膝关节内侧固定平台单髁假体放置位置优化的有限元分析

目的探讨股骨组件及胫骨组件冠状面位置变化对股骨及胫骨生物力学的影响。方法取1名汉族男性志愿者的左侧膝关节CT及MRI图像,建立正常膝关节三维有限元模型(finite elemental model,FEM)。设计股骨组件及胫骨组件内翻6°、内翻3°、0°、外翻3°、外翻6°,组合成25个膝内侧单髁置换FEM。沿股骨机械轴加载1000 N载荷,观察von Mises云图应力分布,测量外侧间室载荷比例,测量胫骨组件下方松质骨及内侧皮质骨、聚乙烯衬垫上表面、外侧间室股骨软骨高接触应力值。将与中立位(胫骨及股骨假体内外翻0°、胫骨假体后倾5°)比较有统计学意义的指标通过散点图标识,找出点项目密集区和稀疏区,比较两区有统计学意义的项目数量,确定股骨组件、胫骨组件优化位置。结果股骨组件0°位放置时,胫骨从内翻6°至外翻6°各组合的胫骨组件下方松质骨高接触应力差异无统计学意义;胫骨组件0°位放置时,股骨组件内翻6°、外翻6°组件下方松质骨高接触应力值与中立位比较增加(9.21±3.38)MPa和(9.08±4.13)MPa(P<0.05)。股骨、胫骨组件从内翻6°至外翻6°变化时,胫骨下方内侧皮质骨高接触应力值逐渐下降(P<0.05)。股骨组件0°位放置时,胫骨组件从内翻6°至外翻6°各组合聚乙烯衬垫上表面高接触应力值的差异无统计学意义;胫骨组件0°位放置时,股骨组件内翻6°、外翻6°组与中立位组比较分别增加(2.88±2.53)MPa和(3.47±2.86)MPa(P<0.05);股骨及胫骨组件从内翻6°至外翻6°变化时,外侧间室载荷比例及外侧间室股骨软骨高应力值逐渐下降(P<0.05)。稀疏区(股骨或胫骨从内翻3°至外翻3°的所有组合的集合)有统计学意义的指标比例(2.8%,1/36)明显小于密集区(去除稀疏区以外的所有组合的集合)的比例(57.8%,37/64),差异有统计学意义(χ^2=29.61,P<0.001)。结论在下肢力线正常、关节线不变的条件下,膝关节内侧固定平台单髁假体放置位置为股骨组件、胫骨组件内翻、外翻角度不宜超过3°。

Finite element analysis of optimization on placement of medial fixed-bearing unicompartmental knee arthroplasty

Objective To investigate the influence of displacement of femoral and tibial components on the biomechanics of femoral or tibial bone in coronal view.Methods A series of CT and MRI of the left knee joint of a Han male volunteer was taken and a three-dimensional finite element model of the healthy knee joint was established.The femoral component and the tibial component were designed with varus 6°,varus 3°,0°,valgus 3°,and valgus 6°,and were combined into 25 three-dimensional finite element model(FEM)of medial unicompartmental knee arthroplasty.A 1000 N load was applied along the femoral mechanical axis.The von Mises cloud stress distribution was observed.Moreover,the lateral compartment load ratio,the high contact stress of cancellous bone and medial cortical bone below the tibial component,the upper surface of the polyethylene liner,and the femoral cartilage in the lateral compartment was measured.The statistically significant indicators compared with the neutral position(0°varus or valgus of the tibia and the femoral prosthesis,and 5°posterior slope of tibia prosthesis)were identified by scatter plots to find the dense and sparse areas of point items.The optimal position of the femoral component and the tibial component was determined by the number of items with statistical significance in the sparse area.Results When the femoral component was placed at 0°position,there was no significant difference in the high contact stress of cancellous bone below the tibial component in the five groups.When the femoral component was placed at 0°position,the tibial component was 6°varus or 6°valgus and the stress was increased by 9.21±3.38 MPa and 9.08±4.13 MPa(P<0.05),respectively.With the changes of femoral and tibial components from 6°varus to 6°valgus,the high contact stress of the medial cortical bone below the tibia was gradually decreased(P<0.05).When the femoral component was placed at 0°,the tibial component changes from 6°varus to 6°valgus without significant difference in the high contact stress on the upper surface of each group of polyethylene gasket.Compared with the neutral position group,the high contact stress of the 6°varus or 6°valgus group were increased by 2.88±2.53 MPa and 3.47±2.86 MPa,respective ly(P<0.05).The lateral compartment load ratio and the high contact stress of lateral compartment femoral cartilage was gradually decreased(P<0.05),when the femoral and tibial components changed from 6°varus to 6°valgus.The number(2.8%,1/36)of indicators in the sparse area(the combination of all combinations of femur or tibia from 3°varus to 3°valgus)was less than that(57.8%,37/64)in the dense area(set of all combinations except sparse area),and the difference was significant(χ2=29.61,P<0.001).Conclusion It is suggested that the position of the femoral component and the tibial component in fixed medial unicompartmental arthroplasty should not exceed 3°varus or valgus in patients with standard lower limb alignment.