Journal of Biomechanics 36 (2003) 1659–1668 Experimental evaluation of an elastic foundation model to predict contact pressures in knee replacements Benjamin J. Fregly a,b,c, *, Yanhong Bei a , Mark E. Sylvester a a Department of Mechanical & Aerospace Engineering, University of Florida, Gainesville, FL 32611, USA b Department of Biomedical Engineering, University of Florida, Gainesville, FL 32611, USA c Department of Orthopaedics & Rehabilitation, University of Florida, Gainesville, FL 32611, USA Accepted 9 April 2003 Abstract Computational wear prediction is an attractive concept for evaluating new total knee replacement designs prior to physical testing and implementation. An important hurdle to such technology is the lack of in vivo contact pressure predictions. To address this issue, this study evaluates a computationally efficient simulation approach that combines the advantages of rigid and deformable body modeling. The hybrid method uses rigid body dynamics to predict body positions and orientations and elastic foundation theory to predict contact pressures between general three-dimensional surfaces. To evaluate the method, we performed static pressure experiments with a commercial knee implant in neutral alignment using flexion angles of 0, 30, 60, and 90  and loads of 750, 1500, 2250, and 3000N. Using manufacturer CAD geometry for the same implant, an elastic foundation model with linear or nonlinear polyethylene material properties was implemented within a commercial multibody dynamics software program. The model’s ability to predict experimental peak and average contact pressures simultaneously was evaluated by performing dynamic simulations to find the static configuration. Both the linear and nonlinear material models predicted the average contact pressure data well, while only the linear material model could simultaneously predict the trends in the peak contact pressure data. This novel modeling approach is sufficiently fast and accurate to be used in design sensitivity and optimization studies of knee implant mechanics and ultimately wear. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Dynamic modeling; Contact pressure prediction; Total knee replacements 1. Introduction Wear remains a primary factor limiting the life span of total knee replacements (TKRs). Liberated polyethy- lene wear debris can initiate osteolysis (i.e., bone destruction) resulting in pain and implant loosening. Researchers currently have three basic options for studying wear: (1) Analyze implants retrieved after failure, (2) Analyze implants retrieved post-mortem, or (3) Analyze implant wear test results. Ideally, implants prone to failure would be identified before such designs are used in patients. While revision and post-mortem retrievals are valuable for studying insert damage modes (Bartel et al., 1986), they can be difficult to obtain and take years before becoming available (Harman et al., 2001). Physical wear testing is essential, and recent knee simulator designs are becoming more successful at reproducing the wear patterns observed in retrievals (Walker et al., 1997). However, a single test can cost tens of thousands of dollars and take months to run. A computational wear model is an attractive solution to these limitations (Sathasivam and Walker, 1998). Required inputs to such a model are in vivo tibial insert surface kinematics and contact pressures. Deformable body contact analyses, such as finite element analyses (FEA) (Bartel et al., 1986, 1995; Bendjaballah et al., 1997; D’Lima et al., 2001; P! eri ! e and Hobatho, 1998; Otto et al., 2001; Rawlinson and Bartel, 2002, Sathasi- vam and Walker, 1998, 1999), elasticity analyses (Bartel et al., 1986; Jin et al., 1995; Rawlinson and Bartel, 2002), and elastic foundation analyses (Blankevoort et al., 1991; Li et al., 1997; Nun˜o and Ahmed, 2001; Pandy ARTICLE IN PRESS *Corresponding author. Department of Mechanical & Aerospace Engineering, University of Florida, 231 MAE-A Building, PO Box 116250, Gainesville, FL 32611, USA. Tel.: +1-352-392-8157; fax: +1- 352-392-7303. E-mail address: fregly@ufl.edu (B.J. Fregly). 0021-9290/03/$-see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0021-9290(03)00176-3