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Journal of Peking University (Health Sciences) ›› 2022, Vol. 54 ›› Issue (1): 95-99. doi: 10.19723/j.issn.1671-167X.2022.01.015

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Effects of the resin base and retention depth on fracture resistance of molars restored with nano-ceramic endocrowns

LI Zhi1,XU Yong-xiang2,BAO Xu-dong1,△(),WANG Xiao-yan1,△()   

  1. 1. Department of Cariology and Endodontology, Peking University School and Hospital of Stomatology, Beijing 100081, China
    2. Department of Dental Materials, Peking University School and Hospital of Stomatology & National Center of Stomatology & National Clinical Research Center for Oral Diseases & National Engineering Laboratory for Digital and Material Technology of Stomatology & Beijing Key Laboratory of Digtal Stomatology & NHC Research Center of Engineering and Technology for Computerized Dentistry & NMPA Key Laboratory for Dental Materials, Beijing 100081, China
  • Received:2021-05-19 Online:2022-02-18 Published:2022-02-21
  • Contact: Xu-dong BAO,Xiao-yan WANG E-mail:baoxudong@vip.163.com;wangxiaoyan@pkuss.bjmu.edu.cn

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Abstract:

Objective: To compare the effects of resin base and different retention depth on the fracture resistance of mandibular molars restored with nano-ceramic endocrowns. Methods: Forty mandibular molars selected and randomly divided into 5 groups: ① The control group which was consisted of intact teeth, ② the non-resin base group, ③ the 2 mm retention depth group, ④ the 3 mm retention depth group, ⑤ the 4 mm retention depth group, respectively. After tooth preparation, in vitro root canal therapy was conducted, which was followed by endocrown design, production and adhesive of groups ②-⑤. All the samples were under load (N) of the universal mechanical testing machine after embedding. The fracture pattern of each sample was observed under stereomicroscope. Then the microstructure of the fracture surface was observed by scanning electron microscopy. Results: The fracture loads of each group were respectively: the control group fracture load was (3 069.34±939.50) N; experimental groups: fracture load of (2 438.04±774.40) N for the group without resin base; fracture load of (3 537.18±763.65) N for the group with 2 mm retention depth. The fracture load of the retention depth 3 mm group was (2 331.55±766.39) N; the fracture load of the retention depth 4 mm group was (2 786.98±709.24) N. There was statistical significance in the effect of resin base and different retention depth on the fracture loads of molars restored with nano-ceramic endocrown (P<0.05). Repairable fractures in each group were as follows: control group 2/8, non-resin base group 1/8, retention depth of 2 mm group 1/8, retention depth of 3 mm group 2/8, and retention depth of 4 mm group 0/8. The effects of the retention depth and the presence of resin base on the fracture resistance of the resin nano-ceramic endocrowns were statistically significant (P<0.05). Scanning electron microscopy showed more arrest lines and small twist hackles on the fracture surface of the restorations with resin base (retention depths of 2 mm, 3 mm, and 4 mm), with cracks extending towards the root. In addition to the characteristics above, more transverse cracks parallel to the occlusal surface, pointing outwards from the center of the pulp cavity retention, were also observed on the fracture surface of the non-resin base restorations. Conclusion: When molar teeth with nano-ceramic endocrowns are restored, resin base and the retention depth of 2 mm help the teeth to obtain optimal fracture strength.

Key words: Endocrowns, Retention depth, Resin base, Fracture resistance, Resin nano-ceramic

CLC Number: 

  • R783

Figure 1

Fracture modes of teeth restored with endocrown A, repairable fracture; B, irrepairable fracture."

Figure 2

Select samples for SEM observation Radial cracks in fracture section under the stereomicroscope (white arrows)."

Table 1

Fracture modes of control group and experimental groups"

Items Group ① Group ② Group ③ Group ④ Group ⑤
Repairable fracture 2/8 1/8 1/8 2/8 0/8
Irrepairable fracture 6/8 7/8 7/8 6/8 8/8

Figure 3

Samples of fracture A and B, typical irrepairable fracture of the sample without resin base. The central retainer was intact and debonded. C and D, typical irrepairable fracture of the sample with resin base. The endocrown fracture piece and the broken pieces of teeth remianed bonding."

Figure 4

Fracture sections of restorationa with and without resin base The green arrows indicate the crack propagation direction (dcp); the black arrows indicate the characteristic microstructure in the fracture section, and * indicates the fracture surface. A, fracture section of restoration with resin base; B, fracture section of restoration without resin base. LE, lengthwise cracks;LA, lateral cracks."

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