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

The effects of lubrication on the temperature rise and surfacefinish of glass-ionomer cements

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

Biomaterials in Relation to Dentistry, Queen Mary, University of London, Mile End Road, London E1 4NS, UK

a r t i c l e i n f o

Article history:

Received 9 June 2004

Accepted 12 August 2004

Keywords:

Glass-ionomer cement

Finishing

Surface roughness

Lubrication

Temperature rise

a b s t r a c t

Object: Previous work [Jones CS. Factors influencing the finishing of direct filling materials.

PhD Thesis, University of London; 2002] has shown that there is an optimum load, speed and

time that produced the smoothest surface when finishing glass-ionomer cement using each

of four grades of a disc system. This study looks at the effects of lubrication on the

temperature produced in samples of GIC when finished dry and with different lubricants

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

using different lubricants.

Materials and methods: A thermocouple connected so that it permitted the display and

recording of temperature against time was inserted 1mm into the base of samples of a glass-

ionomer cement. The samples were finished and polished using each of the grades of a disc

system in a specially constructed jig that mimicked oral finishing. After roughening, 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 using a profilometer. 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: Walnut oil and petroleum jelly produced significant temperature increases com-

pared to both dry and with water finishing. Lubricated with water significantly reduced the

temperature rise compared to dry. The Ra values of 0.5 mm was obtained for the coarse and a

value of 0.3 mm for the medium discs run without lubrication. With lubrication the Ra

increased although there was little difference between the lubricants. However the photo-

micrographs showed that walnut oil and petroleum jelly caused gross morphological

changes indicating major surface destruction.

Conclusions: The practice of finishing GICs using petroleum jelly or similar lubricant

appears to be detrimental. Further experimental work needs to be done to advise practi-

tioners on finishing GICs to produce the smoothest surface possible.

# 2006 Elsevier Ltd. All rights reserved.

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1. Introduction

Finishing dental fillings is necessary to produce the required

anatomical form, tooth aesthetics and patient comfort.

* 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 reservedoi:10.1016/j.jdent.2004.08.012

Finishing can also eliminate food traps and may enhance

the materials properties.

It has been determined that patients are able to distinguish

differences in roughness values of between 0.25 and 0.5 mm

d.

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Table 1 – Showing the colour coding, particle size andparticle distribution of the disc system

Grade of disc Coarse(black)

Medium(violet)

Fine(green)

Super-fine(red)

Particle size

(mm)

>100 40 30 5–10

Particle

distribution

(No/200 mm2)

6 16 72 750

with their tongues therefore the finish of a restoration should

be as smooth as enamel.2 The final surface of a restoration is

therefore important and may be influenced by factors such as

lubrication.

The Oxford English Dictionary’s3 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

lubricants are water and crude oils. Finishing and polishing

are frictional processes and as such will produce heat. Excess

heat will damage the pulp4 and therefore the need for

lubrication may be important. It could also enhance or

prejudice the finishing 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 Christensen and Dilts5 looking at

thermal changes during polishing, Alpen et al.6 looking at the

effect of overheating on amalgam hardness and Stewart et al.7

looking at the temperature rise in direct filling materials. None

examined the correlation of temperature rise surface finish

and lubrication.

A previous studies by one of the authors1 examining the

range of loads, speeds and times used by practitioners’ when

finishing glass-ionomer cement (GIC) showed they used loads

between 17 and 92 g, speeds between 8000 and 25,000 rpm and

times between 5 and 50 s. It would seem that they need

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

get the best surface finish for glass-ionomer cements.

The mean figures from this practitioner study were used to

conduct a laboratory experiment1 to determine the optimum

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

These results show there is no improvement in the surface

finish following the use of the medium disc. Finer discs render

the surface rougher than before using the disc. However, the

load of 20 g at 15,000 rpm for 20 s was the optimum for the

green, fine, disc and was used in subsequent experiments, as

were these values for the red, super-fine disc.

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.

This current work examined the temperature rise in

samples of GIC when finished dry, and when finished with

different lubricants. The surface achieved with the different

finishing methods was also recorded. These experiments were

all done using the optimum values obtained from the

laboratory study and using the same GIC and disc system.

Table 2 – Optimum results from laboratory study forfinishing glass-ionomer cement

Disc Coarse(black)

Medium(violet)

Fine(green)

Super-fine(red)

Load (g) 40 30 20 20

Speed (rpm) 15000 15000 15000 15000

Time (s) 20 20 20 20

2. Materials

A conventional glass-ionomer cement (Fuji IX, GC Corp.) was

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 particle size and particle distribution. The

lubricants were water, walnut oil and petroleum jelly.

3. Method

Samples of the material were mixed according to the

manufacturers instructions and packed into a specially

constructed brass mould. Producing samples 25 mm long by

6 mm wide by 2 mm deep. The samples were stored in

deionised water in an oven at 37 8C for at least 24 h. They were

pre-roughened to a Ra value of between 2.5 and 3.5 mm. This

was determined using a two-dimensional profilometer (Mitu-

toya Surftest, Japan). Six runs were recorded; the readings

were taken 0.5 mm apart in the axis of the sample. The

traversing length was 4.8 mm with the cut off point of 0.8 mm.

This roughness 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 a white stone.8

All the experiments were then conducted at the loads,

speeds and times from the laboratory experiments and these

values are shown in Table 2.

3.1. Method for temperature rise

The samples were constructed with a thermocouple that was

inserted 1 mm into their base.

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

This comprised a flat bed that could be moved in the

horizontal plane. A brass plate with the attached, pre-

roughened sample was screwed to the flat bed. The sample

with 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.

This permitted temperature variations to be recorded at

second intervals. 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

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

glass-ionomer cement finished using different lubricants

(error bars show standard deviation).

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, firstly run dry

without lubrication then, in turn, lubricated with water,

walnut oil and petroleum jelly.

3.2. Method for surface finish

Five samples were tested for each method and the roughness

recorded as previously described. Any lubricant was removed

by wiping the sample with a tissue.

After each disc had been used one of the five samples was

selected at random and prepared for examination in the

scanning electron microscope.

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

glass-ionomer cement run dry.

The results both for the temperature rise and surface finish

were analysed statistically using non-parametric Mann–

Whitney U-test.

4. Results

4.1. Results for temperature rise

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

the samples un-lubricated. The drop in temperature from the

maximum is when the disc is changed. Notice the long time for

the material to return to its starting temperature.

The maximum temperature for each disc was measured

and the means and standard deviations calculated. Fig. 3

shows the maximum temperature rise for all the polishing

methods and for all the discs. The maximum rise was with

petroleum jelly, followed by walnut oil, then dry and with

water the rise was minimal.

Fig. 4 – Bar chart of the Ra values of samples of glass-

ionomer cement finished using different lubricants (error

bars show standard deviation).

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Fig. 5 – Photomicrographs of samples of glass-ionomer cement finished using the coarse, medium and fine discs run dry.

Field width 75 mm. Note the surface after the medium disc is not as rough as after using the other two discs.

Fig. 6 – Photomicrographs of samples of glass-ionomer cement finished dry, with water, walnut oil and petroleum jelly. Field

width 75 mm. Note the much rougher surfaces with all the finishing methods apart from dry finishing.

To confirm these findings were not brand specific the

experiment was repeated with another disc system (3MESPE

SofLex discs). The results were similar.

The statistical analysis showed that apart from walnut oil

and petroleum jelly using the two coarser discs all were

significantly different.

4.2. Results for surface finish

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 showed that with

the exception of wet and petroleum jelly, walnut oil and

petroleum jelly all were highly significant different.

4.3. SEM views of surfaces

Fig. 5 shows the photomicrographs of the surfaces finished

dry. Of the coarse, medium and fine the smoothest surface is

obtained with the medium disc. Both the coarse and fine discs

show a rougher surface.

Fig. 6 shows the smoothest surface (the sample finished dry

using the medium disc) for comparison and samples finished

wet, with walnut oil and with petroleum jelly. The photo-

micrograph of the sample finished dry shows a smoother

surface.

5. Discussion

As practitioners are aware it is difficult when finishing glass-

ionomer cement to obtain a smooth polished surface. This is in

part due to the incorporation of air during the mixing of the

powder and liquid and also with capsulation where the

mechanical vibration will also include air in the mixture. The

work of Jones et al.9 shows that the viscosity of GIC has an

effect on the number of inclusions in capsulated glass-

ionomer cements.

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It would be expected that with a thermal diffusivity of just

under 0.2 mm2/s the time for the temperature to return to

ambient would be longer than amalgam and this was the case.

A pilot study for the temperature rise amalgam and

composite showed that glass-ionomer cement behaved in a

different way. Firstly when run dry the temperature of

amalgam decreased the finer the disc, composite tended to

increase while GIC had a similar rise for all the discs. Secondly

with amalgam and composite the ranking order for tempera-

ture rise using the lubricants was run dry, petroleum jelly,

walnut oil and the least with water. Glass-ionomer cement’s

ranking order was petroleum jelly, walnut oil, run dry and the

least rise with water.

The same pilot study showed the results for surface

roughness values in the specimens again differed from

glass-ionomer cement in the ranking order. With amalgam

and composite the ranking was run dry, walnut oil, petroleum

jelly and the roughest with water. With glass-ionomer cement

the ranking is run dry, wet, walnut oil and petroleum jelly the

roughest. In view of the make up of glass-ionomer cements it

was unexpected that samples finished dry would produce the

smoothest surface, and that finished wet would lead to a

smoother surface than using the other lubricants. It was

expected that running the discs dry or with water would

damage the surface of glass-ionomer cements to a greater

extent than with oil or petroleum jelly. Fig. 4 showing surface

roughness shows the samples finished dry with the coarse and

medium discs have a surface that is less than half the surface

roughness value of the other finishing methods. The fine disc

used dry although not as good as with the two coarser discs it

is still better than the other finishing methods. Unlike dry

finishing the other finishing methods all improve slightly the

finer the disc.

It might also be that the coarser discs start to desiccate the

surface and the finer discs break the surface down.

One explanation for the improvement in the roughness

value with the two coarser discs when run dry could be that

set glass-ionomer cement is made up of large glass particles

(Fuji IX has particles between <10 and 0.5 mm) and that there

is a considerable amount of matrix between them. This is in

contrast to present day composites (Z100 particles were

measured between 2.2 and 0.18 mm) where the amount of

matrix between the particles is small. With glass-ionomer

cement when using the coarser discs, made up of large sized

abrasive particles, less of the potentially softer matrix is

abraded. However with the finer discs the abrasive particles

are more able to remove the matrix preferentially. Polishing

of the glass particles is possible as the abrasive used on the

disc (Al2O3 MP 2050 8C) have a higher melting point than the

glass (MP 11–1300 8C).10 Fig. 5 shows what appear to be

smoothed glass particles after using the medium disc run dry

and there appears to be some roughening of the surface after

using the fine disc. The preferential removal of the matrix

could possibly be the case after using the fine disc as in the

photomicrograph the glass particle appears to be standing

proud.

It would be expected that the samples abraded while being

lubricated with walnut oil and petroleum jelly would have

temperature rises and surface roughness values between un-

lubricated and water lubricated samples. This was not the case

and it could be that walnut oil and petroleum jelly lubricate the

surface but do not allow the heat produced to dissipate. This

results in a higher temperature, water loss and surface

degradation. The mechanism is still unclear area therefore

further work on the role of lubricants is essential to clarify this,

particularly since these findings are contrary to the manu-

facturers’ recommendations where available.

In addition it is possible that the abraded particles of the

cement are trapped by the viscous oil and jelly. They may then

act in some way together with the lubricant to form a further

abrasive slurry. These abraded particles could be cleared from

the disc more easily by the water and when run dry. However

this did not occur with composite and amalgam in the pilot

study.

As the matrix is susceptible to water, Wilson et al.,11 it is

possible that by using water lubrication the matrix may be

softened and then erroded. This could be verified by examin-

ing the weight loss of water and dry finished specimens.

It seems likely that the ‘‘finishing’’ of glass-ionomer

cements is a process that involves the polishing of the glass

particles and the abrasion of the matrix or the depleted glass

layer. There appears to be a critical balance between this and

the risk of water loss and desiccation. The photomicrographs

(Figs. 5 and 6) show some evidence that support what may be

happening. When the samples are finished dry there is an

improvement as the abrasive size is reduced from the coarse

to the medium disc. The glass particles appear to be ground or

polished smoother and the whole surface is not breaking up.

When used wet the surface was definitely rougher and it

appeared as if some glass particles had been removed. The

surface damage was very much more apparent after using the

walnut oil and petroleum jelly.

6. Conclusions

1. Dentists use too great a load when finishing GICs.

2. T

he best values for surface roughness were when finished

dry.

3. O

nly the two coarser discs improved the surface.

4. F

inishing with water cools the material whereas walnut oil

and petroleum jelly result in more heat than finishing dry.

5. P

etroleum jelly and oil were detrimental to the cement.

6. M

ore advice is needed from the manufacturers to the

practitioners as to the best way to finish GICs.

r e f e r e n c e s

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2. Oxford English Dictionary. Oxford at the Clarendon press;1961 reprint.

3. Jones CS, Billington R, Pearson GJ. The In Vivo perception ofroughness. British Dental Journal 2004;196:42–5.

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

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

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

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

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

9. Jones CS, Pearson GJ, Billington RW, Manterfield J. Effects ofviscosity in capsulated glass-ionomer cements. Journal ofDental Research 1997;76(5):432.

10. Bowden FP, Tabor D. Effect of frictional heating on surfaceflow. In: Bowden FP, Tabor D, editors. The friction and

lubrication of solids. Oxford: Clarendon Press; 1950.11. Wilson AD, Crisp S, Padden JM. The hydration of a glass-

ionomer (ASPA) cement. British Polymer Journal 1981;13:66–70.