Aim of this study was the development of a dynamic FE-framework to identify worst-case size combinations and kinematics in a virtual wear simulator setup covering five daily activities and high, dynamic loads.

Two cruciate sacrificing knee designs (D1 & D2) were tested physically on a wear-testing machine prior the model development using a high demanding, daily activity protocol (HDA) [1]. A simplified FE-setup was generated, reduced to the 3D geometries of the assembly whereas the representation of the mechanical wear simulator conditions and the load transmission was achieved by joint elements. Inertial and other time-related effects of the physical situation were compensated by a system of spring- and damper elements.

Using a time-series signal optimization approach on the anterior-posterior translation and the internal-external rotation results for each activity, 38 variable parameters were varied in between pre-defined limits in a semiautomatic workflow. For each design, two consecutive cycles of a single activity were analysed and the results of the second cycle were used for the optimization. Based on the determined values, a single set of averaged parameter settings was identified that covers all activity cycles sufficiently.

A total of 1010 dynamic analyses were carried out in order to find a sharable set of parameter values.

In this study, an efficient simulation workflow for design evaluation was developed. Therefore, a HDA wear-testing machine was simplified to boundary conditions and stabilizing elements, using a single set of parameters for all activities. The calculated kinematics were in a comparable range to the machine output. Further applications of the method were found in systematic analyses of entire implant systems to achieve consistent kinematics over the size compatibility range in the design process of new implant systems.

Introduction

Acetabular bone defects are still challenging to quantify. Numerous classification schemes have been proposed to categorize the diverse kinds of defects. However, these classification schemes are mainly descriptive and hence it remains difficult to apply them in pre-clinical testing, implant development and pre-operative planning. By reconstructing the native situation of a defect pelvis using a Statistical Shape Model (SSM), a more quantitative analysis of the bone defects could be performed. The aim of this study is to develop such a SSM and to validate its accuracy using relevant clinical scenarios and parameters.

Introduction: Total knee arthroplasty (TKA) has become a successful clinical treatment for patients in regard to relief of pain, correction of deformity and restoration of function with promising long term behaviour [Pradhan et al. 2006].

In TKA the generation of polyethylene wear debris is mainly affected by the factors design of the articulating bearing, contact stresses, kinematics, implant material and surface finish [McEwen et al. 2005].

The objective of our study was to evaluate the in vitro wear behaviour of fixed bearing knee designs in comprehension to the contact mechanics and resultant kinematics for different degrees of congruency.

Material and Methods: Wear simulator testing on 12 TKA devices has been performed according to ISO 14243-1 under load control. The knee replacements were tested in the fixed bearing configurations LC, CR and DD with different degrees of tibio-femoral congruency.

For gravimetric wear assessment the protocol described in ISO 14243-2 has been used, followed by a kinematic analysis of the single test stations.

The articulating contact and subsurface stresses have been investigated in a finite element analysis.

Results: The contact areas are increasing from Search® Evolution LC (144 mm2) to Columbus® CR (235 mm2) and Columbus® DD (279 mm2), whereas the peak surface contact stresses are decreasing from Search® Evolution LC (34.4 MPa) to Columbus® CR (20.9 MPa) and Columbus® DD (18.1 MPa). The estimated amount of wear has decreased from Search® Evolution LC (21.4 mg/million cycles) to Columbus® CR (8.9 mg/million cycles) and Columbus® DD (2.2 mg/million cycles).

The wear rates between the knee design configurations differ substantially and statistically analysis demonstrates a significant difference (p< 0.01) between the test groups in correlation with congruency.

Conclusion: The present study demonstrates the influence of different bearing types on contact stresses, abrasive wear and kinematics for three different degrees of tibio-femoral congruency under elimination of production, material and sterilization parameters.

Corresponding author: Dr.-Ing. Thomas M. Grupp

Research and Development: e-mail: thomas.grupp@aesculap.de