Comparison shows impact of design on kinematics

Computer simulation may hold one key toward streamlining the process of testing new orthopaedic devices, according to Edward A. Morra, MSME, and A. Seth Greenwald, DPhil Oxon. Their display at the 2008 AAOS Annual Meeting examined the use of a computer model to simulate movement and wear in an artificial knee.

According to Mr. Morra, the LifeMOD/KneeSIM, a software application produced by Biomechanics Research Group, Inc. (San Clemente, Calif.), is a validated musculoskeletal modeling system that can be used to compare the kinematic performance and wear points of various artificial knee designs. The software displays two views simultaneously. On the right side is a visual representation of an artificial knee, based on the specifications of the device and patient parameters. The left side contains engineering plots and data with which an engineer might feel more comfortable.

“The two displays are synchronized so that the doctor can see how the knee moves through its range of motion during knee replacement surgery,’” said Mr. Morra. “And the plot quantifies that movement so that implant designers and engineers can make better decisions.”

Like its real-world counterpart, the simulated knee contains bones, ligaments, tendons, muscles, and cartilage. The muscle-driven model is capable of performing almost any activity the user prescribes and will process data based on the specifications of any knee implant components.

“You can specify any implant in the program,” said Mr. Morra, “and the software will make the calculations to balance the implant nicely. It will adjust all of the ligament tensions so it’s a perfect [virtual] surgery every time. […] We’re able to present a compelling visualization that both the doctor and the designer can understand.”

Study results
The research team used KneeSIM to compare the motion of six contemporary total knee arthroplasty (TKA) designs—three posterior sacrificing designs and three cruciate retaining designs—with in vivo kinematic data of a healthy knee. They used the software to create three-dimensional solid models of the femoral, patellar, and tibial insert components for each knee, which were then “implanted” into the KneeSIM joint space according to the manufacturer’s recommended surgical procedure.

The maximum flexion angle achievable with each design was determined through an analysis of deep flexion activity to 160 degrees of knee flexion, with maximum flexion angle being defined by impingement of the posterior femoral bone cut surface with the tibial insert. The component motions were captured as computer animations and compared with the movement of a healthy knee during a deep knee bend activity.

None of the designs studied precisely replicated the kinematics of the healthy knee—a condition that is increasingly identified by patients and physicians as an objective of TKA. As a group, the posterior stabilized designs achieved higher flexion than the cruciate retaining designs, primarily due to the engagement of the femoral cam and tibial post during high flexion. Cruciate retaining designs displayed contact in the central or anterior portions of the insert during high flexion, which diminished their ability to achieve a flexion greater than 105 degrees.

Based on their results, the researchers concluded that a dynamic computer simulation could expand the methodologies available for predicting TKA outcomes, particularly in the area of knee kinematics. “We believe that we can take this tool and design it to be broadly used for testing new components,” said Mr. Morra.

“The Influence of Contemporary Knee Design Geometry on High Flexion Motion: A Kinematic Comparison” was Scientific Exhibit 32 at the 2008 AAOS Annual Meeting. Disclosure information on the presenters can be found in the 2008 Proceedings of the Annual Meeting.

Peter Pollack is a staff writer for AAOS Now. He can be reached at ppollack@aaos.org