Chasing the Ultimate Implant

Chasing the Ultimate Implant

New technologies and design approaches help to chart the course of next-generation

orthopedic technology.

Patrick Treacy and Walter Schmidt

Stryker Orthopaedics

Lesson number one: The patient always comes first. That’s a medtech axiom. Orthopedic implant companies, to be successful, have to find the perfect balance between leveraging the latest manufacturing technology and bringing it to bear for the best patient outcomes. By itself, the latest, greatest and most durable material isn’t enough to make a successful knee replacement, for example. Orthopedic firms have a number of resources to draw upon in the pursuit of re-creating the body’s natural framework and movement.

In this article, two executives from Stryker Corp. discuss that company’s approach to some of today’s orthopedic design challenges.

Challenges in Knee Implant Design

By Patrick Treacy

The push for innovation in total knee replacement design is being driven by a baby boomer population that’s living longer and expecting more than just relief from knee pain. Such patients expect to return to their previously active lifestyles with an implant that has the potential for a greater lifespan than ever. Not only does this mean that their implants need to last longer and be more durable, the implant’s performance also needs to accommodate the activities they enjoy, through increased range of motion and stability.

As a result, the orthopedic industry’s challenge has been to develop both improved implant designs and materials to meet the needs of high-demand patients. A recent publication in the Journal of Bone and Joint Surgery emphasized that materials alone do not compensate for shortfalls in implant design.

Design Philosophies

Historically, knee implants have been designed based on a traditional perspective of knee biomechanics that described knee motion as occurring around changing centers of rotation. Implants that were designed based on this philosophy feature a changing center of rotation and multi-radius saggital geometry. Research and improved technologies recently have enabled a more contemporary understanding of knee anatomy and kinematics.

Surgeons and medical device makers now believe that knee motion actually occurs around a single center of rotation called the flexion-extension axis. Coupled with the circular geometry of the posterior condyles, this axis enables more natural knee kinematics.1 As a result, modern implant designs should be designed to assist in restoring these features by implementing a single axis of rotation and a constant saggital radius through the functional range of motion.

To respond to these issues, surgeons collaborated with engineers to createStryker’s Triathlon knee system. This group based its design goals on the modern understanding of how the knee functions. As a result, the implant system is designed with a single radius, based on a fixed flexion/extension axis, to promote stability and a rapid return to function.
In addition, the implant was designed to allow for “soft tissue guided motion,” which enables the implant to work with the patient’s body, not against it. This design helps reduce the stresses on the implant and may make it easier to move and wear less over time. A range of eight different anthropometric implant sizes captures a wider range of patients and accommodates both male and female anatomies.

In designing Triathlon, one of the goals was to reduce polyethylene wear, thereby potentially allowing for a longer lasting implant.2 With standard polyethylene, the implant has demonstrated a 53 percent reduction in wear compared with other implants.3 Incorporating an advanced bearing surface, the implant also has shown 96 percent less wear in laboratory testing compared with other bearing technology.3-4

Clearly, implant design and improved materials can increase clinical success while allowing patients the potential to return to their active lifestyles more quickly.5 In the future, collaborations in instrument and implant design between surgeons and device makers could have the potential to drive innovation in the clinical and kinematic performance of total knee replacements.

A Three-Dimensional Approachto Implant Design

By Walter Schmidt

As our industry evolves to meet a changing patient population and meet other challenges facing the global healthcare community, the tools we use to design implants also will evolve. Historically, implant design has been derived from collections of two-dimensional radiographs, literature-based data, and/or physical measurements of archived museum specimens. While these databases have played a critical role in implant design, we are now finding that three-dimensional (3-D) design tools can enable us to design implants that help address a wider range of demographic variables, such as age, gender and body mass index. These 3-D design tools may move us another step closer toward our goal of creating implant designs that help improve long-term clinical outcomes.

Creating a New Design Model

Recognizing the need for a next-generation implant design development process, Stryker initiated development of a new 3-D design tool. Its Osteosynthesis division began developing a “Virtual Bone Database” to facilitate the design of pelvic, femoral, tibial and fibular plates, as well as trochanteric nails. This new database, developed in partnership with the company’s Orthopaedics division, includes computer tomography (CT) scans from a diverse population of living patients, spanning the demographics of age, gender, height, weight and ethnicity. This virtual catalog currently contains more than 800 3-D lower extremity CT scans.

The trick is turning a repository of image data into a design tool. The database uses patient demographic information to assist in the design effort, providing engineers with a wider foundation of data upon which to conceptualize new designs. All of the patient data is cataloged according to patient demography. First, individual patient bone segments are imported into computer-aided design programs. Then, these segments can be sorted according to demographic characteristics, enabling engineers to virtually evaluate component shape and fit, via virtual surgery and implantation, within specific population subsets. These subsets can then be combined into single average shapes for collective evaluation.

The database also allows design engineers to address bone density, a critical issue in implant design. Patient bone density information is recorded from CT scan data, enabling engineers to evaluate the best potential position of pegs, keels or screws in trauma, extremities and orthopedic applications.

To support the database, a customized bone dimensioning software tool was developed, which allows design engineers to select a certain bone segment (e.g., a pelvis, femur, tibia, patella, or fibula) and prescribe a patient demographic subset. The design engineer either can define specific lengths or angles from a virtual template, or select from predefined measurements. The software tool pinpoints coordinates on patient-specific bone segment surfaces using template bone locations as points of reference. This exercise enables engineers to evaluate implant designs more thoroughly in a virtual environment, using data that is more demographically specific than ever before.

Potential Design Advantages

Tools like this can allow design engineers to take the design evaluation process one step further toward the goal of helping improve clinical outcomes. The database’s 3-D bone morphological information can be used to design implant shape and help optimize bone coverage, to potentially result in better implant fit, long-term implant stability, and reduced intra-operative fracture risk. Using tools like this, an orthopedic company can develop implants that fit the widest possible patient population, potentially decreasing the need for multiple implants in the operating room or implant customization during surgery.

One of the biggest advantages of the database is the ability to assess potential fit in individual patients. Using finite element analysis computer simulations, engineers can evaluate an implant’s shape and stiffness performance in conjunction with a specific patient bone segment, enabling them to design what could be the best implant fit for specific patient groups. It also enables a design team to address clinical issues such as stress shielding.
Stress shielding is defined as a reduction of local bone density over time as a result of a stress reduction in the bone, which can be caused by implants which are stiffer than the bone that the device is replacing. By analyzing the possible effects of stress shielding on current implant designs, it is easier to develop futuristic designs with the goal of reducing stress shielding.

Building the Futureof Implant Design

Three-dimensional implant design and virtual testing using databases represent one step toward the future of medical device design. Not only do these tools drive research and development efficiency, they help keep our focus in the right place—on the patient and improved clinical outcomes.

Patrick Treacy, is vice president and general manager, Knee Reconstruction and Biomaterials, at Stryker Orthopaedics.

Walter Schmidt is principal engineer, Reconstructive Technologies, for the Kalamazoo, Mich.-based company.

References:

1. Eckhoff, D.G., Bach, J.M., Spitzer, V.M., Reinig, K.D., Bagur, M.M., Baldini, T.H., Flannery, N.M.P. Three-Dimensional Mechanics, Kinematics, and Morphology of the Knee Viewed in Virtual Reality. J Bone Joint Surg Am. 2005; 87:71-80. doi: 10.2106/JBJS.E.00440

2. Mueller-Rath R., Kleffner B., Andereya S., Mumme T., Wirtz D.C. Measures for reducing ultra-high-molecular-weight polyethylene wear in total knee replacement: a simulator study. Biomed Tech (Berl). 2007; 52(4): 295-300.

3. Stryker Test Report RD 06-013

4. R. Tsukamoto, P.A. Williams, H. Shoji, K. Hirakawa, K. Yamamoto, M. Tsukamoto and I.C. Clarke, “Wear of sequentially enhanced 9-Mrad polyethylene in 10 million cycle knee simulation study,” Dept Joint Research Centre, Loma Linda

5. Hitt, K., Harwin, S.F., Greene, K.A. Early Experience with a New Total Knee Implant: Maximizing Range of Motion and Function with Gender-Specific Sizing.