Internal venting of castings to improve marginal seal and retention of castings

BALSHI

tapering toward the lingual aspect and allow appropriate oral hygiene. When the form and surface of the pontic are properly prepared, the pateint is instructed to use interproximal threading devices and dental floss to maintain a plaquefree pontic surface. Patients should also be instructed to carefully cleanse the proximal surface of the adjacent abutment teeth. When the patient has a high lip line, connectors may be extended cervically to esthetically compensate for the slight amount of loss in the vertical height of the interdental papilla. Care must be taken so that these, connectors do not impinge on any part of the remaining papilla. RADIOGRAPHIC FOLLOW-UP CLINICAL EVALUATION

AND

Periodic postoperative radiographic and clinical evaluations should be made to monitor both the abutment teeth and the implanted durapatite crystals (Fig. 12). Patients should be reevaluated periodically. A 2!4year follow-up of nine patients showed no clinical or radiographic change in pontic-residual ridge relationship. SUMMARY A method of preventive ridge augmentation using durapatite granules to prevent residual ridge collapse in the region of fixed partial denture pontics has been described.

I thank the dental technicians at the Fort Washington Dental Laboratory Inc. for their attention to detail in producing the ovoid pontic designed prostheses and to Mr. Ken Orth, C.D.T., for his photographic assistance.

REFERENCES Takei HH. The interdental space. Dent Clin North Am 1980;24: 169-76. 2. Yuodelis RA, Smith DH. Correction of periodontal abnormalities as a preliminary phase of oral rehabilitation. Dent Clin North Am 1976;20:181-97. 3. Langer B, Calagna L. The subepithelial connective tissue graft. A new approach to the enhancement of anterior cosmetics. Int J Periodont Res Dent 1982;2:23-33. 4. Kay HB. Esthetic considerations in the definitive periodontal prosthetic management of the maxillary anterior segment. Int J Periodont Res Dent 1982;3:45-59. 5. Langer B, Calagna L. The subepithelial connective tissue graft. J PROSTHETDENT 1980:44:363-7. 6. Block MS, Kent JN. A comparison of particulate and solid root forms of hydroxylapatite in dog extraction sites. J Oral Maxillofat Surg 1986;44:89-93. 7. Kent JN, Quinn JH, Zide MF. Correction of alveolar ridge deficiencies with nonresorbable hydroxylapatite. J Am Dent Assoc 1982;105:993-1001. 8. Garver DG, Fenster DK. Vital root retention in humans: a final report. J PROSTHETDENT 1980;43:368-73. 9. Stein RS. Pontic-residual ridge relationship: a research report. J PROSTHETDENT 1966;16:251-85. 1.

Reprint requests to: DR. THOMAS J. BALSHI INSTITUTE FOR FACIAL ESTHETICS 467 PENNSYLVANIAAVE. FORT WASHINGTON.PA 19034

Internal venting of castings to improve seal and retention of castings K. J. Bruggers, D.D.S.,*

and H. Bruggers,

University Dentistry,

School of Dentistry,

of North Carolina, New Orleans, La.

D.D.S., M.A.** Chapel

Hill,

lhe integrity of margins of castings and the ability of cements to seal the margin and retain the casting have been investigated since the advent of dental castings. The development of ceramometal crowns with margins that cannot be burnished has renewed interest in the study of marginal integrity. *Graduate Student, Fixed Prosthodontics, University of North Carolina, School of Dentistry. **Associate Dean and Professor, Department of Fixed Prosthodontics, Louisiana State University, School of Dentistry. 270

marginal

N.C.,

and Louisiana

State University,

School of

Jorgensen,’ in studying the film thickness of zinc phosphate cement, postulated that hydraulic pressure develops in the occlusal portion of castings during cementation. This pressure prevents full seating of the casting and created a marginal discrepancy. He found that as pressure was exerted on dental cements a separation of particles from the liquid occurred. The particles tended to accumulate in various regions on the preparation interface, increasing film thickness, especially on the occlusal surface.* Methods advocated to overcome this problem generalSEPTEMBER

1987

VOLUME

58

NUMBER

3

INTERNAL

VENTING

OF CASTINGS

Fig. 1. Additional

wax on line angles.

ly fall into three categories: die relief, stripping, and venting. Fusa yama et a1.3,4used various means of die relief to allow cement to flow more readily, allowing the casting to seat more fully. Research concerning die relief confirmed Fusayama’s contention.5-7 Better control of the amount of relief has been attempted through stripping.‘-” External venting of the crown has been advocated as another means of further seating a casting by reducing hydrostatic pressure near the occlusal surface of the preparation.7z” Hoard et al.” found that venting is successful because it affects the flow of cement, limits the stacking of particles, and relieves small internal pressures. Venting is impractical for most ceramometal crowns because the vent interrupts the integrity of the compressive ceramic covering. Two methods of internal venting that eliminate the need for crown perforation were introduced. Brose et al.13 suggested using orthodontic wire during waxing to develop escape channels in the wax pattern. To remove the wires before casting, it was necessary for the wires to cross the margins, leaving an opening that had to be closed with cement. Tjan et a1.‘4,‘5placed a groove on the axial wall of the preparation after the impression was made but before cementation. This approach raises the questions of pulpal integrity in teeth with large pulps. This study d.etermined that porcelain-fused-to-metal (PFM) crowns internally vented by laboratory methods could maintain marginal integrity and retention. MATERIAL

AND

METHODS

Fifty standardized Ivorine PFM crown preparations (Columbia Dentoform Corp., New York, N.Y.) were used to provide standardization of margin configuration, axial wail length, and taper. Fifty copings were constructed by using two layers of die spacing material (Tru-Fit, George Taub Products, Jersey City, N.J.). Thirty copings were designed so that the castings could THE JOURNAL

OF PROSTHETIC

DENTISTRY

Fig. 2. Relief of castings for vents.

be relieved at the line angles and at the middle of the buccal and lingual walls. The relief was developed by using a No. 1 PKT waxing instrument (Hu-Friedy Mfg. Co., Chicago, Ill.) to apply inlay wax on the occlusal line angles and centered on the external buccal and lingual walls after the coping was waxed to proper thickness (Dr. Peck’s hard purple inlay wax, A.E. Peck Mfg. Co., Costa Mesa, Calif.) (Fig. 1). After waxing was completed, 20 copings were randomly selected for retention testing (10 prepared for venting and 10 with no relief). These 20 copings had a T-bar attached in the central fossa to facilitate testing on the Instron machine (TMS 1102, Instron Corp., Canton, Mass.). All margins were inspected under a stereo microscope (E. Leitz, Inc., Rockleigh, N.J.) before investment (Complete, J.F. Jelenko Co., Armonk, N.Y.). The copings were cast in ceramic alloy (Olympia, J.F. Jelenko Co.) using standard casting techniques. After divestment, the castings were seated on their respective dies. The stereomicroscope was used with a disclosing material (G.C. Fitchecker, G.C. International Corp., Scottsdale, Ariz.) to remove irregularities on the fit surface and to finish the margins on the casting. All of the copings prepared for relief were ground internally, where the extra external wax had been placed, with a No. 1 round bur. A metal thickness of 0.5 mm was attained (Fig. 2), matching the metal thickness of the remainder of the coping where the porcelain was to be applied. Porcelain (Vita VMK-68, Vita Zahnfabrik, Sackingen, West Germany) was applied in the following manner. A wash coat of opaque was followed by a second opaque bake. Body porcelain was applied in two bakes. Final contour was attained and the crowns were glazed. The firing schedule and temperatures formulated by the manufacturer were followed. The glazed crowns were placed on the dies and final marginal finish was attained. 271

BRUGGERS AND BRUGGERS

Table I. Marginal Method Vented Nonvented

discrepancy

data

No. of samples

Range

16 8

Table II. Retention Mean

data

(pm)

(rtm)

Method

No. of samples

Failure force range (kg)

Mean (kg)

16-25 10-80

16.25 39.38

Vented Nonvented

10 10

21.0-51.0 18.0-58.5

37.3 33.5

Pooled variance = 216.676137; SD = 6.37391369; degrees of freedom = 22; 1 = 3.62806921; p = 1.8 X lo-‘. Critical region 1 = 3.1188 @ p 5 .005.

Pooled variance = 113.394445; SD = 4.76223571; degrees of freedom = 18; t = .797944544; p = .5592. Critical region t = 2.1009 @ p 2 0.05.

The glazed crowns were cemented to their respective dies with a standard mix of zinc phosphate cement (Flecks Zn Cements, Mizzy Inc.) mixed on a cooled glass slab by using the technique described by Shillingburg et a1.16A small increment of cement was spatulated for 20 seconds on a wide area of the slab. This portion was allowed to stand for 1 minute. The remaining powder was added in small increments until proper consistency was reached. Finger pressure was used to initially seat the crowns, which were then placed under a static load of 12 lbs for 10 minutes. Six crowns tilted badly during cementation and were eliminated from the study. Forty-four samples were usable, 20 of which were used in retention testing. The remaining 24 samples, eight without internal venting and 16 with internal venting, were embedded in acrylic resin and sectioned by using an ultrafine diamond disk in the following distribution: 4 Unvented sectioned mesiodistal 4 Unvented sectioned faciolingual 8 Vented sectioned mesiodistal 8 Vented sectioned faciolingual These sections were polished on a grinder with aluminum oxide grit (200 to 600) and examined under the stereo microscope at x40 magnification. Film thickness was measured on the occlusal table and at the margins. The remaining 20 crowns, 10 vented and 10 unvented, were separated from their respective dies, and an Instron testing machine with a crosshead speed of 5 mm/90 seconds was ,used to determine their retentive strengths.

veneer cast crowns. Because external occlusal or lingual venting cannot be used in most ceramometal crowns, some type of internal relief has to be initiated. Die spacing will have a variable effect because the thickness of the die spacer is dependent on the type and age of the material used. Internal venting can negate this factor because the size of the channel can be predetermined. This study indicated that the wax pattern can be modified so that the casting can easily be relieved, allowing more apical seating during cementation procedures. The procedure allows for a more conservative preparation and less chair time.

RESULTS Marginal discrepancy data are shown in Table I. A Student t-test demonstrated significant reduction in the thickness of the cement at the margin. A Student t-test was also done on the retention data (Table II). No significant loss of retention was demonstrated in the vented crowns. DISCUSSION Relief channels, venting, and die spacing have proved to be effective in improving the seating of complete 272

CONCLUSIONS Under the conditions of this study, the following conclusions can be made: 1. These castings, when cemented, exhibited a superior marginal fit compared with nonvented castings. 2. No significant retention was lost in using this procedure. We express our appreciation to J. F. Jelenko and Co. for supplying alloy.

REFERENCES 1. Jorgensen KD. Factors affecting the film thickness of zinc phosphate cements. Acta Odontol Stand 1960;18:479-90. 2. Jorgensen KD. Structure of the film of zinc phosphate cements. Acta Odontol Stand 1960;18:491-501. 3. Fusayama T, Ide K, Hosoda H. Relief of resistance of cement of full cast crowns. J PROSTHET DENT 1964;14:95-106. 4. Fusayama T, Ide K, Kurosu A, Hosoda H. Cement thickness between cast restorations and preparation walls. J PROSTHET DENT 1963;13:354-64.

5. 6.

Dodge WW, Weed RM, Baez RJ. The effect of die spacer on casting fit [Abstract]. J Dent Res 1982;62:323. Campagni WV, Preston JD, Reisbick MH. Measurement of paint-on die spacers used for casting relief. J PROSTHET DENT 1982;47:606-11.

7.

8. 9. 10.

Eames WB, O’Neal SJ, Montiero J, Miller C, Roan JD, Cohen KS. Techniques to improve the seating of castings. J Am Dent Assoc 1978;96:432-7. Lorencki SF. A rationale for electro-deplating a cast restoration. Dent Digest 1968;74:249-51. Pate1 MG. The effect of electrochemical milling on margins of MOD inlays. J PROSTHET DENT 1973;30:66-73. Bassett RW, Stauts BM: Evaluation of electro chemical milling (stripping) versus etching with aqua regia. J S Calif Dental Assoc 1966;34:478-83. SEPTEMBER

1987

VOLUME

58

NUMBER 3

INTERNAL

11.

VENTING

OF CASTINGS

Cooper TM, Christensen GJ, Laswell HR, Baxter R. Effect of venting on cast gold full crowns. J PROSTHET DENT 1971;26:621-

DENT 1984;52:50-6.

6.

12.

Hoard RJ, Caputo AA, Contino RM, Koenig ME. Intracoronal pressure during crown cementation. J PROSTHET DENT 1978; 40:520-5. 13. Brose MO, Woelfel JB, Rieger MR, Tanquist DA. Internal channel vents for posterior complete crowns. J PROSTHET DENT 1984;51:755..60. 14.

15. Tjan AHL, Sarkissian R. An internal escape channel: a practical alternative to occlusal venting in full cast crowns. J PROSTHET

Tjan AHL, Sarkissian R, Miller GD. Effect of multiple axial grooves on t.le marginal adaptation of full cast-gold crowns. J

16. Shillingburg HT Jr, Hobo S, Whitsett LD. Fundamentals of fixed prosthodontics. Berlin: Quintessence, 1976;285. Reprmt requests to: DR. HOWARD BRUGGERS LOUISIANA STATE UNIVERSITY SCHOOL OF DENTISTRY NEW ORLEANS, LA 70119

PROSTHET DENT 1981;46:399-403.

Microleakage pattern of a resin-veneered glass-ionomer cavity liner Gary A. Grim, D.M.D., University

of Lcuisville,

M.S.D.,*

and Jeffrey

School of Dentistry,

Louisville,

S. Shay, D.M.D.* Ky.

lhe use of glass-ionomer cement as a cavity liner under composite restorations has generated considerable attention. Some advantages are chemical bonding to enamel and dentin, fluoride release, and protection of dentinal tubules from resin monomer. In addition, glass-ionomer cements can be acid-etched similar to enamel, aiding the retention of composite veneers. One initial report describing glass; ionomers as cavity liners suggested that this procedure was superior to a resin/dentin bond to avert marginal leakage under a composite.’ The mechanical properties of one liner, G-C lining cement (G-C International Corp., Scottsdale, Ariz.) were reported to be comparable to zinc phosphate and polycarboxylate cements.’ Nevertheless, Fuks et al.” examined the marginal adaptation of two glass-ionomer cements in class II cavities and discovered that many specimens exhilbited minute gaps in the tooth/cement interface. In a fstudy involving microleakage of class V restorations, le,akage was observed in glass-ionomer cements without varnish protection and some leakage occurred with the varnish.4 The effect of a glass-ionomer liner on microleakage was reported in a recent study of class V cavities5 The liner was unable to prevent gross leakage in dentin when Enamel bond (3M Co., St. Paul, Minn.) was placed over the liner before restoration with Silux material (3M Co.). The location of the liner was not specified and the

*Associate Professor, Department of Restorative Dentistry THE JOURNAL

OF PROSTHETIC

DENTISTRY

leakage associated with the combination of glass-ionomer liner, dentin bonding agent, and restorative resin was not reported. This in vitro study tested the efficacy of a resinveneered glass-ionomer liner in controlling microleakage in class V restorations. MATERIAL

AND METHODS

A new glass-ionomer liner, Ketac-Bond (ESPEPremier, Norristown, Pa.), was selected for this study. Two composite restorative systems were chosen: a conventional resin bonding agent (Visio-Fil with VisioBond, ESPE-Premier) and a dentin bonding agent (Silux with Scotchbond, 3M Co.). The study was performed on extracted human molar teeth that were hand scaled, cleaned with a slurry of flour of pumice, and stored in tap water at room temperature. Class V cavities were prepared in the cervical third of the lingual and facial surfaces so that the gingival margin extended immediately below the cementoenamel junction. The cavities were prepared with new No. 170 tungsten carbide burs in a high-speed handpiece with a water spray. The preparations were as uniform as possible with the axial wall at a depth of approximately 1 mm at the gingival floor. A short enamel bevel was made with the No. 170 bur. The cavities were washed with a spray of water for 10 seconds and dried with oil-free compressed air. Ten teeth randomly selected were restored by using a glass-ionomer liner as described by McLean et al.’ The 273