Using Simulation Methods for Orthopaedic Implant Design

Historically, implant companies have relied heavily on producing physical prototypes for testing hip joints, shoulder joints, knee replacements and spinal implants for reliability and regulatory approvals. However, orthopaedic device manufacturers are increasingly investing in finite element analysis (FEA) and biomechanics simulation technology to conduct full body simulations of the musculoskeletal system. Target areas of the body include lumbar, fusion, total
hip replacement, total knee replacement and cervical simulation.

Biomechanics simulation technology brings to life the ideas of mechanical and biomedical engineers to make better performing products faster, help them innovate more rapidly and deliver device design concepts to the surgeon. It allows users to visualise in a more realistic manner how the product will work early on in the design phase. Surgeons can more effectively be trained on how devices can be implanted into the body.

Traditional methods

The traditional product development process starts with computer-aided design (CAD) drawings and moves to the use of FEA software to model and run a series of stress analyses on the device and/or surrounding bone to examine high strain areas. This has been a common approach taken by many in the orthopaedics market. The requirements to run FEA include modelling the parts and the environment in which the parts will operate. This can be a time consuming effort in itself.

Once the modelling is complete, the amount of time spent on the analysis can be lengthy, especially when using quasistatic analysis methods to step through simulation in increments to run knee simulations. This process alone can take one or several days to complete. This information is valuable, however, because once engineers have finished FEA, they are then prepared with information that can be used for further analyses such as strength, durability and wear of the device. Once the FEA is complete, engineers can create a hardware prototype and then start laboratory testing. Here, they look at kinematic performance. This process typically takes four to six months to complete. However, the average total design time can take one and a half years because engineers often need to make three or four design iterations and loop the process until clinical trials begin. As a result, the designs are evolutionary and conservative.

Virtual methods

Test methods have been developed that reduce the number of design iterations by replicating the in vitro tests done in the laboratory within a virtual simulation environment that produces the same results as the test laboratory. Once in vitro simulation is modelled and validated, in vivo simulations can be run within the virtual environment to replicate real life events such as climbing stairs, sitting, standing and jumping (Figure 1). The end result is the ability to assess the implant’s performance in real world simulations.

This process enables engineers to look at the entire kinematic performance, including strength and durability using biomechanics simulations, and then export the data to FEA and perform system level simulations that make the overall test process move faster (Figure 2). Once validated, the results of doing virtual testing in a more holistic, iterative way enables engineers to reduce the number of laboratory tests and reach clinical trials faster.

Best of both techniques

In this way, the optimal simulation process uses multibody dynamics simulation and FEA. It unites the best aspects of both methods to minimise computational complexity and expand the scope of multibody dynamics. It will assess important nonlinearities such as polyethylene and cement loads to evaluate how materials are behaving or device components interact with bone such as the tibia.

Traditionally, FEA has been the only approach, but it does not tell engineers what the boundary conditions are and how to load the device properly to ensure that it is accurate and aligned with what is seen in clinical trials. Today, many manufacturers are utilising full body simulations and gathering mechanical loads to apply to FEA analysis to thereby more accurately understand device performance. For example, can the device design or analysis be applied to a 90th percentile male? By undertaking studies on various groups of anatomical models within a virtual environment, engineers can obtain better answers related to device performance and optimise designs to fit many populations of people who range in size, shape and weight. Working in a common tool set and transferring loads effectively between FEA and biomechanics streamlines the process and gives engineers accurate results faster than traditional methods of test.

FEA technology and multibody dynamics simulation can be integrated simultaneously. This allows dynamic boundary conditions and loads to be automatically transferred to the FEA model and small changes and their impact on the dynamics performance (the biomechanical range of motion or performance of the device inside the body) to be assessed. The end result is a fully integrated simulation solution in which engineers visualise full body gait models and witness how loads are being applied.

Future developments

Further refinements are on the way. These relate to the improved integration of FEA and biomechanics and the ability to evaluate fully nonlinear transient analysis in a multibody environment that takes into account polyethylene and cement loads in the highest fidelity within musculoskeletal models. This will be a complete hybrid solution. Current research is exploring kinematic performance, how it affects the musculoskeletal system and patient muscle efficiency and contact forces. Also being investigated are components to assess wear and stress strain in cement and bone, and improved ways to distribute results to engineers across design teams to ensure more efficient virtual testing and design.

Leslie Rickey is the Senior Director, Product Marketing for MSC Software Corp., 2 MacArthur Place, Santa Ana, California 92707, USA, tel. +1 714 540 8900, e-mail: leslie.rickey@mscsoftware.com [3], www.mscsoftware.com/ [4]

MSC Software operates in partnership with LifeModeler Inc. (www.lifemodeler.com [5]).