Journal of Dentistry, 7, No. 1,1979, pp. 52-56.
Printed
in Great Britain
The use of crowns to modify abutment teeth of removable partial dentures 1. Introduction
and partial denture design
B. J. Smith, BDS, MSc, FDS RCS* C. H. Turner, BDS, FDS Rest The Den tat School, The University of Newcastle upon Tyne
ABSTRACT Major modifications of abutment teeth can only be achieved by the use of restorations. Full crowns enable these modifications to be made in conjunction with ideal design and location of the components of removable partial dentures. Part 1 of this paper discusses these relevant components while part 2 will describe the clinical and laboratory procedures.
INTRODUCTION When natural teeth are selected as abutments for removable partial dentures, it is often desirable to modify their axial and occlusal contours. Minor modifications to natural tooth contour can be made by simple. grinding, but these may be limited if the enamel is thin in the areas requiring modification; certainly the enamel should not be completely removed. Indeed, Darling (1959) has stated that any damage to the surface zone of the enamel will render that surface very susceptible to caries. On the other hand, Axinn (1975) considered that simple recontouring of natural teeth was acceptable, provided that the modified area had smooth contours and was treated afterwards with topical flouride. Further, he recommended that clinicahy there should be no evidence of mechanical preparation, and the retentive area should be obvious only when the master cast is placed on a surveyor. Jenkins and Berry (1976) have suggested that an undercut can be created for a minimally or unrestored tooth by the addition of acidetched composite resin to the buccal surface of the tooth. In contrast, major modifications to tooth contour can only be achieved by the use of restorations. When such restorations are already part of the overall treatment plan of the patient, it is practical to coordinate the design of the restorations with that of the proposed partial denture. Moreover, when the contours of the abutment teeth are under the control of the clinician, a partial denture can be designed without compromise. The severest modifications to tooth contour generally require the provision of crowns, and the indications for these may be summarized as follows (after Chandler et al., 1973): 1. For potential abutment teeth that are grossly carious, broken down or heavily restored. 2. When the natural crowns of the potential abutment teeth cannot be altered suffciently by grinding to provide: a. A desired path of insertion and withdrawal.
* Present address: Guy’s Hospital Dental School, London. t Present address: University College Dental School, Mortimer Market, London.
Smith and Turner: Crowns to modify abutment teeth
53
b. Suitable retentive undercuts. c. Suitable guide planes and reciprocal element areas. d. Occlusal rest seats. e. Satisfactory occlusion. 3. When it is desirable to splint abutment teeth. Although a change of proximal contour alone, for instance, can be achieved by use of an inlay, when several of the changes given above need to be made, a full or three-quarter crown is indicated. Chandler et al. (1973) recommended the use of partial veneer crowns, whereas Henderson and Steffel (1973) preferred full coverage crowns. In fact, the latter are technically easier to construct and should certainly be used when a buccal restoration is already present or when a buccal undercut is required. The two parts of this paper will discuss the use of crowns to modify abutment tooth contour as part of the overall treatment plan of a patient requiring clasp-retained removable partial dentures. The authors have excluded precision attachments because they have been described in detail elsewhere (Preiskel, 1973), and the modifications and equipment needed may be complicated and not within the scope of general dental practice. PARTIAL DENTURE DESIGN Tooth-supported partial dentures A partial denture can only be totally tooth-supported when abutment teeth are present at both ends of any edentulous spaces. Then, each denture base may be supported by rigid occlusal rests located at the side of the tooth adjacent to the denture base. This location is desirable to reduce the flexibility of the base and to avoid food impaction between the abutment tooth and the denture base. Only where there is sufficient space between the occlusal surface of an abutment tooth and that of an opposing tooth can an occlusal rest be placed without increasing the vertical separation of the jaws in occlusion. This space is rarely present naturally and must be created by the development of a rest seat in the tooth or in a restoration. Normally, the width of the rest should be at least half the distance between the buccal and lingual cusps. Its length should be the same as the width. On the other hand, when an occlusal contact is deficient, an occlusal rest can serve the additional function of restoring an occlusal contact. Direct retention of the denture by clasps is only possible where the heights of contour of the selected surfaces of the abutment teeth are sufficiently far away from the gingival margins for measured amounts of horizontal undercut to be present for a given path of insertion. These amounts are usually 0.25 mm for cobalt-chromium alloys and 0.5-0.75 mm for gold alloys. Every retentive clasp arm must be reciprocated either with a reciprocal clasp arm or with a portion of the major or minor connector. Reciprocation is necessary to avoid destructive intermittent forces being applied to the clasped tooth when the denture is inserted or removed from the mouth or when the denture moves in the mouth during mastication. The reciprocal component should be in contact with the tooth and be at the same level as the clasp when the clasp first touches the tooth on inserting the denture (Fig. 1, position A), when the clasp is elastically deformed maximally at the height of the contour (Fig. 1, position B) and when the clasp is situated passively in the undercut area (Fig. I, position c). These requirements are present in reverse when the denture is removed from the mouth.
54
Journal of Dentistry, Vol. ~/NO. 1
Fig. 1. Path of insertion of a retentive portion of a clasp arm showing when the clasp first touches the tooth (A), when the clasp is deformed maximally at the height of the contour (3) and when the clasp is situated passively in the undercut (Cl.
Fig. 2. A retentive clasp arm is reciprocated effectively by a reciprocal component when a guide plane is presant. The length of the guide plane must be at least equal in length to the path of insertion of the clasp.
Fig. 3. When a guide plane is absent, reciprocation may be by a reciprocal component of a depth (X) equal to or greater than the path of insertion of the clasp (VI.
Unless the reciprocal component is actually another clasp passing over an identical tooth contour opposite to the retentive clasp, the only tooth contour that will enable a reciprocal component to work effectively is a straight plane parallel to the path of insertion (Fig. 2). This plane must be of sufficient length to ensure effective reciprocation at the times illustrated in Fig 1. This plane will also act as a guide plane. Since most teeth do not have natural guide planes, they usually have to be created. When the abutment teeth are to be crowned, maximum opportunity exists for the creation of guide planes in the restorations. Guide planes are also valuable proximally in defining a single path of insertion (which, incidentally,
Smith and Turner:
Crowns to modify
abutment
teeth
55
improves retention) and for eliminating potential food traps between a denture base and an abutment tooth. Guide planes should be straight occlusogingivally but curved buccolingually and mesiodistally. Proximally, they should extend to within 1 mm of the gingival margin to prevent a stagnation area. In the absence of a guide plane the reciprocal component itself must be of a depth equal to or greater than the total path of insertion of a clasp (Fig. 3). This depth can be provided by a lingual plate or minor connector. Then there will be a line contact between the tooth and the reciprocal component. In this situation the height of contour must of necessity be near the occlusal surface of the tooth to allow for sufficient depth of the reciprocal component and, at the same time, to avoid a space between the tooth and the reciprocal component occlusally. However, this system does not satisfy one criterion of reciprocation since the points of contact of the clasp and the reciprocal component are not at the same level. Theoretically, rotational forces could be applied to the tooth during placement and removal, and with movement of the denture during function. A retentive clasp arm does not have to be located on the buccal surface of an abutment tooth and consequently the reciprocal component on the lingual surface. However, buccal surfaces are generally more suitable for retention because of the difficulty of providing effective reciprocation when the lingual surface is used for retention. A wide reciprocal component such as a lingual plate can only be used on the lingual side of a tooth.
Tooth- and tissue-supported partial dentures When an abutment tooth is present at only one end of an edentulous area, the partial denture base cannot be solely tooth-supported but is at best tooth- and tissue-supported. A comparable situation to that found with the distal edentulous area occurs with a long anterior edentulous area even though abutment teeth are present at both ends. The tooth support of denture bases restoring this type of edentulous area is provided by a rigid occlusal rest located ideally on the side of the tooth away from the edentulous area (Kratochvil, 1963). In this location, displacement of the denture base towards the tissues will cause rotation of the base around the part of the occlusal rest nearest the centre of the tooth. Occlusal forces are then directed down the long axis of the abutment tooth, and any rotation of the tooth will tend to close the contact area (Demer, 1976). In addition, an occlusal rest in this position tends to act as an indirect retainer when the denture base is displaced away from the tissues. It does this by virtue of its position on the opposite side of the fulcrum when the denture base rotates around the tip of the retentive clasp. The tip of the retentive clasp associated with the base should lie on the surface of the tooth adjacent to the denture base so that the tip disengages from the tooth as the denture base is displaced towards the tissues during mastication. This surface may be reached by a bar clasp or a circumferential clasp. The action of the mesial rest as an indirect retainer is enhanced when the tip of the clasp is adjacent to the denture base. The comments made earlier in this paper concening guide planes and reciprocation also apply to the tooth- and tissue-supported partial denture. However, the proximal guide plane should be no longer than 2-3 mm occlusogingivally (Krol, 1973). Sufficient space must be available for the proximal plate to rotate into when the denture base is displaced towards the tissues.
56
Journal of Dentistry, Vol. ~/NO. 1
CONCLUSIONS The three components of a partial denture that are particularly relevant to full crown construction are occlusal rests, retentive clasp arms and reciprocal components. The location of these components for tooth-supported and tooth- and tissue-supported partial dentures have been discussed and the importance of guide planes emphasized. Part 2 of this paper will describe the clinical and laboratory procedures involved in the production of the crowns in coordination with the partial dentures. Acknowledgements We are grateful to Professor Roy Storer and Professor D. N. Allan for helpful comments, and to the Department of Photography of the Medical School of the University of Newcastle upon Tyne for the illustrations. REFERENCES
Axinn
S. (1975)
Preparation
of retentive
areas for clasps in enamel. J. Prosthet. Dent. 34,
405-407.
Chandler
H. T., Brudvik J. S. and Fisher W. T. (1973) Surveyed
crowns. J. Prosthet. Dent.
30,775-780.
Darling A. I. (1959) The pathology and prevention of caries. Br. Dent. J. 107, 287-296. Demer W. J. (1976) An analysis of mesial rest-I-bar clasp designs. J. Prosthet. Dent. 36, 243-253.
Henderson D. and Steffel V. L. (1973) McCracken’s Removable Partial Prosthodontics, 4th ed. St Louis, Mosby, p. 219. Jenkins C. B. and Berry D. C. (1976) Modification of tooth contour by acid-etch retained resins for prosthetic purposes. Br. Dent. J. 141, 89-90. Kratochvil F. J. (1963) Influence of occlusal rest position and clasp designs on movement of abutment teeth. J. Prosthet. Dent. 13, 114-123. Krol A. J. (1973) Clasp design for extension-base removable partial dentures. J. Prosthet. Dent. 29,408-415.
Preiskel H. W. (1973) Precision Attachments
in Dentistry,
2nd ed. London,
Kimpton.
Printed
in Great Britain
The use of crowns to modify abutment teeth of removable partial dentures 1. Introduction
and partial denture design
B. J. Smith, BDS, MSc, FDS RCS* C. H. Turner, BDS, FDS Rest The Den tat School, The University of Newcastle upon Tyne
ABSTRACT Major modifications of abutment teeth can only be achieved by the use of restorations. Full crowns enable these modifications to be made in conjunction with ideal design and location of the components of removable partial dentures. Part 1 of this paper discusses these relevant components while part 2 will describe the clinical and laboratory procedures.
INTRODUCTION When natural teeth are selected as abutments for removable partial dentures, it is often desirable to modify their axial and occlusal contours. Minor modifications to natural tooth contour can be made by simple. grinding, but these may be limited if the enamel is thin in the areas requiring modification; certainly the enamel should not be completely removed. Indeed, Darling (1959) has stated that any damage to the surface zone of the enamel will render that surface very susceptible to caries. On the other hand, Axinn (1975) considered that simple recontouring of natural teeth was acceptable, provided that the modified area had smooth contours and was treated afterwards with topical flouride. Further, he recommended that clinicahy there should be no evidence of mechanical preparation, and the retentive area should be obvious only when the master cast is placed on a surveyor. Jenkins and Berry (1976) have suggested that an undercut can be created for a minimally or unrestored tooth by the addition of acidetched composite resin to the buccal surface of the tooth. In contrast, major modifications to tooth contour can only be achieved by the use of restorations. When such restorations are already part of the overall treatment plan of the patient, it is practical to coordinate the design of the restorations with that of the proposed partial denture. Moreover, when the contours of the abutment teeth are under the control of the clinician, a partial denture can be designed without compromise. The severest modifications to tooth contour generally require the provision of crowns, and the indications for these may be summarized as follows (after Chandler et al., 1973): 1. For potential abutment teeth that are grossly carious, broken down or heavily restored. 2. When the natural crowns of the potential abutment teeth cannot be altered suffciently by grinding to provide: a. A desired path of insertion and withdrawal.
* Present address: Guy’s Hospital Dental School, London. t Present address: University College Dental School, Mortimer Market, London.
Smith and Turner: Crowns to modify abutment teeth
53
b. Suitable retentive undercuts. c. Suitable guide planes and reciprocal element areas. d. Occlusal rest seats. e. Satisfactory occlusion. 3. When it is desirable to splint abutment teeth. Although a change of proximal contour alone, for instance, can be achieved by use of an inlay, when several of the changes given above need to be made, a full or three-quarter crown is indicated. Chandler et al. (1973) recommended the use of partial veneer crowns, whereas Henderson and Steffel (1973) preferred full coverage crowns. In fact, the latter are technically easier to construct and should certainly be used when a buccal restoration is already present or when a buccal undercut is required. The two parts of this paper will discuss the use of crowns to modify abutment tooth contour as part of the overall treatment plan of a patient requiring clasp-retained removable partial dentures. The authors have excluded precision attachments because they have been described in detail elsewhere (Preiskel, 1973), and the modifications and equipment needed may be complicated and not within the scope of general dental practice. PARTIAL DENTURE DESIGN Tooth-supported partial dentures A partial denture can only be totally tooth-supported when abutment teeth are present at both ends of any edentulous spaces. Then, each denture base may be supported by rigid occlusal rests located at the side of the tooth adjacent to the denture base. This location is desirable to reduce the flexibility of the base and to avoid food impaction between the abutment tooth and the denture base. Only where there is sufficient space between the occlusal surface of an abutment tooth and that of an opposing tooth can an occlusal rest be placed without increasing the vertical separation of the jaws in occlusion. This space is rarely present naturally and must be created by the development of a rest seat in the tooth or in a restoration. Normally, the width of the rest should be at least half the distance between the buccal and lingual cusps. Its length should be the same as the width. On the other hand, when an occlusal contact is deficient, an occlusal rest can serve the additional function of restoring an occlusal contact. Direct retention of the denture by clasps is only possible where the heights of contour of the selected surfaces of the abutment teeth are sufficiently far away from the gingival margins for measured amounts of horizontal undercut to be present for a given path of insertion. These amounts are usually 0.25 mm for cobalt-chromium alloys and 0.5-0.75 mm for gold alloys. Every retentive clasp arm must be reciprocated either with a reciprocal clasp arm or with a portion of the major or minor connector. Reciprocation is necessary to avoid destructive intermittent forces being applied to the clasped tooth when the denture is inserted or removed from the mouth or when the denture moves in the mouth during mastication. The reciprocal component should be in contact with the tooth and be at the same level as the clasp when the clasp first touches the tooth on inserting the denture (Fig. 1, position A), when the clasp is elastically deformed maximally at the height of the contour (Fig. 1, position B) and when the clasp is situated passively in the undercut area (Fig. I, position c). These requirements are present in reverse when the denture is removed from the mouth.
54
Journal of Dentistry, Vol. ~/NO. 1
Fig. 1. Path of insertion of a retentive portion of a clasp arm showing when the clasp first touches the tooth (A), when the clasp is deformed maximally at the height of the contour (3) and when the clasp is situated passively in the undercut (Cl.
Fig. 2. A retentive clasp arm is reciprocated effectively by a reciprocal component when a guide plane is presant. The length of the guide plane must be at least equal in length to the path of insertion of the clasp.
Fig. 3. When a guide plane is absent, reciprocation may be by a reciprocal component of a depth (X) equal to or greater than the path of insertion of the clasp (VI.
Unless the reciprocal component is actually another clasp passing over an identical tooth contour opposite to the retentive clasp, the only tooth contour that will enable a reciprocal component to work effectively is a straight plane parallel to the path of insertion (Fig. 2). This plane must be of sufficient length to ensure effective reciprocation at the times illustrated in Fig 1. This plane will also act as a guide plane. Since most teeth do not have natural guide planes, they usually have to be created. When the abutment teeth are to be crowned, maximum opportunity exists for the creation of guide planes in the restorations. Guide planes are also valuable proximally in defining a single path of insertion (which, incidentally,
Smith and Turner:
Crowns to modify
abutment
teeth
55
improves retention) and for eliminating potential food traps between a denture base and an abutment tooth. Guide planes should be straight occlusogingivally but curved buccolingually and mesiodistally. Proximally, they should extend to within 1 mm of the gingival margin to prevent a stagnation area. In the absence of a guide plane the reciprocal component itself must be of a depth equal to or greater than the total path of insertion of a clasp (Fig. 3). This depth can be provided by a lingual plate or minor connector. Then there will be a line contact between the tooth and the reciprocal component. In this situation the height of contour must of necessity be near the occlusal surface of the tooth to allow for sufficient depth of the reciprocal component and, at the same time, to avoid a space between the tooth and the reciprocal component occlusally. However, this system does not satisfy one criterion of reciprocation since the points of contact of the clasp and the reciprocal component are not at the same level. Theoretically, rotational forces could be applied to the tooth during placement and removal, and with movement of the denture during function. A retentive clasp arm does not have to be located on the buccal surface of an abutment tooth and consequently the reciprocal component on the lingual surface. However, buccal surfaces are generally more suitable for retention because of the difficulty of providing effective reciprocation when the lingual surface is used for retention. A wide reciprocal component such as a lingual plate can only be used on the lingual side of a tooth.
Tooth- and tissue-supported partial dentures When an abutment tooth is present at only one end of an edentulous area, the partial denture base cannot be solely tooth-supported but is at best tooth- and tissue-supported. A comparable situation to that found with the distal edentulous area occurs with a long anterior edentulous area even though abutment teeth are present at both ends. The tooth support of denture bases restoring this type of edentulous area is provided by a rigid occlusal rest located ideally on the side of the tooth away from the edentulous area (Kratochvil, 1963). In this location, displacement of the denture base towards the tissues will cause rotation of the base around the part of the occlusal rest nearest the centre of the tooth. Occlusal forces are then directed down the long axis of the abutment tooth, and any rotation of the tooth will tend to close the contact area (Demer, 1976). In addition, an occlusal rest in this position tends to act as an indirect retainer when the denture base is displaced away from the tissues. It does this by virtue of its position on the opposite side of the fulcrum when the denture base rotates around the tip of the retentive clasp. The tip of the retentive clasp associated with the base should lie on the surface of the tooth adjacent to the denture base so that the tip disengages from the tooth as the denture base is displaced towards the tissues during mastication. This surface may be reached by a bar clasp or a circumferential clasp. The action of the mesial rest as an indirect retainer is enhanced when the tip of the clasp is adjacent to the denture base. The comments made earlier in this paper concening guide planes and reciprocation also apply to the tooth- and tissue-supported partial denture. However, the proximal guide plane should be no longer than 2-3 mm occlusogingivally (Krol, 1973). Sufficient space must be available for the proximal plate to rotate into when the denture base is displaced towards the tissues.
56
Journal of Dentistry, Vol. ~/NO. 1
CONCLUSIONS The three components of a partial denture that are particularly relevant to full crown construction are occlusal rests, retentive clasp arms and reciprocal components. The location of these components for tooth-supported and tooth- and tissue-supported partial dentures have been discussed and the importance of guide planes emphasized. Part 2 of this paper will describe the clinical and laboratory procedures involved in the production of the crowns in coordination with the partial dentures. Acknowledgements We are grateful to Professor Roy Storer and Professor D. N. Allan for helpful comments, and to the Department of Photography of the Medical School of the University of Newcastle upon Tyne for the illustrations. REFERENCES
Axinn
S. (1975)
Preparation
of retentive
areas for clasps in enamel. J. Prosthet. Dent. 34,
405-407.
Chandler
H. T., Brudvik J. S. and Fisher W. T. (1973) Surveyed
crowns. J. Prosthet. Dent.
30,775-780.
Darling A. I. (1959) The pathology and prevention of caries. Br. Dent. J. 107, 287-296. Demer W. J. (1976) An analysis of mesial rest-I-bar clasp designs. J. Prosthet. Dent. 36, 243-253.
Henderson D. and Steffel V. L. (1973) McCracken’s Removable Partial Prosthodontics, 4th ed. St Louis, Mosby, p. 219. Jenkins C. B. and Berry D. C. (1976) Modification of tooth contour by acid-etch retained resins for prosthetic purposes. Br. Dent. J. 141, 89-90. Kratochvil F. J. (1963) Influence of occlusal rest position and clasp designs on movement of abutment teeth. J. Prosthet. Dent. 13, 114-123. Krol A. J. (1973) Clasp design for extension-base removable partial dentures. J. Prosthet. Dent. 29,408-415.
Preiskel H. W. (1973) Precision Attachments
in Dentistry,
2nd ed. London,
Kimpton.
