Verdonschot N, Huiskes R
Biomechanics Section, University of Nijmegen, The Netherlands.
J Biomech. 1996 Dec;29(12):1569-75.
Acrylic cement, used to fixate total hip arthroplasty (THA), creeps under dynamic and static loading conditions. As a result, THA stems which are debonded from the cement, may gradually subside, depending on their shape and surface roughness. The purpose of this study was to evaluate the relationship among dynamic load, creep characteristics, interface friction, and subsidence patterns. A laboratory model consisting of a metal tapered cone, surrounded by a cement mantle, was developed. The cone was gradually compressed in the cement by a dynamic, sinusoidal axial force, cycling between 0 and 7 kN for 1.7 million cycles at a frequency of 1 Hz. Subsidence and cement strain were monitored. Two tapers were tested in this way. The relationships among subsidence, creep properties and interface friction were evaluated from a finite element (FE) model, used to simulate the experiments. In this model, the creep properties obtained in dynamic and static, tension and compression experiments measured earlier, were used. The subsidence patterns of both tapers were similar, but one subsided more than the other (380 vs 630 microns). Both subsided stepwise instead of continuous, with a frequency much smaller than that of the applied load. The characteristics of the subsidence and cement-strain patterns could be reproduced by the FE model, but not with great numerical precision. The stepwise subsidence could be explained by slip-stick mechanisms at the interface starting distally and gradually working towards proximal. Variations in friction from 0.25 to 0.50 reduced the total subsidence and the step frequency by about 50%. It was concluded that FE-models used to simulate the mechanical endurance characteristics of THA reconstructions, extended to incorporate cement creep, produce realistic results. These results showed that prosthetic subsidence under dynamic loads occurs due to cement creep. The extent of the subsidence is extremely sensitive to interface friction, hence to small variations in surface roughness and cement constitution. This may explain the relatively large variation of in vivo prosthetic subsidence rates reported in the literature.
用于固定全髋关节置换术（THA）的骨水泥在动态和静态加载条件下会发生蠕变。因此，与骨水泥脱粘的THA柄可能会逐渐下沉，这取决于它们的形状和表面粗糙度。本研究的目的是评估动态载荷、蠕变特性、界面摩擦和下沉模式之间的关系。开发了一个由金属锥形圆锥体和骨水泥套组成的实验室模型。通过动态正弦轴向力使圆锥体在骨水泥中逐渐压缩，轴向力在0至7 kN之间循环，频率为1 Hz，循环170万次。监测下沉和骨水泥应变。用这种方法测试了两个圆锥体。通过用于模拟实验的有限元（FE）模型评估下沉、蠕变特性和界面摩擦之间的关系。在该模型中，使用了先前在动态和静态、拉伸和压缩实验中获得的蠕变特性。两个圆锥体的下沉模式相似，但其中一个下沉比另一个更多（380微米对630微米）。两者都是逐步下沉而不是连续下沉，频率远低于施加载荷的频率。FE模型可以再现下沉和骨水泥应变模式的特征，但数值精度不高。逐步下沉可以通过界面处从远端开始并逐渐向近端发展的粘滑机制来解释。摩擦系数从0.25变化到0.50可使总下沉量和步频降低约50%。得出的结论是，用于模拟THA重建机械耐久性特征并扩展以纳入骨水泥蠕变的FE模型产生了现实的结果。这些结果表明，动态载荷下假体下沉是由于骨水泥蠕变引起的。下沉程度对界面摩擦极其敏感，因此对表面粗糙度和骨水泥成分的微小变化也很敏感。这可能解释了文献中报道的体内假体下沉率相对较大的差异。
