Design of gold supporting structures for fused porcelain restorations

DESIGN OF GOLD SUPPORTING STRUCTURES FOR FUSED PORCELAIN RESTORATIONS GEORGE

STRAUSSBERG,

D.D.S.,

GERRY

KATZ,”

AND MASAHIRO KUWATA’

*

South Orange, N. I.

long been used as primary materials for dental G restorations. They offerhavestrength, ease of fabrication, and good marginal fit. OLD

AND

ITS

ALLOYS

For esthetic reasons, porcelain has also had a long and successful history in prosthodontics. No other material maintains its surface texture and color as long while resembling the natural teeth so well. Properly fused, the two materials eliminate the necessity of making a choice between strength and esthetics. Porcelain-fused-to-gold provides a means of constructing fixed partial dentures that can restore proper anatomic contours that are compatible with the oral tissues while providing a fine esthetic result in a strong restoration. To achieve the best results with restorations of this type, it is important to understand the physical characteristics and the limitations of both materials. Unsupported porcelain is brittle. Therefore, the gold form to which it is bonded must be shaped to give the porcelain adequate support. PRINCIPLESOFBONDINGPORCELAINTO

GOLD

The development of porcelains and metal alloys with matching moduli of thermal expansion has solved most of the problems encountered in obtaining dependable porcelain-on-metal structures. As the porcelains are fused to the underlying metal, they are bonded through the medium of a special metal dispersion “bonding agent.” This agent alloys with the metal substrate? forming a metal-porcelain interface that is highly resistant to separation or peeling. However, while strongly adhering to the supporting metal, the porcelain still suffers surface checks when subjected to excessive deformation. When the metal bends, porcelain must break. For this reason, the structure supporting the porcelain must be designed with sufficient rigidity to avoid flexing of the metal. With a rigid and properly formed supporting structure and an effective bonding agent, the problem of porcelain peeling from the metal is no longer a deterrent factor. RESPONSIBILITY

FORDESIGN

OFPORCELAIN

FUSED-TO-GOLDRESTORATIONS

The most important reason for failure of porcelain-fused-to-metal restorations is poor design of the supporting metal structure. The primary obligation for the *Certified Dental Technician, Director of Ceramicast Laboratories, Inc., East Orange, **Instructor, Aishi School of Dental Technology, Tokyo, Japan. tThe specific technique has been developed by American Thermocraft Corp. and the Ney Co. The materials used are Thermalite 1650 Porcelain and Ney-Oro P-16 gold. 928

N.J. J. M.

Volume Number

16 5

GOLD

AND

PORCELAIN

9X9

RESTORATIONS

finished restoration rests with the dentist and the basic plan for its design must not be delegated to the technician. The dentist must recognize the principles involved and insist that they be followed. He must be aware of the differences between the gold supporting structures that are required for porcelain fused to metal and those needed for other fixed restorations. The more carefully the principles of construction of sound porcelain-on-gold are carried out, the less likely is it that costly failures and re-makes will be encountered. These principles will be described in this article. AVOIDANCE

OF SHARP

CORNERS

ON

THE

GOLD

SUPPORTING

STRUCTURE

The shape of the supporting gold must be such that the covering porcelain is not compelled to produce a sharp angle. Porcelain is strong in compression, but not as strong in tension. Porcelain can fracture much more easily at sharp corners than when it is supported by a curved metal surface. Therefore, sharp metal corners which support a layer of porcelain must be rounded (Fig. 1) . Sharp inside corners in metal must be similarly avoided, but for a different reason. Porcelain shrinks during firing. As it shrinks it may pull away from a sharp corner, possibly leaving a void between the porcelain and the metal (Fig. 2). PORCELAIN

MUST

SURROUND

THE

METAL

The gold supporting structure must be shaped so as to permit the porcelain to “grip” the metal as a hand grips a ball. The gold structure that supports a veneerlike labial surface of porcelain for anterior teeth must allow the porcelain to extend at least over the incisal edge and not simply end at the incisal edge as is typical in plastic veneer construction (Fig. 3). The pressure exerted on the metal by the opposing teeth will deflect the metal and impose a flexing force on the porcelain. Since porcelain resists flexure poorly, it is likely to fracture unless properly extended. An all-porcelain incisal edge will produce a strong restoration with good incisal color. A gold backing which extends to the incisal edge invites fractures and makes it impossible to produce the translucency necessary for a vital appearance.

OUTSIDE CORNERS

ROUNDED DESIGN

Fig.

l.-.4,

the

sharp

outside

Fig.

Z.--A,

the

sharp

inside

corners of the gold roundness of the metal corner in the metal correct shape of the

SHARP INSIDE CORNERS MAY PRODUCE SHRINKAGE VOIDS

supporting structure coping is preferred. may produce gold substructure.

Structure

PREFERRED ROUNDING OF INSIDE METAL CORNERS

must

be

a shrinkage

avoided. void.

B,

the

R, the

930

STRAUSSBERG

nPlCAL PLASTIC VENEER CONSTRUCTION - UNSUITED FOR PORCELAIN

PREFERRED SHAPE FOR WED PORCELAIN

ET

T. Pros. Septembedktober,

AL

TVPKAL COLD BACKING’ FOR PLASTIC VENEER

Den. 1966

RECOMMENDED DESIGN FOR FUSED PORCELAIN

Fig. 3.-A, the gold supporting structure used for a plastic veneer. B, the comparable structure for a porcelain-fused-to-gold restoration. Note the porcelain coverage of the incisal edge. Fig. 4.-A, the design of a posterior restoration with a buccal plastic veneer. B, the design recommended for fused porcelain. Note that the porcelain should be extended occlusally to form the buccal cusps.

The same considerations are followed when designing the gold supporting structure for the posterior teeth. Here too, the shape of the gold should permit the porcelain on the buccal surface to be carried onto the occlusal surface to form the buccal cusps (Fig. 4). ALLOWANCES

FOR

THICKNESS

OF PORCELAIN

The metal structure should be designed so that the porcelain will be of reasonably uniform thickness to provide control of both firing and color. In making the preparation, the mesiodistal sulci of the tooth should be reduced sufficiently to provide adequate space for both the gold and the covering porcelain. If metal is to be used for the center of the occlusal surface, the metal crown should be designed with a definite step to allow space for a layer of masking opaque under the translucent occlusal porcelain (Fig. 5). If the porcelain is tapered gradually, the junction with the gold will not be clean and sharp (inlay-like junction) and the porcelain is more likely to chip. The only place where porcelain is tapered to a thin edge is at the gingival margins. MAINTENANCE

OF A

0.5

MM.

MINIMUM

THICKNESS

OF GOLD

The minimum thickness of gold should be 0.5 mm. If the metal is too thin, it may deform either under the stress of mastication or when the restoration is placed in the mouth or on the die. Such deformation will cause the porcelain to check. The use of No. 26 guage wax provides a casting pattern with the necessary 0.5 mm. thickness of metal. The lingual margin should be cast in metal in the form of a collar. This adds rigidity to the crown. TYPICAL

METAL

COPINGS

FOR

PORCELAIN

FUSED

TO GOLD

The extension of gold on the lingual surfaces of the anterior teeth is determined to a large extent by the nature of the occlusion. With average vertical and horizontal overlap and with adequate space for both gold and porcelain, an in-

GOLD

AND

PORCELAIN

AVOID “FEATHERING” OF PORCELAIN EDGES Fig. posterior porcelain.

5.-A, feathering of porcelain restorations should be designed

931

RESTORATIOiYS

PREFERRED DESIGN FOR METAL ISLANDS

edges should be with a definite

avoided. step

to

R, the provide

metal occlusal stops adequate thickness

in of

68

Fig. 6.-A, the design for an upper anterior lingual surface. B, the lingual aspect illustrates gingival edge. Fig. 7.-A, when the opposing tooth contact is is required past the point of centric occlusal contact. distal and mesial surfaces of the restoration. C’, porcelain is to be avoided. Fig. 8.-A, the opposing tooth contact is near cover the incisal edge and extend as far as possible junction of gold and porcelain is not desirable. the the

restoration the use near B, the the

of

with full porcelain a re-enforcing

the cingulum, the porcelain straight-line

incisal mesially

edge. and

metal

coverage collar

of at

a metal lingual surface may extend around the termination of gold and B, the distally.

porcelain C, the

must straight-line

still

dividual crown may be designed with full porcelain coverage of the lingual surface (Fig. 6 ). Where the occluding contact is near the cingulum or when little space exists, the metal on the lingual surface must extend past the point of tooth contact in centric occlusion to accomodate the slide during mastication (Fig. 7). \Yhen there is inadequate space for combined porcelaiu and metal, then the metal should extend to the contact in centric occlusion (Fig. 8). However, the shape of the metal should still permit porcelain coverage of the incisal edge, and

932

STRAUSSBERG

ET

AL

the porcelain should extend around onto the distal and mesial surfaces. This can be done with no sacrifice of strength and with a definite improvement in the appearance of the restoration. Variations in occlusion and intercuspation also determine the extent of porcelain coverage for posterior restorations (Figs. 9 to 11). In most instances, single crowns can be made with full porcelain coverage with, possibly, a gold occlusal stop. Complete coverage of the lingual surface with metal is rarely necessary (Fig. 11). PHYSICS

OF FIXED

PARTIAL

DENTURES

The design of the gold structure for a fixed partial denture is dependent upon the geometry of the restoration, To appreciate this dependence, it is only necessary to recall a few of the elementary engineering principles discussed by Ty1man.l A fixed partial denture is essentially a beam supported by abutment teeth. A force applied to the center of the beam will cause the beam to deflect. Thus the top layers of the beam are placed in compression and the bottom layers in tension (Fig. 12). The deflection of such a beam varies directly with the cube of its length and inversely with the cube of its thickness (in the direction of stress). In dental terms, as has been stated by Smyd,s “. . . all other things remaining equal, a twotooth bridge will bend eight times as much as a one-tooth span and a three-tooth span will bend twenty-seven times as much.” A simple experiment performed with a strip of wood presents a convincing demonstration of this principle (Fig. 13). To

1OA

10B

1OC

Fig. 9.-A, single posterior crowns with adequate space for opposing tooth contact can usually be restored with full porcelain coverage. B and C, the lingual and occlusal aspects of such a restoration. Fig. 10.-A, where the occlusion does not allow sufficient clearance for an adequate thickness of porcelain, a metal “island” may be used. I3 and C, the lingual and occlusal view of this restoration. Note the sharp, inlaylike appearance of the metal island obtained by providing a step in the metal understructure. Fig. Il.-A, the extensive metal structure required in exceptional situations still permits a substantial extension of porcelain onto the occlusal surface. B and C, the labial and occlusal aspects of this restoration.

GOLD

AND

PORCELAIN

03.3

RESTORATIONS

UPPER AREA IS IN COMPRESSION 136

138

LOWER AREA IN TENSION /

Fig. Fig.

12.-A 13.--A,

diagram a beam

of a stressed beam indicates the parts subjected to tension and compression. placed in this manner will resist stress much better than one placed as in B.

ANTERIOR PONTKS

LINGUAL ASPECT

Figs. 14 to 16.-Typical anterior pontic designs parallel the designs of abutments in Figs. 7 to 9. Note that the same principles are followed: avoidance of sharp inside and outside corners: extension of porcelain around the supporting gold at incisal mesial, and distal edges; and provision of adequate thickness of porcelain. The shaded areas indicate the approximate shape and position of solder joints. Note the use of porcelain in the part that will be in contact with the mucosa.

withstand this stress, the gold framework must provide bulk for strength, particularly in the occluso-gingival direction. In general, the gold supporting structure for pontics should be similar in form to that used for the proximating abutments. For example, if an abutment has a gold lingual surface, the same consistent design should be carried over to the adjacent pontic to provide both maximum strength and uniformity of appearance (Figs. 14 to 17). For esthetic reasons, the design of the gold supporting structure for pontics should allow for a layer of glazed porcelain in the region of the denture bases provided that strength is not sacrificed. Considerable latitude exists in design of a three unit fixed partial denture, especially when an adequate occlusogingival space exists and when the pontic is relatively short mesiodistally. The metal frame can be formed to permit the use of porcelain to contact the residual ridge and along most of the lingual surface (Fig. 37, A j. However, when a longer unsupported span is involved, the cross-section of the metal must be considerably larger (Fig. 17, B). The larger the span, the

934

STRAUSSBERG

ET AL

Se,,teml,er.&~E

%

POSTERIOR PONTICS

UNGUAL ASFfCl

DESIGN ioR SNORT SPAN

Fig. 17.--A, a full porcelain ture is extended

DESIGN FOR LONG SPAN

the typical structure of posterior pontics used in a tied partial denture shows gingival surface and porcelain covering the lingual surface. B, the gold Strutocclusogingivally to provide greater rigidity for a longer span.

more metal should be employed, particularly occlusogingivally, in the buccolingual width increases strength only in direct increase in that dimension. DESIGN

OF GOLD

SOLDER

since an increase proportion to the

JOINTS

Since gold solder has less resistance to the tensile and shearing stresses to which the fixed partial denture is subjected than the cast gold, care should be taken to design the restoration for maximum strength at soldered joints. One way to increase the strength of a solder joint is to use the maximum available crosssectional area. This can be accomplished by specifying a diagonal solder connection through the heaviest section of the pontic instead of through the smaller interproxima1 area (Fig. 18). An additional advantage of the diagonal connection is that it reduces the shearing stress upon the solder. In a fixed partial denture involving a span of two or more pontics, the two solder joints which connect the central units to their abutments are made in ‘opposite directions (Fig. 19). This “keystone” configuration produces the maximum strength. The metal connection between the two abutments is subjected to considerable shearing stress. For this reason, the joints should be made as large ancl as secure as possible. This can be done by designing the two proximating abutment crowns with high gold lingual surfaces to provide the maximum area for connection. The coping for the second abutment can be cast with a lingual extension that overlaps the lingual shoulder of the first abutment. This provides a strong solder joint not ‘only at the interproximal connection, but also all around the lingual attachment (Fig. 19, B); Whenever possible, a long fixed partial denture should be attached to two abutments at each end. The reason for this becomes clear upon examination of Fig. 20, which diagrams the forces acting upon the abutments. In a fixed partial denture with a single abutment (Fig. 20, A), occlusal stress

GOLD

AND

PORCELAIK

935

RESTORATIONS

DIAGONAL SOLDER CONNECTION THROUGH PONllC \

Fig. l&-The gold supporting structure for a the location of the diagonal connection. Note that metal coverage on the distal surface.

Fig. 19.--A, the diagonal pontic section of fixed partial connection between the first

Fig. 20.-A span upon the abutments.

diagrammatic roots of the

“keystone” dentures. and second

three the

tooth molar

solder connections are B, note the lingual bar mesial abutments of the

representation abutments and

on

indicates the the connections

fixed crown

partial denture is strengthened

used in a gold structure provided to reinforce fixed partial denture.

effects of between

deflection the first

indicates by full

the

on and

for the solder

a long second

produces a torque or turning effect upon the root of the abutment tooth. This torque produces excessive mesiodistal forces which are physiologically destructive to the periodontal ligament and the surrounding bone. When a second abutment is affixed to the restoration (Fig. 20, B), the center of rotation shifts so that the forces upon the roots are now applied in a vertical direction which is more acceptable to the supporting tissues. The design of suitable abutments for a restoration involving a long unsupported span frequently presents a problem. Multiple abutments are required not only to reduce the stress against periodontal membranes but also to provide resistance to deflecting stresses. When anterior teeth are replaced by a fixed partial denture, multiple abutments are frequently required to resist the torque resulting from the relative position of the teeth (Fig. 21) .

STRAUSSBERG ET AL

936

Fig. Z.--The

torque created by stress on an unsupported anterior resisted by adequate abutments.

September-

fixed

restoration

Pros. Den. d ctober, 1966

should

be

SUMMARY

Improvements in preparation and laboratory techniques have made possible a generation of porcelain-fused-to-metal restorations that combine great strength, long life, and excellent esthetics. To obtain the optimum results inherent in porcelain-fused-to-gold restorations, the dentist must understand and respect the physical characteristics of the materials and guide the design and fabrication of the restoration so as to exploit their strengths and compensatefor their weaknesses. The gold framework upon which the porcelain will be fused must be sufficiently rigid in all its parts to support the porcelain without excessive flexure. The framework must provide sufficient space for adequate thickness of the layers of opaque and translucent porcelain. The units of the fixed partial denture must be securely and rigidly connected by sound, well-designed solder joints. The illustrations East Orange, N. J.

in this article

by courtesy of the American

Thermocraft

Corporation,

REFERENCES 1. Tylman, S. D.: Theory and Practice of Crown and Bridge Prosthesis, ed. 3, St. Louis, 1954, The C. V. Mosby Company, pp. 159-165. 2. Smyd, E. S. : The Mechanics of Dental Structure, J.A.D.A. 44:187-193, 1952. 3. Smyd, E. S.: Mechanics of Dental Structures: Guide to Teaching Dental Engineering at Undergraduate Level, J. PROS. DENT. 2:668-692, 1952. 505 S. ORANGE AVE. SOUTH ORANGE, N. J.