Newsletter Volume 12 - 4th Quarter 2006
Maximizing Performance of a Total Knee System
CONTENTS
Case Study: Maximizing Performance of a Total Knee System
Software: NEW! LifeMOD/KneeSIM™ v2007
BRG announces LifeMOD Employment Referral Network.
Publications: Journal Papers, Magazine Articles and Book Chapters
This issue of the newsletter builds on last month's issue on quantifying performance by identifying human function signatures. In this month's issue, Maximizing Performance of a Total Knee Replacement System, we introduce our total knee development solution, LifeMOD/KneeSIM. This product was developed in a 10 year collaboration with most of the top orthopaedic companies in the world.
Building on the notion of human signatures and standardization in the orthopaedics industry, we would like to announce that the Orthopaedic Research Laboratories, Cleveland Ohio, directed by A. Seth Greenwald, has chosen LifeMOD/KneeSIM to develop a series of total knee replacement system (TKR) testing and validation protocols. Using the human function signature methodology described in this newsletter, they will be introducing a standard system of measurement for into an industry which has always had a need for performance quantification.
Finally, with the many employment inquiries from students and scientists, as well as many inquiries from our industry clients we have decided to link up the two with the LifeMOD Employment Referral Network.
CASE STUDY: MAXIMIZING PERFORMANCE
OF A TOTAL KNEE SYSTEM
Introduction
How does one replicate normal knee function with metal and plastic? How can congruent geometric surfaces replicate the stabilizing function of the cartilage, menisci and cruciate ligaments while providing the patient with good range of motion? This is the challenge facing the engineers, surgeons, and scientists who develop total knee replacement systems.
In an industry which typically relies on clinical data of past patient performance, fluoroscopy studies with patients, and cadaver experiments, designs evolve very slowly. In the replaced knee, the contacting congruent surfaces of the femur, tibia and patella not only perform the general load bearing tasks of the natural knee, but also add the stabilizing factor of the missing menisci and cruciate ligaments.
To achieve an optimal design, engineers must understand the physics of the knee system and how it functions in the body. In order to make significant and timely advances in patient satisfaction, manufacturers simply cannot wait for each design iteration to be manufactured and implanted in a cadaver for a series of mechanical tests which supply limited design feedback.
Virtual in vivo simulation
What would change if one could seamlessly compare the kinematic and kinetic performance of TKR designs to competitors' and to a functioning natural knee very quickly? Imagine a virtual product development environment to optimize functional performance while maintaining product durability and design robustness. Consider being able to brainstorm and communicate design and alignment sensitivities to investigators and surgeons with both animations and dynamic graphs. Of course, results must be assured to be reliable, using continuously validated test protocols and providing comparable results with fluoroscopy and other test data.
This virtual in vivo simulation is the main focus of a new product, LifeMOD/KneeSIM. By integrating virtual in vivo simulation into the established engineering design cycle, design innovation can be accelerated resulting in increased patient satisfaction. To explain how this can be accomplished, we will follow the steps outlined in last month's newsletter, "Quantifying Performance using Human Function Signatures".
Step 1. Develop the appropriate model
The approach used to model the human response in LifeMOD/KneeSIM is to simulate
the function of the Purdue/Oxford Rig knee mechanical simulator shown in Figure
1. The knee simulator model applies a quadriceps and hamstrings load across the
joint to match a user-entered hip flexion angle history. External reactions such as
bodyweight and ground reaction forces are applied through actuators at the hip and
ankle respectively. The motion of the patella with respect to the femoral component
and the femoral component with respect to the tibia is retrieved using a virtual
spatial linkage. This virtual machine is capable of applying loads across the joint
similar to a deep knee bend, walking or lunging. This configuration is chosen for
the virtual in vivo simulation model for the following reasons:
- Versatile function, can simulate several activities
- Easily validated, virtual machine very similar to physical machine
- Repeatable, simulation results reflect only design changes in knee system.
Figure 1. LifeMOD/KneeSIM virtual Purdue/Oxford Rig. The virtual model is driven using the same inputs as the physical simulator, providing an excellent validation platform.
Step 2. Make sure the model is valid
LifeMOD/KneeSIM is continuously validated by several major orthopaedic companies
against data from physical test machines and live patient fluoroscopy studies. In
addition, the results from LifeMOD/KneeSIM are easily comparable to published studies
using the same coordinate reference systems and data reporting methods.
Figure 2. Here are the results for one manufacturer's validation study. The simulation results (dotted lines) are compared against patients which have the implanted component system for a lunge activity (solid lines). The data to be compared are the "low points" or sulcus of each condyle of the femoral component.
Step 3. Define a small set of test events
The test events to be performed with the virtual simulator are double deep knee bend,
gait and lunge activities. The results from all three activities can be compared
against human patients in publications involving fluoroscopy studies.
Figure 3. The virtual Purdue/Oxford rig is extremely versatile and can be used to drive gait, deep knee bend and lunge simulations.
Step 4. Define the performance signature
There are two types of signatures which would benefit TKR design: kinematic, or how
the components move during a certain activity and kinetic, or the forces present
in the device itself and the surrounding tissues and bones. Both signatures may be
used to compare device performance against the natural knee and competitor products
and designs.
Figure 4. This is an example of one set of data used to quickly characterize the kinematic (motion) performance of an implant concept. The "barbells" in the model are rigidly attached to the femoral component and each sphere is located at the center of the small posterior radii. By plotting the anterior-posterior travel of the spheres, a notion of femoral rollback and internal/external rotation is conveyed. These data may then be compared to an intact knee as a "gold standard".
Understanding the Physics of Pathological Motion - Paradoxical Motion and Lateral Pivot
In order to design new high-performance knee systems, the physics of previous design pathologies must first be understood.
Motion in the knee during flexion or any activity is governed by forces. The geometric shape of the bearing surfaces can have a predominant effect on these forces and can influence or guide the motion of the knee.
Using LifeMOD/KneeSIM, advanced TKR designers can identify and quantify which force vectors were responsible for the motion of the components in the animation. These forces result from contact between the bearing surfaces of the components, ligament forces, and muscle forces. Understanding the location, direction and magnitudes of all forces present during a set of activities provides the designer with greater knowledge of the physics of knee function.
To illustrate the relationship between component geometry and forces, we examine two well known issues of pathological motion in certain TKR implants today: paradoxical motion and lateral pivot (figure 5). During a deep knee bend activity, the main tissue force present is the quadriceps muscle. Due to the Q-angle in the joint (left picture, figure 5), the muscle reactions induce a torque which serves to internally rotate the tibia. In other words, the forces and torques attempt to reduce the Q-angle during high forces across the joint. With a dual contacting bearing surface such as the tibiofemoral joint, the dominant center of rotation of the tibia can occur about either the medial side (medial pivot) or the lateral side (lateral pivot). In a healthy knee during the initial stages of flexion, the femur first translates posteriorly and pivots about the medial side (center picture, figure 5).
With the geometric configuration of many existing TKR systems, the femur is positioned too far posteriorly during extension. When the knee is flexed the femur moves anterior (instead of posterior, hence paradoxical) due to the muscle forces. Once in this position the femur pivots laterally due to the geometric conformity of the tibial component.
Figure 5. In the normal knee the Q-Angle is formed by the line of the quadriceps force and the patellar ligament axis (left). During flexion, forces direct the femur to roll back because of the anterior position of the femur on the tibia. As these forces increase, the quadriceps mechanism begins to reduce the Q-angle by applying external rotation torque to the femur through the patella (center). In some conventional TKR's (right), since the articular surfaces are not anatomically accurate the femur sits posterior on the tibia and must translate forward due to tissue forces. Then, during flexion, the femur pivots laterally to minimize the Q-angle.
Once the force environment of this phenomenon is identified and understood, TKR designers may begin to explore design changes which could affect this abnormal knee motion. Whether it is changing the geometric conformity of the condyle compartments, or changing the location and geometry of the cam mechanism, the resulting motion and the force environment of each iteration may be instantly examined.
The force environment is communicated to the surgeon, engineer, designer using animation of scaling force vectors and a set of characterization data plots. As an example, figure 6 displays a deep knee bend simulation at 50 degrees of flexion. From the animation and data it can be understood which force contributed to the observed motion.
Figure 6. The force environment for a deep knee bend at 50 degrees of flexion. The dominant tissue forces are the quad muscle forces and the patellar ligament force. The contact forces include the components of the tibiofemoral contact force and the cam engagement contact force, and the patellofemoral contact force.
The product performance envelope - optimizing function while maintaining robustness and durability
Optimized knee function is not the only consideration for engineers developing a high performance design. The TKR design must also be robust or relatively insensitive to surgical alignment error and patient anatomical differences. In the new virtual design paradigm, many orthopaedic companies run LifeMOD/KneeSIM in a variability analysis loop (Figure 7) to determine the margin of error and patient variability for the design.
And of course durability is paramount; the TKR design must be long lasting and resistant to wear and breakage. This is accomplished by exporting the forces and displacements from the LifeMOD/KneeSIM functional simulation as boundary conditions and loadings for finite element analyses.
Figure 7. Using LifeMOD/KneeSIM to perform a variability analysis. Variables include component locations/orientations, ligament and muscle attachment locations, tissue mechanical properties and pre-tensions. From each simulation, the function signatures are compared with the norm to determine the safe placement envelope.
How does a design compare to others?
With a standard system of measurement for the complex behavior of the total knee replacement system, engineers may now benchmark existing product designs and competitor product designs. By laser scanning physical components, CAD geometry files can be created for import into LifeMOD/KneeSIM for functional simulation and performance characterization.
Figure 8 compares four characteristics of a PCL sacrificing design (red curve) and a PCL retaining design (blue dashed curve). It can be observed from the top plot that tibial internal rotation is impeded for the PCL sacrificing design because of the cam engagement. The cam also encourages much more femoral rollback than the PCL retaining design. As a result of both these effects, the quadriceps force and subsequently the patella shear (shear force between the patella component and the patella bone) is greater for the PCL sacrificing design.
Figure 8. Demonstrating the functional differences between a PCL sacrificing design (red curve) and a PCL retaining design (blue dashed curve).
Conclusion
Virtual in vivo simulation represents a very powerful design tool in the development of total knee replacement designs. In addition to accelerating design innovation, it is now being used in an effort to standardize performance characterization for government-based certification efforts. All these activities will greatly benefit manufacturers, the surgical community, and ultimately, the patient.
SOFTWARE
We would like to introduce LifeMOD/KneeSIM, a complete virtual product development solution for Total Knee Replacement Systems. The product, developed in collaboration with the world's leading orthopaedic companies, can be used for virtual simulation to ensure proper tibial rotation, validate femoral rollback, minimize patella shear, maximize quadriceps efficiency and increase durability and design robustness. LifeMOD/KneeSIM has proven itself in the design of PCL/ACL sacrificing/retaining, fixed/mobile bearing, and unicondylar component systems. With simulation times of less than five minutes to obtain results, hundreds of design iterations may be characterized in a day.
LifeMOD continues to be the leading human modeling solution integrated into many top international organizations' product design processes. With its automated and "wizard-like" modeling process, LifeMOD's ease-of-use is unmatched in the industry. At the same time, built on the de facto standard for motion analysis, MSC.Software's ADAMS, LifeMOD uniquely provides this ease-of-use on top of a true industry tested physics engine. This foundation ensures that your product, whether interacting with the human model, or inside the human model, can be captured with state of the art accuracy and capability.
With six anthropometric body size databases, LifeMOD™ can model a broad range of ages, heights, and weights. Databases included are GeBod, US Army NATICK, and PeopleSize for the UK, USA, Japan and China. Specific human or even non-human models can also be created to offer the greatest flexibility to suit your needs.
Inverse and forward dynamics with trainable muscles or joints enable you to take motion capture data beyond a simple replay, to true predictive analysis, enabling your design and development teams to explore their ideas. The comprehensive diagnostics that can be reported from LifeMOD ensure that your team will not be left guessing what is driving your product or human model performance.
LifeMOD's Hybrid III crash dummy adds even more capability by enabling injury and crash simulations to quickly perform analyses and drive safety design. And there's so much more. Learn more by visiting our web page or by contacting us at newsletter@lifemodeler.com
SERVICES
The Biomechanics Research Group, Inc. is a service-based organization with many major commercial successes in utilizing LifeMOD human simulation in the design process. We are expert in the development of the appropriate models, simulation cases and human response signature development to accelerate innovation of your product and greatly reduce time to market. Contact us to see how we have done this in many industries.
OTHER NEWS
This month we are excited to announce that the Orthopaedic Research Laboratories, in Cleveland Ohio, under the direction of A. Seth Greenwald, D. Phil. (Oxon) has chosen LifeMOD/KneeSIM to develop a new knee testing functional protocol. This capability to be developed by Ed Morra, MSME, will complement their existing efforts to provide functional performance data to a significant number of national and international manufacturers, the Food and Drug Administration (FDA) and the orthopaedic surgeon community. All these efforts are directed toward an optimization of surgical techniques, orthopaedic devices and surgeon education to improve patient outcome. See Press Release.
Throughout the years, we have received many employment inquiries from student graduates, and professionals seeking employment to utilize their skills with LifeMOD. Also, we have been repeatedly asked for references by our top industry clients for LifeMOD-trained talent. We have decided to combine both these needs in the LifeMOD Employment Referral Network. If you are a LifeMOD trained investigator or an industry professional in need of a LifeMOD trained investigator please contact us.
New Publication: "3D Motion Analysis of Golf Swings, Development and validation of a golf-specific test set-up" Nils Betzler, Stefan Kratzenstein, Fabian Schweizer, and Kerstin Witte, Ninth International Symposium on the 3D Analysis of Human Movement, 2006, See PDF.
New Publication:"The Effects of Racket Inertia Tensor on Elbow Loadings and Racket Behavior for Central and Eccentric Impacts", Steven M. Nesbit, Michael Elzinga, Catherine Herchenroder and Monika Serrano, Journal of Sports Science and Medicine, Vol 5, Issue 2, 2006, See PDF
New Publication: "Effect of Guided Knee Motion and High Flexion TKA on Kinematics, Implant Stresses, and Wear" Michael D. Ries, Jan M.K. Victor, Johan Bellemans, MD, Langdorp Belgium, Jason Otto, Brian W. McKinnon, Amit Parikh, Jeff A. Sprague, Abraham Salehi, American Academy of Orthopaedic Surgeons, 2006, See PDF.
New Publication: "Sport Biomechanical Analysis using Full-body Computer Models" Steven M. Nesbit, and M.X. Ribadeneira, Modeling and Simulation 2003, See PDF.
New book chapter: "Chapter 24: The Virtual Knee" in Total Knee Arthroplasty, Springer 2005, See PDF.
Check out our new model repository! We would like to sincerely thank those who have contributed to the LifeMOD™ body of knowledge. We pledge to do our best to expand the technical capabilities of LifeMOD while developing new ways to educate the community.
If you would like further information on our software and services, please give us a call.
Copyright© 2006 LifeModeler, Inc.