Australian Dental Journal, February, I977
11
Volume 22, No. I
Some aspects of non-precious metal alloys for ceramic restorations L. Stevens
Aasm,cr--Coniment is niade regarding the cost, related to working properties, of non-precious metal ;rlloys compared to precious metal alloys for ceramics and Iheir possiblc potential for the production of allergic response.
cost Thc cost of materials is only onc of several factors which iletermine the cost of restorative ilcnt;il treatment. This single factor must be of considerable niagnitude to greatly influence the cost of a restoration. With respect to porcelain fused to metal restorations, the smaller cost of the non-precious metal alloys compared to precious metal alloys is claimed to result in a considerable reduction in the overall fee charged. Some comniercinl dental laboratories base their charges on the cost of construction of such restorations plus the cost of the alloy used. Provided that the fee charged for construction remains the same, regardless of the alloy used, then non-precious metal alloy restorations have a distinct cost advantage over precious metal alloy restorations a s claimed; i t is well documented that the former are niorc difficult and time consuming to work than the latterl.2. Therefore it is difficult to see that a truc cost comparison is being niade between thc two types of restoration. There may be several reasons why commercial dental laboratories charge the same fee for construction of porcelain fused to metal restorations
regardless o f the alloy used. Of these i t might be that the principal reason is related to profitability. Increase i n out-piit of non-precious metal :alloy restorations would allow ii laboratory to absorb any extra cost duc to increase in time involved or tlifficulty in construction. In the short term, since the patient, the dentist, the commercial laboratory owner and the manufacturer of the alloy all benefit financially, increasing numbers {if such restorations are likely to be niade. If this trend occurs. then in the long term the use o f precious metal alloys will be depressed and the routine use of nonprecious metal alloys established. In that situation reassessnient of fees would be based on all pertinent factors affecting construction of the porcelain fused lo metal crown and the true cost would emerge. The cost of ;I dental material is ;a legitimate factor to consider in its selection provided it is related to all other properties. Published work indicates :a favourablc comp:arison of the two alloys with respect to mechanical properties“.” and bond strength of porcclain to alloy”. The non::
MoRa, J. P., a n d Jenkins. W. A,-Stelus report on h x s c metal crown a n d hridge ;illoys. J.A.D.A.. XY:3, 652-655
(Sept.) 1Y74. MoR;i. J . I’.. Lugassy, A . A , , Guckes. ,A. 13.. ;tiid Geltlcmiin. L.-An evaluntion of noiiprecious lrlloys f o r ilsc with porcrlarn veneers. l’;:rt I. 1’hysic;il pt‘opeities. J. I’ros. 13.. 3O:4. 424-431 ( 0 c t . J 1973. ZSccd. I. K.. and McLenn. .I. W.-Thc strength of iiiclal8
1
Phillips, R . \\‘.-Skinner.’?; science of dental ninteriuls. I’hil;idelphia. W. B. S:iiinders Co.. 7th ed.. 1973 ( p . 54Y). castiiigs in dcntistry. In, Symposium dental biomateri;ils-rese~irch priorilies. Edit.. Wachtel. I-. W.. Dhew puhlic:ition No. ( N I H ) 74-548, 1973 (PP. 27-44).
Asgar, K.-Met:il
ceramic bonds with hasc niet:ils cont:iining chroniiiim. A preliniiim-y report. Brit. D. J., 1 3 2 5 232-234 (March, 21) 1972.
12
Australian Dental Jouinal, February, 1977
precious metal alloys are suggested to be biologicnlly conipatible~. Allergic potential Uio-conipatability might need t o be further investigated regarding the production of tissue reactions. Hedegardi found, with gold alloys. that homogenization of the cast alloy appeared t o lessen discoloration of the alloy and reduce certain subjective symptoms and lesions of the niucous membrane. Galvanic phenomena association with the heterogenous structure of t h e alloy probably produced these effects. Homogenization of cast gold alloys is a simple procedure and is recommended prior t o age hardenings. T h e precious nietal alloys, apart f r o m solid solution hardening. owe their strength and hardness t o their property of age hardenability which is complex in nature!’. Honiogenization can be effected by heating t h e alloy to just below its liquidus temperature for ii suitable period and quenching. Age hardening can be carried out by heating at a suitable temperature for a suitable time. This occurs during firing of t h e porcelain veneer. T h u s ii precious metal alloy can be made homogeneous a n d age hardened after casting. T h e non-precious metal alloys a r e used as cast. They derive their strength and hardness from solid solution hardening and the presence of precipitates in the a s cast condition. T h e y a r e inhomogeneous, as cast, and can not be homogenized without loss of precipitate and subsequent loss of strength and hardness. Loss of strength and hardness can occur during firing of t h e porcelain veneer. It seems likely that base metal alloys, despite their ability to passivate, might have higher corrosion rates in the oral environnient than dental gold alloys. lnhoniogeneity and corrosion a r e related and corrosion products formed could cause tissue response. T h e non-precious metal alloys a r e either cobaltchromium o r nickel-chromium based systems1(’. It is well known that nickel is capable of producing sensitivity and allergic response to subsequent exposure’’. Nickel in coins12.l:i, jewellery14.1G and ~~
tap waterll; can produce sensitization and allergic response. Reports have been made concerning contact reactions with mucous membranes due to alloys containing nickel, chromium and cobaltl7. Cobalt and nickel sometimes sensitize siniultaneouslylS. Fisher17 considers that metallic chromium is not it sensitizer and that allergic reaction attributed to metallic chromium objects is due t o their nickel content. H e states that nickel readily penetrates the micro-pores in chrome-plated objects. Allergic reaction to these metals coninionly o c c t ~ r sin hot climates and in warm moist areas of t h e body where secretions a r e present. This would enhance galvanic effects and it m a y be that corrosion products, rather than t h e nietal itself, combine with some bodily constituent t o f o r m a complex antigen”. Considering the reputation of these metals a s antigens it would be of interest to carry out allergy tests, with the non-precious nietol alloys, on pcrsons known to be sensitive t o these metals. Also to determine whether o r not theve alloys can produce allergic sensitization. Working properties It has been suggested that, :is ii group. the nun-precious nietal alloys do not possess thc ability to be cast into fine section compared to the precious ceramic alloysl!l. This has been confirnied by ;I project recently completed in this Department. T h e marginal fit of restorations should be such that it minimum cement line is obtained. With vertically inserted restorations, t o obtain :I ccnient line determined only by the properties of t h e cement. it is necessary t o incorporate axinl and occl usnl t ole riince together with ii horizon t a I shoulder rather than ii bevel o r a chamfer at the margin. lorgensen”l indicates that congruently fitting. vertically inserted restorations would have a thick cenient line the width being determined by the taper angle of t h e preparation and the thickness of the film of cement o n t h e axial walls. Bevels and chamfers a r e preferred a t cavo-surface margins21 f o r several reasons. One is to allow Black, H.-Dermatitis from nickel and copper in coins. Contact Dermatitis Newsletter No. 12: 396. 1972. Foussereau. J.-Allergy to nickel in gold alloy. Contact Dermatitis Newsletter N o 4: 60. 196X. 13 Caul. L. E.-Development of :Illergic nickel dcrmotitis from earrings. J.A.M.A.. 2OW. 186-188 (Apl.) 1967. 10 Fregert. S.-Nickel in tapw:iter. Contact Dermatitis Newsletter No, 9: 202. 1971, 17 Fisher. A. A.-Contact dermatitis. Philadelphia. Lea and Febiger. 2nd ed., 1973 (p. 315). ‘6 Rudzki, E.. and Kohutnicki. A . J.-Sensitization t o cobalt and chromium and to cohiilt and nickel in nien and women. Contact Ikrmatitis Newsletter Nu. 9: l Y 6 , IY71. 13 Andersen. J. N.-Applied dental materials. Oxford, Blackwell Scientific Publications, 4th ed., 1972 (p. 85). :(I Jorgensen. K. D.-The grain size of zinc phosphate cements. Acta. Odont. Scand.. 21 :3, 255-270 (June) 1963. ?*Tylman, S. D.-Theory and practice of crown and fixed partial prosthodontics (bridge). St. Louis. T h e C. V. Mosby Company, 6th ed., I970 (pp. 297-299). I:’
“ M o l f ; ~ .J . P.. Guckes. A. D., Okawa. M. T.. and Lilly. G . E.-An evaluation of nonprecious alloys for use with porcelain veneers. P;irt I I . Industrial safety and biocompalibility. J. Pros. D., 30:4, 432-441 (Oct.) 1973. 7 Hedegard, 6.-Studies in crown and bridge prosthesis. Homogenization of dental alloy castings. A clinical study. Stockholm, Royal School of Dentistry, 1959. 6 Anderson. J. N.-Applied dental materials. Oxford, BI:ickwell Scientific I?ublications, 4th ed., 1972 (p. 66). Leinfelder. K. F.. O’Brien. W. J.. Ryge. G.. and Fmhurst. C. W.-l-l;irdening of high-fusing gold alloys. J . I>. Res., 45:2. 392-396 (March/Apl.) 1966. 1’) Phillips, R. W.-Skinner’s science of dental materials. Philadelphia, W. 6. Saunders Co.. 7th ed., 1973 (p. 549). 11 Rostenberg. A,, and Perkins. A. J.-Nickel and cobalt dermatitis. J . Allergy, 22:466-474, 1951. 12 Bettley, F. R.-Nickel coin dermatitis, Contact Dermatitis Newsletter No. 9 : 198, 1971.
”
Australian Dental Journal, February, 1977 burnishing of alloy to tooth to give the most intimute fit. However this would tend towards congruency bctween the restoration and the tooth in t h e area of the bevel o r chamfer. This would result in a thick cement line, the width being determined by the angle of the bevel o r chamfer and the thickness of the film of cement in congruent areas. Under these conditions burnishing is contraindicated and with improvements in casting accuracy m a y not be necessary: percentage elongation is not a true measure of t h e capacity. Although the non-precious metal alloys have high values of percentage elongation they also have high values of yield strength. As a result, the property of burnishing under clinical conditions is possibly inferior t o that of the precious metal nlloys. Undoubtedly the principal reason for using bevels and chamfers is t o finish the marginal sections in ennniel where possible". Apart from conservation of tooth structure, clinical observation suggests that incidence and the effect of any recurrent decay is less when bcvelled o r chamfered margins a r e placed in enamel compared to shoulder-type margins which extend into dentine. Should marginal discrepancy occtir at it finely chamfered o r bevelled margin, as appears to be m o r e likely t o happen with the non-precious metal alloys. there would be an increase in thickness of the cement line which would predispose to recurrent decay. Since ability t o cast these alloys i n thin sections and their limited capacity for burnishing i t might be better t o design preparations f o r non-precious metal alloys with marginal shoulders rather than chamfers o r bevels. Recently letters were sent to five commercial tlental laboratories in Brisbane asking whether construction of porcelain t o non-precious nietal alloy was m o r e demanding than porcelain t o precious nietal alloy with respect to equipment, '
. I .
:'I
Shillinburg. H. T.. Hobo. S.. and Fisher. I>.W.-Preparalions f o r cast gold restorations. Berlin, D i e Quintessenr, I974 fn. ,, - 261. -.,G u i d e to dental materials and devices. Chicago. American Dental Association. 6th ed., 1972 (p. 37).
13 time, knowledge and skill, and whether an increase in charge was made to compensate a n y extra demand. Three replies were received. All indicated that non-precious metal alloys wcrc not more difficult t o use in the construction of the restorations and that the charge for Construction of ceramic-nietul restorations was the same except for the cost of the alloy. These three coniiiierci;il tlental laboratories a r c cqiiippcd with modern equipment :ind employ expert tradesmen. As a result, even though the nonprecious metal alloys have bcen shown t o be inferior to precious metal alloys in some aspects of handling, these laboratories a r e able t o produce satisfactory restorations. T h e restorations a r e satisfactory as indicated by the small number of complaints from dentists regarding them and by their increasing acceptance by the profession. However i t is clear that. in less experienced hands ; t i i d without appropriate cquipnient. the non-precious metal alloys are disudvantagctl regarding working propcrties. Conclusion T h e introduction o f nun-precious metal alloys reprcsents an advance in dental treatment. They are still very new and were rcgarded a s experimental only three yews ago2:{. Since then their use has become relatively well established and successful judged o n this basis. However it is importnnt to consider that the precious metal alloys of various types have been proven clinically over a long period and despite their. apparent cost disadvnntnge should not be regarded iis superseded. T h e non-precious metal alloys should be regardcd :is additional materials. This suggested status woultl have t o be reconsidered should the known allergic effects of the metals comprising them become apparent. Dental School, University of Queenslond, T u r b o t Street. Brisbane, Q'ld. 4000.
11
Volume 22, No. I
Some aspects of non-precious metal alloys for ceramic restorations L. Stevens
Aasm,cr--Coniment is niade regarding the cost, related to working properties, of non-precious metal ;rlloys compared to precious metal alloys for ceramics and Iheir possiblc potential for the production of allergic response.
cost Thc cost of materials is only onc of several factors which iletermine the cost of restorative ilcnt;il treatment. This single factor must be of considerable niagnitude to greatly influence the cost of a restoration. With respect to porcelain fused to metal restorations, the smaller cost of the non-precious metal alloys compared to precious metal alloys is claimed to result in a considerable reduction in the overall fee charged. Some comniercinl dental laboratories base their charges on the cost of construction of such restorations plus the cost of the alloy used. Provided that the fee charged for construction remains the same, regardless of the alloy used, then non-precious metal alloy restorations have a distinct cost advantage over precious metal alloy restorations a s claimed; i t is well documented that the former are niorc difficult and time consuming to work than the latterl.2. Therefore it is difficult to see that a truc cost comparison is being niade between thc two types of restoration. There may be several reasons why commercial dental laboratories charge the same fee for construction of porcelain fused to metal restorations
regardless o f the alloy used. Of these i t might be that the principal reason is related to profitability. Increase i n out-piit of non-precious metal :alloy restorations would allow ii laboratory to absorb any extra cost duc to increase in time involved or tlifficulty in construction. In the short term, since the patient, the dentist, the commercial laboratory owner and the manufacturer of the alloy all benefit financially, increasing numbers {if such restorations are likely to be niade. If this trend occurs. then in the long term the use o f precious metal alloys will be depressed and the routine use of nonprecious metal alloys established. In that situation reassessnient of fees would be based on all pertinent factors affecting construction of the porcelain fused lo metal crown and the true cost would emerge. The cost of ;I dental material is ;a legitimate factor to consider in its selection provided it is related to all other properties. Published work indicates :a favourablc comp:arison of the two alloys with respect to mechanical properties“.” and bond strength of porcclain to alloy”. The non::
MoRa, J. P., a n d Jenkins. W. A,-Stelus report on h x s c metal crown a n d hridge ;illoys. J.A.D.A.. XY:3, 652-655
(Sept.) 1Y74. MoR;i. J . I’.. Lugassy, A . A , , Guckes. ,A. 13.. ;tiid Geltlcmiin. L.-An evaluntion of noiiprecious lrlloys f o r ilsc with porcrlarn veneers. l’;:rt I. 1’hysic;il pt‘opeities. J. I’ros. 13.. 3O:4. 424-431 ( 0 c t . J 1973. ZSccd. I. K.. and McLenn. .I. W.-Thc strength of iiiclal8
1
Phillips, R . \\‘.-Skinner.’?; science of dental ninteriuls. I’hil;idelphia. W. B. S:iiinders Co.. 7th ed.. 1973 ( p . 54Y). castiiigs in dcntistry. In, Symposium dental biomateri;ils-rese~irch priorilies. Edit.. Wachtel. I-. W.. Dhew puhlic:ition No. ( N I H ) 74-548, 1973 (PP. 27-44).
Asgar, K.-Met:il
ceramic bonds with hasc niet:ils cont:iining chroniiiim. A preliniiim-y report. Brit. D. J., 1 3 2 5 232-234 (March, 21) 1972.
12
Australian Dental Jouinal, February, 1977
precious metal alloys are suggested to be biologicnlly conipatible~. Allergic potential Uio-conipatability might need t o be further investigated regarding the production of tissue reactions. Hedegardi found, with gold alloys. that homogenization of the cast alloy appeared t o lessen discoloration of the alloy and reduce certain subjective symptoms and lesions of the niucous membrane. Galvanic phenomena association with the heterogenous structure of t h e alloy probably produced these effects. Homogenization of cast gold alloys is a simple procedure and is recommended prior t o age hardenings. T h e precious nietal alloys, apart f r o m solid solution hardening. owe their strength and hardness t o their property of age hardenability which is complex in nature!’. Honiogenization can be effected by heating t h e alloy to just below its liquidus temperature for ii suitable period and quenching. Age hardening can be carried out by heating at a suitable temperature for a suitable time. This occurs during firing of t h e porcelain veneer. T h u s ii precious metal alloy can be made homogeneous a n d age hardened after casting. T h e non-precious metal alloys a r e used as cast. They derive their strength and hardness from solid solution hardening and the presence of precipitates in the a s cast condition. T h e y a r e inhomogeneous, as cast, and can not be homogenized without loss of precipitate and subsequent loss of strength and hardness. Loss of strength and hardness can occur during firing of t h e porcelain veneer. It seems likely that base metal alloys, despite their ability to passivate, might have higher corrosion rates in the oral environnient than dental gold alloys. lnhoniogeneity and corrosion a r e related and corrosion products formed could cause tissue response. T h e non-precious metal alloys a r e either cobaltchromium o r nickel-chromium based systems1(’. It is well known that nickel is capable of producing sensitivity and allergic response to subsequent exposure’’. Nickel in coins12.l:i, jewellery14.1G and ~~
tap waterll; can produce sensitization and allergic response. Reports have been made concerning contact reactions with mucous membranes due to alloys containing nickel, chromium and cobaltl7. Cobalt and nickel sometimes sensitize siniultaneouslylS. Fisher17 considers that metallic chromium is not it sensitizer and that allergic reaction attributed to metallic chromium objects is due t o their nickel content. H e states that nickel readily penetrates the micro-pores in chrome-plated objects. Allergic reaction to these metals coninionly o c c t ~ r sin hot climates and in warm moist areas of t h e body where secretions a r e present. This would enhance galvanic effects and it m a y be that corrosion products, rather than t h e nietal itself, combine with some bodily constituent t o f o r m a complex antigen”. Considering the reputation of these metals a s antigens it would be of interest to carry out allergy tests, with the non-precious nietol alloys, on pcrsons known to be sensitive t o these metals. Also to determine whether o r not theve alloys can produce allergic sensitization. Working properties It has been suggested that, :is ii group. the nun-precious nietal alloys do not possess thc ability to be cast into fine section compared to the precious ceramic alloysl!l. This has been confirnied by ;I project recently completed in this Department. T h e marginal fit of restorations should be such that it minimum cement line is obtained. With vertically inserted restorations, t o obtain :I ccnient line determined only by the properties of t h e cement. it is necessary t o incorporate axinl and occl usnl t ole riince together with ii horizon t a I shoulder rather than ii bevel o r a chamfer at the margin. lorgensen”l indicates that congruently fitting. vertically inserted restorations would have a thick cenient line the width being determined by the taper angle of t h e preparation and the thickness of the film of cement o n t h e axial walls. Bevels and chamfers a r e preferred a t cavo-surface margins21 f o r several reasons. One is to allow Black, H.-Dermatitis from nickel and copper in coins. Contact Dermatitis Newsletter No. 12: 396. 1972. Foussereau. J.-Allergy to nickel in gold alloy. Contact Dermatitis Newsletter N o 4: 60. 196X. 13 Caul. L. E.-Development of :Illergic nickel dcrmotitis from earrings. J.A.M.A.. 2OW. 186-188 (Apl.) 1967. 10 Fregert. S.-Nickel in tapw:iter. Contact Dermatitis Newsletter No, 9: 202. 1971, 17 Fisher. A. A.-Contact dermatitis. Philadelphia. Lea and Febiger. 2nd ed., 1973 (p. 315). ‘6 Rudzki, E.. and Kohutnicki. A . J.-Sensitization t o cobalt and chromium and to cohiilt and nickel in nien and women. Contact Ikrmatitis Newsletter Nu. 9: l Y 6 , IY71. 13 Andersen. J. N.-Applied dental materials. Oxford, Blackwell Scientific Publications, 4th ed., 1972 (p. 85). :(I Jorgensen. K. D.-The grain size of zinc phosphate cements. Acta. Odont. Scand.. 21 :3, 255-270 (June) 1963. ?*Tylman, S. D.-Theory and practice of crown and fixed partial prosthodontics (bridge). St. Louis. T h e C. V. Mosby Company, 6th ed., I970 (pp. 297-299). I:’
“ M o l f ; ~ .J . P.. Guckes. A. D., Okawa. M. T.. and Lilly. G . E.-An evaluation of nonprecious alloys for use with porcelain veneers. P;irt I I . Industrial safety and biocompalibility. J. Pros. D., 30:4, 432-441 (Oct.) 1973. 7 Hedegard, 6.-Studies in crown and bridge prosthesis. Homogenization of dental alloy castings. A clinical study. Stockholm, Royal School of Dentistry, 1959. 6 Anderson. J. N.-Applied dental materials. Oxford, BI:ickwell Scientific I?ublications, 4th ed., 1972 (p. 66). Leinfelder. K. F.. O’Brien. W. J.. Ryge. G.. and Fmhurst. C. W.-l-l;irdening of high-fusing gold alloys. J . I>. Res., 45:2. 392-396 (March/Apl.) 1966. 1’) Phillips, R. W.-Skinner’s science of dental materials. Philadelphia, W. 6. Saunders Co.. 7th ed., 1973 (p. 549). 11 Rostenberg. A,, and Perkins. A. J.-Nickel and cobalt dermatitis. J . Allergy, 22:466-474, 1951. 12 Bettley, F. R.-Nickel coin dermatitis, Contact Dermatitis Newsletter No. 9 : 198, 1971.
”
Australian Dental Journal, February, 1977 burnishing of alloy to tooth to give the most intimute fit. However this would tend towards congruency bctween the restoration and the tooth in t h e area of the bevel o r chamfer. This would result in a thick cement line, the width being determined by the angle of the bevel o r chamfer and the thickness of the film of cement in congruent areas. Under these conditions burnishing is contraindicated and with improvements in casting accuracy m a y not be necessary: percentage elongation is not a true measure of t h e capacity. Although the non-precious metal alloys have high values of percentage elongation they also have high values of yield strength. As a result, the property of burnishing under clinical conditions is possibly inferior t o that of the precious metal nlloys. Undoubtedly the principal reason for using bevels and chamfers is t o finish the marginal sections in ennniel where possible". Apart from conservation of tooth structure, clinical observation suggests that incidence and the effect of any recurrent decay is less when bcvelled o r chamfered margins a r e placed in enamel compared to shoulder-type margins which extend into dentine. Should marginal discrepancy occtir at it finely chamfered o r bevelled margin, as appears to be m o r e likely t o happen with the non-precious metal alloys. there would be an increase in thickness of the cement line which would predispose to recurrent decay. Since ability t o cast these alloys i n thin sections and their limited capacity for burnishing i t might be better t o design preparations f o r non-precious metal alloys with marginal shoulders rather than chamfers o r bevels. Recently letters were sent to five commercial tlental laboratories in Brisbane asking whether construction of porcelain t o non-precious nietal alloy was m o r e demanding than porcelain t o precious nietal alloy with respect to equipment, '
. I .
:'I
Shillinburg. H. T.. Hobo. S.. and Fisher. I>.W.-Preparalions f o r cast gold restorations. Berlin, D i e Quintessenr, I974 fn. ,, - 261. -.,G u i d e to dental materials and devices. Chicago. American Dental Association. 6th ed., 1972 (p. 37).
13 time, knowledge and skill, and whether an increase in charge was made to compensate a n y extra demand. Three replies were received. All indicated that non-precious metal alloys wcrc not more difficult t o use in the construction of the restorations and that the charge for Construction of ceramic-nietul restorations was the same except for the cost of the alloy. These three coniiiierci;il tlental laboratories a r c cqiiippcd with modern equipment :ind employ expert tradesmen. As a result, even though the nonprecious metal alloys have bcen shown t o be inferior to precious metal alloys in some aspects of handling, these laboratories a r e able t o produce satisfactory restorations. T h e restorations a r e satisfactory as indicated by the small number of complaints from dentists regarding them and by their increasing acceptance by the profession. However i t is clear that. in less experienced hands ; t i i d without appropriate cquipnient. the non-precious metal alloys are disudvantagctl regarding working propcrties. Conclusion T h e introduction o f nun-precious metal alloys reprcsents an advance in dental treatment. They are still very new and were rcgarded a s experimental only three yews ago2:{. Since then their use has become relatively well established and successful judged o n this basis. However it is importnnt to consider that the precious metal alloys of various types have been proven clinically over a long period and despite their. apparent cost disadvnntnge should not be regarded iis superseded. T h e non-precious metal alloys should be regardcd :is additional materials. This suggested status woultl have t o be reconsidered should the known allergic effects of the metals comprising them become apparent. Dental School, University of Queenslond, T u r b o t Street. Brisbane, Q'ld. 4000.
