Undergoing a total knee replacement can be an anxiety provoking experience – but it doesn’t have to be! Often this apprehension is due to unknowns about the surgery and the implant itself – so education and information can go a long way to alleviate some of those fears. In this post, we will try to address one common topic of discussion that comes up with knee replacement patients: Total knee replacement implants are not organs or tissue, they are synthetic, which may leave you asking, “What, exactly, is going to be put into my body?”
During knee replacement surgery, the surgeon surgically removes the damaged bone and cartilage of the joint and replaces it with smooth, artificial implants, eliminating painful bone-on-bone contact. There is a common misconception that this involves cutting off the entire end of the bone – this is not the case. As demonstrated in the illustration below, the surgery involves resurfacing or “capping” the ends of the bone, as opposed to completely cutting off the ends.,
Almost all total knee replacements are composed of a four-part system: each component is made out of a specific material based on its unique role and challenges faced in the body.
The femoral (thigh bone) side is a single element that resurfaces/replaces the bottom of the thigh bone and provides the top half of the new joint’s bearing surface.
The tibial (shin) side has two elements and resurfaces/replaces the top of the shin bone. This portion of the implant is made up of a tray attached directly to the bone and a plastic insert that snaps into this tray and provides the lower half of the new joint’s bearing surface.
Finally, the patellar (kneecap) component replaces the surface of the kneecap, which rubs against the femur. The patella protects the joint, and the newly resurfaced patellar button is designed to slide smoothly on the front of the joint. In some cases, surgeons do not resurface the patella, though my practice is to resurface it in almost all cases as I feel it can be a source of persistent pain if it is not resurfaced.
There are a variety of accepted materials used for total knee implants, but all materials used in building implants usually meet the following criteria:
- Implants must be able to duplicate the structures they intend to replace, meaning they need to bear weight and stress without breaking and move smoothly against each other.
- Implants must be able to retain their strength and shape for a long period of time. Older models have shorter lifespans of 15-20 years, but newer advances in technology have extended implant lifetime.
- Implants must be made of materials that are biocompatible and inert so that your body will not reject the implant.
The Femoral Component
The femoral component is typically made of the toughest materials. Traditionally, cobalt chromium (CoCr) has been the material of choice for knee implants because of its strength and relative hardness; however, some patients are allergic to elements, such as nickel, found in CoCr.
An alternative to CoCr is a ceramicised metal alloy, OXINIUM. The smooth, hard surface of an OXINIUM implant is not a coating, but the result of a process which allows oxygen to absorb into the zirconium metal, which changes only the surface from metal to ceramic, retaining all of the durability of the underlying metal. In addition to being more durable than a true ceramic, this metal implant’s ceramicised surface is more than twice as hard and therefore may be more resistant to the kind of scratching and abrasion that can cause a CoCr implant to wear out more quickly.
The Tibial Component and Spacer
The tibial component is generally made from titanium, known for its strength and lightness. The tibial component is responsible for holding the spacer insert in place. The tibial spacer, or insert, acts as a cushion between the tibial and femoral components. The insert is made from a plastic called ultra high molecular weight polyethylene, and is designed so that the femoral component can smoothly articulate with it.
The Patellar Component
Like the tibial spacer, the patellar component, or ‘button’, is also made from a plastic called ultra high molecular weight polyethylene.
Some surgeons choose not to resurface the patella with a patellar component. The native patella rubs on the femur and can deteriorate over time, just like the femur and tibia. My usual protocol is to resurface the patella as I feel that it can be a source for lingering pain after surgery if it is not replaced.
How to choose
There are many different total knee implant designs on the market. These different implant designs and their materials carry various advantages and disadvantages. It is important to discuss the type of implant you will be getting with your orthopedic surgeon. Their recommendation will be based on your size and weight, bone health, and the level of activity you hope to resume after surgery. It is also best that your surgeon is familiar with the manufacturer and type of implant that they’ve chosen for you. If you feel encouraged to do your own research, make sure the recommended implant has good long term data supporting its use and durability. Ultimately, the more comfortable and confident you and your doctor feel about the decision, the better.
My Approach to TKA
Total knee arthroplasty technology has been rapidly evolving over decades. My approach involves the use of a minimally invasive, muscle sparing surgical technique, combined with customized instrumentation that is uniquely 3-D printed for each specific patient undergoing surgery. This technology, called VISIONAIRE Patient Matched Instrumentation, has been shown to be more accurate and more efficient in various studies.
The most common implant I personally use is called the JOURNEY II CR, manufactured by Smith + Nephew. I have worked as a consultant for the Journey II knee with Smith-Nephew. I have helped to design its modern instrumentation, and have taught numerous other surgeons about this knee system via lectures and hands on cadaveric labs. The implants that comprise this system are designed to mimic the shapes and motion of a normal knee. Your native knee is not a simple hinge-type joint that just bends back and forth. Instead, as you go from extension to flexion and back again, there are subtle rotational movements around the knee joint that occur in addition to and in conjunction with the simple hinge-type bend. There is laboratory data that suggests that the Journey II knee may be more kinematically matched to a normal knee than other traditional knee implants systems. In simple terms, this means that a Journey II replaced knee may more closely mimic the motion and dynamics of a normal human knee.
Another benefit of this system is the unique material that can be used for the femoral component, referenced earlier, called OXINIUM. The OXINIUM does not contain any nickel, lowering the risk of allergic reaction, and is biocompatible. There is also data to support that it may provide better wear characteristics compared to other knee systems.
Ultimately, the most important part of having a successful knee replacement surgery and a safe rapid recovery that gets you back to your life quickly is having a system and team in place that allows for a seamless experience. It takes a big group of medical professionals – including nurses, physician assistants, medical assistants, physical therapists, surgical technologists, and many others to make the process work well. I am extremely fortunate to get to work with the best team members in the business, and our collective focus is always centered around putting the patient first. This means that your comfort, safety, and wellbeing are our top priority at all times. We are always available to help answer any questions you or your family may have before, during, or after your surgery, and take great pride in providing top notch surgical care while creating a positive patient and family experience from start to finish.
