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Total Knee Replacement Implant Design & Rationale
The cost of TKA prosthesis would constitute about 30% of the total cost of a TKA replacement surgery, and therefore patients are commonly worried about the right choice for their joint replacement. The cost of the prosthesis is a topic that becomes very confusing to the patient who has mixed feelings that they want the “BEST IMPLANT,” but at a “REASONABLE PRICE.” This section will try to clarify the present understanding of the prosthetic designs and their effect on outcomes. Patients get varied information that some models are superior to others; some metals used are better than the other; some plastic liners give great survivorship than the competitors. Surgeons generally offer implants that are either “standard” or “latest” based on the launch of the device; it’s differential pricing in the specific market, and the availability in that country. When you search on the internet for information regarding what is the best prosthesis for either yourself or your parents, you are more “INFORMED” and more “CONFUSED” about the implant choices.
Let us start by explaining the commercial aspect of product development and its influence on pricing for their products. The best example of this would be the mobile phone manufacturing industry. When a company brings in the next generation of mobile phones, they drop the price of the old one, increase the cost of the new one, and add features that are attractive to the consumer. The problem is to decide which new functionality is added and is it worth it to you as a consumer. Let us explain a little bit about innovation itself. Innovation can be broadly divided into quantum, incremental, and cosmetic innovation. Quantum innovation is evident to everybody. At the beginning of this century, there was hardly any mobile phone with “touch screen,” but by 2010, the entire mobile phone industry had tilted or was almost swept away by the “touch Screen” magic. After that, there have been various innovations like increased speed, battery life, camera, and picture qualities, which can be called “incremental innovation.” They do feature in your choice as a consumer while making purchase decisions because those features do affect your “functionality.” There is a significant chunk, which is “cosmetic” innovation where they talk of blinking to take pictures, gems embedded on your phone, or shiny back covers to make it attractive, which have only limited utility but sway your emotions and desires to buy a phone.
Many times companies use advertisements to push their “cosmetic” innovation as “incremental” innovation and thus nudge you to buy their product. The companies, which produce medical devices, use the same strategy to remain profitable many times push their “cosmetic” innovations as “incremental” innovation. They sell that idea to both the general consumer as well as surgeons who are often surrogate decision-makers for their patients. The sophistication with which new features are launched and marketed makes it difficult even for a joint replacement surgeon to pick the right value addition- let alone an orthopedic surgeon. There have been few “quantum innovation” in joint replacement like the advent of low friction arthroplasty principle, infection reduction strategies of Dr. John Charnley that were so self-evident that a vast majority of surgeons adopted it immediately. There have been “incremental innovation” like metal backing of the tibial baseplate, improvement in liner locking mechanism, which have helped with modularity during a knee replacement, but unfortunately, there has been a surge in “cosmetic innovation.” Nobel Laureate Dr. Albert Szent-Gyorgi once said, “Discovery consists of looking at the same thing as everyone else and thinking something different. It is very apt in modern society where a proper student of science would be critical of work being done and look at innovations for their true purpose. The big problem for even a knowledgeable surgeon is going to be to differentiate incremental innovation from cosmetic innovation. The other problem is to see if that incremental innovation is worth it in a given situation looking at the “cost-effectiveness” of that value addition. Our strong recommendation is to openly discuss with your operating Surgeon regarding the implant choices and their underlying rationale for choosing a particular design.
The history of joint replacement has seen a tremendous amount of churning, and what went out of favor has sometimes returned with minor readjustments. Today the standard TKA consists of a metal-backed tibial component on which a machined high-grade plastic will lock-in. The femoral component is made of metals like cobalt chrome, hardened titanium, or ceramized metal called -oxinium.
As we described earlier, the survivorship of both the components for the tibial side has done well. Some pooled data of survivorship (collected from a large number of patients in an extensive registry like Australian registry, Mayo clinic registry) has shown that the all poly tibial components have done better than the metal-backed tibia. All poly tibia decreases the Surgeon’s intraoperative flexibility in decision making. From a patient’s perspective, it does not make any difference.
Many patients question if the joint replacement components are individually customized to the patient. It is not true. Firstly, a knee CT or MRI scan is performed. The images are shared with the company, which produces cutting block based on this data. Finally, the surgery is performed with the provided cutting blocks, but the implants are used in the standard fashion. Comparative studies have shown no difference in standard knee replacement and custom knee replacement. The femur or tibial deformity is an indication for custom joint replacement.
As the name implies- navigation is a tool for smooth, accurate arrival to a predetermined destination. Navigation technology uses infra-red signals from devices, and sensors capture them and then help in making precise bone cuts in joint replacement surgery. Navigated TKA has been around for more than a decade and is beneficial for accurate placement of the individual components. Though it was adapted more than a decade ago, its use has not been widespread. It is expensive; both disposable or large non-disposable sets would add about 20,000-25,000 rupees to the operational cost.
Meanwhile, it helps in accurate placement of components; it has not been shown to increase implant survivorship or better symptomatic outcome to the patient. The question then is, why use it? The answer is that it helps in ACCURATE placement of components. Studies have shown that even expert surgeons place components within three degrees of the neutral mechanical axis in only 80% of the time while navigation can help to improve that accuracy to 90-95% of the time.
A few companies have come up with the idea of high flexion with design modifications of the prosthesis that technically permits deep flexion. But, many studies have questioned the possibility of deep flexion and also are worried about the possibility of higher failure rates than a conventional prosthesis. A patient opting for a high flexion knee design should be aware of the chance that they might not get that deep flexion. Implant design is just one factor in the postoperative Range of motion. The primary determinant of postoperative flexion is the pre-operative Range of motion in the same patient. They should also realize that high flexion prosthesis increases the cost of the implant by almost 10-15%.
Standard TKA implant provides good range of motion without need for any high flex design. Principles have to be appropriately followed
Designing a Joint replacement specifically for a woman sounds exciting and intuitive. There is the anatomical difference between men and women, but again patients who have received unisex prosthesis have done very well. The gender knee increases the cost of surgery by 10-15%.
Lifetime warranty appears like a car ad, which states that it will deliver a fuel efficiency of 25 KMPL. How many of us trust the car to provide that fuel efficiency knowing well that it is done “Under standard Test conditions”? Real-life fuel efficiency will be much lower than what is advertised. The problem with ads depicting medical devices could be potentially misleading and lead to wrong choices. There are “advertorials,” which have started appearing in the general press, which states that a knee prosthesis will last a lifetime. The data on that is derived with certain assumptions; a joint replacement prosthesis tested in a ‘Simulator,” which tries to imitate the daily motion of knee bending and straightening. This “wear simulator’ testing has shown that even at 30 million cycles, the plastic component of the prosthesis has not worn out. Thus they assume that it will last 30 years by the thumb rule that an individual will walk 1 million steps in a year. Unfortunately, 1 million steps in a year is an assumption. Some people might end up walking 3-5 million steps in a single year, and the plastic wearing out is just one cause of the failure of a knee replacement. There are many other causes, and the companies do not even advertise that. So, the tactic of marketing it as either 30 years or a lifetime warranty is just a “marketing tool” not based out on reality.
The other common question which patients have is the name of the prosthesis, and they come asking for a particular prosthesis or are worried about a prosthesis that it has been “Recalled.” Companies pick names that are attractive or implicative that lets you live a very active lifestyle. The other problem is that of prosthesis “recall”; it is more of a batch recall due to some late fault detection rather than the entire brand issue. Almost all prostheses and companies have been sued, especially in the United States. When you search for a specific prosthesis, it is virtually certain that you will find one or two law-related links in them. It is always advisable to be informed about the prosthetic choice and have a frank discussion with your Surgeon regarding the same. Some designs of the prosthesis have shown to do well early on in the first 4-5 years of life. Later problems could be noted due to either material issues or design issues. So, when it comes to joint replacement, the “Latest” implant does not guarantee the “Best result” as a problem can surface later on. You must find the right Surgeon with the PROSTHESIS AND ITS INFLUENCE ON OUTCOME