tion of the fracture compared with completely V8

resistance of a wrought cast post and cores

J. S. Ryther, DDS, MS,a J. M. Leary, DDS, MS,b S. A. Aquilino, and A. M. Diaz-Arnold, DDS, MSd The University of Iowa, College of Dentistry, Iowa City, Iowa

DDS,

post

MS,

The intraradicular fracture of a post and core presents a difficult and often unrestorable situation. Attempts to prevent this problem involved the use of materials with increased physical properties. The use of a preformed wrought post with a core cast to it may provide a more fracture-resistant foundation. This investigation compared a commercially available noble alloy-wrought post that bad a gold core cast to it with completely cast specimens from three different alloys. A standard pattern was designed to simulate a post and core that would be used in a tooth of limited length and canal space. Patterns were cast and fixed to custom holding devices. Each specimen was subjected to a shearing force until catastrophic failure in a universal testing machine. Significant differences were noted among the groups. (J PROSTHET DENT 1992;68:443-8.)

eeth critical to restorative treatment are frequently coronally deficient yet retained by a sound root. The restoration of these teeth with a casting after endodontic therapy is possible with a post and core foundation. Several studies have examined the use of a core cast to a base metal post. These studies revealed problems with corrosion and reorganization of the grain structure.lM3 Bergman et a1.4 demonstrated that when a highnoble alloy-wrought wire was incorporated into a specimen, the unit exhibited more favorable mechanical properties than did completely cast specimens. Removable partial denture frameworks have been cast to wrought wire clasps while successfully retaining the wrought grain structure of the clasp.5v 6 The length, surface configuration, and diameter of posts have been examined with regard to retention and stress distribution.7-‘2 However, no information could be found concerning the rigidity and fracture resistance of various post and core systems with a wrought alloy as a post. The use of a wrought post with a core cast to it may provide increased resistance to post fracture in certain situations. This investigation examined the fracture resistance of four material variations of a standardized post and core foundation (Table I). The purpose was to evaluate the fracture resistance of a wrought post with a core cast to it compared with conventionally cast posts and cores.

MATERIAL

AND

METHODS

The specimens used in this experiment were designed to approximate the size and shape of a post and core that

aclinical Assistant Professor, Department of Prosthodontics. bAssociate Professor, Department of Family Dentistry. CAssociate Professor, Department of Prosthodontics. dDepartment of Family Dentistry. 10/1/36725 THE

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Table I. Material cores Post

variations

of experimental Core

post and TYPO

Wrought No-Ox alloy Cast Gold

Cast gold

Cast to post

Cast gold

Cast silverpalladium Cast wrought alloy

Cast silverpalladium Cast wrought alloy

All cast post and core All cast post and core

All cast post and core

could be used in a tooth of a limited diameter and root length. The design of the experimental post and core was determined through evaluation of several studies (Fig. 1).7-‘3 A transition zone was designed to simulate the portion of the post and core that would contain the antirotational device and provide the additional bulk that is present in the clinical appiication of this custom technique (Fig. 1). Two pattern variations were fabricated: a completely cast pattern and a wrought post with a core cast to it. The completely cast specimens were made from a plastic pattern (Plastic Lab burnout post, Whaledent International, New York, N.Y.) of dimensions similar to those of the wrought alloy post. The plastic laboratory burnout post or the wrought alloy post was seated into a brass mold (Medical Engineering, University of Iowa, Iowa City) machined to standardize the post and core patterns (Fig. 2). Autopolymerizing methyl methacrylate resin (Duralay, Reliance Dental Manufacturing Co., Worth, Ill.) was used to develop the remainder of the patterns (Fig. 3). The specimens that were to have a gold core (Midas, Jelenko Dental Health Care Products, Armonk, N.Y.) cast to a wrought alloy post (Noble Alloy, Non-Oxidizing Vented 443

RYTHER

ET AL

CORE

TRANSITION ZONE

4mm L-J-4

POST ‘I’ lmm

Fig. 2. Brass mold block.

II. Manufacturer’s recommended burnout, and casting temperature

crucibles,

Table

h-

Alloy

lmm

Fig. 1. Experimental

post design.

Post, Whaledent) substituted this post for the plastic burnout post. Completed patterns were vacuum-invested with the specific investment (High Temp, Whip Mix Dental Manufacturing Co., Louisville, Ky.) recommended by the manufacturer of the wrought post. The casting rings, with the exception of the completely cast No-Ox alloy (Whaledent) were placed into a cold burnout oven (Accu-Therm 1000, Jelenko) programmed to rise to 950’ F at the rate of ZOO/ min. When that temperature was reached, a heat soak of 1 hour was allowed before casting. The patterns to be cast from the No-Ox alloy were placed in a cold burnout oven

444

Gold* SilverPalladium No-Ox

Investment burnout (” F)

950 950 1450

Alloy preheat (” F)

Alloy casting (” W

1400

1750

1800 2150

Carbon Quartz

1960

2360

Quartz

Crucible selection

programmed to rise to 1450’ F at 20”/min. When that temperature was reached, a l-hour heat soak followed. Four groups of 10 patterns were cast with three alloys (Table I): Group 1: A noble alloy wrought post with a core of gold cast to it Group 2: Completely cast post and cores cast from a gold alloy Group 3: Completely cast post and cores cast from a silver-palladium alloy (Albacast, Jelenko)

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Fig.

AND

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SYSTEMS

3. Duralay

application.

Fig,

Fig.

Fig.

4. Completed

casting.

Group 4: Completely cast post and cores cast from the same alloy (No-Ox) as the noble alloy-wrought post The specimens were cast in an induction casting machine (Autocast digital casting machine, Unitek Corp., Monrovia, Calif.) at the manufacturers’ recommended casting temperature for each alloy (Table II). The specimens were allowed to bench cool at room temperature and then devested. After devesting, all sprues were removed and the specimens trimmed flush with the occlusal surface of the core area (Fig. 4). Holding devices for specimen testing were made with autopolymerizing resin (Fastray, Harry J. Bosworth Co., Skokie, Ill.) in phenolic ring forms (Phenolic Rings, Buehler

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5. Creation

of transition

zone.

6. Specimen before testing.

Ltd., Lake Bluff, Ill.). A Para-Post drill (P-42-4, Whaledent) was used to place a 1 mm pilot hole entirely through each ring along its long axis. The same reamer (Medical Engineering, University of Iowa) used to prepare the core and transition zone areas of the brass mold block was used to prepare the transition zone in the resin-filled rings (Fig. 5). The reamer was mounted in a drill press and the 1.0 mm post space was used as a pilot. Each casting specimen was checked for a passive fit while assuring intimate contact of its transition zone to the junction of the core within the holding device. The core area was not embedded. With the use of a cool glass slab, zinc phosphate cement (Flecks zinc-phosphate cement, Mizzy, Cherry Hill, N.J.) powder and liquid were properly apportioned and mixed. Cement was applied to the post and transition zone areas of the casting sample and to the canal space of the holding ring. The castings were then seated with firm finger pressure for 30 seconds. A 10 kg mass was then applied to the occlusal surface for 20 minutes.

445

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Fig.

ET AL

7. Specimen failure.

The specimens were placed in a specially constructed mounting apparatus (Fig. 6). This apparatus held the specimens centered at 135 degrees to the long axis of the sample post and core in relation to the crosshead of the Instron Universal testing machine (Instron Corp., Canton, Mass.). A 500 kg load cell and a crosshead speed of 0.5 cm/ min were used. A shearing force was applied until catastrophic failure occurred (Fig. 7).

I

II

III’

Cast Gold

Cast SilverPalladium

Cast No-Ox

IV Cast No-Ox Alloy

RESULTS

‘numbers in parentheses are standard svor of mean

The mean fracture strength, standard deviations, standard error of the means, and minimum and maximum values were calculated for the four groups of post and cores (Table III and Fig. 8). An analysis of variance (ANOVA) of the four groups of post and cores at the 95% confidence level demonstrated that a significant difference existed between material variations of the experimental post and cores (Table IV). Duncan’s multiple range test was completed to determine differences present between specific material variations (Table V). The completely cast gold post and cores were significantly more fracture resistant than the other material variations. For the completely cast silver-palladium and the cast to No-Ox alloy wrought post and core, material variations were statistically equivalent when fracture resistance was compared, These variations were, however, less fracture resistant than the completely cast gold post and cores. The variation of post and cores cast from the No-Ox alloy was significantly less resistant to fracture than the other three groups (Table V).

Fig. 8. Graph of mean fracture stre ngth.

DISCUSSTON This study investigated the fracture resistance of four groups of post and core specimens when a force was directed at 135 degrees to the long axis of the post. This angulation was selected because it was close to the ideal interincisal angle of 131 degrees as determined by the

446

Steiner Orthodontic Analysis and has been used by other authors.14, l5 In this study all samples failed during testing at the same location, the junction of the core and transition zone. One of the post and core groups (group 1) was a nonoxidizing wrought noble alloy metal post with a gold core cast to it. Several authors have successfully cast a gold alloy to noble alloy wire and found that it was possible to develop a metallic bond while retaining the physical properties of the wrought wire.4-6 This, however, required a strict protocol consisting of the use of a carbon-free and chloride-free investment, the use of a nonoxidizing wrought wire, and maintenance of a low burnout temperature for a limited time to reduce reorganization of the wrought wire grain structure. This investigation strictly adhered to these criteria. The lower fracture resistance of the wrought No-Ox posts when compared with the gold alloy posts appears to indicate that either the wrought grain structure was reorganized during the burnout and casting procedure or that that the wrought alloy posts were inherently weaker than the gold alloy cast posts. The decrease in fracture resistance of the cast No-Ox alloy posts when compared with the wrought No-Ox alloy posts supports the premise that the

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Table

III.

Mean fracture strength, standard deviation, standard error of the means, minimum and maximum values

fMPa) Mean fracture strength

GrOUp

Cast to No-Ox

991.7 1207.8

Cast to gold Cast to Silver-palladium Cast to No-Ox alloy

999.2 725.7

SD

SEM

158.5 218.4 258.2 220.8

50.1 69.1 81.7

Table IV. ANOVA: Material variations of experimental post and cores Variable

Degrees of freedom

Sum of squares

3 36

1170787.2 1694189.8 --

Material Error Total

39

F value

8.29

69.8

824.4 736.9

2864977.0

*Indicates that a difference less than 0.05 is statistically significant.

1186.6 1498.9 1461.4 1011.8

699.5 399.7

Table V. Duncan’s multiple range test for material variations of the experimental post and cores* Mean fracture strength (MW

PR>F 0.0003*

Maximum fracture strength

Minimum fracture strength

Cast gold Cast silver palladium Cast to No-Ox Cast No-Ox alloy

Grouping?

1207.8 999.2 991.7

A B B

725.7

C

*a = 0.05, df = 5, n = 10. TMeans with the same letter are not significantly different.

wrought nature of the No-Ox posts was maintained at least in part during the fabrication of the samples. Casting the No-Ox alloy samples was difficult because this alloy was designed to be wrought and not cast. The silver-palladium alloy posts also exhibited a lower fracture resistance when compared with the gold alloy specimens. All specimens were ben’ch cooled to room temperature after casting. Heat-hardening the gold alloy significantly improves its physical prolperties. However, heathardening had no appreciable effect on the physical properties of the silver-palladium alloy. A comparison of the physical properties of these two alloys in the hardened condition as reported by the manufacturer supports the increased fracture resistance of the gold alloy specimens found in this investigation. In the clinical application of the post and core, a fracture of a cemented unit presents a difficult situation. Often retrieval of the apical portion of the post results in removal of additional dentin, further compromising the prognosis of the restoration. In addition, a post that bends without fracturing may result in an unrestorable condition.i6 It appears from the experimental samples that a more gradual transition zone perhaps approaching the shape of a tapered post may provide a more fracture-resistant post. The use of a gold-based alloy post and care may also provide increased resistance to fracture of the post when used in teeth with a limited canal space. CONCLUSIONS The following conclusions are suggested by the statistical analysis of the data from this study: 1. The completely cast gold alloy post and core speci-

THE JOURNAL OF PROSTHETIC DENTISTRY

mens were statistically more resistant to fracture than the other three groups. 2. The No-Ox wrought post with a gold core cast to it and the completely cast silver-palladium post and cores were statistically equal in resistance to fracture. They were, however, statistically less resistant to fracture than the completely cast gold post and cores. 3. The completely cast No-Ox alloy post and cores were statistically the least resistant to fracture of the four groups of post and cores. REFERENCES 1. Harty FJ, Leggett LJ. A post crown technique using nickle-cobaltchromium post. Br Dent J 1972;132:394-9. 2. Legget,t L. Restoration of non-vital posterior teeth. J Br Endod Sot 1979;12:73-83. 3. Turner CH, Willoughby FW. An investigation of the process of casting cores on to preformed wrought dental posts. J Oral Rehabil1983;10:17785. 4. Bergman M, Holmlund L, Wictorin L. Noble metal alloy wires in cast posts. Odont Revy 1974;25:273-87. 5. Gilson TD, Asgar K, Peyton F. The quality of union formed in casting gold to embedded attachment metals. J PROSTHETDENT 1965;15:46473. 6. Asgar K, Peyton F. Casting dental alloys to embedded wires. J PROSTHETDENT 1965;15:312-21. 7. Colley IT, Hampson EL, Lehman ML. Retention of post crowns: an assessment of relative efficiency of posts of different shapes and stress. Br Dent J 1968;124:63-9. 8. Johnson JK, Sakumura J. Dowel form and tensile force. J PROSTHET DENT 1978;40:645-9. 9. Standlee JP, Caputo AA, Hanson EC. Retention of endodontic dowels: effect of cement, dowel length, diameter and design. J PROSTHETDENT 1978;39:417-22. 10. Ruemping DR, Lund MR, Schnell RJ. Retention of dowels subjected to tensile and torsional Forces. J PROSTHETDENT 1979;41:159-62.

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16. Lovdahl PE, Nicholls JI. Pin retained amalgam cores vs. cast-gold dowel-cores.J PROSTHETDENT1977;38:507-14.

11. Standlee JP, Caputo AA, Collard E, Pollack E. Analysis of stress distribution by endodontic posts. Oral Surg Oral Med Oral Pathol 1972;33:953-60. 12. Davy DT, Dilley GL, Krejci RF. Determination of stress patterns in root-filled teeth incorporating various dowel designs. 3 Dent Res 1981;60:1301-10. 13. Black GV. Descriptive anatomy of the human teeth. Philadelphia: Willmington Dental Manufacturing Co., 1890:16-21. 14. Steiner CC. The use of Cephalometrics as an aid to planning and assessing orthodontic treatment. Am J Orthod 1960;46:721-35. 15. Caputo AA, Standlee JP. Pins and posts: why, when, and how. Dent Clin North Am 1976;20:299-311.

Cement luting platinum foil J. G. Wall,

DDS,

thickness MS,a M. H. Reisbick,

The Ohio State University

College of Dentistry,

Reprint

Contributing author J. M. S. Clancy, DDS, MS, Assistant Professor, The University of Iowa, College of Dentistry, Iowa City, Iowa

beneath DMD, Columbus,

requests to:

DR.JEFFREY SRYTHER COLLEGEOFDENTISTRY UNIVERSITYOFIOWA IOWACITY, IA 52242

porcelain

MS,b and K. G. Espeleta,

veneers

made on

DDSe

Ohio

Porcelain laminate veneers were made using a platinum foil matrix and were subsequently cemented to mandibular anterior Cymel teeth. Cement film thickness was measured in six predetermined locations. Repeated measures analysis of variance and single degree of freedom contrasts delineated a significant difference between marginal openings at the incisal edge where foil is folded and in four of the other vertical areas (132 versus 74.1 ym). Marginal cement film thickness of veneers made on platinum foil is less than that reported for veneers made on a refractory investment. (J PROSTHET DENT 1992;68:448-50.)

he marginal integrity of complete porcelain restorations, such as the inlay or crown, has been questioned because of unavoidable errors that result from porcelain firing shrinkage.l The benchmark for this statement is the fact that cement film thickness is frequently evaluated by American Dental Association (ADA) specification standards that suggest that an acceptable film thickness for precision restorations is 50.025 mm.2 Accordingly, porcelain laminate veneers are often considered to have relatively large marginal openings-a concern of most clinicians. Sorensen et a1.3 compared marginal openings between veneers made on platinum foils and those made on refractory casts and found that those made on foil were significantly smaller. Since both fabrication methods are popular and useful, the one that minimizes marginal openings (all else being equal) would be the method of choice. The purpose of this investigation was to measure the

Presented at the Carl 0. Boucher Prosthodontic Conference. aAssistant Professor, Section of Restorative and Prosthetic Dentistry. bprofessor and Chairperson, Section of Restorative and Prosthetic Dentistry. CPredoctoral student. 10/l/39133

448

luting space under porcelain laminate veneers that were fabricated on platinum foils and that were cemented to typical tooth preparations on Cymel mandibular incisors (Columbia Dentoform Corp., Long Island City, N.Y.). Of special interest were the marginal gaps where the foils had more than one thickness as a result of conventional folds (tinners joints).

MATERIALS

AND

METHODS

Nine mandibular central incisor teeth were identically prepared with a pantographic milling device to receive porcelain laminate veneer restorations made from a highleucite porcelain (Optec VP, Jeneric Gold Co., Wallingford, Conn.). Tooth undercuts were blocked out with modeling plastic before adapting platinum foil (0.001 inch thick). Complete tooth coverage with platinum foil produced more predictably visual marginal fits than mere facial coverage. Proximal foil joints could be kept lingual to the margins; however, there was always a double thickness of foil at the lingual incisal edge and on proximal incisal corners. After adaptation and burnishing, the foils were removed, air abraded at 40 psi with aluminum oxide, and cleansed with water. The high-leucite porcelain was applied in three body bakes and a glaze. Shade Al body porcelain was used for the first two bakes and white enamel was used for the third

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Evaluation of the fracture resistance of a wrought post compared with completely cast post and cores.

The intraradicular fracture of a post and core presents a difficult and often unrestorable situation. Attempts to prevent this problem involved the us...
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