. I • Clasp arms are of either the supra-bulge or infra-bulge variety; that is, they approach re tentive undercuts from the occlusal stop area or from the tissue or base area. Supra-bulge clasps are circumferential. Infra-bulge clasps are of the bar type
Fig. 2 • Lower drawing shows that highly elastic wrought arms may extend more deeply into un dercuts than can cast termini. Circumferential clasps are difficult to adjust to passivity, prevent normal massage of gingiva by deflecting food and usually make an unesthetic display of metal
Fig. 3 • Infra-bulge "T " bar. This clasp arm type permits universal application. Either one or both termini of the T may extend into undercuts; being a push type arm, it provides excellent retention
Fig. 4 • " U " clasp arm is similar to the T but has less contact with tooth surface and permits normal food movement over the tooth surface and free gingiva. Very retentive
STEFFEL . . . VOLUME 66, JU N E 1963 • 75/807
sible need for the stress-breaker. In those rare instances in which the supporting tissues are flabby, highly displaceable, and movable, or where they offer practically no support, some sort of movable connec tions between the appliance and tooth attachments must be employed. Such problems are few and constitute the ex ception. C L A SP CONSIDERATIONS
U p to this point, various aspects pertain ing to the removable partial denture in general and to the clasp partial denture in particular have been covered. Now attention will be focused on the direct retainer or clasp itself. A clasp (Fig. 1) is an extracoronal retainer contacting a tooth either continuously or intermit tently more than half way around and providing support and fixation for a par tial denture. A clasp cannot perform its functions unless it has all its parts. Ac cordingly, it is not a complete clasp un less it has a rest or occlusal stop to pro vide support. Assistance by the teeth in bearing stresses perpendicular to the oc clusal plane is so necessary that clasps without positive stops against teeth will not be considered here. The distance that a retentive clasp arm can safely spring repeatedly into and out of an undercut without undergoing metallic fatigue and breakage depends on (1) the elasticity of the metal itself, (2) the taper of the clasp arm, and (3) the distance traversed by the arm (its length). Assuming that a clasp arm is elastic, if it is then accurately tapered, it will spring throughout its en tire length. The longer it is, the greater will be the arc through which it can safely move at its free retentive terminus. It follows, therefore, that the longer the tapered clasp arm, the better it will spring, the more often it can be safely flexed without becoming fatigued, and the less traumatizing it will be to abut ment teeth. It is regrettable that the cast circum ferential clasp (Fig. 2 ), most destructive to tooth surfaces and most abusive to peri
odontal structures as well as to alveolar tissue supports, is the one most widely used. The bar clasp (Fig. 3 -7 ), con versely, is generally much to be preferred from the standpoint of the patient’s wel fare because o f: 1. The longer clasp arms. 2. Stress absorbing spring action dur ing appliance shifts. 3. The ease with which clasp passivity can be effected to allow for physiologic stimulation through slight denture move ment in function. 4. Minimum tooth contact insuring the smallest caries incidence. 5. Usually better esthetics due to less display of metal. 6. The excellent partial denture fixa tion afforded. 7. The allowance by the open clasp arm of normal gum massage afforded by foods passing over them. Several bar clasp designs allow their elasticity to function in two different ways (Fig. 6 ). For example, the upright portion of a bar clasp L arm will spring or flex laterally on engaging an undercut while the horizontal part of the arm ly ing along the gum tissue will be undergo ing torsion bar springing. It would be unfair to leave the impression that bar clasps present no disadvantages. Such clasps require more metal; they are more difficult and time-consuming to fabricate, they are bulkier and much more con ducive to food entrapment than other clasps. They are also quite costly by com parison. Impression methods, mouth prepara tions, surveying and designing of partial dentures and so forth, are not within the scope of this paper. However, certain ad ditional clasp types which have practical value and which merit daily utilization in the fabricating of partial dentures will be discussed. The clasps shown and dis cussed up to this point and which are the ones mostly used today, are designs pro viding two elastic termini for the engage ment of retentive undercuts. They do not have a rigid reciprocal arm or surface for denture stabilization and for the purpose
76/808 • THE JOURNAL OF THE AMERICAN DENTAL ASSOCIATION
of gliding in contact with the tooth oppo site to the retentive arm while its terminus is being forced over a bulge into an un dercut and vice versa (Fig. 8 -1 5 ). If pro vided, such reciprocal provision certainly can do no harm.
movement of lips and food, and has only one point of contact with the tooth surface. It requires that the reciprocal arm go far around on the lingual aspect for over half way round tooth en gagement (to little broken circle)
However, is that rigid reciprocal factor as indispensable as many have claimed? Theory may have taken precedence over clinical results. No research has been re ported to prove the harm, if any, result ing when a reciprocal bar is not used along with clasp arms at divergent heights on a tooth (Fig. 8 ). If a clasped tooth were under constant tension, movement and trauma would be the inevitable sequel. But since clasps are adjusted to passivity, opposite arms work against one another only a brief moment on insertion or on removal of the appliance. During the time that the denture is in place, functional stresses utilize the clasp com ponents independently, depending on the direction of the force. Points of clasp con tact being at different heights should then make no measurable difference. Generally but not always, I prefer to use clasps with two elastic retentive ter mini, whenever feasible (Fig. 16, 17). Such retainers allow for double adjust ment, provide additional retention and, because both arms possess elasticity, are less harsh and traumatizing to abutment teeth. CON CLUSIO N
Fig. 6 • " L " and " C " clasp arms. The L terminus has only one point of contact with tooth surface. It is springy, allows easy cleaning, is not trau matic to abutment tooth and permits physiologic movement of the base. C arm lacks metal display and possesses mostly good qualities but must have a long reciprocal arm on lingual (to broken circle)
The use of clasp partial prostheses is a responsibility of the dental profession. If this phase were omitted from dental serv ices, dentists would not be fulfilling their obligations to the public. The problem cannot be dodged simply because it is difficult; also, this work cannot be dele gated to any other group. Therefore den tists must study and learn more about partial dentures since there is a definite need for this type of service and, because, in many instances, no other means of oc clusal reconstruction is nearly as effective as the clasp partial denture.
STEFFEL . . . VOLUME 66, JU N E 1963 • 77/809
Fig. 7 • Mesiodistal clasp has limited use, gener ally on upper cuspids. Clasp is excellent esthetically and is best employed when the clasped tooth has been restored with a three-quarter or veneer crown to provide proper contour, mesial arm clearance, and insurance against caries
Fig. 9 • Upper drawing shows same situation as in Figure 8. Lower drawing illustrates how un toward stress laterally on a tooth can be offset and nullified by an upright which will provide con stant reciprocal straight edge stabilization during either insertion or removal
Fig. 8 • Effect of having opposing clasp arms at different levels. On removal, right clasp termini would be exerting greatest pressure against con vexities after clasp arm reciprocation on the left had been lost
Fig. 10 • Another method of providing a non undercut reciprocating surface— with a threequarter crown or by full coverage
78/810 • THE JOURNAL OF THE AMERICAN DENTAL ASSOCIATION
Fig. I I • Embrasure saddle clasp4 arm. Vertical lingual strut provides stabilizing effect as the buccal clasp arm passes over distobuccal bulge
Fig. 12 • A distobuccal clasp5 arm engagement similar to Figure I I and with a vertical lingual strut having no occlusal stop. Clasps of type illus trated by Figures I I and 12 are kind to abutment teeth and allow for physiological stimulation of sub-basal tissues
Fig. 14 • Drawing shows clasping of teeth in se quence.7 This can only be done as on left or right when clasped teeth are in contact. Rigid recipro cation is provided at the mesial aspect of the first bicuspids
Fig. 13 • Lingual and buccal views of another type of clasp6 with rigid lingual arm and with only an elastic wrought component adapted to buccal undercut for retention Fig. 15 • Clasping teeth in sequence. Appliance is well stabilized and unsightly display of metal is avoided
STEFFEL.,. VOLUME ¿4, JU N E 1963 • 79/811
Fig. 16 • Ideal way to provide bilateral stabilization for unilateral extension by crowning and properly contouring a tooth for clasping on side opposite to free-end base. This eliminates bulk, prevents sub sequent caries and provides ideal clasp retention, each clasp having two retentive termini
Physiologic stimulation of teeth and ridge tissues is deemed advisable. Physi ologists agree on this premise. Traumatic stresses as well as dysfunction, on the other hand, are known to be harmful and destructive. Clasps, properly designed, constructed of an elastic material, and adjusted to passivity, permit a greater range of movement of the appliance dur ing the masticatory cycle than has the removable partial denture which obtains its fixation from slotted attachments. These slight advantageous shifts, how ever, will be limited and controlled by proper clasp adjustment. Accordingly, it is my opinion that a clasp with a rounded, but positive, oc clusal rest and elastic arms which are exerting no pressure at all (passive)
against the tooth when at rest will allow in all directions a moderate working movement of the base attached to it. Therefore the abutment tooth does not always have to “go along” with all the action of the saddle— principally the first part of any work motion. This is most desirable since the mucous membrane over the ridges is capable of more displaceability than are the periodontal structures. This controlled, but permitted, movement of a removable partial den ture during the chewing cycle and at other times (up and down, laterally, anteroposteriorly, and rotary) as is possible with ideal clasping, distributes the masti catory loads to teeth and other supporting tissues without trauma to any specific re gion and effects an ideal physiologic stim ulation leading to robust health for all the structures involved.
19 West Fifth Avenue
Fig. 17 • Embrasure double clasp is not at all ideal or necessary, is bulky to the tongue and highly conducive to caries (see Figure 16)
*Professor and chairman, division of rem ovable partial prosthodontics, Co llege of Dentistry, O hio State Univer sity, Columbus, Ohio. 1. Steffel, V. L. Fundamental principles involved in partial denture design. J.A .D .A . 42:534 M ay 1951. 2. Steffel, V. L. Relining rem ovable partial dentures for fit and function. J . Pros. Den. 4:496 Ju ly 1954. 3. Elliott, F. C. Removable partial denture prosthesis. J.A .D .A . 40:676 June 1950. 4. DeVan, M. M. Preserving natural teeth through the use of clasps. J . Pros. Den. 5:208 M arch 1955. 5. G irado t, R. L. History and developm ent of partial denture design. J.A .D .A . 28:1399 Sept. 1941. 6. A ppleg ate, O . C., and Nissle, R. O . Keeping the partial denture in harmony with biologic limitations. J.A .D .A . 43:409 O ct. 1951. 7. Steffel, V. L. Planning rem ovable partial dentures. J . Pros. Den. 12:524 May-June 1962.
CLASP PARTIAL DENTURES
In most instances, the clasp partial den ture is the restoration of choice in oc clusal reconstruction. The slight denture movement
allowed
by
passive
clasps
avoids overworking of abutment teeth and tends to stimulate and preserve ridges. The controlled, permitted move ment of a removable partial denture dur ing the chewing cycle distributes the masticatory loads to teeth and other sup porting tissues without trauma to any specific region. The bar clasp is much to be preferred to the cast circumferential clasp from the standpoint of the patient’s welfare.
“ More than forty years of careful obser vation and experience has led me to be lieve that when partial restorations are to be made, the modern clasp affords the most conservative and the most nearly universally applicable means of anchor age.” The foregoing quotation is from a July, 1934, treatise by the late F. Ewing Roach. Now, 26 years later, with im provements in methods and materials, this statement rings even more true than when it was first given consideration. This is possible because “A Clasp Partial is an appliance which derives its fixation from Extracoronal Attachments or Clasps, can be removed at will by the patient, and which supplies one or more teeth and adjacent structures in a partially edentulous arch.” Its external attach ments often require minimal tooth prep arations. It may rest entirely on remain ing teeth or obtain part of its support from the mucosa and ridge underlying
a base with a positively sustaining abut ment at only one end. When properly designed and when constructed of the highest grade of materials, the clasp par tial denture constitutes a long range ve hicle of function, an appliance the fabri cation of which falls within the sphere of abilities of the average dentist and the cost of which is quite apt to be within the scope of the patient’s economic status. From the standpoint of its providing a satisfactory service to the greatest pos sible segment of society, the clasp partial denture may be said to be to partial prosthodontics what the amalgam restoration is to operative dentistry. In most in stances, therefore, after all factors have been meticulously explored, the clasp par tial denture becomes the restoration of choice over other possibilities. DIRECT RETA IN ERS
The functions of clasps and precision at tachments, both direct retainers, are the same. However, these functions— support, stabilization and retention— are not at tained in the same manner, nor to the same degree of effectiveness by both types of retainers. With slotted attachments, functional stresses directed parallel to the long axes of the abutment teeth as well as those aimed laterally and anteroposteriorly are all efficiently supported and stabilized by the total fibers of the perio dontal structures as there results slight tooth movement, bodily. With clasps, on the contrary, stresses pointing parallel to long axes may use some or all of the in vesting fibers (depending on the shape of the rest seat), but only part of them
72/804 • THE JOURNAL OF THE AMERICAN DENTAL ASSOCIATION
when resisting horizontal shifts of an ap pliance. This is because clasps tend to tip abutment teeth rather than to move them en masse. Therefore, it is sometimes necessary to employ additional clasps to provide necessary stabilization without overworking abutment teeth and ridges. The secret of success in rendering this service seems to rest with the exercising of good judgment: namely, to utilize the precision attachment partial denture where it must be used and to construct a partial restoration with clasp fixation where it is thought feasible or where it is indicated for best results. Surely, both types assume an important role in oral rehabilitation. FU N C T IO N S
At this point, the expectations of direct retainers will be explored in order to de cide how well clasps “measure up.” The functions of a removable partial denture are: 1. The restoration of an ideal cosmetic effect harmonized with a desirable facial expression. 2. The preservation of the remaining teeth and of all other supporting regions and structures. 3. The restoration and preservation of occlusion and masticating efficiency, im paired by either malposition of remaining teeth or by loss of teeth. These functions seem adequately to en compass the problems. Generally speak ing, the problems can all be well solved by the use of the clasp retainer. It is possible to design clasps so as to avoid a display of metal. Also, the slight denture movement allowed by passive clasps avoids overworking of abutment teeth and tends to stimulate and preserve ridges. Finally, excessive cutting away of tooth structure is avoided by a clasping procedure. The “ diagnostic tripod”— the patient present, the roentgenograms illuminated, and mounted study casts at hand— merits emphasizing. By this thorough means of
diagnosing, it is possible to determine what qualities will be most desirable and possible of attainment in the partial den ture, and knowing the basic functions of clasps, the dentist can then design the clasps and the entire appliance intelli gently. FU N D A M E N TA L PRIN C IP LE S
Certain fundamental principles1 apply to all removable partial dentures, whether clasp retained or otherwise. Several gen eralizations seem in order. There are cer tain requirements for all removable par tial dentures if they are to be successful. Among these are the necessity for cross arch stability of the framework; typical design of saddles; selection of teeth which will enhance esthetics, but which are smaller posteriorly in the occlusal aspect than natural teeth; a generally preferred combination overall structure of metal and plastic bases, since such saddles lend themselves best to maximum ridge cover age, and to relining2 and servicing needs; and the use of plastic teeth adjacent to direct retainers, in order to minimize breakage. Regardless of the type of direct retain ers used, partial denture framework must be rigid. T o attain this quality, the bars should be sufficiently stiff to resist spring ing or bending under unilateral and bi lateral masticatory stresses. Stress loads, then, applied on one side of the arch will be transmitted to the opposite side with a complete and unmitigated bi lateral distribution of these forces. Only the indirect retainer, which is considered so necessary with clasp partial dentures, is generally omitted from the internal at tachment framework design. Further, it is never conducive to in ferior or sublevel partial denture service to splint teeth together rigidly by means of crowns or M O D gold inlays. Doing so insures a better overall sharing of the work load by consecutive teeth. Also, the splinted abutment teeth efficiently resist tipping and are prevented from being
STEFFEL . . . VOLUME 66, JU N E 1963 • 73/805
drawn out of contact with the adjacent teeth. The employment of large saddles for intimate tissue contact with as much sur face as will be tolerated by the patient contributes to maximum stress load dis tribution. This principle applies to all removable partial dentures that involve ridge tissues for part of the support. In selected instances (rarely necessary for the maxillary arch), already large and efficient bases or extension saddles can have their function additionally enhanced by a relining2 procedure. A more inti mate adaptation of a basal seat to its sup porting tissues always can be obtained by a meticulously developed relining im pression. Wherever it seems feasible, crowns or inlays should be applied to abutment teeth to provide perfectly contoured and more positive rest seats, to align the crown portion of leaning teeth into a long axis harmony with other abutments, to estab lish coronal shapes and undercuts ideal for engagement of clasp components, and to cut down the incidence of possible subsequent caries. It would be extremely radical daily practice, however, to insist that such operative procedures be carried out routinely, that is, in every instance involving clasp partial service. More often than not, teeth can be utilized as clasped abutments just as nature has provided them or as they have been previously filled and crowned, without additional extensive precious metal restorations. Fillings, crowns, and so forth, can then be placed later during the years of service of the removable partial denture if and when caries does occur. When to do what will be decided by the educated and ex perienced dentist. S T R E SS -B R E A K E R S
The use of elastic spring stress-breakers or stress equalizers and the introduction of hinges between clasps and distal exten sion bases as a matter of regular policy is to be emphatically condemned. Such
gadgets generally defeat the purposes for which they are intended, and are not suc cessful in shielding abutment teeth against trauma or against a failure to preserve the alveolar ridges. They are subject to metallic fractures that require frequent repairs and they often detract from the bilateral inflexibility of the partial den ture. The result is bilateral instability. Spring stress-breakers do not positively limit with a stop the movement of a saddle tissueward under a load. As a re sult, the ridges usually are overworked and traumatized with excessive resorption sequelae; the teeth do not do their full share of providing support and resisting excessive shifting of a restoration; soft tis sues distally adjacent to abutments are pinched and strangulated; the appliance feels bulky to the tongue and presents ad ditional areas for the trapping of food, and the abutment teeth often are sub jected to a tipping strain as the opposite ends of the stress-breaking wires spring in an arc. A hinged stress-breaker allows move ment of the distal end of a saddle from side to side as well as up and down. So the efficient distribution of stress loads to all possible areas in an arch is nullified. With such a design and construction, ridges undergo rapid shrinkage, while the patient suffers daily from a sense of inse curity due to the looseness and lateral wobbling of the extension bases. Remov able partial dentures are complex enough at their simplest; they are subject to breakage even at their best. Partial den tures, therefore, should always be con structed as simply as possible consistent with function. As Elliott® stated: “ . . . the majority of partial dentures can be planned in a simple manner and still ren der as satisfactory service as many of the more complex designs requiring the use of precision type retainers and stressbreakers.” A tooth which is so weak and infirm as to indicate the need for some type of stress-breaking linkage to pamper it should, perhaps, be sacrificed. With the tooth gone, there could then be no pos-
SUPRA BULGE
INFRA BULGE Fig. I • Clasp arms are of either the supra-bulge or infra-bulge variety; that is, they approach re tentive undercuts from the occlusal stop area or from the tissue or base area. Supra-bulge clasps are circumferential. Infra-bulge clasps are of the bar type
Fig. 2 • Lower drawing shows that highly elastic wrought arms may extend more deeply into un dercuts than can cast termini. Circumferential clasps are difficult to adjust to passivity, prevent normal massage of gingiva by deflecting food and usually make an unesthetic display of metal
Fig. 3 • Infra-bulge "T " bar. This clasp arm type permits universal application. Either one or both termini of the T may extend into undercuts; being a push type arm, it provides excellent retention
Fig. 4 • " U " clasp arm is similar to the T but has less contact with tooth surface and permits normal food movement over the tooth surface and free gingiva. Very retentive
STEFFEL . . . VOLUME 66, JU N E 1963 • 75/807
sible need for the stress-breaker. In those rare instances in which the supporting tissues are flabby, highly displaceable, and movable, or where they offer practically no support, some sort of movable connec tions between the appliance and tooth attachments must be employed. Such problems are few and constitute the ex ception. C L A SP CONSIDERATIONS
U p to this point, various aspects pertain ing to the removable partial denture in general and to the clasp partial denture in particular have been covered. Now attention will be focused on the direct retainer or clasp itself. A clasp (Fig. 1) is an extracoronal retainer contacting a tooth either continuously or intermit tently more than half way around and providing support and fixation for a par tial denture. A clasp cannot perform its functions unless it has all its parts. Ac cordingly, it is not a complete clasp un less it has a rest or occlusal stop to pro vide support. Assistance by the teeth in bearing stresses perpendicular to the oc clusal plane is so necessary that clasps without positive stops against teeth will not be considered here. The distance that a retentive clasp arm can safely spring repeatedly into and out of an undercut without undergoing metallic fatigue and breakage depends on (1) the elasticity of the metal itself, (2) the taper of the clasp arm, and (3) the distance traversed by the arm (its length). Assuming that a clasp arm is elastic, if it is then accurately tapered, it will spring throughout its en tire length. The longer it is, the greater will be the arc through which it can safely move at its free retentive terminus. It follows, therefore, that the longer the tapered clasp arm, the better it will spring, the more often it can be safely flexed without becoming fatigued, and the less traumatizing it will be to abut ment teeth. It is regrettable that the cast circum ferential clasp (Fig. 2 ), most destructive to tooth surfaces and most abusive to peri
odontal structures as well as to alveolar tissue supports, is the one most widely used. The bar clasp (Fig. 3 -7 ), con versely, is generally much to be preferred from the standpoint of the patient’s wel fare because o f: 1. The longer clasp arms. 2. Stress absorbing spring action dur ing appliance shifts. 3. The ease with which clasp passivity can be effected to allow for physiologic stimulation through slight denture move ment in function. 4. Minimum tooth contact insuring the smallest caries incidence. 5. Usually better esthetics due to less display of metal. 6. The excellent partial denture fixa tion afforded. 7. The allowance by the open clasp arm of normal gum massage afforded by foods passing over them. Several bar clasp designs allow their elasticity to function in two different ways (Fig. 6 ). For example, the upright portion of a bar clasp L arm will spring or flex laterally on engaging an undercut while the horizontal part of the arm ly ing along the gum tissue will be undergo ing torsion bar springing. It would be unfair to leave the impression that bar clasps present no disadvantages. Such clasps require more metal; they are more difficult and time-consuming to fabricate, they are bulkier and much more con ducive to food entrapment than other clasps. They are also quite costly by com parison. Impression methods, mouth prepara tions, surveying and designing of partial dentures and so forth, are not within the scope of this paper. However, certain ad ditional clasp types which have practical value and which merit daily utilization in the fabricating of partial dentures will be discussed. The clasps shown and dis cussed up to this point and which are the ones mostly used today, are designs pro viding two elastic termini for the engage ment of retentive undercuts. They do not have a rigid reciprocal arm or surface for denture stabilization and for the purpose
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of gliding in contact with the tooth oppo site to the retentive arm while its terminus is being forced over a bulge into an un dercut and vice versa (Fig. 8 -1 5 ). If pro vided, such reciprocal provision certainly can do no harm.
movement of lips and food, and has only one point of contact with the tooth surface. It requires that the reciprocal arm go far around on the lingual aspect for over half way round tooth en gagement (to little broken circle)
However, is that rigid reciprocal factor as indispensable as many have claimed? Theory may have taken precedence over clinical results. No research has been re ported to prove the harm, if any, result ing when a reciprocal bar is not used along with clasp arms at divergent heights on a tooth (Fig. 8 ). If a clasped tooth were under constant tension, movement and trauma would be the inevitable sequel. But since clasps are adjusted to passivity, opposite arms work against one another only a brief moment on insertion or on removal of the appliance. During the time that the denture is in place, functional stresses utilize the clasp com ponents independently, depending on the direction of the force. Points of clasp con tact being at different heights should then make no measurable difference. Generally but not always, I prefer to use clasps with two elastic retentive ter mini, whenever feasible (Fig. 16, 17). Such retainers allow for double adjust ment, provide additional retention and, because both arms possess elasticity, are less harsh and traumatizing to abutment teeth. CON CLUSIO N
Fig. 6 • " L " and " C " clasp arms. The L terminus has only one point of contact with tooth surface. It is springy, allows easy cleaning, is not trau matic to abutment tooth and permits physiologic movement of the base. C arm lacks metal display and possesses mostly good qualities but must have a long reciprocal arm on lingual (to broken circle)
The use of clasp partial prostheses is a responsibility of the dental profession. If this phase were omitted from dental serv ices, dentists would not be fulfilling their obligations to the public. The problem cannot be dodged simply because it is difficult; also, this work cannot be dele gated to any other group. Therefore den tists must study and learn more about partial dentures since there is a definite need for this type of service and, because, in many instances, no other means of oc clusal reconstruction is nearly as effective as the clasp partial denture.
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Fig. 7 • Mesiodistal clasp has limited use, gener ally on upper cuspids. Clasp is excellent esthetically and is best employed when the clasped tooth has been restored with a three-quarter or veneer crown to provide proper contour, mesial arm clearance, and insurance against caries
Fig. 9 • Upper drawing shows same situation as in Figure 8. Lower drawing illustrates how un toward stress laterally on a tooth can be offset and nullified by an upright which will provide con stant reciprocal straight edge stabilization during either insertion or removal
Fig. 8 • Effect of having opposing clasp arms at different levels. On removal, right clasp termini would be exerting greatest pressure against con vexities after clasp arm reciprocation on the left had been lost
Fig. 10 • Another method of providing a non undercut reciprocating surface— with a threequarter crown or by full coverage
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Fig. I I • Embrasure saddle clasp4 arm. Vertical lingual strut provides stabilizing effect as the buccal clasp arm passes over distobuccal bulge
Fig. 12 • A distobuccal clasp5 arm engagement similar to Figure I I and with a vertical lingual strut having no occlusal stop. Clasps of type illus trated by Figures I I and 12 are kind to abutment teeth and allow for physiological stimulation of sub-basal tissues
Fig. 14 • Drawing shows clasping of teeth in se quence.7 This can only be done as on left or right when clasped teeth are in contact. Rigid recipro cation is provided at the mesial aspect of the first bicuspids
Fig. 13 • Lingual and buccal views of another type of clasp6 with rigid lingual arm and with only an elastic wrought component adapted to buccal undercut for retention Fig. 15 • Clasping teeth in sequence. Appliance is well stabilized and unsightly display of metal is avoided
STEFFEL.,. VOLUME ¿4, JU N E 1963 • 79/811
Fig. 16 • Ideal way to provide bilateral stabilization for unilateral extension by crowning and properly contouring a tooth for clasping on side opposite to free-end base. This eliminates bulk, prevents sub sequent caries and provides ideal clasp retention, each clasp having two retentive termini
Physiologic stimulation of teeth and ridge tissues is deemed advisable. Physi ologists agree on this premise. Traumatic stresses as well as dysfunction, on the other hand, are known to be harmful and destructive. Clasps, properly designed, constructed of an elastic material, and adjusted to passivity, permit a greater range of movement of the appliance dur ing the masticatory cycle than has the removable partial denture which obtains its fixation from slotted attachments. These slight advantageous shifts, how ever, will be limited and controlled by proper clasp adjustment. Accordingly, it is my opinion that a clasp with a rounded, but positive, oc clusal rest and elastic arms which are exerting no pressure at all (passive)
against the tooth when at rest will allow in all directions a moderate working movement of the base attached to it. Therefore the abutment tooth does not always have to “go along” with all the action of the saddle— principally the first part of any work motion. This is most desirable since the mucous membrane over the ridges is capable of more displaceability than are the periodontal structures. This controlled, but permitted, movement of a removable partial den ture during the chewing cycle and at other times (up and down, laterally, anteroposteriorly, and rotary) as is possible with ideal clasping, distributes the masti catory loads to teeth and other supporting tissues without trauma to any specific re gion and effects an ideal physiologic stim ulation leading to robust health for all the structures involved.
19 West Fifth Avenue
Fig. 17 • Embrasure double clasp is not at all ideal or necessary, is bulky to the tongue and highly conducive to caries (see Figure 16)
*Professor and chairman, division of rem ovable partial prosthodontics, Co llege of Dentistry, O hio State Univer sity, Columbus, Ohio. 1. Steffel, V. L. Fundamental principles involved in partial denture design. J.A .D .A . 42:534 M ay 1951. 2. Steffel, V. L. Relining rem ovable partial dentures for fit and function. J . Pros. Den. 4:496 Ju ly 1954. 3. Elliott, F. C. Removable partial denture prosthesis. J.A .D .A . 40:676 June 1950. 4. DeVan, M. M. Preserving natural teeth through the use of clasps. J . Pros. Den. 5:208 M arch 1955. 5. G irado t, R. L. History and developm ent of partial denture design. J.A .D .A . 28:1399 Sept. 1941. 6. A ppleg ate, O . C., and Nissle, R. O . Keeping the partial denture in harmony with biologic limitations. J.A .D .A . 43:409 O ct. 1951. 7. Steffel, V. L. Planning rem ovable partial dentures. J . Pros. Den. 12:524 May-June 1962.