Ed Littlewood, from Renishaw’s medical and dental products division, discusses medical applications of 3D printing and the potential of the technology to improve procedures and aid patient recovery.
3D printing is used in industrial applications for prototyping and manufacturing. The technology uses a range of materials including polymers, ceramics, resins, stainless steel, cobalt chrome and titanium. Metal 3D printing is also known as additive manufacturing (AM). The additive manufacturing process produces a 3D object from a digital computer-aided design (CAD) file. Objects are built in layers, adding material until they are complete. This method offers great design flexibility, which means that highly accurate, bespoke and customised objects can be produced at low cost.
The medical application of additive manufacturing is growing and current uses include: craniomaxillofacial (CMF) devices, hip and knee implants, spinal fusion implants, heart stents, neurological drug delivery and external prosthetic limbs. A significant benefit of the technology is the ability to customise and personalise the items produced so that they are patient specific.
A number of UK NHS hospitals have used additive manufacturing to improve predictability, accuracy, safety and efficiency. Advances in the technology have inspired a number of surgeons to commission additive manufactured, patient specific implants (PSIs) and surgical guides.
Patient specific implants (PSIs)
Additive manufacturing of patient specific implants uses a digital workflow that can benefit several stages of the process including planning. To produce a patient specific implant, data is acquired from a patient scan, such as a CT or MRI scan. The patient’s data is then imported into CAD, prepared for manufacture, made and then finished.
One such application of additive manufactured implants is in craniotomy and cranioplasty, when a patient has a piece of skull removed to accommodate swelling caused by injury, tumour or stroke. The patient then requires the surgical repair of the bone defect to restore the skull. The surgical repair can be done by replacing the original bone section, or by using a custom implant. If the surgeon chooses a custom implant, it is important that the implant fits correctly, particularly for aesthetic purposes. The customisation possible with additive manufacturing allows accurate bespoke implant production. This is an advantage for this application because of the irregular shapes of skulls, which makes implants difficult to standardise. Furthermore, the skull and brain are complex and difficult areas to visualise.
A traditional approach to PSI for cranioplasty would be to form the titanium into shape using a hydraulic press; this would be formed on a model taken from an impression of a patient’s skull. The impression is taken over the skin, which means it is subject to some inaccuracy as the implant will be placed directly on the bone. Alternatively, a plate formed from polymethyl methacrylate (PMMA) could be created during the surgery, which adds time to the procedure, or it could be formed prior to surgery, although this requires a more complex sterilising method, as PMMA cannot be autoclaved.
Using a custom, additive manufactured implant offers potential for decreased surgical time and improved implant fit by addressing some of the disadvantages of the aforementioned traditional approaches. However, the real innovation of additive manufacturing for PSI comes in the shape of custom guides. When implanting a PSI, in some cases it might not be obviously located due to a lack of landmarks on the patient’s bone. In this situation, the surgeon can use a placement guide to locate the implant into its correct position.
Surgeons can also use an additive manufactured cutting guide during a surgery to harvest bone accurately. Finally, using a cutting guide can also allow bones to be cut with better precision, giving greater surface area contact between mating surfaces, which leads to better osseointegration. Adopting additive manufacturing technology can help reduce surgery time.
Additive manufacturing for cranioplasty
In a recent example, a patient presented with a meningioma caused by a benign growth on the left side of the cranium. The CT scan revealed that the growth was expanding into the skull. The patient required a craniotomy to remove the tumour and a cranioplasty to rebuild the skull.
PDR, a design consultancy, developed a PSI cranial plate and custom guide for the craniotomy. Due to the design precision additive manufacturing offered, the implant fit the exact specification given by the surgeon, which resulted in a good aesthetic outcome with the implant matching the patient’s cranial contours. The patient was discharged in four days and was found to be complication free.
Additive manufacturing for mandibular reconstruction
A recent procedure at the University Hospital of Wales required complex reconstructive surgery. The surgeons used a digital workflow and pre-planning to optimise productivity. The patient required a mandible reconstruction due to cancer of the lower jaw, which involved the removal of the left side of the jaw.
A section of bone and vascular tissue was removed from the fibula to reconstruct the sectioned jaw. For the operation to be successful, a perfect fit between the two harvested fibula sections and the two remaining healthy sections of the jaw was required. A mandibular plate was constructed to hold the sections together.
The operation involved additive manufactured cutting and drilling guides, additive manufactured implants and a pre-planned surgical approach. The cutting guides were used to harvest the best section of bone and soft tissue to prevent morbidity and establish a healthy blood supply to aid recovery.
Using additive manufacturing to produce the implant and guides resulted in the perfect fit of bone segments. The complex surgery was delivered with high precision, which aided patient safety. The pre-defined cutting and drilling guides reduced the risks that a freehand operation could present.
The Renishaw Healthcare Centre of Excellence in Miskin, near Cardiff in Wales contains a facility for the manufacture of medical products under the ISO13485 quality management system. The facility is focused on the production of craniomaxillofacial specific implants, jigs and guides. Anatomical models are manufactured to complement implant manufacturing in polycarbonate using a fusion deposition machine (FDM) machine.
3D printed models can also be used for training and teaching, as tissue characteristics can be replicated for normal and pathological examples. This bypasses the traditional training approach and can accelerate the training pathway, giving surgeons the opportunity to practise on complex or uncommon pathologies.
Projects are underway to increase the uptake of additive manufactured craniomaxillofacial PSIs. These include the award winning project Additive-manufacture for Design-led Patient Treatment (ADEPT), which aims to simplify the design of CMF implants by increasing the automation in design and offering reduced costs relative to existing subscription based design software packages.
3D printing is a rapidly expanding technology that offers benefits in efficiency, accuracy and ease of customisation. These applications will extend in future, perhaps even to the 3D bioprinting of tissue and organs. Once the technology’s potentials are realised, 3D printing will become increasingly used in medical procedures.
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