From tailoring preprocedural planning in patients with heart failure to building layers of cardiac tissue to create a tiny 'heart', 3D printing (or additive manufacturing) has shown promise in various applications in cardiology. So, how is the technology currently being applied in orthopedics?
Medscape spoke with Timothy M. Wright, PhD, who works in the department of biomechanics at the Hospital for Special Surgery (HSS) in New York City, to learn more about how 3D printing may revolutionize the design of orthopedic devices and improve biomechanical performance in joint replacements.
Can you give us an overview of the role of additive manufacturing, or 3D printing, in orthopedics?
We've been replacing joints that were destroyed by arthritis with metal and plastic parts for nearly 50 years. Just the fact that so many people know someone who has had a total joint replacement tells you about how successful it's been.
Nonetheless, there are problems yet to be solved in orthopedic surgery, and you're always looking for the best solutions. When a new technology emerges, you want to be able to decide whether it's going to help you.
So along comes additive manufacturing, or 3D printing, which has had a remarkable impact on many industries.
Those sorts of components are extremely difficult to build by conventional technology, but 3D printing and the resolution that people can use with it can now handle that. That kind of technology opens up the design space, because if you can think about a structure in three dimensions, you literally can build it with 3D printing, whereas you may not be able to machine it on a milling machine or turn it on a lathe—both of these conventional approaches are limited in the geometries that you can make with them.
A Valuable Adjunct to Conventional Techniques
This also implies that you can build in performance features with the 3D printing that you wouldn't necessarily be able to include by typical machine creation.
You begin to ask yourself how to apply this revolutionary technology to solve some of the problems we haven't solved before. In orthopedics, there are general and specific applications.
For a long time, when someone had a joint replacement, implant components were attached to the bone with a "grout" called polymethylmethacrylate (PMMA). PMMA is a polymer that you can mix up and within about 7 minutes, after it becomes doughy, it hardens. It's a wonderful grout; it fills even the smallest interstice between the implant and the bone and rigidly fixes the two together. But PMMA has disadvantages—for example, it can be abrasive against the bone if the implant starts to loosen.
For many years now, especially in hip replacement, the components were made with porous metallic coatings. The idea was to eliminate the cement. You place the porous part of the implant directly against the bone, and the bone grows to fill in the porosities. Now you have permanent biological fixation, and you don't have to worry about the potential side effects of bone cement. That has worked very well.
So, a general application of 3D printing is to change the texture of those coatings. For example, you can make them a lot rougher. Why do you need a much rougher surface? Because when doctors perform the surgeries, they rely on only a mechanical fit at first. They literally bang the implant component home into the bone and get rigid initial fixation. Then the patient's bone fixes itself to the implant as the bone grows into this porous coating. That initial fixation in the operating room is crucial, and one way to ensure that you get a stable initial fixation is to have a high coefficient of friction between the implant and the bone.
With 3D printing, you can also make porous structures that have very high porosity, higher than what you could make with conventional methods. Implant companies and designers are taking advantage of the ability to improve that initial fixation by increasing both the coefficient of friction and the porosity.
Answering the 'What Ifs'
To me, the more intriguing part of additive manufacturing is to try to solve the specific problems that we simply haven't been able to solve before. For example, the engineers in my department and our surgical colleagues at HSS have been working on replacing the base of the thumb, the carpometacarpal (CMC) joint. It turns out (and I didn't know this, having spent most of my career thinking about hips and knees) that arthritis is highly prevalent in this joint, especially in women.[1,2] This can become quite debilitating. As the joint surfaces wear away, it becomes unstable, such that certain tasks (eg, turning a key in a lock or taking a lid off a jar or a pill bottle) become nearly impossible for these patients.
Several potential solutions have been tried over the past 40 or 50 years. Silicone spacers don't work very well. The bone is abrasive and chews up the silicone, and the patient experiences a very aggressive reaction to the silicone debris in the surrounding tissue. Some surgeons try to interpose soft tissue. They use a nearby tendon or something similar to form a pillow between the two bone surfaces. That doesn't work for very long, and it doesn't really stabilize the joint.
Metal implants have been developed. The metal is held into the metacarpal bone by a metallic stem that goes down the medullary canal. The problem with very small bones is that they are thin-walled and the metal is very stiff, much stiffer than the bone.
So as patients are going about their daily activities, the loads are relatively high, and these metal implants loosen. The bone is bending around the implant because the implant is too stiff.
What if I could make some parts of the implant porous and some solid? What if I could make the implant more compliant and still maintain its strength, maybe by having the bone grow into that porosity? I could create the porosity with 3D printing, and the porosity could also be in the middle of the implant and not even touch the surfaces of the bone.
Do you envision a time when 3D printing will replace conventional methods of making implants?
I think people can get carried away and believe that additive manufacturing is going to be a solution to everything.
I could 3D print anything, but why would I? For example, if I need to make a thousand little plastic parts, I could injection mold them for pennies a part. In that case, 3D printing would be much more expensive. Would it be just as nice? Yes, but why would I do that when I have conventional ways of making those parts?
What we are trying to do in orthopedics is think about the problems that we can solve with this technology, recognizing it's not going to be a solution for everything. But there are specific problems we can solve with 3D printing that we haven't been able to solve until we had this technology.
The other specific design area is for patients who have total hip and knee replacements that have failed. The patient may get an infection in the surrounding bone, or the implant mechanically loosens over time. Now the patient needs to have a second joint replacement in the same joint.
By the time the surgeon goes in and removes those implant components, and often just because of the failure itself, a lot of the bone tissue has been destroyed. This is especially a problem in total hip replacements, where the surgeon essentially prepares the acetabulum (the socket of your hip joint) and replaces it with metal and plastic. If that loosens, you must go in and do a revision surgery, but the surgeon will be faced with a lot of bone loss.
Implant systems for standard, first-time joint replacements come in sizes, just like shoes (eg, a "size 9" shoe, a "size 7" implant). But when you're faced with these difficult revision cases, often there is nothing off the shelf that can help.
It's similar to what we just talked about in the finger. A natural pelvis is relatively compliant—it deforms even when you sit down—but now you're faced with major bone loss in that pelvis. You want to replace that bone with something. Right now, we are limited to relatively big, bulky, stiff metal implants to try to fill up that space.
Along comes 3D printing, and it's the same intriguing challenge as with the finger problem. Can we figure out a way to take advantage of this new technology and make the implant porous where we might want bone to grow into it, or where we might want it to be more compliant? Can we still fix the plastic component to it? Can we give patients optimal solutions for their individual problems?
Thus, additive manufacturing will be very important in patient-specific implants, especially in revision surgeries, where something has failed and you know there is nothing off the shelf that will work. In reality, if we could prove that there is value in the custom, 3D printing solution, it could prove to be both economical and feasible for these patients.
We have a long way to go, but if you think about it, this is a kind of precision medicine. Whereas now people are being screened for the most effective treatment based on their genetic makeup to prescribe patient-specific drug therapies, we can assess the stage of your arthritis and your expectations for lifestyle, and we can begin to think about ways of doing precision surgery.
Proving the Added Value of Additive Manufacturing
In that way, the future of additive manufacturing could be to bend the curve so that many more patients get customized solutions. A patient may not need a patient-specific implant today, but we could show that by getting one early, the patient actually felt better or had better function in the long term. Once we prove the value, then we may be able to extend the applications of additive manufacturing.
It is a fascinating time to be in a field where we have designed with conventional materials for such a long time. And it isn't that we're changing the materials. We're not. We are just changing how we make them, and it opens up all these possibilities. It enables us to look at this multidimensional space in a different context in terms of design specifications and geometry.
To what extent is additive manufacturing being used now?
Some commercial products have been on the market for more than a decade, using additive manufacturing to make those unique porous coatings that we spoke about earlier. The technology has been used commercially in several standard applications to enhance fixation.
Now we have started using it in such areas as the elbow. Elbows were always cemented. We are beginning to use 3D printing to make elbows that can be implanted without cement. For the carpometacarpal joint at the base of the thumb, we are in much earlier in the process, but we're applying optimization routines to explore the role of additive manufacturing in solving those problems.
Whether the increased coefficient of friction that we were talking about carries over to a true clinical advantage remains to be seen. We will need to follow a lot of patients over a fair amount of time to determine whether that is an advantage. But it is certainly not a disadvantage, so why not try it? I believe that is why these implants have gotten regulatory approval—because they are relatively safe. The materials are the same; we are just changing one aspect of the manufacturing technology.
It may take a long time to demonstrate the value. Again, we have to follow these patients to see how they do and whether they have better outcomes than with our conventional techniques.
So the bar will be lower in situations where we don't have a solution. But it will take years to show that the 3D printing works better than what is currently out there, right?
Correct. For example, for implants that are fixed biologically by getting the bone to grow into the porous coating, it turns out you don't need the bone to grow into 100% of the implant surface. It is a relatively small percentage. So, what advantage does 3D printing provide? Is the fixation stronger? Faster? More durable?
This is more important than ever, given the cost-consciousness in the healthcare industry. For now, 3D printing of implants is considerably more expensive. If the price doesn't come down and they provide no added value, then we may be saying 10 years from now, "Well, it was a great experiment, but we really didn't have to go to that length." But in those same 10 years, we may get to an inflection point where the 3D printed implants are cheaper to make and their outcomes are superior to those with conventional implants, significantly raising the overall value. But we are not there yet.
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Cite this: Will 3D Printing Revolutionize Orthopedic Implant Surgery? - Medscape - Jun 27, 2019.