d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 500–505

avai lab le at www.sc iencedi rec t .com

journa l homepage: www. int l .e lsev ierhea l th .com/ journa ls /dema

Surface roughness of impression materials and dentalstones scanned by non-contacting laser profilometry

Jose M. Rodriguez, Richard V. Curtis, David W. Bartlett ∗

Restorative Group, Kings College London Dental Institute, Guy’s Hospital, London, UK

a r t i c l e i n f o

Article history:

Received 26 February 2008

Accepted 14 October 2008

Keywords:

Surface roughness

Non-contacting laser profilometry

Impression materials

Dental stones

a b s t r a c t

Objective. To analyze differences in the way dental materials digitize on a non-contacting

laser profilometer (NCLP).

Methods. Three Type IV dental stones and 15 impression materials were mixed according

to the manufacturer’s instructions and expressed against a glass block to record its surface

characteristics. From each material an area of 6 × 40 mm was scanned on the NCLP and the

Ra, Rq and Rt roughness values measured from 20 randomly selected transverse profiles.

The surface of the impression materials was subsequently poured in MoonstoneTM (Bra-

con Ltd., Etchingham, England) dental stone and the same roughness values obtained from

these casts. Differences in roughness values from the dental materials were compared using

ANOVA and differences in roughness between impression materials and the MoonstoneTM

casts compared using paired t-tests.

Results. There were significant differences in roughness values between individual materials

within each type (impression material or dental stone) (p < 0.05). The roughness of the dental

stones varied between Ra = 0.87 and 0.99 �m, Rq = 1.09 and 1.23 �m, and Rt = 5.70 and 6.51 �m.

The roughness values of the impression materials varied between Ra = 0.75 and 4.56 �m;

Rq = 0.95 and 6.27 �m and Rt = 4.70 and 39.31 �m. Darker materials showed higher rough-

ness values compared to lighter materials (p < 0.05). The roughness of the MoonstoneTM

casts varied between Ra = 0.80 and 0.98 �m; Rq = 1.01 and 1.22 �m, and Rt = 5.04 and 6.38 �m.

Roughness values of some impression materials were statistically significantly lower when

the surface was reproduced in MoonstoneTM (p < 0.01).

Significance. Digitization of dental materials on optical profilometers was affected by color

and transparency.

emy

out the use of electroconductive coating [11]. However, in

© 2008 Acad

1. Introduction

Surface mapping systems have been used in dental researchto measure tooth wear in vitro [1,2], in situ [3,4], in vivo [5,6],

and to measure wear of restorations [7]. Contacting or opticalprofilometers record, from the surface of teeth or materi-als, sequential profiles which are combined to form a digital

∗ Corresponding author at: Guy’s Hospital, Tower Wing, Floor 25, GreatE-mail address: david.bartlett@kcl.ac.uk (D.W. Bartlett).

0109-5641/$ – see front matter © 2008 Academy of Dental Materials. Pudoi:10.1016/j.dental.2008.10.003

of Dental Materials. Published by Elsevier Ltd. All rights reserved.

‘cloud’ of points representing the surface topography. Pro-filometers cannot directly scan teeth in vivo so scans aremade of casts obtained from impressions [5,8], casting theimpressions in gypsum [9], in epoxy resin [6,10] with or with-

Maze Pond, London SE1 9RT, UK. Tel.: +44 207 188 5390.

vitro, profilometers are able to scan extracted teeth directlythus eliminating possible inaccuracies introduced through thecasting procedure [2,3,12,13].

blished by Elsevier Ltd. All rights reserved.

d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 500–505 501

Table 1 – Type IV dental stones used in the study.

Dental stone Manufacturer Color Lot/Exp

TM

Japan

cbltitmapdleolcit

cd[edat

2

2

Tfcamit1mttem

acSsuTs

FujiRock GC Europe, Leuven, Belgium.MoonstoneTM Bracon Ltd., Etchingham, England.SuperRockTM Noritake Gypsum Co. Ltd., Nagoya,

To date there is no consensus on which type of profilometerreates the optimum scan for measurement of tooth wear asoth contacting and non-contacting have been reported in the

iterature [5–7,12,14]. Contacting profilometers cannot pene-rate features smaller than the diameter of the probe whichs usually around 100–500 �m. They also record the surfaceopography by applying a preset force onto the sample which

ay distort flexible materials such as impression materials orbrade hard surfaces such as dental stones. The use of opticalrofilometers overcomes some of these disadvantages as theyo not touch the surface of the sample and the diameter of the

ight spot is usually below 100 �m. Optical systems may useither a triangulation laser sensor which detects the deflectionf a laser spot on a CCD camera or they may use a white light or

aser sensor to record the surface topography using the confo-al principle [15]. A recognized disadvantage of these systemss that the measurement is affected by microgeometry, reflec-ivity and inclination of the surface of the sample [16].

Laser profilometers emit light at a specific wavelength thatould be absorbed or reflected by the sample in dissimilar waysepending upon the color and transparency of the material

17,18]. The aim of this study was to investigate the influ-nce of color and transparency of impression materials andental stones on the accuracy of surface reproduction usingn optical profilometer. The null hypothesis is that color andransparency of materials do not affect digitized optical scans.

. Materials and methods

.1. Reproduction of surface detail by the NCLP

hree Type IV dental stones (Table 1) were vacuum-mixedollowing the manufacturer’s instructions and poured onto alean and smooth glass block to reproduce the surface char-cteristics using a protocol described by DeLong et al. (2001)odified for this study. The stones were prevented from mix-

ng by a specially made barrier and the slab that contained allhree dental stones was called “dental stone slab”. In addition5 different impression materials (Table 2), selected from fiveanufacturers, were expressed in the same manner against

he glass block to record the surface characteristics. In totalhree impression slabs, approximately 130 mm by 90 mm andach containing five materials were made from the impressionaterials.The surface of individual materials (impression materials

nd dental stones) were scanned on a Xyris 2000TL non-ontacting laser profilometer (NCLP-Taicaan® Technologies-outhampton, UK) over an area of 6 mm × 40 mm with a

tep-over distance of 50 �m. Samples moved along the X-axisnderneath the sensor at a maximum table speed of 1.5 mm/s.he NCLP had a 785 nm wavelength laser triangulation sen-or with a spot diameter of 30 �m and an axis and sensor

Pastel yellow 0606271/Jun 2009Peach 06040580/n-a

. Brown 60476/Apr 2009

resolution of 0.1 �m. From the scan of each material the Ra,Rq and Rt surface roughness values were obtained from 20random transverse profiles after applying a 0.8 mm cut offGaussian filter using surface metrology software BoddiesTM

v1.81 (Taicaan® Technologies-Southampton, UK). The rough-ness results from the scans of the three dental stones castagainst the glass block showed that MoonstoneTM (BraconLtd., Etchingham, England) had the lowest roughness values(Table 3) so this dental stone was used to cast the impressionslabs. The three impression material slabs were then pouredin MoonstoneTM (Bracon Ltd., Etchingham, England) and theindividual casts from each impression material were scannedon the NCLP using the same settings described above and thesame roughness values obtained from 20 random transverseprofiles.

2.2. The accuracy of the NCLP

The accuracy of the NCLP in measuring Ra was tested byscanning an impression made of a 6 �m Ra roughness stan-dard (Taylor Hobson-Reference Specimen Type 112/1534). Theimpression was taken in ExtrudeTM light-bodied (Kerr Corpo-ration, Romulus, MI, USA) and scanned on the NCLP using thesame settings described above. The Ra roughness value wasobtained from 20 random transverse profiles after applyingthe same filter and cut off.

Outcome data for roughness values of the impres-sion materials and MoonstoneTM (Bracon Ltd., Etchingham,England) casts were not normally distributed so data isdescribed using medians and inter-quartile ranges. The dif-ferences in roughness values between impression materialsor dental stones were assessed using analysis of variance(ANOVA). Roughness values of the impression materials anddental stones required logarithmic transformations priorto analysis. Where statistical significance was indicated,differences between individual groups were tested using post-ANOVA Scheffe tests. Statistical significance was inferredwhere p < 0.05. The difference in roughness values from theimpression materials and their MoonstoneTM (Bracon Ltd.,Etchingham, England) casts were tested using paired t-tests,and to compensate for multiple testing, p < 0.01 was regardedas indicating statistical significance.

3. Results

3.1. Reproduction of surface detail by the NCLP

The mean surface roughness values and standard deviations

of the three type IV dental stones are shown in Table 3;these varied between Ra = 0.87 and 0.99 �m, Rq = 1.09 and1.23 �m, and Rt = 5.70 and 6.51 �m. There were variation inthe roughness values (Ra, Rq, and Rt) for all dental stones with

502 d e n t a l m a t e r i a l s 2 5 ( 2 0 0 9 ) 500–505

Table 2 – Impression materials used in the study.

Impression material Manufacturer Consistency/color Lot/Exp

AffinisTM heavy-bodied Coltene-Whaledent. Alstätten, Switzerland. Heavy/brown-red 0096188/Jul 2008AffinisTM light-bodied Coltene-Whaledent, Alstätten, Switzerland. Light/green 0090562/May 2008AquasilTM ultra monophase DECA Dentsply Caulk, Milford, DE, USA. Medium/purple 060406/Apr 2008AquasilTM LV light-bodied LV Dentsply Caulk, Milford, DE, USA. Light/blue 060303/Mar 2009AquasilTM putty Dentsply DeTrey, Konstanz, Germany. Putty/blue 0607000513/Dec 2008DoricTM monophase Davis Schottlander & Davis Ltd., Letchworth, England. Medium/purple 570894/Aug 2008ExpressTM light-bodied 3M ESPE, St Paul, MN, USA. Light/blue 5HLF2H3/Aug 2008ExpressTM putty 3M ESPE, St Paul, MN, USA. Putty/clay 6JM21/Oct 2008ExtrudeTM light-bodied Kerr Corporation, Romulus, MI, USA. Light/blue 61137/May 2009ExtrudeTM putty Kerr Corporation, Romulus, MI, USA. Putty/purple 549546/May 2008ImpregumTM 3M ESPE, St Paul, MN, USA. Medium/purple 262218/Feb 2009PresidentTM JET light-bodied Coltene-Whaledent. Alstätten, Switzerland. Light/green 0095975/Jul 2008

Swit, USA, USA

PresidentTM putty Coltene-Whaledent. Alstätten,Take1TM heavy-bodied Kerr Corporation, Romulus, MITake1TM light-bodied Kerr Corporation, Romulus, MI

SuperRockTM (Noritake Gypsum Co. Ltd., Nagoya, Japan) show-ing statistically significant higher roughness values comparedto FujiRockTM (GC Europe, Leuven, Belgium) and MoonstoneTM

(Bracon Ltd., Etchingham, England) (p < 0.05). SuperRockTM

(Noritake Gypsum Co. Ltd., Nagoya, Japan) was also the darkestdental stone tested. FujiRockTM (GC Europe, Leuven, Belgium)and MoonstoneTM (Bracon Ltd., Etchingham, England) showedsimilar roughness values between themselves (p < 0.05).

Table 4 shows the median and inter-quartile ranges ofthe roughness values (Ra, Rq, and Rt) for the 15 impressionmaterials and their MoonstoneTM (Bracon Ltd., Etchingham,England) casts scanned on the non-contacting laser profilome-ter (NCLP). Fourteen impression materials were addition curedsilicones and one a polyether (ImpregumTM [3M ESPE, St Paul,MN, USA]) and varied in color from dark purple to light blue(Table 2). Four impressions were putty consistency, two heavy-bodied, three medium bodied and six light-bodied. Therewas variation in the roughness values (Ra, Rq, and Rt) forall impression materials with some showing statistically sig-nificant higher roughness values than others (Table 4). Thedata for roughness values (Ra, Rq, and Rt) from each dif-ferent impression material had a consistent result. Overalltheir median varied between Ra = 0.75 and 4.56 �m; Rq = 0.95and 6.27 �m, and Rt = 4.70 and 39.31 �m. The median for theMoonstoneTM (Bracon Ltd., Etchingham, England) casts variedbetween Ra = 0.80 and 0.98 �m; Rq = 1.01 and 1.22, and Rt = 5.04and 6.38 �m.

There were statistically significant differences (p < 0.05)

between Ra, Rq and Rt; and the consistency of the impressionmaterials with the highest values shown by hard bodied fol-lowed by medium bodied followed by putty consistency. Therewas no statistically significant difference between putty and

Table 3 – Mean and (standard deviation) of surfaceroughness values (Ra, Rq, and Rt) of Type IV dentalstones.

Dental stone Ra (�m) Rq (�m) Rt (�m)

FujiRockTM 0.94 (0.08) 1.18 (0.09) 6.27 (0.77)MoonstoneTM 0.87 (0.06) 1.09 (0.07) 5.70 (0.72)SuperRockTM 0.99 (0.10) 1.24 (0.13) 6.51 (1.01)

zerland. Putty/mustard 0109448/Jan 2009. Heavy/blue 61230/Aug 2009. Light/orange 61094/Apr 2009

light-bodied materials (p = 0.93 for Ra; p = 0.87 for Rq, p = 1.00for Rt).

There were statistically significant differences in rough-ness values (Ra, Rq and Rt) for the color of the impressionmaterials. The darker materials (AquasilTM Ultra MonophaseDECA [Dentsply Caulk, Milford, DE, USA] and DoricTM

Monophase [Davis Schottlander & Davis Ltd., Letchworth, Eng-land]) showed higher roughness values (Ra, Rq, and Rt) valuesthan the lighter colored materials (ExpressTM [3M ESPE, StPaul, MN, USA] putty and light-bodied; PresidentTM putty[Coltene-Whaledent. Alstätten, Switzerland] and ExtrudeTM

light-bodied [Kerr Corporation, Romulus, MI, USA]) (p < 0.05).There were no statistically significant differences between thelightest colored impression materials.

No statistically significant differences were observed in theroughness values (Ra, Rq, and Rt) measured from the impres-sion materials when compared to their MoonstoneTM casts forExpressTM putty [3M ESPE, St Paul, MN, USA], ExtrudeTM light-bodied [Kerr Corporation, Romulus, MI, USA] and PresidentTM

putty [Coltene-Whaledent. Alstätten, Switzerland] (p > 0.01).However, the roughness values obtained from ExpressTM

light-bodied (3M ESPE, St Paul, MN, USA) were statistically sig-nificantly lower compared to its MoonstoneTM (Bracon Ltd.,Etchingham, England) cast (p < 0.01). All other impressionmaterials showed statistically significant higher roughnessvalues compared to their MoonstoneTM (Bracon Ltd., Etching-ham, England) casts (p < 0.05). The roughness values [Ra, Rq,and Rt] obtained from all of the MoonstoneTM (Bracon Ltd.,Etchingham, England) casts of impression materials were sim-ilar to those measured from the MoonstoneTM (Bracon Ltd.,Etchingham, England) from the dental stone slab.

3.2. The accuracy of the NCLP

The mean and (standard deviation) of the Ra roughness valueobtained from the impression made of the 6 �m roughnessstandard was 5.99 �m (0.29).

4. Discussion

The color and opacity of the impression and stones affectedroughness values scanned on the NCLP and this seemed to be

d e n t a l m a t e r i a l s 2 5

Tabl

e4

–M

edia

nan

d(i

nte

rqu

arti

lera

nge

s)of

Ra,

Rq

and

Rt

surf

ace

rou

ghn

ess

valu

esof

imp

ress

ion

mat

eria

lssc

ann

edon

the

non

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tact

ing

lase

rp

rofi

lom

eter

(Tai

caan

TM

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tham

pto

n)a

nd

thei

rM

oon

ston

eTM

cast

s.

Imp

ress

ion

mat

eria

lIm

pre

ssio

nm

ater

ialR

a(�

m)

Moo

nst

oneT

M

cast

Ra

(�m

)Im

pre

ssio

nm

ater

ialR

q(�

m)

Moo

nst

oneT

M

cast

Rq

(�m

)

Imp

ress

ion

mat

eria

lR

t(�

m)

Moo

nst

oneT

M

cast

Rt

(�m

)

Affi

nis

TM

hea

vy-b

odie

d1.

74(1

.66–

1.82

)0.

80(0

.77–

0.88

)2.

23(2

.14–

2.41

)1.

04(0

.95–

1.08

)11

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(10.

72–1

4.34

)5.

04(4

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5.52

)A

ffin

isT

Mli

ght-

bod

ied

1.16

(1.0

4–1.

29)

0.84

(0.7

9–0.

90)

1.46

(1.3

4–1.

66)

1.07

(1.0

2–1.

13)

7.41

(6.6

8–9.

36)

5.52

(5.0

5–6.

01)

Aq

uas

ilT

MLV

ligh

t-bo

die

d1.

86(1

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1.96

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36(2

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2.49

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08(0

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25(4

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5.75

)A

qu

asil

TM

pu

tty

1.04

(0.9

3–1.

11)

0.92

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98)

1.35

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49)

1.13

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20)

7.18

(6.7

2–9.

23)

5.57

(5.2

7–6.

03)

Aq

uas

ilT

MD

ECA

4.56

(4.3

1–5.

09)

0.93

(0.8

4–0.

99)

6.27

(5.9

0–6.

80)

1.15

(1.0

5–1.

22)

39.3

1(3

3.95

–46.

36)

5.74

(5.2

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31)

Dor

icT

Mm

onop

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00(1

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70(2

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)1.

06(0

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)16

.10

(14.

47–2

2.36

)5.

56(4

.95–

5.97

)Ex

pre

ssT

Mp

utt

y0.

83(0

.74–

0.88

)0.

89(0

.80–

0.95

)1.

05(0

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12(1

.02–

1.19

)5.

30(4

.90–

5.61

)5.

63(5

.13–

6.17

)Ex

pre

ssT

Mli

ght-

bod

ied

0.78

(0.7

5–0.

81)

0.86

(0.8

3–0.

90)

0.97

(0.9

3–1.

01)

1.08

(1.0

5–1.

14)

4.82

(4.3

8–5.

23)

5.36

(4.9

9–5.

84)

Extr

ud

eTM

ligh

t-bo

die

d0.

75(0

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0.80

)0.

81(0

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0.85

)0.

95(0

.90–

1.00

)1.

01(0

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1.06

)4.

71(4

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)5.

13(4

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23(7

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9.20

)6.

38(5

.17–

6.86

)Im

pre

gum

TM

1.23

(1.1

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30)

0.91

(0.8

6–0.

99)

1.60

(1.4

8–1.

64)

1.14

(1.0

9–1.

21)

8.38

(7.5

0–9.

17)

5.90

(5.1

1–6.

35)

Pres

iden

tTM

JET

ligh

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die

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26(1

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15(1

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esid

entT

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utt

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0.99

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96(0

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16(5

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24(5

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ke1T

Mli

ght-

bod

ied

1.66

(1.5

4–1.

92)

0.89

(0.8

5–0.

92)

2.14

(2.0

5–2.

51)

1.11

(1.0

5–1.

17)

12.5

8(1

0.30

–14.

06)

5.57

(5.2

3–6.

26)

Take

1TM

hea

vy-b

odie

d1.

72(1

.64–

1.88

)0.

88(0

.82–

0.92

)2.

29(2

.08–

2.51

)1.

07(1

.01–

1.14

)13

.93

(11.

63–1

7.05

)5.

65(5

.13–

6.01

)

( 2 0 0 9 ) 500–505 503

an interaction with the laser sensor. DeLong et al. (2001) inves-tigated different impression materials scanned on an opticalwhite light scanner and observed no relationship between thedigitizing properties and the color of impression materials.Ours is the first study to describe the effect of the color andtransparency of a material on laser scanners.

The methodology used in this study to test dental mate-rials has been previously described by DeLong et al. (2001)and was adapted for this study. The glass slab was chosenas its surface was highly polished and showed no porosi-ties under an optical microscope. The transparency and highreflectivity of the glass slab prevented scanning by the NCLP.This would have been ideal as it would have allowed directcomparison of the roughness values taken from the glassslab to those of the impression materials. The choice ofimpression materials tested on this study reflected a rangeof manufacturers’ products commonly used in restorativedentistry and currently available on the market. They wereselected to represent a varied range of colors. The dentalstones were tested to evaluate the influence of the opac-ity on digitization by the non-contacting laser profilometer(NCLP).

Roughness values (Ra being the mean displacement of theprofile, Rq the root mean square of all the values in the profileand Rt the maximum peak to valley distance in the profile) pro-vide a method to quantify surface detail and were affected bylight absorption and reflection from the surface of the mate-rial. The roughness values obtained from the impression ofthe Ra roughness standard showed that the NCLP was accu-rate in calculating Ra. The 6 �m Ra roughness standard is avertical reference tool. The NCLP measured a mean 5.99 �m(S.D. 0.29) showing a mean difference of 0.01 �m indicatingexcellent accuracy. The NCLP was capable of detecting verticalfeatures less than the laser spot diameter because the verticalresolution of the sensor was 0.1 �m. The 30 �m spot diame-ter of the laser emitted by the NCLP limited the detection ofhorizontal features but was still capable of detecting verticalfeatures down to 0.1 �m.

The absorption of light by a surface is a property influ-enced by the color of the surface and the wavelength of thelight projected onto it. The sensor of the NCLP had a 785 nmwavelength laser which is almost at the infrared end of thespectrum. This wavelength is absorbed more by colors locatedat the same end of the spectrum. This would explain whydarker colored impression materials showed higher rough-ness values. If an impression material absorbed color at thesame wavelength of the laser then the surface would not bescanned [17].

During the manufacturing process different amounts offiller particles and dies are added to impression materials.They are used to alter the consistency and increase thecontrast between the materials and the oral tissues. Theamount and type of filler and the color of the dies changethe opacity of the material. This in turn may allow or preventthe laser spot from penetrating the surface of the mate-rial thus affecting roughness values by creating artefacts. It

has been previously reported that optical scanners shouldonly be used to scan opaque (non-transparent) materials[18,19]. The results from the present study confirmed thisfinding but we also observed that color influenced digitizing.

l s 2

r

504 d e n t a l m a t e r i a

We observed a correlation between heavy-bodied impressionmaterials and roughness values. These findings should beinterpreted carefully as the heavy-bodied consistency mate-rials tested in this study were all dark. To evaluate the effectof consistency differently colored products should be furthertested.

The dental stones showed similar roughness valuesbetween themselves and significantly lower roughness val-ues than the impression materials. As the dental stoneswere opaque then the roughness findings were more consis-tent between themselves. The dental stone with the highestroughness values was also the darkest in color (SuperRockTM

[Noritake Gypsum Co. Ltd., Nagoya, Japan]), following the sametrend seen on the impression materials. MoonstoneTM (Bra-con Ltd., Etchingham, England) showed the lowest roughnessvalues and was therefore chosen to cast the impression mate-rials.

The reproduction of impression materials in dental stoneinfluenced the roughness values. AquasilTM Ultra MonophaseDECA [Dentsply Caulk, Milford, DE, USA] showed the highestroughness values but these were reduced when the surfaceof the impression material was cast in MoonstoneTM (Bra-con Ltd., Etchingham, England). Directly scanning AquasilTM

Ultra Monophase DECA [Dentsply Caulk, Milford, DE, USA]would create artefacts but if it were cast in MoonstoneTM

(Bracon Ltd., Etchingham, England) they would be eliminated.The results for ExpressTM putty (3M ESPE, St Paul, MN, USA),PresidentTM putty (Coltene-Whaledent. Alstätten, Switzer-land) and ExtrudeTM light-bodied (Kerr Corporation, Romulus,MI, USA) showed no statistically significant differences in theroughness values obtained from the impression materials ortheir MoonstoneTM (Bracon Ltd., Etchingham, England) castsindicating that either an impression or its MoonstoneTM castmay be scanned successfully without adverse effects. In thecase of ExpressTM light-bodied (3M ESPE, St Paul, MN, USA) theroughness values obtained from the impression material werelower than those obtained from the MoonstoneTM (Bracon Ltd.,Etchingham, England) cast.

It is crucial to the accuracy and reproducibility of digitalscanning to test the behaviour and standardize the materialsprior to conducting any research, as in tooth wear measure-ment studies. Some of the materials tested in this studyshowed roughness values around 30 �m which could be mis-interpreted as a feature in wear studies where a thresholdof 30 �m is sometimes set. We recommend that materi-als used for research studies involving optical profilometersare selected based on their optical properties to optimizethe digitizing process. The testing procedure described heremay be used to test any materials scanned on optical scan-ners.

5. Conclusion

Dental materials (impressions or stones) showed statisticallysignificant differences in roughness values (Ra, Rq, and Rt).

These differences were related to the color and transparencyof the materials rather than to real differences in rough-ness. color and transparency of materials affect digitized laserscans.

5 ( 2 0 0 9 ) 500–505

Acknowledgements

The authors would like to thank 3M and Dentsply Caulk forkindly supplying the impression materials and Dr Ron Wil-son for his assistance with the statistical analysis. This studywas supported by the Guy’s and St Thomas’ Charity Grant No.G050202.

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