ABSTRACT
BACKGROUND:For teeth with normal dental crown height,pulp cavity retention crown restoration with different depths of the pulp cavity and different repair materials affects the stress and flexural strength of tooth tissue.For short crown molar defects,the research on pulp cavity repair mainly focuses on clinical observation and in vitro flexural strength experiments. OBJECTIVE:To establish a three-dimensional finite element model for short crown molar restored by the endocrown after root canal treatment to analyze the effects of different pulp cavity retention depths and different repair materials on the distribution and size of dentin equivalent stress. METHODS:Based on establishing the complete model of the short crown mandible first molar,a three-dimensional finite element model was established for repairing the distal adjacent defect of the short crown molar with different pulp cavity retention depths(h=2,3,4 mm)and different repair materials(zirconia,lithium disilicate).Under the oblique loading,the equivalent stress distribution was observed.The peak value of dentin equivalent stress and the mean value of equivalent stress near the bottom of the mesial pulp cavity wall were calculated. RESULTS AND CONCLUSION:(1)Equivalent stress concentration areas:The stress of complete short crown molar and restored models mainly concentrated in the mesial root mesial neck and mesial root lingual neck.The stress concentration area was found in the mesial pulp cavity wall corresponding to the bottom layer of restored models,and the stress concentration was obvious in the 4 mm retention depth group.(2)Under the same repair material,the peak value of dentin equivalent stress was the lowest at 3 mm for all models after repair.The average value of equivalent stress near the bottom of the mesial pulp cavity wall was lowest at 3 mm.(3)Under the same retention depth,there was no significant difference between the two materials in the dentin equivalent stress peak and the mean value near the bottom of the mesial pulp cavity.(4)The results showed that under the conditions of this experiment,the endocrown was used to repair the defect of the short crown molar and the retention depth was 3 mm,which was more beneficial to protect the remaining dental tissue.The selection of zirconia or lithium disilicate as the repair material had little effect on the dentin stress.
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.