Analysis of the geometry of finishing crown restorations

lines for full

David F. Pascoe, B.D.S., D.D.S., M.S.D.* State

University

of New

York,

Health

Sciences

Center,

School

I

n 1963 Rosner’ published the concept of reducing the marginal opening of castings by the use of a beveled finishing line. Since that time the concept has been quoted repeatedly in the literature as an authoritative reference.2-‘o It has become, in effect, a widely advocated panacea compensating for all casting ills. A trigonometric analysis of the casting to-margin relationship is presented here, along with a report of an investigation to determine the validity of some of the theoretical conclusions reached.

THEORETICAL ANALYSIS In Rosner’s article’ it was reported that the marginal opening of a vertically displaced casting (d) is related to the displacement (D) by the formula: d = D * cos a

(0

where (Y = the angle of bevel (Fig. 1). From Fig. 1 it can be seenthat this theorem is valid only when the following are true: (1) the initial adaptation of the casting is exact, and (2) there is a purely vertical displacement of the casting such that point Y, is directly above Y,. This situation can arise as the result of (1) the cement film thickness, or (2) a defect on the internal surface of the casting. If the distance between the axial wall of the tooth and casting is designated j it is then possible to derive the following equations: D-!!l-

(ii)

cos 0

But since, from (i), D = f*cosa *Assistant

Professor,

0022.3913/78/0240-0157$00.60/09

L cosa

f- cosa

=d*cosO,ord= Department



(iii)

cos 0 of Restorative

1978

The

C. V.

Dentistry.

Mosby

Co.

of Dental

Medicine,

Stony

Brook,

N. Y

Thus a direct relationship can be shown between the internal discrepancy and marginal opening. Previous articIes”-‘J have reported that 10 degrees is the maximum acceptable taper. Must recommend marginal bevel of approximately 45 degrees’,2. ‘, ‘(.h and no more than a 40 /J marginal opening.*” If a cement meets ADA Specification No. 6 and has a film thickness of 25 ,u, then: F-25~

0 = 85 degrees a = 45 degrees And then, from equation (iii),

d = 25.~0~ 45 = 203 y cos a5 From this it can be seenthat the marginal opening would be 203 p. This is considerably more than the 40 /.Lthat has been advocated as acceptable.” It has been reported’,’ that even if a perfect extracoronal wax pattern were invested and cast with perfect compensatory expansion, the casting would have slightly smaller internal dimensions than the original tooth. Further, it is reasonable to suppose that in attempting to fabricate a casting with perfect fit, the errors in casting technique would result in a normal distribution producing an equal number of oversize and undersize castings.’ Therefore oversize and undersize castings need to be examined. It is assumedthat any change in dimensionsof the gingival surface of the casting is insignificant. From Fig. 2? showing an undersize restoration resting on the axial wall of the preparation, it can be seenthat: D = tan 0.g (iv) Moreover, D, = tan (Y*g Cv) D, = (tan 0 - tan a}g (vi) = (iv) - (v) d = cos a * D, (vii)

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DILA a

Fig. 1. An exactly fitting casting which is displaced vertically along the long axis of the preparation. By substitution of (vi) in (vii), d=cosa*g(tanO--tana)

Fig. 2. An undersize of the preparation. (viii)

Assuming there is only 0.2% inaccuracy in the compensatory expansion on a 10 mm diameter casting, then g will be 10 p on each side. Using the previous values for (Y and 8 and equation (viii), then: d = cos 45 * 10 (tan 85 - tan 45) = 74 p From this it is apparent that the marginal opening of an undersize casting is unsatisfactory even prior to cementation. It has also been suggested”’ that a modified or oversize casting will allow a sufficient space for cement film thickness on the axial walls of the preparation and enable the margins to be brought into close approximation. Fig. 3 depicts an oversize casting resting on the shoulder of the preparation. Under these cohditions D is independent of h and will be determined by the film thickness of the cement. However,

158

casting is resting on the axial surface

d=-------

Assuming

h * sin (Y sin 0

ct = 45 degrees and 8 - 85 degreies, then

d=

hmsin45 =h.O71 sin 85

A beveled margin will open proportionately to the oversize of the casting, but the shoulder will not vary. In order to predict the opening following cementation it is necessary to combine the derived figures: (iii): Beveled margin of a perfectly fitting cemented casting = 203 p (iii) + (viii): Beveled margin of an undersize cemented casting = 203 + 74 = 277 p ADA specification: Shoulder margin of an oversize cemented casting = 25 p

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1978

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GEOMETRY

OF FINISHING

LINES

THEORETICAL CONSIDERATIONS 1. Asgar’l has shown the necessity of fabricating slightly oversize castings to compensate for investment roughness. 2. An allowance must be made for the cement film thickness if the casting is to be fully seated (iii). 3. If an error is to be made, then it is preferable to have a casting too large rather than too small, which would inherently produce a large marginal opening (viii).

THEORETICAL CONCLUSION Since an oversize casting is indicated to satisfy all three considerations, it is logical to assume that minimal marginal opening will be exhibited by the shoulder configuration.

INVESTIGATION OF BEVELED AND SHOULDER MARGINS Method Fifteen identical stone dies* were poured in a silicone mold of a typical tooth preparation with a shoulder on one side and a beveled shoulder on the other. The dies were divided into three groups of five and individually identified. A silicone mold was constructed to fcc’iitate the construction of wax patterns that were as identical as possible. The dies were lubricated atid inserted into the strengthened mold, and wax was injected around them one at a time. The pattern and the die were removed together from the mold, and the margins were rewaxed and trimmed. The patterns were invested in three groups. Previous experience with the investment usedt resulted in the liquid/powder ratios selected. Group A was invested with 14 cc of special liquid for each 1 gm package of investment to produce undersize castings. Group B was invested with 9 cc of liquid to produce oversize castings. Group C, used to provide a control group, was invested with 11 cc, the quantity routinely used to produce clinically acceptable castings in the laboratory. The investment molds were burned out at the same time in an oven at 1200” F, and the castings were made with new Type III gold alloy.$ The castings were recovered, pickled, and exam*Silky Rock, Whip-Mix Corp., Louisville, Ky. tceramigold, Whip-Mix Corp., Louisville, Ky. $Harmony Hard, Williams Cold Refining Co., Inc., Buffalo, N. Y.

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Fig. 3. An oversize casting is resting on the shoulder of the preparation. ined under a stereomicroscope for nodules or other irregularities which might prevent complete seating on the dies. The dies were soaked in plastic polymer* for 8 hours and then placed in aluminum foil containers. Each casting was filled with activated pafymer and gently placed on its die. The casting and die were then placed in a jig which applied !%I0gm of force to seat the casting on its die. The assemblies were placed in an incubating oven and allowed to cure without movement for 12 hours. The incubator was then turned on and curing completed at 5.5” C for 6 hours. The embedded die-casting assemblies were sectioned 0.5 mm on either side of the buccolingual centerline using a hard-tissue sectioning machine *Bioplastic, Wards Natural N. Y.

Science Establishment,

Rochester.

PASCDE

Table I. Summary of casting-to-die measurement (in microns) Shoulder 1

Type Undersize (Group A) Mean S.D. Oversize (Group B) Mean S.D. Normal (Group C) Mean S.D. *See Results

Bevel

2 399.5" 119i3*

500.3 66.4 7.8 7.5

31.2 20.6

106.3 106.8

63.7 40-9

section.

Table II. Statistical data Groups tested B, vs. B, 8, vs. C, 8, vs. C2

Fig. 4. The locations of the measurements of the

sectionedcastingssummarizedin Table I are identified by the numberedpositions. which cut a slot approximately 0.75 mm wide.* The four surfacesof the sectioned assembly were finished under running water using progressively finer grades of abrasive paper. Measurements of the casting-todie distance were made at the points indicated in Fig. 4 using a micrometer slide microscope. The hillvalley technique advocated by Smith’? was used to derive an average casting-to-die distance. Three measurementswere made for each reading used in the calculations. Results The overall pattern of the results (Table I) corresponds exactly with the theoretical prediction. A slightly undersize casting produced a very large vertical displacement so that the bevel did not overlap, resulting in a large marginal opening. Consequently, the distance from the tip of the casting to the closest part of the die was entered in column 2 of Table I. Some of the control castings were excellent, but even a small variation from oversize produced marked marginal openings (Fig. 2). Under these conditions, as predicted by Rosner,’ the beveled ‘Bronwill

160

Scientific,

Rochester,

N. Y.

Test Paired t Unpaired Unpaired

Observed t value

t&5

cunciuslun

4.99 4.12 6.02

2.09 2.02 2.02

S&&cant Significant Significant

f f

Note: Group A is not included obtained are not directly comparable C. (See Results section)

because the measurements to those from groups B and

margin resulted in a smaller marginal opening than did the shoulder. The oversize castings, as predicted from Fig. 3, resulted in the shoulder margin presenting the smallest opening. The statistical analysis summarized in Table II confirms that at the p = .Ol level the shoulder opening of these castings was significantly smaller than in the undersize castings, the conventional control castings, and the oversize castings with a beveled margin. This can be seen in Figs. 5 and 6. Discussion The large standard deviation of those castings attempting to fit inclined planes is indicative of the clinical problem. The numerical values derived from equations (iii) and (viii) combined indicate the extent of marginal openings which are found as a direct result of the clinical techniques used. Many articles have demonstrated the value of venting as a method of decreasing the cement film thickness beneath cemented restorations. Jorgensensx has also shown the problems associatedwith asymmetrical seating of crowns as a result of filtration phenomena. Another set of measurementswere made on the axial walls of

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Fig, 5. The photomicrographs illustrate the typical situation found with an undersize A . 1AS houlder finishing line. B, The corresponding beveled shoulder.

ca.sting.

Fig. 6. The photomicrographs illustrate the typical marginal openings found on a sli ghtly ovel -siz e casting. A, The shoulder finishing line. B, The corresponding beveled shoulde ‘r.

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the preparations and analyzed, which ensured that no significant tipping of the castings was caused by the polymer in this investigation. The limited applicability of Rosner’s concept has been demonstrated. A small degree of variability is inherent in even extremely well-controlled casting techniques.” It is therefore important to err in the direction of oversize castings with shoulder margins if the number of unacceptable marginal openings is to be minimized.

7. 8. 9.

10.

11.

CONCLUSIONS The geometry exhibiting the least marginal discrepancy in this investigation was the shoulder of the slightly oversize casting (Fig. 4). This was significantly better (p = .05) than the oversize bevel, the control shoulder, or the control bevel.

12. 13. 14.

The author would tike to thank the Media Services, State University of New York at Stony Brook, for the illustrations.

15.

REFERENCES

16.

1. 2. 3. 4.

5. 6.

162

Rosner, D.: Function, placement, and reproduction of bevels for gold castings. J PROSTHET DENT 13:1161, 1963. Barnes, I. E.: The production of inlay cavity bevels. Br Dent J 137:379, 1974. Baum, L., and Klooster, J.: Advanced Restoration Dentistry. Philadelphia, 1973, W. B. Saunders Company, p 168. Eames, W. B., and Little, R. M.: Movement of gold at cavosurface margins with finishing instruments. J Am Dent ASSM: 75:151, 1967. Gilmore, H. W., and Lund, M. R.: Operative Dentistry, ed 2. St. Louis, 1973, The C. V. Mosby Company, p 531. Ingraham, R., Bassett, T. W., and Koser, J. R.: An Atlas of Cast Gold Procedures, ed 2. Orange, N.J., 1964, Uni-Tro College Press, p Il.

17.

18.

Nemetz, M: Tissue management in fixed prosthodonties. j PROSTHET DENT 31:632, 1974. Rosenthal, D.: Function, placement, and reproduction of bevels for gold castings. J PROSTHET DENT 13: 1161, 1963. Charbenau, G. T., Cartwright, C. B., Cornstock, F. W.. Kahler, F. W., Margeson, R. D., Snyder, D. T., and Dennison, J. B.: Principles and Practice of Operative Dentistry. Philadelphia, 1975, Lea & Febiger, Publishers. p 364. Sturdevant, S. M., et al: The Art and Science of Operative Dentistry. New York, 1968, McGraw-Hill Book Company, Inc., p 312. Eames, W. B., O’Neal, S. J., and Miller, L. B.: Variables in the seating of castings. J Dent Res 43, 1974. (Abstract NO. 545) Christensen, G. J.: Marginal fit of gold inlay castings. J PRQSTHET DENT 16:297, 1966. Asgar, K.: Casting Alloys in Dentistry. National Bureau of Standards, Special Publication No. 354, 1972, p 63. Suffert, L. W., and Mahler, D. B.: Reproducibility of gold castings made by present day dental casting techniques. .J Am Dent Assoc 50:1, 1966. Jorgensen, K. D.: The relationship between retention and convergence angle in cemented veneer crowns. Acta Odontot Stand 13:35, 1955. Krug, R. S., and Markley, M. R.: Cast restorations with gold foil like margins. J PROSTHET DENT 22~54, 1969. Smith, B. G.: The effect of surface roughness of prepared dentin on the retention of castings. J PROSTHET DENT 23~187, 1970. Jorgensen, K. D.: Factors affecting the film thickness of zinc phosphate cements. Acta Odontol Stand 18:479, 1960.

Reprint requests to: DR. DAVID F. PA~COE STATE HEALTH SCHOOL

UNIVERSITY

OF NEW

YORK

SCIENCES CENTER OF DENTAL MEDICINE

STONY BROOK,

N. Y. 11794

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1978

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Analysis of the geometry of finishing lines for full crown restorations.

Analysis of the geometry of finishing crown restorations lines for full David F. Pascoe, B.D.S., D.D.S., M.S.D.* State University of New York, H...
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