j o u r n a l o f d e n t i s t r y 3 5 ( 2 0 0 7 ) 3 6 – 4 2

The effects of lubrication on the temperature rise andsurface finish of amalgam and composite resin

C.S. Jones *, R.W. Billington, G.J. Pearson

Queen Mary, University of London, Biomaterials in Relation to Dentistry, Medical Science Building, Queen Mary University Mile End Road,

London E1 4NS, United Kingdom

a r t i c l e i n f o a b s t r a c t

avai lable at www.sc iencedi rec t .com

journal homepage: www. int l .e lsev ierhea l th .com/ journals / jden

Article history:

Received 2 November 2005

Received in revised form

7 April 2006

Accepted 10 April 2006

Keywords:

Amalgam

Composite resin

Polishing

Surface roughness

Lubrication

Temperature

It was thought that when finishing and polishing direct filling materials lubrication would

affect the surface roughness and temperature rise in samples of amalgam and composite.

Object: Previous work by the authors has shown that there is an optimum load, speed and

time that produced the smoothest surface when finishing amalgam and composite resin

using each of four grades of a disc system. This work was undertaken to examine the

effects on temperature rise in samples of amalgam and composite resin of finishing dry

compared to finishing with different lubricants. The experiments all used these optimum

loads, speeds and times. It also compares the surface finish produced using different

lubricants.

Materials and methods: A high copper amalgam and a hybrid composite resin were finished

using the four grades of abrasive discs. Samples produced were 25 mm long by 6 mm wide by

2 mm deep. A thermocouple was inserted 1 mm into the base of the samples. The thermo-

couple was connected via an electronic thermometer to a computer that permitted the

display and recording of temperature against time. After roughening, the samples were

finished and polished in a specially constructed jig that mimicked oral finishing. The pre-

determined optimum loads, speeds and times were used sequentially for each of the four

grades of disc. Five samples were tested for each method of finishing. Firstly, run dry, then in

turn lubricated with water, walnut oil and petroleum jelly. After the use of each abrasive disc

the surface roughness was measured. One of the five samples was selected at random and

prepared for examination in the scanning electron microscope. All results were subjected to

non-parametric statistically analyses.

Results: With both materials the temperature rise was greatest when run dry, followed by

petroleum jelly, walnut oil and the least was when lubricated with water.

With these two materials the surface roughness correlates negatively with the tem-

perature rise. The smoothest surface being achieved when finished dry.

Conclusions: To obtain the smoothest surface finish amalgam and composite should be

finished dry but further work is needed to assess the effect of the temperature rise found in

the materials on the pulp.

# 2006 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: +44 1932 788701; fax: +44 1932 700060.E-mail address: cjonesstheaker@ntlworld.com (C.S. Jones).

0300-5712/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.jdent.2006.04.006

j o u r n a l o f d e n t i s t r y 3 5 ( 2 0 0 7 ) 3 6 – 4 2 37

1. Introduction

The surface finish of dental fillings is important for many

reasons—these include:

� a

chieving the desired anatomical form and shape;

� a

esthetics;

� p

atient comfort: that is being in a state not to cause irritation

to surrounding tissue;

� th

e elimination of food traps;

� to

enhance the material’s properties.

It has been determined that patients are able to distinguish

differences in roughness values of 0.25 and 0.5 mm with their

tongues therefore the finish of a restoration should be as s-

mooth as enamel.1 The final surface of a restoration is ther-

efore important and may be influenced by factors such as

lubrication.

The Oxford English Dictionary’s definition of lubrication is

‘‘that which is employed to reduce friction by interposing a

film between rubbing parts’’. The lubrication system must

continuously replace the film. Industrially the commonest l-

ubricants are water and crude oils. Finishing and polishing are

frictional processes and as such will produce heat. Excess heat

will damage the pulp2 and therefore the need for lubrication

may be important. It could also enhance or prejudice the fi-

nishing process.

A search of the dental literature resulted in very little work

having been done on the science of finishing of dental

materials and very few references to temperature changes

during polishing apart from:

(a) C

T

G

Ab

Pa

Pa

hristensen and Dilts3 looked at thermal changes during

polishing;

(b) A

lpin et al.4 looked at the effect of overheating on amalgam

hardness;

(c) S

tewart et al.5 measured the temperature rise in direct

filling materials.

None examined the correlation of temperature rise surface

finish and lubrication.

A previous study by the present authors,6 examined the

range of loads, speeds and times used by practitioners when

finishing amalgam and composite resin. This showed they

used loads between 9 and 114 g, speeds between 5000 and

25,000 rpm and times between 3 and 76 s. It seemed that they

needed guidance as to the optimum loads, speeds and times to

use to get the best surface finish.

The mean figures from this practitioner study were used to

conduct a laboratory experiment7 to determine the optimum

load, speed and time to produce the best surface finish. These

results show that with both amalgam and composite the

roughness improves as the disc become finer. The Ra value

able 1 – The colour coding, particle size and particle distribu

rade of disc Coarse (black) M

rasive material SiC

rticle size (mm) >100

rticle distribution (no./200 mm2) 6

recorded for amalgam is 0.52 mm for the coarse disc and

0.07 mm for the super-fine and for composite 0.23 and 0.06 mm

for the same discs. In fact this Ra value achieved for composite

of 0.06 mm compares well with the Ra value of 0.03–0.04

recorded for a polished glass surface.8

In clinical dentistry the common lubricants are water and

petroleum jelly and in this study it was decided to investigate

these and also the engineering standard—oil. If this oil were to

be used clinically it would have to be biologically safe. For this

reason in this study a light vegetable oil was chosen for the

investigation. Walnut oil was selected as walnut shells are

used in the tumble polishing of gold jewellery.

The aim of this current work was to examine the

temperature rise in samples of amalgam and composite resin

when finished dry, and when finished with different lubricants.

The surface achieved with the different finishing methods was

also to be recorded. These experiments were all done using the

optimum values obtained from the laboratory study and using

the same materials and disc system.

2. Materials

A high copper amalgam (Dispersalloy, Dentsply, Milford, USA)

and a hybrid composite resin (Z100 3M, St. Paul, USA) were

finished using the four grades of a disc system (Super-Snap

discs, Shofu Dental). Table 1 shows the colour coding of the

discs, their abrasive material, the size of that abrasive and the

particle distribution. The lubricants were water, walnut oil and

petroleum jelly.

3. Method

As directed by the manufacturers the amalgam capsules were

mixed in a Silamat for 4–6 s then packed into a specially

constructed brass mould. The surface was smoothed with

hand instruments. Similarly the composite was packed into

the mould covered with a glass slide and finger pressure used

to expel excess material before being cured with a light (Luxor

ICI) for 40 s (the manufacturers recommended curing time)

The samples produced were 25 mm long by 6 mm wide by

2 mm deep. These were then stored in deionised water in an

oven at 378 C for at least 24 h. They were then attached to a

brass plate with modelling (sticky) wax then pre-roughened to

a Ra value of between 2.5 and 3.5 mm. This figure matched

clinical conditions where some adjustment of the surface has

been necessary after removal of the matrix. This surface

roughness value is similar to that achieved after using an

Arkansas white stone9 that is made from ‘‘Novaculite’’ a

silicate material.

tion of the disc system

edium (violet) Fine (green) Super-fine (red)

Al2O3 Al2O3 Al2O3

40 30 5–10

16 72 750

j o u r n a l o f d e n t i s t r y 3 5 ( 2 0 0 7 ) 3 6 – 4 238

Fig. 1 – Diagram of the set-up of the laboratory test jig with

lubrication for testing the temperature rise in samples

while being finished.

Fig. 1 shows the laboratory jig for finishing the samples.

This comprised a flat bed that could be moved in the

horizontal plane. The brass plate with the sample was screwed

to the flat bed. The flat bed was capable of reciprocal

movements of 20 mm, 30 times a minute. A handpiece with

the disc was attached to an articulated arm. This arm was

capable of vertical movement in one plane. Attached to the

arm was a platform to which weights could be added. The

handpiece and disc were adjusted so that they were at the

point of balance when the disc was just in contact with the

surface of the sample. Loads could then be applied to the

handpiece by placing weights on the platform. The speed of

the handpiece could be varied. Each test speed was confirmed

using a stroboscopic light. A stopwatch controlled the time of

application.

The reservoir of the lubricant was adjusted to produce a

constant flow of the lubricant to the surface of the sample.

When using petroleum jelly this was constantly smeared over

the surface of the sample.

Five samples were tested for each lubricant. All the

experiments were then conducted at the loads, speeds and

times from the laboratory experiments and these values are

shown in Table 2. For all the experiments the discs were only

used once then discarded.

Table 2 – Optimum results from laboratory study forfinishing amalgam and composite resin

Disc Coarse(black)

Medium(violet)

Fine(green)

Super-fine(red)

Amalgam

Load (g) 80 40 40 20

Speed (krpm) 17.5 22 22 20

Time (s) 15 17 15 15

Composite

Load (g) 30 30 20 20

Speed (krpm) 16 17.5 17.5 17.5

Time (s) 22 22 22 22

4. Method for temperature rise

The samples were constructed with a thermocouple inserted

in the middle of the specimen and 1 mm from the surface

being abraded.

The wire from the thermocouple was connected via a pre-

calibrated electronic thermometer to a computer that per-

mitted the display and recording of temperature against time.

Temperature variations were recorded at 1 s intervals.

5. Method for surface finish

Five samples were tested for each method and the roughness

recorded using a profilometer (Mitutoya Surftest, Japan). Any

lubricant was removed by wiping the sample with a paper

tissue; every care was taken to remove the lubricant in order

not to effect the surface roughness testing. Six runs were

recorded; the readings were taken 0.5 mm apart in the long

axis of the sample. The traversing length was 4.8 mm with the

cut-off point of 0.8 mm. After each disc had been used one of

the five samples was selected at random and prepared for

examination in the scanning electron microscope.

6. Results for temperature rise

Fig. 2 shows as an example the traces for the temperature rise

in a sample of amalgam and composite unlubricated. The drop

in temperature from the peak values recorded relates to disc

changes. The composite took an extended period of time (over

100 s) to cool down to its starting temperature whereas

amalgam quickly reached its starting temperature.

The maximum temperature for each disc was measured

and the means and standard deviations calculated. Table 3

and Fig. 3 shows the maximum temperature rise for all the

polishing methods and for all the discs. The maximum rise

was unlubricated discs followed by petroleum jelly then

walnut oil. With water as a lubricant the rise was minimal for

amalgam. No temperature rise at all was noted with the

composite.

Fig. 2 – Temperature tracings for the finishing of samples of

amalgam and composite run dry.

j o u r n a l o f d e n t i s t r y 3 5 ( 2 0 0 7 ) 3 6 – 4 2 39

Table 3 – The temperature rise in degrees Celsius duringfinishing using different lubricants

Disc Coarse(black)

Medeium(violet)

Fine(green)

Super-fine(red)

Amalgam dry 26 20 16 11

Composite dry 11 16 13 14

Amalgam wet 6 4 3 1

Composite wet 0 0 0 0

Amalgam WO 11 8 7 7

Composite WO 4 6 7 8

Amalgam PJ 15 11 9 8

Composite PJ 7 11 11 14

Fig. 3 – Bar chart of the temperature rises in samples of

amalgam and composite finished using different

lubricants (error bars show standard deviation).

Table 4 shows the results for the non-parametric

statistical analysis (Mann–Whitney U test). When looked

at statistically these results show that, for amalgam, apart

from using the coarse disc, there is no difference in the

temperature rise between dry run, walnut oil and petroleum

jelly. However, when using water spray there was a

difference between this and the other finishing methods

( p � 0.01).

For composite the statistical results were similar in that

samples finished with water spray, temperature rises were

different from those using the other finishing methods

(p � 0.01). With the coarser discs there was a difference was

either p � 0.01 or p � 0.05 between those finished unlubricated

and those using walnut oil and petroleum jelly with the latter

being cooler. With both these lubricants the temperature

attained rose as the fineness of the disc increased. There was a

lower rise in temperature with the samples finished with

walnut oil compared to those finished with petroleum jelly

(p � 0.05).

The recovery period for the samples is taken as the time for

the temperature to return to the starting temperature (as

shown in Fig. 2). It was found that with amalgam, apart from

samples lubricated with water that had no recovery period,

the recovery period was short, 15–25 s. However, the rate of

recovery for the samples lubricated with walnut oil and

petroleum jelly was slightly longer than when finished

unlubricated. Composite had a long recovery time, between

1 and 2 min.

Table 4 – The statistical analysis of temperature variations wi

Lubrication Amalgam

Wet WO

Dry coarse <0.01 <0.05 >

Wet coarse – <0.01 <

WO coarse – – >

Dry medium <0.01 >0.05 >

Wet medium – <0.01 <

WO medium – – >

Dry fine <0.01 >0.05 >

Wet fine – <0.01 <

WO fine – – <

Dry S-fine <0.01 >0.05 >

Wet S-fine – <0.01 <

WO S-fine – – >

7. Results for surface finish

Table 5 and Fig. 4 shows the Ra value achieved with the

four methods of finishing and shows that finishing the

surface dry produced the smoothest surface. Statistical

analysis confirmed this (Table 6). For amalgam, a much

higher Ra value is obtained for the coarsest disc apart from

when finished dry. The Ra value of those specimens

finished dry was one-half that of the other finishing

methods.

8. SEM views of surfaces

Fig. 5 shows the photomicrographs of the amalgam surfaces

finished with the four methods using the coarse disc. These

show that the smoothest surface is produced appears to be

when no lubricant is used.

Similarly for composite (Fig. 6) shows the smoothest

surface is the sample finished dry.

th different lubricants

Composite

PJ Wet WO PJ

0.05 <0.01 <0.01 <0.05

0.01 – <0.01 <0.01

0.05 – – <0.05

0.05 <0.01 <0.05 <0.05

0.01 – <0.01 <0.01

0.05 – – <0.05

0.05 <0.01 <0.01 >0.05

0.01 – <0.01 <0.01

0.05 – – <0.05

0.05 <0.01 >0.05 >0.05

0.01 – <0.01 <0.01

0.05 – – <0.05

j o u r n a l o f d e n t i s t r y 3 5 ( 2 0 0 7 ) 3 6 – 4 240

Table 5 – The surface roughness values (Ra) in micro-meters achieved after finishing using different lubricants

Disc Coarse(black)

Medeium(violet)

Fine(green)

Super-fine(red)

Amalgam dry 0.52 0.30 0.10 0.07

Composite dry 0.23 0.12 0.09 0.06

Amalgam wet 1.22 0.45 0.22 0.17

Composite wet 0.37 0.34 0.26 0.20

Amalgam WO 0.96 0.35 0.22 0.25

Composite WO 0.38 0.19 0.10 0.10

Amalgam PJ 0.88 0.39 0.20 0.10

Composite PJ 0.36 0.25 0.16 0.11

Fig. 4 – Bar chart of the Ra values of samples of amalgam

and composite finished using different lubricants (error

bars show standard deviation).

9. Discussion

The samples are by necessity much larger than a restoration

but using the specially constructed jig with the thermocouple

inserted into it’s base gives an indication of the temperature

changes that might be experienced at the amelodentinal

junction when finishing a restoration in the mouth.

It is possible that if too great a temperature rise is generated

by the finishing of amalgam restorations excess mercury may

be brought to the surface, thus weakening the restoration.10

Temperatures may exceed the 65 8C required for mercury

liberation.11 High temperatures may lead to melting of the

resin component of composite resins. Excess heat production

will also damage the pulp.3 For these reasons the effects of

lubrication may be of some importance.

The rise in temperature of the materials due to frictional

forces generated during the finishing process was as expected

for the two materials. For amalgam, with a high thermal

diffusivity of 9.6 mm2/s, it would be expected the temperature

would rise rapidly but would also be dissipated quickly. When

finished with water spray the amalgam specimen cooled very

rapidly. This is also illustrated by the rapid fall in temperature

observed on changing discs (Fig. 2). Walnut oil was only

moderately useful in reducing the temperature rise in the

specimens and there was little difference in temperature rise

between petroleum jelly and finishing dry. With both walnut

oil and petroleum jelly the greater viscosity of these materials

may lead to greater accumulation of swarf between and over

Table 6 – The statistical analysis of roughness variations with

Lubrication Amalgam

Wet WO

Dry coarse <0.01 <0.01 <

Wet coarse – <0.01 <

WO coarse – – >

Dry medium <0.01 >0.05 >

Wet medium – <0.05 >

WO medium – – >

Dry fine <0.01 <0.01 <

Wet fine – >0.05 >

WO fine – – >

Dry S-fine <0.01 <0.01 <

Wet S-fine – >0.05 <

WO S-fine – – <

the abrasive particles on the discs, the lubricant acting as a

binder. This is likely to result in the abrasive becoming less

effective.

The roughness values achieved using the lubricants

shows that finishing dry, especially with the coarser discs,

achieves the smoothest results for all the materials. The

smoothness of the finish appears to be related to the

temperature rise that occurs in the sample. The results

show that the highest temperature measured within the

sample is when the polishing disc is run unlubricated. The

next greatest rise observed was with petroleum jelly,

followed by walnut oil. The smallest increase was noted

when water acted as the lubricating agent. The ranking order

for the surface roughness values is similar with the

smoothest surface being produced when unlubricated and

the roughest when run wet. The likely explanation for this is

that a higher temperature is more likely to produce surface

smearing and thus a smoother surface. It would appear that

this higher temperature when finishing amalgam leads to the

production of the Beilby layer.12 In addition the viscous

nature of both walnut oil and petroleum jelly makes them

different lubricants

Composite

PJ Wet WO PJ

0.01 <0.01 <0.01 <0.01

0.01 – >0.05 >0.05

0.05 – – >0.05

0.05 <0.01 <0.01 <0.01

0.05 – <0.01 <0.01

0.05 – – >0.05

0.01 <0.01 >0.05 <0.01

0.05 – <0.01 <0.05

0.05 – – <0.01

0.01 <0.01 <0.01 <0.01

0.01 – <0.01 <0.01

0.01 – – >0.05

j o u r n a l o f d e n t i s t r y 3 5 ( 2 0 0 7 ) 3 6 – 4 2 41

Fig. 5 – Photomicrographs of samples of amalgam using finished dry, with water, walnut oil and petroleum jelly using the

coarse disc (field width 700 mm).

likely to bind abraded particles together blocking the

sluiceways on the surface of the disc making it less effective

in smoothing the surface. It is possible, that with lubrication,

a longer finishing time would produce the level of smooth-

ness achieved without lubrication.

With composite resin (thermal diffusivity 0.4 mm2/s) the

high temperatures reached and the long recovery period for

Fig. 6 – Photomicrographs of samples of composite finished dry

disc (field width 90 mm).

the temperature to return to ambient has clinical significance.

This increase in temperature, during the finishing process, will

be transmitted to the pulp/dentine interface and will take

longer to dissipate. When discs were used without lubrication

and when using both walnut oil and petroleum jelly, the

temperature increased with the fineness of the disc. This is

because the temperature, in the time the disc is changed to a

, with water, walnut oil and petroleum jelly using the fine

j o u r n a l o f d e n t i s t r y 3 5 ( 2 0 0 7 ) 3 6 – 4 242

finer one, does not return to the starting temperature,

enhancing the temperature rise. This may be described as a

staircase effect. Unlike amalgam the low thermal diffusivity of

composite does not allow the specimen to return to the resting

temperature before further heating commences.

As with amalgam, there would seem to be no advantage in

using petroleum jelly to reduce the temperature rise when

finishing composite.

The photomicrographs of the composite show that the

surface of the samples is smoother when finished unlubri-

cated than by any other method. The roughest surface is

produced on the samples finished with water. With all the

methods irregular shaped voids appear and it would seem that

particles have been ‘‘pulled out’’ of the surface. However,

finished dry seems to produce fewer voids. When finished

with walnut oil and petroleum jelly the surface has a more

particulate appearance. This is probably the result of the

matrix being ground away preferentially and the filler

particles becoming exposed leading to the surface produced

having a greater roughness value. It would seem that the

higher temperature at the surface when the sample is finished

dry may cause localised softening and melting. This may lead

to smearing of the resin over any exposed particles and so this

particulate appearance is not so noticeable and the surface is

smoother. It is also possible that the increased temperature

may have some effect on the resin properties leading

potentially to further cross-linking of the resin phase. This

would theoretically result in a harder surface that might be

more resistant to wear.

10. Clinical significance

It would seem the ideal technique that would produce the

smoothest finish while not raising the temperature to a level

where pulpal damage would occur is the use of intermittent

finishing with the disc run dry as advocated by Stewart et al.5

Future work in this area, that has been sadly neglected, is

required to evaluate the optimal values of load, speed and time

when using lubrication and investigate possible effects of

amount of lubricant used. The use of alternatives to sanding

discs should be pursued: relating the surface properties such

as hardness for lubricated and unlubricated surfaces.

11. Conclusions

1. The lowest values for surface roughness for both amalgam

and composite were when finished dry.

2. R

educed temperatures resulting from lubrication do not

outweigh the adverse effects on roughness.

r e f e r e n c e s

1. Jones CS, Billington R, Pearson GJ. The in vivo perception ofroughness. British Dental Journal 2004;196:42–5.

2. Zach L, Cohen G. Pulp response to externally applied heat.Oral Surgery 1965;19:515–30.

3. Christensen G, Dilts WE. Thermal changes during dentalpolishing. Journal of Dental Research 1968;47(5):690–3.

4. Alpin AW, Cantwell KR, Sorenson FM. Effect of overheatingon amalgam hardness. Journal of Dental Research1967;46(6):1420–4.

5. Stewart GP, Bachman TA, Hatton JF. Temperature rise due tofinishing of direct restorative materials. American Journal ofDentistry 1991;4(1):23–8.

6. Jones CS, Billington R, Pearson GJ. Interoperator variabilityduring polishing. Quintessence International 2006;37(3):183–90.

7. Jones CS, Billington R, Pearson GJ. Laboratory study of theloads, speeds and times to finish and polish directrestorative materials. Journal of Oral Rehabilitation2005;32:686–92.

8. Jones CS, Pearson GJ. The force speed and time to polishcomposite. Journal of Dental Research 2000;79. Abstract 3636,p. 598.

9. Pearson GJ, Knibbs PJ. Finishing an anhydrous glass-ionomer cement (an in vitro and in vivo study). RestorativeDentistry 1987;3:35–9.

10. Eames WB, Minnock JB, Wasden L. Thermal response ofamalgam to polishing instruments. JADA 1966;73:1111–3.

11. Anusavice Kenneth J. Phillip’s science of dental materials.10th ed. Philadelphia: W.B. Saunders Co.; 1996.

12. Beilby G. Aggregation of flow and solids. London: Macmillanand Co.; 1921.