关键词: 改良式牙冠延长术, 牙本质肩领, 纤维桩核冠, 残根, 抗折性能

Abstract: Objective    To study the effects of modified crown lengthening procedure combined with ferrule height on the fracture resistance of mandibular first premolar with oblique fractures restored with fiber post and core crown. Methods    Totally forty mandibular first premolars（all single-root canals）extracted at the Department of Oral and Maxillofacial Surgery，School and Hospital of Stomatology，Hebei Medical University from April to July 2024 were selected. Following conventional root canal treatment，the crowns were severed at a height of 3.0 mm above the lingual enamel-cementum junction and subsequently cut from the lingual section to the buccal at 1.0 mm above the buccal enamel-cementum junction to create the lingual-buccal oblique fractured root models. They were randomly divided into 5 groups（Groups A to E），with 8 in each group. Group A：the control group，designed with a half-wrapped dentin ferrule measuring 2.0 mm on the lingual side and 0 mm on the buccal side；Group B：simulated modified crown lengthening procedure，designed with a dentin ferrule measuring 2.0 mm on the lingual side and 1.0 mm on the the buccal side；Groups C，D and E：simulated modified crown lengthening procedure，designed with a full-wrapped dentin ferrule measuring 2.0 mm，3.0 mm and 4.0 mm respectively at the residual root neck. After restoration with preformed glass fiber posts，resin cores and cobalt-chromium metal full crown，the specimens were embedded in self-curing resin blocks. The clinical crown-root ratios（C/R）of the completed Groups A to E were 0.75，0.83，0.91，1.10 and 1.33，respectively. The embedded specimens were placed on a universal testing machine and loaded（the loading direction was at an angle of 150° to the long axis of the tooth）until fracture. The fracture load values and fracture mode of the specimens were recorded. Results    The fracture loads of the specimens in Groups A to E were（1.45 ± 0.10）kN，（1.63 ± 0.07）kN，（1.67 ± 0.09）kN，（0.95 ± 0.12）kN and（0.69 ± 0.18）kN，respectively. There was a statistically significant difference in the fracture loads among the groups（F = 112.047，P < 0.001）. Further pairwise comparisons between groups revealed that there was no statistically significant difference between Group B and Group C（q = 0.494，P = 0.957），but there were statistically significant differences among the other groups（all P < 0.05）. Among the fracture mode of each group，only 2 specimens in Groups D and E had fracture surfaces below the cervical one-third of the root，while the fracture surfaces of the other groups were above the cervical one-third，or there were fractures and detachments of the post-core. There was no significant difference in the fracture mode among all groups（χ2 = 6.321，P = 0.642）. Conclusion    In simulating the effects of modified crown lengthening procedure combined with ferrule height on the fracture resistance of mandibular first premolars following fiber post-core crown restoration，it is found that variations in dentine ferrule height significantly influence the resistance of these residual roots after restoration. Specifically，when a fully wrapped dentine ferrule measuring 1.0 - 2.0 mm is prepared at the residual root neck and the crown-to-root ratio remains below 1，optimal fracture resistance of the teeth can be ensured.

Key words: modified crown lengthening procedure, dentin ferrule, fiber post-core crown, residual roots, fracture resistance