Variation in Tooth Wear in Young Adults Over a Two-year Period Issue 3

Variation in tooth wear in young adults over a two-year period

Maria R. Pintado, MPH/Gary C. Anderson, DDS, b Ralph DeLong, DDS, MS, PhD, c and William H. Douglas, BDS, MS, PhD d School of Dentistry, University of Minnesota, Minneapolis, Minn.

Statement of problem. Although all the processes of loss of hard tissue are important, attrition on the occlusal surfaces commands our attention. Purpose of study. The enamel wear rate of 18 young adults over 2 consecutive years was measured independently by volume loss and mean depth loss. Any significant differences in tooth wear resulting from gender and a clinical diagnosis of bruxism were identified. Material and methods . A strict protocol for dental impressions provided epoxy models, which were digitized with a null point contact stylus. AnSur software provided a complete morphologic description of changes in the wear facets. Results. The mean loss for all teeth measured was 0,04 mm 3 by volume and 10.7 pm by depth for the first year. Conclusions. These numbers were approximately doubled at 2 years of cumulative wear. (J Prosthet Dent 1997;77:313-20.)

P i n d b o r g t reserved the term "am'ition" for the surface loss of hard tissue resulting from tooth-to-tooth contact, either occlusal or interproximal. Although all the processes of loss of hard tissue are important, attrition on the occlusal surfaces commands our attention for a number of reasons. (1) Dental attrition probably has articular significance for the temporomandibular joint and, in extreme cases, it may be responsible for loss ofocclusal vertical dimension. 2,3 (2) Attrition leads to the loss of essential anatomic form and possibly to a change in masticatory efficiency. 4 (3) Continued dental attrition can lead to a breach ofocclusal enamel, exposing dentin. This may lead to an acceleration of the loss of hard tissue because of the inferior wear-resistant properties of dentin. The pulp may become involved in extreme conditions, s

The substance of this report was delivered at the annual meeting of the American Equilibration Society, Chicago, Ill., February 1996.

Supported by the MN Dental Research Center for Biomaterials and Biomechanics and the Minnesota Clinical Dental Research Cen- ter, National Institutes of Health No. P30-DE09737.

~Associate Professor, Department of Oral Science. bAssociate Professor, Department of Restorative Sciences. cChairman and Associate Professor, Department of Restorative Sci-

ences. dProfessor, Department of Oral Science, and Director, Minnesota

Dental Research Center for Biomateria[s and Biomechanics.

More generally, the study of dentai attrition fits into the broad anthropologic studies o f mammalian tooth wear, with particular reference to the primitive culture studies ofBeyron, 6 the anatomic studies ofGreaves, 7 the microscopic studies of Teaford and Tylenda, s and the mechanistic studies o f Young and Robson. 9 Luke and Lucas 4 believed that a low physiologic rate of enamel wear was optimal because it favored the retention of the cusp form in the human dentition. This was in contra- distinction to the flat plane views of Berry and Poole. ~0 Pronounced tooth wear has also long been associated with parafunctional bruxing activity, ~-~3 and in the same vein parafunctional bruxism was thought to be the result of occlusal triggers. ~4 More recent investigations suggest that bruxism is a behavioral pattern generated in the central nervous system. 15,~6 Regardless o f etiology, however, there is little disagreement that bruxing activity can contribute to tooth wear. There is also some evidence of gender effect on dental attrition, with greater attrition reported in males than in females. ~7,~8

Method is also an issue in dental attrition in the quest for a more quantitative database. Molnar et al. 19 found a loss of cusp height of 800 pm at 18 years of age in a g r o u p o f c o n t e m p o r a r y Aus t ra l i an abor ig ines . Lambrechts et al. 2° reported a steady annual wear rate 0 f 2 9 gm for molars and 18 pm for premolars in 21

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THE JOURNAl OF PROSTHETIC DENTISTRY PINTADO ET AI.

subjects extending over 48 months. In a smaller study Roulet et al. 21 found a loss of 33 ~am for mandibular molars.

The literature seems to suggest that the number of studies on quantitative enamel wear remains quite small, which indicates that there is a need to increase the knowledge base in this respect. Furthermore, it is common, even in quantitative studies, to measure wear and divide by the number of years to give an annualized clinical wear. Although this is useful, it would be interesting to know the rate of change of tooth wear in consecutive years. In addition, there appears to be little quantitative information on the extent of tooth wear resulting from gender differences and parafunctional bruxism at a quantitative level. Knowledge of such effects on wear would be useful in the assessment of diagnosis, prognosis, and treatment outcomes.

The objective of this report was to expand the clinical database of enamel wear by studying 18 young adults over a 2-year period. The secondary objectives were to identify any significant differences in tooth wear caused by gender differences and a clinical diagnosis of bruxism.

M A T E R I A L A N D M E T H O D S

This study protocol was submitted and approved by the University Committee on the Use of Human Sub- jects in Research. Eighteen subjects, dental students between 22 to 30 years old, were examined clinically. The choice of candidates was constrained by availability for a period of 2 years.

Clinical criteria

All subjects had a full complement of maxillary and mandibular teeth. All postcrior teeth had occlusal contact with their antagonists, which was confirmed by shimstock. 22 The presence and location of all dental res- torations were recorded during clinical examination. Teeth restored with crowns were excluded from the study. Thc test teeth to be measured were free of caries and significant periodontal attachment loss at the time of baseline data collection. These included the maxillary and mandibular canines, second premolars, and the first molar teeth, which provided three major tooth types in the dental arch at which measurements could be made.

The current clinical diagnosis of bruxism behavior is principally dependent on history, tooth wear, tooth fracture, tooth mobility, and clinical evidence of masticatory muscle pain--although this process is still the subject of debate26,23 Tooth wear was the dependent variable to be measured. None of the subjects exhibited evidence of tooth fracture or mobility. Bruxers and nonbruxers were clinically identified through a history and clinical examination of the masticatory system.24 No effort was made to distinguish diurnal and nocturnal activity.

The historical questionnaire included six items tradi- tionally believed to suggest bruxing activity. 1~,16,2s Sub-

jects who were classified as bruxers gave a positive re- sponse to at least two of the six items: 1. Has anyone heard you grinding your teeth at night? 2. Is your jaw ever fatigued or sore on awakening in the

morning? 3. Arc your teeth or gums ever sore on awakening in

the morning? 4. Do you ever experience temporal headaches on awak-

ening in the morning? 5. Are you ever aware of grinding your teeth during the

day? 6. Are you ever aware of clenching your teeth during

the day? The teeth were divided by subject's gender (male, fe-

male), parafunctional habit (bruxer, nonbruxer), and tooth type (canine, premolar, molar) for analysis with a three-way analysis of variance (ANOVA) with i2 groups ( 2 x 2 x 3 ) .

Impression technique

The measurement of wear was accomplished through impression procedures performed at baseline, 1 year, and 2 years. At baseline and subsequent appointments, irre- versible hydrocolloid scavenger impressions were made to remove plaque and any other substance that covered the teeth. Immediately after, impressions were made with polyvinyl siloxane (Express, 3M, St. Paul, Minn.) supported in a quadrant metal tray. Baseline ("before") and follow-up ("after") impressions were taken at the agreed intervals. Impressions from the 18 young adult subjects were washed in a soap solution, rinsed, air dried, and boxed in the traditional manner (Express, 3M). The impressions were used to make epoxy replicas (Epoxy- Die, Ivoclar, Schaan, Liechtenstein), which were then digitized.

Digi t izat ion and analysis

The epoxy replicas were digitized on a profiling system that uses a null point contact stylus as described by DeLong et al. 2a and Pintado et al. 27 The precision and accuracy of this profiling system (digitized data plus computer alignment) was determined by following a method similar to that suggested by Hewlett et al. 28 The upper hemisphere of a grade 5 precision ball bearing with a diameter of 4.0000 mm _+ 0.0002 mm was profiled with two different spherical-tipped styli: a ruby stylus (R) with a diameter of 300 _+ 2.5 pm and a tungsten carbide stylus (T) with a diameter of 110 -+ 5 gm. Six digital images of the ball were created, three for each stylus, by measuring the Z height of the ball every 50 pm in the X and Y directions. The images were aligned to an image of the ball mathematically generated on the same XY grid with AnSur software (Regents , Universi ty o f Minnesota , Minneapolis , Minn.). Precision is defined as the root-mean-squared difference between the aligned digital images. Accu-

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PINTADO ET AL THE JOURNAL OF PROSTHETIC DENTISTRY

Table I. Mean volume loss (in cubic millimeters), enamel-to-enamel tooth wear

All teeth Canines Premolars Molars Bruxers Nonbruxers Females Males

Number 196 72 66 58 91 105 83 113

Year 1 annual

Mean 0,044 0.093 0.016 0.016 0.073 0.019 0.022 0,061

SD 0.251 0.409 0.030 0.045 0,365 0.040 0.038 0.329

Year 2 annual

Mean 0.054 0.080 0.031 0.047 0.073 0.037 0,037 0.065

SD 0.216 0.337 0.055 0.085 0.307 0.059 0,078 0.275

2 Year cumulative

Mean 0.098 0.173 0.047 0.063 0.146 0.056 0.059 0./26

SD 0,331 0.530 0.063 0.096 0.477 0.071 0.087 0.429

A% 22.7 -14.0 93.8 193.8 0.0 94.7 68.2 6.6

A%, Percentage annual change in wear from first to second year,

Table II. Mean depth (in micrometers), enamel-to-enamel tooth wear

All teeth Canines Premolars Molars Bruxers Nonbruxers Females Males

n 196 72 66 58 91 105 83 113

Year 1 annual

Mean 10.74 16.74 8.90 5.38 12,32 9,37 8,94 12.06

SD 24.25 36.39 12.46 10.30 32.82 12.89 12.73 30.02

Year 2 annual

Mean 9.34 10.94 6.59 10.50 10.64 8.22 6.93 11.12

SD 16.96 24.65 7.88 10.94 23.88 6.02 8.98 20.57

2 Year cumulative

Mean 20.08 27.68 15.49 15,88 22.96 17.59 15.87 23.18

SD 29.59 43.96 14.74 15.02 40.59 14.23 15.57 36.39

A% -13,0 -34.7 -25.9 95.1 -13.6 -12.3 -22,6 -7.8

/k%, Percentage annual change in wear from first to second year.

racy is defined as the magnitude of the root-mean- squared difference between the aligned digital images and the mathematic image. Both the precision and accuracy depend on the surface angle. The mean accuracy and precision in micrometers for the surface angles from 0 degrees (horizontal) to 60 degrees are R = 4, T = 5 and R = 3, T = 3, respectively. For all angles less than 60 degrees the accuracy is better than 7 btm and the precision is better than 5 ~am with either stylus.

Approximately 25,000 coordinate points were col- lected from each occlusal surface by use of a tungsten carbide stylus. "Before" and "after" images were aligned with AnSur software in three dimensions and with 6 degrees o f f reedom by minimizing the root -mean- squared difference between the two images. The aligned occlusal surfaces were rendered with use of gray scaling to produce a near photographic quality image of the numeric database in which the wear facets could be clearly seen. Individual profiles were used to tag the wear areas and the following morphologic measurements were made: volume loss (in cubic millimeters) and mean depth loss (in micrometers). The anatomic location of the morphologic change on the occlusal surface was auto- matically outlined by the software.

R E S U L T S

The descriptive statistics for clinical tooth wear by volume and by mean depth are presented in Tables I and II, categorized by tooth type, clinical bruxism, and gender. The wear data are shown for each year, and the percentage annual change (A%) from the first to the second year of wear is indicated. The cumulative wear for the 2-year period is also shown.

The mean loss for all teeth measured was 0.04 mm 3 by volume and 10.7 ~am by depth for the first year of measurement . These numbers were approximately doubled at 2 years of cumulative wear. The canines ap- pear to have experienced the heaviest wear at both I year and 2 years.

A three-way ANOVA was performed on volume wear and depth wear to identify differences between simple effects such as tooth type, bruxers/nonbruxers, and gender type. Interactive effects between different factors were also examined. All F values and corresponding p values were calculated. The 1-year results for the post hoc multirange testing of the ANOVA are shown for the simple effects of tooth type, bruxism, and gender in Table 11I by volume and in Table IV by mean depth. Similar results for 2 cumulative years of wear are shown

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Table III. Fisher's PLSD* at 1 year: volume

Effect Mean difference Critical difference p Value

Canine, molar 0.076 0.085 0.080 (NS) Canine, premolar 0.077 0.082 0.068 (NS) Molar, premolar 0.001 0.087 0.990 (NS) Bruxer, nonbruxer 0.053 0.069 0.130 (NS) Female, male -0.039 0.070 0.267 (NS)

NS, Not significant. *Significance level 5%.

Table IV. Fisher's PLSD* at 1 year: mean depth


Canine, molar 11.362 8.151 0.007 (S) Canine, premolar 7.846 7.873 0.051 (S) Molar, premolar -3.516 8.315 0.405 (NS) gruxer, nonbruxer 2.951 6.617 0.380 (NS) Female, male -3.116 6.678 0.359 (NS)

5, Significant; N5' not significant. *Significance level 5%.

Table V. Fisher's PLSD* at 2 years (cumulative): volume


Canine, molar 0.110 0.111 0.052 (S) Canine, premolar 0.126 0.107 0.021 (S) Molar, premolar 0.016 0.113 0.777 (NS) Bruxer, nonbruxer 0.091 0.090 0.048 (S) Female, male -0.068 0.091 0.144 (NS)

5, Significant; NS, not significant.

*Significance level 5%.

Table VI. Fisher's PLSD* at 2 years (cumulative): mean depth

Effect Mean difference Criticaldifference pValue

Canine, molar 11.795 9.939 0.020 (S) Canine, premolar 12.189 9.549 0.013 (S) Molar, premolar 0.394 10.138 0.939 (NS) Bruxer, nonbruxer 5.375 8.068 0.190 (NS) Female, male -7.308 8.143 0.078 (NS)

5, Significant; NS, not significant. *Significance level 5%.

in Tables V and VI. Most o f the significant differences emerged after 2 years of cumulative wear: between canines on the one hand and molars and premolars on the other and between bruxers and nonbruxers. Illustrations of wear with particular reference to occlusal anatomy are shown as gray-scaled renditions of the numeric database in Figures 1 and 2.

D I S C U S S I O N

The fairly low annualized clinical tooth wear rates in this study of 0.04 m m 3 by volume and 10 pm by mean depth are somewhat lower than those o fLambrech t s et al. 2° and Roulet et al., 2~ who made individual measurements on selected parts o f the occlusal surface. The different sets o f data are probably compatible because the mean depth is integrated over the entire contact area in this study. The low wear rate is also in agreement with the longitudinal findings of Carlsson ct al. 29 and the cross-sectional findings of H u g o s o n et at.17 These data are also consistent with the arguments of Luke and Lucas 4 for a biologic advantage of cusp retention. At this rate it would not be expected that the depth of enamel would be breached by wear during the life o f the tooth. How- ever, these are average values and that individual cases can far exceed this wear rate and should be considered separately. The high SDs in this study are reflections of true biologic variation.

As previously stated, the c o m m o n practice of calcu- lating an annualized wear rate assumes that the actual wear rate is constant year by year. This can be shown in a simple way by considering the general equation for wear30 :

V / L = K F / H (1)

where V is the volumc of material removed as two surfaces, pressed together by a normal force (F), slide against each other over a distance L; H is the hardness of the softer o f the two materials; and K is a constant that de- pends on the type of wear and the geometry involved. I f this equation is applied to chewing, V would be the volume of tooth structure removed, F would be the mean force of mastication, H would be the hardness of enamel, and L would be the mean excursive path multiplied by the total number of chewing cycles during the test period. For healthy individuals with stable diets and stable dentitions, it is reasonable to assume that the hardness of enamel, the mean force of mastication, the mean excursive path, and the number of chewing cycles are relatively constant over time intervals o f a few months or years. This means that H, F, and L will be relatively constant during the test period; thus the volume of enamel removed year by year is relatively constant.

The most c o m m o n method o f report ing wear rates in dentistry is by depth. This is most likely because of his- toric reasons (it was easier to measure depth) and because of the direct relationship of depth to the vertical dimension of occlusion or facial height. Wear rates by volume and wear rates by depth are related. The volume of material removed is equal to the area o f wear (A) times the mean change in depth (D) in the wear area. Thus equation (1) can be rewritten in terms of D:

V / L = D A / L = K F / H

rearranging to

D = K F L / ( H A ) = C o n s t / A (2) On the basis o f the preceding argument K, F, L, and

H are constant; thus the wear rate by depth can only be



Fig. 1. Gray-scaled computer image of incisal aspect of maxillary right canine. Brighter colors represent greater wear depth in micrometers and uneven depth of wear across wear facet. Profile view 46 shows [abiopalatal extent of wear (green, baseline; red, 2 years).

constant if the wear area is constant. Intuitively, the area of wear on teeth is expected to increase with time. From equation (2) this means that the change in depth (D) is expected to decrease in time; namely, the mean depth of the wear region should increase at a decreasing, not a constant, rate.

I f the preceding arguments are true, then the relative change in wear by volume between two consecutive years (A%) would be zero or near zero and the change by mean depth would be less than zero. The value turned out to be +22.7% (Table I) when measured by volume and - 13.0% when measured by mean depth (Table II) . Al- though there is not perfect agreement, the trends are supported. I t must be remembered that the preceding arguments apply to wear over all o f the teeth, whereas this study only considered the canines, second premolars, and first molars. Still, this cautions against the use of annualizing procedures and also indicates that there are important differences in wear when measured by volume and depth, which have much to do with the mean- ing of occlusion.

There was a modest reduction in the volume rate of

wear (A%) in canines in the second year (-14.0%) (Table I). However, there was a great increase in the annual volume wear rate (A%) for the premolars and molars (94% and 194%, respectively) at the second year. Clearl)% on the basis o f tooth type, the annual rate of wear when measured by volume is not constant, even for 2 consecutive years.

The most interesting differences in Tables I and I I lie in the comparisons between wear measured by volume loss and wear measured by mean depth on the same tooth. These differences are explained by changes in the area of contact during the wear process. When the area of contact increases, the wear measured by depth is less than that measured by volume, for the simple reason that the larger area requires a smaller depth to express the same volume.

As an example, during the first year, if 0.1 m m 3 of enamel was lost with a mean depth of 0.010 mm, then the area o f wear is I0 .0 m m 2. I f another 0.1 m m 3 of enamel was lost during the second year and the area of wear covered 20.0 m m 2, then the change in the mean depth during the second year would be 0.005 mm. Thus

MARCH 1997 317


Fig. 2. Occlusal view of mandibular right second premolar. Gray-scaled computer image represents baseline data, and color overlay represents wear facet at 2 years from baseline. Uneven wear is noted across buccal wear facet, and coronal profile shows goodness-of-fit and wear facet at 2 years.

wear measured by volume increased 100% in the second year, whereas wear measured by mean depth increased only 50%; or by use of the above terminology, A% for volume is zero and A% for depth is -50%. The wear measured by volume is perceived to be occurring at a much faster rate than wear measured by depth. This is a simple requirement of geometry or, to put it in clinical terms, a matter o f simple anatomy.

I f the area of contact is reduced during the second interval, then the converse is true and the wear measured by volume is less than that measured by depth. Intuitively, the wear area is expected to always increase with time, at least until the tooth has worn so severely that it has worn beyond its heights o f contour so that the cross-sectional area is now decreasing. However, the area of wear can decrease with substantially less than catastrophic wear on the tooth. I t is dangerous to focus only on one tooth or one area when evaluating wear. Wear is a phenomenon under control o f the masticatory system, and all the teeth must be considered. As one

area wears, it changes the relationship of the teeth, bring- ing other areas into contact. These other contacting areas can alter the functional path and thus the wear area on the tooth under consideration. Figures 1 and 2 illus- trate this phenomenon and demonstrate that not all the wear facets were actively wearing at the same rate and that the area of wear became less with time. This is a salutary lesson to those engaged with clinical occlusal studies not to assume that the entire visible wear facet is actively losing contour. A wear facet can express uneven activity, reduced activity, or become completely inactive.

Some of these differences between wear measured by volume and by depth are highlighted in Table II. Here the percentage of change (A%) by depth was negative in almost every case (except the molar), although we know that the volume loss as a result o f wear was increasing. The answer to this apparent discrepancy has already been alluded to and lies in the fact that as the volume wear was increasing the area of contact also was increasing, leading to a smaller increase in loss from depth alone. These are



important features that need to be incorporated into a common understanding of the clinical wear of teeth.

Wear measured by volume loss is perhaps best regarded as a material property of enamel. Any volume can be expressed by an infinite number of area x depth combi- nations. Thus volume is unconstrained geometrically and may assume any shape, whereas wear reported as depth is a morphologic parameter because the facet is described in one direction. As a result, depth measurements alone cannot describe the total amount of wear that the tooth has undergone, although generalized wear expressed as depth may be an indicator o f loss of facial height. For these reasons, reporting wear by volume and by mean depth is particularly useful because it describes the entire process o f occlusal wear.

Hal f the subjects ( 9 /18 ) in this study were classified as bruxers by the stated criteria. Although reports of the prevalence o f bruxism vary greatly, the 50% prevalence identified in this sample does not seem unreasonable in a group of dentally aware young adults.

The statistical results for wear at the end of 1 year are shown by volume and by mean depth in Tables III and IV. By volume, no statistical differences were noted in any of the categories. This is the consequence of relatively low wear over 1 year and a high biologic variation. As far as simple effects are concerned, there was a weak significance between the higher wear rate of the canine over the molar (p = 0.080) and the canine over the premolar teeth (p = 0.068). By depth o f wear after 1 year, the canines showed significantly more change than the molars and premolars (p = 0.007 and p = 0.051, respectively; Table IV) because o f the smaller variation with depth measurements. There was no difference on the basis of either gender or bruxism after 1 year of wear.

After 2 years of cumulative wear the greater loss by volume and by depth of the canines over the molar and premolar teeth show stronger significance (Tables V and VI). Also at 2 years the difference between bruxers and nonbruxers is now significant by volume (p = 0.048). However, when the same wear data are analyzed by depth, certain similarities and differences emerge (Table VI). The higher wear rates of the canines over molars and premolars are even more marked (p = 0.020 and p = 0.013, respectively). However, the differences between bruxers and nonbruxers in depth of wear are not significant (p = 0.190) (Table VI). This is a reconfirma- tion o f the essential difference between wear loss as measured by depth and that measured by volume. The significant difference in volume of wear loss between bruxers and nonbruxers in Table V was accompanied by a large increase in the surface area o f wear facets. When this large area was divided into the volume increase, the results indicated a relatively minor increase in depth of wear, which did not reach significant levels. Thus volume is more sensitive than depth when wear is assessed.

Regarding the investigation o f the impact o f bruxism

on tooth wear in this study, it must be acknowledged that the criteria used for the diagnosis ofbruxism, which included clinical diagnosis through tooth wear, is still subject to controversy and discussion. 16,23 However, the addition o f quantifiable measures of tooth wear, as of- fered in this study, may significantly improve current diagnostic criteria for bruxism.

Although the means for males were higher after 2 )rears of cumulative wear, the difference when measured by depth was still not quite significant on the basis of gender (p = 0.078) (Table VI). On the basis o f the histo- logic features of the hard tissues alone, there is probably no reason to suppose any difference in the inherent wear rates o f females and males. However, the greater upper body strength of males may be consistent with a greater strength of the muscles of mastication and a greater masticatory force, 3~,32 which would be consistent with a greater rate o f enamel wear. But, as noted, the higher mean wear rates for males in this study did not reach significant levels. This may be more o f a problem with the high rate of variation.

C O N C L U S I O N S

There were significant differences between wear measured by volume and wear measured by mean depth over a 2-year period in a population of 18 young adults. A steady wear rate of 0.04 mm 3 per year by volume and 10 }am per year by depth, averaged over all teeth, was experienced. However, there were important differences on the basis o f anatomic position in the arch, with the canines showing the most wear. When measured by volume, the greater wear of bruxers over nonbruxers became significant after 2 years. There was no significant difference in tooth wear on the basis of gender in this young, but small, population.

It was noted that high-wearing enamel couples wore out of trouble, as evidenced by the decreasing loss when measured by mean dcpth. This may indicate that facial height is highly conserved, even in a young population with a high wear rate. In high-wearing situations a large increase in the area of contact was observed. This increase in area is consistent with a loss of contour and progress toward a fiat cuspal morphologic structure. Therefore it might be said that, although facial height is conserved, the price to be paid is loss ofcuspal structure in high-wearing occlusions. In a progressive situation, when all cuspal structure is gone and the teeth are relatively fiat, wear loss by volume and depth will be more closely correlated again and facial height will be more quickly lost. This is the situation typical of older age, and it will be exacerbated if the enamel is breached and occlusal dentin is exposed.

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27. Pintado MR, Conry JP, Douglas WH. Measurement of sealant volume in vivo using image~processing technology. Quintessence Int 1988;19:613- 7.

28. Hewlett ER, Orro ME, Clark GT. Accuracy testing of three-dimensional digitizing systems. Dent Mater 1992;8:4943.

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31. Tate GS, Throckmorton GS, El]is E Ill, Sinn DP, Blackwood DJ. Estimated masticatory forces in patients before orthognathic surgery. J Oral Maxillofac Surg 1994;52:130-6.

32. Tate GS, Throckmorton GS, Ellis E Ill, Sinn DP. Masticatory performance, muscle activity, and occlusal force in preorthognathic surgery patients. J Oral Maxil[ofac Surg 1994;52:476-81.

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New produc t news

The January and July issues of the Journal carry information regarding new products of inter- est to prosthodontists. Product information should be sent l month prior to ad closing date to: Dr. Glen P. McGivney, Editor, SUNY at Buffalo, School of Dental Medicine, 345 Squire Hall, Buffalo, NY 14214. Product information may be accepted in whole or in part at the discretion of the Editor and is subject to editing. A black-and-white glossy photo may be submitted to accom- pan3; product information.

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Variation in Tooth Wear in Young Adults Over a Two-year Period Issue 3

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