J. Dent 1991;
296
19: 296-300
An investigation into the incidence of voids in indirect composite inlays formed using different packing techniques M. A. Wilson and R. D. Norman* Depattmenr of Resrorarive Dentistry, University University,
Alron, Illinois,
of Manchester
Dental
Hospital,
UK and *Southern
Illinois
USA
ABSTRACT An investigation is described into the incidence of voids in indirect composite inlays formed using different packing techniques and different composite systems. From the results it was found that packing inlays under a 6 bar air pressure prior to light curing produced significantly fewer voids than layering or bulk packing techniques. KEY WORDS: J. Dent. 1991; 1991)
Dental composites, Inlays, Technique 19:
296-300
(Received 25 September
1990;
reviewed 30 October 1990;
accepted 11 April
Correspondence should be addressed to: Dr M. A. Wilson, Department of Restorative Dentistry, University of Manchester Dental Hospital, Higher Cambridge Street, Manchester Ml 5 6FH, UK.
INTRODUCTION Composite restoratives are now widely used in the restoration of posterior teeth (Christensen, 1989). It has been reported however that posterior composites perform least well when the restorations are large or in situations of heavy occlusal loading (Wilson et al., 1986). Difficulties may also be encountered with polymerization shrinkage during polymerization, causing stresses between tooth cusps (Causton et al., 1985), and increasing the possibility of microleakage (Fayyad and Shortall, 1987). These difficulties and problems encountered when attempting to place, contour and finish large restorations using such technique-sensitive materials have in part resulted in the emergence of the indirect composite inlay. With the introduction of secondary and super (heat) cure techniques, many of the difficulties encountered with direct composite restorations have been largely overcome, and in vitro tests would seem to suggest that the physical properties of composite restoratives may be improved by secondary curing (Wendt, 1987) and super curing, and that microleakage will be reduced (Robinson et al., 1987). The presence of porosities within dental composites has been well documented by Ogden (1985) and Van Dijken et @ 1991 Butterworth-Heinemann 0300-5712/91/05029h-05
Ltd.
al. (1986). The adverse effects of porosities contained within direct composite restorations has been reported by Asmussen and Jorgensen (1982) Ogden (1985) Leinfelder et al. (1985) and McCabe and Ogden (1987) and manipulative procedures which increase the likelihood of incorporation of porosities into the material should be avoided (Chadwick et al., 1989). There are at present several composite systems which are produced specifically for the production of indirect composite inlays, relying on one of three different methods of placement. Placement techniques include: 1. The layering technique. This is the method which is also used when placing direct composite resins. The method allows the material to be cured in increments, thereby reducing the effects of polymerization shrinkage, and also ensures that the composite at the base of deep cavities is polymerized. Restorations are built up in increments or layers. Each layer (< 2.5 mm) is cured by visible light from a light source for > 20 s according to manufacturers’ instructions. This technique is recommended by Coltene (C) (Coltene AG, Alstatten. Switzerland), and Kulzer (K) (Kulzer and Co. GmbH. Wehrheim. Germany). for their respective indirect inlay systems.
Wilson and Norman: Voids in indirect composite inlays
2. Bulk packing.
In this method the composite is packed into the die/preparation in one increment and then light cured. This technique has the major problem that in deep cavities it is not possible to be certain that all parts of the composite inlay are polymerized prior to removal. However, this technique has been suggested for the manufacture of direct inlays by: Hussey (1988) Jordan and Suzuki (1989) and for small restorations by Kanca (1989). 3. Placement under pressure. The material Isosit (I) (Ivoclar/Vivadent, Schaan, Liechtenstein) is different from all other indirect composite resin systems in that it is super (heat) cured (120°C) in a water-bath at 6 bar pressure (James and Yarovesky, 1983). The manufacturers of this material claim that this technique prevents the formation of porosities within the inlays. To date no published work has reported investigations of different systems in terms of the presence of voids within composite inlays, comparing different placement techniques. The effect of pressure on the production of porosities has also not been investigated, and in particular the need for pressure applied via a water-bath. The purpose of this study was to determine which method of formation produces the smallest number and area of voids within composite inlays of a variety of composite materials. Materials advocated for use as indirect inlays and a limited selection of other composite restoratives were tested. In addition, since the method of manufacture of a composite, or the method whereby it is loaded into a syringe could result in the entrapment of air within the material, potentially making void production in the inlay more likely, specimens of composite from all the syringe-loaded systems were investigated.
MATERIALS
AND METHODS
Six composite materials were selected for use in this study for the production of inlays, two are composites recommended for direct applications in the restoration of posterior teeth, and one is an anterior composite restorative which may
(Table I), Three of these materials are recommended
Table 1. The materials used in the manufacture of inlays Code 1. S R-lsosit Inlay/Onlay lvoclar A G, Schaan/Liechtenstein 2. Brilliant DI Coltene AG, Alstatten, Switzerland 3. Kulzer Kulzer Et Co GmbH, Dental Division, Wehrheim, Germany 4. Herculite X R Kerr Manufacturing Company, Romulus, Ml, USA 5. Occlusin 6. Opalux ICI Dental, Macclesfield, UK
I C K
H
oc OP
297
also be used in the restoration of small cavities in nonstress-bearing situations in posterior teeth (Wilson and Wilson, 1988). Inlays were manufactured in a brass mould which had been milled to the shape of a large mesio-occluso-distal inlay. The inlays produced were 10 mm long, 5 mm wide, 3 mm deep at the occlusal isthmus and 5 mm deep in each of the proximal boxes. The walls of the inlay had a 5” taper, and the mould was designed to allow the walls of the cavity to be removed so that access to all parts of the inlay was possible. Three methods of forming inlays were used: Method 1 - layering technique. Method 2 -bulk placement at atmospheric pressure. Method 3 -bulk placement with pressure (material I being introduced into the die and then placed into a water-bath according to manufacturer’s instructions for use). Layering technique The composite was placed in the mould in layers < 2.5 mm thick. A 2 mm depth increment of composite was placed into each box and cured for 60 s, this was followed by two layers over the occlusal portion. The layers in the boxes were cured for 60 s each, but because the occlusal area was larger than the diameter of the distal end of the tibreoptic of the curing light, each occlusal layer was cured for 2 min. A total curing time of 6 min was required for each inlay. Bulk placement
at atmospheric
pressure
The composites were placed into the moulds using stainless steel hand instruments (flat plastics and a G packer) with a smearing/tamping action. After packing the composites, C, K, H, OP and OC were light cured for 4 min from the occlusal surface, and 1 min for each side of the two boxes, giving a total curing time of 8 min. Composite I, being heat cured rather than light cured, was exposed to 120°C for 10 min. Bulk placement
under pressure
The composites C, K, H, OP and OC were placed into the moulds in a similar manner to that described for bulk placement at atmospheric pressure. The mould was placed inside a pressure chamber and all light excluded. The pressure chamber was just large enough to accommodate the mould and had a plexiglass windowjust above the specimens which could be covered to prevent light curing. The pressure chamber was sealed and the internal air pressure raised to 6 bar. This pressure was held for 10 min then the cover was removed from the window and the inlays cured with a light source. To compensate for the greater distance from the light source and the presence of the plexiglass, each occlusal surface was cured for eight 1-min exposures to light. The pressure was dropped and
298
J. Dent 1991; 19: No. 5
RESULTS Voids in the composite from the syringe
resin as extruded
Voids were only found within the materials OP, C and H. The area of the voids was large for these materials, the largest area being 247 mm2. Distribution of worst areas of porosity within the inlays
Fig. 7. Typical examples of inlay specimens.
the mould removed. The composite I was placed into the moulds and cured in water at 120°C for 10 min under 6 bar pressure according to the manufacturer’s instructions. Five inlays were made from each material using the three packing methods (Fig. 1). The only exception was composite I, which could not be readily manufactured using a layering technique. Inlays which had surface irregularities or voids were discarded and replaced. The inlays were embedded in clear acrylic resin and sectioned horizontally in the mesiodistal plane using a jeweller’s lathe, A no. 10H diamond wheel was mounted into the chuck and the specimens cut under a steady stream of water. For each inlay three sections were prepared. The first section was just below the surface of the inlay, and sections 2 and 3 were cut approximately 1 mm and 2 mm deeper respectively into the body of the inlay. The specimens were examined under a light microscope at a magnification of X 3. The area containing most porosity within each inlay was identified, and a black and white Polaroid photograph was taken. The sites where the porosities occurred within each inlay section were recorded on a line diagram of the inlay. Each processed film was then scanned using a computer-aided image analyser, and the number and area of the voids measured. In addition, the composites which had syringes as primary packaging were examined for voids by extruding the composite from the tube, discarding the first 2 cm and then curing the following 5 cm. These cured specimens were sectioned and any voids noted. Due to the tub presentation of composite I, it was not considered possible to remove and cure the material for this assessment without introducing a significant handling error.
With the layering techniques, for all materials the voids tended to occur along the junction between the layers of the materials. With bulk placement techniques voids were most commonly found along the position of the axiopulpal line angle, except for material (I), where the voids were found uniformly scattered in all parts of the inlay. Inlays placed with the aid of pressure showed the smallest number of voids, and these were mainly in the occlusal portion of the inlay. The number
of voids
The results showing the mean number of voids using the different methods of placement are shown in Table ZZ.No significant difference was found between the number of voids occurring in the sections of each inlay. A mean figure for the number of voids occurring in the sections of each inlay. A mean figure for the number of voids in an inlay was calculated. Using the Kruskal-Wallis nonparametric analysis of variance, for an overall comparison of the three methods of construction of the inlay, a highly significant difference was found to exist between the methods (P < 0.00005). Differences between different methods for each material were compared using the Mann-Whitney ZJ test, the results are presented in Table ZZZ In all materials in which the three placement techniques were compared, it was found that there was a highly significant difference between method 3 (placement under pressure) and method I (layering), and also a significant difference between method 2 (bulk placement) and method 3 (placement under pressure). No significant difference could be found between method 1 (layering) and method 2 (bulk placement), except for material C. The area of the voids No significant differences could be found between the values for the area of voids in the different sections in the
Table II. The number of voids using different packing techniques
Material code
:C OP C K
I
Method 7 No. of sections Mean s.d.
12 15 13 15 12
2.58 2.60 1.53 3.46 2.00
-
1.37 1.84 0.87 1.50 1.12
Method 2 No. of sections Mean s.d.
14 15 13 15 15 14
2.40 3.21 2.15 2.33 1.46 10.0
1.80 1.12 1.28 1.44 0.74 2.63
Method 3 No. of sections Mean s.d.
15 15 15 15 15
0.66 0.33 0.26 0.66 0.46 0.80
0.72 0.61 0.45 1.04 0.74 1.56
Wilson
Table 111.Comparison of the number of packing methods for each material Material code H
oc
OP
I
No. of sections 12 14 14 15 15 12
voids for the three
Method of packing
P = 0.0001 P = 0.0002 P = 0.9145
1
P = 0.00005
13 13 13 15 15 13
: 2 3 3 1
P = 0.1634
15 15 15 15 15 15
: 2 3 3 1
P = 0.0306
12 15 15 15 15 12
: 2 3 3 1
P=O.2134 P = 0.0010
15 14
:
P = 0.00005
P = 0.00005
P = 0.00005 P = 0.0001
P = 0.0003 P = 0.00005
P = 0.0005
inlays. The mean area of the voids investigated in the inlay sections is shown in Table IV. It should be noted that with method 3 some sections did not contain any voids (Table V). When the area of the voids was compared in those sections containing voids, using the Kruskal-Wallis test it was found that no significant difference existed between the methods except for material C. Specific comparisons between methods I,2 and 3 for material C are shown in Table VI.
Table IV. The area of voids (pm2) using different
Material code
GC OP C K
I
Method 1 No. of sections Mean s.d.
12 15 13 15 12
8.19 7.30 8.06 7.82 7.32
-
0.45 0.71 0.84 0.36 2.33
in indirect
composite
inlays
299
It was disappointing to find voids within certain composites even before any attempt had been made to form inlays. When the area of the voids was examined, it was apparent that the three materials with the largest void areas when using bulk placement at atmospheric pressure were the same materials found to have voids present in the material which extruded from the syringe. This would seem to indicate that the presence of voids in composite in a syringe can result in voids in bulk packed inlays. Although in this study a very small sample was examined, it was considered that voids occurring within the composite could adversely affect cured inlays. It was not surprising to find that with the layering method of placement, the voids were most commonly found at the junction of each layer. Although the effects of polymerization shrinkage may be minimized by this technique, there is a risk of air entrapment between the layers of composite which could adversely affect the longterm success of the inlay. Bulk placement at atmospheric pressure would appear to overcome the problem of trapping air at layer interfaces, but voids may be found adjacent to the axiopulpal line angles. Voids in this area could result in stress fractures. The generalized voids in material (I) were probably incorporated into the material during manipulation and packing. The material was very viscous and required spatulation before insertion into the die. Although the area of the voids was extremely large, it should be remembered that this is not the technique recommended by the manufacturers. Bulk packing of inlay cavities, as advocated by Hussey (1988) has disadvantages in large restorations as curing via a conventional light source has a light cure depth of approximately 2.5 mm using the manufacturer’s recommended times. Inlays which are any deeper or are being packed into dies which are made from materials which do not transmit light, will result in poorly cured or uncured resin at the base of the inlay which would distort if the inlay had to be removed from the die for a secondary cure. In terms of number of voids, no differences could be found between the numbers of voids occurring within different sections of the inlays. The voids appeared to be evenly distributed throughout the depth of the inlays and were not found to be a surface phenomenon. Method 3,
P = 0.4135
: 2 z
Voids
DISCUSSION
Probability
1 2 2 3 3 1
15 15 15 15 15 15
and Norman:
packing techniques
Method 2 No. of sections Mean s.d.
14 15 13 15 15 14
8.12 7.76 8.32 8.66 7.97 21.14
0.39 0.50 0.66 0.64 0.60 0.89
Method 3 No. of sections Mean s.d,
8 4 4 7 5 6
7.81 8.02 8.01 7.51 8.02 8.04
0.88 0.72 0.68 1.26 0.41 0.82
300
J. Dent 1991; 19: No. 5
Table V. Inlay sections not containing any voids Material code
Method
7
Method
2
Method
3
oc OP
7 (46.6%) 11 1 1 (73.3%) (73.3%)
C K
8 (53.3%) 10 (66.6%)
H
I
It is concluded that packing composite inlays using air pressure at 6 bar before light curing produced significantly fewer voids than layering techniques or bulk packing. Further testing is now required to discover if mechanical properties are affected by this packing technique.
9 (60%)
References Table VI. Separate comparisons of area of voids in those sections containing voids for three packing methods Material code C
No. of sections 15 15 15 7 7 15
Method of packing
Probability
P = 0.001 1 P = 0.0219 P = 0.6982
placement under pressure, produced fewer voids than any other method, and was found to be statistically different from the other two methods. No statistical significance could be found in terms of number of voids between bulk placement at atmospheric pressure and layering techniques. The position of the voids in the inlays formed by bulk placement at atmospheric pressure was adjacent to the axiopulpal line angle. Voids occurring at this site could be expected to adversely affect the long-term prognosis for the inlay. When the area of the voids was considered, method 3 produced the most inlays with no voids identified. Where voids did occur there was no significant difference between the area of the voids using the three different methods for packing the dies. It is of interest that the benefits of lower porosity values can be obtained for all the composite resins investigated with the air pressure technique, and without having to use a water bath as in the Ivoclar/Vivadent system. It is now necessary to carry out physical tests on the inlays of light-cured composites fabricated by the method incorporating the application of air pressure prior to primary curing. In addition, further investigations are indicated to determine optimum pressure to ensure homogeneity within the material and adaptation of the inlays to the dies. The use of an inert atmosphere may also be found to be beneficial in reducing the number of voids before curing the composite.
Asmussen E. K. and Jorgensen K. D. (1982) Fatigue strength of some resinous materials. &and. J. Dent. Res. 92, 257-261. Causton B., Miller B. and Sefton J. (1985) The deformation of cusps by bonded posterior composite restorations-an in vitro study. Br. Dent J. 59, 397-400. Chadwick R. G., McCabe J. F., Walls A. W. G. et al. (1989) The effect of placement technique upon the compressive strength and porosity of a composite resin. J. Dent 17, 230-233. Christensen G. J. (1989) Alternatives for the restoration of posterior teeth. Int. Dent. J. 39, 155-161. Fayyad M. A. and Shortall A. C. C. (1987) Microleakage of dentine-bonded posterior composite restorations. J. Dent. 5, 61-12. Hussey D. L. (1988) Direct hybrid composite inlays. Rest. Dent 4, 28-31. James D. F. and Yarovesky U. (1983) An esthetic inlay for posterior teeth. Quinfessence Int. 14, 1-7. Jordan R. E. and Suzuki M. (1989) Direct hybrid composite inlay technique. Esthetic Dent. 1, 57-62. Kunca J. (1989) The single visit heat processed indirect composite resin inlay. Esthetic Dent. 1, 31-34. Leinfelder K. F., McCartha C. D. and Visniewski J. F. (1985) Posterior composites: a critical review. J. Ala. Dent. Assoc. 69, 19-25. McCabe J. F. and Ogden A R. (1987) The relationship between porosity, compressive fatigue limit and wear in composite restorative materials. Dent Mater. 3, 9-12. Ogden A R. (1985) Porosity in composite resins-an Achilles’ heel? .I Dent. 13, 331-340. Robinson P. B., Moore B. K. and Swartz M. L. (1987) Comparison of microleakage in direct and indirect composite resin materials in-vitro. Oper. Dent. 12, 113-l 16. Van Dijken J. W. V., Ruyter I. E. and Holland R. I. (1986) Porosity in posterior composite resins. Stand. J. Dent. Res. 94,471-478. Wendt S. L. (1987) The effect of heat used as secondary cure upon the physical properties of three composite resins 2 Wear, Hardness and color stability. Quintessence Int. 18, 125-131. Wilson N. H. F., Wilson M. A. and Smith G. A. (1986) A clinical trial of a visible light-cured posterior composite restorative: 3 year results. Quintessence Int 17, 643-6.52. Wilson M. A. and Wilson N. H. F. (1988) A clinical trial of composite resin restoratives in premolars. J. Dent. Res. 68, 630.
296
19: 296-300
An investigation into the incidence of voids in indirect composite inlays formed using different packing techniques M. A. Wilson and R. D. Norman* Depattmenr of Resrorarive Dentistry, University University,
Alron, Illinois,
of Manchester
Dental
Hospital,
UK and *Southern
Illinois
USA
ABSTRACT An investigation is described into the incidence of voids in indirect composite inlays formed using different packing techniques and different composite systems. From the results it was found that packing inlays under a 6 bar air pressure prior to light curing produced significantly fewer voids than layering or bulk packing techniques. KEY WORDS: J. Dent. 1991; 1991)
Dental composites, Inlays, Technique 19:
296-300
(Received 25 September
1990;
reviewed 30 October 1990;
accepted 11 April
Correspondence should be addressed to: Dr M. A. Wilson, Department of Restorative Dentistry, University of Manchester Dental Hospital, Higher Cambridge Street, Manchester Ml 5 6FH, UK.
INTRODUCTION Composite restoratives are now widely used in the restoration of posterior teeth (Christensen, 1989). It has been reported however that posterior composites perform least well when the restorations are large or in situations of heavy occlusal loading (Wilson et al., 1986). Difficulties may also be encountered with polymerization shrinkage during polymerization, causing stresses between tooth cusps (Causton et al., 1985), and increasing the possibility of microleakage (Fayyad and Shortall, 1987). These difficulties and problems encountered when attempting to place, contour and finish large restorations using such technique-sensitive materials have in part resulted in the emergence of the indirect composite inlay. With the introduction of secondary and super (heat) cure techniques, many of the difficulties encountered with direct composite restorations have been largely overcome, and in vitro tests would seem to suggest that the physical properties of composite restoratives may be improved by secondary curing (Wendt, 1987) and super curing, and that microleakage will be reduced (Robinson et al., 1987). The presence of porosities within dental composites has been well documented by Ogden (1985) and Van Dijken et @ 1991 Butterworth-Heinemann 0300-5712/91/05029h-05
Ltd.
al. (1986). The adverse effects of porosities contained within direct composite restorations has been reported by Asmussen and Jorgensen (1982) Ogden (1985) Leinfelder et al. (1985) and McCabe and Ogden (1987) and manipulative procedures which increase the likelihood of incorporation of porosities into the material should be avoided (Chadwick et al., 1989). There are at present several composite systems which are produced specifically for the production of indirect composite inlays, relying on one of three different methods of placement. Placement techniques include: 1. The layering technique. This is the method which is also used when placing direct composite resins. The method allows the material to be cured in increments, thereby reducing the effects of polymerization shrinkage, and also ensures that the composite at the base of deep cavities is polymerized. Restorations are built up in increments or layers. Each layer (< 2.5 mm) is cured by visible light from a light source for > 20 s according to manufacturers’ instructions. This technique is recommended by Coltene (C) (Coltene AG, Alstatten. Switzerland), and Kulzer (K) (Kulzer and Co. GmbH. Wehrheim. Germany). for their respective indirect inlay systems.
Wilson and Norman: Voids in indirect composite inlays
2. Bulk packing.
In this method the composite is packed into the die/preparation in one increment and then light cured. This technique has the major problem that in deep cavities it is not possible to be certain that all parts of the composite inlay are polymerized prior to removal. However, this technique has been suggested for the manufacture of direct inlays by: Hussey (1988) Jordan and Suzuki (1989) and for small restorations by Kanca (1989). 3. Placement under pressure. The material Isosit (I) (Ivoclar/Vivadent, Schaan, Liechtenstein) is different from all other indirect composite resin systems in that it is super (heat) cured (120°C) in a water-bath at 6 bar pressure (James and Yarovesky, 1983). The manufacturers of this material claim that this technique prevents the formation of porosities within the inlays. To date no published work has reported investigations of different systems in terms of the presence of voids within composite inlays, comparing different placement techniques. The effect of pressure on the production of porosities has also not been investigated, and in particular the need for pressure applied via a water-bath. The purpose of this study was to determine which method of formation produces the smallest number and area of voids within composite inlays of a variety of composite materials. Materials advocated for use as indirect inlays and a limited selection of other composite restoratives were tested. In addition, since the method of manufacture of a composite, or the method whereby it is loaded into a syringe could result in the entrapment of air within the material, potentially making void production in the inlay more likely, specimens of composite from all the syringe-loaded systems were investigated.
MATERIALS
AND METHODS
Six composite materials were selected for use in this study for the production of inlays, two are composites recommended for direct applications in the restoration of posterior teeth, and one is an anterior composite restorative which may
(Table I), Three of these materials are recommended
Table 1. The materials used in the manufacture of inlays Code 1. S R-lsosit Inlay/Onlay lvoclar A G, Schaan/Liechtenstein 2. Brilliant DI Coltene AG, Alstatten, Switzerland 3. Kulzer Kulzer Et Co GmbH, Dental Division, Wehrheim, Germany 4. Herculite X R Kerr Manufacturing Company, Romulus, Ml, USA 5. Occlusin 6. Opalux ICI Dental, Macclesfield, UK
I C K
H
oc OP
297
also be used in the restoration of small cavities in nonstress-bearing situations in posterior teeth (Wilson and Wilson, 1988). Inlays were manufactured in a brass mould which had been milled to the shape of a large mesio-occluso-distal inlay. The inlays produced were 10 mm long, 5 mm wide, 3 mm deep at the occlusal isthmus and 5 mm deep in each of the proximal boxes. The walls of the inlay had a 5” taper, and the mould was designed to allow the walls of the cavity to be removed so that access to all parts of the inlay was possible. Three methods of forming inlays were used: Method 1 - layering technique. Method 2 -bulk placement at atmospheric pressure. Method 3 -bulk placement with pressure (material I being introduced into the die and then placed into a water-bath according to manufacturer’s instructions for use). Layering technique The composite was placed in the mould in layers < 2.5 mm thick. A 2 mm depth increment of composite was placed into each box and cured for 60 s, this was followed by two layers over the occlusal portion. The layers in the boxes were cured for 60 s each, but because the occlusal area was larger than the diameter of the distal end of the tibreoptic of the curing light, each occlusal layer was cured for 2 min. A total curing time of 6 min was required for each inlay. Bulk placement
at atmospheric
pressure
The composites were placed into the moulds using stainless steel hand instruments (flat plastics and a G packer) with a smearing/tamping action. After packing the composites, C, K, H, OP and OC were light cured for 4 min from the occlusal surface, and 1 min for each side of the two boxes, giving a total curing time of 8 min. Composite I, being heat cured rather than light cured, was exposed to 120°C for 10 min. Bulk placement
under pressure
The composites C, K, H, OP and OC were placed into the moulds in a similar manner to that described for bulk placement at atmospheric pressure. The mould was placed inside a pressure chamber and all light excluded. The pressure chamber was just large enough to accommodate the mould and had a plexiglass windowjust above the specimens which could be covered to prevent light curing. The pressure chamber was sealed and the internal air pressure raised to 6 bar. This pressure was held for 10 min then the cover was removed from the window and the inlays cured with a light source. To compensate for the greater distance from the light source and the presence of the plexiglass, each occlusal surface was cured for eight 1-min exposures to light. The pressure was dropped and
298
J. Dent 1991; 19: No. 5
RESULTS Voids in the composite from the syringe
resin as extruded
Voids were only found within the materials OP, C and H. The area of the voids was large for these materials, the largest area being 247 mm2. Distribution of worst areas of porosity within the inlays
Fig. 7. Typical examples of inlay specimens.
the mould removed. The composite I was placed into the moulds and cured in water at 120°C for 10 min under 6 bar pressure according to the manufacturer’s instructions. Five inlays were made from each material using the three packing methods (Fig. 1). The only exception was composite I, which could not be readily manufactured using a layering technique. Inlays which had surface irregularities or voids were discarded and replaced. The inlays were embedded in clear acrylic resin and sectioned horizontally in the mesiodistal plane using a jeweller’s lathe, A no. 10H diamond wheel was mounted into the chuck and the specimens cut under a steady stream of water. For each inlay three sections were prepared. The first section was just below the surface of the inlay, and sections 2 and 3 were cut approximately 1 mm and 2 mm deeper respectively into the body of the inlay. The specimens were examined under a light microscope at a magnification of X 3. The area containing most porosity within each inlay was identified, and a black and white Polaroid photograph was taken. The sites where the porosities occurred within each inlay section were recorded on a line diagram of the inlay. Each processed film was then scanned using a computer-aided image analyser, and the number and area of the voids measured. In addition, the composites which had syringes as primary packaging were examined for voids by extruding the composite from the tube, discarding the first 2 cm and then curing the following 5 cm. These cured specimens were sectioned and any voids noted. Due to the tub presentation of composite I, it was not considered possible to remove and cure the material for this assessment without introducing a significant handling error.
With the layering techniques, for all materials the voids tended to occur along the junction between the layers of the materials. With bulk placement techniques voids were most commonly found along the position of the axiopulpal line angle, except for material (I), where the voids were found uniformly scattered in all parts of the inlay. Inlays placed with the aid of pressure showed the smallest number of voids, and these were mainly in the occlusal portion of the inlay. The number
of voids
The results showing the mean number of voids using the different methods of placement are shown in Table ZZ.No significant difference was found between the number of voids occurring in the sections of each inlay. A mean figure for the number of voids occurring in the sections of each inlay. A mean figure for the number of voids in an inlay was calculated. Using the Kruskal-Wallis nonparametric analysis of variance, for an overall comparison of the three methods of construction of the inlay, a highly significant difference was found to exist between the methods (P < 0.00005). Differences between different methods for each material were compared using the Mann-Whitney ZJ test, the results are presented in Table ZZZ In all materials in which the three placement techniques were compared, it was found that there was a highly significant difference between method 3 (placement under pressure) and method I (layering), and also a significant difference between method 2 (bulk placement) and method 3 (placement under pressure). No significant difference could be found between method 1 (layering) and method 2 (bulk placement), except for material C. The area of the voids No significant differences could be found between the values for the area of voids in the different sections in the
Table II. The number of voids using different packing techniques
Material code
:C OP C K
I
Method 7 No. of sections Mean s.d.
12 15 13 15 12
2.58 2.60 1.53 3.46 2.00
-
1.37 1.84 0.87 1.50 1.12
Method 2 No. of sections Mean s.d.
14 15 13 15 15 14
2.40 3.21 2.15 2.33 1.46 10.0
1.80 1.12 1.28 1.44 0.74 2.63
Method 3 No. of sections Mean s.d.
15 15 15 15 15
0.66 0.33 0.26 0.66 0.46 0.80
0.72 0.61 0.45 1.04 0.74 1.56
Wilson
Table 111.Comparison of the number of packing methods for each material Material code H
oc
OP
I
No. of sections 12 14 14 15 15 12
voids for the three
Method of packing
P = 0.0001 P = 0.0002 P = 0.9145
1
P = 0.00005
13 13 13 15 15 13
: 2 3 3 1
P = 0.1634
15 15 15 15 15 15
: 2 3 3 1
P = 0.0306
12 15 15 15 15 12
: 2 3 3 1
P=O.2134 P = 0.0010
15 14
:
P = 0.00005
P = 0.00005
P = 0.00005 P = 0.0001
P = 0.0003 P = 0.00005
P = 0.0005
inlays. The mean area of the voids investigated in the inlay sections is shown in Table IV. It should be noted that with method 3 some sections did not contain any voids (Table V). When the area of the voids was compared in those sections containing voids, using the Kruskal-Wallis test it was found that no significant difference existed between the methods except for material C. Specific comparisons between methods I,2 and 3 for material C are shown in Table VI.
Table IV. The area of voids (pm2) using different
Material code
GC OP C K
I
Method 1 No. of sections Mean s.d.
12 15 13 15 12
8.19 7.30 8.06 7.82 7.32
-
0.45 0.71 0.84 0.36 2.33
in indirect
composite
inlays
299
It was disappointing to find voids within certain composites even before any attempt had been made to form inlays. When the area of the voids was examined, it was apparent that the three materials with the largest void areas when using bulk placement at atmospheric pressure were the same materials found to have voids present in the material which extruded from the syringe. This would seem to indicate that the presence of voids in composite in a syringe can result in voids in bulk packed inlays. Although in this study a very small sample was examined, it was considered that voids occurring within the composite could adversely affect cured inlays. It was not surprising to find that with the layering method of placement, the voids were most commonly found at the junction of each layer. Although the effects of polymerization shrinkage may be minimized by this technique, there is a risk of air entrapment between the layers of composite which could adversely affect the longterm success of the inlay. Bulk placement at atmospheric pressure would appear to overcome the problem of trapping air at layer interfaces, but voids may be found adjacent to the axiopulpal line angles. Voids in this area could result in stress fractures. The generalized voids in material (I) were probably incorporated into the material during manipulation and packing. The material was very viscous and required spatulation before insertion into the die. Although the area of the voids was extremely large, it should be remembered that this is not the technique recommended by the manufacturers. Bulk packing of inlay cavities, as advocated by Hussey (1988) has disadvantages in large restorations as curing via a conventional light source has a light cure depth of approximately 2.5 mm using the manufacturer’s recommended times. Inlays which are any deeper or are being packed into dies which are made from materials which do not transmit light, will result in poorly cured or uncured resin at the base of the inlay which would distort if the inlay had to be removed from the die for a secondary cure. In terms of number of voids, no differences could be found between the numbers of voids occurring within different sections of the inlays. The voids appeared to be evenly distributed throughout the depth of the inlays and were not found to be a surface phenomenon. Method 3,
P = 0.4135
: 2 z
Voids
DISCUSSION
Probability
1 2 2 3 3 1
15 15 15 15 15 15
and Norman:
packing techniques
Method 2 No. of sections Mean s.d.
14 15 13 15 15 14
8.12 7.76 8.32 8.66 7.97 21.14
0.39 0.50 0.66 0.64 0.60 0.89
Method 3 No. of sections Mean s.d,
8 4 4 7 5 6
7.81 8.02 8.01 7.51 8.02 8.04
0.88 0.72 0.68 1.26 0.41 0.82
300
J. Dent 1991; 19: No. 5
Table V. Inlay sections not containing any voids Material code
Method
7
Method
2
Method
3
oc OP
7 (46.6%) 11 1 1 (73.3%) (73.3%)
C K
8 (53.3%) 10 (66.6%)
H
I
It is concluded that packing composite inlays using air pressure at 6 bar before light curing produced significantly fewer voids than layering techniques or bulk packing. Further testing is now required to discover if mechanical properties are affected by this packing technique.
9 (60%)
References Table VI. Separate comparisons of area of voids in those sections containing voids for three packing methods Material code C
No. of sections 15 15 15 7 7 15
Method of packing
Probability
P = 0.001 1 P = 0.0219 P = 0.6982
placement under pressure, produced fewer voids than any other method, and was found to be statistically different from the other two methods. No statistical significance could be found in terms of number of voids between bulk placement at atmospheric pressure and layering techniques. The position of the voids in the inlays formed by bulk placement at atmospheric pressure was adjacent to the axiopulpal line angle. Voids occurring at this site could be expected to adversely affect the long-term prognosis for the inlay. When the area of the voids was considered, method 3 produced the most inlays with no voids identified. Where voids did occur there was no significant difference between the area of the voids using the three different methods for packing the dies. It is of interest that the benefits of lower porosity values can be obtained for all the composite resins investigated with the air pressure technique, and without having to use a water bath as in the Ivoclar/Vivadent system. It is now necessary to carry out physical tests on the inlays of light-cured composites fabricated by the method incorporating the application of air pressure prior to primary curing. In addition, further investigations are indicated to determine optimum pressure to ensure homogeneity within the material and adaptation of the inlays to the dies. The use of an inert atmosphere may also be found to be beneficial in reducing the number of voids before curing the composite.
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