## 3 Discussion and Conclusions  
This case study reports different resorptive patterns developed in two central upper incisors from a single individual by means of SEM and stereomicroscopy imaging. The two teeth share the same history. The outcome of this case report indicates that an unpredictable pattern of resorption may account for the poor prognosis when teeth are replanted outside the current guidelines for the management of dental avulsion. It is worth noting that in the case of reimplantation of multiple teeth sharing the same history, the bad outcome of one tooth would not necessarily occur to the other ones. A possible explanation for the observed differences in the two dental roots reactions can be searched in the factors affecting those teeth that are managed correctly. The major risk factors are the extra-oral time, the storage circumstances, the age of the patient, the open root apex, and the timing of pulp extirpation after replantation; all these have been widely documented in association with external root resorption onset and type [4; 5; 6; 9]. This case report presented at the baseline all the already known risk factors, such as delayed replantation, incorrect storage, and the timing of pulp extirpation after replantation. Several studies investigated the effect of dry storage on periodontal ligament (PDL) cells' viability and subsequent onset of inflammatory resorption. [PERSON] and [PERSON] [5] have shown that periodontal ligament cells can be expected to survive a dry time of 15 min or less but are unlikely to survive a dry time of greater than 60 min [4; 10]. Milk is a good storage media and it has been shown to preserve PDL cells for up to 8 h, however, [PERSON]'s balanced salt solution (HBSS), saliva, and saline are also indicated to keep periodontal tissue under good state until replantation [5; 11]. The evidence shows that teeth with damage, contamination, and/or that are kept in dry storage longer than 15 min, increase the risk of early resorption [4]. The onset of the inflammatory resorption is more rapid than the replacement one, and the overall rate of root resorption in teeth with extended extraoral periods is significantly higher in younger patients [5; 6]. [PERSON] et al. examined the factors associated with avulsion injury in 51 children with 62 avulsed permanent incisors, in order to clarify the association between factors like history of the accident, concomitant injuries, age, gender, apical maturity of the root, replantation rate, storage media, extra-alveolar duration, endodontic treatment, compliance, and recall appointments response and the outcomes of the replantation. They showed that immature incisors exhibited significantly more complications compared with mature teeth with a lower survival rate [6]. The findings of [PERSON] et al. were in accordance with the results of a previous study by [PERSON] et al., that showed that the progression of root resorption in teeth with prolonged extraoral time is strictly related to the age of the patient, with a higher rate in patients 17-39 years old at the time of the avulsive trauma, compared with 8-16 years old patients [12]. Figure 7: SEM images of tooth 11: (**A**) image at 60\(\times\) magnification showing the boundary between the dentine under resorption (D) and the preserved root cement (Ce); (**B**) image at 50\(\times\) magnification; other than the boundary described before, some craters penetrating the dentine are visible (arrows). The dentine-cement the boundary is visible. Moreover, a recent prospective clinical study on 165 patients documented that the risk of developing severe inflammatory resorption before the onset of endodontic therapy is directly affected by the pulpectomy time and is inversely proportional to age [13]. Similarly to the variability observed for external resorption incorrectly managed cases, we show an unpredictable pattern of resorption also in those cases not fulfilling the recommended criteria. This may have an impact on the root surface prognosis.