The interrelationship between the periodontium and the

margins of artificial crowns: a review

 
Ridwaan Omar, BSc,BDS,LDSRCS,MSc,FRACDS;
Kamal Fareed, BD5, MS
Department of Restorative Dental Sciences, King Saud University, College of Dentistry, P.O. Box 60169, Riyadh 11545, Saudi Arabia.

Opinions have changed from the assumption that only poor restorative treatment leads to periodontal disease. Presently, it is believed that even clinically sound restorations may be an important etiological factor in the initiation of periodontal disease. The two aspects which are probably most intimately related to the supporting tissues are the manner in which the restoration establishes marginal contact with the tooth and the external contour of the restoration. In this paper, the objectives of margin placement will be reviewed; features of variously proposed designs will be compared and contrasted in the light of current materials and techniques.

Crown margins play a significant role in the survival of restorations in the oral cavity. The margins of restorations should satisfy the following: [1] they must fit as closely as possible to the finish line of the preparation to minimize the exposure of cement to the orai fluids; [2] they must have sufficient strength to withstand functional forces; [3] they should be as well finished and polished as possible; [4] they should be located in an area of easy access for both the patient and the dentist, whenever possible; [5] the biocompatibility of the restorative material is a further consideration.

Review of Literature

The margins of crowns should be as acute as possible for a closer fit.1-7 In complete ceramic ven­ eer crowns the shoulder margin produces the greatest potential for marginal opening. This open­ ing is reduced by the placement of a bevel. The more acute the angle of the margin, the more closely it fits at the margins.3 However, if it is made too acute, it has lower strength. An angle of 30 to 45 degrees is considered to be optimal.2, 8 McLean and Wilson9 state that the margins must reach an angle of 10 to 20 degrees for bevels of ceramometal crowns to reach a significant level of closure of margins. It is axiomatic, however, that crowns fail to seat by several microns.10

Several methods have been used to improve the seating of crowns during cementation: compensatory expansion of investments, internal etching, electro-chemical stripping, die spacing, venting and internal escape channels.11-17

Ostlund18 and Grajower and Lewinstein19 mathematically studied the geometry of different margin designs with allowances for the compensat­ ory expansion of the investment and for a finite thickness of cement. Both studies concluded that a slightly oversized crown on a shoulder margin would produce the best marginal fit and that bevels in such crowns would increase the marginal open­ ing.

Kay et al20 used a computer model to simulate the cementation of full crowns and test a number of variables relating to die spacing, cement setting rate and marginal form. They stated that in the presence of 15 microns axial and occlusal relief, the 90-degree shoulder preparation was vastly superior to the shoulder and bevel preparation.

The reverse was true without die relief.Omar21 investigated the effect of various margi­ nal configurations on fit, in vitro, using scanning electron microscopy. Under controlled laboratory conditions he showed that there was a significant difference between the mean marginal openings of a shoulder-bevel ceramometal crown [mean mar­ ginal opening = 23 ± 7 um] and the porcelain jacket crown [mean marginal opening =76 ± 21um]. There was a less significant difference between the mean marginal openings of a shoul­ der-bevel ceramometal crowns [mean marginal opening = 23 ± 7u] and the facially butted porcelain ceramometal crown [mean marginal opening = 33 ± 8 um].

Belser et al22 in a clinical study measured the marginal openings of three types of ceramometal crowns. They found no significant difference between bevelled metal margins, metal butt mar­ gins or porcelain butt margins before or after cementation.

El-Ebrashi et al23 reported that the rounded shoulder produced the least stress concentration of seven designs tested by loading on the occlusal surface. Chamfer finish lines are preferred for posterior veneer preparations. This finish line has a more acute angle at the margin and has been shown experimentally to exhibit the least stress, thereby protecting the underlying cement from adverse stresses.24

In the past, the gingival sulcus was considered to be a sterile area and the concept of "extension for prevention" was in vogue for restoration margins. Loe showed that the placement of margins below the gingival crest was a major contributory factor in the progression of periodontal disease.25

Plaque retention has been shown to increase with restorations with inadequate marginal adapta­ tion. An adverse periodontal reaction is created when such margins are placed near or within the gingival sulcus.26-29 The intensity of the inflammatory response increases around crowns with sub­ gingival margins when compared to those with supragingival margins.28, 30-37 Conventional
hygiene aids do not effectively remove plaque from within the sulcus.38

Richter and Ueno39 conducted a clinical study with 12 crowns designed to have subgingival and supragingival margins on the facial surface. They found no difference in [1] the health of the gingiva, [2] change in sulcus depth, [3] gingival contour, and [4] plaque accumulation up to a period of three years. They suggested that the fit and finish of the full crown restorations may be more significant to gingival health than the location of the finish line. In spite of this, they have suggested supragingival placement of margins for the following reasons: [1 ] crowns in this study had excellent margins and contours which is quite different from clinical observations of many cast gold crowns; [2] though no statistical differences were found according to the specific criteria evaluated, slight differences were noted invariably in favour of the supragingival placement; [3] clinical service beyond three years could show significant differences.

Therefore, according to contemporary thinking, it is important to keep crown margins supragingival or, at least, at the crest of the gingiva whenever possible.27, 28, 31, 32, 36, 40, 41.Clinicians often feel it is necessary to place crown margins subgingivally due to [1] esthetic require­ ments, [2] old subgingival restorations, [3] caries, abrasion or erosion, and [4] short clinical crowns. When placing margins subgingivally several authors suggest protection of the "biologic width" from mechanical trauma. The term "biologic width" represents the attachment complex and has a length of 2 or 3 mm.41-46 Some authors recommend placing the margin to the base of the sul­
cus.42, 45 Others suggest placement of the margin 0.5 mm44 and 1 mm above the base of the sul­
cus43, 46

Newcomb28 conducted a clinical study to evaluate the effect of various levels of subgingival placement of margins on gingival inflammation [0.25 mm, 0.5 mm, 0.75 mm and 0.1 mm from the base]. He showed that the nearer a subgingival margin approaches the base of the gingival crevice, the more likely it is that severe gingival inflamma­ tion will occur. The least inflammation was observed when subgingival margins are placed at the gingival crest or just into the gingival crevice.

Block47 stated that it is very difficult to locate reference points, such as the alveolar crest, base of the sulcus or histologic width, accurately from a clinical standpoint. The dimension of the junctional

epithelium could be as long as 5 mm or more in length. If the guidelines of "biologic width" are used, then the restoration could unintentionally be placed within the attachment. Thus, such assump­ tions could be destructive to the health of the tis­ sues. An alternative term to the "biologic width", the "sub-crevicular attachment complex" has been suggested.47 He recommends that margins should be placed no more than 0.5 mm into the sulcus. If the sulcus depth is 1 mm or less, then the margin should terminate at or above the gingival margin.

Surgical lengthening of the clinical crown has been suggested as a way to place margins supragin-givally in cases of short clinical crowns, caries, restorations, and high alveolar crest level.47-49 This procedure involves the reflection of a mucogingival flap and, possibly, the removal of bone to ensure that there is a distance of 3-4 mm between the crest of the bone and the margin of the preparation. Post-operatively, as long as 20 weeks may be required for the establishment of a stable gingival margin at the new location.50 Surgical lengthening is obviously not appropriate in teeth where the sup­ port would be reduced to such a degree which would affect the stability of the teeth, as in the molar regions.51

Finally, considering the biocompatibility of the materials used for the restoration, the customary gold, acrylic, and porcelain could cause adverse tissue reactions [inflammatory response] especially when the surfaces are not polished and rough.52, 53

When comparing gold alloy restorations with four alternative alloy systems, as long as the margi­ nal adaptation is maintained, the periodontal status was favourable.54-56 However, a report of rapid bone loss adjacent to crown margins of non-precious alloys in two patients, related to nickel hyper­ sensitivity, has recently appeared.57 Therefore, this may be considered when one uses non-precious alloys.

Glass ceramic restorations showed markedly reduced bacterial colonization compared with nat­ ural contralateral teeth [p<.001 ].58

Summary and Conclusions

From the review of the literature it is quite clear that crown margins must be kept supragingival or at least at the crest of the marginal gingiva; subgingi­ val margins, if placed, should have excellent margi-

nal integrity for maintaining periodontal health.

With the development of newer materials, the biocompatibiiityofthe material of such restorations must be tested prior to clinical application.

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Address reprint requests to:
Dr. R. Omar,
Department of Restorative Dental Sciences,
P.O. Box 60169, Riyadh 11545, Saudi Arabia.