选择性激光熔化打印可摘局部义齿圆环形卡环固位力的有限元分析

Finite element analyses of retention of removable partial denture circumferential clasps manufactured by selective laser melting

目的:通过有限元分析对比评价不同设计的选择性激光熔化(selective laser melting,SLM)打印钴铬、纯钛、钛合金(Ti-6Al-4V)可摘局部义齿圆环形卡环的固位力,并分析卡环在脱位过程中的应力分布。方法:设计A(体部宽度/厚度为1.9 mm/1.1 mm,卡环臂聚合度为0.8)和B(尺寸A的1.2倍)两种圆环形卡环尺寸,再针对上述尺寸分别设计进入0.25 mm(A1,B1)和0.50 mm(A2,B2)两种倒凹深度,建立上述4种形态的卡环和基牙的有限元模型,将测量得到的SLM金属(钴铬、纯钛、钛合金)的密度和弹性模量赋予卡环,将釉质的密度、弹性模量、泊松(Poisson)比赋予基牙。设置卡环臂尺寸为A且进入0.25 mm倒凹的铸造钴铬合金卡环为对照组(C),并在有限元模型中赋予铸造钴铬合金的密度和弹性模量。所有金属的泊松比设定为0.33。在有限元模型中,沿脱位方向对卡环施加5 N的初始载荷,计算卡环沿脱位方向的最大位移,若卡环没有完全脱位,则每次在卡环上重新施加比上一次多5 N的载荷,直至卡环能够完全脱位,从而得出不同组卡环的固位力区间。经过上述有限元模型测试,筛选出与对照组卡环固位力区间相同的SLM金属卡环,进一步进行实体的就位/脱位实验验证,并通过有限元分析其von Mises应力分布。结果:有限元分析发现B1、B2形态的SLM金属(钴铬、纯钛、钛合金)卡环,A2形态的SLM钴铬卡环的固位力均大于对照组卡环。A1形态的SLM纯钛和钛合金卡环的固位力小于对照组卡环。A1形态的SLM钴铬卡环、A2形态的SLM纯钛和钛合金卡环的固位力与对照组卡环相当(15~20 N),进一步的实体测试表明,这3组SLM金属卡环固位力分别为(21.57±5.41)N、(19.75±4.47)N、(19.32±2.04)N,差异无统计学意义(P>0.05),这3组卡环中,除SLM钛合金卡环外,SLM钴铬和纯钛卡环的最大von Mises应力均超过各自的屈服强度。结论:对于SLM工艺加工制作的卡环,卡环臂尺寸和进入倒凹深度均相同的钴铬卡环的固位力大于纯钛和钛合金卡环,可通过调整进入倒凹深度和卡环臂尺寸来调整SLM金属卡环的固位力。考虑到义齿在患者口内的长期稳定使用,如采用SLM工艺加工卡环,建议设计卡环臂尺寸为A,进入倒凹量为0.50 mm,并采用钛合金材料制作。

Objective:To compare the retentions of different designs of cobalt-chromium(Co-Cr),pure titanium(CP Ti),and titanium alloy(Ti-6Al-4V)removable partial denture(RPD)circumferential clasps manufactured by selective laser melting(SLM)and to analyze the stress distribution of these clasps during the removal from abutment teeth.Methods:Clasps with clasp arm size A(1.9 mm width/1.1 mm thickness at the body and 0.8-taper)or B(1.2 times A)and 0.25 mm or 0.50 mm undercut engagement were modeled on a prepared first premolar die,named as designs A1,A2,A3,and A4,respectively.The density and elastic modulus of SLM-built Co-Cr,CP Ti,and Ti-6Al-4V were measured and given to different groups of clasps.The density,elastic modulus,and Poisson’s ratio of enamel were given to the die.The control group was the cast Co-Cr clasp with design A1,to which the density and elastic modulus of cast Co-Cr alloy were given.The Poisson’s ratio of all metals was 0.33.The initial 5 N dislodging force was applied,and the maximum displacement of the clasp along the insertion path was computed.The load was reapplied with an increment of 5 N than in the last simulation until the clasp was completely dislodged.The retentive force range of different groups of clasps was obtained.The retentive forces of the SLM-built Co-Cr,CP Ti,and Ti-6Al-4V clasps with equivalent computed retentive force range to the control group were validated through the insertion/removal experiment.The von Mises stress distributions of these three groups of SLM-built clasps under 15 N loads were analyzed.Results:SLM-built Co-Cr,CP Ti,and Ti-6Al-4V clasps with designs B1 or B2,and Co-Cr clasps with design A2 had higher retentive forces than those of the control group.SLM-built CP Ti and Ti-6Al-4V clasps with design A1 had lower retentive forces than those of the control group.SLM-built Co-Cr clasp with design A1 and SLM-built CP Ti and Ti-6Al-4V clasps with design A2 had equivalent retentive forces to those of the control group.The insertion/removal experiment showed that the measured retentive forces of these three groups of SLM-built clasps were(21.57±5.41)N,(19.75±4.47)N,and(19.32±2.04)N,respectively.No statistically significant measured retentive force difference was found among these three groups of SLM-built clasps(P>0.05).The maximum von Mises stress of these three groups of SLM-built clasps exceeded their responding yield strength except for the Ti-6Al-4V one.Conclusion:SLM-built Co-Cr circumferential clasps had higher retention than CP Ti and Ti-6Al-4V ones with the same clasp arm size and undercut engagement.The retention of SLM-built circumferential clasps could be adjusted by changing the undercut engagement and clasp arm size.If SLM-built circumferential clasps are used in clinical practice,the Ti-6Al-4V clasp with clasp arm size A and 0.50 mm undercut engagement is recommended considering the long-term use of RPD in the patient’s mouth.