Time to saturation point in tooth-bleaching protocols

Background The commonly described saturation bleaching point, discussed in textbooks and scientific articles, has not been objectively described in the literature. This in vitro study aimed to characterize the color change pattern over time and determine the time to clinically perceptible stabilization (saturation point) for bleaching. Methods Sixty extracted human molars were randomly assigned to in-office bleaching (IOB) (35% hydrogen peroxide, 50 minutes per session) or at-home bleaching (AHB) (10% carbamide peroxide, 3 hours daily) for 8 cycles (sessions for IOB or weeks for AHB). Tooth color was assessed by spectrophotometry (change in CIELAB color difference [ΔE ab ], change in CIEDE2000 color difference [ΔE 00 ], change in whiteness index for dentistry [ΔWI D ]) and change in shade guide units (ΔSGUs). Median time to stabilization was defined as the first cycle after which intercycle changes remained below the perceptibility threshold (PT) and acceptability threshold (AT), confirmed in a subsequent cycle. Color stabilization time was assessed by Kaplan-Meier analysis with log-rank testing. Adjusted Cox and Deming regressions were used to estimate hazard ratios and protocol conversion factors, respectively. Results Using PT, stabilization was reached in 13 of 30 specimens (AHB) and 11 of 30 specimens (IOB) for CIELAB color difference, and median time was not reached. Similar findings were observed for ΔE 00 and ΔWI D . ΔSGUs stabilized in all specimens (median time to stabilization, 3 or 4 cycles). Using AT, most specimens reached stabilization (median time to stabilization, 5-7 cycles), with no significant difference between protocols. A moderate association was observed ( R 2 = 0.59), with 1 IOB session producing approximately 0.85 of the color change achieved by 1 AHB week. Conclusions Color stabilization was threshold dependent, with no difference between protocols. Most specimens stabilized after 5-7 cycles under AT and after 3-4 cycles under PT, using ΔSGUs. On average, 1 IOB session produced 0.85 times the color change observed after 1 AHB week. Practical Implications Decisions on bleaching duration should be guided by visible bleaching response rather than by a fixed number of cycles.