An advanced joint simulation technology by Advanced Mechanical Technology, Inc (AMTI), Watertown, MA, is showing researchers how new prosthetic designs will perform inside the human body. Virtual Soft Tissue (VST), a highly detailed and programmable computer model, accounts for the missing biological structure of the knee during simulation testing of joint implants.

"We've created something that allows an implant to react the way it would as an integrated part of the knee," says Bruce White, CEO of AMTI and VST patent recipient.

Total knee replacement (TKR), the most common total joint procedure, is designed to replace the articular cartilage, which may have eroded over time or become damaged by extreme activity or accident. A largely successful procedure, TKR has reduced pain and improved mobility for a population of several million individuals. In response to patient and medical demand, designers and manufacturers are working toward greater range of motion and longer service life while also striving to ensure patient safety.

An AMTI ADL knee simulator performs a wear test on a set of knee implants. Equipped with Virtual SoftTissue (VST), these wear testing machines help implant manufacturers and researchers evaluate the performance and durability of new implant designs.

Simulator machines are designed to test the function and durability of implants by recreating the loads and movements of almost any activity so the response of the implant can be evaluated. Over the past two decades, the role of joint motion simulation has evolved from a scientific tool into an integral part of the design, manufacturing, and quality assurance processes of implant device production.

"For fully realistic simulation, you need an accurate constraint model, information about the way the soft tissues surrounding the knee joint would respond to the joint’s motion," says White. "Soft tissue may share as much as 50% of the shear load for the joint, so a more accurate and detailed constraint model will give you more accurate test results."

Soft tissue constraint was originally represented with simplistic linear models and mechanical springs. However, researchers found that wear results obtained using this method did not match what they observed in patients.

"The constraining force of the soft tissue is complex and asymmetrical, making it a challenge to represent," says White. "But VST is a sophisticated model and gives us the clearest picture we have of how a new prosthetic device will respond after it is integrated with the remaining biological structure of the knee."

That deep understanding of implant performance has become even more important as the average age of implant recipients has fallen over the past decade. This younger, more active population is driving the development of implants that offer enhanced performance and longer service life. These goals are naturally at odds: the more movement an implant allows, the greater the challenge to long-term durability. To evaluate the subtle design changes that can strike a balance, manufacturers require high-fidelity test environments.

"Implants and implant testing technology have a certain synergy—one spurring the other on to its next evolutionary stage," says White. "In an industry that has always placed patient safety above all other considerations, simulation testing must keep pace with that development."