Chang et al.

Int Chin J Dent 2005; 5: 22-32.

Validity of the M-L scale and Vivadent scale in clinical wear study
Shu-cheng Chang, DDS, MS,a Shiro Suzuki, DDS, PhD,b and Yu-Hwa Pan, DDS, MScabMackay Memorial Hospital, Taipei, Taiwan, Clinical Research and Department of Prosthodontics,c
University of Alabama at Birmingham, Birmingham, USA, and Dental Department, Chang-Gung
Memorial Hospital, Keelung, Taiwan
Purpose: The purpose of this study was to evaluate the validity of two commonly used visual inspection
methods, the M-L scale and the Vivadent scale, in a clinical wear study.
Materials and Methods: Sixty study casts were evaluated using both the Micro-Vu metrology system andvisual inspection methods. Values obtained by using the visual inspection methods were correlated to thosemeasured by the Micro-Vu metrology system. The same principles were applied to prepare specimens to test the
precision of the proposed technique.
Results: This new impression technique in conjunction the Micro-Vu system was proved to be precise. The
mean of the differences between repeat measurements was 1.26 µm with a standard deviation of 0.8 µm. When
comparing the validity of the two scales, the M-L scale was found to be better than the Vivadent scale. Values
obtained by using visual inspection methods were larger than values obtained by using the Micro-Vu metrology
system. Overall values obtained by using the M-L scale were smaller than those obtained by the Vivadent scale.
The effect of evaluators and the interactions between influencing factors were prominent as to significantly
reverse the effect of some evaluators.
Conclusion: Based upon the results of this study, it was concluded that the validity of the currently used visualinspection methods was considerably influenced by many factors, the desirability of continuing these types ofmeasurements for definitive wear assessment should be reconsidered. (Int Chin J Dent 2005; 5: 22-32.)
Key Words: Baldwin illusion, mental average procedure, M-L scale, validity, Vivadent scale.

Introduction
Occlusal wear has been a major clinical concern for all restorative materials that are used for restorations ofposterior teeth. Assessment of the amount of occlusal loss is an important issue for any new resin composite
restorative material. According to the original guidelines for the acceptance of a posterior composite by theAmerican Dental Association in 1984,1 the wear loss should be less than 250 µm after 5 years of clinical service.
These guidelines were revised in 1989, and the posterior composites were classified into two categories.2 The
Category-A composites are for unrestricted use in posterior teeth, while Category-B composites are restricted forsmall, conservative cavity preparations in which the occlusal stress is limited. To be qualified as a Category-A
composite, after an initial 6-month period, no more than 50 µm wear should occur during the following 2 years,
and less than 100 µm wear during a 4-year period. For a Category-B composite, the corresponding amounts ofwear should be less than 125 µm and 175 µm, respectively. Wear requirements were based on the average wearof individual restorations. The revised guidelines published in 1991 and 1996 excluded the possibility of acomposite being accepted for restricted use.3,4
Measurement of clinical wear can be divided into two methodologies, direct and indirect measurements. A
direct evaluation method applies to wear evaluation techniques that employ direct visual observation byevaluators. The indirect methods allow evaluators to evaluate information recorded in the patient's mouth.
Indirect methods can be further divided into two categories, non-instrumental and instrumental methods. Fromthe literature, many wear evaluation methods have been proposed and are summarized in Table 1.5-34 Thesemethods differ widely in their ability to generate quantitative values, their availability, the cost of equipment,
and the time and effort required for evaluations. As the instrumental methods are often costly, non-instrumental

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Int Chin J Dent 2005; 5: 22-32.

methods are more feasible in large clinical studies.33 ADA guidelines in 1989 restricted the evaluation of wear
to indirect methods only and required that the precision and accuracy of the method used should be stated.2
Until the present time, it remains unclear which indirect non-instrumental method provides the best precision andvalidity. Validity is the extent to which the study measures what it is intended to measure; precision is the
reproducibility of the study results.35 Most indirect non-instrumental methods for clinical wear compare thedistance between the worn restorative surface and its original margin with a set of standards. In order todetermine the validity of non-instrumental methods, reliable instrumental methods must be explored thatmeasure the same distance. Some studies had been performed to compare the precision of different scales.15,36,37
However, due to the difficulties of measuring this distance as well as the multi-factorial character of the indirect
non-instrumental methods themselves, only a few of them focused on validity.17 None of these studies had been
designed to compare the validity of different scales.
Table 1.

Direct and indirect methods in clinical wear measurements.

Category/ Methods

Technique

Reference

USPHS
Clinical ranking
Modified USPHS

Visual comparison
Visual comparison
Visual comparison

5
6
7-9

Indirect/ non-instrumental
Photographic method
Total ranking model
Categorial scoring

Photograph comparison
Model comparison
Model comparison

6, 1011, 12
11

Leinfelder’s method (I)SEM photograph rating
M-L scale
Vivaden scale

Model compared to scale
SEM photo comparison
Model compared to scale
Model compared to scale

12
13
14
15

Step wedge meothod
Sabri/Boghosian scale
Indirect/ instrumental
Handelman’s method

Model compared to scale
Model compared to scale

16
17

Coping impression for volume

18

Dennison’s method
Urquiola’s method
Vrijhoef’s method
Santucci’s method

Coping impression for volume
Coping impression for volume
Coping impression for volume
Topographic micrometer for depth

19
20
21
22

Leinfelder’s method (II)Profilometry
Roulet’s method
Eick’s method

Traveling microscope
Profiling for depth
Computer 3D occlusal mapping
Stereo-camera with reference system1223-25
26
27

Williams’ method
Lambrechts’ method
DeLong’s method
Michigan system

Laser occlusal mapping
3D measuring microscopeComputer graphics and servohydraulics
Computer-graphic measuring system

28
29
30
31

Reflex microscope
Impression sectioning
SEM photogrammetry
Chadwick’s method

Optical measuring system
SEM photograph for depth
SEM photography with computer
Stereomicroscope with camera

32
25
25
8

Laser profilometry
Mehl’s method

Laser profiling for depth
Optical 3D device

33
34

Direct23

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Int Chin J Dent 2005; 5: 22-32.

Reviewing the literature involving the use of instrumental evaluation for wear of composite resins revealed
that most of the studies focused on the volume of material loss as the main factor to judge wear.18-21,26-32,34
However, Chadwick et al.8 considered that it may be misleading to use this approach because a large restoration
with a minimal depth of material wear loss may have an apparent volume loss that is greater than a small
restoration that shows a considerable depth of wear loss. They suggested that calculating the depth of material
loss might be a fairer way of comparing the clinical performance of materials. Few techniques have been
advocated to measure the depth of wear loss.8,12,22-25,33 Among them, only Chadwick et al.,8 Leinfelder et al.,12and Bayne et al.33 measured the depth of wear in the clinical situation.
Factors that might influence the result of the measurement when an indirect method is applied include:
different scales,15,36,37 different evaluators,15,38 use of a mental average procedure,15,16,33 the Baldwin illusion,17,39
and degree of experience gained with repeat measurements.38
The objectives of this study were: a) to introduce a new instrumental method to measure the amount of wear
loss in a clinical study, and b) to investigate the validity of two commonly used indirect wear evaluation methods,
the M-L scale and the Vivadent scale. It was expected that this study would provide a better understanding of
the relationship between these two methods, and the underlining factors that might influence their validity.

Materials and Methods
Sixty study casts, including upper or lower molars, which were representative of varying degrees of occlusal
wear, were chosen from several long-term wear studies previously conducted in the Research Clinic of the
University of Alabama, School of Dentistry. To reduce the variables and preserve the original casts for futurestudies, all casts were duplicated with a silicone- based impression material (Express, 3M Co., St. Paul, MN,
USA) and poured with dental stone (Silky Rock, Whip-Mix Corp., Louisville, KY, USA). The reason forchoosing this dental stone is its low expansion value (0.09%). The buccal cusps of all sixty experimental casts
were sorted by their height. The lowest 20 casts were coded as “L”, low cusp height; the medium 20 casts werecoded as “M”, medium cusp height; and the highest 20 casts were coded as “H”, high cusp.
The definition of wear in this study is the mean distance from the cavosurface margin to the occlusal surfaceof the composite resin, as described by Leinfelder et al.12 This kind of wear of the specimens is a contact-freearea wear24 and caused almost entirely by three-body abrasion by abrasive particles in the masticatory slurry.16
To fulfill the objectives of this study, the selected experimental casts were evaluated using two methodologies,
instrumental and non-instrumental evaluations.
For the instrumental evaluation, the Micro-Vu optical metrology system (Model Qubix 8x6, Micro-Vu
Corporation, CA, USA) was used to measure wear on the negative replicas of sixty casts (Fig. 1). The Micro-Vu
metrology system is a three dimensional non-contacting measuring tool. It consists of three main parts: themeasuring stage, the InSpec Vision System and the InSpec Metrology Software. The motorized stage is anelectro-mechanical assembly that sends accurate x, y, and z coordinates information to the measuring software.
The InSpec Vision system has the capability of quantitative measurements of the desired features on a specimen.
By taking the positional information from the measuring stage, the InSpec metrology software is able todetermine the size and location of the features of interest and execute appropriate functions as needed. For the x
and y axes, the resolution is 0.5 µm and repeatability is 1.0 µm.
For each experimental cast, three impressions were taken with a silicone impression material (Express).

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Int Chin J Dent 2005; 5: 22-32.

Sectioning of the impression was accomplished with a sharp #12 surgical blade.

All sections were in a

bucco-lingual direction. After one arbitrary cut was made on the first replica, the sectioned replica was used tomark the designated points in the buccal and lingual margins on the stone model prior to further sectioning. For
the other two negative replicas, three equally spaced sections were made on both the mesio-buccal area and the
disto-lingual area of the margins (Fig. 2). This sectioning of multiple impressions is advantageous as it produces
thick sections and minimizes the risk of distortion and loss of accuracy.

MB

B

L

DL

L

1
2
Fig. 1. Micro-Vu optical metrology system.
Fig. 2. Scheme for evaluation. Two points (designated as points of the buccal margin, B, and the lingual
margin, L) are at the intersections of the arrow and margins. Two areas (mesio-buccal area, MB, and disto-lingualarea, DL) are bolded. Arrow denotes the position of arbitrary section on the first replica. Solid lines are sections of
the second negative replica and dash lines are sections of the third one.

All sections of negative replicas for a tooth were then placed on a glass slide and measured by using theMicro-Vu system. The profiles of wear facets were magnified 35 to 365 times as necessary to create an image ofa sufficient size on the monitor in order to identify the location of the cavosurface margin and the worn
restorative surface. The distance between these two locations was calculated by the computer software. The
data obtained by examining the sections through designated buccal and lingual points for each cast were coded
as “point” data. Four measurements for each area were averaged then coded as “area” data.
As a precision test for the measurement technique, a similar procedure with exactly the same materials forclinical casts was performed on 18 cylinders with known depths of prepared depressions ranging from 0 to 1,000µm. Impressions were taken and negative replicas were sectioned as described for wear specimens. Threeparallel sections with one at the diameter of circular depressions were made for each negative replica. Sections
facing the diameter of the replicas were placed on glass slides and measured by the Micro-Vu metrology system.
For each section, two depths of depression were measured. Seventy-two measurements were obtained. Repeat
measurements of all impressions were performed at a different time period.
For the non-instrumental evaluation two methods, the M-L scale and the Vivadent scale, were investigated inthis study. Moffa and Lugassy first developed the M-L scale in 1986.14 The M-L standards are 18 cylinders
with flat-bottomed circular depressions of known depth centered on the flat end surfaces, representing
incremental defects at intervals ranging from 0 to 1,000 µm. The difference between each scale in the series is
25 µm when the wear is less than 100 µm, 50 µm when the wear is between 100 µm to 500 µm, and 100 µm
when the wear is more than 500 µm. The Vivadent scale is an 18-category standard scale that consists of toothshaped, tooth-sized dies with restoration-like incremental defects ranging from 25 µm to 1,000 µm. The extent
of the artificial material loss at the margins is constant. The difference between each standard in the series is 25

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Int Chin J Dent 2005; 5: 22-32.

µm when the wear is less than 200 µm, 50 µm when the wear is between 200 µm to 400 µm, and 100 µm when
the wear is more than 400 µm. Both the incremental depth pattern of the M-L scale and that of the Vivadentscale reflect the capacity of evaluators to visually detect smaller differences when the actual amount of existing
loss is less.15
Five dentists denoted C, L, N, P, and S were trained to use both scales following a training program. Amongthem, only evaluator S had previous experience using the M-L scale for evaluating wear on clinical casts. The
standards for both scales were duplicated and labeled randomly. For the M-L scale, evaluators had to evaluate
thirty-six stone replicas and achieve 90% accuracy prior to proceeding. Evaluators then evaluated ten additional
clinical casts with known amounts of wear and were required to get 80% accuracy before they proceeded toevaluate the experimental casts. For the Vivadent scale, a similar training program was followed with the only
difference being that replicas of the Vivadent scale were used.
Each evaluator was instructed to conduct evaluations in a standardized manner and received a protocol clearly
describing the procedures and necessary information. A high intensity incandescent lamp was positioned in
front of the evaluator and the experimental casts and arranged to emphasize shadows cast by exposed walls of
the cavity preparation and to aid in evaluation of the degree of wear. No amplifier was used.
For each experimental cast, two points on the buccal and lingual margins and two areas (mesio-buccal anddisto-lingual area) were evaluated (Fig. 2).

A red mark was placed near the point being evaluated, andcomparisons could be made with standard scales and coded as such. For those two areas, a "mental average
procedure” was utilized.
After taking the training program for the M-L scale, evaluators examined all experimental casts with thatscale. At least a 24-hour lapse was required before evaluating experimental casts with the other scale to preventthe possibility that evaluators might remember the values from the previous evaluation. The same rules applied
for evaluating casts using the Vivadent scale. Repeat measurements were done after a 1-week lapse. Before a
repeat measurement was made, a new training program was performed.

After finishing their evaluation,

evaluators were asked three subjective assessment questions regarding which scale was easier to learn, faster touse, and their confidence that they had used it correctly.
The experience factor was determined by the improvement of validity of repeat measurements with the same
scale. The effect of the mental average procedure was tested by the difference between the evaluations of points
and areas. The mental average procedure was employed only when evaluating areas. Therefore, the difference
between the results obtained from the area and point evaluations indicated the effect of the mental average
procedure. The effect of “Baldwin illusion” was judged by the influence of cusp height.
Correlation and regression analyses were used for the precision test of the impression technique with theMicro-Vu system. A combination of analysis of variance (ANOVA) and multivariate analysis of variance(MANOVA) were used to analyze the validity. The confidence level for hypothesis tests was set at 0.05.
Scheffe’s test was applied when the F-test was significant. The dependent variable tested was the difference
between square root of values obtained by Micro-Vu system and the square root of values obtained by evaluators
using two different scales. This variable was tested to show approximately a normal distribution. “Validity” isrepresented by the mean of this variable. The closer to zero this mean is, the better is the validity. A negativevalue of mean indicates that the values obtained by using non-instrumental scales were larger than that obtainedby the Micro-Vu system.

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Int Chin J Dent 2005; 5: 22-32.

Results
Instrumental evaluation
The correlation between the results obtained by the Micro-Vu system and the depths of cylinders was veryhigh. R2 for the first and repeat measurements were 0.9966 and 0.9970, respectively. The mean of thedifference between the first measurement of replicas and second measurement was 1.26 µm with a standard
deviation of 0.8 µm.
Three different types of profiles of impression replicas were observed with the Micro-Vu metrology system(Fig. 3): flat floors of worn restorations, curved floors of worn restorations and marginal degradation. When acurved floor of composite was observed, higher magnification was used to define the locations of cavosurface
margin and worn surface. The depth of marginal degradation was excluded as a part of the wear facet.

abcFig. 3. Buccal profiles of wear facet seen on Micro-Vu system. a) wear on a flat worn composite
Surface; b) wear on a curved worn surface; and c) marginal degradation.
Results of the instrumental evaluations of 60 experimental casts are shown in Table 2. The distribution ofwear on 120 points and the averaged wear on 120 areas are shown in Table 3. Mean of point values was larger
than that of area values. The range of measurements was 1,000 µm. Sixty percent of all measurements were lessthan 300 µm. The standard deviations of measurements were large due to their wide range and distribution.
Table 2.
Point
Area
Total
Table 3.

Measurements of wear with the Micro-Vu system.
Number120
120
240

Mean (µm)
172.43149.25160.84

S.D. (µm)
237.51202.33220.47

Std error (µm)
21.68
18.47
14.23

Range (µm)
1,000
1,000
1,000

Distribution of the wear on points and averaged wear on areas.

Wear (µm) 0-99 100-199 200-299 300-399 400-499 500-599 600-699 700-799 800-899 900-999 1,000

Total

Number

240

124

7

16

9

4

1

3

1

1

2

9

Non-instrumental evaluation
Table 4 is the complete model for this study. The effect of sixty casts, five evaluators, two scales, and themental average procedure (MAP) were all statistically significant. The overall effect of repeat measurement was
not statistically significant (p=0.2252). The Baldwin illusion was not included in this model because only buccal
cusps were ranked. Before further discussion of the results, the importance of the interaction between factors is
noteworthy. No interaction higher than 3-way was significant.

27

Chang et al.
Table 4.

Int Chin J Dent 2005; 5: 22-32.

Full model of ANOVA used in this study.

SourceMain effectMethodEvaluator
MAP
RepeatCast
Two-way interaction
Method x Evaluator
Method x MAP
Method x Repeat
Method x Cast
Evaluator x MAP
Evaluator x Repeat
Evaluator x CastMAP x Repeat
MAP x Cast
Repeat x Cast
Three-way interaction
Method x Evaluator x MAP
Method x Evaluator x Repeat
Method x Evaluator x Cast
Method x MAP x Repeat
Method x MAP x Cast
Method x Repeat x Cast
Evaluator x MAP x Repeat
Evaluator x MAP x Cast
Evaluator x Repeat x CastMAP x Repeat x Cast
Four-way interaction
Method x Evaluator x MAP x Repeat
Method x Evaluator x MAP x Cast
Method x Evaluator x Repeat x Cast
Evaluator x MAP x Repeat x Cast
Method x MAP x Repeat x Cast
Five way interaction
Method x Evaluator x MAP x Repeat x Cast

df

F

Pr>F

1
4
1
2
59

9.19
75.04
13.15
1.49
38.71

0.0024*0.0001*0.0003*0.22520.0001*

4
1
2
59
4
8
236
2
59
118

32.73
1.68
7.18
2.43
9.83
3.86
2.78
0.72
12.71
0.98

0.0001*0.19470.0008*0.0001*0.0001*0.0002*0.0001*
0.48840.0001*0.5537

4
8
236
2
59
118
8
236
472
118

0.15
14.24
1.64
0.02
0.45
0.64
0.47
0.74
0.71
0.2

0.96390.0001*0.0001*
0.98190.99990.99910.8774
0.9987
1
1

8
236
472
472
118

0.18
0.41
0.7
0.23
0.17

0.9941
1
1
1
1

472

0.2

1

*p<0.05, statistically significant difference.

Table 5.

Comparison of difference on influencing factors.

FactorMethodVariabledf
1, 7,198

F
14.33

Pr>F
0.0097*

Vivadent
M-L
Evaluator

4, 7,19556.34<0.001*

C
L
N
P
S
MAP

1, 7,198

9.59

2, 7,197

1.09

2, 1,797

32.57

H
M
L
*p<0.05, statistically significant difference.
** Means with the same letter are not significantly different.

28-0.32998-0.09902

0.060948
0.065158-0.57643b-0.07122c-0.88753c-0.53516c
0.99785a

0.093480.08765
0.122030.09059
0.09374-0.07641-0.35259

0.062857
0.063294-0.23088-0.28561-0.12700

0.078920
0.078661
0.074219

0.84400a-0.00440b-0.79752c

0.15365
0.131310.14565

0.337

1
2
3
Cusp height

S.D.

0.0020*MAP
NMAP
Repeat measurement

Mean<0.001*

Chang et al.

Int Chin J Dent 2005; 5: 22-32.

The results of overall difference between methods, evaluators, with mental average procedure (MAP) or without
mental average procedure (NMAP), repeat measurements, and cusp height are presented in Table 5. The only
factor that was not significant is the effect of repeat measurements (p=0.337).
The results of most important interactions between influencing factors are shown in Table 6. Interactions
among method, evaluator, and the mental average procedure (MAP) were all statistically significant.
Table 6.

Comparison of most important interactions between influencing factors.

FactorMethod by evaluator

Variabledf
9, 7,190

F
37.25

Pr>F
<0.001*

Vivadent x C
Vivadent x L
Vivadent x N
Vivadent x P
Vivadent x S
M-Lx C
M-Lx L
M-Lx N
M-Lx P
M-Lx S
MAP by evaluator

9, 7,190

29.56

3, 7,196

5.84

Vivadent x MAP
Vivadent x NMAP
M-L x MAP
M-L x NMAP

S. D.

-1.06788
0.33673-0.73395-0.42581
0.24101-0.08498-0.47918-1.04111-0.64450
1.75469

0.133290.11886
0.16845
0.12306
0.12446
0.128620.127120.176540.132930.13449-0.56281-0.18185-0.32234-0.53333
1.21831-0.59005
0.03941-1.45271- 0.53698
0.777400.126530.129150.17962
0.12132
0.127130.13773
0.11847
0.162650.13465
0.13739-0.241290.08848-0.41867-0.28651

0.085006
0.092478
0.08734
0.091628<0.01*

MAP x CMAP x LMAP x NMAP x P
MAP x S
NMAP x C
NMAP x L
NMAP x N
NMAP x P
NMAP x S
Method by MAP

Mean

0.006**p<0.05, statistically significant difference.

The correlation of the answers to the three subjective assessment questions and the scale with the better
validity used by individual evaluator are listed in Table 7. Except for Evaluator S, the more confidence that theevaluator had toward the scale, the more valid were the data obtained by using that scale.
Table 7.

Answers for three subjective assessment questions.

Evaluator
Easier to learn
Faster evaluation
More confidence on scale
More valid on scaleC
Vivadent
M-L
M-L
M-L

L
M-L
M-L
Vivadent
VivadentN
M-L
Vivadent
Vivadent
VivadentP
M-L
Vivadent
Vivadent
Vivadent

S
Vivadent
M-L
M-L
Vivadent

Discussion
Instrumental evaluation
The impression technique with the Micro-Vu optical metrology system was shown to be ideal for evaluating
clinical wear due to the high resolution (0.5 µm) and repeatability (1.0 µm) of the system and the small standard
deviation of this technique (0.8 µm). In addition, the ability of viewing the profile of restorations along with
their wear facets at a proper magnification facilitates measuring wear on different shaped restorations withdifferent marginal degradation patterns (Fig. 3). When the worn composite surface is flat, it is easy to measure29Chang et al.

Int Chin J Dent 2005; 5: 22-32.

the occlusal wear (Fig. 3a). However, it is difficult when it is curved since the junction between the floor ofcomposite and enamel becomes more obscured (Fig. 3b).

By increasing the power of magnification, thelocations of floor and margin can be identified easily. Marginal degradation is characterized by a breakdown at
the resin-enamel wall interface. Reduced tensile strength due to decreased particle size of composite and weakbonding strength of bonding agent are believed to play main roles in marginal degradation.40 Because of thedifference in the mechanisms involved, marginal degradation (Fig. 3c) should not be confused with wear. With
the aid of the Micro-Vu metrology system, this mistake is prevented.
The technique proposed in this study is similar to the technique used by Leinfelder et al.12 In this present
study, more sections were made and a different viewing and measuring system was used. It is quite clear that thecomputer-aided system used in this study may achieve a more favorable performance than an optical travelingmicroscope.
Roberts and Soderholm's study showed that among three techniques compared, cross-sectioned impression
technique, SEM photometric techniques and profilometer evaluation, the standard error for the impression
technique was the smallest.32 Our study also indicated that this technique was very precise.
Non-instrumental evaluation
For comparison of the overall validity of the non-instrumental evaluation methods, three major findings were:a) the M-L scale was more valid than the Vivadent scale, b) the overall values obtained by using the M-L scale
were smaller than values by the Vivadent scale, and c) the values obtained by both the M-L scale and theVivadent scale were larger than the values measured by the Micro-Vu system (Table 5). To our knowledge, noprevious study has been published that compared the validity of different scales. Our second major finding was
the opposite of Bryant’s study15 and was also different from the study by Taylor et al.,38 whose results showedthat the values obtained by the M-L scale and the Vivadent scale were equivalent. The third finding disagreed
with Winkler et al.17 who found that the visual estimates of wear using the clinical casts were significantly lower
than those using mechanical measurement.
Data obtained by different evaluators were significantly different (p<0.001). The data obtained by Evaluator
L were most valid while those by Evaluator S were least valid. The effect of the evaluator factor was so
influential as to shift the direction of the results when its interactions with other factors were considered. For
example, the overall validity of the M-L scale was better than that of the Vivadent scale. When the evaluator
effect was further analyzed, this statement was true only for Evaluator C. For the other four evaluators, thevalidity of the M-L scale was worse than that of the Vivadent scale (Table 6). Our results support the findings
by Taylor et al.36 that the controlling limitation for non-instrumental methods is the evaluators’ ability todiscriminate wear.
For the mental average procedure, Taylor et al.36 and Bryant15 stated that because the amount of occlusal loss
was usually uneven along the margins, the evaluator presumably performed this procedure for both the casts ofthe experimental restorations and standards while making the comparison. Surprisingly, using the mental
average procedure increased the validity of the results (p=0.0020). However, when the effect of this procedure
was analyzed for its interaction with the evaluator effect (Table 6), the results were different. In fact, the mental
average procedure only increased the validity for Evaluator N. For the other four evaluators, the mental average
procedure either had no influence or decreased their validity. Again, the interaction of evaluator effect was so
powerful as to reverse the overall effect. For the two methods investigated in this study, their interaction with30

Chang et al.

Int Chin J Dent 2005; 5: 22-32.

the mental average procedure was also significant (p=0.006).

Regardless of whether the mental average

procedure was employed, the Vivadent scale was more valid than the M-L scale. Regardless of which scale wasused, evaluations with the mental average procedure were less valid than without it. The former result seems to
be contradictory to the result of overall validity, for which the M-L scale was more valid. The reason is that
evaluators tended to give smaller values, even smaller than values obtained by the Micro-Vu system, when using
the Vivadent scale while the mental average procedure was not indicated (mean=0.088).

Therefore, ourconclusion for the mental average procedure is that this procedure might influence different evaluators to acertain extent and the validity of different methods as well.
The effect of experience was tested by repeat measurements. Evaluators gained experience by repeat training
programs and measurements.

There was no significant difference among repeat measurements (p=0.337).

Although some authors suggested that experienced evaluators achieved higher intra-evaluator and inter-evaluatoragreement,15 our study showed that experience did not increase the validity of the results. In other words, morepractice or experience might benefit the precision of the measurements, but not the validity for a particular scale.
Baldwin first studied the effect of size contrast in 1895. In general, size contrast means that any stimulus
surrounded by larger stimuli seems smaller than it really is.39 Winkler et al.17 speculated that the Baldwin
illusion had an influence for underestimation of the depth of wear at the margin. The results showed that cusp
height played an important role in this study (p<0.001). The validity for the evaluations of medium height cusps
was best among the three different cusp height categories. The effect of the Baldwin illusion was not significant.
As far as subjective assessment is concerned, both scales were equally favored (Table 7). This is different
from the evaluators in Bryant’s study who were all more confident with the Vivadent scale.15 The relationshipbetween the confidence level and validity showed that the validity increased with subjective confidence except
for Evaluator S, who had previous experience with the M-L scale.

Conclusion
The impression technique in conjunction with the Micro-Vu optical metrology system for measuring wear ofcomposite resins utilized in this study was proven to be highly precise. The ability to view the profile of a wear
facet on a thin section as well as to magnify it for detailed appreciation was advantageous for assessing wear.
This new technique is recommended for use in future studies involving wear loss evaluation.
For non-instrumental methods, the three major findings in this study were: a) different scales, evaluators, andthe mental average procedure influenced the validity of the results, b) the validity was not improved by practiceor experience, and c) interactions between factors, especially with evaluators, were high, which in turn definitely
affected the result obtained by using non-instrumental evaluation methods.

Therefore, the desirability ofcontinuing these types of evaluation techniques to assess wear should be reconsidered.
Acknowledgment
The authors would like to thank Dr. Jack Lemons and Mr. Preston Beck for their advise and assistance when using theMicro-Vu system, and Drs. Adam Clark, Samir Nanjapa, Winnie Lee for their efforts with evaluations.

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Reprint request to:
Dr. Shiro Suzuki
Department of Prosthodontics, University of Alabama at Birmingham, School of Dentistry
1919 7th Avenue South, Birmingham, AL 35294-0007, USA
Fax: +1-205-975-6108 E-mail: shiro@mail.dental.uab.edu
Received January 19, 2005. Revised February 13, 2005. Accepted February 16, 2005.
Copyright ©2005 by the Editorial Council of the International Chinese Journal of Dentistry.

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