5425, E-Mail flippert@iu.edu 2 Objectives: To investigate the enamel and dentin surface loss caused by the interaction between abrasives in toothpaste and toothbrush filament stiffness. Methods:The study followed a 2 (high/low-level abrasive; silica) × 3 (filament stiffness; soft/medium/hard) × 2 (cycling time; 3/5d) factorial design. Polished bovine enamel and dentin specimens (n=8 each per group) were subjected to 5d of erosion/abrasion cycling: erosion (5min, 4×/d, 0.3% citric acid, pH 3.75), abrasion (15s, 2×/d, 45 strokes each, 150g load, automated brushing machine), fluoride treatment (15s with abrasion and 45s without abrasion; 275ppm F as NaF in abrasive slurry) with exposure to artificial saliva between erosion and abrasion/F exposure (1h) and all other times (overnight).Non-contact profilometry was used to determine surface loss (SL) after 3 and 5d of cycling. Data were analyzed using three-way ANOVA (factors: abrasive/filament stiffness/time) with separate analyses conducted for enamel and dentin.Results: For enamel, only 'cycling time' was found to affect surface loss with 5>3d. Overall, there was little SL (mean range: 0.76-1.85µm). For dentin (mean SL range: 1.87-5.91µm), significantly higher SL was found for 5 vs. 3d, with particularly large differences for hard stiffness/high-level abrasive, and medium stiffness/medium abrasive. Hard stiffness resulted in significantly higher SL than medium stiffness for high abrasive after 5d, with no other significant stiffness differences. High abrasive had significantly higher SL than medium abrasive overall with strong effects for all combinations, except medium stiffness after 5d. Conclusion:The interplay between abrasivity and filament stiffness appears to be more relevant for dentin than enamel.3