Statement of problem

Crown-lengthening procedures (CLPs) and orthodontic eruption procedures (OEPs) improve the biomechanical properties of residual root restoration. However, their use is limited by clinic time, cost, and crown-root ratio. An inner shoulder retention form (ISRF) overcomes these limits; however, whether ISRF meets the biomechanical requirements is unclear.

Purpose

The purpose of this in vitro and finite element analysis (FEA) study was to evaluate the effects of 3 residual root treatments (ISRF, CLP, and OEP) on premolar fracture resistance and stress distribution after post-and-core restoration.

Material and methods

Thirty-two extracted mandibular second premolars were screened and randomly divided into 4 groups (n=8): a 2-mm ferrule restoration group (NPR2; control group) and 3 experimental groups (0.5×0.5-mm ISRF [ISRF0.5], 2-mm CLP [CLP2], and 2-mm OEP [OEP2]). After removal of the crown, endodontic treatment, and embedding and restoration, the specimens were loaded on a universal tester (crosshead speed of 0.1 mm per minute) at 30 degrees to the long axis of the tooth until fracture. Fracture resistance and pattern were analyzed. Four groups were added in FEA: 1-mm ferrule restoration group (NPR1), 1.0×1.0-mm ISRF (ISRF1), 1-mm CLP (CLP1), and 1-mm OEP (OEP1). The maximum principal stress peak (σmax), maximum displacement, and stress distribution were evaluated.

Results

Mean fracture load ±standard deviation was 796.23 ±155.61 N (NPR2), 650.32 ±150.43 N (ISRF0.5), 385.38 ±149.92 N (CLP2), and 542.93 ±79.34 N (OEP2); these differences were statistically significant (F=12.724; P<.001). The main fracture patterns were decementation from the lingual crown margin and oblique root fracture in ferrule restoration groups and post-and-core decementation and oblique fracture starting from the middle proximal surface in the ISRF groups without ferrules. In FEA, the results of NPR2, ISRF0.5, CLP2, and OEP2 were consistent with the in vitro experiments and provided stress distribution and displacement information.

Conclusions

Teeth with crown dentin ferrules showed higher fracture resistance and lower stress concentration. For residual roots, ISRF and OEP showed no differences, but CLP had lower fracture resistance and higher stress concentration.

中文翻译：

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核后修复后3种不同的残留根处理的效果：体外抗断裂实验和有限元分析。

问题陈述

牙冠加长程序（CLP）和正畸喷发程序（OEP）可改善残余牙根修复的生物力学特性。但是，它们的使用受到临床时间，成本和冠根比率的限制。内肩保留形式（ISRF）克服了这些限制；但是，尚不清楚ISRF是否满足生物力学要求。

目的

这项体外和有限元分析（FEA）研究的目的是评估3种残余牙根处理（ISRF，CLP和OEP）对后磨牙后核心复位后抗磨前抗裂性和应力分布的影响。

材料与方法

筛选32颗下颌第二前磨牙，随机分为4组（n = 8）：2毫米套圈修复组（NPR2；对照组）和3个实验组（0.5×0.5毫米ISRF [ISRF0.5]） ，2毫米CLP [CLP2]和2毫米OEP [OEP2]）。取下牙冠，进行牙髓治疗以及包埋和修复后，将标本以与牙齿长轴成30度的角度放在万能测试仪（十字头速度为每分钟0.1毫米）上直至破裂。分析了耐断裂性和图案。FEA中添加了四组：1-mm套圈修复组（NPR1），1.0×1.0-mm ISRF（ISRF1），1-mm CLP（CLP1）和1-mm OEP（OEP1）。评估了最大主应力峰值（σmax），最大位移和应力分布。

结果

平均断裂载荷±标准偏差为796.23±155.61 N（NPR2），650.32±150.43 N（ISRF0.5），385.38±149.92 N（CLP2）和542.93±79.34 N（OEP2）; 这些差异具有统计学意义（F = 12.724; P <.001）。套圈修复组的主要骨折类型为舌冠边缘切开和斜根骨折； ISRF组中，无套圈的是从核心近端表面开始的核后切开和斜骨骨折。在FEA中，NPR2，ISRF0.5，CLP2和OEP2的结果与体外实验一致，并提供了应力分布和位移信息。

结论

带有冠状牙本质套环的牙齿显示出较高的抗断裂性和较低的应力集中。对于残留根，ISRF和OEP没有差异，但是CLP具有较低的抗断裂性和较高的应力集中。