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case_images: Preoperative intraoral frontal (a) and lateral (b) views showing the preexisting upper second premolar crown (tooth 1.5).
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case_images: Lateral and occlusal views of the preexisitng restoration are shown in Figures 1(a) and 1(b).
case_images: Periapical radiograph showing previous endodontic and prosthetic treatments on upper second premolar.
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case_images: The previously treated abutment (>10 years ago, by other colleagues) was originally restored with a metal cast post-and-core extending up to the coronal third of the root; the intraoral periapical X-ray revealed a partially treated and/or filled root canal space, accompanied by a slight radiolucency at the mesial side of the apex (Figure 2).
case_images: Periapical radiograph showing immediate outcome of nonsurgical endodontic retreatment performed at our dental office.
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case_images: Standardized endodontic procedures included the use of stainless steel manual files and rotary Ni-Ti instruments (ProTaper  Universal Series, Dentsply Maillefer), along with NaOCl and EDTA irrigations; the single-cone gutta-percha technique was chosen for final obturation (Figure 3) of the root canal (finishing file instrument and gutta-percha point: size F1, ProTaper  Universal, Dentsply Maillefer).
case_images: Postoperative intraoral frontal (a) and lateral (b) views showing good integration of the new veneered-zirconia crown on tooth 1.5.
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case_images: The definitive crown was adhesively luted as previously reported: briefly, the inner surface of zirconia framework was pretreated with low-pressure (1 bar) 50 mum alumina sandblasting and ultrasonically cleaned; an MDP-based, dual polymerizing luting agent (Clearfil  SA, Kuraray Medical Ltd.) was subsequently applied for final cementation (Figures 4(a) and 4(b)).
case_images: Frontal (a) and lateral (b) views 7 years after crown delivery: an overall dental deterioration is visible associated with soft tissue modifications.
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case_images: About seven years later, in 2015, some modifications of the soft tissues were noted, as multiple gingival recession developed on upper posterior teeth: however, the margin at the premolar restoration was just slightly affected (0.5 mm recession, Figures 5(a) and 5(b)). Minor signs of wear were identified at some locations (slight indentations at incisal margins of left central incisor, left lateral incisor, and canine: Figure 5(a)); the zirconia-porcelain crown, however, was free from chippings and visible cracks.
case_images: Development of clinical symptoms 9 years after crown delivery: the radiograph revealed a periapical radiolucency, with widening of periodontal ligament/lamina dura modifications.
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case_images: Diffuse widening of the periodontal ligament (in comparison with X-ray at time of root canal filling) and lamina dura modifications near the apex were observed from radiographic analysis, despite the apparently well-performed root canal therapy (Figure 6).
case_images: Apical resection of premolar root showing vertical bone resorption at the cortical vestibular side.
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case_images: A root-end resection was performed to ensure the removal of apical ramifications and/or residual bacterial contamination as potential aetiologic factors (Figure 7).
case_images: Three (a) and five (b) months after endodontic surgery, the clinical scenario did not improve: a progressive radiolucency involved the distal areas of the tooth and proximal peaks.
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case_images: The clinical scenario did not improve after apical surgery: during subsequent months (July and September 2017), the radiographic follow-up revealed a progressive radiolucency also involving the distal areas of the tooth and proximal peaks (Figures 8(a) and 8(b)).
case_images: Clinical (a) and radiological (b) presentation of the vertical root fracture: splitting of the premolar root into two halves is clearly visible.
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case_images: On December 2017, a vertical fracture associated with separation of root fragments came to light both clinically and radiographically (Figures 9(a) and 9(b)).
case_images: Inner (a) and outer (b) views of the two fragments produced by the longitudinal fracture, retrieved after tooth extraction.
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case_images: Two main tooth fragments were retrieved from the extraction procedure: a larger one, formed by the crown, post and gutta-percha obturation and a smaller slice with unoccupied root canal space, detached from all the other restorative materials (Figures 10(a) and 10(b)).
case_images: Extraoral close readaptation of fragments: the fracture runs for the entire bucco-lingual length of the tooth, as shown by separation on the vestibular (a) and palatal (b) sides. The fracture line was off-centered from the canal when observed on the axial plane (c).
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case_images: The fracture was running for the entire bucco-lingual root length: it was considered "complete" in the horizontal extent or type "A" according to the classification of von Arx and Bosshardt, being visible from both the vestibular and palatal sides (Figures 11(a) and 11(b)). The fracture was off-centered in the axial plane (i.e., asymmetric involvement of the root), locating itself mainly outside the root canal space in the apical third (Figure 11(c)).
case_images: An incomplete cracking line (arrow) involved the coronal area of the small fragment.
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case_images: A detailed analysis of the fragments shows a close adaptation between post/endodontic filling and the dentinal walls, which also appeared of adequate thickness; in addition, an incomplete cracking line was visible on the coronal third of the small fragment (Figure 12).