Van Rietbergen B, Huiskes R, Weinans H, Sumner D R, Turner T M, Galante J O
Biomechanics Section, University of Nijmegen, The Netherlands.
J Biomech. 1993 Apr-May;26(4-5):369-82. doi: 10.1016/0021-9290(93)90001-u.
A major problem threatening the long-term integrity of total hip replacement is the loss of proximal bone often found around noncemented stems in the long term. It is generally accepted that 'stress shielding' is the cause for this problem: after implantation of the prosthesis the surrounding bone is partially 'shielding' from load carrying and starts to resorb. One of the proposed answers to this problem is the application of press-fitted stems. These smooth-surfaced implants are thought to provoke higher proximal bone loading, and, hence, less stress shielding than bonded implants, because they are wedged into the femur every time when loaded. However, in a two-year experiment in dogs, similar amounts of resorption of the proximal cortex were found around press-fitted and bonded implants. The question arises how similar resorption patterns can develop under completely different stress conditions, and whether this phenomenon can be explained by adaptive bone remodeling theories based on Wolff's law. In the present study an answer was sought for this question. An advanced iterative computer simulation model was used to analyze the remodeling process in the animal experiment. Three-dimensional finite element models were constructed from the animal experimental configuration, in which smooth, press-fitted stems were applied unilaterally in the canine. The FE model was integrated with iterative remodeling procedures, validated in earlier studies. In the model an appropriate non-linear representation of the loose bone-implant interface was realized, also capable of simulating the proximal interface gap that was found around the uncoated implants. The simulation models predicted similar amounts of proximal bone loss and distal bone densification as found in the animal model. Hence, the cortical bone loss could indeed be predicted by the strain-adaptive bone remodeling theory. By unraveling the simulation process, the question stated above could be answered. Densification of the distal bone bed during the initial remodeling process was found to cause reduced axial stem displacement (elastic subsidence), decreasing the wedging effect of the stem and, hence, decreasing the loading of the proximal bone, resulting in proximal bone loss. Hence, whereas in the case of bonded stems the proximal resorption process develops monotonously to a new equilibrium, the process around smooth, press-fitted stems develops nonmonotonously. This is due primarily to the unbonded interface conditions and the development of a proximal fibrous membrane. The remodeling process then gradually causes the stem to be jammed in the distal diaphyses (proximal 'stress bypass').
威胁全髋关节置换长期完整性的一个主要问题是长期以来在非骨水泥柄周围经常出现的近端骨丢失。人们普遍认为“应力遮挡”是这个问题的原因：假体植入后，周围的骨头部分地“免受”承载负荷的影响，并开始吸收。针对这个问题提出的一个解决方案是应用压配式柄。这些表面光滑的植入物被认为会引起更高的近端骨负荷，因此，与粘结植入物相比，应力遮挡更少，因为每次加载时它们都会楔入股骨。然而，在一项对狗进行的为期两年的实验中，在压配式和粘结式植入物周围发现近端皮质的吸收量相似。问题是在完全不同的应力条件下如何出现相似的吸收模式，以及这种现象是否可以用基于沃尔夫定律的适应性骨重塑理论来解释。在本研究中，试图找到这个问题的答案。使用了一个先进的迭代计算机模拟模型来分析动物实验中的重塑过程。根据动物实验配置构建了三维有限元模型，其中在犬类动物中单侧应用了光滑的压配式柄。有限元模型与迭代重塑程序相结合，该程序在早期研究中得到了验证。在模型中，实现了对松质骨 - 植入物界面的适当非线性表示，也能够模拟在未涂层植入物周围发现的近端界面间隙。模拟模型预测的近端骨丢失和远端骨致密化量与动物模型中发现的相似。因此，皮质骨丢失确实可以通过应变适应性骨重塑理论来预测。通过剖析模拟过程，可以回答上述问题。发现在初始重塑过程中远端骨床的致密化会导致轴向柄位移（弹性下沉）减少，从而降低柄的楔入效应，进而降低近端骨的负荷，导致近端骨丢失。因此，对于粘结柄，近端吸收过程单调发展到一个新的平衡，而对于光滑的压配式柄，其周围的过程则非单调发展。这主要是由于未粘结的界面条件和近端纤维膜的形成。然后，重塑过程逐渐导致柄卡在远端骨干中（近端“应力旁路”）。
