Variability and Robustness in Enamel Hardness Testing: a Scoping Review

Objectives: This review examines the strength of experimental designs in studies evaluating enamel surface hardness in de- and remineralization. We focus on key methodological factors such as randomization, blinding, sample size, and calibration, as well as the variability in enamel surface hardness, considering factors like sample preparation.
Methods: Literature from 1988-2023 was searched using the Medline, EMBASE, and Scopus databases to identify relevant studies in English. Out of an initial 16,193 studies identified, 325 were selected for full-text review, and 54 studies were ultimately included for data extraction. The review process adhered to Arksey and O’Malley’s five-stage framework.
Results: We found significant differences in methodologies used across studies, particularly in the preparation and testing of enamel samples. Sample type varied, with square slabs being the most common (55.6%), followed by half (16.7%) and whole (16.7%) crowns. Third molars were most frequently used (44.4%). Saw machines were commonly employed for cutting (51.9%), and polishing was often done using handpieces (42.6%). Surface hardness was primarily measured using Vickers indenters (81.5%), with loads of 100 to 200 grams applied for 10 to 15 seconds. Demineralization was most frequently performed using pH cycling models (68.5%). We also identified an increase in the number of these types of studies over the last two decades with most studies coming from India, USA, and Brazil. Several methodological weaknesses were noted, including low reporting rates for randomization (5.6%) and blinding (16.7%), raising concerns about potential bias. Furthermore, 79.6% of the studies did not report a sample size calculation.
Conclusions: This review describes the variability and methodological shortcomings in studies assessing enamel surface hardness in de- and remineralization research. It emphasizes the need for more transparency, standardization, and rigor in areas such as randomization, blinding, calibration, and sample size calculations to enhance the reliability and reproducibility of future research.