## 4 Discussion  
It was previously speculated that the pink teeth phenomenon might occur due to asphyxia such as in cases of drowning, hanging, and strangulation [5, 6, 10, 12, 13, 16, 22]. It has also been speculated that the discoloration might be caused by hemoglobin, carbon monoxide, or the effect of bacteria that produce red pigment [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22]. However, the precise mechanism has yet to be established. In particular, the relationship with the cause of death, if any, remains insufficiently explained. In our previous study [26] of 324 unidentified cadavers, we confirmed that pink teeth tended to occur more frequently in the anterior teeth and in relatively young victims, findings that were consistent with other reports [5, 10, 12, 19, 20, 21]. Thus, our findings did not produce novel evidence for an association with cause of death. In many reports to date, teeth with a pinkish tone that can be discriminated from the color of normal teeth have been simply regarded as pink teeth without consideration for the  
Figure 7: Association of SPTC (**a**) and SPTR (**b**) with time after death in drowning cases (18 cases, n = 2–10). (**a**) SPTC between 2 weeks and 1 month and for 2–6 months showed significant increases compared with \(\leq\)1 week (\(p\) < 0.01); (**b**) Although there were no significant differences, SPTR tended to decrease with time after death. \({}^{**}\)\(p\) < 0.01. SPTC, Score of Pink Teeth Color; SPTR, Score of Pink Teeth Region. degree and region of discoloration. Moreover, no studies have objectively scored discoloration on pink teeth. Therefore, we speculated that classification and evaluation of the degree and region of discoloration with reference to color criteria similar to that used in the field of prosthodontic dentistry might produce novel findings for clarifying the pink teeth phenomenon. We visually scored the degree and region of discoloration by using SPTC and SPTR, respectively, and analyzed the associations of these scores with various cadaveric factors in 68 cases of pink teeth (Table 1). In our sample, pink teeth were present in approximately twice as many male cadavers (67.6%) as female cadavers (29.4%), and were most common in those 40-59 years of age (29.4%). We examined the associations of SPTC and SPTR with sex and with age. As shown in Figure 3, there were no significant differences. In our previous study on 324 unidentified cadavers including 68 cases of pink teeth, we confirmed that pink teeth were more likely to be seen in younger people than in older people, possibly because the dental tubules become narrower and the volume of the pulp chamber decreases in secondary dentine with advancing age [10; 12; 19; 20; 21], but there was no significant difference in age among the cases with pink teeth. In time after death, the value of SPTC for 2-6 months showed a significant increase compared with \(\leq\)1 week (Figure 4a). This result might be due to the decomposition process. The value of SPTC after 7 months showed a significant decrease compared with 2-6 months and that of SPTR after 7 months showed a significant decrease compared with \(\leq\)1 week, which might have been affected by fading in the coloration (Figure 4), especially considering that cases after 7 months had begun to skeletonize or mummify. In the cause of death, there were no significant differences between SPTC and SPTR (Figure 6). However, when the time after death was investigated among 18 drowning cases, the value of SPTC \(\leq\)1 week showed a significant increase compared with 2 weeks and 1 month and for 2-6 months (Figure 6a), suggesting a strong association with cadaveric decomposition. Although the value of SPTR did not show a significant difference, it seemed to decrease over time (Figure 6b). The discoloration was most pronounced at the root just below the cervical region (Grade 1) with time, as shown in the results of Figures 4 and 6, and also the coloration tended to remain at the cervical region as fading occurred elsewhere.