## 2 Case Presentation
#### Stereomicroscopy and SEM Images  
After extraction, the organic material of the tooth was removed by exposure to an aqueous-based solution containing 2.5% NaOCl, for 24 h. The samples were dehydrated and prepared for stereomicroscopy (Zeiss Axio Zoom V16 binocular stereo microscope, oculars PI 10\(\times\)/23, objective 1\(\times\)/0.25 FWD 56 mm, Zeiss, Jena, Germany) and SEM evaluation, to illustrate the morphological characteristics of the two roots. Stereo microscopes allow us to generate three-dimensional images of the examined surface through optical microscopy in reflected light. In detail, we overlapped a series of \(\sim\)100 photographs captured by moving on the vertical axis in multifocal mode. This examination allowed some areas of major interest to be further investigated through scanning electron microscopy (Hitachi TM3000-Tabletop Scanning Electron Microscope; magnifications 50 to 2000 at 15 Kv acceleration voltage, Hitachi Ltd, Tokyo, Japan).  
Figures 2 and 3 show stereomicroscopy images of the roots of teeth 11 and 21, respectively. The buccal side is more affected than the lingual one, with the gutta-percha filling exposed for more than 50% of its length in both roots (Figures 2B and 3B).  
In the two cases, two different patterns of root resorption were observed, as follows:  
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The resorptive pattern characterizing the root of the tooth 21 shows high heterogeneity (Figure 5). In this tooth, the external replacement resorption invaded the root by resorbing all the cement and, irregularly, the dentine, that assumed a spurs appearance (Figure 5A,B). At higher magnifications (Figure 5C), these spurs are characterized by the presence of resorptive dentine lacunae and by the presence of treatment agents used by the dental practitioner before replantation. The dentine at the base of the spurs and at higher magnification (Figure 6) shows the same resorptive lacunae with dentine tubules surrounded by a hypermineralized collar of peritubular dentine;
5. The microscopic structure of tooth 11 (Figure 7) is characterized by the same pattern of resorptive dentine lacunae as tooth 21 and by the same appearance of the dentine tubules (i.e., surrounded by a hypermineralized collar of peritubular dentine). Differently from tooth 21, here the cement preserves in some areas, preventing the resorptive reaction.  
Figure 4: Scanning electron microscopy (SEM) images of tooth 21: (**A**) image at 60\(\times\) magnification showing the dentine resorption cavity at the cervical palatal margin; (**B**) stereomicroscopy image of the same area; and (**C**) SEM magnification (800\(\times\)) of the bottom of the cavity showing the resorptive reaction in the dentine. E = enamel, Ce = cement, D = dentine.  
Figure 3: Stereomicroscopic images of tooth 21 root. (**A**) mesial view; (**B**) buccal view; (**C**) distal view; and (**D**) lingual view, * = isolated resorption cavity.  
Figure 5: SEM images of the apical third of the root of tooth 21, buccal view: (**A**) heterogeneous external replacement resorption characterizing the root, 50\(\times\) magnification; (**B**) magnification of the dentine spurs (150\(\times\) magnification); and (**C**) 500\(\times\) magnification highlighting resorptive area of the dentine. * = treatment agents residuals, DL = dentine resorption lacunae.  
Figure 6: SEM images of the apical third of the root of tooth 21, buccal view: (**A**) image at 300\(\times\) magnification showing the dentine resorption as a series of concaves surfaces; the dentine tubules are cut in different planes depending from the resorptive progression front; (**B**) magnification of the same area at 1800\(\times\). White star indicates a large resorptive lacuna; large white arrow point at a dentinal tubule hole; small white arrow indicates the hypermineralized pertubular dentine.