The evolution of tooth wear indices
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Tooth wear—attrition, erosion and abrasion—is perceived internationally as an ever-increasing problem. Clinical and epidemiological studies, however, are difficult to interpret and compare due to differences in terminology and the large number of indices that have been developed for diagnosing, grading and monitoring dental hard tissue loss. These indices have been designed to identify increasing severity and are usually numerical. Some record lesions on an aetiological basis (e.g. erosion indices), others record lesions irrespective of aetiology (tooth wear indices); none have universal acceptance, complicating the evaluation of the true increase in prevalence reported. This article considers the ideal requirements for an erosion index. It reviews the literature to consider how current indices have evolved and discusses if these indices meet the clinical and research needs of the dental profession.
KeywordsTooth wear Tooth wear indices
There is both a clinical and a scientific need to be able to measure tooth wear, and the literature abounds with many methods which can be broadly divided into quantitative and qualitative in nature. Quantitative methods tend to rely on objective physical measurements, such as depth of groove, area of facet or height of crown. Qualitative methods, which rely on clinical descriptions, can be more subjective if appropriate training and calibration are not carried out but which, with correct safeguards, can be valuable epidemiological tools. In a clinical intra-oral examination, there will be an inclination towards descriptive assessment measures, such as mild, moderate or severe, rather than quantitative measurement, which is easier to perform reliably on a model or in the laboratory. Such methods tend to be more sensitive but do not lend themselves readily to clinical use—especially in epidemiology, where fieldwork data collection is often carried out in an environment lacking sophisticated equipment.
Quantitative and qualitative methods typically utilise grading or scoring systems designed to identify increasing severity or progression of a condition; these are described as indices and are usually numerical. An ideal index should be simple to understand and use, clear in its scoring criteria and be demonstrably reproducible. Its application should be useful for research into the aetiology, prevention and monitoring of a condition, essentially being an epidemiological and clinical tool.
Review of the literature reveals the fact that many different tooth wear indices have been developed for clinical and laboratory use all over the world. Unfortunately, the production of so many indices does not allow for ready comparison of results between different working groups, and this is especially important in epidemiology when trying to define the prevalence of a condition. Confusion is further generated in the literature as the majority of researchers, in their attempts to quantify the amount of tooth tissue loss due to tooth wear, have historically concentrated on one aetiology only, and these indices tend to be surface limited. Often, the wear patterns described do not appear to reflect the aetiology suggested, and this relates to lack of uniformity with tooth wear terminology and translation errors. Many diagnostic indices do not properly reflect the morphological defects, and there is little international standardisation. All of these factors complicate the comparison of data and evaluation of the efficacy of preventive and therapeutic measures.
The literature identifies separate indices for use in clinical and laboratory situations and specific indices for attrition, abrasion, erosion and multifactorial tooth wear. There are common threads to all of the indices, such as descriptive diagnostic criteria and criteria for quantifying the amount of hard tissue loss. These generally consider the size of the affected area—as a proportion of a sound surface and/or the depth of tissue loss—often expressed as a degree of dentine exposure.
The clinical measurement of erosion
The earliest indices shared common, arbitrary criteria, relying on descriptive terms such as slight, mild, moderate, severe and extensive. Restarski et al.  developed a six-point grading system to evaluate the severity of erosive destruction observed on the lingual surfaces of rat and puppy molars, but concerns were raised with regards to reproducibility. With vague criteria definitions, variability in recording is expected. Each animal was allocated a total score, calculated by summing the mean molar quadrant scores. Whilst producing simple data for analysis, it is acknowledged that averaging scores in this manner leads to the loss of much data. If the number of teeth severely affected is small, the erosion score will be low; but this could mask a significant, localised clinical problem .
Eccles index for dental erosion of non-industrial origin 
Early stages of erosion, absence of developmental ridges, smooth, glazed surface occurring mainly on labial surfaces of maxillary incisors and canines
Dentine involved for less than one third surface; two types
Type 1 (commonest): ovoid–crescentic in outline, concave in cross section at cervical region of surface. Must differentiate from wedge shaped abrasion lesions
Type 2: irregular lesion entirely within crown. Punched out appearance, where enamel is absent from floor
More extensive destruction of dentine, affecting anterior teeth particularly. Majority of lesions affect a large part of the surface, but some are localised and hollowed out
Lingual or palatal
Dentine eroded for more than one third of the surface area. Gingival and proximal enamel margins have white, etched appearance. Incisal edges translucent due to loss of dentine. Dentine is smooth and anteriorly is flat or hollowed out, often extending into secondary dentine
Incisal or occlusal
Surfaces involved into dentine, appearing flattened or with cupping. Incisal edges appear translucent due to undermined enamel; restorations are raised above surrounding tooth surface
Severely affected teeth, where both labial and lingual surfaces are extensively involved. Proximal surfaces may be affected; teeth are shortened
Greater accuracy was introduced by Xhonga and Valdmanis  who divided erosions into four levels by measurement with a periodontal probe: none, minor (less than 2 mm), moderate (up to 3 mm) and severe (greater than 3 mm). They further differentiated types of erosion by morphological descriptions, such as wedge, saucer, groove and atypical. They did not address the problem of inter- or intra-examiner variability.
The clinical measurement of tooth wear
It is perhaps significant that the earliest index documented by Broca  and used as a foundation for the development of further indices graded horizontal or oblique patterns of occlusal wear without presupposing the aetiology. Smith and Knight  introduced the more general concept of measuring tooth wear per se, irrespective of the cause, and since then more recent indices have been developed or modified from Smith and Knight that do not rely on a prior diagnosis and are more clinically relevant. Most of these stress the importance of user training sessions and calibration exercises.
Smith and Knight tooth wear index 
No loss of enamel surface characteristics
No loss of contour
Loss of enamel surface characteristics
Minimal loss of contour
Loss of enamel exposing dentine for less than one third of surface
Loss of enamel just exposing dentine
Defect less than 1 mm deep
Loss of enamel exposing dentine for more than one third of surface
Loss of enamel and substantial loss of dentine
Defect less than 1–2 mm deep
Complete enamel loss–pulp exposure–secondary dentine exposure
Pulp exposure or exposure of secondary dentine
Defect more than 2 mm deep–pulp exposure–secondary dentine exposure
This index was the first one designed to measure and monitor multifactorial tooth wear; a further pioneering feature was the ability to distinguish acceptable and pathological levels of wear, by comparison with threshold normal values for the age groups studied. Tooth wear was defined as pathological if the teeth became so worn that they do not function effectively or seriously mar the appearance—before they are lost through other causes—or the patient dies. Results from inter- and intra-reproducibility exercises were within a range regarded as acceptable for epidemiological purposes, and the index appears simple to use clinically—intra-orally or from models and photographs. However, some problems have been identified with the TWI, including the time necessary to apply to a whole dentition, amount of data generated and the comparisons with threshold levels for each age group; the thresholds proposed were high, erring towards understatement rather than exaggerations of pathological wear. Full use of the index as a research tool is not feasible without computer assistance.
A sign of professional confidence in this index is its adopted use by a number of different investigators in the UK looking at tooth wear prevalence and severity [3, 20, 25, 27], aetiology and risk [1, 21]. Other researchers have used it with modifications pertaining to the particular age group being studied. Millward et al.  made adjustments to study erosion in the primary and secondary dentitions, excluding cervical surfaces. Subjects were then grouped together to produce three classifications: no or mild erosion—no score >1, moderate—at least one tooth in dentition score 2, severe—at least one tooth in dentition score 3 or 4.
In a survey of elderly people, Steele et al.  combined low wear scores representing small losses of enamel and used the worst surface score per tooth as an overall tooth score. Donachie and Walls [7, 8] outlined various flaws in the tooth wear index as an epidemiological tool in the ageing population and suggested a need to increase the sensitivity of TWI at extremes of tooth wear, to take account of the capacity of the elderly to have adequate function in cases of significant wear. They suggested altering threshold values, amplifying scoring criteria and creating a sixth point to distinguish between exposure of secondary dentine and frank pulp exposure.
Development of indices
No erosion. Surface with a smooth, silky glazed appearance, possible absence of developmental ridges
Loss of surface enamel. Intact enamel cervical to the erosive lesion; concavity on enamel where breadth clearly exceeds depth, thus distinguishing it from toothbrush abrasion. Undulating borders of the lesion are possible and dentine is not involved
Involvement of dentine for less than half of tooth surface
Involvement of dentine for more than half of tooth surface
No erosion. Surface with a smooth, silky glazed appearance, possible absence of developmental ridges
Slight erosion, rounded cusps, edges of restorations rising above the level of adjacent tooth surface, grooves on occlusal aspects. Loss of surface enamel. Dentine is not involved
Severe erosions, more pronounced signs than in grade 1. Dentine is involved
O’Sullivan  proposed a new index for the measurement of erosion specifically in children. The index was qualitative with a broad attempt at quantification noting whether less or more than half of the surface was affected. Every tooth was examined and assigned a three-digit score relating to the site of erosion, severity (grade 0–5) and area of surface affected. O’Brien  reported the use of a partial recording system for measuring erosion in children in UK children’s dental health surveys, where only the facial and lingual surfaces of the primary and permanent maxillary incisor teeth were scored for erosion. Again, the criteria were mostly qualitative and descriptive, with a broad attempt made to quantify the area involved.
Simplified scoring criteria for TWI 
No wear into dentine
Dentine just visible (including cupping) or dentine exposed for less than 1/3 of surface
Dentine exposure greater than 1/3 of surface
Exposure of pulp or secondary dentine
However, despite calibration and training, difficulties were experienced diagnosing dentine exposure in the epidemiological field and there is some debate as to the significance of dentinal cupping when exposed dentine does not relate to significant amounts of tissue loss . In a recent study of Ganss et al. , teeth were visually and histologically examined for presence of exposed dentine, and the correlation in accuracy between the two examinations was poor. The diagnosis of exposed dentine is obviously important but may not be accurate from visual examination alone.
Oilo et al.  concentrated on a different type of scoring system, with criteria based on treatment need. They criticised the use of indices that used a nonlinear scoring method, claiming calculated mean wear scores can be misleading. Their index was based on Ryge and Snyder’s  system for evaluating the quality of restorations and had five categories divided into two broad camps; Romeo, Sierra and Mike were satisfactory, whilst Tango and Victor indicated unacceptable levels of wear requiring treatment. All groups except Romeo were subdivided according to degree of dentine exposed and clinical findings such as pain, sensitivity and fracture of restorations, giving the impression of a cumbersome system. Dahl et al.  modified it with the introduction of even more categories, with an aim to establish subjective dental criteria for present and future evaluations of tooth wear and the need for treatment. In practice, these indices require experience for reliable use; individuals with differing clinical backgrounds will not get consistent, objective results.
Indices that attempt to visualise, measure and monitor the amount of worn enamel or exposed dentine by difficult direct clinical interpretation  and indirectly on accurate, serial study casts which must be retained [5, 11, 29, 32] have been described. Larsen et al.  recommended a new clinical index based on a combination of clinical examination, photographs and study casts, with complicated qualitative and quantitative criteria. Plaque-free teeth were clinically examined and photographed prior to taking silicone impressions for epoxy resin casts. They considered clinical and photographic data to be supplemental with final wear classification based on visual inspection of casts at ×10 magnification. Each tooth surface was scored, with six grades of erosion severity modelled from Smith and Knight ; the index and its criteria are complicated and time consuming.
Review of the literature on indices for tooth wear (or erosion) is confusing; there are too many indices proposed and used, with lack of standardisation in terminology. There are many epidemiological studies reported, but it is difficult to quantify the increases in prevalence reported internationally, as results are not easily comparable. It is doubtful whether any of the indices described are sensitive enough to monitor all but the most severe changes in tooth wear clinically and these cannot be used to measure a rate of wear . It is a challenge to try to develop a simple index that can be used clinically to assess progression of wear.
To date, there is not one ideal index that can be used for epidemiological prevalence studies, clinical staging and monitoring, and it may be necessary to accept that one simple index does not yet exist to meet all requirements of both clinical and research teams. There should, however, be an aim for indices that can be relevant to both fields and can be used internationally in order to strengthen knowledge of dental erosion.
Conflict of interest statement
The author declares that she has no conflict of interest.
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