Dunne N J, Orr J F
Department of Mechanical and Manufacturing Engineering, Dublin City University, Dublin, Republic of Ireland.
Proc Inst Mech Eng H. 2000;214(6):645-58. doi: 10.1243/0954411001535679.
Cemented fixation of hip replacements is the elective choice of many orthopaedic surgeons. The cement is an acrylic polymer which grouts the prostheses into the medullary cavity of the femur. Cement pressure is accepted as a significant parameter in determining the strength of cement/bone interfaces and hence preventing loosening of the prostheses. The aim of this work was to allow optimal design of the intramedullary stem of a hip prosthesis through knowledge of the flow characteristics of curing bone cement which can be used to predict pressures achieved during insertion of the femoral stem. The viscosity of the cement is a vital property determining the cement flow and hence cement interdigitation into bone. The apparent viscosities, nu(a), of three commercial bone cements were determined with respect to time by extrusion of the curing cement through a parallel die of known geometry under selected pressures. Theoretical models were developed and implemented in a computer program to describe cement flow in three models each of increasing complexity: (a) a simple parallel cylinder, (b) a tapered conical mandrel and (c) an actual femoral prosthesis, the latter models being complicated by extensional effects as annular areas increase. Predicted pressures were close to those measured experimentally, maximum pressures being in the range 10-160 kPa which may be compared with a threshold of 76 kPa proposed for effective interdigitation with cancellous bone. The theoretical model allows the prosthesis/bone geometry of an individual patient to be evaluated in terms of probable pressure distributions in the medullary cavity during cemented fixation and can guide stem design with reference to preparation of the medullary canal. It is proposed that these models may assist retrospective studies of failed components and contribute to implant selection, or to making informed selection from options in custom hip prosthesis designs to achieve optimum cement pressurization.
髋关节置换的骨水泥固定是许多骨科医生的首选方法。骨水泥是一种丙烯酸聚合物，它将假体灌浆到股骨的髓腔中。骨水泥压力被认为是确定骨水泥/骨界面强度从而防止假体松动的一个重要参数。这项工作的目的是通过了解固化骨水泥的流动特性来优化髋关节假体髓内柄的设计，这些特性可用于预测股柄插入过程中所达到的压力。骨水泥的粘度是决定骨水泥流动以及因此决定骨水泥与骨相互交错程度的关键特性。通过在选定压力下将固化骨水泥从已知几何形状的平行模具中挤出，测定了三种市售骨水泥的表观粘度随时间的变化。开发了理论模型并在计算机程序中实现，以描述三种复杂度不断增加的模型中的骨水泥流动：(a) 简单的平行圆柱体，(b) 锥形心轴，(c) 实际的股骨假体，后一种模型由于环形区域增加而受到拉伸效应的影响。预测压力与实验测量值接近，最大压力在10 - 160 kPa范围内，可与为实现与松质骨有效相互交错而提出的76 kPa阈值相比较。该理论模型可以根据骨水泥固定过程中髓腔内可能的压力分布来评估个体患者的假体/骨几何形状，并可参考髓腔准备情况指导柄的设计。建议这些模型可有助于对失败部件进行回顾性研究，并有助于植入物的选择，或在定制髋关节假体设计的选项中做出明智选择，以实现最佳的骨水泥加压。
