Clinical Study on the Efficacy of 35% Hydrogen Peroxide Gel According to Exposure Time (40 min vs. 20 min) by Spectrophotometry

Objective To analyze the chromatic effect on CIELAB coordinates and the clinical effectiveness of in‐office whitening treatment (perceptible changes) with 35% hydrogen peroxide (HP) gel according to the number of clinical sessions and exposure time (40 min vs. 20 min) in the three thirds of the tooth. Materials and Methods Seventy‐four participants were assigned to two groups: Group 1 ( n = 37) received 40‐min applications, and Group 2 ( n = 37) received 20‐min applications, both undergoing two in‐office bleaching sessions with 35% HP, with a 1‐week interval. CIELAB coordinates (L*, a*, b*) were recorded in the three thirds using the Cobra‐Raypicker RP‐0201 spectrophotometry at baseline (T0), after the first (T1), and the second session (T2). Separate two‐way ANOVAs were performed for each color coordinate (L*, a*, b*) at each tooth third, and color differences (Δ E ab , Δ E 00 ) were calculated and compared between exposure groups with the Mann–Whitney U test and between sessions with the Wilcoxon signed‐rank test. Results Significant increases in L* and decreases in a* and b* were observed in all thirds after bleaching, with greater changes after two sessions, with the largest changes in b and the smallest in a *. No significant differences were observed between Group 1 and Group 2. The greatest change was in b*, with large effect sizes, while a* showed the smallest variation. After one session, perceptible color change (Δ E ab and Δ E 00 ) occurred in over 80% of participants in Group 1 and 75% in Group 2, increasing to nearly 100% in both groups after two sessions. Conclusion The 35% HP treatment significantly alters the color coordinates of the three thirds of the tooth, with perceptible color changes in most patients. The treatment is more effective with two sessions than with one, but its effectiveness does not change when reducing the 35% HP exposure time from 40 to 20 min per session. The largest chromatic change in all thirds is the reduction in yellowness (b*), while lightness (L*) increases and redness–greenness (a*) shows smaller changes. Clinical Significance This study suggests that reducing application time to 20 min with 35% HP in‐office is sufficient to achieve clinically perceptible color change while optimizing chair time and potentially reducing tooth sensitivity risk. The greatest effect was observed in the reduction of yellowness (b*), suggesting that teeth with more yellowish tones may show greater whitening benefit.