Manufacturing technology 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. Source: NS Medical 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 pro- inflammatory 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
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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-by- layer 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-for- manufacture 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.
Extra features
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
Medical Device Developments /
www.nsmedicaldevices.com
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