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Evaluating Interfacial Gaps for Esthetic Inlays
EVALUATING INTERFACIAL GAPS FOR ESTHETIC INLAYS S.J. O ’NEAL,
M.S.; ROBERT L. MIRACLE; KARL F. LEIN FELDER, D.D.S., M.S.
© se of posterior composite resins has increased considerably in the last several years. Their continuing acceptance is related in part to major improvements in their physical and mechanical characteristics. Wear rates of some current formulations are as much as 10 times less than their predecessors.1,2 While the manufacturers have concentrated on developing better restorative systems, they have done little to minimize technique sensitivity. Consequently, the composite resin restoration has many clinical problems. These include a significantly higher incidence of secondary caries as compared to amalgam, microleakage and less-than-ideal marginal integrity.3'6 In addition, because of the complexities associated with the insertion and finishing techniques, many clinicians have difficulty in establishing proper anatomic form, proximal contour and contact. To minimize these problems, the composite resin inlay/onlay system was developed. Introduced in Europe several years ago, this new approach for enhancing the performance of posterior composite resin has been adopted by many clinicians.7 48
JADA, Vol. 124, December 1993
A
B
S
T
R
A
C
T
Because of some inadequacies associated w ith the direct fill posterior com posite resin, the
This study was designed to determine the dimensional interfacial gap of several resin and ceramic inlay/onlay systems and the rate of wear for different types of luting agents.
inlay/onlay form of th e same m aterial o r ceram ic agents has been introduced. This clinical investigation m easured the w e a r rate of several types of luting agents with both resin and ceram ic restorative systems and identified several factors related to w e a r of the cem enting agent.
One of the greatest forces behind the acceptance of the composite resin inlay has been the work of Wendt, who showed th a t heat treating enhances the resin hardness and wear resistance.89 Such a procedure also further reduced the incidence of microleakage.10 While many clinicians consider the composite resin inlay as an acceptable restoration, little or no information has been published about the role of the interfacial gap between the restoration and the wall of the preparation.
MATERIALS AND METHODS
Four different types of inlay systems were included in this study. In addition, six com posite resin luting agents of varying particle sizes were investigated (Tables 1, 2). C linical procedure. A series of adult patients received about 230 inlay/onlays of varying compositions under controlled clinical conditions (Table 1). At least 10 restorations from each group were evaluated for interfacial gap width as well as cement loss. Specimens were selected on a randomized basis. The various cements and mean particle sizes are presented in Table 2. All procedures were carried out by a team of clinical investigators who have been long-term participants in an ongoing program of clinical research. The inlay/onlay materials included both composite and ceramic agents. Four different procedural techniques were evaluated:
RESEARCH TABLE 1
INLAY SYSTEMS INCLUDED IN THE STUDY. Manufacturer
Inlay system
...............' .........' C eree '
Type
Number of restorations
S ie m e n s
C eram ic
120
C erin a te
D en -M at
Ceram ic
50
B r illia n t
C o lten e
C om posite
30
P -5 0
3M Co.
C om posite
30
1
direct, indirect chairside die, indirect laboratory processed, andCAD-CAM. D irect. In each instance, standard inlay/onlay cavity preparations were generated. In the case of onlays, the cusps were reduced by at least 2 millimeters. Under such conditions, a straight bevel about 1.5 to 2.0 mm was generated on the buccal or lingual surface. When the cavity preparation was completed, an alcoholbased separation agent (Coltene) was painted on all prepared walls. After matrixing and wedging, the composite resin (Brilliant, Coltene) was inserted in segments not exceeding 2 mm. After the m aterial was condensed, the surface was exposed to the curing light for at least 20 seconds. The final occlusal component was lightcured for 60 seconds. At this point, the restoration was removed from the preparation and then subjected to dry heat processing for 5 minutes a t 250 F (125 C). Next, the restoration was reinserted in the cavity preparation, adjusted for fit and cemented. The occlusal surface
was then contoured to anatomic form, followed by surfacing and polishing. Occlusal adjustments were completed immediately after surfacing and before the final polishing process. Indirect chairside die. After the cavity preparation was completed, a poly vinylsiloxane impression was generated (Express, 3M). After removal, the impression was rinsed and dried, followed by dry heating for five minutes at 120 C. It was allowed to bench
cool for three minutes. A working die was fabricated using an experimental epoxy resin (3M), which showed a slight negative dimensional change when cured. The base portion of the working cast was formed with a special thermoplastic resin and a strip of plastic with Yelcro-type tags on the surface. The flexible strip served as a hinge for orientation of the dye after scoring. The entire model was immersed in cold w ater for three minutes until the die and base were fully hardened. After separation of the model and trimming of the die, a primer and separator were applied to the internal walls of the prepared replica. The inlay was fabricated by bulk loading of P50 composite into the epoxy die and curing for 60 seconds. The inlay was then trimmed to the appropriate contours and polished with Sof-Lex disks (3M). At this point, the inlay was heated for an additional
TA B L E 2
CEMENTS I D MEAN PARTICLE SIZES. Luting agent
D uo-C em en t
Manufacturer
Inlay system
Particle size (microns)
V iv a d en t
Ceree
0 .0 5
C olteneC em en t
C olten e
B r illia n t
0.6
P -50 E x p erim en ta l
3M Co.
P -50
1-3
MierofillP on tic
K u lzer
Ceree
1-3
D en -M at
C erin a te
1-3
L.D. C aulk
Ceree
1-3
U ltrab on d C aulk E x p erim en ta l
JADA, Vol. 124, December 1993
49
RESEARCH TABLE 3
AVERAGE GAP D Material
Width
Depth
X C e rin a te
216
(8 2 )
105
(50)
P -50
181
(55)
74
( 21 )
Ceree
169
(48)
80
( 22 )
C oltene
112
(38)
43
(1 3 )
Vertical bar represents values which were not statistically different (P< .05) of the interfacial gap width and depth.
Figure 1. Mean gap dimension for the four different inlay systems at 24 months.
Figure 2. Distribution for Coltene according to interfacial gap dimensions. More than half showed gaps of 50 microns or less.
50
JADA, Vol. 124, December 1993
five minutes at 120 C. The restoration was cemented with an experimental hybrid dual cured cement (3M). Laboratory-processed inlays. When the preparation was complete, the impression was registered using a poly vinylsiloxane material (Reprosil, L.D. Caulk Co.). The impression in this case was cast with a die stone (Vel Mix, Kerr). To test the hypothesis of the manufacturer th at the clinical longevity of the inlay is independent of gap dimension, a special process was used. By applying five layers of a die spacer on the die stone replica before fabricating the refractory dye, it was possible to increase the width of the interfacial gap considerably. The dies containing several layers of a die spacer were again impressed but this time cast with a refractory material. The ceramic restorations were fabricated directly on these working dies. All inlays were fabricated by Den-Mat’s ceramic laboratory and cemented according to the m anufacturers’ recommen dations. Only the cerinate inlays were processed in this manner and cemented with U ltradent (Table 2). CAD-CAM processed inlays. After the cavity preparation was generated, the surfaces were imaged with the Cerec CAD-CAM system.1112 The restorations were electronically designed, followed by milling in accordance with the technique recommended by the manufac turer. After fabrication of the restoration, it was cemented using one of three different luting agents. These included two hybrids and a microfill (Table 2). Only one ceramic
RESEARCH'
Figure 3. Distribution according to interfacial gap dimensions for P-50 inlays. The gap size is considerably larger, resulting In a shift of the curve to the right.
Figure 4. Vertical wear rates o f four cements; mean gap depth at 24 months.
system Dicor (Dentsply) was used. Cem entation. Before cementation, the enamel walls of all cavity preparations were etched with a 37 percent concentration of H3P04 in gel form. After washing and drying, bonding agents supplied by the respective m anufacturers were painted on the surface of the cured inlay. Next, the inlay preparation was surfaced with the primer and adhesive agent specifically recommended by the various manufacturers. The composite
resin luting agent was placed in the prepared cavity, followed by complete seating of the inlay/onlay. Immediately after the residual cement was removed from the margins, the restored tooth was exposed to visible light-cure radiation for about three minutes. In the case of the Cerinate porcelain, a hydrofluoric acid gel was used to etch the restoration surfaces. An ammonium bifluoride gel was used for Dicor inlays, and a 30 percent concentration of HF with conventional porcelain.
After washing and drying, the etched surfaces were coated with a thin layer of silane coupling agent. E valuation. Vinyl polysiloxane (L.D. Caulk Co.) impressions were generated of each restored tooth at baseline, a t six months and again at the end of the first and second years. These then were cast with an epoxy resin (EpoxyDye, Ivoclar North America). A diamond blade was used to cut thin sections buccolingually in the areas along the inclined planes. Each section was then evaluated for loss of cementing agent at an X10 magnification. Measurements were made both of the width as well as the depth of the interfacial defect. This information was used for determining the following conditions: ■“ mean horizontal and vertical gap dimensions; *■ rate and extent of wear of the luting agent; ■* possible relationships between wear and type of luting agent; *■*possible relationship between horizontal width of the gap and vertical wear or loss of material. An analysis of variance was used to assess differences between interfacial gap dimensions and restorative system. Special grouping was determined by a Scheffe test. All tests were carried out at a P < .05 level of compliance. RES U LTS
The width of the interfacial gap varied considerably (Figure 1) for the various systems investigated in this clinical study. Table 3 gives the average horizontal (width) and vertical (depth) gap dimensions for all inlay systems. The mean JADA, Vol. 124, December 1993
51
RESEARCH
Figure 5. Vertical wear rates of three cements including two hybrids and one microfill. The two uppermost curves represent the wear rates of the two hybrid luting systems used with the CAD-CAM systems. The lower curve represents the wear rate of the Dual-Cure microfill, also included in the CAD-CAM study.
Figure 6. Relationship between vertical wear and horizontal gap dimension.
interfacial gap for the direct Coltene inlays was 112 microns. The CAD-CAM inlays had a mean interfacial gap of 169 microns. By comparison, the Cerinate gap dimension averaged 216 microns; for the P-50 inlays, the gap was 181 microns. Standard deviation values for each system and statistical significance are shown in Table 3. In the Coltene material, more 52
JADA, Vol. 124, December 1993
than half of the margins had interfacial gaps of 50 microns or less (Figure 2). But some gap dimensions were considerably greater. A similar distribution for another P-50 inlay material is shown in Figure 3. The gap size is considerably larger, resulting in a shift of the curve to the right. The mean vertical loss (wear) of luting agents in two years is presented in Figure 4. Most of
the wear occurred during the first six to 12 months. At that time, the wear rate tended to level off. Hybrid cements showed the greatest am ount of wear. Figure 5 shows wear rates of two hybrid luting systems and a microfill—both used with the CAD-CAM system. Again, most of the wear occurred during the first 12 months of clinical service. The same relationship was reported by Kawai and others for an in vitro evaluation of the cement wear in Cerec restorations.13 One of the most interesting observations of the study related to the relationship between the width of the interfacial gap and the vertical loss of the cement (Figure 6). This relationship for the four different inlay systems for two years is depicted in Figure 6. Findings include: ■■ the ratio of depth to width ranged from 38 to 49 percent; ■» the depth/width ratio generally increased during the first year and then tended to level out; *■ the depth/width ratio was greatest for the hybrids as compared to the submicron sized filled luting agents (the same relationship noted by Kawai and others.13) The material presented in Figure 7 shows th a t vertical loss of the microfills did not exceed more than 45 percent of the horizontal gap width. The hybrids, however, approximated 65 percent. A careful examination of the inlay-tooth interface using scanning electron microscopy revealed a uniform loss of substance. The pattern of wear is similar to th a t shown by composite resins in Class I
RESEARCH cavity preparations.14A typical wear pattern of the luting agent is presented in Figure 8. In the scanning electron microscope, some particles can be seen along the edge of the margins as well as in the area where the composite resin luting agent was worn. It is conjectured th at the food particles actually caused the wear of the composite resin luting agent. D IS C U S S IO N
Our study results show a relationship between wear of the luting agent and a number of factors. The first of these is the width of the interfacial gap. The greater the gap width, the greater the loss of the cement. The wear rate, which appears to be fairly linear, normally tends to taper off at the end of the first year. The relationship between gap dimension and loss of cement can be attributed to the presence of food bolus. If the gap dimension is sufficiently narrow, the food particles will not be able to stress the surface mechanically during the masticatory process. Once the gap dimension exceeds 100 microns, the possibility of the food particles contacting the cement surface increases and causes wear. As the horizontal gap increases, the vertical gap increases: there is a defect available for microbial growth and subsequent secondary caries. When a vertical depth approaches 50 to 75 microns, the mechanical defect is large enough to predispose an in creased number of restorations to secondary decay. Clinical data support the theory th a t marginal widths greater than 100 to 150 microns are assoc iated with greater marginal
80
o o T— 60 X
sz T3 £ Q. CD
Q
40
y
/ 20
-
/ A
*
yT
■----- ■ Hybrid • Al
0J (
• Mean A Submicron i
i
Time in Years Figure 7. Relationship between vertical wear of the cement and composition. The lowest curve represents the mean depth/width ratios for the microfill cements. The uppermost curve represents the depth/width ratio of the hybrid luting agents in the same period. The middle curve represents mean values for all cements.
%
Figure 8. SEM of margin between tooth (top) and composite resin (R) inlay (bottom) (original magnification X300). Occlusal surface (E) is positioned in the upper left portion— the inlay in the lower right. The interfacial gap is located in the center with black arrows indicating its width. Hollow areas indicate food particles.
leakage and an increase in secondary decay.15 The apparent cessation of wear after the first year or so can also be related to action of the food bolus against its surface. Once the gap becomes sufficiently deep, it is likely
that the food bolus translating across the occlusal table cannot sufficiently contact the cement to abrade its surface. Another interesting finding relates to the size of the filler particle of the cement. Without exception, the hybrid cements JADA, Vol. 124, December 1993
53
RESEARCH
Figure 9. Diagrammatic illustration explaining wear and type of cement.
wore away faster than those in which the filler particle was submicron in size. A possible explanation for the difference in the amount of wear is shown in Figure 9. During mastication, the food bolus is forced across the occlusal table. It is conjectured th a t if the filler particles of the luting agent are sufficiently large, they tend to extrude above the surface. During translation of the bolus, some of the masticatory energy is transferred to the extruding filler particles, which in turn transm it it to the surrounding resin matrix. Eventually this energy causes microcracking of the resin material. As the cement is weakened, it becomes more susceptible to wear. When the filler particles are small enough, the coefficient of friction is lowered considerably. Under the same condition, the stress transfer is minimized which results in better wear resistance. C O N C L U S IO N
Based on results from this 54
JADA, Vol. 124, December 1993
study, the following conclusions can be made: ■“ The width of the interfacial gap varies considerably among various systems. The vertical loss of wear of luting agents tends to occur linearly during the first year and then generally levels off. It is directly related to the width of the interfacial gap. Microfills or those containing submicron-sized fillers are considerably more wearresistant than hybrids/luting agents. ■» The depth/width ratio of the interfacial gap ranges from about 38 to 49 percent depending on the type of cementing material. ■
Dr. O’Neal is profes sor of dentistry. Mr. Miracle is a
Department of
senior dental
Restorative
student, School of
Dentistry, School of
Dentistry, University
Dentistry, University
of Alabama at
of Alabama at
Birmingham.
Birmingham.
1. Ruyter IE, Sjovik Kleven IJ. Monomers and filler content of resin-based crown and bridge materials. Dent Mater 1987;3:315-21. 2. Strohaver RA, Mattie DR. A scanning electron Dr. Leinfelder is a microscope member of the JADA comparison of editorial board and microfilled fixed chairman, Depart prosthodontic resins. ment of Bio J Prosthet Dent materials, School of 1987;57:559-65. Dentistry, University 3. Jones RM, of Alabama at Goodacre CJ, Moore Birmingham, UAB BK, Dykema RW. A Station, Box 49, comparison of the Birmingham, Ala. physical properties of 35294-0007. four prosthetic Address reprint veneering materials. requests to Dr. J Prosthet Dent Leinfelder. 1989;61:38-44. 4. Eick JD, Welch FH. Polymerisation shrinkage of posterior composite resins and its possible influence on postoperative sensitivity. Quintessence Int 1986;17:103-11. 5. Roulet JF. The problems associated with substituting composite resins for amalgam: a status report on posterior composites. J Dent 1989;16:101-13. 6. Jansen EK. Contraction patter of composite resins in dentine cavities. Scand J Dent Res 1982;90:480-3. 7. Burke FJT, Watts D, Wilson NHF, Wilson MA. Current status and rationale for composite inlays and onlays. Conservative Dent 1991;269-73. 8. Wendt SL. The effect of heat used as a secondary cure upon the physical properties of three composite resins. Parts I,II. Quintessence Int 1987;18:351-6. 9. Wendt SL. Time as a factor in the heat curing of composite resins. Quintessence Int 1989;20:359-63. 10. Robinson P, More B, Swartz M. Comparison of microleakage in direct and indirect composite resin restorations in-vitro. Oper Dent 1987;12:113-6. 11. Leinfelder KF, Isenberg BP, Essig ME. A new method for generating ceramic restorations: a CAD-CAM system. JADA 1989;118:703-7. 12. Isenberg BP, Essig ME, Leinfelder KF, Mueninghoff LA. Clinical evaluation of CEREC CAD-CAM restorations (Abstract no. 1597). J Dent Res 1990:69:1597. 13. Kawai K, Isenberg BP, Leinfelder K. Effect of gap dimension on composite resin cement wear (Abstract no. 1402). J Dent Res 1992;72:691. 14. Leinfelder KF. Evaluation of clinical wear of posterior compositions. In: International symposium of posterior composite resin, dental restorative materials. The Netherlands: Peter Szulc; 1985:501-10. 15. O’Neal SJ, Leinfelder KF, Wright W, Gerbo L. Clinical evaluation of two types of composite resin inlay systems (Abstract no. 1527). J Dent Res 1991;70:457.
M.S.; ROBERT L. MIRACLE; KARL F. LEIN FELDER, D.D.S., M.S.
© se of posterior composite resins has increased considerably in the last several years. Their continuing acceptance is related in part to major improvements in their physical and mechanical characteristics. Wear rates of some current formulations are as much as 10 times less than their predecessors.1,2 While the manufacturers have concentrated on developing better restorative systems, they have done little to minimize technique sensitivity. Consequently, the composite resin restoration has many clinical problems. These include a significantly higher incidence of secondary caries as compared to amalgam, microleakage and less-than-ideal marginal integrity.3'6 In addition, because of the complexities associated with the insertion and finishing techniques, many clinicians have difficulty in establishing proper anatomic form, proximal contour and contact. To minimize these problems, the composite resin inlay/onlay system was developed. Introduced in Europe several years ago, this new approach for enhancing the performance of posterior composite resin has been adopted by many clinicians.7 48
JADA, Vol. 124, December 1993
A
B
S
T
R
A
C
T
Because of some inadequacies associated w ith the direct fill posterior com posite resin, the
This study was designed to determine the dimensional interfacial gap of several resin and ceramic inlay/onlay systems and the rate of wear for different types of luting agents.
inlay/onlay form of th e same m aterial o r ceram ic agents has been introduced. This clinical investigation m easured the w e a r rate of several types of luting agents with both resin and ceram ic restorative systems and identified several factors related to w e a r of the cem enting agent.
One of the greatest forces behind the acceptance of the composite resin inlay has been the work of Wendt, who showed th a t heat treating enhances the resin hardness and wear resistance.89 Such a procedure also further reduced the incidence of microleakage.10 While many clinicians consider the composite resin inlay as an acceptable restoration, little or no information has been published about the role of the interfacial gap between the restoration and the wall of the preparation.
MATERIALS AND METHODS
Four different types of inlay systems were included in this study. In addition, six com posite resin luting agents of varying particle sizes were investigated (Tables 1, 2). C linical procedure. A series of adult patients received about 230 inlay/onlays of varying compositions under controlled clinical conditions (Table 1). At least 10 restorations from each group were evaluated for interfacial gap width as well as cement loss. Specimens were selected on a randomized basis. The various cements and mean particle sizes are presented in Table 2. All procedures were carried out by a team of clinical investigators who have been long-term participants in an ongoing program of clinical research. The inlay/onlay materials included both composite and ceramic agents. Four different procedural techniques were evaluated:
RESEARCH TABLE 1
INLAY SYSTEMS INCLUDED IN THE STUDY. Manufacturer
Inlay system
...............' .........' C eree '
Type
Number of restorations
S ie m e n s
C eram ic
120
C erin a te
D en -M at
Ceram ic
50
B r illia n t
C o lten e
C om posite
30
P -5 0
3M Co.
C om posite
30
1
direct, indirect chairside die, indirect laboratory processed, andCAD-CAM. D irect. In each instance, standard inlay/onlay cavity preparations were generated. In the case of onlays, the cusps were reduced by at least 2 millimeters. Under such conditions, a straight bevel about 1.5 to 2.0 mm was generated on the buccal or lingual surface. When the cavity preparation was completed, an alcoholbased separation agent (Coltene) was painted on all prepared walls. After matrixing and wedging, the composite resin (Brilliant, Coltene) was inserted in segments not exceeding 2 mm. After the m aterial was condensed, the surface was exposed to the curing light for at least 20 seconds. The final occlusal component was lightcured for 60 seconds. At this point, the restoration was removed from the preparation and then subjected to dry heat processing for 5 minutes a t 250 F (125 C). Next, the restoration was reinserted in the cavity preparation, adjusted for fit and cemented. The occlusal surface
was then contoured to anatomic form, followed by surfacing and polishing. Occlusal adjustments were completed immediately after surfacing and before the final polishing process. Indirect chairside die. After the cavity preparation was completed, a poly vinylsiloxane impression was generated (Express, 3M). After removal, the impression was rinsed and dried, followed by dry heating for five minutes at 120 C. It was allowed to bench
cool for three minutes. A working die was fabricated using an experimental epoxy resin (3M), which showed a slight negative dimensional change when cured. The base portion of the working cast was formed with a special thermoplastic resin and a strip of plastic with Yelcro-type tags on the surface. The flexible strip served as a hinge for orientation of the dye after scoring. The entire model was immersed in cold w ater for three minutes until the die and base were fully hardened. After separation of the model and trimming of the die, a primer and separator were applied to the internal walls of the prepared replica. The inlay was fabricated by bulk loading of P50 composite into the epoxy die and curing for 60 seconds. The inlay was then trimmed to the appropriate contours and polished with Sof-Lex disks (3M). At this point, the inlay was heated for an additional
TA B L E 2
CEMENTS I D MEAN PARTICLE SIZES. Luting agent
D uo-C em en t
Manufacturer
Inlay system
Particle size (microns)
V iv a d en t
Ceree
0 .0 5
C olteneC em en t
C olten e
B r illia n t
0.6
P -50 E x p erim en ta l
3M Co.
P -50
1-3
MierofillP on tic
K u lzer
Ceree
1-3
D en -M at
C erin a te
1-3
L.D. C aulk
Ceree
1-3
U ltrab on d C aulk E x p erim en ta l
JADA, Vol. 124, December 1993
49
RESEARCH TABLE 3
AVERAGE GAP D Material
Width
Depth
X C e rin a te
216
(8 2 )
105
(50)
P -50
181
(55)
74
( 21 )
Ceree
169
(48)
80
( 22 )
C oltene
112
(38)
43
(1 3 )
Vertical bar represents values which were not statistically different (P< .05) of the interfacial gap width and depth.
Figure 1. Mean gap dimension for the four different inlay systems at 24 months.
Figure 2. Distribution for Coltene according to interfacial gap dimensions. More than half showed gaps of 50 microns or less.
50
JADA, Vol. 124, December 1993
five minutes at 120 C. The restoration was cemented with an experimental hybrid dual cured cement (3M). Laboratory-processed inlays. When the preparation was complete, the impression was registered using a poly vinylsiloxane material (Reprosil, L.D. Caulk Co.). The impression in this case was cast with a die stone (Vel Mix, Kerr). To test the hypothesis of the manufacturer th at the clinical longevity of the inlay is independent of gap dimension, a special process was used. By applying five layers of a die spacer on the die stone replica before fabricating the refractory dye, it was possible to increase the width of the interfacial gap considerably. The dies containing several layers of a die spacer were again impressed but this time cast with a refractory material. The ceramic restorations were fabricated directly on these working dies. All inlays were fabricated by Den-Mat’s ceramic laboratory and cemented according to the m anufacturers’ recommen dations. Only the cerinate inlays were processed in this manner and cemented with U ltradent (Table 2). CAD-CAM processed inlays. After the cavity preparation was generated, the surfaces were imaged with the Cerec CAD-CAM system.1112 The restorations were electronically designed, followed by milling in accordance with the technique recommended by the manufac turer. After fabrication of the restoration, it was cemented using one of three different luting agents. These included two hybrids and a microfill (Table 2). Only one ceramic
RESEARCH'
Figure 3. Distribution according to interfacial gap dimensions for P-50 inlays. The gap size is considerably larger, resulting In a shift of the curve to the right.
Figure 4. Vertical wear rates o f four cements; mean gap depth at 24 months.
system Dicor (Dentsply) was used. Cem entation. Before cementation, the enamel walls of all cavity preparations were etched with a 37 percent concentration of H3P04 in gel form. After washing and drying, bonding agents supplied by the respective m anufacturers were painted on the surface of the cured inlay. Next, the inlay preparation was surfaced with the primer and adhesive agent specifically recommended by the various manufacturers. The composite
resin luting agent was placed in the prepared cavity, followed by complete seating of the inlay/onlay. Immediately after the residual cement was removed from the margins, the restored tooth was exposed to visible light-cure radiation for about three minutes. In the case of the Cerinate porcelain, a hydrofluoric acid gel was used to etch the restoration surfaces. An ammonium bifluoride gel was used for Dicor inlays, and a 30 percent concentration of HF with conventional porcelain.
After washing and drying, the etched surfaces were coated with a thin layer of silane coupling agent. E valuation. Vinyl polysiloxane (L.D. Caulk Co.) impressions were generated of each restored tooth at baseline, a t six months and again at the end of the first and second years. These then were cast with an epoxy resin (EpoxyDye, Ivoclar North America). A diamond blade was used to cut thin sections buccolingually in the areas along the inclined planes. Each section was then evaluated for loss of cementing agent at an X10 magnification. Measurements were made both of the width as well as the depth of the interfacial defect. This information was used for determining the following conditions: ■“ mean horizontal and vertical gap dimensions; *■ rate and extent of wear of the luting agent; ■* possible relationships between wear and type of luting agent; *■*possible relationship between horizontal width of the gap and vertical wear or loss of material. An analysis of variance was used to assess differences between interfacial gap dimensions and restorative system. Special grouping was determined by a Scheffe test. All tests were carried out at a P < .05 level of compliance. RES U LTS
The width of the interfacial gap varied considerably (Figure 1) for the various systems investigated in this clinical study. Table 3 gives the average horizontal (width) and vertical (depth) gap dimensions for all inlay systems. The mean JADA, Vol. 124, December 1993
51
RESEARCH
Figure 5. Vertical wear rates of three cements including two hybrids and one microfill. The two uppermost curves represent the wear rates of the two hybrid luting systems used with the CAD-CAM systems. The lower curve represents the wear rate of the Dual-Cure microfill, also included in the CAD-CAM study.
Figure 6. Relationship between vertical wear and horizontal gap dimension.
interfacial gap for the direct Coltene inlays was 112 microns. The CAD-CAM inlays had a mean interfacial gap of 169 microns. By comparison, the Cerinate gap dimension averaged 216 microns; for the P-50 inlays, the gap was 181 microns. Standard deviation values for each system and statistical significance are shown in Table 3. In the Coltene material, more 52
JADA, Vol. 124, December 1993
than half of the margins had interfacial gaps of 50 microns or less (Figure 2). But some gap dimensions were considerably greater. A similar distribution for another P-50 inlay material is shown in Figure 3. The gap size is considerably larger, resulting in a shift of the curve to the right. The mean vertical loss (wear) of luting agents in two years is presented in Figure 4. Most of
the wear occurred during the first six to 12 months. At that time, the wear rate tended to level off. Hybrid cements showed the greatest am ount of wear. Figure 5 shows wear rates of two hybrid luting systems and a microfill—both used with the CAD-CAM system. Again, most of the wear occurred during the first 12 months of clinical service. The same relationship was reported by Kawai and others for an in vitro evaluation of the cement wear in Cerec restorations.13 One of the most interesting observations of the study related to the relationship between the width of the interfacial gap and the vertical loss of the cement (Figure 6). This relationship for the four different inlay systems for two years is depicted in Figure 6. Findings include: ■■ the ratio of depth to width ranged from 38 to 49 percent; ■» the depth/width ratio generally increased during the first year and then tended to level out; *■ the depth/width ratio was greatest for the hybrids as compared to the submicron sized filled luting agents (the same relationship noted by Kawai and others.13) The material presented in Figure 7 shows th a t vertical loss of the microfills did not exceed more than 45 percent of the horizontal gap width. The hybrids, however, approximated 65 percent. A careful examination of the inlay-tooth interface using scanning electron microscopy revealed a uniform loss of substance. The pattern of wear is similar to th a t shown by composite resins in Class I
RESEARCH cavity preparations.14A typical wear pattern of the luting agent is presented in Figure 8. In the scanning electron microscope, some particles can be seen along the edge of the margins as well as in the area where the composite resin luting agent was worn. It is conjectured th at the food particles actually caused the wear of the composite resin luting agent. D IS C U S S IO N
Our study results show a relationship between wear of the luting agent and a number of factors. The first of these is the width of the interfacial gap. The greater the gap width, the greater the loss of the cement. The wear rate, which appears to be fairly linear, normally tends to taper off at the end of the first year. The relationship between gap dimension and loss of cement can be attributed to the presence of food bolus. If the gap dimension is sufficiently narrow, the food particles will not be able to stress the surface mechanically during the masticatory process. Once the gap dimension exceeds 100 microns, the possibility of the food particles contacting the cement surface increases and causes wear. As the horizontal gap increases, the vertical gap increases: there is a defect available for microbial growth and subsequent secondary caries. When a vertical depth approaches 50 to 75 microns, the mechanical defect is large enough to predispose an in creased number of restorations to secondary decay. Clinical data support the theory th a t marginal widths greater than 100 to 150 microns are assoc iated with greater marginal
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■----- ■ Hybrid • Al
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• Mean A Submicron i
i
Time in Years Figure 7. Relationship between vertical wear of the cement and composition. The lowest curve represents the mean depth/width ratios for the microfill cements. The uppermost curve represents the depth/width ratio of the hybrid luting agents in the same period. The middle curve represents mean values for all cements.
%
Figure 8. SEM of margin between tooth (top) and composite resin (R) inlay (bottom) (original magnification X300). Occlusal surface (E) is positioned in the upper left portion— the inlay in the lower right. The interfacial gap is located in the center with black arrows indicating its width. Hollow areas indicate food particles.
leakage and an increase in secondary decay.15 The apparent cessation of wear after the first year or so can also be related to action of the food bolus against its surface. Once the gap becomes sufficiently deep, it is likely
that the food bolus translating across the occlusal table cannot sufficiently contact the cement to abrade its surface. Another interesting finding relates to the size of the filler particle of the cement. Without exception, the hybrid cements JADA, Vol. 124, December 1993
53
RESEARCH
Figure 9. Diagrammatic illustration explaining wear and type of cement.
wore away faster than those in which the filler particle was submicron in size. A possible explanation for the difference in the amount of wear is shown in Figure 9. During mastication, the food bolus is forced across the occlusal table. It is conjectured th a t if the filler particles of the luting agent are sufficiently large, they tend to extrude above the surface. During translation of the bolus, some of the masticatory energy is transferred to the extruding filler particles, which in turn transm it it to the surrounding resin matrix. Eventually this energy causes microcracking of the resin material. As the cement is weakened, it becomes more susceptible to wear. When the filler particles are small enough, the coefficient of friction is lowered considerably. Under the same condition, the stress transfer is minimized which results in better wear resistance. C O N C L U S IO N
Based on results from this 54
JADA, Vol. 124, December 1993
study, the following conclusions can be made: ■“ The width of the interfacial gap varies considerably among various systems. The vertical loss of wear of luting agents tends to occur linearly during the first year and then generally levels off. It is directly related to the width of the interfacial gap. Microfills or those containing submicron-sized fillers are considerably more wearresistant than hybrids/luting agents. ■» The depth/width ratio of the interfacial gap ranges from about 38 to 49 percent depending on the type of cementing material. ■
Dr. O’Neal is profes sor of dentistry. Mr. Miracle is a
Department of
senior dental
Restorative
student, School of
Dentistry, School of
Dentistry, University
Dentistry, University
of Alabama at
of Alabama at
Birmingham.
Birmingham.
1. Ruyter IE, Sjovik Kleven IJ. Monomers and filler content of resin-based crown and bridge materials. Dent Mater 1987;3:315-21. 2. Strohaver RA, Mattie DR. A scanning electron Dr. Leinfelder is a microscope member of the JADA comparison of editorial board and microfilled fixed chairman, Depart prosthodontic resins. ment of Bio J Prosthet Dent materials, School of 1987;57:559-65. Dentistry, University 3. Jones RM, of Alabama at Goodacre CJ, Moore Birmingham, UAB BK, Dykema RW. A Station, Box 49, comparison of the Birmingham, Ala. physical properties of 35294-0007. four prosthetic Address reprint veneering materials. requests to Dr. J Prosthet Dent Leinfelder. 1989;61:38-44. 4. Eick JD, Welch FH. Polymerisation shrinkage of posterior composite resins and its possible influence on postoperative sensitivity. Quintessence Int 1986;17:103-11. 5. Roulet JF. The problems associated with substituting composite resins for amalgam: a status report on posterior composites. J Dent 1989;16:101-13. 6. Jansen EK. Contraction patter of composite resins in dentine cavities. Scand J Dent Res 1982;90:480-3. 7. Burke FJT, Watts D, Wilson NHF, Wilson MA. Current status and rationale for composite inlays and onlays. Conservative Dent 1991;269-73. 8. Wendt SL. The effect of heat used as a secondary cure upon the physical properties of three composite resins. Parts I,II. Quintessence Int 1987;18:351-6. 9. Wendt SL. Time as a factor in the heat curing of composite resins. Quintessence Int 1989;20:359-63. 10. Robinson P, More B, Swartz M. Comparison of microleakage in direct and indirect composite resin restorations in-vitro. Oper Dent 1987;12:113-6. 11. Leinfelder KF, Isenberg BP, Essig ME. A new method for generating ceramic restorations: a CAD-CAM system. JADA 1989;118:703-7. 12. Isenberg BP, Essig ME, Leinfelder KF, Mueninghoff LA. Clinical evaluation of CEREC CAD-CAM restorations (Abstract no. 1597). J Dent Res 1990:69:1597. 13. Kawai K, Isenberg BP, Leinfelder K. Effect of gap dimension on composite resin cement wear (Abstract no. 1402). J Dent Res 1992;72:691. 14. Leinfelder KF. Evaluation of clinical wear of posterior compositions. In: International symposium of posterior composite resin, dental restorative materials. The Netherlands: Peter Szulc; 1985:501-10. 15. O’Neal SJ, Leinfelder KF, Wright W, Gerbo L. Clinical evaluation of two types of composite resin inlay systems (Abstract no. 1527). J Dent Res 1991;70:457.