Periodontal dictates for esthetic ceramometal crowns

E S T H E T I C D E N T I S T R Y

R. Sheldon Stein, DMD

Periodontal health must be the crucial con­sideration in any restorative treatment plan. The interaction between the crown and the investing tissue contributes to the health of the gingiva. The dentist must consider the tooth’s position, shape, function, and physi­cal state as well as the means by which it is restored, when yielding to the demands of the patient for an improved esthetic appearance.

P eriodontal disease usually is attrib­uted to calcific deposits associated

with root surfaces of teeth. H ow ever, the co m p lica ted in te ra c tio n b e tw e en the crown and the investing tissues1 (Fig 1) also contributes to the pathological condi­tion.

Ideally, all margins of crowns should be supragingival. Crowns conceived in the presence of periodontal disease and en­larged, edem atous, inflamed gingiva can attain only limited esthetic appeal and will exacerbate the periodontal disturbance. It is true that improperly fabricated crowns will contribute to periodontal disease, and it is equally true that periodontal disease may destroy otherwise excellent crowns.

A mutual protective complex m ust exist between restoration and periodontium .2 No subgingival tooth preparation (crown) should be placed when gingival disease is present. Furtherm ore, to attain the most predictable, consistent, and best esthetic re s to ra tio n , the crow n shou ld no t be placed unless the tissue is in an absolute state of health.

In view of the demands o f the currently cosm etically concerned dental public ,3

many factors involved in tooth-tissue in­teraction are overlooked, resulting in a violation of the factors essential to the success of restorative procedures in clini­cal practice. One o f the most com mon er­rors concerns preoccupation with color. Tooth form, size, and position often are sacrificed in the attem pt to achieve what is thought to be a cosmetically pleasing re­sult that, more often than not, represents concessions to patient’s demands (Fig 2). When cosmetic appearance is the primary factor, most restorative procedures will fail. The cosmetic appearance as repre­sented by color (shade matching), is only one of the esthetic considerations in the development o f a harmonious and effec­tive natural tooth composition.

It is an inflexible dictum that there can­not be an esthetic or successful restoration in the presence of periodontal disease. Esthetic dentistry is the science that at­tem pts to deduce from nature the rules and principles o f art. In that nature will con­t r i b u t e to th e o b j e c t i v e , n a t u r a l phenom ena must be studiously observed with regard to teeth and their surrounding tissues. This means the form, size, posi­tion, arrangem ent, com position, texture, and, to be sure, color of teeth must be precisely evaluated in each case. Thus, this discussion focuses on anterior crowns (although virtually the same principles apply to other areas of the mouth) and considers those factors that have a signifi­cant role in what is generally accepted as esthetic dentistry.

Tooth form

The common categories o f tooth form are described by the familiar morphological

terms: square, tapered, and ovoid, with modifications and com binations of these. How ever, the shape of the patient’s face or frame of lips may affect the perception of form. One com m on attribute is the mor­phological a rch itec tu re at the gingival third o f a crown when the adjacent gingiva is healthy. It is not only a question of ap­pearance. The most crucial link between tooth form and gingival health is the design of the cervical third of the crown. The need for the proper “ em ergence profile” cannot be overem phasized (Fig 3).4-8 This linkage between the em ergence profile and transient planes affords a natural appear­ance to the basic tooth forms and, in addi­tion, supplies the properly deflective func­tional contours. This com plem entary rela­tionship betw een form and function is es­sential to meet biologic dem ands for an extended time (Fig 4).

Careful appraisal o f the facial aspect of an anterior tooth in relationship to the gin­giva shows a trigonal outline rather than the sem ilunar image perceived by most dentists and technicians (Fig 5). The facial surface o f the cervical third o f the crown is made up of two rhomboids o f somewhat different dimensions because of the tri­gonal nature of the gingival outline (Fig 6). Obedience to this rule, deduced from the natural appearance, not only leads to a m ore esthe tic resu lt but also com pels sounder preparation of the tooth for a biocompatible crown that would be dif­ficult, if not im possible, to achieve by any other approach.

As said before (but perhaps not often enough), restorations constructed to ac­com modate gingiva enlarged and distorted by chronic or acute inflammatory changes will be esthetically unsatisfactory , will

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Fig 1 ■ Gingival inflammation and enlargement as­sociated with ceramometal crowns placed 6 months pre­viously. Clinical probing disclosed appreciable misfit of crown margin and root surface.

Fig 2 ■ Advanced periodontal problem concerning the maxillary central incisor restored with a crown 2 years earlier. The patient insisted on restorative treatment to close midline diastema. Extension of form and contour beyond physiological limits is apparent. Elongation and migration led to superimposed trauma from occlusion. Orthodontic movement of all anterior teeth or crowns on both central incisors might have been a better solution.

place an undue burden on tissue mainte­nance, and will lead to premature tooth instability. The relationship of tooth form to the periodontal complex does not begin and end at the gingival margin but extends to the occlusal/incisal aspect. Obedience to the p ro p e r em ergence profile will strongly influence the bulk of the ultimate crown and, accordingly, the depth and shape of the preparation. The tooth prepa­ration should be a prototype of the antici­pated crown form (Fig 7).

In th e s im p les t ca se o f ind iv idual crow ns, the ideal crown should look as if it belongs th e re , appearing harm onious, natural, inconspicuous, and in concert w ith the o ther teeth (Fig 8). All these phrases and adjectives apply to the same objective, but as these attributes cannot, in and o f them selves, provide methodolog­

ical advice, the param eters of a crown should be examined in more detail. Invari­ably, attention to detail makes for worthy achievem ents in both art and science.

Crown size

Size and form are intimately related, as the support of gingival health is of paramount import. The constraints that apply in con­structing a crown should not be viewed as inhibition but rather as a positive guide.

Height or length of a crown is influenced by the distance between the incisal table and the apex of the cissoid angle of the trigone (the highest point of the gingival outline). The natural design often is al­tered by periodontal d isease, the sub­sequent treatm ent of which exposes root surfaces. At that point the patient will seek

esthetic treatm ent, particularly when the maxillary gingival architecture shows un­s igh tly ro o t su rfaces and em b ra su re spaces (Fig 9).

If the incisal relationship is edge-to- edge, there is little possibility for a de­crease in vertical dimension. Only slight amelioration can be accom plished with o r­thodontic treatm ent, endodontic therapy, and modification of root surfaces. Again, it is worth noting that the margin of an ideal crow n should be supragingival. (W here m any te e th are invo lved , an acrylic labial facade may be used.)

When the patient has a sufficient over­bite, the dentist has more options and can shorten the crown vertically (endodontic therapy may be required) and remodel the exposed root to simulate an acceptable crown form.

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Fig 3 ■ Top and middle, close examination shows either a straight line or concave contour at the cervical third of crown form . P roper emergence profile provides biocompatible contour with gingival tissue. Bottom, overcontour at gingival aspect causes pressure atrophy of gingival tissues, resulting in an inflammatory re­sponse.

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Fig 4 ■ Top left, preoperative view of patient with periodontal involvement and 5- to 6-mm pocket on mandibular anterior teeth. Periodontal surgery was done to elimi­nate pockets. Top right, postoperative appearance 2 weeks after final insertion of restorations. Vertical dimension is not increased but tooth height appears shortened as gingival margin is located apically after periodontal therapy. Bottom left and right, postoperative appearance of maxillary and mandibular teeth sextants 11 years after treatment. Patient maintains meticulous oral hygiene.

As to the appropriate width of a crown, it should be realized that a crown made to fill a m esiodistal space that is too wide will inevitably be overcontoured and provoke periodontal disorders. If, how ever, the space is too narrow , it is reasonable to assum e th e re is generally insufficient in te r r o o t s p a c e to a c c o m m o d a te a biocom patible crown or pontic. In both instances, orthodontic treatm ent is the preferred remedy. Any com prom ise pre­dicts failure.

A nother dimension of size that deserves m ention is depth . This term connotes translucency, although perhaps “ opales­cen ce” m ore accurately describes the phenom ena that give a crown or tooth vib­rancy and vitality. Opalescence can be achieved only through meticulous atten­tion to the art and science o f ceramics (Fig 10).

Position

A nother esthetic problem is that of mal- posed teeth in lingual or labial version. The former often presents an anatomic cul-de-sac tha t ham pers effective oral hygiene, leading to inflammation of sur­rounding gingiva. Efforts to align the tooth by exaggerating its bulk invariably result in overcontouring the body to incisal as­pect of the crown. Inevitably, such distor­tion of the coronal anatom y will com ­prom ise periodontal health. The latter problem of labial version is often accom ­panied by scuffing, caused by toothbrush abrasion of the adjacent gingiva. Where the labial version is slight, minor ortho­dontic treatm ent may help resolve the situation, although orthodontic correction could also produce a diminished interroot space. If that is the case, major orthodon­

tic correction can be undertaken. If this is not possible, it is best to weigh the advan­tages of selective extraction, remembering the maxim: “ A tooth would rather have good pontic than a poor tooth relationship as its neighbor!” 5

An example is the dilemma posed by the attem pts to modify a peg-shaped lateral incisor. To achieve a size and form that looks good, overcontouring seems un­avoidable. This in turn will lead to a nega­tive periodontal response. In such a case the intensity of the patient’s desire must be weighed against the thoroughness o f his or her com prehension of the liabilities. The issue is a complex anatomical one and not predictably remedied by a crown. N or can it be any better resolved by laminate ve­neers o r bonding. Rather, its ultimate fail­ure is predicated on the distortion of the em ergence profile and the interrelation-

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Fig 5 ■ Top, various tooth forms with healthy tissue show trigonal gingival outline. Middle, diagram contrasts semilunar gingival outline generally visualized by dentist and technician (top) with actual trigonal outline (bottom) noted in natural dentitions. Middle depiction shows common invasive tissue error made during crown preparation when outline is visualized as semilunar. Bottom, four maxillary incisor crowns display trigonal outline of gingival outline when tissue is in “ absolutely healthy state” as guide for clinical delineation of margin area during tooth preparation.

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Fig 6 ■ Top and middle, overlay analysis of anterior incisors depicts rhomboidal forms in relationship to emergence profile and transient planes whereby trigone is formed.

Fig 7 ■ Proper tooth preparation shows sufficient cer­vical reduction, vertical and lingual oblique axial planes, and concave lingual reduction, all confirmed by incisal table aligned to middle of proximal aspect. These features enable a technician to fabricate a crown of ideal proportions, form, position, and inclination.

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Fig 8 ■ Top left, ceramometal crown with gold collar on mandibular right first premolar shows ideal emergence profile 4 years after insertion. Top right and middle left, buccal and lingual views of ceramometal crown with gold collar on mandibular left first molar shows biocompatible tissue response and overall esthetic appearance as result of proper emergence profile 5 years after insertion. Middle right and bottom left, pre- and postoperative appearance of maxillary right lateral, central, and left central incisors. Excellent gingival adaptation 5 years after highfusing (2,400 F) porcelain veneer crowns were placed. Appearance is natural because of harmonious blend of color and form in addition to healthy tissue response in concert with remaining teeth. Bottom right, crown on lateral incisor was inserted 7 years earlier. Gold collar is concealed subgingivally and margin ends at epithelial attachment.

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Fig 9 ■ Top, after periodontal therapy, roots are ex­posed and interproximal spaces are exaggerated. Mid­dle, when overbite relationship permits, foregoing case can be corrected by shortening of clinical crown and raising of contact areas to create a more esthetic appear­ance. Bottom, edge-to-edge relationship is corrected by redesigning and root coloration of clinical crowns. Re­sult is not as esthetically pleasing as in middle figure, but is an acceptable compromise to some patients.

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Fig 10 a Maxillary and mandibular posterior quadrants restored with ceramometal crowns show match with natural counterparts when ceramic art is meticulously observed.

Fig 11 ■ Top left, flaking of porcelain at margins occurs during try-on of six-unit anterior bridge with multiple butt joint preparations. Top right, section of five-unit anterior fixed partial denture with 1-mm gold collars. Bottom left, internal view of crowns with adequate chamfer shoulder and bevel. Bottom right, 10-year postopera­tive appearance of fixed partial denture, extending from maxillary right lateral incisor to left canine.

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Fig 12 ■ Left, cross section of root at gingiva. Irregularities of outline must be removed during preparation to ensure precise adaptation of crown margin (orig mag x 100). Right, microscopic analysis of beveling to remove root circumference ir­regularities with multifluted bur (Illustration, courtesy of Dr. Peter Scharer, Zurich, Switzerland).

ship between gingiva and restorative mar­gin. (M oreover, several new and popular techniques have an additional difficulty: subgingival finishing of margins.)

Tooth preparation

Materials and laboratory methods used to attain a precise marginal fit have been im­proved. Although attention to the careful delineation of the crown preparation itself remains a constant requirem ent and de­spite the availability of various excellent im pression materials, a good impression o f a poor preparation does not ensure a good restoration.

Strategies of crown preparation are di­verse, but one common denom inator is provision of a marginal area in sufficient dimension so it will accom m odate restora­tive materials and be of proper form, fit, and color. The debate on how to prepare a tooth for a crown is often reduced to a choice between a bevel o r a butt joint. Some clinicians advocate a shoulder bevel on the proximal and lingual aspects, and a butt joint on the labial surface.

The bevel provides a circum ferential guide plane for the definitive seating of multiple units by permitting minute tooth m ovem ent in any direction. The butt jo in t does not lend itself to multiple-unit resto­rations because of the probability that a tolerant yet accurate parallelism between units d im inishes exponen tia lly as the

num ber of units increases. In such cases, there is flaking or chipping of porcelain at the margins.

T h e a p p e a l o f th e b u tt jo in t and avoidance of a gold collar is, with rare exceptions, overcome by a properly de­signed cham fered shoulder with bevel. This preparation provides a more precise fit (F ig l l ) . 6 An added bonus is the smoothing o f irregularities of the root sur­face as the bevel (and only the bevel) is placed in the sulcus (Fig 12).

Access to the sulcus is easily attained by the proper use o f retraction cord or elec­trosurgery if tissues are healthy. The labial cham fer-shouldercan be redefined so that the labial bevel is reduced to half its size. The argument that subsequent recession of the gingiva will expose the gold collar can be effectively refuted because there is a distinct dem arcation between porcelain and root surface color when recession un­covers the butt joint.

Marginal finish

The finish of a crown, both the porcelain and the metal, is crucial. The smoothness of the surfaces affects the deposition of plaque.7 The desirable finish of porcelain is described as clam shell, as such a tex­ture diffracts light ideally and provides a su rface sm oo thness th a t d iscou rages plaque formation. The finely textured sur­face can be attained with use of disks and

survives self-glazing. Use of liquid over­glaze is discouraged. The overglaze tech­nique produces a rougher surface because m icroscopic bubbles are formed during the procedure and subsequently rupture.8 A self-glazing p ro ce ss9 p rese rves the clam-shell surface achieved in the pre­glaze porcelain finishing.

As for the finishing of metal, a smooth, clean surface is essential. It is not widely appreciated that some agents (rouge and tripoli) used to achieve this goal leave a residue, such as copper oxide, which is toxic to soft tissues. W ater-soluble polish­ing agents are available.

Recent investigations indicate that an alarming num ber of the new formula alloys may be to x ic .10 " Locally, the toxicity is seen as a black discoloration of the gingiva adjacent to the metal (Fig 13). The com­plete clinical significance of this reaction is not known, but its presence indicates that dentists and technicians should carefully select alloys, and m anufacturers should also be precise regarding com position. Gingival discoloration of this nature de­fea ts the es th e tic ach ievem ent o f the crown.

Cementation

The final esthetic result may be com ­promised during cem entation. N ot only must excess subgingival cem ent be com­pletely rem oved but the scratches and im-

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Fig 13 ■ “ Black gingiva” response to cytotoxic alloy occurred 3 months after insertion. Similar response occurred with a fixed partial denture placed 6 months earlier. Removal of prostheses showed base metal alloy was used in bothcases.

perfections induced by its removal must also be eliminated to avoid consequent plaque deposition and gingival inflamma­tion. M oreover, care must be taken during later prophylaxes as ultrasonic or hand scalers can m ar metal surfaces at the m ar­gins. If this occurs, the defects can be smoothed with a fine flour o f pumice.

A lso, tem porary cem entation o f the finished crown is recommended because it provides for esthetic réévaluation as well as assessm ent o f gingival responses. Un­fortunately, such a procedure may be im­practical when all-ceramic crow ns are in­serted.

Conclusion

The to o th ’s position, shape, function, physical state, and the means by which it is restored help to determ ine, by their in­teraction, whether periodontal injury will occur in tooth and tissue if an esthetic crown is placed. Tissue demands must be considered throughout all restorative pro­cedures to achieve a durable and esthetic restoration. Gingival health is the ultimate criterion of all restorations.

Dr. Stein is an assistant dean, clinical affairs, and research professor o f prosthetic dentistry, Boston University Goldman School of Graduate Dentistry, and maintains a private practice, 50 Staniford St, Bos­ton, 02114. Address requests for reprints to the author.

1. Stein, R.S., and Glickman, I. Prosthetic consid­erations essential for gingival health. Dent Clin North Am 177-188, 1960.

2. Stein, R.S. Mutual protective complex of dental

restorations. In Laney, W.R., and Gibilisco, J.A. Diagnosis and treatment in prosthodontics. Philadel­phia, Lea & Febiger, 1983.

3. Watson, J.F ., and Crispin, B.J. Margin place­ment of esthetic veneer crowns. Attitudes of patients and dentists. J Prosthet Dent 455:499-501, 1981.

4. Stein, R.S. The emergence profile. Jap J Techn 15(1 ):30-39, 1987.

5. Stein, R.S. Pontic-residual ridge relationship: a research report. J Prosthet Dent 16(2):251-285, 1966.

6. Panno, F.V., and others. Evaluation of the 45- degree labial bevel with a shoulder preparation. J Pros­thet Dent 56(6):655-661, 1986.

7. Swartz, M., and Phillips, R. Comparison of

bacterial accumulations on rough and smooth enamel surfaces. J Periodontol 28(10):303-307, 1957.

8. Stein, R.S., and Kuwata, M. A dentist and a dental technologist analyze current ceramo-metal pro­cedures. Dent Clinic North Am 21(4):729-749, 1977.

9. Stein, R.S., and Abdullah, B. Modified stain technique for ceramo-metal restorations. Thesis, Bos­ton University, 1984.

10. Stein, R.S.; Hitti, F .R .,and Duval, M. Possible cytotoxicity of dental alloys. Thesis, Boston Univer­sity, 1984.

11. Bergman, M. Erosion in the oral cavity — po­tential local and systemic effects. Int Dent J 36(1):41- 44, 1986.

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