Journal of Dentistry,
7, No. 2, 1979,
pp. 105-l
10. Printed in Great Britain
The abrasivity of finishing agents used on composite filling material G. J. Pearson, PhD, BDS, LDS J. J. Messing, BDS, FDS Department of Conservative Dentistry, University College Hospital Dental School, London
ABSTRACT A selection of finishing agents was used on standardized discs of one composite, and the abrasivity of each agent was recorded using a Talysurf machine and scanning electron microscope. The character of the various surface finishes is described and the method of production discussed.
INTRODUCTION Since the introduction of composite filling materials to the United Kingdom in 1970, a number of attempts has been made to assess the most appropriate methods of finishing the materials clinically after flash and excess have been removed. Many of the methods originally suggested are now considered to be unsatisfactory because of the destruction of adjacent enamel, as documented by Hannah and Smith (1973). Various methods of evaluation of the surface finish have been used (Hannah and Smith, 1973; Heath and Wilson, 1976; Verrall et al., 1976). The majority of workers consider the least abrasive material for trimming composites to be fine sanding discs or strips, although the type of filling material used may affect the finish achieved (Heath and Wilson, 1976). There is some discrepancy in the degree of roughness which is produced by the particles and finishing methods adopted (Hannah and Smith, 1973; Ribbons and Pearson, 1973; Heath and Wilson, 1976; Verrall et al., 1976). However, all workers do agree that the basic finishing methods do not produce a surface which is ideal. MATERIALS AND METHODS This study reports an evaluation of the abrasivity of eight finishing materials on one composite restorative. Discs of the composite material (Concise) 15 cm in diameter and 1 mm thick were prepared using a PTFE mould. A smooth surface was obtained by covering the top surface of the mould with a glass microscope slide held in place by a 250-g weight. The surface obtained under a glass slide was found to be similar to that produced by a Mylar strip in a simulated clinical situation (Heath and Wilson, 1976). The discs were allowed to cure for 30 minutes at room temperature. The glass slide was removed and each disc was then finished with one of a selection of instruments (Table Z) using a conventional contra angle handpiece running at a constant speed of 2000 rev/min for a standard time of 1 minute. The force applied to the handpiece was similar to that used in the mouth clinically, which, for any one operator, is reproducible (Campbell-Keys and Verrall, 1978). The pressure applied using discs is self-limiting, as above a given load the discs flex. Surface profile analyses of the discs of composite material were taken using a Talysurf 4 and four Ra values for each disc were recorded. The Ra value is defined as the arithmetical average value of the departure of the profile above and below a mean reference line. A
106
Journal of Dentistry, Vol. ~/NO. 2
Tab/e /. Details of the finishing materials used
Finishing
material
Manufacturer
Description
Abrasive particle size (range p m)
Tungsten carbide burr
E.L.A. Dial Instruments and Accessories Ltd. London
Plain tapered fissure
White stone (Shofu)
Shofu Dental Corporation, California,USA
Cone-shaped
430-21
White stone (Alpine)
A.D. Trade Distributors, London
Cone-shaped
402-23
Rubber Wheel (Aaba)
Identoflax, W. Germany
Diameter 1’6 cm
Sand disc Medium Fine
Amalgamated Dental Co., London
Diameter 1’6 cm
Sof-Lax disc Fine Super-fine
3M Company, Minnesota, USA
Diameter 1’6 cm
-
96-30 43-10
3-9 l-7
Tab/e Il. Mean Ra values produced by different finishing materials
Finishing
methods
Mean Ra value (transverse length 1 ml
rd.
Tungsten carbide burr
1.7
0.3
White stone (Shofu)
1.9
04
White stone (Alpine)
1.7
0.1
1.8
0.2
Fine sand disc
1.1
0.3
Medium sand disc
1.5
0.1
Fine Sof-Lax disc
0.7
0.2
Surer-fine Sof-Lax disc
04
0.2
Rubber
wheel
representative tracing of each finishing technique was also taken. The discs were then plated with gold (thickness 80-12OA) in an evaporation coater and examined in a scanning electron microscope (IS1 3A) using an accelerating voltage of 15 kV. An indication of the size of the abrasive particles used in the finishing procedures is given in Table I. RESULTS Mean Ra values produced by the eight different methods are shown in Table I. The roughest surface was created by the White Shofu stone, the profile being slightly more irregular than from the rubber wheel, tungsten carbide burr and white Alpine stone (Fig. 1).
107
Pearson and Messing: Abrasivity of finishing agents
F,ne -------
Sof-Lex dlrc Super-fine %f-LeFi disc
Fig. 1. Representative surface profile tracings of discs of composite material after finishing with eight different types of finishing agents.
Fig. 2 Surface of composite resin after finishing with a Shofu stone. Field of view 504 /Im.
Fig. 3. Surface of composite resin after finishing with white stone. Note the filler particles standing proud of the surface. Field of view 48 /lrn.
fig. 4. Surface of composite resin after finishing with an Aaba wheel. Field of view 612 m.
Fig. 5. Surface of composite resin after finishing with a medium sanding disc. Field of view 510 elm.
Journal of Dentistrv, Vol. ~/NO. 2
Fig. 6. Surface of composite resin after finishing with a fine sanding disc. Field of view 591 ,urn.
Fig. 7. Surface of composite resin after finishing with a fine Sof-Lex disc. Field of view 574 j&n.
Fig, 8. Surface of composite resin after finishing with a super-fine Sof-Lex disc. Field of view 564
Fig. 9. Surface of composite resin after finishing with a fine Sof-Lex disc. The filler particles are not clearly defined and a resin layer covers the surface. Field of view 46 j&n.
Pm.
Fig. 10. Fracture surface of restoration showing the evan filler/resin distribution from the surface downwards.
Pearson and Messing. Abrasivity
of finishing
agents
109
The medium sand disc produced a smoother surface overall than these, but small irregularities occurred. The remaining three agents, fine sand and fine and super-fine Sof-Lex discs, created a generally undisturbed surface profile. Considerable differences were achieved in overall surface profiles. The drum- or cone-shaped finishing instruments caused greater disturbance than the disc-shaped finishing agents. SEM photographs of the various methods showed that with white stone and tungsten carbide burrs a considerable number of inorganic filler particles was exposed and stood proud of the surface (Figs. 2, 3). The rubber wheel and medium sand disc formed an irregular surface but maintained the flat surface profileof the specimen (Figs. 4, 5). The three finer finishing agents produced a smooth surface with a flat surface profne (Figs. 6-9), with resin covering the particles. The untreated surfaces of a test disc were compared with a composite restoration placed in a cavity in an extracted tooth using a Mylar strip. Both the test disc and the restoration were fractured and the fracture surfaces compared. It was found that the filer particle distribution observed just beneath the finished surface of the test disc was similar to that observed in the deeper portion of the restoration. The loss of less than lc( from the surface of the specimens would result in exposure of the filler particles. It was felt that any abrasive would expose filler particles after only a few seconds (Fig. IO).
DISCUSSION There is a considerable variation in the character of the surfaces produced by the techniques tested, but the surface profiles fall into broad groups. In the first group there was a loss of the flat contour of the surface overall, whereas the second showed short period irregularities or roughness which in some cases were quite large. For the specimens where there was a loss of form or flatness the cause was probably, in part, the shape of the instruments used to finish the materials. There is a smaller area of contact with the drum-shaped burrs than the disc-shaped instruments and minor movements of the handpiece would result in uneven trimming. It appears that, allowing for this factor, the roughness of the surface is due to the relative size of the abrasive particles used, the finer the abrasive, the smoother being the finish. A closer examination of the finished surfaces of the material also revealed that in four cases (rubber wheel, sanding discs and Sof-Lex discs) the majority of the inorganic filler particles were not uncovered. When white stones and tungsten carbide burrs were used, particles were left protruding from the surface. It was found that considerable heat was generated when disc type finishing materials were used. The appearance of the composite material following use of these devices indicated that smearing of the resin had occurred. Bowden (1947) has demonstrated that the basic action of polishing is the liberation of considerable heat locally. This results in hot spots and a smear layer is formed over the surface of the filler particles due to softening of the material. This could well have occurred in the present test owing to the relatively low melting temperature of the resin. Both patterns were seen when the rubber wheel and medium sanding discs were used, where the abrasive material has chipped away the surface, but at the same time resin has been smeared over the remaining exposed inorganic filler particles. The factors likely to influence the surface finish of composite are thus the type and size of the abrasive material used, the shape of the finishing instrument and the amount of heat generated during finishing procedures.
110
Journal of Dentistry, Vol. ~/NO. 2
Acknowledgements The authors wish to thank Mr Richard Verrall of the Materials Science Department, University of Sussex, for his help in the use of the Talysurf, and also Mr Charles Day for preparation of the photographs. We are indebted to the 3M Company for the provision of the experimental polishing discs. REFERENCES
Bowden F. R. (1947) Science News IV. Harmondsworth, Middx, Penguin, pp. 15 1- 154. Campbell-Keys W. and Verrall R. J. (1978) The finishing of composite restorations in the surgery. Dent. Update 5, 105- 110. Hannah C. McD. and Smith G. A. (1973) The surface finish of composite restoration materials. Br. Dent. J. 135, 483-489. Heath J. R. and Wilson M. J. (1976) Surface roughness of restorations. Br. Dent. J. 140, 131-137. Ribbons J. and Pearson G. J. (1973) A composite filling material (two year clinical assessment). Br. Dent. J. 134, 389-391. Verrall R., Campbell-Keys W. and Pearson G. J. (1976) Int. Assoc. Dent. Res. Abstr. No. 158.
7, No. 2, 1979,
pp. 105-l
10. Printed in Great Britain
The abrasivity of finishing agents used on composite filling material G. J. Pearson, PhD, BDS, LDS J. J. Messing, BDS, FDS Department of Conservative Dentistry, University College Hospital Dental School, London
ABSTRACT A selection of finishing agents was used on standardized discs of one composite, and the abrasivity of each agent was recorded using a Talysurf machine and scanning electron microscope. The character of the various surface finishes is described and the method of production discussed.
INTRODUCTION Since the introduction of composite filling materials to the United Kingdom in 1970, a number of attempts has been made to assess the most appropriate methods of finishing the materials clinically after flash and excess have been removed. Many of the methods originally suggested are now considered to be unsatisfactory because of the destruction of adjacent enamel, as documented by Hannah and Smith (1973). Various methods of evaluation of the surface finish have been used (Hannah and Smith, 1973; Heath and Wilson, 1976; Verrall et al., 1976). The majority of workers consider the least abrasive material for trimming composites to be fine sanding discs or strips, although the type of filling material used may affect the finish achieved (Heath and Wilson, 1976). There is some discrepancy in the degree of roughness which is produced by the particles and finishing methods adopted (Hannah and Smith, 1973; Ribbons and Pearson, 1973; Heath and Wilson, 1976; Verrall et al., 1976). However, all workers do agree that the basic finishing methods do not produce a surface which is ideal. MATERIALS AND METHODS This study reports an evaluation of the abrasivity of eight finishing materials on one composite restorative. Discs of the composite material (Concise) 15 cm in diameter and 1 mm thick were prepared using a PTFE mould. A smooth surface was obtained by covering the top surface of the mould with a glass microscope slide held in place by a 250-g weight. The surface obtained under a glass slide was found to be similar to that produced by a Mylar strip in a simulated clinical situation (Heath and Wilson, 1976). The discs were allowed to cure for 30 minutes at room temperature. The glass slide was removed and each disc was then finished with one of a selection of instruments (Table Z) using a conventional contra angle handpiece running at a constant speed of 2000 rev/min for a standard time of 1 minute. The force applied to the handpiece was similar to that used in the mouth clinically, which, for any one operator, is reproducible (Campbell-Keys and Verrall, 1978). The pressure applied using discs is self-limiting, as above a given load the discs flex. Surface profile analyses of the discs of composite material were taken using a Talysurf 4 and four Ra values for each disc were recorded. The Ra value is defined as the arithmetical average value of the departure of the profile above and below a mean reference line. A
106
Journal of Dentistry, Vol. ~/NO. 2
Tab/e /. Details of the finishing materials used
Finishing
material
Manufacturer
Description
Abrasive particle size (range p m)
Tungsten carbide burr
E.L.A. Dial Instruments and Accessories Ltd. London
Plain tapered fissure
White stone (Shofu)
Shofu Dental Corporation, California,USA
Cone-shaped
430-21
White stone (Alpine)
A.D. Trade Distributors, London
Cone-shaped
402-23
Rubber Wheel (Aaba)
Identoflax, W. Germany
Diameter 1’6 cm
Sand disc Medium Fine
Amalgamated Dental Co., London
Diameter 1’6 cm
Sof-Lax disc Fine Super-fine
3M Company, Minnesota, USA
Diameter 1’6 cm
-
96-30 43-10
3-9 l-7
Tab/e Il. Mean Ra values produced by different finishing materials
Finishing
methods
Mean Ra value (transverse length 1 ml
rd.
Tungsten carbide burr
1.7
0.3
White stone (Shofu)
1.9
04
White stone (Alpine)
1.7
0.1
1.8
0.2
Fine sand disc
1.1
0.3
Medium sand disc
1.5
0.1
Fine Sof-Lax disc
0.7
0.2
Surer-fine Sof-Lax disc
04
0.2
Rubber
wheel
representative tracing of each finishing technique was also taken. The discs were then plated with gold (thickness 80-12OA) in an evaporation coater and examined in a scanning electron microscope (IS1 3A) using an accelerating voltage of 15 kV. An indication of the size of the abrasive particles used in the finishing procedures is given in Table I. RESULTS Mean Ra values produced by the eight different methods are shown in Table I. The roughest surface was created by the White Shofu stone, the profile being slightly more irregular than from the rubber wheel, tungsten carbide burr and white Alpine stone (Fig. 1).
107
Pearson and Messing: Abrasivity of finishing agents
F,ne -------
Sof-Lex dlrc Super-fine %f-LeFi disc
Fig. 1. Representative surface profile tracings of discs of composite material after finishing with eight different types of finishing agents.
Fig. 2 Surface of composite resin after finishing with a Shofu stone. Field of view 504 /Im.
Fig. 3. Surface of composite resin after finishing with white stone. Note the filler particles standing proud of the surface. Field of view 48 /lrn.
fig. 4. Surface of composite resin after finishing with an Aaba wheel. Field of view 612 m.
Fig. 5. Surface of composite resin after finishing with a medium sanding disc. Field of view 510 elm.
Journal of Dentistrv, Vol. ~/NO. 2
Fig. 6. Surface of composite resin after finishing with a fine sanding disc. Field of view 591 ,urn.
Fig. 7. Surface of composite resin after finishing with a fine Sof-Lex disc. Field of view 574 j&n.
Fig, 8. Surface of composite resin after finishing with a super-fine Sof-Lex disc. Field of view 564
Fig. 9. Surface of composite resin after finishing with a fine Sof-Lex disc. The filler particles are not clearly defined and a resin layer covers the surface. Field of view 46 j&n.
Pm.
Fig. 10. Fracture surface of restoration showing the evan filler/resin distribution from the surface downwards.
Pearson and Messing. Abrasivity
of finishing
agents
109
The medium sand disc produced a smoother surface overall than these, but small irregularities occurred. The remaining three agents, fine sand and fine and super-fine Sof-Lex discs, created a generally undisturbed surface profile. Considerable differences were achieved in overall surface profiles. The drum- or cone-shaped finishing instruments caused greater disturbance than the disc-shaped finishing agents. SEM photographs of the various methods showed that with white stone and tungsten carbide burrs a considerable number of inorganic filler particles was exposed and stood proud of the surface (Figs. 2, 3). The rubber wheel and medium sand disc formed an irregular surface but maintained the flat surface profileof the specimen (Figs. 4, 5). The three finer finishing agents produced a smooth surface with a flat surface profne (Figs. 6-9), with resin covering the particles. The untreated surfaces of a test disc were compared with a composite restoration placed in a cavity in an extracted tooth using a Mylar strip. Both the test disc and the restoration were fractured and the fracture surfaces compared. It was found that the filer particle distribution observed just beneath the finished surface of the test disc was similar to that observed in the deeper portion of the restoration. The loss of less than lc( from the surface of the specimens would result in exposure of the filler particles. It was felt that any abrasive would expose filler particles after only a few seconds (Fig. IO).
DISCUSSION There is a considerable variation in the character of the surfaces produced by the techniques tested, but the surface profiles fall into broad groups. In the first group there was a loss of the flat contour of the surface overall, whereas the second showed short period irregularities or roughness which in some cases were quite large. For the specimens where there was a loss of form or flatness the cause was probably, in part, the shape of the instruments used to finish the materials. There is a smaller area of contact with the drum-shaped burrs than the disc-shaped instruments and minor movements of the handpiece would result in uneven trimming. It appears that, allowing for this factor, the roughness of the surface is due to the relative size of the abrasive particles used, the finer the abrasive, the smoother being the finish. A closer examination of the finished surfaces of the material also revealed that in four cases (rubber wheel, sanding discs and Sof-Lex discs) the majority of the inorganic filler particles were not uncovered. When white stones and tungsten carbide burrs were used, particles were left protruding from the surface. It was found that considerable heat was generated when disc type finishing materials were used. The appearance of the composite material following use of these devices indicated that smearing of the resin had occurred. Bowden (1947) has demonstrated that the basic action of polishing is the liberation of considerable heat locally. This results in hot spots and a smear layer is formed over the surface of the filler particles due to softening of the material. This could well have occurred in the present test owing to the relatively low melting temperature of the resin. Both patterns were seen when the rubber wheel and medium sanding discs were used, where the abrasive material has chipped away the surface, but at the same time resin has been smeared over the remaining exposed inorganic filler particles. The factors likely to influence the surface finish of composite are thus the type and size of the abrasive material used, the shape of the finishing instrument and the amount of heat generated during finishing procedures.
110
Journal of Dentistry, Vol. ~/NO. 2
Acknowledgements The authors wish to thank Mr Richard Verrall of the Materials Science Department, University of Sussex, for his help in the use of the Talysurf, and also Mr Charles Day for preparation of the photographs. We are indebted to the 3M Company for the provision of the experimental polishing discs. REFERENCES
Bowden F. R. (1947) Science News IV. Harmondsworth, Middx, Penguin, pp. 15 1- 154. Campbell-Keys W. and Verrall R. J. (1978) The finishing of composite restorations in the surgery. Dent. Update 5, 105- 110. Hannah C. McD. and Smith G. A. (1973) The surface finish of composite restoration materials. Br. Dent. J. 135, 483-489. Heath J. R. and Wilson M. J. (1976) Surface roughness of restorations. Br. Dent. J. 140, 131-137. Ribbons J. and Pearson G. J. (1973) A composite filling material (two year clinical assessment). Br. Dent. J. 134, 389-391. Verrall R., Campbell-Keys W. and Pearson G. J. (1976) Int. Assoc. Dent. Res. Abstr. No. 158.
