Journal of Oral Rehabilitation, 1978, Volume 4, pages 111-115
Wear of combinations of acrylic resin and porcelain, on an abrasion testing machine
ALAN HARRISON University of Wales
Department of Restorative Dentistry, Dental School,
Summary
Wear tests of various combinations of acrylic resin and porcelain were made using a machine which was designed to test materials under conditions similar to those of masticatory function by simulating the loads, sliding distances, and contact times encountered in the human masticatory cycle. The results showed that the amount of wear of the two materials worn in combination depended on the nature of the surrounding medium and on the surface roughness of the opposing material. Acrylic resin showed good wear resistance provided no third party abrasive or opposing hard, rough surface was present. When a mild abrasive was incorporated in the system, the acrylic resin vs acrylic resin combination wore almost seven times more than porcelain vs porcelain. Clinical experience would suggest that this is a reasonably sound order of wear. Introduction
When testing the laboratory wear of materials, it is important that the results correlate with the clinical situation. For example, it is possible under artificial wear conditions to show that porcelain abrades faster than acrylic resin (Mahalick, Knap & Weiter, 1971), although we know that clinically this is not the case. Porcelain, because of its brittle nature, has a tendency to crack and fracture under relatively small loads. Boddicker (1947) used loads of 6-25 lb to maintain occlusal contact between opposing complete dentures and noted that porcelain teeth cracked and shattered under these conditions. Thomson (1965) demonstrated the unsuitability of standard wear testing equipment using specimens of acrylic resin and porcelain and showed cracking of the porcelain surface and abrasion by accumulated debris. There have been few studies in the laboratory of wear of combinations of acrylic resin and porcelain and the results are contradictory. Greenwood (1952) noted the unexpectedly high rate of wear of acrylic resin teeth against porcelain teeth. His conclusions were that acrylic resin teeth wear most rapidly against natural teeth, less against porcelain teeth and least against acryhc resin teeth. Myerson (1957), Correspondence: Dr Alan Harrison, Department of Restorative Dentistry, Dental School, Heath Park, Cardiff CF4 4XY, Wales.
0305-182X/78/0400-0111 $02.00
© 1978 Blackwell Scientific Publications
111
112
A. Harrison
however, reported on combinations of porcelain-to-resin and claimed that this combination was more resistant to abrasion than were resin-to-resin artificial teeth. He showed that the wear was considerably less in the water-lubricated abrasion of the porcelain versus plastic teeth than in the plastic versus plastic teeth. As abrasive matter was introduced into the system more wear took place in the plastic versus porcelain combination and its superiority was narrowed. The wear of combinations of acrylic resin and porcelain has already been studied under clinical conditions (Harrison, 1976) and therefore further study was necessary to investigate these combinations in the laboratory. Preparation of specimens Since normal manufactured porcelain and acrylic resin teeth were used on the dentures in the clinical trial, in order to make a valid comparison with results from the machine, it was necessary to use materials of similar composition, manufactured under the same conditions. Pin and plate specimens of acrylic resin and porcelain were made using normal assembly line techniques and materials by two dental manufacturing companies.* The pin specimens were cylindrical in shape, 4-5 mm in diameter and approximately 4-0 mm in length. The plate specimens were 2-0 cm long, 1-0 cm wide and approximately 3-0 mm thick. Test method Wear tests were made using a previously described machine which is designed to test materials under conditions similar to those of masticatory function by simulating the loads, sliding distances, and contact times encountered in the human masticatory cycle (Harrison & Lewis, 1975). Combinations of acrylic resin and porcelain pin and plate specimens were worn against each other for 5 days under a pressure of 0-27 N/mm^. Tests were run with water in the test bed, no abrasive was introduced. The same combinations of acrylic resin and porcelain pin and plate specimens were repeated using calcium pyrophosphate (RDA 500 standard abrasive) in suspension in carboxylated methyl cellulose and water, as a mild abrasive medium. The time was reduced to 20 h. The same series was repeated under the same conditions except that 600 mesh carborundum powder in suspension in carboxylated methyl cellulose and water was used as a severe abrasive medium. The pin and plate combinations investigated were: (i) acrylic resin pin vs acrylic resin plate; (ii) acrylic resin pin vs glazed porcelain plate (roughness index 0-5 |U.m); (iii) acrylic resin pin vs sandblasted porcelain plate (roughness index 6-0 fj-m); (iv) porcelain pin (glazed) vs porcelain plate (glazed). The pin specimens of each material were mounted on the lower ends of the loading rods of the abrasion testing machine. The combined length of rod and specimen was measured using a bench micrometer accurate to 0-0002 mm, the mean value of five determinations being recorded. Each group of specimens was then worn on the machine in the various combinations mentioned above. The mean length of each rod and specimen was again determined and the loss calculated. The mean wear for * Dentsply Ltd., Brighton, Sussex, England, and Dentsply International, York, Pennsylvania, U.S.A.
Wear of acrylic resin and porcelain in combination
113
the group was calculated from the wear of each of the five specimens within the group. With water only in the system it is pumped across the plate specimens mounted on the test bed of the machine, the pin and plate specimens are therefore immersed in a constant flow of water. The abrasives in suspension are held in individual baths around each plate specimen. Results
The results of the loss of material from the pin specimens are shown in Table I. The combinations tested in a water only environment show least wear. Only acrylic resin pins vs sandblasted porcelain of roughness index 6-0 /xm, to simulate porcelain which has lost its glaze, showed significant wear. The insertion of a mild abrasive, calcium pyrophosphate, into the system affects the wear rates of the various combinations. There was little difference between acryhc resin versus acrylic resin and acrylic resin versus sandblasted porcelain. The acrylic resin vs glazed porcelain wore marginally less and the porcelain vs porcelain wore least of all.
Table 1. Laboratory results of the wear of combinations of acrylic resin and porcelain Environment
Specimen
Pin
Plate
Acrylic resin Acrylic resin Acrylic resin Porcelain
Acrylic resin Porcelain glazed Porcelain sandblasted Porcelain
Water only (5 days) Wear s.d. (mm)
Mild abrasive (20 h) Wear s.d. (mm)
Severe ;abrasive (20 h) Wear (mm) s.d.
Nil
—
0-0706t
0-009
0 -4497
0-0037*
0-0033
0-04181
0-0081
0 -7606§ 0-0105
0-1692
0-0647
0-0644t1: 0-0189
0 -6214§ 0-0542
0-0028*
0-0013
0-0124
0-0063
1 -2069
0-0458
0-0568
*, t, X, § Similar superscripts indicate no significant difference at the 1 % level.
The order of wear was completely upset by the incorporation of a severe abrasive into the system. Acrylic resin versus acrylic resin wore least; there was little difference between acrylic resin versus glazed and sandblasted porcelain, and the porcelain to porcelain specimens wore most. Discussion
It appears from the results that the two important features causing alterations in the wear rates are the nature of the opposing surface and the intervening abrasive slurry, i.e. the formation of two- or three-body wear processes, or a combination of the two. With water only in the system the wear is restricted to a two-body process since any loose particles are swept away by the constant irrigation of the water bath
114
A.Harrison
current. Thus, the amount of wear in this system is very much less in all combinations than in any other system. The acrylic resin shows good wear resistance provided no abrasive material is present. The tests were run continuously for five days, which is approximately equivalent to 5 months in the mouth, there being no measurable wear after this time. Although the resin is soft relative to porcelain, if there are no intermediate hard particles or an opposing hard, rough surface, it seems to possess very adequate wear resistance. Obviously, food and other factors in the mouth play a major part in the wear process, since all the clinical cases investigated with acrylic resin teeth wore in this time (Harrison, 1976). In water only, sandblasted porcelain caused considerably more wear to acrylic resin than a glazed porcelain surface. This situation can be equated to rubbing the acrylic resin against a sheet of abrasive paper. It does illustrate that when opposing combinations of acrylic resin and porcelain teeth are used on dentures, as in the case of Myerson and Sears' teeth,* the porcelain teeth should maintain their original glaze for as long as possible. If alterations are necessary to improve the occlusion, either in the laboratory or clinic, the opposing acrylic resin teeth should be ground. When a mild abrasive is incorporated in the system the acrylic resin vs acrylic resin combination wore almost 7 times more than porcelain vs porcelain. Clinical experience would suggest that this is a reasonably sound order of wear. The wear rates of acrylic resin vs acrylic resin and acrylic resin vs porcelain combinations are evened up considerably by the abrasive present. The sandblasted porcelain caused slightly more wear to the opposing acrylic resin than glazed porcelain, although the difference is only significant at the 5% level. These results correlate well with the clinical data and it would seem likely that in the mouth the wear processes which occur are a combination of two-body wear and three-body wear with only mild abrasives present, since the results with severe abrasives in this series clearly indicate a total reversal of clinically accepted and observed wear characteristics. The order of wear produced by the use of a severe abrasive rnedium (600 grit carborundum) is obviously artificial. The hard particles have a severe effect on the porcelain and cause rapid loss from its surface. It may be that the softer, more elastic acrylic resin can absorb more of the energy when ground against those particles and is, therefore, not affected to the same extent as the porcelain. Conclusions
Acrylic resin shows good wear resistance provided no third party abrasive or opposing hard, rough surface is present. The clinical situation is attained by the use of a mild abrasive slurry; severe abrasives are not commonly part of the normal masticatory process. In water only, sandblasted porcelain caused considerably more wear to the opposing acrylic resin surfaces than a glazed porcelain surface. When acrylic resin and porcelain teeth are used in opposing combinations on dentures the porcelain teeth should not be ground, either in the laboratory or in the clinic. The abrasion testing machine used with the appropriate mild abrasive slurry can effectively duplicate the clinical results which were proposed by Harrison (1976). * Myerson Tooth Corporation, Cambridge, Mass., U.S.A.
Wear of acrylic resin and porcelain in combination
115
Acknowledgments I am indebted to Mr K. J. White of Dentsply Ltd., Brighton, England, and Mr F. F. Koblitz, of Dentsply International, York, Pennsylvania, U.S.A., for their help and assistance in the manufacture of the acrylic resin and porcelain test pieces. References BoDDiCKER, V.S. (1947) Abrasion tests for artificial teeth. Journal of the Americal Dental Association, 35, 793. GREENWOOD, A . R . T . (1952) The wear resistance of synthetic resin teeth. M.Sc. thesis. Commonwealth of Australia Department of Health. HARRISON, A. (1976) Clinical results of the measurement of occlusal wear of complete dentures. Journal of Prosthetic Dentistry, 35, 504. HARRISON, A. & LEWIS, T.T. (1975) The development of an abrasion testing machine for dental materials. Journal of Biomedicai Materials Research, 9, 341. MAHALICK, J.A., KNAP, F.J. & WEITER, E.J. (1971) Occlusal wear in prosthodontics. Journal of the American Dental Association, 82, 154. MYERSON, R.L. (1957) The use of porcelain and plastic teeth in opposing complete dentures. Journal of Prosthetic Dentistry, 7, 625. THOMSON, J.C. (1965) Attrition of acrylic resin teeth. Dental Practitioner, 15, 233.
Manuscript accepted 24 January 1977
Wear of combinations of acrylic resin and porcelain, on an abrasion testing machine
ALAN HARRISON University of Wales
Department of Restorative Dentistry, Dental School,
Summary
Wear tests of various combinations of acrylic resin and porcelain were made using a machine which was designed to test materials under conditions similar to those of masticatory function by simulating the loads, sliding distances, and contact times encountered in the human masticatory cycle. The results showed that the amount of wear of the two materials worn in combination depended on the nature of the surrounding medium and on the surface roughness of the opposing material. Acrylic resin showed good wear resistance provided no third party abrasive or opposing hard, rough surface was present. When a mild abrasive was incorporated in the system, the acrylic resin vs acrylic resin combination wore almost seven times more than porcelain vs porcelain. Clinical experience would suggest that this is a reasonably sound order of wear. Introduction
When testing the laboratory wear of materials, it is important that the results correlate with the clinical situation. For example, it is possible under artificial wear conditions to show that porcelain abrades faster than acrylic resin (Mahalick, Knap & Weiter, 1971), although we know that clinically this is not the case. Porcelain, because of its brittle nature, has a tendency to crack and fracture under relatively small loads. Boddicker (1947) used loads of 6-25 lb to maintain occlusal contact between opposing complete dentures and noted that porcelain teeth cracked and shattered under these conditions. Thomson (1965) demonstrated the unsuitability of standard wear testing equipment using specimens of acrylic resin and porcelain and showed cracking of the porcelain surface and abrasion by accumulated debris. There have been few studies in the laboratory of wear of combinations of acrylic resin and porcelain and the results are contradictory. Greenwood (1952) noted the unexpectedly high rate of wear of acrylic resin teeth against porcelain teeth. His conclusions were that acrylic resin teeth wear most rapidly against natural teeth, less against porcelain teeth and least against acryhc resin teeth. Myerson (1957), Correspondence: Dr Alan Harrison, Department of Restorative Dentistry, Dental School, Heath Park, Cardiff CF4 4XY, Wales.
0305-182X/78/0400-0111 $02.00
© 1978 Blackwell Scientific Publications
111
112
A. Harrison
however, reported on combinations of porcelain-to-resin and claimed that this combination was more resistant to abrasion than were resin-to-resin artificial teeth. He showed that the wear was considerably less in the water-lubricated abrasion of the porcelain versus plastic teeth than in the plastic versus plastic teeth. As abrasive matter was introduced into the system more wear took place in the plastic versus porcelain combination and its superiority was narrowed. The wear of combinations of acrylic resin and porcelain has already been studied under clinical conditions (Harrison, 1976) and therefore further study was necessary to investigate these combinations in the laboratory. Preparation of specimens Since normal manufactured porcelain and acrylic resin teeth were used on the dentures in the clinical trial, in order to make a valid comparison with results from the machine, it was necessary to use materials of similar composition, manufactured under the same conditions. Pin and plate specimens of acrylic resin and porcelain were made using normal assembly line techniques and materials by two dental manufacturing companies.* The pin specimens were cylindrical in shape, 4-5 mm in diameter and approximately 4-0 mm in length. The plate specimens were 2-0 cm long, 1-0 cm wide and approximately 3-0 mm thick. Test method Wear tests were made using a previously described machine which is designed to test materials under conditions similar to those of masticatory function by simulating the loads, sliding distances, and contact times encountered in the human masticatory cycle (Harrison & Lewis, 1975). Combinations of acrylic resin and porcelain pin and plate specimens were worn against each other for 5 days under a pressure of 0-27 N/mm^. Tests were run with water in the test bed, no abrasive was introduced. The same combinations of acrylic resin and porcelain pin and plate specimens were repeated using calcium pyrophosphate (RDA 500 standard abrasive) in suspension in carboxylated methyl cellulose and water, as a mild abrasive medium. The time was reduced to 20 h. The same series was repeated under the same conditions except that 600 mesh carborundum powder in suspension in carboxylated methyl cellulose and water was used as a severe abrasive medium. The pin and plate combinations investigated were: (i) acrylic resin pin vs acrylic resin plate; (ii) acrylic resin pin vs glazed porcelain plate (roughness index 0-5 |U.m); (iii) acrylic resin pin vs sandblasted porcelain plate (roughness index 6-0 fj-m); (iv) porcelain pin (glazed) vs porcelain plate (glazed). The pin specimens of each material were mounted on the lower ends of the loading rods of the abrasion testing machine. The combined length of rod and specimen was measured using a bench micrometer accurate to 0-0002 mm, the mean value of five determinations being recorded. Each group of specimens was then worn on the machine in the various combinations mentioned above. The mean length of each rod and specimen was again determined and the loss calculated. The mean wear for * Dentsply Ltd., Brighton, Sussex, England, and Dentsply International, York, Pennsylvania, U.S.A.
Wear of acrylic resin and porcelain in combination
113
the group was calculated from the wear of each of the five specimens within the group. With water only in the system it is pumped across the plate specimens mounted on the test bed of the machine, the pin and plate specimens are therefore immersed in a constant flow of water. The abrasives in suspension are held in individual baths around each plate specimen. Results
The results of the loss of material from the pin specimens are shown in Table I. The combinations tested in a water only environment show least wear. Only acrylic resin pins vs sandblasted porcelain of roughness index 6-0 /xm, to simulate porcelain which has lost its glaze, showed significant wear. The insertion of a mild abrasive, calcium pyrophosphate, into the system affects the wear rates of the various combinations. There was little difference between acryhc resin versus acrylic resin and acrylic resin versus sandblasted porcelain. The acrylic resin vs glazed porcelain wore marginally less and the porcelain vs porcelain wore least of all.
Table 1. Laboratory results of the wear of combinations of acrylic resin and porcelain Environment
Specimen
Pin
Plate
Acrylic resin Acrylic resin Acrylic resin Porcelain
Acrylic resin Porcelain glazed Porcelain sandblasted Porcelain
Water only (5 days) Wear s.d. (mm)
Mild abrasive (20 h) Wear s.d. (mm)
Severe ;abrasive (20 h) Wear (mm) s.d.
Nil
—
0-0706t
0-009
0 -4497
0-0037*
0-0033
0-04181
0-0081
0 -7606§ 0-0105
0-1692
0-0647
0-0644t1: 0-0189
0 -6214§ 0-0542
0-0028*
0-0013
0-0124
0-0063
1 -2069
0-0458
0-0568
*, t, X, § Similar superscripts indicate no significant difference at the 1 % level.
The order of wear was completely upset by the incorporation of a severe abrasive into the system. Acrylic resin versus acrylic resin wore least; there was little difference between acrylic resin versus glazed and sandblasted porcelain, and the porcelain to porcelain specimens wore most. Discussion
It appears from the results that the two important features causing alterations in the wear rates are the nature of the opposing surface and the intervening abrasive slurry, i.e. the formation of two- or three-body wear processes, or a combination of the two. With water only in the system the wear is restricted to a two-body process since any loose particles are swept away by the constant irrigation of the water bath
114
A.Harrison
current. Thus, the amount of wear in this system is very much less in all combinations than in any other system. The acrylic resin shows good wear resistance provided no abrasive material is present. The tests were run continuously for five days, which is approximately equivalent to 5 months in the mouth, there being no measurable wear after this time. Although the resin is soft relative to porcelain, if there are no intermediate hard particles or an opposing hard, rough surface, it seems to possess very adequate wear resistance. Obviously, food and other factors in the mouth play a major part in the wear process, since all the clinical cases investigated with acrylic resin teeth wore in this time (Harrison, 1976). In water only, sandblasted porcelain caused considerably more wear to acrylic resin than a glazed porcelain surface. This situation can be equated to rubbing the acrylic resin against a sheet of abrasive paper. It does illustrate that when opposing combinations of acrylic resin and porcelain teeth are used on dentures, as in the case of Myerson and Sears' teeth,* the porcelain teeth should maintain their original glaze for as long as possible. If alterations are necessary to improve the occlusion, either in the laboratory or clinic, the opposing acrylic resin teeth should be ground. When a mild abrasive is incorporated in the system the acrylic resin vs acrylic resin combination wore almost 7 times more than porcelain vs porcelain. Clinical experience would suggest that this is a reasonably sound order of wear. The wear rates of acrylic resin vs acrylic resin and acrylic resin vs porcelain combinations are evened up considerably by the abrasive present. The sandblasted porcelain caused slightly more wear to the opposing acrylic resin than glazed porcelain, although the difference is only significant at the 5% level. These results correlate well with the clinical data and it would seem likely that in the mouth the wear processes which occur are a combination of two-body wear and three-body wear with only mild abrasives present, since the results with severe abrasives in this series clearly indicate a total reversal of clinically accepted and observed wear characteristics. The order of wear produced by the use of a severe abrasive rnedium (600 grit carborundum) is obviously artificial. The hard particles have a severe effect on the porcelain and cause rapid loss from its surface. It may be that the softer, more elastic acrylic resin can absorb more of the energy when ground against those particles and is, therefore, not affected to the same extent as the porcelain. Conclusions
Acrylic resin shows good wear resistance provided no third party abrasive or opposing hard, rough surface is present. The clinical situation is attained by the use of a mild abrasive slurry; severe abrasives are not commonly part of the normal masticatory process. In water only, sandblasted porcelain caused considerably more wear to the opposing acrylic resin surfaces than a glazed porcelain surface. When acrylic resin and porcelain teeth are used in opposing combinations on dentures the porcelain teeth should not be ground, either in the laboratory or in the clinic. The abrasion testing machine used with the appropriate mild abrasive slurry can effectively duplicate the clinical results which were proposed by Harrison (1976). * Myerson Tooth Corporation, Cambridge, Mass., U.S.A.
Wear of acrylic resin and porcelain in combination
115
Acknowledgments I am indebted to Mr K. J. White of Dentsply Ltd., Brighton, England, and Mr F. F. Koblitz, of Dentsply International, York, Pennsylvania, U.S.A., for their help and assistance in the manufacture of the acrylic resin and porcelain test pieces. References BoDDiCKER, V.S. (1947) Abrasion tests for artificial teeth. Journal of the Americal Dental Association, 35, 793. GREENWOOD, A . R . T . (1952) The wear resistance of synthetic resin teeth. M.Sc. thesis. Commonwealth of Australia Department of Health. HARRISON, A. (1976) Clinical results of the measurement of occlusal wear of complete dentures. Journal of Prosthetic Dentistry, 35, 504. HARRISON, A. & LEWIS, T.T. (1975) The development of an abrasion testing machine for dental materials. Journal of Biomedicai Materials Research, 9, 341. MAHALICK, J.A., KNAP, F.J. & WEITER, E.J. (1971) Occlusal wear in prosthodontics. Journal of the American Dental Association, 82, 154. MYERSON, R.L. (1957) The use of porcelain and plastic teeth in opposing complete dentures. Journal of Prosthetic Dentistry, 7, 625. THOMSON, J.C. (1965) Attrition of acrylic resin teeth. Dental Practitioner, 15, 233.
Manuscript accepted 24 January 1977
