The interval nature of an ordinal scale for measuring the marginal fracture of amalgam

The clinical performance of amalgam alloys over time has been assessed by measuring the extent of marginal fracture of restorations made from these alloys. Scales of photographs of restorations exhibiting varying degrees of marginal fracture have been used to make these assessments. The purpose of this study was to determine the relationship between the ordinal scale numbers of five photographs composing a commonly used scale (Mahler and Marantz, 1979) and the average width in micrometers of marginal fracture of the restorations in the photographs of this scale. In addition, a comparison was made between parametric and non-parametric statistical methods when applied to marginal fracture data. The results showed that four of the five photographs of this scale demonstrated a significant linear regression with marginal fracture width (R² = 0.997; p = 0.002). The last photograph of the scale, which proved to be an outlier, was not used in the regression and was accommodated by an extrapolation procedure. Using previously gathered clinical data on the marginal fracture behaviour of five amalgam alloys, the use of parametric statistical procedures (ANOVA and Scheffé's multiple comparison test) proved to be more discriminatory than the use of non-parametric parametric procedures (Kruskal-Wallis and Dunn's Multiple comparison test) when tested at the same overall confidence level. Thus, having a photographic scale of an interval nature removes any doubt using the more powerful technique of parametric statistics to evaluate the marginal fracture behavior of dental amalgams. In addition, based on these data in the form of marginal fracture widths and Maryniuk's (1990) survey of dentists' replacement decisions, a restoration having a marginal fracture width of approximately 100 μm or more, is at risk of being replaced.