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Gingival crown margin configurations: A review and discussion. Part I: Terminology and widths
Gingival crown margin configurations: A review discussion. Part I: Terminology and widths A. J. Hunter, B.D.S., M.D.&., Dunedin, New Zealand
D.R.D.R.C.S.,
and A. R. Hunter,
and
B.D.S., M.D.S.
The terms bevel, chamfer, and shoulder are widely used to describe crown margin designs. However, as no clear definition of the essential feature(s) of each design has been universally accepted, the same term often describes margins of widely differing width andlor configuration. Similarly “bevel angles” are not consistently defined. While tradition favors the use of thinner marginal designs, many of the reasons advanced for their superiority are questionable in the light of contemporary research. Use of marginal widths beyond the absolute minimum demanded of the material may contribute to overcoming some of the persistent problems identified with fixed prosthodontic replacement of natural teeth. These include overcontour, porcelain debonds, poor esthetics, and fit. It is suggested that the problems associated with underpreparation and the potential advantages of wider preparations need reemphasis. (J PROSTHET DENT 1990;64:548-52.)
T
00th preparation for fixed prosthodontics requires a decision regarding the marginal configuration. The design dictates the shape and bulk of the casting and influences the fit at the margin.l Although many factors such as materials, esthetics, and access influence this selection, most dentists probably have a “preferred” design, However, there is disagreement about what constitutes ideal marginal geometry and width. These articles review the issues involved and discuss the implications of research into the clinical longevity of crowns related to the marginal design. Part I discusses terminology and margin width, while Part II reviews the theoretical and practical marginal geometry.
TERMINOLOGY
AND DEFINITIONS
At present there is no universally accepted basis for classifying margin designs, and many texts avoid verbal definitions in favor of illustrations.1-6 Unfortunately, this approach does not clearly distinguish the “essential” feature(s) of each design and therefore illustrations of chamfers or shoulders are often misleading (Fig. I). Chamfers have been defined on the basis of their marginal width and/or geometry. Jacobsen and Robinson7 stated that a width greater than 0.3 mm at its cervical termination precluded any margin being called a chamfer. Others’? 6 g consider an obtuse angled gingival termination or curved cross-sectional shape to be fundamental for chamfers. Bell et al.1° described a chamfer preparation with a reduction of 1.5 mm, rounded internal line angles, and a cavosurface angle of 135 degrees. However, their shoulder preparations were identical except for cavosurface angle, and this feature was crucial in classifying marginal designs. However, illustrated examples demonstrating rounded in-
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ternal line angles and cavosurface junctions of approximately 90 degrees have also been termed chamfersir, r2 Similar problems arise with attempts to define. shoulders. Many classify a flat shoulder as one that makes a 90degree angle with the vertical axial wall of the preparation.2l 8,g,I3 This classification presumably relies on width to determine the marginal type, while the angulation from the vertical axial wall indicates marginal direction (for example, 135-degree shoulder). With any classification based on width, substantial differences arise with changes in marginal angulation. Some shoulders may exhibit slip joint geometry (135-degree shoulders) but others may not (go-degree shoulders). Shoulders have also been defined using marginal geometry, where the discriminating features are an external cavosurface angle of 90 degrees and a corresponding butt joint of restoration/tooth at the margin.‘? 3l5,lo Depending on the apical extent of the preparation, and the axial profile of the tooth, a go-degree shoulder using this classification may often appear similar to a 135-degree shoulder as defined by McLean and Wilson.i3 Bevels are another recommended marginal design because they incorporate geometric principles to minimize marginal discrepencies. l, 12,14,I5 It is important to realize that bevel angulation may be assessed from different planes (Fig. 2). Some authorities define the angle of the bevel as the angle between the created bevel surface and the preparation surface that has been bevelled,13*I4916-18while others describe it as the angle between the created bevel surface and the external surface of the tooth.3, lo*igs2i The former definition of bevel angulation will be used throughout these articles. For convenience, Kuwata22 classified margins on the basis of their “margin angle.” This is the angle formed between a vertical projection from the external surface of the tooth and its prepared surface (Fig. 2, angle b). He
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Fig. 1. Margin illustrations. a, Long chamfer. b and c, Chamfers; note differing exit profiles. d, Flat or go-degree shoulder, based on the preparation axial wall. e, Sloped shoulder (on axial wall) or go-degree shoulder, based on cavosurface angle. considered that margin angles between 0 and 30 degrees gave bevelled margins, those between 31 and 60 degrees were chamfers, and those between 61 and 90 degrees were shoulders. A practical system for classifying margins was advanced by Pardo, l2 who broadly categorized margins into the two main classes of inclined vertical or horizontal. A go-degree shoulder was considered a horizontal margin and the 135degree shoulder was considered an inclined vertical design. Although this classification emphasized the importance of exit configuration while ignoring margin width, in practice the horizontal designs are often wider. Another source of confusion is the different terminology for describing discrepancies between the casting and the tooth. These discrepancies may be the result of inaccuracies inherent in crown fabrication; an inability to correctly seat the crown during cementation, or a combination of these factors. These discrepancies are commonly measured at the external margin of the crown or at right angles to the two opposing surfaces (Fig. 3). The former has been described as the “vertical discrepancy,“14 the “marginal opening measured verticaIly,“23 or the “minimal marginal cement thickness.“16 The discrepancy at right angles to the two opposing surfaces has been referred to as the “marginal opening, “15t24 the “horizontal discrepancy,“14 or the “marginal adaptation.“23 The least confusing terminology is the use of the term “seating” to describe the vertical discrepancy at the external margin, and the term “sealing” to describe the horizontal discrepancy observed at right angles to the two opposing surfaces.11s21
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Fig. 2. Bevel angles. Angle a is the angle formed by the created bevel surface with the surface that has been bevelled. Angle b is the angle formed by the created bevel surface with the external surface, also called margin angle. 180 degrees -a = b.
MARGINAL
WIDTH
For simplicity, concepts related to marginal width and geometry can usually be discussed independently. The minimum marginal width of a crown is largely determined by the dental materials used, while the maximum width is limited by the desire to preserve both a suitable coronal core and pulp vitality. Within these general parameters, marginal widths may vary, with designs exhibiting minimal axial reduction favored when possible.25 These include regular chamfers and knife edges (including feather and chisel edges) that would seldom be wider then 0.3 mm.
Marginal
widths
below 0.3 mm
The cited advantages of thin margins are that these designs allow for intraoral finishing of the crown margins, are conservative, and their geometric configuration improves the marginal seal.1,6-8 Because minimal reduction is required, preparation of knife-edge margins has been considered to.be relatively easy.s,s,26However, others consider that achieving a definite finish line with a knife-edge configuration is also difficult and requires careful preparation.1,4 Crown fabrication is also a problem, as these margins are often indistinct on the impression and die.4, 7 In addition, overcontoured restorations are commonly produced to obtain sufficient bulk of restorative material to ensure functional rigidity. l, 7 Although sometimes recommended for routine use,26knife-edges are not a preferred design72l5 and should be restricted to specific clinical sit549
or with an explorer, with the efficacy of the examination becoming increasingly questionable the more subgingival the margin. Even if objective evaluation were possible, limited access for instruments precludes marginal finishing.5s 13,23,2gl33Where accesspermits, disks and stones can be effective in achieving marginal closure, but burnishers appear ineffective in reducing discrepancies.34g35 Although margin finishing of complete cast crowns during cementation is often recommended,20*2s,2gv33 the time available is limited as these procedures must be completed while the cement remains fluid.13, 33 The limited ductility and high yield strength of many alloys also limits the effectiveness of finishing procedures. Although Gilboe and Thayer 27accepted that type 3 gold possessedlimited ductility, they maintained that thin cervical margins did allow for bending of the metal to improve the marginal seal. Conversely, Behrend23 considered that burnishing of margins was impossible with the current hard alloys. Gardiner15 also concluded that burnishing was not a practical way of achieving marginal closure for type 3 gold and ceramometal alloys. Others2g*36also consider ceramometal alloys unsuitable for burnishing, while McLean and Wilson13 noted that the pressures and distortion of metal, associated with the technique, could crack porcelain bonded to these alloys. Alloys with Brine11 hardness numbers greater than 84 or 100 are unsuitable for burnishing.5s l5 Fig. 3. Diagram illustrating discrepancies between castings and teeth. a, Seating discrepancy. 6, Sealing discrepancy.
uations such as their use in extremely bell-shaped teeth.‘, 2,8,27 Greater axial reduction (up to 0.3 mm7) is required for chamfers that many consider the “ideal ” gingival finish line for cast restorations.l* 4,6s~25This increased width allows easier evaluation of the margin at all stages and reduces the chances of an overcontoured casting, while allowing adequate rigidity. While possessing these practical advantages, chamfers retain the benefits of the thinner knife-edge design. Many consider that thin margins (including bevels) are desirable because they allow for marginal closure through finishing procedures.‘? 5-7*20*27-2gThese procedures include spinning and swaging, the use of disks and stones to move metal to the margins, and burnishing to bend the crown margin toward the tooth. Stiffler in 1980 reported a 50% reduction in the marginal opening if accessible, and where the type of alloy permitted the use of rotary instruments for burnishing (cited by Scharer2g). However, others consider intraoral finishing procedures for gingival margins difficult and impractica16* 15,21,23The dentist’s ability to objectively evaluate the cervical margins of crowns appears to be limited, particularly in approximal and subgingival locations.28p30-32Assif et a1.32believed that there was a direct relationship between the location of restorative margins and the ability to evaluate them by sight 550
Marginal
widths
greater
than 0.3 mm
Margin widths greater than 0.3 mm, often referred to as shoulders, have been unpopular, particularly if a chamfer could be placed. Tylman and Malonesstated that shoulders were difficult to prepare on posterior teeth, and that their routine use was difficult to justify when stress analysis, microleakage, and pulpal response were considered. Others agreed, and considered shoulders unnecessarily destructive,‘, ’ bulky,6 and unsuitable for marginal finishing unless modified with a beve1.l. 4*7s33134 The thickened wax patterns resulting from wider margins also exhibit increased casting shrinkage, although this can be compensated for by effective die spacing.3’ Despite their unpopularity, the advantages of wider margins over thinner designs should be considered. These advantages include improved control of crown contours and esthetics, increased structural rigidity, and clearer impressions and dies. According to Gilboe and Thayer,27 an advantage of bevelled shoulder preparations were that they allowed the incorporation of physiologic contours in both the temporary and final crown. Increased tooth reduction also facilitates contour changes in the crown, permitting guide planes for removable prostheses, or flattening contours to facilitate plaque control.8, 27 Some believe that with good oral hygiene, minor changes in crown contour ( + 1 mm) do not significantly affect gingival health.%, 3gHowever, increases in crown contour usually compromise gingival health by impeding plaque removal29 ‘921,23,4o-42Fankhauser, cited by Scharer,2g in NOVEMBER
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1979 reported that minor changes in contour led to increased gingival bleeding and that many ceramometal crowns were overcontoured by a mean value of 0.5 mm. Overcontoured crowns are a common problem,41 particularly approximallf13, 44 and with ceramometal restorations.2 Karlsso# discovered that a “significant” number of ceramometal restorations were overcontoured approximally, and others have reported that 25% to 80% of complete cast crowns are overcontoured.45-47 Parkinson47 reported that although cast crowns were less overcontoured than ceramometal restorations, differences in plaque scores were not significant, both being significantly higher than in the uncrowned controls. These studies indicated that the problem of overcontour was not confined to ceramometal restorations and that small increases in contour can adversely influence gingival health. Grass0 et a1.46suggested that unsuitable embrasures and contours were a greater problem with crowns than were inadequate gingival cavosurface margins. Another advantage of greater axial reduction is that it permits a greater thickness of restorative material, thus ensuring rigidity during function.27 Crown flexure predisposes to cement failure, and with ceramometal crowns there is the additional hazard of porcelain failure.48, 4gAlthough Karlsson44 and Coornaert et a1.4greported comparatively low rates of porcelain fracture, Walton et a1.50 reported that porcelain fractures and “debonds” were the second highest cause of failure with ceramometal crowns. The rate of porcelain failure due to flexure appears to be related to the thickness of the metal substructure and to occlusal factors, particularly bruxism.44, 48*4g,51 Distortion of ceramometal substructures can occur during crown fabrication.2p 13x*a 51-54This distortion has been attributed to temperature-related creep of the alloy53v55,56 or to a thermal expansion mismatch between porcelain and alloy. 13*52Whatever the exact cause, this distortion is a common problem. Karlsson44 found that although ceramometal and acrylic resin veneered crowns had a similar “frequency” of poor marginal adaptation, the size of the discrepancy was significantly greater with ceramometal crowns. He attributed this finding to weak metal substructures that were distorted during the fabrication process. Many consider that increasing the bulk of metal, particularly at the internal line angles, increases resistance to marginal distortion.15, 31,52,53 Thus increased marginal widths (shoulders) have usually been recommended as a method of minimizing these discrepancies51, 5s The desire to enhance esthetics without sacrificing strength is a crucial consideration when using ceramometal restorations, but a high proportion of ceramometal restorations are judged unacceptable by patients due to poor esthetics.50 Good marginal integrity, suitable margin location, and harmonious contour are all important in achieving optimal esthetics.23s43,48,57,58 Overcontoured crowns are unlikely to blend with natural counterparts due to their poor shape and adverse effect on gingival tissues.ss*43High reflectance of the opaque layer, particularly at the margins, THE
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is another common esthetic problem with ceramometal restorations’s All these problems can be minimized by providing “adequate” marginal width to accommodate both metal and porcelain, without overcontouring. Unfortunately, there are variations in opinion regarding the margin reduction required for ceramometal restorations, with suggestions ranging from 1 to 2 mm. ‘32,5,4s+4gBecause optimal esthetics depend on adequate reduction, it may be desirable to increase the marginal width of preparations toward the upper limit of the recommended range. The effect of increased margin widths on the ease of the clinical procedures is disputed. Some consider flat or sloped shoulders difficult to prepares l8 or impractical with small teeth or when excessive root surfaces are exposed.5g According to Gilboe and Thayer,27 the increased width associated with bevelled shoulders facilitates the placement of gingival retraction cord, while the increased bulk of impression material at the gingival margin results in more accurate impressions and dies. Increased bulk is useful where the impression material possesseslimited tear strength. In contrast, Schweikert2’j and Panno et a1.5g stated that shoulder preparations made impression making difficult, especially for multiple preparations. Perhaps the greatest effect of increased margin widths clinically is the ease of evaluating preparations, impressions, and dies. Shoulders are commonly well defined’ and according to some create the clearest finish line.g Picard reiterated that a poor impression, even of a good preparation, will produce a poor die and ultimately lead to a poor final result. The ability to accurately evaluate margins, particularly at the impression stage is crucial to ensuring an acceptable crown and is a major advantage of wider preparations. CONCLUSION Tradition has favored thin marginal designs, with the consequence that there is a tendency to “underprepare” teeth. Inadequate preparation is cited as the reason for many of the problems associated with crowns.2p21*23,2g,43,44 The consequences of underpreparation are usually worse than those associated with preparations that are wider than the absolute minimum required by the restorative material. Thin finish lines have been advocated in the belief that they allow marginal closure through intraoral finishing and contribute to the maintenance of pulpal vitality. However, a review of the literature indicates that the efficacy of intraoral finishing procedures should be reassessed. The ability of dental technicians to produce anatomic crown contours while working to the minimum possible thickness of material has also been overestimated. If maintaining a normal emergence profile is important, then wider margins allow easier fabrication of appropriately contoured crowns while also improving rigidity and esthetics. While conservation of tooth structure is desirable, this principle is not an absolute contraindication to increasing marginal width beyond the minimum to accommodate the materials being used. Experience with shoulder prepara551
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tions suggests that margin widths exceeding 0.3 mm are not usually incompatible with pulpal vitality. If the advantages of increased preparation were stressed, rather than the conservation of tooth tissue, dentists might be encouraged to increase marginal widths where possible. REFERENCES 1. Shillingburg HT, Hobo S, Whit&t LD. Fundamentals of fixed prosthodontics. 2nd ed. Chicago: Quintessence Publishing Co, 1981:90-3,118. 2. Roberts DH. Fixed bridge prostheses. 2nd ed. Bristol: John Wright & Sons, 1989:112-4. 3. Carbeneau GT, Cartwright CB, Comstock FW, et al. Principles and practice of operative dentistry. 2nd ed. Philadelphia: Lea and Febiger Publishing Co, 1981:377-a. 4. Gihnore HW, Lund MR, Bates DJ, Vernetti JP. Operative dentistry. 4th ed. St Louis: CV Mosby Co, 1982:263-4,333-4. 5. Baum L, Phillips RW, Lund MR. Textbook of operative dentistry. 2nd ed. Philadelphia: WB Saunders Co, 1985:532-3,573. 6. Pitt-Ford TR. The restoration of teeth. Oxford: Blackwell Scientific Publications, 1985:185-6. 7. Jacobsen PH, Robinson PB. Basic techniques and materials for conservative dentistry. 3. Restoration of the broken down posterior tooth. J Dent 1981$X01-8. 8. Tylman SD, Malone WFP. Tylman’s theory and practice of fixed prosthodontics. 7th ed. St Louis: CV Mosby Co, 1978:112-3,116-7,214. 9. Picard HM. A manual of operative dentistry. 5th ed. New York: Oxford University Press, 1983:231-2. 10. Bell CJ, Bowden JR, Saund P, Smith M, Stephenson RI. The Dicer castable ceramic crown. Dent Practice 1987;25:20-5. 11. Gavelis JR, Morency JD, Riley ED, Sozio RB. The effect of various finish line preparations on the marginal seal and occlusal seat of full crown preparations J PROSTH~ DENT 1981;45:138-45. 12. Pardo GI. A full cast restoration design offering superior marginal characteristics. J PROSTHET DENT 1982;38:539-43. 13. McLean JW, Wilson AD. Butt joint versus bevelled gold margin in metal-ceramic crowns. J Biomed Mater Res 1980;14:239-50. 14. Rosner D. Function, placement, and reproduction of bevels for gold castings. J PROSTHETDENT 1963;13:1160-6. 15. Gardiner FM. Margins of complete crowns-literature review. J PROSTHET DENT 1982;48:396-400. 16. Grajower R, Lewinstein I. A mathematical treatise on the fit of crown castings. J PROSTHETDENT 1983;49:663-73. 17. Ostlund LE. Cavity design and mathematics: their effect on gaps at the margins of cast restorations. Operative Dent 1985;10:122-37. 18. Donovan T, Prince J. An analysis of margin configurations for metalceramic crowns. J PROSTHETDENT 1985;53:153-7. 19. Rosenstiel E. The marginal fit of inlays and crowns. Br Dent J 1964;117:432-42. 20. Kashani HG, Khera SC, Gulker IA. The effects of bevel angulation on marginal integrity. J Am Dent Assoc 1981;103:882-5. 21. Gulker I. Margins. NY State Dent J 1985;51:213-7. 22. Kuwata M. Gingival margin design of abutments for ceramometal restorations. 2. Quintessence Dent Technol 1979;10:27-38. 23. Behrend DB. Crown margins and gingival health. Ann R Auat Co11Dent Surg 1984;8:138-45. 24. Pascoe DF. An evaluation of the marginal adaptation of extracoronal restorations during cementation. J PROSTH~ DENT 1983;49:657-62. 25. Fusayama T, Ide K, Hosada H. Relief of resistance of cement of full cast crowns. J PROSTHETDENT 1964;14:95-106. 26. Schweikert EO. Feather-edged or knife-edged preparation and impression technique. J PRO~THETDENT 1984;52:243-6. 27. Gilboe DB, Thayer KE. Bevelled shoulder concept: full gold crown preparation. Can Dent Assoc J 1980;46:519-23. 28. Christensen GJ. Marginal fit of gold inlay castings. J PROSTHETDENT 1966;16:297-305.
29. Scharer P. In: Dental ceramics. Proceedings of the First International Symposium on Ceramics. Chicago: Quintessence Publishing Co, 1983:295-7,301-3,311. 30. McLean JW, van Fraunhofer JA. The estimation of cement film thickness by an in viva technique. Br Dent J 1971;131:107-11. 31. Belser UC, MacEntee MI, Richter WA. Fit of three porcelain-fused-to-
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metal marginal designs in viva: a SEM study. J PROSTHETDENT 198$53:24-g. 32. Assif D, Antopolski B, Helft M, KafIe I. Comparison of methods of clinical evaluation of the marginal fit of complete cast crowns. J PROSTHET DENT 1985;54:20-4.
33. Rosenstiel E. To bevel or not to bevel? Br Dent J 1975;138:389-92. 34. Metzler JC, Chandler HH. An evaluation of techniques for finishing margins of gold inlays. J PROSTHET DENT 1976;36:523-31. 35. Sarrett DC, Richeson JS, Smith GE. Scanning electron microscopy evaluation of four finishing techniques on margins of gold castings. J PROSTHET DENT 1983;50:784-92.
36. Miller L. In: Dental ceramics. Proceedings of the First International Symposium on Ceramics. Chicago: Quintessence Publishing Co, 1983:X3,178,183. 37. Finger W. Effect of thickness of peridental restorations on the casting precision. Stand J Dent Res 1980,88:455-g. 38. Erhlich J, Hochman N. Alterations on crown contour--effect on gingival health in man. J PROWHET DENT 1980;44:523-5. 39. Parkinson CF, Schaberg TV. Pontic design of posterior fixed partial prostheses: is it a microbial misadventure? J PROBTHET DENT 1984;51:51-4. 40. Perel ML. Axial crown contours. J PROSTHET DENT 19’71;25:642-9. 41. Yuodelis RA, Weaver JD, Sapkos S. Facial and lingual contours of artificial complete crown restorations and their effects on the periodontium. J PROSTHET DENT 1973;29:61-6. 42. Becker CM, Kaldahl WB. Current theories of crown contour, margin placement and pontic design. J PROSTHET DENT 1981;45:268-77. 43. Palomo F, Peden J. Periodontal considerations of restorative procedures. J PROSTHET DENT 1976;36:387-93. 44. Karlsson S. A clinical evaluation of fixed bridges, 10 years following insertion. J Oral Rehabil 1986;13:423-32. 45. Grasso JE, Nalbandian J, Sanford C, Balit H. The quality of restorative dental care. J PROSTHET DENT 1979;42:571-8. 46. Grasso JE, Nalbandian J, Sanford C, Balit H. Effect of restoration quality on periodontal health. J PROSTHET DENT 1985;53:14-9. 47. Parkinson CF. Excessive crown contours facilitate endemic plaque niches. J PROSTHRT DENT 197&35:424-g. 48. Comcowich WL. In: Dental ceramics. Proceedings of the First International Symposium on Ceramics. Chicago: Quintessence Publishing Co, 1983265.8. 49. Coornaert J, Adriaens P, De Boever J. Long-term clinical study of porcelain-fused-to-gold restorations. J PROSTHET DENT 1984;51:338-42. 50. Walton JN, Gardiner FM, Agar JR. A survey of crown and fixed partial denture failures: length of service and reasons for replacement. J PROSTHET DENT 1986;56:416-20. 51. Hobo S, Shillingburg HT. Porcelain-fused-tometab tooth preparation and coping design. J PROSTHET DENT 1973;30:28-35.
52. Shillingburg HT, Hobo S, Fisher DW. Preparation design and margin distortion in porcelain-fused-to-metal restorations. J PROSTHETDENT 1973;29:276-84. 53. Faucher RR, Nicholls JI. Distortion related to margin design in procelain-fused-to-metal restorations. J PROSTHET DENT 1980;43:149-55. 54. Wanserski DJ, Sobczak KP, Monaco JG, McGivney GP. An analysis of margin adaptation of all-porcelain facial margin ceramometal crowns. J PROSTHET DENT 1986;56:289-92.
55. Bridger DW, Nicholls JI. Distortion of ceramometal fixed partial dentures during the firing cycle. J PR~~THET DENT 1981;45:507-14. 56. Buchanan WT, Svare CW, Turner KA. The effect of repeated firings and strength on marginal distortion in two ceramometal systems. J PROSTHET DENT 1981;45:502-6.
57. Henry PJ. Pontic form in fixed partial dentures. Au& Dent J 1971;16: l-7. 58. Goldstein RE. Esthetic principles for ceramometal restorations. Dent Clin North Am 1977;21:802-22. 59. Panno FV, Vahidi F, Gulker I, Ghalili KM. Evaluation of the 45-degree labial bevel with a shoulder preparation. J PROSTHRT DENT 1986; 561655-61. Reprint requests to: DR. ALAN J. HUNTER 21 PACIFIC ST. DUNEDIN NEW ZEALAND
NOVEMBER
1990
VOLUME
64
NUMBER
6
D.R.D.R.C.S.,
and A. R. Hunter,
and
B.D.S., M.D.S.
The terms bevel, chamfer, and shoulder are widely used to describe crown margin designs. However, as no clear definition of the essential feature(s) of each design has been universally accepted, the same term often describes margins of widely differing width andlor configuration. Similarly “bevel angles” are not consistently defined. While tradition favors the use of thinner marginal designs, many of the reasons advanced for their superiority are questionable in the light of contemporary research. Use of marginal widths beyond the absolute minimum demanded of the material may contribute to overcoming some of the persistent problems identified with fixed prosthodontic replacement of natural teeth. These include overcontour, porcelain debonds, poor esthetics, and fit. It is suggested that the problems associated with underpreparation and the potential advantages of wider preparations need reemphasis. (J PROSTHET DENT 1990;64:548-52.)
T
00th preparation for fixed prosthodontics requires a decision regarding the marginal configuration. The design dictates the shape and bulk of the casting and influences the fit at the margin.l Although many factors such as materials, esthetics, and access influence this selection, most dentists probably have a “preferred” design, However, there is disagreement about what constitutes ideal marginal geometry and width. These articles review the issues involved and discuss the implications of research into the clinical longevity of crowns related to the marginal design. Part I discusses terminology and margin width, while Part II reviews the theoretical and practical marginal geometry.
TERMINOLOGY
AND DEFINITIONS
At present there is no universally accepted basis for classifying margin designs, and many texts avoid verbal definitions in favor of illustrations.1-6 Unfortunately, this approach does not clearly distinguish the “essential” feature(s) of each design and therefore illustrations of chamfers or shoulders are often misleading (Fig. I). Chamfers have been defined on the basis of their marginal width and/or geometry. Jacobsen and Robinson7 stated that a width greater than 0.3 mm at its cervical termination precluded any margin being called a chamfer. Others’? 6 g consider an obtuse angled gingival termination or curved cross-sectional shape to be fundamental for chamfers. Bell et al.1° described a chamfer preparation with a reduction of 1.5 mm, rounded internal line angles, and a cavosurface angle of 135 degrees. However, their shoulder preparations were identical except for cavosurface angle, and this feature was crucial in classifying marginal designs. However, illustrated examples demonstrating rounded in-
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ternal line angles and cavosurface junctions of approximately 90 degrees have also been termed chamfersir, r2 Similar problems arise with attempts to define. shoulders. Many classify a flat shoulder as one that makes a 90degree angle with the vertical axial wall of the preparation.2l 8,g,I3 This classification presumably relies on width to determine the marginal type, while the angulation from the vertical axial wall indicates marginal direction (for example, 135-degree shoulder). With any classification based on width, substantial differences arise with changes in marginal angulation. Some shoulders may exhibit slip joint geometry (135-degree shoulders) but others may not (go-degree shoulders). Shoulders have also been defined using marginal geometry, where the discriminating features are an external cavosurface angle of 90 degrees and a corresponding butt joint of restoration/tooth at the margin.‘? 3l5,lo Depending on the apical extent of the preparation, and the axial profile of the tooth, a go-degree shoulder using this classification may often appear similar to a 135-degree shoulder as defined by McLean and Wilson.i3 Bevels are another recommended marginal design because they incorporate geometric principles to minimize marginal discrepencies. l, 12,14,I5 It is important to realize that bevel angulation may be assessed from different planes (Fig. 2). Some authorities define the angle of the bevel as the angle between the created bevel surface and the preparation surface that has been bevelled,13*I4916-18while others describe it as the angle between the created bevel surface and the external surface of the tooth.3, lo*igs2i The former definition of bevel angulation will be used throughout these articles. For convenience, Kuwata22 classified margins on the basis of their “margin angle.” This is the angle formed between a vertical projection from the external surface of the tooth and its prepared surface (Fig. 2, angle b). He
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Fig. 1. Margin illustrations. a, Long chamfer. b and c, Chamfers; note differing exit profiles. d, Flat or go-degree shoulder, based on the preparation axial wall. e, Sloped shoulder (on axial wall) or go-degree shoulder, based on cavosurface angle. considered that margin angles between 0 and 30 degrees gave bevelled margins, those between 31 and 60 degrees were chamfers, and those between 61 and 90 degrees were shoulders. A practical system for classifying margins was advanced by Pardo, l2 who broadly categorized margins into the two main classes of inclined vertical or horizontal. A go-degree shoulder was considered a horizontal margin and the 135degree shoulder was considered an inclined vertical design. Although this classification emphasized the importance of exit configuration while ignoring margin width, in practice the horizontal designs are often wider. Another source of confusion is the different terminology for describing discrepancies between the casting and the tooth. These discrepancies may be the result of inaccuracies inherent in crown fabrication; an inability to correctly seat the crown during cementation, or a combination of these factors. These discrepancies are commonly measured at the external margin of the crown or at right angles to the two opposing surfaces (Fig. 3). The former has been described as the “vertical discrepancy,“14 the “marginal opening measured verticaIly,“23 or the “minimal marginal cement thickness.“16 The discrepancy at right angles to the two opposing surfaces has been referred to as the “marginal opening, “15t24 the “horizontal discrepancy,“14 or the “marginal adaptation.“23 The least confusing terminology is the use of the term “seating” to describe the vertical discrepancy at the external margin, and the term “sealing” to describe the horizontal discrepancy observed at right angles to the two opposing surfaces.11s21
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Fig. 2. Bevel angles. Angle a is the angle formed by the created bevel surface with the surface that has been bevelled. Angle b is the angle formed by the created bevel surface with the external surface, also called margin angle. 180 degrees -a = b.
MARGINAL
WIDTH
For simplicity, concepts related to marginal width and geometry can usually be discussed independently. The minimum marginal width of a crown is largely determined by the dental materials used, while the maximum width is limited by the desire to preserve both a suitable coronal core and pulp vitality. Within these general parameters, marginal widths may vary, with designs exhibiting minimal axial reduction favored when possible.25 These include regular chamfers and knife edges (including feather and chisel edges) that would seldom be wider then 0.3 mm.
Marginal
widths
below 0.3 mm
The cited advantages of thin margins are that these designs allow for intraoral finishing of the crown margins, are conservative, and their geometric configuration improves the marginal seal.1,6-8 Because minimal reduction is required, preparation of knife-edge margins has been considered to.be relatively easy.s,s,26However, others consider that achieving a definite finish line with a knife-edge configuration is also difficult and requires careful preparation.1,4 Crown fabrication is also a problem, as these margins are often indistinct on the impression and die.4, 7 In addition, overcontoured restorations are commonly produced to obtain sufficient bulk of restorative material to ensure functional rigidity. l, 7 Although sometimes recommended for routine use,26knife-edges are not a preferred design72l5 and should be restricted to specific clinical sit549
or with an explorer, with the efficacy of the examination becoming increasingly questionable the more subgingival the margin. Even if objective evaluation were possible, limited access for instruments precludes marginal finishing.5s 13,23,2gl33Where accesspermits, disks and stones can be effective in achieving marginal closure, but burnishers appear ineffective in reducing discrepancies.34g35 Although margin finishing of complete cast crowns during cementation is often recommended,20*2s,2gv33 the time available is limited as these procedures must be completed while the cement remains fluid.13, 33 The limited ductility and high yield strength of many alloys also limits the effectiveness of finishing procedures. Although Gilboe and Thayer 27accepted that type 3 gold possessedlimited ductility, they maintained that thin cervical margins did allow for bending of the metal to improve the marginal seal. Conversely, Behrend23 considered that burnishing of margins was impossible with the current hard alloys. Gardiner15 also concluded that burnishing was not a practical way of achieving marginal closure for type 3 gold and ceramometal alloys. Others2g*36also consider ceramometal alloys unsuitable for burnishing, while McLean and Wilson13 noted that the pressures and distortion of metal, associated with the technique, could crack porcelain bonded to these alloys. Alloys with Brine11 hardness numbers greater than 84 or 100 are unsuitable for burnishing.5s l5 Fig. 3. Diagram illustrating discrepancies between castings and teeth. a, Seating discrepancy. 6, Sealing discrepancy.
uations such as their use in extremely bell-shaped teeth.‘, 2,8,27 Greater axial reduction (up to 0.3 mm7) is required for chamfers that many consider the “ideal ” gingival finish line for cast restorations.l* 4,6s~25This increased width allows easier evaluation of the margin at all stages and reduces the chances of an overcontoured casting, while allowing adequate rigidity. While possessing these practical advantages, chamfers retain the benefits of the thinner knife-edge design. Many consider that thin margins (including bevels) are desirable because they allow for marginal closure through finishing procedures.‘? 5-7*20*27-2gThese procedures include spinning and swaging, the use of disks and stones to move metal to the margins, and burnishing to bend the crown margin toward the tooth. Stiffler in 1980 reported a 50% reduction in the marginal opening if accessible, and where the type of alloy permitted the use of rotary instruments for burnishing (cited by Scharer2g). However, others consider intraoral finishing procedures for gingival margins difficult and impractica16* 15,21,23The dentist’s ability to objectively evaluate the cervical margins of crowns appears to be limited, particularly in approximal and subgingival locations.28p30-32Assif et a1.32believed that there was a direct relationship between the location of restorative margins and the ability to evaluate them by sight 550
Marginal
widths
greater
than 0.3 mm
Margin widths greater than 0.3 mm, often referred to as shoulders, have been unpopular, particularly if a chamfer could be placed. Tylman and Malonesstated that shoulders were difficult to prepare on posterior teeth, and that their routine use was difficult to justify when stress analysis, microleakage, and pulpal response were considered. Others agreed, and considered shoulders unnecessarily destructive,‘, ’ bulky,6 and unsuitable for marginal finishing unless modified with a beve1.l. 4*7s33134 The thickened wax patterns resulting from wider margins also exhibit increased casting shrinkage, although this can be compensated for by effective die spacing.3’ Despite their unpopularity, the advantages of wider margins over thinner designs should be considered. These advantages include improved control of crown contours and esthetics, increased structural rigidity, and clearer impressions and dies. According to Gilboe and Thayer,27 an advantage of bevelled shoulder preparations were that they allowed the incorporation of physiologic contours in both the temporary and final crown. Increased tooth reduction also facilitates contour changes in the crown, permitting guide planes for removable prostheses, or flattening contours to facilitate plaque control.8, 27 Some believe that with good oral hygiene, minor changes in crown contour ( + 1 mm) do not significantly affect gingival health.%, 3gHowever, increases in crown contour usually compromise gingival health by impeding plaque removal29 ‘921,23,4o-42Fankhauser, cited by Scharer,2g in NOVEMBER
1990
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CROWN
MARGINAL
CONFIGURATIONS.
PART
I
1979 reported that minor changes in contour led to increased gingival bleeding and that many ceramometal crowns were overcontoured by a mean value of 0.5 mm. Overcontoured crowns are a common problem,41 particularly approximallf13, 44 and with ceramometal restorations.2 Karlsso# discovered that a “significant” number of ceramometal restorations were overcontoured approximally, and others have reported that 25% to 80% of complete cast crowns are overcontoured.45-47 Parkinson47 reported that although cast crowns were less overcontoured than ceramometal restorations, differences in plaque scores were not significant, both being significantly higher than in the uncrowned controls. These studies indicated that the problem of overcontour was not confined to ceramometal restorations and that small increases in contour can adversely influence gingival health. Grass0 et a1.46suggested that unsuitable embrasures and contours were a greater problem with crowns than were inadequate gingival cavosurface margins. Another advantage of greater axial reduction is that it permits a greater thickness of restorative material, thus ensuring rigidity during function.27 Crown flexure predisposes to cement failure, and with ceramometal crowns there is the additional hazard of porcelain failure.48, 4gAlthough Karlsson44 and Coornaert et a1.4greported comparatively low rates of porcelain fracture, Walton et a1.50 reported that porcelain fractures and “debonds” were the second highest cause of failure with ceramometal crowns. The rate of porcelain failure due to flexure appears to be related to the thickness of the metal substructure and to occlusal factors, particularly bruxism.44, 48*4g,51 Distortion of ceramometal substructures can occur during crown fabrication.2p 13x*a 51-54This distortion has been attributed to temperature-related creep of the alloy53v55,56 or to a thermal expansion mismatch between porcelain and alloy. 13*52Whatever the exact cause, this distortion is a common problem. Karlsson44 found that although ceramometal and acrylic resin veneered crowns had a similar “frequency” of poor marginal adaptation, the size of the discrepancy was significantly greater with ceramometal crowns. He attributed this finding to weak metal substructures that were distorted during the fabrication process. Many consider that increasing the bulk of metal, particularly at the internal line angles, increases resistance to marginal distortion.15, 31,52,53 Thus increased marginal widths (shoulders) have usually been recommended as a method of minimizing these discrepancies51, 5s The desire to enhance esthetics without sacrificing strength is a crucial consideration when using ceramometal restorations, but a high proportion of ceramometal restorations are judged unacceptable by patients due to poor esthetics.50 Good marginal integrity, suitable margin location, and harmonious contour are all important in achieving optimal esthetics.23s43,48,57,58 Overcontoured crowns are unlikely to blend with natural counterparts due to their poor shape and adverse effect on gingival tissues.ss*43High reflectance of the opaque layer, particularly at the margins, THE
JOURNAL
OF
PROSTHETIC
DENTISTRY
is another common esthetic problem with ceramometal restorations’s All these problems can be minimized by providing “adequate” marginal width to accommodate both metal and porcelain, without overcontouring. Unfortunately, there are variations in opinion regarding the margin reduction required for ceramometal restorations, with suggestions ranging from 1 to 2 mm. ‘32,5,4s+4gBecause optimal esthetics depend on adequate reduction, it may be desirable to increase the marginal width of preparations toward the upper limit of the recommended range. The effect of increased margin widths on the ease of the clinical procedures is disputed. Some consider flat or sloped shoulders difficult to prepares l8 or impractical with small teeth or when excessive root surfaces are exposed.5g According to Gilboe and Thayer,27 the increased width associated with bevelled shoulders facilitates the placement of gingival retraction cord, while the increased bulk of impression material at the gingival margin results in more accurate impressions and dies. Increased bulk is useful where the impression material possesseslimited tear strength. In contrast, Schweikert2’j and Panno et a1.5g stated that shoulder preparations made impression making difficult, especially for multiple preparations. Perhaps the greatest effect of increased margin widths clinically is the ease of evaluating preparations, impressions, and dies. Shoulders are commonly well defined’ and according to some create the clearest finish line.g Picard reiterated that a poor impression, even of a good preparation, will produce a poor die and ultimately lead to a poor final result. The ability to accurately evaluate margins, particularly at the impression stage is crucial to ensuring an acceptable crown and is a major advantage of wider preparations. CONCLUSION Tradition has favored thin marginal designs, with the consequence that there is a tendency to “underprepare” teeth. Inadequate preparation is cited as the reason for many of the problems associated with crowns.2p21*23,2g,43,44 The consequences of underpreparation are usually worse than those associated with preparations that are wider than the absolute minimum required by the restorative material. Thin finish lines have been advocated in the belief that they allow marginal closure through intraoral finishing and contribute to the maintenance of pulpal vitality. However, a review of the literature indicates that the efficacy of intraoral finishing procedures should be reassessed. The ability of dental technicians to produce anatomic crown contours while working to the minimum possible thickness of material has also been overestimated. If maintaining a normal emergence profile is important, then wider margins allow easier fabrication of appropriately contoured crowns while also improving rigidity and esthetics. While conservation of tooth structure is desirable, this principle is not an absolute contraindication to increasing marginal width beyond the minimum to accommodate the materials being used. Experience with shoulder prepara551
HUNTER
tions suggests that margin widths exceeding 0.3 mm are not usually incompatible with pulpal vitality. If the advantages of increased preparation were stressed, rather than the conservation of tooth tissue, dentists might be encouraged to increase marginal widths where possible. REFERENCES 1. Shillingburg HT, Hobo S, Whit&t LD. Fundamentals of fixed prosthodontics. 2nd ed. Chicago: Quintessence Publishing Co, 1981:90-3,118. 2. Roberts DH. Fixed bridge prostheses. 2nd ed. Bristol: John Wright & Sons, 1989:112-4. 3. Carbeneau GT, Cartwright CB, Comstock FW, et al. Principles and practice of operative dentistry. 2nd ed. Philadelphia: Lea and Febiger Publishing Co, 1981:377-a. 4. Gihnore HW, Lund MR, Bates DJ, Vernetti JP. Operative dentistry. 4th ed. St Louis: CV Mosby Co, 1982:263-4,333-4. 5. Baum L, Phillips RW, Lund MR. Textbook of operative dentistry. 2nd ed. Philadelphia: WB Saunders Co, 1985:532-3,573. 6. Pitt-Ford TR. The restoration of teeth. Oxford: Blackwell Scientific Publications, 1985:185-6. 7. Jacobsen PH, Robinson PB. Basic techniques and materials for conservative dentistry. 3. Restoration of the broken down posterior tooth. J Dent 1981$X01-8. 8. Tylman SD, Malone WFP. Tylman’s theory and practice of fixed prosthodontics. 7th ed. St Louis: CV Mosby Co, 1978:112-3,116-7,214. 9. Picard HM. A manual of operative dentistry. 5th ed. New York: Oxford University Press, 1983:231-2. 10. Bell CJ, Bowden JR, Saund P, Smith M, Stephenson RI. The Dicer castable ceramic crown. Dent Practice 1987;25:20-5. 11. Gavelis JR, Morency JD, Riley ED, Sozio RB. The effect of various finish line preparations on the marginal seal and occlusal seat of full crown preparations J PROSTH~ DENT 1981;45:138-45. 12. Pardo GI. A full cast restoration design offering superior marginal characteristics. J PROSTHET DENT 1982;38:539-43. 13. McLean JW, Wilson AD. Butt joint versus bevelled gold margin in metal-ceramic crowns. J Biomed Mater Res 1980;14:239-50. 14. Rosner D. Function, placement, and reproduction of bevels for gold castings. J PROSTHETDENT 1963;13:1160-6. 15. Gardiner FM. Margins of complete crowns-literature review. J PROSTHET DENT 1982;48:396-400. 16. Grajower R, Lewinstein I. A mathematical treatise on the fit of crown castings. J PROSTHETDENT 1983;49:663-73. 17. Ostlund LE. Cavity design and mathematics: their effect on gaps at the margins of cast restorations. Operative Dent 1985;10:122-37. 18. Donovan T, Prince J. An analysis of margin configurations for metalceramic crowns. J PROSTHETDENT 1985;53:153-7. 19. Rosenstiel E. The marginal fit of inlays and crowns. Br Dent J 1964;117:432-42. 20. Kashani HG, Khera SC, Gulker IA. The effects of bevel angulation on marginal integrity. J Am Dent Assoc 1981;103:882-5. 21. Gulker I. Margins. NY State Dent J 1985;51:213-7. 22. Kuwata M. Gingival margin design of abutments for ceramometal restorations. 2. Quintessence Dent Technol 1979;10:27-38. 23. Behrend DB. Crown margins and gingival health. Ann R Auat Co11Dent Surg 1984;8:138-45. 24. Pascoe DF. An evaluation of the marginal adaptation of extracoronal restorations during cementation. J PROSTH~ DENT 1983;49:657-62. 25. Fusayama T, Ide K, Hosada H. Relief of resistance of cement of full cast crowns. J PROSTHETDENT 1964;14:95-106. 26. Schweikert EO. Feather-edged or knife-edged preparation and impression technique. J PRO~THETDENT 1984;52:243-6. 27. Gilboe DB, Thayer KE. Bevelled shoulder concept: full gold crown preparation. Can Dent Assoc J 1980;46:519-23. 28. Christensen GJ. Marginal fit of gold inlay castings. J PROSTHETDENT 1966;16:297-305.
29. Scharer P. In: Dental ceramics. Proceedings of the First International Symposium on Ceramics. Chicago: Quintessence Publishing Co, 1983:295-7,301-3,311. 30. McLean JW, van Fraunhofer JA. The estimation of cement film thickness by an in viva technique. Br Dent J 1971;131:107-11. 31. Belser UC, MacEntee MI, Richter WA. Fit of three porcelain-fused-to-
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metal marginal designs in viva: a SEM study. J PROSTHETDENT 198$53:24-g. 32. Assif D, Antopolski B, Helft M, KafIe I. Comparison of methods of clinical evaluation of the marginal fit of complete cast crowns. J PROSTHET DENT 1985;54:20-4.
33. Rosenstiel E. To bevel or not to bevel? Br Dent J 1975;138:389-92. 34. Metzler JC, Chandler HH. An evaluation of techniques for finishing margins of gold inlays. J PROSTHET DENT 1976;36:523-31. 35. Sarrett DC, Richeson JS, Smith GE. Scanning electron microscopy evaluation of four finishing techniques on margins of gold castings. J PROSTHET DENT 1983;50:784-92.
36. Miller L. In: Dental ceramics. Proceedings of the First International Symposium on Ceramics. Chicago: Quintessence Publishing Co, 1983:X3,178,183. 37. Finger W. Effect of thickness of peridental restorations on the casting precision. Stand J Dent Res 1980,88:455-g. 38. Erhlich J, Hochman N. Alterations on crown contour--effect on gingival health in man. J PROWHET DENT 1980;44:523-5. 39. Parkinson CF, Schaberg TV. Pontic design of posterior fixed partial prostheses: is it a microbial misadventure? J PROBTHET DENT 1984;51:51-4. 40. Perel ML. Axial crown contours. J PROSTHET DENT 19’71;25:642-9. 41. Yuodelis RA, Weaver JD, Sapkos S. Facial and lingual contours of artificial complete crown restorations and their effects on the periodontium. J PROSTHET DENT 1973;29:61-6. 42. Becker CM, Kaldahl WB. Current theories of crown contour, margin placement and pontic design. J PROSTHET DENT 1981;45:268-77. 43. Palomo F, Peden J. Periodontal considerations of restorative procedures. J PROSTHET DENT 1976;36:387-93. 44. Karlsson S. A clinical evaluation of fixed bridges, 10 years following insertion. J Oral Rehabil 1986;13:423-32. 45. Grasso JE, Nalbandian J, Sanford C, Balit H. The quality of restorative dental care. J PROSTHET DENT 1979;42:571-8. 46. Grasso JE, Nalbandian J, Sanford C, Balit H. Effect of restoration quality on periodontal health. J PROSTHET DENT 1985;53:14-9. 47. Parkinson CF. Excessive crown contours facilitate endemic plaque niches. J PROSTHRT DENT 197&35:424-g. 48. Comcowich WL. In: Dental ceramics. Proceedings of the First International Symposium on Ceramics. Chicago: Quintessence Publishing Co, 1983265.8. 49. Coornaert J, Adriaens P, De Boever J. Long-term clinical study of porcelain-fused-to-gold restorations. J PROSTHET DENT 1984;51:338-42. 50. Walton JN, Gardiner FM, Agar JR. A survey of crown and fixed partial denture failures: length of service and reasons for replacement. J PROSTHET DENT 1986;56:416-20. 51. Hobo S, Shillingburg HT. Porcelain-fused-tometab tooth preparation and coping design. J PROSTHET DENT 1973;30:28-35.
52. Shillingburg HT, Hobo S, Fisher DW. Preparation design and margin distortion in porcelain-fused-to-metal restorations. J PROSTHETDENT 1973;29:276-84. 53. Faucher RR, Nicholls JI. Distortion related to margin design in procelain-fused-to-metal restorations. J PROSTHET DENT 1980;43:149-55. 54. Wanserski DJ, Sobczak KP, Monaco JG, McGivney GP. An analysis of margin adaptation of all-porcelain facial margin ceramometal crowns. J PROSTHET DENT 1986;56:289-92.
55. Bridger DW, Nicholls JI. Distortion of ceramometal fixed partial dentures during the firing cycle. J PR~~THET DENT 1981;45:507-14. 56. Buchanan WT, Svare CW, Turner KA. The effect of repeated firings and strength on marginal distortion in two ceramometal systems. J PROSTHET DENT 1981;45:502-6.
57. Henry PJ. Pontic form in fixed partial dentures. Au& Dent J 1971;16: l-7. 58. Goldstein RE. Esthetic principles for ceramometal restorations. Dent Clin North Am 1977;21:802-22. 59. Panno FV, Vahidi F, Gulker I, Ghalili KM. Evaluation of the 45-degree labial bevel with a shoulder preparation. J PROSTHRT DENT 1986; 561655-61. Reprint requests to: DR. ALAN J. HUNTER 21 PACIFIC ST. DUNEDIN NEW ZEALAND
NOVEMBER
1990
VOLUME
64
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