Abstract
Introduction
Typical failure of cementless total hip arthroplasty is the lack of initial stability. Indeed, presence of motion at the bone implant-interface leads to formation of fibrous tissue that prevents bone ingrowth, which in turn may lead to loosening of the implant. It has been shown that interfacial micromotion around 40 produces partial ingrowth, while micromotion exceeding 150 completely inhibits bone ingrowth. Finite element analyses (FEA) are widely used to evaluate the initial stability of cementless THA in pre-clinical validation. Untill now, most FE models developed to predict initial stability of cementless implants were performed based on static load, by selecting the greatest load at a particular time of the cycle activity, but in fact the hip is exposed to varied load during the activity. The aim of this study is to investigate the difference in the predicted micromotion induced by static, quasi-static and dynamic loading conditions.
Materials & Methods
Finite element analysis (FEA) was performed on a Profemur®TL implanted into a composite bone. The implant orientation was validated in a previous study [3]. All materials were defined as linear isotropic homogeneous. Static and dynamic FEA was performed for the loading conditions defined by simulating stair-climbing. In the static analysis, the applied resultant force (calculated with a body weight of 836N) were 951N and 2107N to simulate the abductor muscle and the hip joint contact forces, respectively [4]. In the dynamic analysis, the applied resultant force can be seen on Fig. 1. The initial stability was extracted on 54 points (Fig. 2) located on the plasma spray surface by calculating the difference between the final displacement of the prosthesis and the final displacement of the composite bone.
Results
The mean micromotion predicted with the static loading conditions is 32μm with a maximum of 76μm whereas the maximum micromotion predicted with dynamic loading conditions is 36μm with a maximum of 86μm. Micromotion predicted with dynamic load greater than the micromotion predicted with static load on 35 out of 54 points. In the superior portion of the prosthesis, micromotion predicted with static loading condition is greater on medial posterior and in lateral anterior faces. In the inferior portion, the micromotion
Discussion
Micromotion predicted by the dynamic loading condition is greater than that predicted with static loading condition. Moreover, 22 points are in the range of 50–150μm (range for partial osseointegration) with dynamic condition, whereas only 16 points are in this range with static condition. On the posterior inferior face, all points are in this range with the dynamic condition, whereas only 2 with static condition. However micromotion predicted at all points either by static or dynamic conditions are lower than 150μm, the threshold value with regard to osseointegration.
