Th ear of enamel opposing materials: An in vitro study Ralph DeLong, DDS, MS, PhD,a Maria William H. Douglas, BIDS, MS, PhDb Biomaterials
shaded
R. Pintado,
MPH,a
ceramic
restorative
and
Research Center,University of MinnesotaSchoolof Dentistry, Minneapolis,Minn.
The wear rate of intact human enamel opposed by Olympia porcelain gold, Dicer, Ceramco porcelain, and externally shaded Dicer and Ceramco was investigated with an artificial oral environment. The enamel-material couples were subjected to 300,000 masticatory cycles at a maximal occlusal force of 13.4 N while they were continuously bathed with 37” C deionized water. Both the enamel and material surfaces were analyzed by use of a three-dimensional surface monitoring computer program, AnSur, to record the removal of the material and the maximal loss of vertical height. The enamel opposing the externally shaded materials abraded two to five times more than that opposing the unshaded materials and 10 to 15 times more than enamel opposing gold. The wear rates for enamel opposing the gold and unshaded Dicer were similar both in the removal of material and in the loss in vertical height. (J PROSTHET DENT 1992;68:42-8.)
T
he demandfor estheticsin dental restorative materials hascontributed to a dramatic increasein porcelain occlusalsurfaces.r,2 This can have seriousconsequencesif the porcelain restoration opposesan enamel surface, becausein vitro studies have repeatedly shownthe excessive wear of enamelopposingporcelairi.3-5This inordinate wear hasbeen clinically documented b:yWiley,2 who usedqualitative methods. Dicer material (Dentsply/York Division, Dentsply International, York, Pa.), a castable glass,has beensuggestedasan esthetic alternative for porcelain and it is less abrasive against enamel surfaces. The reduced wear hasbeenattributed to the similar hardnessvaluesfor enameland Dicer and the shadingporcelainsof Dicer that incorporate minimal amounts of abrasives.6 Unfortunately, this claim is difficult to document becausein vivo evaluations of enamel wear are arduous. A closed-loop,servohydraulic artificial oral environment was developedat the University of Minnesota Dental Schoolto resolve this problem.7 This system accurately reproduces the forces, movements, and environmental conditions of the human oral cavity consistent with current physiologic knowledge. Measurement of the articular contact wear of various restorative materials subjectedto this artificial oral environment was well correlated with the corresponding contact wear recorded clinically.s-10The wear of enamel opposedby Dicer and by two different porcelain systems was alsoinvestigated in the artificial environment. There was a 50% reduction in the total enamel removed when opposedby unshadedDicer material compared with the unshadedporcelains.The majority of ceramic restorations
aAssociate Professor. bProfessor. 10/l/37420
42
D+S
P
D G
0
150 Cycles
300 (xl
000)
1. Volume lossof enamelopposingdifferent restorative materials in an artificial oral environment. (G, Gold; D, Dicer; P, porcelain; D + S, Dicer -+-shading; P + S, porcelain + shading.) Vertical lines identify curves that are not significantly different (p > 0.05). Fig.
are externally shadedbefore placements,sothis study determined if external shadingsubstantially altered the wear of enamel.The wear of enamelopposinga porcelain fusedto-metal gold surface served as a control, becausemetal occlusal surfaces have been recommended with metal/ ceramic restorations to minimize enamel wear.
METHODS
AND
MATERIALS
Two ceramicsystemsweretested: a castableglasssystem (Dicer, Corning Glass,Works,Corning, N.Y.) and a porcelain-fused-to-metal (PFM) system using a conventional dental porcelain (CeramcoII body porcelain, CeramcoInc.,
JULY1992
VOLUME68
NUMBER1
ENAMEL
Table
WEAR
AND
I. Mean
CERAMIC
enamel volume
Cycles
loss in cubic millimeters
Gold
150K 300K
0.014 0.025
PFM, Porcelain fused-to-metal. *Reproduced with permission
Table
RESTORATIONS
11. Mean
Dicer*
f 0.005 f 0.006
from DeLong
maximum
0.033 0.055
loss of vertical
height
deviation
PFM*
* 0.014 i 0.020
R, et al. Dent Mater
Gold
Cycles
f 1 standard
0.091 0.162
Dicer
f 0.025 i- 0.057
0.155 0.255
+ shading
i 0.051 + 0.102
PFM
0.178 0.366
+ shading
?I 0.026 + 0.074
1989;5:266-71
in millimeters
L 1 standard
Dicer*
PFM*
deviation Dicer
+ shading
PFM
+ shading
Enamel 150K 300K
0.061 0.085
i 0.017 +- 0.020
0.076 k 0.024 0.098 + 0.024
0.148 0.187
k 0.032 + 0.055
0.224 0.294
-t 0.044 + 0.063
0.202 0.284
t 0.034 + 0.046
0.070 0.094
* 0.015 +- 0.018
0.152 i 0.035 0.192 f 0.045
0.272 0.352
f 0.049 + 0.089
0.248 0.345
t 0.051 t 0.069
0.273 0.388
_t 0.053 f 0.072
Combined 15OK 300K Abbreviations *Reproduced
as in Table I. from DeLong R, et al. Dent Mater
1989;5:266-71.
Johnson & Johnson Co., East Windsor, N.J.). Ceramic disks were designed as the mandibular element of the artificial oral environment. Ten PFM circular disks, 12 mm in diameter and 3 mm thick, were prepared by a commercial dental laboratory according to the manufacturer’s recommendations. The disks were initially polished to a flat surface using 240 grit and then 500 grit aluminum oxide sandpaper, and were finally cleaned in an ultrasonic cleaner. Five of the samples received external shading and all the samples were glazed according to the manufacturer’s recommendations. The Dicer disks were 10 mm in diameter, 2 mm thick, and were prepared by the Corning Glass Works. Five of the disks had external shading applied and five were in a natural glazed condition. The Dicer disks were polished on one side by the manufacturer. Five disks of dental porcelain gold (Olympia, Penwalt/ Jelenko, Armonk, N.Y.), 10 mm in diameter and 1 mm thick, were fabricated following the manufacturer’s recommendations. The gold disks were polished on one side using traditional dental polishing techniques. Extracted maxillary third molars were used as the maxillary component of the artificial oral environment and were stored in deionized water at 4’ C. The maxillary specimens were prepared by removing the buccal cusps and isolating the mesiolingual cusp. The mandibular and maxillary elements were mounted in nylon rings using a chemically cured composite resin and were stored in deionized water at 37” C for at least 24 hours. The maxillary molars were mounted so the enamel had functional contacts with the opposing disks. Each enamel-material couple was subjected to 300,000 defined masticatory cycles in the artificial oral environment that has been extensively described.7-11 The masticatory parameters maintained during each cycle were: max-
THE
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DENTISTRY
imal occlusal force at 13.4 N; lateral excursion at 0.62 mm; cuspal contact time at 0.23 second; and a chewing rate of 4 Hz. Deionized water at 37’ C was continually circulated over the wear surface. Before each test the occlusal surfaces of each enamel cusp and disk were traced using a digital technique. The X, Y, and Z coordinates of 12,800 surface points were collected and arranged into 50 profiles. The profiles were assembled with computerized graphics to represent the surface. Complete occlusal mappings were repeated at 150,000 cycles and at 300,000 cycles. Changes in contour were determined by comparing the “before” and “after” surfaces, using the computer program An&r (Regents of the University of Minnesota, Minneapolis, Minn.).12,i3 The maximal depth and volume of the material removed were calculated for both the enamel cusp and disk. The qualitative alterations of the occluding surfaces were examined with scanning electron microscopic (SEM) photomicrographs.
RESULTS The mean values for the total volume of enamel removed are listed in Table I, and for comparison, the unshaded Dicar and unshaded Ceramco (PFM) volumes from DeLong et. al.ll are included in this table; the results are shown graphically in Fig. 1. The mean values for the maximal loss of vertical height for the enamel and the opposing material are presented in Table II and are displayed graphically in Figs. 2 and 3. The combined loss of height was determined by summing the maximal depths of wear facets for the enamel and opposing material. An analysis of variance was computed with one betweensubject variable or material type and one within-subject variable or number of cycles for each of the three conditions: enamel volume loss, maximal loss of enamel vertical
43
DELONG,
PINTADO,
AND
DOUGLAS
400
300
200
100
0
0 150
0
0
300
Cycles
(x
1000)
Fig. 2. Maximal depth of enamel wear facet by different restorative materials in an artificial oral environment. Abbreviations same as in Fig. 1. Vertical lines identify curves that are not significantly different (p > 0.05).
Table III. Volumetric millimeters/cycle Material
Enamel Enamel Enamel Enamel Enamel
wear factors in cubic
coluple
K, x lo3
vs vs Dicer gold
vs PFM vs Dicer -t shading vs PFM + shading
Normalized
0.184 0.083 0.541 0.851
1219 .
2.2 1.0 6.5 10.3 14.7
K,*
1
height, and combined loss of vertical height. A significant difference was noted (p < 0.0001) in all instances, and pairwise comparisons were performed using the NewmanKeuls test. Those materials that were not significantly different at the 5% confidence level are indicated in the figures by vertical bars. Theoretical models of tooth wear imply that the volume of occlusion wear increases in direct proportion to functional time. This has been confirmed by research on a variety of dental materials opposed by natural enamel in an artificial mouth environment.5,s-1’ It can be also stated that the volumetric wear of a material is defined as the total volume loss (V) divided by the total lateral excursion (L), and is directly proportional to the normal force (W) applied to the material and inversely proportional to the hardness (H) of the material. wear rate = V/L = K X W/H
where K is a generalized coefficient of wear that depends on the geometry and the mechanism of wear. Like the coeffi-
44
300
(xl 000)
Fig. 3. Combined loss of vertical height by different restorative materials in an artificial oral environment. Abbreviations same as in Fig. 1. Vertical lines identify curves that are not significantly different (p > 0.05).
cient of friction, K is dimensionless paring the occlusal wear of different excursion (L,) and the occlusal force artificial oral environment for all expression can be simplified and the in the number of cycles (N) can be
and is useful for commaterials. The lateral were consistent in the materials. Thus the volume loss recorded written as:
V = K,N where
Bracket indicates no significant difference aPthe 5 % confidence level. Abbreviations as in Table 1. *Normalized to the enamel versus gold couple, which is set to equal 1.
Volumetric
150
Cycles
K, = L,KW/H
and L = NL,.
K, is a volumetric wear factor representing the material removed during a chewing cycle. In the artificial oral environment, K, is a constant and the relationship between the volume of material removed and the number of cycles is linear. A linear regression analysis was performed for each enamel-material couple and the correlation coefficients ranged between 0.86 and 0.97. The mean wear factors K, that were calculated from the slopes of the regression lines are designated in Table III. A one-way analysis of variance calculated for the enamel volumetric wear factors revealed a significant difference (p < 0.0001) between the materials. A comparison of the pairs was also performed for the different materials using the Newman-Keuls test. Those wear factors that were not significantly different at the 5% confidence level are indicated by a vertical bracket in Table III.
DISCUSSION Two methods are commonly used to report the wear of restorative materials, the volume of material removed (volume loss) and loss of vertical height (height loss). The
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Fig. 4. SEM photomicrographs of enamel wear facet created when opposing Olympia gold (left, original magnification x300; bar = 100 pm) and unshaded Dicer material (right, original magnification X 150; bar = 200 pm) in the artificial oral environment after 300,000 masticatory cycles. Both samples exhibited a polished appearance without grooves.
!atter is more relevant clinically, because it correlates with the diminished vertical dimension of occlusion. The former method is an easier method for evaluation of wear because of its linearity with time. Volume
Loss
The artificial oral environment has demonstrated a greater correlation with clinical wear studies assuming that 250,000 cycles simulates 1 year of clinical wear with normal intraoral conditions. In the artificial oral environment, as depicted in the results, the material loss was linear with time and was verified in Fig. 1, if each enamel-material couple was calculated using linear regression analysis. The correlation coefficients ranged from 0.86 to 0.97, indicating a high degree of linearity for each couple. This linearity was also expected to be valid clinically because the mean chewing forces and patterns for a patient do not change substantially with time. For these reasons, volumetric wear investigations in an artificial oral environment can predict clinical conditions. This has been confirmed by direct clinical comparisons.8-10 The volumetric wear recorded for enamel in this research supported the manufacturer’s claim that Dicer material creates less enamel wear than porcelain. The different wear of enamel caused by different occluding materials can be compared by using the normalized volumetric wear factors. The wear factors, K, that are determined from the slopes of the wear curves represent the enamel removed in each chewing cycle. Dental gold is generally considered the ideal
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DENTISTRY
restorative material for opposing enamel, so the wear factors were based on gold to allow comparison of the different wear rat,es (Table III). Enamel abraded three times less if opposed by unshaded Dicer material than if opposed by unshaded porcelain. The Dicer material created twice the wear of enamel as gold, but this difference was not significant (p > 0.05). Unshaded Dicer material appears an excellent alternative to porcelain if opposing enamel contacts the restoration and esthetics are critical. The wear coefficient for enamel opposed by shaded Dicer was 70 % of the coefficient for enamel opposed by the shaded porcelain. This supported the manufacturer’s claim that shaded Dicar wears enamel less than shaded porcelain. However, the wear coefficient for enamel opposed by shaded Dicer was five times larger than-the coefficient for unshaded Dicer and nearly twice as large as the coefficient for unshaded porcelain. This indicated that the wear of enamel was significantly increased with the addition of shade. Thus shading should be avoided where enamel contacts the restoration, and this is true for both Dicer material and porcelain. Height
loss
The enamel removed by a ceramic restoration is dramatically increased with the addition of shading. While the removal of enamel can be crucial, it may not have an immediate clinical effect on the occlusion. A considerable amount of enamel can be removed with limited loss in vertical dimension if the wear is distributed to an extensive 45
DELONG,
Fig. 5. SEM photomicrographs (original magnification X150; bar enamel wear facet created opposing Ceramco porcelain (left), shaded (center), and shaded Dicer material (right) in the artificial oral 300,000 masticatory cycles. Grooves characteristic of abrasive wear photomicrographs.
Fig. 6. SEM photomicrograph
of enamel wear facet created by shaded Dicer. Arrow indicates reduced enamel wear after shade was removed by abrasive wear. (Original magnification ~40; bar = 500 pm.)
area on the occlusal surface. Therefore it is crucial to record the loss of vertical height. Fig. 2 demonstrates the maximal loss in vertical height of the enamel wear facet as a function of the number of cycles. The values were determined
46
PINTADO,
AND
DOUGLAS
= 200 pm) of the Ceramco porcelain environment after were visible in the
by comparing the original unworn surface with the abraded surface using the surface analysis program AnSur. The greatest vertical loss between the two surfaces at the wear facet was used to record the loss in vertical height. The loss in vertical height for the disks was reported in a similar manner. The curves for the loss of vertical height (Figs. 2 and 3) illustrated a progressive decrease in wear with time, but the volume curves were linear. Conversely, this loss in vertical height has been represented by a quadratic equation.5, g The loss in the vertical height by enamel wear can be divided into three groups (Fig. 2) that are statistically distinct (p < 0.05). Group 1, that created the least enamel wear, contained gold and Dicer material; group 2 consisted of unshaded porcelain; and group 3, that was responsible for the greatest amount of wear, involved the shaded Dicer material and shaded porcelain. These results were similar to those for the volume loss, but the wear of enamel caused by the shaded materials was not statistically different (p > 0.05). The combined loss in vertical height of the couples is critical clinically because it is a direct measure of the loss in the vertical dimension of occlusion. When the greatest loss of vertical height for the enamel and the corresponding opposing material were combined, a distinction between groups 2 and 3 was impossible-that is, porcelain, shaded porcelain, and shaded Dicer material all created a similar loss in vertical dimension. However, the loss in the
JULY
1992
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CERAMIC
RESTORATIONS
Fig. 7, SEM photomicrograph of wear facet created on shaded Dicer material. Arrow indicates surface after complete shade removal, exposing unshaded Dicer material. (Original magnification x40; bar = 500 ym.)
vertical dimension of occlusion resulting from the enamelDicer material couple was significantly greater than that for the enamel-gold couple @ < 0.05). When considering both forms of wear analysis, volume loss and height loss, the material couples can be divided into two groups. The first group consisted of gold and Dicar material and the second group included porcelain and the shaded materials. The SEM photomicrographs of the enamel wear facets verified this classification. The enamel facets created by gold and Dicer material (Fig. 4) were similar in appearance but quite different from those resulting from porcelain or the shaded materials (Fig. 5). The former were smooth and with a polished appearance when viewed microscopically. The wear facets caused by the second group exhibited large grooves that were indicative of a more abrasive wear process.5 The wear characterized by the shaded Dicer material was unique because it appeared self-limiting (Fig. 6). There is a central region of enamel that abraded at a slower rate and this attribute was confirmed on the shaded Dicer material disk that revealed that this facet has two distinct wear processes (Fig. 7). The outer surface of the wear facet represents the usual abrasive wear, while the inner central region of the wear facet is similar to that of the unshaded Dicar material. Thus once the shade has been worn through, a shaded Dicer restoration performs in a similar fashion to the unshaded Dicer restoration. This same effect was not apparent with any shaded porcelain restorations. The elevation of wear rates can be confirmed by the SEM photomicrographs of the facets on the shaded materials. A relativeiy soft, glassy phase surrounds the harder metal
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JOURNAL
DF
PROSTHETLC
DENTISTRY
Fig. 8. SEM photomicrograph of the trailing edge of a shaded Dicer material wear facet. Preferential wearing of the shading exposed abrasive particles (arrow) incorporated in the shaded material. (Original magnification X1100; bar = 20 pm.)
Fig. 9. SEM photomicrograph of the edge of a wear facet on shaded Dicer material showing porosity with application of shading (arrow). (Original magnification x430; bar = 50 Rm.)
oxide particles in the shading materials for both Dicer and ceramic porcelain, and this wears preferentially, leaving the irregular particles exposed (Fig. 8). Second, the addition of the shading commonly incorporates porosity in the restoration, and as the shades wear the porosity is exposed, leaving a “cheese grater”-appearing surface (Fig. 9). Research is needed to clarify whether either of these factors
47
DELONG,
accounts for the inordinate shaded restorations.
wear of enamel
attributed
to
CONCLUSIONS The addition of external shading to restorative porcelain caused an elevation in the wear of enamel two to five times more than that of unshaded restorative porcelain, and 10 to 15 times greater than that of enamel opposing gold. The ranking of these materials according to the magnitude of enamel removed with time was: gold I Dicer material < Porcelain < Dicer material + shading < Porcelain + shading A ranking
for loss of vertical
dimension
of occlusion
was:
gold < Dicer material < Porcelain = Dicer material + shading = Porcelain + shading Either ranking suggested that ceramic restorations should not have external shade applied to contacting occlusal surfaces. However, the wear created by the shaded Dicer restorations appeared self-limiting, but this property was not evident for shaded porcelain. REFERENCES 1. Christensen GJJ. The uses of porcelain-fused-to-metal restorations in current dental practice: a survey. J PROSTHET DENT 1986;56:1-3. 2. Wiley MG. Effects of porcelain on occluding surfaces of restored teeth. J PROSTHET DENT 1989;61:133-7. 3. Mahalick JA, Knap FJ, Weiter EJ. Occlusal wear in prosthodontics. J Am Dent Assoc 1971;82:154-9. 4. Monasky GE, Taylor DF. Studies on the wear of porcelain, enamel and gold. J PROSTHET DENT 1971;25:299-306.
48
PINTADO,
AND
DOUGLAS
5. DeLong R, Douglas WH, Sakaguchi RL, Pintado MR. The wear of dental porcelain in an artificial mouth. Dent Mater 1986;2:214-9. 6. Grossman DG. Cast glass ceramics. Dent Clin North Am 1985;29:72539. 7. DeLong R, Douglas WH. Development of an artificial oral environment for testing of dental restoratives: bi-axial force and movement control. J Dent Res 1983;62:32-6. 8. Coffey JP, Goodkind RJ, DeLong R, Douglas WH. In vitro study of the wear characteristics of natural and artificial teeth. J PROSTHET DENT 1985;54:273-80. 9. Sakaguchi RL, Douglas WH, DeLong R, Pintado MR. The wear of a posterior composite in an artificial mouth: a clinical correlation. Dent Mater 1986;2:235-40. 10. DeLong R, Sakaguchi RL, Douglas WH, Pintado MR. The wear of dental amalgam in an artificial mouth: a clinical correlation. Dent Mater 1985;6:238-42. 11. DeLong R, Sasik C, Pintado MR, Douglas WH. The wear of enamel when opposed by ceramic systems. Dent Mater 1989;5:4:266-71. 12. DeLong R, Pintado MR, Douglas WH. Measurement of change in surface contour by computer graphics. Dent Mater 1985;1:27-30. 13. DeLong R, Douglas WH. Quantitative assessment of anatomical change in the human dentition. In: Pederson PC, Onaral B, eds. Proceedings of the Twelfth Annual International Conference of the IEEE Engineering in Medicine and Biology Society. Piscataway, N.J.: IEEE, 1990:2054-5. 14. Archard JF. Contact and rubbing of flat surfaces. 3 Appl Physiol 1953; 24:981-S. 15. Greenwood JA, Williamson JB. Contact of nominally flat surfaces. Proc R Sot (Land) 1966;A295:300. 16. Burton RA. Friction and wear. In: Szeri AR, ed. Tribology. New York: McGraw-Hill, 1980:19-23. Reprint requests to: DR. RALPH DELONG UNIVERSITY OF MINNESOTA SCHOOL OF DENTISTRY BIOMATERIALS RESEARCH CENTER 16-212 Moos HEALTH SCIENCES TOWER 515 DELAWARE STREET SE MIXNEAPOLIS, MN 55455
JULY
1992
VOLUME
68
NUMBER
1
shaded
R. Pintado,
MPH,a
ceramic
restorative
and
Research Center,University of MinnesotaSchoolof Dentistry, Minneapolis,Minn.
The wear rate of intact human enamel opposed by Olympia porcelain gold, Dicer, Ceramco porcelain, and externally shaded Dicer and Ceramco was investigated with an artificial oral environment. The enamel-material couples were subjected to 300,000 masticatory cycles at a maximal occlusal force of 13.4 N while they were continuously bathed with 37” C deionized water. Both the enamel and material surfaces were analyzed by use of a three-dimensional surface monitoring computer program, AnSur, to record the removal of the material and the maximal loss of vertical height. The enamel opposing the externally shaded materials abraded two to five times more than that opposing the unshaded materials and 10 to 15 times more than enamel opposing gold. The wear rates for enamel opposing the gold and unshaded Dicer were similar both in the removal of material and in the loss in vertical height. (J PROSTHET DENT 1992;68:42-8.)
T
he demandfor estheticsin dental restorative materials hascontributed to a dramatic increasein porcelain occlusalsurfaces.r,2 This can have seriousconsequencesif the porcelain restoration opposesan enamel surface, becausein vitro studies have repeatedly shownthe excessive wear of enamelopposingporcelairi.3-5This inordinate wear hasbeen clinically documented b:yWiley,2 who usedqualitative methods. Dicer material (Dentsply/York Division, Dentsply International, York, Pa.), a castable glass,has beensuggestedasan esthetic alternative for porcelain and it is less abrasive against enamel surfaces. The reduced wear hasbeenattributed to the similar hardnessvaluesfor enameland Dicer and the shadingporcelainsof Dicer that incorporate minimal amounts of abrasives.6 Unfortunately, this claim is difficult to document becausein vivo evaluations of enamel wear are arduous. A closed-loop,servohydraulic artificial oral environment was developedat the University of Minnesota Dental Schoolto resolve this problem.7 This system accurately reproduces the forces, movements, and environmental conditions of the human oral cavity consistent with current physiologic knowledge. Measurement of the articular contact wear of various restorative materials subjectedto this artificial oral environment was well correlated with the corresponding contact wear recorded clinically.s-10The wear of enamel opposedby Dicer and by two different porcelain systems was alsoinvestigated in the artificial environment. There was a 50% reduction in the total enamel removed when opposedby unshadedDicer material compared with the unshadedporcelains.The majority of ceramic restorations
aAssociate Professor. bProfessor. 10/l/37420
42
D+S
P
D G
0
150 Cycles
300 (xl
000)
1. Volume lossof enamelopposingdifferent restorative materials in an artificial oral environment. (G, Gold; D, Dicer; P, porcelain; D + S, Dicer -+-shading; P + S, porcelain + shading.) Vertical lines identify curves that are not significantly different (p > 0.05). Fig.
are externally shadedbefore placements,sothis study determined if external shadingsubstantially altered the wear of enamel.The wear of enamelopposinga porcelain fusedto-metal gold surface served as a control, becausemetal occlusal surfaces have been recommended with metal/ ceramic restorations to minimize enamel wear.
METHODS
AND
MATERIALS
Two ceramicsystemsweretested: a castableglasssystem (Dicer, Corning Glass,Works,Corning, N.Y.) and a porcelain-fused-to-metal (PFM) system using a conventional dental porcelain (CeramcoII body porcelain, CeramcoInc.,
JULY1992
VOLUME68
NUMBER1
ENAMEL
Table
WEAR
AND
I. Mean
CERAMIC
enamel volume
Cycles
loss in cubic millimeters
Gold
150K 300K
0.014 0.025
PFM, Porcelain fused-to-metal. *Reproduced with permission
Table
RESTORATIONS
11. Mean
Dicer*
f 0.005 f 0.006
from DeLong
maximum
0.033 0.055
loss of vertical
height
deviation
PFM*
* 0.014 i 0.020
R, et al. Dent Mater
Gold
Cycles
f 1 standard
0.091 0.162
Dicer
f 0.025 i- 0.057
0.155 0.255
+ shading
i 0.051 + 0.102
PFM
0.178 0.366
+ shading
?I 0.026 + 0.074
1989;5:266-71
in millimeters
L 1 standard
Dicer*
PFM*
deviation Dicer
+ shading
PFM
+ shading
Enamel 150K 300K
0.061 0.085
i 0.017 +- 0.020
0.076 k 0.024 0.098 + 0.024
0.148 0.187
k 0.032 + 0.055
0.224 0.294
-t 0.044 + 0.063
0.202 0.284
t 0.034 + 0.046
0.070 0.094
* 0.015 +- 0.018
0.152 i 0.035 0.192 f 0.045
0.272 0.352
f 0.049 + 0.089
0.248 0.345
t 0.051 t 0.069
0.273 0.388
_t 0.053 f 0.072
Combined 15OK 300K Abbreviations *Reproduced
as in Table I. from DeLong R, et al. Dent Mater
1989;5:266-71.
Johnson & Johnson Co., East Windsor, N.J.). Ceramic disks were designed as the mandibular element of the artificial oral environment. Ten PFM circular disks, 12 mm in diameter and 3 mm thick, were prepared by a commercial dental laboratory according to the manufacturer’s recommendations. The disks were initially polished to a flat surface using 240 grit and then 500 grit aluminum oxide sandpaper, and were finally cleaned in an ultrasonic cleaner. Five of the samples received external shading and all the samples were glazed according to the manufacturer’s recommendations. The Dicer disks were 10 mm in diameter, 2 mm thick, and were prepared by the Corning Glass Works. Five of the disks had external shading applied and five were in a natural glazed condition. The Dicer disks were polished on one side by the manufacturer. Five disks of dental porcelain gold (Olympia, Penwalt/ Jelenko, Armonk, N.Y.), 10 mm in diameter and 1 mm thick, were fabricated following the manufacturer’s recommendations. The gold disks were polished on one side using traditional dental polishing techniques. Extracted maxillary third molars were used as the maxillary component of the artificial oral environment and were stored in deionized water at 4’ C. The maxillary specimens were prepared by removing the buccal cusps and isolating the mesiolingual cusp. The mandibular and maxillary elements were mounted in nylon rings using a chemically cured composite resin and were stored in deionized water at 37” C for at least 24 hours. The maxillary molars were mounted so the enamel had functional contacts with the opposing disks. Each enamel-material couple was subjected to 300,000 defined masticatory cycles in the artificial oral environment that has been extensively described.7-11 The masticatory parameters maintained during each cycle were: max-
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JOURNAL
OF PROSTHETIC
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imal occlusal force at 13.4 N; lateral excursion at 0.62 mm; cuspal contact time at 0.23 second; and a chewing rate of 4 Hz. Deionized water at 37’ C was continually circulated over the wear surface. Before each test the occlusal surfaces of each enamel cusp and disk were traced using a digital technique. The X, Y, and Z coordinates of 12,800 surface points were collected and arranged into 50 profiles. The profiles were assembled with computerized graphics to represent the surface. Complete occlusal mappings were repeated at 150,000 cycles and at 300,000 cycles. Changes in contour were determined by comparing the “before” and “after” surfaces, using the computer program An&r (Regents of the University of Minnesota, Minneapolis, Minn.).12,i3 The maximal depth and volume of the material removed were calculated for both the enamel cusp and disk. The qualitative alterations of the occluding surfaces were examined with scanning electron microscopic (SEM) photomicrographs.
RESULTS The mean values for the total volume of enamel removed are listed in Table I, and for comparison, the unshaded Dicar and unshaded Ceramco (PFM) volumes from DeLong et. al.ll are included in this table; the results are shown graphically in Fig. 1. The mean values for the maximal loss of vertical height for the enamel and the opposing material are presented in Table II and are displayed graphically in Figs. 2 and 3. The combined loss of height was determined by summing the maximal depths of wear facets for the enamel and opposing material. An analysis of variance was computed with one betweensubject variable or material type and one within-subject variable or number of cycles for each of the three conditions: enamel volume loss, maximal loss of enamel vertical
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400
300
200
100
0
0 150
0
0
300
Cycles
(x
1000)
Fig. 2. Maximal depth of enamel wear facet by different restorative materials in an artificial oral environment. Abbreviations same as in Fig. 1. Vertical lines identify curves that are not significantly different (p > 0.05).
Table III. Volumetric millimeters/cycle Material
Enamel Enamel Enamel Enamel Enamel
wear factors in cubic
coluple
K, x lo3
vs vs Dicer gold
vs PFM vs Dicer -t shading vs PFM + shading
Normalized
0.184 0.083 0.541 0.851
1219 .
2.2 1.0 6.5 10.3 14.7
K,*
1
height, and combined loss of vertical height. A significant difference was noted (p < 0.0001) in all instances, and pairwise comparisons were performed using the NewmanKeuls test. Those materials that were not significantly different at the 5% confidence level are indicated in the figures by vertical bars. Theoretical models of tooth wear imply that the volume of occlusion wear increases in direct proportion to functional time. This has been confirmed by research on a variety of dental materials opposed by natural enamel in an artificial mouth environment.5,s-1’ It can be also stated that the volumetric wear of a material is defined as the total volume loss (V) divided by the total lateral excursion (L), and is directly proportional to the normal force (W) applied to the material and inversely proportional to the hardness (H) of the material. wear rate = V/L = K X W/H
where K is a generalized coefficient of wear that depends on the geometry and the mechanism of wear. Like the coeffi-
44
300
(xl 000)
Fig. 3. Combined loss of vertical height by different restorative materials in an artificial oral environment. Abbreviations same as in Fig. 1. Vertical lines identify curves that are not significantly different (p > 0.05).
cient of friction, K is dimensionless paring the occlusal wear of different excursion (L,) and the occlusal force artificial oral environment for all expression can be simplified and the in the number of cycles (N) can be
and is useful for commaterials. The lateral were consistent in the materials. Thus the volume loss recorded written as:
V = K,N where
Bracket indicates no significant difference aPthe 5 % confidence level. Abbreviations as in Table 1. *Normalized to the enamel versus gold couple, which is set to equal 1.
Volumetric
150
Cycles
K, = L,KW/H
and L = NL,.
K, is a volumetric wear factor representing the material removed during a chewing cycle. In the artificial oral environment, K, is a constant and the relationship between the volume of material removed and the number of cycles is linear. A linear regression analysis was performed for each enamel-material couple and the correlation coefficients ranged between 0.86 and 0.97. The mean wear factors K, that were calculated from the slopes of the regression lines are designated in Table III. A one-way analysis of variance calculated for the enamel volumetric wear factors revealed a significant difference (p < 0.0001) between the materials. A comparison of the pairs was also performed for the different materials using the Newman-Keuls test. Those wear factors that were not significantly different at the 5% confidence level are indicated by a vertical bracket in Table III.
DISCUSSION Two methods are commonly used to report the wear of restorative materials, the volume of material removed (volume loss) and loss of vertical height (height loss). The
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Fig. 4. SEM photomicrographs of enamel wear facet created when opposing Olympia gold (left, original magnification x300; bar = 100 pm) and unshaded Dicer material (right, original magnification X 150; bar = 200 pm) in the artificial oral environment after 300,000 masticatory cycles. Both samples exhibited a polished appearance without grooves.
!atter is more relevant clinically, because it correlates with the diminished vertical dimension of occlusion. The former method is an easier method for evaluation of wear because of its linearity with time. Volume
Loss
The artificial oral environment has demonstrated a greater correlation with clinical wear studies assuming that 250,000 cycles simulates 1 year of clinical wear with normal intraoral conditions. In the artificial oral environment, as depicted in the results, the material loss was linear with time and was verified in Fig. 1, if each enamel-material couple was calculated using linear regression analysis. The correlation coefficients ranged from 0.86 to 0.97, indicating a high degree of linearity for each couple. This linearity was also expected to be valid clinically because the mean chewing forces and patterns for a patient do not change substantially with time. For these reasons, volumetric wear investigations in an artificial oral environment can predict clinical conditions. This has been confirmed by direct clinical comparisons.8-10 The volumetric wear recorded for enamel in this research supported the manufacturer’s claim that Dicer material creates less enamel wear than porcelain. The different wear of enamel caused by different occluding materials can be compared by using the normalized volumetric wear factors. The wear factors, K, that are determined from the slopes of the wear curves represent the enamel removed in each chewing cycle. Dental gold is generally considered the ideal
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restorative material for opposing enamel, so the wear factors were based on gold to allow comparison of the different wear rat,es (Table III). Enamel abraded three times less if opposed by unshaded Dicer material than if opposed by unshaded porcelain. The Dicer material created twice the wear of enamel as gold, but this difference was not significant (p > 0.05). Unshaded Dicer material appears an excellent alternative to porcelain if opposing enamel contacts the restoration and esthetics are critical. The wear coefficient for enamel opposed by shaded Dicer was 70 % of the coefficient for enamel opposed by the shaded porcelain. This supported the manufacturer’s claim that shaded Dicar wears enamel less than shaded porcelain. However, the wear coefficient for enamel opposed by shaded Dicer was five times larger than-the coefficient for unshaded Dicer and nearly twice as large as the coefficient for unshaded porcelain. This indicated that the wear of enamel was significantly increased with the addition of shade. Thus shading should be avoided where enamel contacts the restoration, and this is true for both Dicer material and porcelain. Height
loss
The enamel removed by a ceramic restoration is dramatically increased with the addition of shading. While the removal of enamel can be crucial, it may not have an immediate clinical effect on the occlusion. A considerable amount of enamel can be removed with limited loss in vertical dimension if the wear is distributed to an extensive 45
DELONG,
Fig. 5. SEM photomicrographs (original magnification X150; bar enamel wear facet created opposing Ceramco porcelain (left), shaded (center), and shaded Dicer material (right) in the artificial oral 300,000 masticatory cycles. Grooves characteristic of abrasive wear photomicrographs.
Fig. 6. SEM photomicrograph
of enamel wear facet created by shaded Dicer. Arrow indicates reduced enamel wear after shade was removed by abrasive wear. (Original magnification ~40; bar = 500 pm.)
area on the occlusal surface. Therefore it is crucial to record the loss of vertical height. Fig. 2 demonstrates the maximal loss in vertical height of the enamel wear facet as a function of the number of cycles. The values were determined
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PINTADO,
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= 200 pm) of the Ceramco porcelain environment after were visible in the
by comparing the original unworn surface with the abraded surface using the surface analysis program AnSur. The greatest vertical loss between the two surfaces at the wear facet was used to record the loss in vertical height. The loss in vertical height for the disks was reported in a similar manner. The curves for the loss of vertical height (Figs. 2 and 3) illustrated a progressive decrease in wear with time, but the volume curves were linear. Conversely, this loss in vertical height has been represented by a quadratic equation.5, g The loss in the vertical height by enamel wear can be divided into three groups (Fig. 2) that are statistically distinct (p < 0.05). Group 1, that created the least enamel wear, contained gold and Dicer material; group 2 consisted of unshaded porcelain; and group 3, that was responsible for the greatest amount of wear, involved the shaded Dicer material and shaded porcelain. These results were similar to those for the volume loss, but the wear of enamel caused by the shaded materials was not statistically different (p > 0.05). The combined loss in vertical height of the couples is critical clinically because it is a direct measure of the loss in the vertical dimension of occlusion. When the greatest loss of vertical height for the enamel and the corresponding opposing material were combined, a distinction between groups 2 and 3 was impossible-that is, porcelain, shaded porcelain, and shaded Dicer material all created a similar loss in vertical dimension. However, the loss in the
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Fig. 7, SEM photomicrograph of wear facet created on shaded Dicer material. Arrow indicates surface after complete shade removal, exposing unshaded Dicer material. (Original magnification x40; bar = 500 ym.)
vertical dimension of occlusion resulting from the enamelDicer material couple was significantly greater than that for the enamel-gold couple @ < 0.05). When considering both forms of wear analysis, volume loss and height loss, the material couples can be divided into two groups. The first group consisted of gold and Dicar material and the second group included porcelain and the shaded materials. The SEM photomicrographs of the enamel wear facets verified this classification. The enamel facets created by gold and Dicer material (Fig. 4) were similar in appearance but quite different from those resulting from porcelain or the shaded materials (Fig. 5). The former were smooth and with a polished appearance when viewed microscopically. The wear facets caused by the second group exhibited large grooves that were indicative of a more abrasive wear process.5 The wear characterized by the shaded Dicer material was unique because it appeared self-limiting (Fig. 6). There is a central region of enamel that abraded at a slower rate and this attribute was confirmed on the shaded Dicer material disk that revealed that this facet has two distinct wear processes (Fig. 7). The outer surface of the wear facet represents the usual abrasive wear, while the inner central region of the wear facet is similar to that of the unshaded Dicar material. Thus once the shade has been worn through, a shaded Dicer restoration performs in a similar fashion to the unshaded Dicer restoration. This same effect was not apparent with any shaded porcelain restorations. The elevation of wear rates can be confirmed by the SEM photomicrographs of the facets on the shaded materials. A relativeiy soft, glassy phase surrounds the harder metal
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Fig. 8. SEM photomicrograph of the trailing edge of a shaded Dicer material wear facet. Preferential wearing of the shading exposed abrasive particles (arrow) incorporated in the shaded material. (Original magnification X1100; bar = 20 pm.)
Fig. 9. SEM photomicrograph of the edge of a wear facet on shaded Dicer material showing porosity with application of shading (arrow). (Original magnification x430; bar = 50 Rm.)
oxide particles in the shading materials for both Dicer and ceramic porcelain, and this wears preferentially, leaving the irregular particles exposed (Fig. 8). Second, the addition of the shading commonly incorporates porosity in the restoration, and as the shades wear the porosity is exposed, leaving a “cheese grater”-appearing surface (Fig. 9). Research is needed to clarify whether either of these factors
47
DELONG,
accounts for the inordinate shaded restorations.
wear of enamel
attributed
to
CONCLUSIONS The addition of external shading to restorative porcelain caused an elevation in the wear of enamel two to five times more than that of unshaded restorative porcelain, and 10 to 15 times greater than that of enamel opposing gold. The ranking of these materials according to the magnitude of enamel removed with time was: gold I Dicer material < Porcelain < Dicer material + shading < Porcelain + shading A ranking
for loss of vertical
dimension
of occlusion
was:
gold < Dicer material < Porcelain = Dicer material + shading = Porcelain + shading Either ranking suggested that ceramic restorations should not have external shade applied to contacting occlusal surfaces. However, the wear created by the shaded Dicer restorations appeared self-limiting, but this property was not evident for shaded porcelain. REFERENCES 1. Christensen GJJ. The uses of porcelain-fused-to-metal restorations in current dental practice: a survey. J PROSTHET DENT 1986;56:1-3. 2. Wiley MG. Effects of porcelain on occluding surfaces of restored teeth. J PROSTHET DENT 1989;61:133-7. 3. Mahalick JA, Knap FJ, Weiter EJ. Occlusal wear in prosthodontics. J Am Dent Assoc 1971;82:154-9. 4. Monasky GE, Taylor DF. Studies on the wear of porcelain, enamel and gold. J PROSTHET DENT 1971;25:299-306.
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5. DeLong R, Douglas WH, Sakaguchi RL, Pintado MR. The wear of dental porcelain in an artificial mouth. Dent Mater 1986;2:214-9. 6. Grossman DG. Cast glass ceramics. Dent Clin North Am 1985;29:72539. 7. DeLong R, Douglas WH. Development of an artificial oral environment for testing of dental restoratives: bi-axial force and movement control. J Dent Res 1983;62:32-6. 8. Coffey JP, Goodkind RJ, DeLong R, Douglas WH. In vitro study of the wear characteristics of natural and artificial teeth. J PROSTHET DENT 1985;54:273-80. 9. Sakaguchi RL, Douglas WH, DeLong R, Pintado MR. The wear of a posterior composite in an artificial mouth: a clinical correlation. Dent Mater 1986;2:235-40. 10. DeLong R, Sakaguchi RL, Douglas WH, Pintado MR. The wear of dental amalgam in an artificial mouth: a clinical correlation. Dent Mater 1985;6:238-42. 11. DeLong R, Sasik C, Pintado MR, Douglas WH. The wear of enamel when opposed by ceramic systems. Dent Mater 1989;5:4:266-71. 12. DeLong R, Pintado MR, Douglas WH. Measurement of change in surface contour by computer graphics. Dent Mater 1985;1:27-30. 13. DeLong R, Douglas WH. Quantitative assessment of anatomical change in the human dentition. In: Pederson PC, Onaral B, eds. Proceedings of the Twelfth Annual International Conference of the IEEE Engineering in Medicine and Biology Society. Piscataway, N.J.: IEEE, 1990:2054-5. 14. Archard JF. Contact and rubbing of flat surfaces. 3 Appl Physiol 1953; 24:981-S. 15. Greenwood JA, Williamson JB. Contact of nominally flat surfaces. Proc R Sot (Land) 1966;A295:300. 16. Burton RA. Friction and wear. In: Szeri AR, ed. Tribology. New York: McGraw-Hill, 1980:19-23. Reprint requests to: DR. RALPH DELONG UNIVERSITY OF MINNESOTA SCHOOL OF DENTISTRY BIOMATERIALS RESEARCH CENTER 16-212 Moos HEALTH SCIENCES TOWER 515 DELAWARE STREET SE MIXNEAPOLIS, MN 55455
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