Design and analysis of robust total joint replacements: finite element model experiments with environmental variables.

Computer simulation of orthopaedic devices can be prohibitively time consuming, particularly when assessing multiple design and environmental factors. Chang et al. (1999) address these computational challenges using an efficient statistical predictor to optimize a flexible hip implant, defined by a midstem reduction, subjected to multiple environmental conditions. Here, we extend this methodology by: (1) explicitly considering constraint equations in the optimization formulation, (2) showing that the optimal design for one environmental distribution is robust to alternate distributions, and (3) illustrating a sensitivity analysis technique to determine influential design and environmental factors. A thin midstem diameter with a short stabilizing distal tip minimized the bone remodeling signal while maintaining satisfactory stability. Hip joint force orientation was more influential than the effect of the controllable design variables on bone remodeling and the cancellous bone elastic modulus had the most influence on relative motion, both results indicating the importance of including uncontrollable environmental factors. The optimal search indicated that only 16 to 22 computer simulations were necessary to predict the optimal design, a significant savings over traditional search techniques.

Preclinical cost analysis of orthopaedic implants: a custom versus standard cementless femoral component for revision total hip arthroplasty.

A preclinical cost analysis method was introduced to assess the cost effectiveness of using a custom implant instead of standard "off-the-shelf" implants for revision total hip arthroplasty. Finite element models of proximal femur-implant systems were constructed and an array of environmental factors, including loads and bone properties, was incorporated into a computer experiment to evaluate relative motion between implant and bone. Implant performance related cost was then determined from relative motion measures using a quality loss function. Unit manufacturing cost was added to implant performance cost to determine the cost difference between the two implants. The reduction in relative motion achieved by the custom implant with respect to an equivalent-lengthed standard implant justified its additional unit manufacturing costs. In response to these results and suggestions by surgeons, we increased the length of the standard implant by 50 mm and performed an identical series of analyses. We found that increasing the stem length to 120 mm substantially decreased the relative motion of the standard implant to values less than for the custom implant. This case study provides preliminary evidence that a surgical inventory consisting of longer-stemmed standard implants or modular distal stems is more cost effective than designing custom devices on a case-by-case basis. Additional design studies are warranted before generalizing such a claim.

Robust optimization of total joint replacements incorporating environmental variables.

Direct search techniques for the optimal design of biomechanical devices are computationally intensive requiring many iterations before converging to a global solution. This, along with the incorporation of environmental variables such as multiple loading conditions and bone properties, makes direct search techniques infeasible. In this study, we introduced new methods that are based on the statistical design and analysis of computer experiments to account efficiently for environmental variables. Using data collected at a relatively small set of training sites, the method employs a computationally inexpensive predictor of the structural response that is statistically motivated. By using this predictor in place of the simulator (e.g., finite element model), a sufficient number of iterations can be performed to facilitate the optimization of the complex system. The applicability of these methods was demonstrated through the design of a femoral component for total hip arthroplasty incorporating variations in joint force orientation and cancellous bone properties. Beams on elastic foundation (BOEF) finite element models were developed to simulate the structural response. These simple models were chosen for their short computation time. This allowed us to represent the actual structural response surface by an exhaustive enumeration of the design and environmental variable space, and provided a means by which to validate the statistical predictor. We were able to predict the structural response and the optimal design accurately using only 16 runs of the computer code. The general trends predicted by the BOEF models were in agreement with previous three-dimensional finite element computer simulations, and experimental and clinical results, which demonstrated that the important features of intramedullary fixation systems were captured. These results indicate that the statistically based optimization methods are appropriate for optimization studies using computationally demanding models.

Cemented femoral stem performance. Effects of proximal bonding, geometry, and neck length.

The effects of proximal bonding, distal stem geometry, and femoral neck length on cement and interface stresses were determined to understand better their role in clinical performance. The effects of stem design were compared with the effects of environmental variables, patient weight, and patient activity. Finite element models were used to determine peak cement and interface stresses, and an experimental layout was used to separate design and environmental effects. Bonding reduced cement mantle stresses by 35% to 60%, to levels below the cement fatigue strength. A flat sided implant provided more torsional resistance, reducing shear stresses at the proximal cement-prosthesis interface by 22% to 73% with respect to a distal round implant. Neck length had minimal effects on stresses compared with bonding or implant geometry. Cement-bone interface stresses were more sensitive to patient activity than to the design variables. Therefore, claims that a strong cement and prosthesis bond may be harmful to the bone-cement interface are unjustified based on these results. The best combination of design variables was a proximally bonded, flat sided implant with neck length left to the surgeon's discretion. This combination was most effective at protecting the cement mantle and prosthesis interface and perhaps the cement-bone interface by minimizing stresses associated with cement debris generation.