To manufacture precise, patient-specific products in the medical industry, one technique that is gaining popularity is 3D printing. Ed Littlewood, Marketing Manager at 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.

At Madame Tussauds, the famous waxwork museum in London, there is a careful procedure when making a celebrity figure; it involves around 15 artists working on each model for three to four months. The detailed process requires 250 precise measurements before the figure is made; this means that the end result is a lifelike, near-exact replica.

Additive manufacturing or 3D printing is used in a variety of industrial applications for prototyping and manufacturing. The technology uses a range of materials including polymers, ceramics, resins, stainless steel, cobalt chrome and titanium. The additive manufacturing process produces 3D objects from a computer-aided design (CAD) file. Objects are built in layers, adding material until the manufactured part is complete. This method offers great design flexibility, which means that highly accurate, bespoke and customised devices can be produced at low cost compared to traditional manufacturing.

The medical applications of additive manufacturing are growing continually. 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 additively manufactured patient-specific implants and surgical guides.

Patient-specific implants

Creating patient-specific implants via additive manufacturing entails the use of 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 computed tomography (CT) or magnetic resonance imaging (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 craniectomy 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 can make implants difficult to standardise. Furthermore, the skull and brain are complex and difficult areas to visualise.

A traditional approach to patient-specific implants 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, though this requires a more complex sterilising method, as PMMA cannot be autoclaved.

Using a custom, additively 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 patient-specific implants comes in the shape of custom guides. When implanting a patient-specific implant, in some cases it might not obviously locate 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 additively manufactured cutting guide during a surgery to remove 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, as observed in the examples below.

Additive manufacturing for cranioplasty

In one 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 craniectomy to remove the tumour and a cranioplasty to rebuild the skull.

PDR, a world-leading design consultancy, designed a patient-specific cranial plate and custom guide for the craniotomy. Thanks to the design freedom additive manufacturing offered, the implant fitted the precise specification given by the surgeon, which resulted in a good aesthetic outcome with the implant matching the patient’s cranial contours exactly. 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 in the UK required complex reconstructive surgery. The patient required a mandible reconstruction due to cancer of the lower jaw, which involved the removal of the left side of the jaw.

The surgeons used a digital workflow and pre-planning to optimise productivity. A section of bone and vascular tissue was removed from the patient’s fibula (leg bone) to reconstruct the sectioned jaw. For the operation to be successful, a perfect fit was required between the two harvested fibula sections and the two remaining healthy sections of the jaw. A mandibular plate was constructed to hold the sections together.

The operation involved additively manufactured cutting and drilling guides, additively 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 patient-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

Outside of patient-specific implant production, 3D printing is used to make models for surgical preparation. This provides surgeons with a tangible, 3D model of a specific patient. The surgeon can use this model to simulate an operation – a significant improvement on the 2D information of a scan. This can be useful to surgeons because by allowing them to perform a ‘dry run’ of the procedure – they can confirm the implant design is suitable, meaning that if there is any difficulty in placing the implant it can be resolved before the surgery, rather than during.

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.

Looking forward

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.