• Professor Alister Hart

Health

Q&A: Prof Alister Hart

Consultant orthopaedic surgeon at The London Clinic explores the impact of 3D printing on the world of orthopaedics

Interview: Viel Richardson
Portrait: Orlando Gili

What was it that first attracted you to orthopaedic surgery?
I specialise in hip and knee problems and I love the fact it is a very technical field. It combines the complexities of working with load-bearing and moving parts of the human body and the processes and materials involved in designing and fitting surgical implants. I love the topographical nature of the field. The complexities are what interested me—I think all medics like problem-solving.

What kind of problems do you face?
How do you get a hip replacement to last for a lifetime? Why does one particular type of implant fail and another succeed? What is the best way to secure an implant? Which materials are best for them? These are just a few of the questions we ask, and the answer can vary from patient to patient. What did the patient do to the implant, and can we learn from their experiences, both good and bad? These are complex questions, which we need more research to understand.

Are you involved in this research?
Yes, working as a surgeon I was encountering significant problems with orthopaedic implants, which got me interested in research. The more involved I got in research, the more interesting it became—and now I spend half my time in research and the other half in surgery.

What has been the biggest change in the field in your time?
The biggest change has been the advent of 3D printing technology. It fits with the broader idea of personalised medicine and has made inroads in orthopaedics in recent years. There is still some resistance to its use in certain scenarios, but the possibilities it opens up for the creation of implants are incredible.

How is it impacting on the field?
Traditionally the implants we use in hip and knee replacement surgery are mass-produced in a range of standard sizes. As a surgeon, you choose the closest size to the one you need and then do your best to fit it in the optimal position. But because there is an almost infinite variety of human sizes, this can sometimes be difficult, and occasionally impossible. 3D printing technology allows us to create implants tailored to specific patients.

Can you give us an example?
There are examples where someone’s joint is malformed, either from birth or through trauma or disease. One patient had an accident 20 years ago which smashed his pelvis and left it in an abnormal position. He came to us because his mobility was deteriorating and he was in increasing pain. We designed and fitted a bespoke hip implant and he is now walking without crutches for the first time in 11 years. In this case, the standard implants were simply not feasible, so a custom-made implant was the only option.

Is this the only way you use the technology?
No. We are also using 3D printer technology to make guides that help with the planning and performance of implant surgery. For example, if we are taking the head off a thigh bone in a hip ball replacement procedure, we need to know precisely where and at what angle to make the cut. The more accurate this is, the easier it is to place the stem of the implant in the right position.

We use CT scanning images to precisely plan the cut. Using these scans we can design physical guides that fit snugly to the patient’s bone. Once in place, they are used to guide the cutting instrument, resulting in a very accurate cut.

How does this technology work with standard implants?
Using guides actually gets the best out of the standard implants by making it easier to fit them in the most advantageous position. It adds a few minutes to each operation but can add as many as 20 years to the life of the implant. This is commonly done in knee operations and less so with the hip, but we still probably have more experience using it in hip operations than anybody else in the world. 

Professor Alister Hart

How does the design process work?
First we take a CT scan that will give us an extremely detailed map of the area and image of the bones. With that information I design the implant and plan the length and directions of the screws needed to hold the implant in place. Then we send a 3D image of the design with the implant and all the specifications to the facility who will print the implant.

How does the communication between you and the implant manufacturer work?
We exchange 3D images, which you can rotate to examine the design from all angles, then we discuss the specifics via Skype. After this, they print a model in plastic which they send to me. This way I can check it is exactly what I want before giving the go-ahead. As well as a final check, the model is extremely useful in planning the procedure with any other surgeons involved. This is important because, especially with hip replacement surgery, we are working close to some extremely important arterial routes, nerve pathways and internal organs, which could easily get damaged.

Is there a preferred material for orthopaedic implants?
Titanium is our preferred material for two reasons. The first is that it is very friendly to bones. Implants made from titanium will stick to bone, which means you can design something that will perfectly mirror the topography of the bone. The other metals we use are cobalt chrome and stainless steel, neither of which will directly stick to bone, so you need some type of cement between the two.

Why not use only titanium?
Titanium is softer than the others and not strong enough to withstand the load-bearing pressures of a knee or hip joint. We have to use other materials such as ceramic, stainless steel or cobalt chrome for those load-bearing parts. These are connected to a titanium support structure. Most implants will be made of more than one material to harness the best qualities for fixation and the best qualities for movement and longevity.

So are there any downsides to this technology?
The whole process takes longer and it’s a bit more complex. You need extremely accurate imaging or the whole process won’t work, so that is an extra step for the patient. Then you have to design each implant individually and arrange to get them printed.

Another downside is perhaps more surprising. If you print a perfectly fitted implant but fail to fit it in precisely the right position, it will actually be worse for the patient than fitting a standard implant. An implant shaped to fit the bone precisely will only fit in one position, like putting a hand into a glove. There is only one way to wear a glove, with every finger in the corresponding finger of the glove, whereas mittens can accept fingers that are injured, swollen and even overlapping. If a bespoke printed implant does not fit exactly, most of it will not actually be in contact with the bone. In the case of hip implants, this means that some of the implant edges will protrude from the bone where they can catch on muscles and tendons. So the extra level of accuracy from the bespoke implant demands an extra level of accuracy from the surgeon.

Where do you think this technology will be in five years’ time?
We are at the start of a revolution, no question. This will be used for planning operations to make them easier and the implants last longer. The planning of the procedure is just as important as the implants themselves and 3D printing makes it easier to get both of these more accurate, which has a hugely beneficial impact on the longevity of the implant.

None of what we have talked about would be possible without good imaging, and as imaging technology improves, the process will become even more effective, cheaper and more widespread. Further research will also discover new materials and new ways of securing the implants. It is a very exciting time to be an orthopaedic surgeon.