As human life expectancy continues to increase, so does the need to ensure it’s of the highest quality possible. Bone and joint diseases are one of the more common medical issues that come with ageing, and in many cases they can necessitate full or partial replacement using an artificial implant. Given that a second replacement – implanted in what’s known as revision surgery – carries a higher risk of complications than the first, there’s a strong impetus on making the first last for as long as possible. One way to do this is by improving the implants themselves, which are broadly categorised into two groups: permanent joint replacements and temporary fracture fixation devices.

To date, three general materials categories have been used to manufacture orthopaedic devices: biocompatible metals, polymers and ceramics. None are without their issues, however. While titanium has extremely good biocompatibility and strength, for example, an abrasion of the titanium oxide layer can lead to the release of particles into the surrounding tissues, which can cause a proinflammatory response and result in implants loosening in the long run. Polymers, on the other hand, are subject to deformation under load, a process commonly known as ‘creep’. Although many early iterations of joint replacements were made of polymers, the risk of creep and progressive wear that comes with the material has largely disqualified it from use nowadays. Finally, ceramic has a high strength and is less prone to the wear and tear that occurs with other materials due to friction, but its low toughness gives it a higher risk of fracturing – which is especially dangerous during revision surgery due to the debris.

“[3D printing] adds significant value to complex cases, such as revision surgery where standardised implants are not readily available.”

Despite the drawbacks, metal is the most common choice for implants, with popular choices including surgical stainless steel, cobalt-chromium and commercially pure or titanium-based alloys like aluminium and vanadium. But the material isn’t the only important factor it comes to measuring an implant’s success, especially when it comes to quality-of-life variables like physical function and limb alignment. That’s where the value of swapping an off-the-shelf approach for one that’s custom tailored to individual patients comes in, and research teams around the world are striving for ways to make this an option in the clinic, primarily thanks to the use of 3D printing.

A perfect fit

At the University of Birmingham’s Healthcare Technologies Institute, a team of researchers led by Sophie Cox, associate professor in healthcare technologies, is leveraging an advanced metal 3D printing technology called selective laser melting to create customised implants. “3D printing offers a unique opportunity to create implants that are customised to the patient’s anatomy,” says Cox. “This adds significant value to complex cases, such as revision surgery where standardised implants are not readily available.”

Generally, the 3D printing process involves the production of a solid object in a layer-bylayer fashion according to a digital model. The use of high-resolution medical imaging data allows surgeons to create 3D models that can be shaped, aligned and cut to a patient’s anatomical requirements.

The benefits of the process are obvious. For a start, 3D printing enables a designer to quickly modify and develop products in a design-formanufacture workflow. The speed of 3D printing also means prototypes can be tested for accuracy, with input from members of the surgical team. Another use case for 3D printing in orthopaedic surgery is in the planning, as it’s possible to print a replica of a patient’s joint so that surgeons can see what they’re up against before a procedure. This could be particularly useful as a teaching tool, as well as in complex joint replacements, where surgeons can assess which aspects are likely to make the operation difficult, and plan accordingly.

For years, orthopaedic specialists have been developing technology to model, design and create reconstructions of bone defects caused by both tumours and trauma. The lead times were very long, often up to six months. With 3D printing, however, the turnaround time from design to creation can be as little as six weeks.

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Cox was recently awarded a UKRI Future Leaders Fellowship for her work, giving her and the team £1.2m over an initial four-year period to further develop implants that are better suited to the mechanical properties of bone, whilst exhibiting new biologically functional properties, such as antimicrobial alloys. But beyond creating the perfect fit, the team are also focused on improving an understanding of the 3D printing process such that it can be used in a broader range of applications.

“Our team,” says Cox, “is leading the way in understanding how to tailor the surface of customised 3D-printed implants so that they maximise the potential for new bone regrowth while preventing bacterial adhesion, which could result in an infection. To achieve this, the team are exploring and developing new approaches to implant design, including the possibility to incorporate beneficial factors, such as antibiotics or growth factors.”

“Our team is leading the way in understanding how to tailor the surface of customised 3D printed implants so that they maximise the potential for new bone regrowth while preventing bacterial adhesion.”

“We are also looking to use novel ways to move the laser while 3D printing, such that we can manufacture intricate porous structures that mechanically behave like bone,” Cox adds. One of the challenges that must be overcome for a successful implant is osteoconduction – where new bone grows on the surface of it – and the porosity of that surface plays a significant role in how osteoconductive it is. “Using these porous structures within orthopaedic devices provides opportunities for new tissue to grow onto or into the implant, which can improve implant performance through the creation of a strong interface between the device and patient’s surrounding bone tissue,” Cox explains. “We’re only just beginning to understand how these technologies can be best used, and there’s certainly lots more innovation to come in the 3D printing space.”

Taking a more macro view, the future for the 3D printing market looks extremely encouraging. In 2022, the global market size was valued at £19.84bn and is projected to grow at a CAGR of 23.3% between 2023 and 2030. Of course, healthcare is one of many industries included to reach that prediction, but there is currently a vast amount of research and development being carried out in the sector, and the growing demand for prototyping applications is likely to further drive the growth of the market.

Former US President Barack Obama asserted in his 2013 State of the Union speech that 3D printing had the potential to revolutionise the way we make things. The evidence required to substantiate whether that assertion was accurate would fill an entire book – but in orthopaedic departments at least, it seems like there’s a high likelihood it will be borne out in the future.

Needless to say, Cox is very optimistic – and excited – about the potential 3D printing holds for the medical sector. “3D printers have become much more advanced in recent years and now the industry is moving into a rapid phase of innovation,” she says. “There is great potential for us to improve implant design, materials and push forward new concepts that aim to advance orthopaedic surgeries using 3D printing.”

How do joint replacements perform?

Although cases of rheumatoid arthritis, osteonecrosis (bone death), fractures and bone tumours can require surgical intervention, osteoarthritis is the leading reason for the 450,000 total hip replacements and 754,000 knee replacements performed each year in the US. In the UK, the number of replacements is 160,000, split almost equally between hip and knee implant procedures.

Total hip and knee arthroplasty are both reasonably successful operations, with a 5% failure rate at the 10-year mark of an implantation. One meta-analysis including 58,932 total hip replacements in patients with a mean age of 69 estimated that three-quarters of hip replacements last between 15 and 20 years, and just over half of hip replacements last 25 years in patients with osteoarthritis. The same researchers looked at 7,232 knee replacements in another meta-analysis and found 82% of total knee replacement implants lasted 25 years.

The option of revision surgery to swap the prosthesis for another is there when joints do fail before their bearer dies, but the second joint is never as effective as the first. Reasons for this can vary, but tend to be related to the quality of the surrounding bones, which degrades naturally with age – especially true in patients with osteoarthritis. Another factor is the complexity of modifying implants to adapt them to a bone structure with greater degradation.

Couple this with the fact that revision surgery often requires more time in the operating theatre than an initial implantation, which presents a greater risk to elderly patients anaesthetised for the process, and there’s a clear and present requirement for the next generation of artificial joints to last longer.