The past 20 years of innovation in knee replacement have led to increased durability and clinical longevity of implants, reduced early complications, decreased surgical time from incision to closure and better targeting of clinical indication.
The primary drivers of innovation are patients’ and healthcare providers’ desire for long-term clinical survivorship and a return to normal, healthy function without pain. This is measured by the rate of revision surgeries caused by early- to mid-term failure of the implant, infection, loss of fixation, significant pain or unacceptable functional outcomes. In short, surgeons and patients alike want one surgery to restore normal function without pain for the life of the patient. While total hip replacement is nearly “there,” and many of the materials, instrumentation and surgical innovations are common to both hip and knee, industry continues to grapple with the estimated 20% of patients who are dissatisfied with their knee replacement.
This article reflects on past and current trends in materials to increase durability of articulation and reliability of implant fixation; design solutions to improve kinematics (and reduce pain); computer-driven advances in design and manufacturing and surgical tools to increase accuracy, reliability and efficiency of procedures.
Material innovations in knee arthroplasty and the primary contributing factor to increased implant survivorship is the long-running incremental improvement in ultra-high molecular weight polyethylene (UHMWPE). Collectively, the innovations to polyethylene are manifested as increased resistance to wear and oxidative degradation of the tibial- and patellar-bearing surfaces. Improved resistance to wear has been achieved by radiation cross-linking in the range of 50 to 130 kGy (electron beam, gamma radiation), addition of stabilizing additives (vitamin E) to the resin or infused into the molded/extruded material, and the industry’s better understanding, optimization and control of the process variables of temperature, pressure and time in the molding (and extruding) of the polyethylene itself. This is manifested in increased toughness and resistance to degradative processes. An interesting aspect of the history of UHMWPE in our industry is the constancy of the base resin itself, with the manufacturing method, molecular weight, chemistry and particle size essentially unchanged since its introduction to orthopedics in the 1960s.
On the metal side of the articulating equation are improved surface finishing and cleanliness and better control of the microstructure of cobalt chrome (CoCr) castings, which collectively reduce abrasive damage and wear of both articulating surfaces. Materials innovations during the past 30 years include ceramic surfaces such as Endotec’s historical application of titanium nitride coating, Smith & Nephew’s zirconium oxide (Oxinium, formed from underlying Zr2.5Nb alloy) and more recently Aesculap’s Advanced Surface Technology (Zr, Cr nitride layered coating). The high hardness and chemical stability inherent to these ceramic surfaces are more resistant to abrasive damage and degradative processes and isolate—or eliminate—the CoCrMo alloy from the biological environment. It remains to be seen if these technologies actually work clinically which, if proven with data showing increased survivorship, would drive wider adoption of these and similar materials innovations.
The past 30+ years include incremental improvement in engineered surfaces and coatings for increasing initial and long-term fixation of the implants to bone, with and without bone cement. Innovation in surface coatings can be traced to the 1970s when large, regular sintered bead porous coatings were developed for cementless fixation, soon followed by smaller and irregular beaded sintered titanium and CoCr porous coatings, and then plasma-sprayed titanium and HA porous (and dense) coatings in the 1990s. More recently, starting in the late 1990s and proliferating in the 2000s, are trabecular-structured titanium and tantalum surfaces which are generally characterized as 1 to 2mm thick, 50% to 80% porous with pore sizes ranging from 0.2 to 0.7mm and compressive modulus much closer to cortical bone than the base metal itself. Additive manufacturing has made these porous-structured surfaces available to anyone willing to invest the time and money into product development.
Device companies—large, medium and start-ups alike—continue to introduce new articulation designs that provide for more intrinsically stable, natural kinematic motion of the reconstructed knee. This gradual shift in philosophy away from traditional cruciate-retaining and posterior-stabilized (cruciate-sacrificing) designs is being driven by the 20% patient-dissatisfaction issue, with optimization of natural kinematics being in the forefront of surgeons’ and engineers’ minds.
Historically, Wright Medical/MicroPort Orthopedic’s medial pivot knee systems and more recently, Medacta’s medial stabilized knee are examples of this philosophy. Better matching of implant size and shape to the anatomy of patients’ knees has been ongoing for many years, as manifested by products such as Zimmer’s gender-specific and Persona knee systems in the 2000s and 2010s, ConforMIS’ unicompartmental and total knee systems of the past 15 years and more recently, Bodycad’s unicompartmental knee system. The industry’s drive to patient-specific, customized designs can actually be traced back to Techmedica in the 1980s, but cost containment and limitations of computer-aided design and manufacturing at that time contributed to that company’s end. Fast-forward to the 2010s, and sophisticated subtractive and additive manufacturing technologies are enabling patient-tailored design and manufactured solutions to become more commonplace.
In the world of design innovation in knee replacement, we need to ask, “What about the patella?” Surgeons and development engineers alike have tried for decades, and for the most part failed, to improve the reliability, accuracy, simplicity and speed of this part of the surgery. One innovation in patella surgery is choosing not to resurface a perfectly healthy, well-tracking native patella, as is more commonplace in Europe. This change in thinking will further be enabled by knee designs with more natural kinematics and patient-specific, anatomical fit.
Innovation in procedures is underpinned by computer technology enabling cost-effective manufacture of sophisticated implants and instruments, operative use of surgical tools and robotic surgery. A common goal is reliable, increased accuracy of implant position to plan. In theory, robots and computer-assisted surgery systems provide high- and low-volume surgeons alike the same ability to position implants as the designer intended; the same is true for patient-specific implants and instruments.
Computer-aided and robotic systems add cost to the instrumentation needed to implant a conventional system of knee implants, but usually with some consequent reduction in supporting instrument inventory. Patient-specific instruments and implants eliminate most or all reusable instruments needed to support a case, and in turn reduce the cost of surgery assuming similar pricing to conventional knee systems. The past three to four decades of corrective knee surgery have been marked by devices and procedures better targeting the clinical indication; examples include high tibial osteotomy (opening and closing wedge), unicompartmental knee arthroplasty, patella-femoral arthroplasty and therapies/devices for repair of focal defects of the cartilage. Debate continues as to the effectiveness of these procedures in terms of patient quality of life, cost effectiveness and in considering the first operation or revision to total knee replacement.
What Does the Future Hold?
The following thoughts are predicated upon my belief that hardware device solutions will always be a large proportion of surgical procedures to treat orthopedic maladies. Why? Because purely regenerative, biological solutions are destined to fail in patients who are incapable of changing lifestyle behaviors that cause chronic disease states and degenerative change to bone and soft tissues. With this being said, technologies in biologics and drugs that increase the speed and quality of surgical knee repair will contribute to further incremental improvement in materials, design and surgery technologies.
Materials: To further increase implant longevity and clinical survivorship, I foresee:
- Continued incremental improvement in durability of UHMWPE with highly cross-linked vitamin E formulations becoming universal by 2025. Additionally, expect all-ceramic and ceramic-coated femoral components to make further inroads in the coming years.
- Fertile ground for potential new or improved UHMWPE resin.
- Elastomeric materials and composite structures better mimicking the elastic/plastic properties of cartilage with the durability of UHMWPE, and better matching materials properties to targeted clinical indication.
- Porous structured and engineered surfaces (physical, chemical, structural) that incorporate bioactive modifications to speed osteointegration and maintain fixation over the life of the patient, and also reduce the potential for infection, which in turn will increase the percentage of cementless procedures in total knees (as it was in total hip replacement 20+ years ago).
Design: To increase patient satisfaction and reduce knee pain towards a universally “forgotten knee,” expect:
- Continued evolution of articulating design towards more normal, natural, patient-specific biomechanics via intrinsic stability by geometry (and new materials that further enable design-driven innovation), and surgical alignment and placement, with the later becoming reality as we better understand mechanical versus anatomical versus dynamic/kinematic alignment of the knee.
- Design-driven solutions that can be realized from materials that mimic the mechanical and physical properties of cartilage and enable retention or repair of patients’ ligaments and soft tissues.
- Better patella resurfacing options (implants, instruments and surgical, and designs that better enable retention of the native patella).
- Further proliferation of patient-specific knee implants and instruments that fit and clinically function better than standardized systems, and eliminate supporting instrument and implant inventories.
- Potential marriage of computer-aided and robotic surgery with patient-specific implants, and further reduction in reusable instruments.
Surgery: To increase accuracy, reliability and speed, and reduce inventory/instruments, expect:
- Device companies to engage in further development of robotic technology, patient-specific implant systems and single-use instrumentation. Computer-aided and guided surgery systems are included in this milieu. Robotic systems will become cost efficient and eventually increase the speed of surgery over current practice. As seemingly applied to all things in technology, the cost of robotics will decrease over time as the market becomes more competitive and saturated.
- As surgeons’ ability to target specific indications continues to improve, expect further segmentation of product offerings designed for more narrowly targeted clinical indications ranging from infection resistant/preventing limb and joint salvage devices to more ligament-preserving total knee and unicompartmental knee options, to the repair of focal cartilage defects with synthetic implant solutions.
Robert A. Poggie, Ph.D. is President of BioVera, Inc., a consulting company specializing in FDA and Health Canada regulatory strategy and execution, with expertise in biomaterials, applied research and device testing. His previous employers include Smith & Nephew, Implex, Zimmer and Pipeline Orthopaedics.
Read more from Dr. Poggie on BONEZONE®.
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