Patient selection
This study was approved by the bioethics committee of the School of Stomatology of Zhejiang University (202134). The research team therefore had access to patients' electronic records. One blinded senior examiner identified patients with teeth that had received CAD/CAM zirconia post-cores by screening the patient archives at the Department of Prosthodontics. The patients had volunteered and provided signed informed consent for dental prostheses consisting of integrated CAD/CAM zirconia post-cores and all-ceramic crowns.
The case inclusion criteria were as follows: (a) no loosening of affected teeth; (b) completed root canal treatment; (c) no obvious clinical symptoms; (d) no periodontal inflammation. While the patient exclusion criteria were as follows: (a) patients that were seriously ill, had mental illnesses, incapacitated, lliterate, or mental retardation, etc.; (b) prisoners, children, and pregnant women; (c) smokers; (d) patients < 18 years of age at the time of the first visit.
The final study samples consisted of 400 endodontically-treated teeth in 342 patients which were retrospectively evaluated. Periodontal treatment, root canal treatment, and occlusal adjustments were conducted when necessary before prosthetic treatment. All preparations were made in accordance with unified process to minimize the study's heterogeneity and increase reliability by using CAD/CAM zirconia post-cores from the same manufacturer (Dentsply, Sirona, Germany) and restricting two senior operators.
Prosthetic protocol
Before restoration, the root canal fillings were evaluated radiographically with regard to length, density, adaptation to the root canals, and periapical conditions. All teeth were prepared for post restorations in accordance with unified operation method. We determined the working length according to the root canal length and marked it on the reamer. Then, we drilled in at low speed in the direction of the root canal and pulled the tape out until the desired working length. According to the length, shape and diameter of the root of the affected tooth, the corresponding type of P drill was used as the final drill needle for preparation to the predetermined working length. The post was longer than the height of the clinical crown and retained 5 mm of canal filling at least at the apex. A ferrule with a minimum height of 1.5 mm was also prepared consistent with the residual hard tissue.
Thereafter, impressions were made using polysiloxane impression material (3 M, Maplewood, MN, USA) and a type wax was created and scanned after spraying scanning powder (DS-EX Proceed; Shining 3D, Zhejiang, China). The deputy cast post was designed using the CAD/CAM system (Ceramill; Amann Girrbach, Charlotte, NC, USA), and the zirconium oxide block (Dentsply, Sirona, Germany) was cut with a grinding apparatus (Ceramill Motion 2, Amann Girrbach). Finally, a sintering furnace (Infire HTC speed; Dentsply Sirona, York, PA, USA) was used. All post-cores were polished and cemented conventionally using resin cement (RelyX™ Unicem, 3M,US). After curing, dental preparation and conventional crown restoration were performed. All affected teeth in this study were restored using zirconia all-porcelain crowns made by CAD/CAM integration (Dentsply, Sirona, Germany and 3M, Lava, USA.). All crowns were sand-blasted and cemented conventionally using resin cement (RelyX™ Unicem, 3M, US). The production process with different tooth positions has been shown in Figs. 1, 2 and 3.
Data collection and analysis
The follow-up examinations, conducted by two prosthodontists, consisted of the evaluation of the patients’ perceptions, clinical measurements, and radiographic examinations. The success criteria were established as follows: (1) probing depths ≤ 3 mm at six aspects; (2) lack of mobility of the crown or post; (3) lack of tooth sensitivity to horizontal and vertical percussion; (4) no sign of periapical inflammation; and (5) no breakage of the detachment of the post and crown [20].
Failure cases were divided into catastrophic and noncatastrophic complications. Catastrophic complications were defined as tooth extractions due to root fracture, severe periodontitis, or postfracture. Noncatastrophic complications included percussion sensitivity or other signs of periapical inflammation, fracture of the core or crown, post debonding, and crown dislodgement.
The longevity of the zirconia post was measured up to the day of teeth extraction, catastrophic complications, or clinical examination if no catastrophic complications occurred. Survival rate was assessed using Kaplan–Meier analysis by Graphpad Prism 8.0 and SPSS, and survival percentages were calculated up to 5 years of follow-up due to the small sample size after that. Cases without any complications were considered successful to access success rate, and any differences between tooth positions and gender were examined by Cox–Mantel Test with post hoc contrasts by Bonferroni test. The statistical significance for all tests was set at P < 0.05.