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Several erosion indices have been developed based on the Eccles (1979) erosion index and Smith and Knight’s (1984b) tooth wear index. These include the Linkosalo and Markannen (1985) index, Lussi et. al’s (1991) index, O’Brien’s (1994) index, O’Sullivan’s (2000) index and Larsen et. al’s (2000) index. These indices aim to monitor erosion by ascribing a numerical score to affected teeth surfaces, corresponding to the severity of erosive TSL. The indices can also be used in vitro on study models as well as in vivo.

Interestingly, Wetselaar et al. (2009) assessed the reliability of a newly developed TSL scoring index to detect TSL both in vivo and on study models in vitro. They found that using this scoring system in vivo resulted in more reliable TSL scores, especially for buccal and palatal surfaces, compared to the in vitro study model counterpart. On the other hand, they reported occlusal and incisal TSL assessment of

study models had a reliability of ‘fair to good’ and ‘excellent’. The authors of this study mentioned the advantages of this scoring index over traditional indices in that it measures TSL in easy-to-use small steps and allows for measuring more extensive TSL levels.

Because there is “ o agreed co se sus o a u iversally accepted tooth wear i dex” (Bartlett and Dugmore, 2008), an attempt was made to use a standardised and reproducible index that is easily used by GDPs (Young et al., 2008). Thus, the basic erosive wear examination (BEWE) index was proposed (Bartlett et al., 2008). However, as with other indices, it is not able to detect small changes of 17.6 - 108.2 µm/6 months caused by the erosion (Bartlett et al., 1997). These conventional indices are only beneficial for epidemiological studies because they measure erosion in a crude way that is rarely reproducible or reliable, rendering them subjective, inaccurate and variable across dentists (Hall et al., 1997; Azzopardi et al., 2000). Therefore, dentists monitoring erosive TSL using such indices should bear in mind the problem of intra-examiner reproducibility and remember the precise diagnostic criteria for the index used (O'Sullivan and Milosevic, 2008).

There are currently six non-destructive methods in research and development for use

in vivo to directly monitor erosive TSL. These methods are QLF (Pretty et al., 2004),

OCT (Wilder-Smith et al., 2009), spectroradiometry (Krikken et al., 2008) (measuring light reflectance from enamel), reflectance confocal microscopy (RCM) (Contaldo et al., 2013), computer automated design-computer automated machinery (CAD-CAM) and ultrasound (Fukukita et al., 1985; Huysmans and Thijssen, 2000; Louwerse et al., 2004; Bozkurt et al., 2005; Tagtekin et al., 2005; Toda et al., 2005;

Harput et al., 2009; Hua et al., 2009; Hughes et al., 2009; Dwyer-Joyce et al., 2010; Harput et al., 2011)

Some of these methods were able to detect the demineralised superficial enamel layer, such as QLF (Field et al., 2010), while other methods were able to measure the crater depth (e.g. ultrasound) relative to a stable reference in vitro. These non-destructive methods may offer a solution for measuring and monitoring the enamel layer chair-side.

Quantitative light-induced fluorescence is an optical method to detect early caries that measures the difference in fluorescence between sound and unsound teeth via a hand-held probe (de Josselin de Jong et al., 1995). Quantitative light-induced fluorescence has been investigated in vitro to detect and quantify artificially-induced erosive lesions with good results (Pretty et al., 2004). A drawback with this method is that it requires an intact reference area to compare it to the relative loss in fluorescence from the eroded surface (Huysmans et al., 2011), which rarely occurs in erosive TSL. In addition, it shows a ‘trend’ of fluorescence from teeth and does not yield an enamel layer thickness (Field et al., 2010). Thus, further research is required to assess its feasibility in a clinical environment and to validate its use in vivo to know if it is able to quantify and monitor erosive TSL reliably.

Another interesting in vivo study on GORD patients was completed by Wilder-Smith and co-workers (2009) in which they were able to measure a decrease in enamel thickness of 15 ±0.17 µm using an OCT scanner. The OCT device was able to detect enamel erosion in GORD patients taking omeprazole or a placebo (Wilder-Smith et

impression material that had 3 mm holes drilled into it (where the OCT probe was placed) for optimal positioning.

Before the enamel thickness scans were performed on each patient, the stent was placed inside the patient’s mouth to accurately position the OCT probe on the scan site. Again, this is time consuming and will need to be remade if a patient receives a new restoration (cast or direct), as this alters the seating of the stent. On the other hand, spectroradiometry measures the relative ‘yellowish’ colour of enamel that emanates from underlying dentine (Krikken et al., 2008). This requires the superficial enamel layer to wear away to some extent until a ‘difference’ in colour reflectance can be detected. In addition, the spectroradiometer was bulky and cumbersome to manoeuvre around teeth.

Contaldo and co-workers (2013) investigated a hand-held RCM device, originally used in dermatology, to image the surface and subsurface topography of enamel in

vivo. However, the device could not image more than 300 µm into enamel and was

bulky, which did not allow for imaging teeth other than central incisors. Further research and development is required to render RCM a clinically viable approach to monitor erosive TSL.

The CAD-CAM approach has developed at a fast pace bringing with it refinements in technology where in vivo 3D images of teeth are now replacing conventional study models. In fact, DeLong (2006) argues that the best method for accurately measuring wear of any material in vitro and in vivo is sequential 3D imaging. The 3D images (e.g. the baseline image and the image after 6 months) are then superimposed on each other and the ‘difference’ is calculated by a process known as image registration (DeLong, 2006). Al-Omiri et al. (2010) compared the accuracy of a new CAD-CAM

laser scanning machine against a toolmaker’s microscope and Smith and Knight’s index in detecting TSL over a six-month period. They found that Smith and Knight’s index was the least sensitive in quantifying TSL and was not able to monitor progression in most cases. However, CAD-CAM scanners are too cumbersome for routine use in a dental setting and are costly (Al-Omiri et al., 2010). These scanners also require further development to render them fully capable of monitoring erosive TSL (Huysmans et al., 2011).