## 4 Discussion  
The use of a spectrophotometer to evaluate the color changes that are generated in a tooth when it is being bleached is an objective method that allows us to establish numerically the degree of whitening achieved in a tooth at a given moment of treatment, and allows us to follow the evolution of the color changes that occur [16; 17; 21]. The systematic review by [PERSON] et al. provides much evidence for the similar color modifications achieved by in-office tooth whitening effects, whitening realized by patients at home and combinations of the two [5]. Other research suggests that combined tooth whitening treatments reduce the total treatment time needed to achieve the desired results in comparison with home whitening alone [1; 2]. In this way, the time of contact between the bleaching products used and the patient's oral tissues used in self-application by patients is shortened [6]. Treatments of shorter duration are always preferable, making combined treatments more convenient for patients [7]. At the same time, a reduction in the number of visits to the clinic will reduce the cost of whitening treatment carried exclusively in-office [8; 9]. Regarding the existing literature, little research has looked into the correlation between the degree of whitening achieved in an in-office session and the number of weeks of home whitening needed to achieve the same outcome, a fact that makes a comparison of the results obtained in the present study difficult. [PERSON] et al. [10] did not find differences in the efficacy of a single session in-office whitening treatment compared with another with two sessions, and a third with one in-office session and one-week home whitening realized by the patient; however, this study did not evaluate color by means of spectrophotometry. At the same time, [PERSON] et al. [8] and [PERSON] et al. [11] showed that combined treatment achieved an appreciable color modification (evaluated by shade guides). Quantifying color modification in combined whitening using spectrophotometers, [PERSON] et al. [12] found in-office whitening, home whitening and combined treatments equally effective. Comparing different whitening techniques, [PERSON] et al. [13] showed that combined treatment obtained a statistically significant color change with a considerable increase in L*. [PERSON] et al. [14] found greater effects with combined whitening compared with in-office whitening alone. In the present work, no differences were found between the two study groups regarding the color modification obtained at the end of treatment. This implies that there was no need to perform a second in-office session using a bleaching product of high concentration. Eliminating the second session could reduce the risk of generating the undesirable effects on both oral soft tissues and dental tissues that this type of product can provoke [15]. Nevertheless, the significant reduction in the number of weeks of home whitening occasioned by the second in-office session (8.2 weeks in the two-session compared with 11.6 weeks in the one-session group) makes the second session recommendable in  
Figure 8: (**a**) Initial photograph of a Group 2S patient. (**b**) Photograph of Group 2S patient at the end of the second in-office session. (**c**) Photograph of Group 2S patient at the end of home whitening. cases of moderate/severe discoloration, which would otherwise require prolonged periods of home treatment. The second session will allow color stabilization to be reached in fewer weeks of home treatment, which releases the patient from the risks associated with prolonged exposure to a whitening agent [8; 9; 15], as well as dietary and oral hygiene restrictions, and reduces the overall cost of treatment as it requires smaller quantities of home whitening agent and fewer follow-up check-ups.