Marginal fit of gold inlay castings

OPERATIVE

MARGINAL

DENTZSTRY

FIT

OF GOLD

INLAY

CASTINGS

GORDON J. CHRISTENSEN, D.D.S.,M.S.D.* University

of Washington,

School of Dentistry,

Seattle, Wash.

1.i

70R MANY YEARS the gold inlay has been regarded by dentists as one of the finest

restorations for treating moderate to large carious lesions. However, it has also been accepted that a marginal opening in gold inlays is unavoidable because of the fi‘!m thickness of the cementing medium. Dentists who are unaware of the technique for finishing inlays condemn them because of the solubility of the cementing medium. The purposes of the study were : (1) to measure microscopically the marginal opening, termed clinically acceptable by ten experienced restorative dentists, (2) to determine the ability of these ten operators to evaluate the marginal adaptation of vjsually accessible and inaccessible gold inlay margins by explorer, and (3) to determine if gold inlay margins may be closed past the film thickness of zinc phosphate cement by proper finishing and polishing. Although ADA standards indicate that the maximum film thickness of cement is 40 p,l several studies have consistently shown lesser values2-” Jorgensen2 stated that film thickness is related to the configuration, length, and taper of cavity walls as well as the viscosity of the cementing medium. Different methods of evaluating film thickness have shown varied values. Norman, Swartz, and Phillips4 showed marginal opening values as low as 3 p. Marginal cement thickness has been reported by Kurosu and Ide5 to be between 30 and 40 p on inlays and up to 90 p on crowns. Bjorndal and Sahs6 have reported that the gold cannot be burnished to cover the cement line in cast gold restorations. Most studies show marginal zinc phosphate cement thickness to be between 20 and 40 FL. MATERIALSAND

METHODS

Ten extracted human biscuspid teeth (Figs. 1 and 2) were mounted adjacent to one another in acrylic resin denture material to a simulated normal bone level. Soft denture-reline material was applied to the acrylic resin to simulate the normal gingival level. The teeth were mounted so that they could be taken out by removing a section of orthodontic wire which ran through a hole in the root of each tooth to hold it in position. The orthodontic wire was sealed in place during the Presented before International *Present affiliation, Chairman Kentucky, Lexington, Ky.

Association for Dental Research, in Toronto, Canada. and Assistant Professor of Operative Dentistry, University

of

CHRISTENSEN

298

Fig. I.-Ten human teeth mounted in acrylic resin and soft denture of the orthodontic wire allows removal of the teeth.

Fig. Z.-Occlusal

view of experimental

J. Pros. Den. March-April, 1966

reline material.

Removal

model.

experiment. Mesioocclusodistal inlay preparations which covered both buccal and lingual cusps were cut in each tooth. Direct wax patterns were carved, and type B gold alloy castings were made for each tooth, The marginal fit of the inlays was measured microscopically before finishing procedures. The ten castings were produced and eventually cemented by adhering to the following requirements and techniques : Wax Pattern and Casting.-An accurate wax pattern which showed no marginal discrepancy to the naked eye was mandatory. With the exception of a slight proximal excess for adequate contact areas, the occlusal and proximal surfaces had the exact contour and anatomy expected in the finished restoration. An investing and casting technique which insured undistorted, bubble-free, well-fitted castings was necessary. When an unpolished casting was tried on a tooth, and it showed an obvious marginal discrepancy, the casting was remade because attempts to burnish or “spin” gold into voids in excess of the slightest visually detectable space usually end in mutilation of the tooth and an unavoidable cement line in the final restoration. Finishing the Casting.-The occlusal and proximal surfaces of the castings stage prior to trying the inlay on the were finished and smoothed to the “tripoli” tooth. Finishing was accomplished to 1 mm. from the margins, since no burnishing or finishing should be attempted on margins before trying the casting on the tooth.

MARGINAL

FIT

OF GOLD INLAY

299

CASTINGS

BURNISHING

initial

Fig. 3.-Gentle burnishing of the margins seating of the inlay on the tooth.

prior

to finishing

procedures

and after

the

BURNISHING CONTACT

Fig. 4.-A sharp cleoid carver coronal groove areas.

is used to burnish

and smooth margins

in constricted

inter-

After adjusting contact areas and occlusion, placing the rubber dam, and seating the inlay firmly in place, marginal adaptation was carefully evaluated by sight and by using a sharp explorer. Finishing the Margins.----Slightly open margins were burnished with a suitable instrument (Fig. 3). Intercoronal inlays with occlusal margins in groove areas were burnished and smoothed using small, smooth, pointed stones and a small, sharp discoid-cleoid carver (Fig. 4). Abrasive discs, on a small-headed nonscrew type of mandrel* or a No. 303-g mandrel, were used to finish margins. Initial marginal smoothing was done with g inch extra-fine garnet discs at low speed. The disc must rotate from gold to tooth to effect marginal adaptation. An orange-wood peg (Figs. 5 and 6) or similar device on which the simulated patient closed was continually used during finishing *Sproule

mandrel,

right

angle,

Hepworth,

Vancouver,

British

Columbia.

300

CHRISTENSEN

MARGINAL AREAS TO BE FINISHED BY DISCS

Fig. shown

J.-Three-eights while the patient

Fig. 6.-An gingival margins.

inch garnet and cuttle discs are used occludes on a short orange-wood stick.

extra-narrow,

fine

cuttle

finishing

strip

to finish

is “pointed”

the margins

and

used

in the areas

to finish

the

procedures to keep the inlay fully seated. These G inch discs will finish the marginal areas as shown in Fig. 5. To smooth these areas, futher, two other types of lesser abrasive discs were used prior to cementation. These were coarse 3/s inch cuttle and medium cuttle discs. The gingival margins were finished at this stage. An 18 inch extra-narrow, fine-cuttle finishing strip cut to a point as shown in Fig. 6 was carefully introduced interproximally. A gentle stripping of this margin effectively reduced the degree of marginal opening in the gingival areas,

Volume 16 Number 2

MARGINAL

FIT

OF GOLD INLAY

301

CASTINGS

INLAY

Fig. 7.-With the margins finished to a medium cuttle an inlay holder, and the proximal surfaces are smoothed.

stage, the inlay

is held by hand in

The inlay was then removed from the tooth, held in an inlay holder* (Fig. 711,and the proximal surface was carefully disced with M inch fine cuttle discs to remove the roughness produced by the preceding manipulation (Fig. 8). The final smoothing and polishing of the proximal surfaces were accomplished with a small felt wheel and a suitable abrasive compound and polish. Cementing the Inlay.-The inlay was cemented and held in place by the simulated patient closing on the orange-wood peg. Immediate, gentle marginal hurnishing with a broad burnisher was applied to force marginal gold, which may be slightly displaced by hydraulic pressure, against the tooth structure. Slight discing, with fine g inch cuttle discs, was carried out on all accessible margins. The inlay was polished with a suitable moist polishing abrasive+ on a soft rubber cup as recommended by G. D. Stibbs.z A smoother surface may be attained by using one of several finer polishing abrasives3 on a soft rubber cup. Radiographs were taken of the completed inlays (Fig. 9). A list of instructions and a scoring form was duplicated for distribution to ten dentists who were on the faculty at the University of Washington in the Departments of Operative Dentistry and Fixed Partial Dentures. They were requested to evaluate the marginal adaptation of the ten inlays using an explorer and the pro*Crescent Dental Manufacturing Company. 18303 Centriforce abrasive, American Optical Company. tProfessor and Chairman, Department of Operative Dentistry Dentures, University of Washington, Seattle, Wash. IR309W Centriforce abrasive, American Optical Company.

and Professor

of Fixed

Partial

CHRISTENSEN

302

SLIGHTLY ROUGH AREAS TO BE SMOOTHED BEFORE CEMENTATION

Fig. 8 .-Areas

where additional

“out-of-the-mouth”

discing and polishing

is usually

necessary.

Fig. 9.- Roentgenograms of the ten completed test inlays. Although all preparations significant bevels, note the apparent blocking of the bevel by some of the castings.

had

MARGINAL

FIT

OF

GOLD

IiXLAY

CASTINGS

303

vided radiographs. Each inlay was divided into eight sections of margin. The scoring 01 the margins was to be done according to each dentist’s standard of clinical acceptability. A grade of 3 was assigned to clinically acceptable margins. A 1 or 2 score was assigned to poorer margins and a 4 or 5 score to better ones. A sample scoring section is shown in Table I. The ntarginal opening for each section was measured, microscopically. The measurements were reliable in increments of approximately 5 microns. The microscopic opening of margins and the scores assigned each section of margin were compiled on a master sheet. R:ESULTS

The 80 sections of margin were divided for evaluation purposes into 40 prosima1 and 20 accusal margins which were accessible by sight and by explorer, and 20 gingival margins which were accessible only by explorer and roentgenograms. ‘L’he relationship of the microscopic opening to the mean score assigned each section of margin by the ten dentists was studied by correlating the microscopic opening (abscissa) and the mean score (ordinate) in a linear regression. The three groups (occlusal, proximal, and gingival ) related as follows : pvobabilify of regressionby analysis of variance proximal p < .Ol (very significant relationship between scores assigned by op craters and actual microscopic marginal opening). occlusal p < .05 (significant relationship shown). gingival p > .05 (no relationship shown between the score assignedthe margins and the actual microscopicmarginal opening). Gingival margins with openings up to 119 ,LLwere accepted while proximal or occlusal margins with as little as 26 p opening were rejected. Occlusal margins afforded the greatest range of microscopic opening (2-5 1 I*). Therefore, using the linear regression prediction formula for the group (1~ = -0.022, y = 3.8654 0.022X), a mean score of X = 3, or least clinically acceptable margins, relates to a y value of 39 ,u. Therefore, the least acceptable occlusal margin was projected to be 39 p open. (Table II). Microscopic marginal openings were present on all margins prior to the beginning of finishing procedures. After proper discing and finishing on the teeth, there were sections of margin where no openittg could be detected using the optical arrangement utilized. Gingival margins where proper finishing is difficult had greater openings than easily accessible margins. To show, numerically, the degree of variation in the scoring of the three groups of margins, a coefficient of variation was computed for each group (Table III). There was a greater degree of scoring variation in the gingival areas. DISCUSSION

Considering that gold inlay cavity preparation, and preservation of the dental pulp are all carried remains the variable factor in the clinical success men who participated in this experiment, all of

restoration of tooth morphology, out properly, marginal adaptation or failure of gold inlays. The ten whom successfully use cast gold

304

CHRISTENSEN TABLE

BETTER ~___

MARGINAL SECTION

5

I WORSE CLINICALLY ACCEPTABLE

4

I

__

---

3

2

I

1

X

x

X

TABLE

II.

THE ACCEPTABLE

AREA

RANCE@CORE

N I

SCORE

EACH GROUP OF MARGINS

MEAN OPENING

RANGE

I

Gingival Proximal Occlusal

20 40 20

E

N

74 1

III.

MEAN SCORE

34-119 /.I 9-34 /.l 2- 51 /.l

26 P 21 LJ

3-4

TABLE

AREA

~)FOR

DEGREE

OF VARIATION

STANDARD

DEVIATION

COEFFICIENT

OF VARIATION

/

Gingival Proximal Occlusal

ii 20

3.40 4.00 3.30

1.01 0.74 .0.73

0.30 0.19 0.22

restorations routinely, have pointed out several important considerations in their accumulated clinical experience. During finishing procedures, an attempt should be made to have maximum visual access to gold inlay marginal areas. Extra-heavy dark-colored rubber dam retracts the interproximal gingiva markedly and provides a color contrast with the tooth. Normally extended gingival margins which are located under gingiva may be exposed to visual access in this way. The use of rubber dam clamps will further retract buccal and lingual tissue and stretch the interproximal rubber dam. This consequently retracts the interproximal gingival tissue to a greater degree if needed. Proper finishing of gold castings will minimize the degree of opening at margins. The cementing of inlays without proper discing and finishing on the tooth is inviting failure. Gold castings may be finished past the reported film thickness of zinc phosphate cement, if proper time and care are used in the finishing procedure. Those who disfavor the gold inlay should evaluate qualitatively the marginal adaptation of their finished gold inlays, and if it is found that this quality could be improved, a change in finishing technique is suggested.

MARGINAL

FIT

OF

GOLD

INLAY

CASTINGS

305

CONCLUSIONS

1. The least acceptable, visually accessible margin was computed from a linear regression prediction formula to be open 39 p. The range of opening of 40 barely clinically acceptable margins was from 2 to 51 p. 2. There is a direct relationship between the microscopic opening of visually accessible occlusal and proximal gold inlay margins and the experienced restorative dentists’ ability to evaluate this opening clinically by sight and explorer. 3. These ten restorative dentists were not able to evaluate consistently the marginal opening of gingival areas by explorer or roentgenogram. They disagreed n:.ore on the clinical acceptability of gingival margins than on any other marginal area. 3. Margins may be closed past the reported film thickness of zinc phosphate cements if proper finishing techniques are used. 5. Explorer examination of visually accessible gold inlay margins is superior to, and more reliable than, explorer or roentgenographic examination of visually inaccessible margins. REFERENCES

1. American Dental Association: Guide to Dental Materials, ed. 2, Chicago, 1964-1965,American Dental Association, p. 104. 2. Jorgensen, I(. D.: Factors Affecting the Film Thickness of Zinc Phosphate Cements, Acta

odont. scandinav. 18:479-490, 1960. 3. Fusayama, T., and Iwamoto, T.: Relationship Between Retaining Force of Inlays and Film Thickness of Zinc Oxyphosphate Cement, J. D. Res. 39:756, 1960. (Abst.). 4. Norman, R. D., Swartz, M. L., and Phillips, R. W.: Studies on Film Thickness, Solubility and Marginal Leakage of Dental Cements, J. D. Res. 42:950-958, 1963. 5. Kurosu, A. and Ide, K.: Cement Thickness Between Cast Restorations and Preparation Walls, Bull. Tokyo M. & D. Univ. 8:337-338, 1961. (Abs). 6. Bjorndal, A. M., and Sahs, E. A.: Comparative Microphotographic Study of Marginal -4daptation of Amalgam and Gold Inlay Restoration, Iowa D. J. 46:1’2-14, 1960. COLLEGE OF DENTISTRY UNIVERSITY OF KENTUCKY LEXINGTON, KENTUCKY 40506