Introduction 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 26 27 9 10 1 Table 1 10 Class Surface Criteria Class I   Early stages of erosion, absence of developmental ridges, smooth, glazed surface occurring mainly on labial surfaces of maxillary incisors and canines Class II Facial 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 Class IIIa Facial 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 Class IIIb 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 Class IIIc 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 Class IIId All Severely affected teeth, where both labial and lingual surfaces are extensively involved. Proximal surfaces may be affected; teeth are shortened 33 The clinical measurement of tooth wear 4 30 30 2 Table 2 30 Score Surface Criteria 0 B/L/O/I No loss of enamel surface characteristics C No loss of contour 1 B/L/O/I Loss of enamel surface characteristics C Minimal loss of contour 2 B/L/O Loss of enamel exposing dentine for less than one third of surface I Loss of enamel just exposing dentine C Defect less than 1 mm deep 3 B/L/O Loss of enamel exposing dentine for more than one third of surface I Loss of enamel and substantial loss of dentine C Defect less than 1–2 mm deep 4 B/L/O Complete enamel loss–pulp exposure–secondary dentine exposure I Pulp exposure or exposure of secondary dentine C 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. 3 20 25 27 1 21 18 31 7 8 Development of indices 2 14 15 17 23 10 30 15 17 3 Table 3 16 17 Surface Score Criteria Facial 0 No erosion. Surface with a smooth, silky glazed appearance, possible absence of developmental ridges 1 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 2 Involvement of dentine for less than half of tooth surface 3 Involvement of dentine for more than half of tooth surface Occlusal/lingual 0 No erosion. Surface with a smooth, silky glazed appearance, possible absence of developmental ridges 1 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 2 Severe erosions, more pronounced signs than in grade 1. Dentine is involved 23 22 2 30 4 Table 4 2 Score Criteria 0 No wear into dentine 1 Dentine just visible (including cupping) or dentine exposed for less than 1/3 of surface 2 Dentine exposure greater than 1/3 of surface 3 Exposure of pulp or secondary dentine 12 12 24 28 6 13 5 11 29 32 14 30 Conclusion 19 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.