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Mid-Journey on the Road to the Forgotten Knee

In the last 45 years, numerous achievements have been made in modern total knee replacement (TKR) in areas of materials, design and computer technology. As it stands today, primary TKR reliably results in a durable joint with low potential for revision, but most patients sense the artificial knee and some studies estimate that about 20% are dissatisfied with their TKR.

TKR traces its origin to the early 1970s at the Hospital for Special Surgery, where John Insall collaborated with surgeons Chitiranjan Ranawat and Allan Inglis, along with engineer Peter Walker, Ph.D., to develop the total condylar knee. In the mid-1970s, cruciate sacrificing (posterior stabilized) total knee and resurfacing designs for the patellofemoral joint were also developed. The biomaterials that made up the implant components at that time remain the same today, with polyethylene for tibial and patellar bearing surfaces, CoCr alloy for the femur and titanium or CoCr alloy for the tibial base plate (or “all-poly”), with PMMA-cement dominating fixation to bone.

The design of articulating surfaces has ranged from unconstrained to fully constrained with everything in-between, including mobile bearings, rotating platforms, nearly flat on radiused femurs, posterior-stabilized, conforming femorotibial articulation and medial pivot. Throughout most of this history, knees have been mechanically aligned with compensation in component position to better match anatomical rotation and joint line, with ligament balancing and attention to flexion-extension gaps being controlled in planning pre-operatively and via surgery intra-operatively.

Also noteworthy from the past 15+ years are products and procedures focused on localized pathologies of the knee, with unicompartmental knee surgery becoming a mainstay in knee arthroplasty, and cellular, drug and traditional materials solutions to osteochondral defects in various R&D phases in the U.S. today.

Before we predict what TKR will look like in the future, let’s review notable changes to the procedure over its lifetime and ways that materials of choice have changed (or remain unchanged, in many cases).

Biomaterials, Then and Now

Knee arthroplasty surgery has been dominated by CoCr alloy for the femoral surface replacement, with Oxinium (Smith & Nephew’s surface-oxidized zirconium metal alloy) being the one main exception, titanium-nitride coating CoCr and titanium alloys as another minor exception, and a few attempts at all-ceramic femoral components that were not fully commercialized.

Reasons for the dominance of metal alloys in general, and CoCr alloy in particular for bearing surfaces, include high fatigue strength, wear resistance and biocompatibility. Titanium alloy has mostly been applied to tibial base plates and metal-backed patellas, with Trabecular Metal (Zimmer Biomet’s chemically vapor deposited, cancellous structured tantalum metal) being a notable exception, where the option for cemented fixation or bone ingrowth into a porous coated or structured surface is desired. CoCr alloy is also applied to the tibial base plate and is essential for mobile bearing knees. 

Polyethylene remains the bearing material of choice for the tibial and patellar bearing surfaces, with the main change in the past 10 years being higher levels of cross-linking for increased wear resistance and vitamin E and other anti-oxidant additives for reducing oxidative degradation. PMMA bone cement remains the dominant material for fixation of devices to bone, with cementless bone fixation occurring in an ever-slowly-increasing percentage of primary cases.

The theoretical case for ceramic-coated and all-ceramic knees has focused on reduced polyethylene wear via a harder, more wear-resistant hydrophilic surface, in addition to the reduced potential for immune response to metals. Note that scientific literature is divergent on theory translating to clinical results. 

Polyethylene remains the bulwark for the more compliant side of articulation, and for good reason. Except for the period of the late 1980s through early 2000s, when poor methods of polyethylene consolidation/molding led to abnormally high rates of delamination, fatigue wear and osteolysis, cross-linked and stabilized polyethylene materials have translated into durable and reliable bearing implants for knee arthroplasty.

The elimination of bone cement from knee arthroplasty has long been sought by many surgeons and biomedical engineers, with various scientific literature highlighting the failures and successes. Generally speaking, component designs that are inherently stable with respect to the bone, and in turn limit relative micro motion during the early bone-healing phase, as well as porous materials with demonstrated bone ingrowth capability, have done well clinically.

Computer-Based Advances

Computer hardware and software are the underpinnings of the most visible and notable changes in the practice of knee arthroplasty. This began with computer-aided surgery in the early 2000s, patient-specific surgical guides and manufactured implants in the mid-2000s and robotic surgery within this decade. Improvements in imaging techniques, hardware and translation to surgeons and engineers also underlie these advances.

Orthosoft, acquired by Zimmer and branded Zimmer CAS, was one of the early commercial examples, with ConforMIS being a notable commercial pioneer of patient-specific implants for knee arthroplasty. Stryker’s Mako is the elephant in the room for robotic-based surgery, with Zimmer Biomet (and more recently Bodycad) providing patient-specific implant and resection guide solutions to knee arthroplasty. And let us not forget companies such as OrthAlign and OrthoSensor, as well as Smith & Nephew’s Navio, that focus on intra-operative balancing and/or alignment of knee components and forces. 

These advances have a common goal of more reliable and accurate positioning of the implants relative to the pre-operative plan and matched to the patient’s soft tissues and kinematics. This raises one of the more debated topics of the day: the long-held and commonly practiced mechanical alignment of the knee, versus the more recent (but also historical) trend towards kinematic alignment, which might be more accurately termed three-dimensional, anatomic alignment. The former’s clinical success for the patient strongly relies on ligament balancing, especially for unconstrained and semi-constrained articulations. The latter facilitates better positioning of implants relative to the patient’s natural kinematics, with properly designed articulating surfaces, and can lead to a more naturally stable, kinematically normal functioning knee.

Computer and imaging technology, married with rapid manufacturing that includes additive manufacturing, and intra-operative devices that ensure implant positioning to plan, provide the opportunity to better align the knee to the patient’s specific anatomy and kinematic function. Current research, development and surgical practice will further our understanding of the variables and their interaction that dictate the patient’s sense of knee pain and function. In turn, this will lead to improvement and adoption of implant design concepts and surgical practice that delivers a knee replacement device that is forgotten by the patient. 

What Will TKR Look Like in 2042?

Given the past 45 years of progress in design, biomaterials, computer and imaging technology, what does the future hold? With the incremental evolutionary change inherent to product development in orthopaedics, and the topics of the day that include infection prevention, early healing/recovery, patient-centric focus, cost-consciousness and the quest for the forgotten knee, we can expect the following:

Materials Focus

  • Polyethylene will continue to be the mainstay of tibial and patellar bearing surfaces, with additives to stabilize against oxidative degradation and cross-linking to resist wear being commonplace. The past 30 years have seen research into a more naturally compliant, tough and wear resistant material that resembles the combined mechanical properties of the meniscus and cartilage with the wear resistance of polyethylene. If such a material is developed in coming years, this would be a game-changer and allow for design of the ultimate natural knee joint.
  • CoCrMo will continue for the foreseeable future as the mainstay in femoral components, with all-ceramic and ceramic coated knees on the upswing due to more cost-efficient processes and better materials properties relative to CoCr.
  • Cementless fixation of knees will continue to increase, as was seen for hips in the 1990s. Learning from the past, inherently stable designs (relative to bone) and porous structured surfaces facilitated by additive manufacturing will continue to underpin biological fixation in knee arthroplasty.
  • Materials with inherently infection-resistant surface texture and chemistry will become available.

Design, Manufacturing and Innovation

  • The adoption of patient-specific resection guides and implants will continue to increase, as will intra-operative methods to improve alignment and ligament balance, in addition to robotic assisted or robotic bone resection.
  • It is also probable that patient-specific devices and intra-operative assist systems will converge for an optimal solution for implant and surgery for the patient.
  • Additive manufacturing will continue its upward trend, especially underpinning patient-specific cutting guides and cementless implants.
  • The debate of mechanical versus kinematic alignment will be resolved, and implant design will evolve toward resting natural knee function. This suggests that more geometrically constrained knee designs that replicate knee kinematic function will be on the upswing.
  • Designs inherently unnatural, such as rotating platform and mobile bearing knees, will disappear in 25 years. In turn, mechanical alignment will be less practiced. This prediction is more probable via invention of a wear resistant, elastomeric, tough material that mimics the compliance of cartilage and meniscus and its self-healing durability with respect to wear and fatigue. Polyethylene serves part of the equation, but is not compliant and deformable relative to the tissues it replaces and in response to dynamic loading that in part guides knee motion.

The future is so bright in knee arthroplasty, I gotta wear shades, as the song said. The convergence of clinical and biomechanical research, product development, advanced imaging, design, manufacturing and devices that improve surgery will lead us to the forgotten knee within 25 years.

*Main article image courtesy of Shutterstock

Robert A. Poggie, Ph.D., is President of BioVera. His previous employers and functions include Smith & Nephew, Implex, Zimmer and Pipeline Orthopaedics, with responsibilities in applied research, biomaterials, clinical research, medical education and regulatory affairs. He can be reached by This email address is being protected from spambots. You need JavaScript enabled to view it..