Objective: Numerous studies using various evaluation techniques have shown substantial variations in the degree of bristle end-roundness of commercially available toothbrushes. The aim of the present study was to compare the effects of the examination angle, the bristle selection and two analyzing techniques assessing bristle end-rounding measurements.
Methodology: The study was conducted in two parts. Part One used five randomly selected tufts from 20 brushes that were scanned with an electron micrograph (SEM; 45x) at two viewing angles (45 degrees and 90 degrees). Those bristle tips that were visible on both viewing angles were then judged by 1) a direct comparison to a grading scale, and 2) a shape factor (SF) analysis. In Part Two of the study, SEM images of five bristles from different tufts obtained from 40 brushes were taken at a viewing angle of 45 degrees, and five bristles from different locations within a tuft were also judged by both the direct comparison to a grading scale method, and the SF analysis, to assess bristle location.
Results: The SF values and the direct comparison percentage of rounded bristles did not differ because of the viewing angle (45 degrees or 90 degrees; p > 0.05; Mann-Whitney test), but differed significantly at various SF thresholds (percentage of bristles above or below a certain degree of end-roundness) when bristles from Part One were compared to Part Two (p < 0.01, adjusted chi-square-test). Location of the bristles from either the edge of a tuft or those located in the inner part had no effect on the assessments (p > 0.05). The results from the subjective direct comparison grading did not differ from those found with the SF analysis (p > 0.05).
Conclusion: SF analysis is a time-consuming method for assessing bristle end-rounding, and can be as accurately done by direct comparison to a grading scale. One viewing angle (45 degrees) of bristles from different locations within a tuft can also be used to accurately assess a brush's level of end-roundness.