Be specific! Custom implants focus on the individual patient

No two patients are the same, so why should their implants be the same? Customised implants allow for personalised healthcare, helping to ensure that each patient gets the very best treatment. In orthopaedics, it pays to be patient specific.

Every implant procedure involves a degree of customisation to ensure an acceptable surgical implementation of the device in combination with the unique anatomy of each patient. The question is how and where this customisation occurs - by the surgeon in the operating theatre, or by design during the device manufacture?

Patient-specific implants (PSIs) use the latter approach. Here we put the focus on pre-operative planning, using medical imaging data and additive manufacturing (AM) to obtain a precise, customised implant fit. The payback for this up-front effort is better patient outcomes, reduced surgery and rehabilitation times, and lower overall costs.

This article focuses on recent developments in medical device design and manufacture that are making personalised healthcare a reality.

Limitations of standard implants and surgical techniques

Orthopaedic implants are medical devices that are designed to replace a missing joint or bone, or to support bone that has been damaged as a result of illness or trauma. Standard implants are produced in a range of sizes to suit different groups of people, but will not perfectly match the anatomy of any one individual patient. Either the implant or the patient's bone must be adapted by the surgeon to ensure a precise fit.

This consumes valuable operating theatre time whilst the device is cut and bent into shape, or whilst the patient's bone is ground back to match the fixings on the device. Often this freehand process will be iterative, requiring several adjustments to achieve an acceptable fit. Healthy bone can be lost unnecessarily during this process.

Furthermore, with traditional implants, the hospital must stock a large range of devices of various sizes and shapes, to provide an acceptable match with each patient’s anatomy. This has a significant cost implication and capital is tied up with inventory rather than being available to directly help patients.

By contrast, additive manufactured PSIs ensure that an excellent fit is achieved by design. This means that less time is required during surgery to fit the device, and less work is needed to remove healthy tissue during device fixation, reducing the risk of surgical complications. It is also no longer necessary to stock a range of standard products, lowering healthcare group costs.

Controlled surgical bone removal

In some cases, modification of the patient's bone structure is an essential part of the procedure. For instance, the patient's bone may have become distorted or damaged such that corrective surgery is required. In other instances, parts of the bone structure must be removed to access diseased tissue within. Such procedures require careful cutting of the existing bone, often followed by re-construction with the aid of an implant.

Custom AM devices can be helpful in these instances, providing perfectly fitting cutting guides to help the surgeon to cut and prepare the bone in exactly the right position, once again minimising surgery time and eliminating opportunities for human error.

Re-construction procedures can require the deformed bones to be cut, re-positioned and locked into a corrected configuration. Patient-specific 3D-printed re-positioning guides, coupled with PSIs to lock the sectioned bones into their new positions, provide an attractive solution.

Software tools underpin the new digital workflow

New software tools underpin the design and manufacture of PSIs. From computed tomography (CT) data to finished implant, streamlined software simplifies the design process.

A good example of this is ADEPT, a 3 year project focused on advancing the use of digital design and 3D printing technologies within craniomaxillofacial (CMF) surgery. The project was funded by the UK’s innovation agency, Innovate UK, and the Engineering and Physical Sciences Research Council (EPSRC).

Video above - click on the 'play' button to see the ADEPT craniomaxillofacial design workflow

The digital design workflow for a cranioplasty is as follows:

• CT data is imported into ADEPT where scan preparation tools aid bone and tissue identification
• A 3-dimensional model of the unique patient anatomy is created
• A 'mirror' tool maintains symmetry during implant design by identifying pairs of anatomical locations and a central point
• The boundary of the custom implant is defined by selecting points on the 3D model
• Flexibility splits can be defined on the base surface of the implant if required
• A unique identifying 'tag' can be added for traceability
• Fluid transfer and suture holes can be added either automatically or manually
• Counter-sunk screw holes are placed around the implant periphery
• Finally, the implant design is reviewed before sending for additive manufacture

Image above - ADEPT craniomaxillofacial implant design software.

We now move to preparation of the AM build file:

• The implant model is imported into QuantAM build preparation software
• Areas requiring supports during manufacture are identified and support structures are defined
• Multiple implants and surgical guides can be grouped together on a build plate for a more efficient production process.
• The supported model is sliced and the laser scan paths to solidify the component are generated, ready to be sent to the AM machine

Image above - QuantAM additive manufacturing build preparation software.

The final stage is additive manufacture, in which the build file is transferred to the AM machine. Once the build is complete, post-processing completes the production sequence, including support removal, stress-relieving heat treatment and surface finish enhancement.

So, orthopedic surgery can benefit significantly from a digital workflow and pre-planning processes to produce personalised implants, optimising operating theatre productivity with excellent surgical results. Let's look at some applications of these techniques.

Cranial surgery

In a patient presenting with a meningioma (a slow-growing tumour in the membrane surrounding the brain), a common treatment involves a craniectomy to remove the growth, followed by a cranioplasty to rebuild their skull. With careful planning, these can be performed as a single procedure, guided by CT imagery and deploying 3D-printed devices to help with both stages of the surgery. In this case, the device design work was performed by the International Centre for Design and Research (PDR), based at Cardiff Metropolitan University in Wales,

From the patient's CT data, a surface model of the skull can be constructed. The surgeon can then define the limits of the region to be removed in the craniectomy, from which a 3D-printed cutting guide can be designed. Aids such as a handle and indicators for orientation can be added at this stage.

Image above - a model of a patient's skull showing a growth that is to be removed during a craniectomy (left hand image). A 3D printed cutting guide is used to assist with the removal of the affected region (right hand image).

Once the tumour has been removed, the skull must be re-sealed with a cranial plate. A 3D model of the cranial plate, which covers and overlaps the aperture created during the craniectomy, can be generated by mirroring the healthy side of the patient's cranium. Redundant fixation features, splits and fluid transfer perforations can be added at the design stage to give the surgeon the maximum discretion during the operation.

Image above - comparison before and after cranioplasty surgery, with a customised 3D printed metal implant.

Refer to Digital evolution of cranial surgery for a case study that explores this topic in more detail.

Facial reconstruction

?Reconstructive surgery following cancer or trauma can present some complex challenges that can be met by using PSIs and custom surgical tools. For instance, an oral cancer patient needing surgery to remove part of their lower jaw may also require reconstruction using sections of their fibula bone. The operation requires a perfect fit between harvested sections of fibula bone and the remaining healthy sections of the jaw. A mandibular plate implant will be required to hold the re-constructed jaw sections together, in order for the bone to heal and knit together.

The first step is to harvest healthy bone from the fibula of just the right size and shape to replace the cancerous jaw tissue. A 3D-printed cutting guide (left hand image, below), also designed by PDR, is used to remove precise sections of bone and vascular connective tissue. It is important to identify the best bone and soft tissue to harvest, to ensure a healthy blood supply in the rebuilt jaw and so aid a fast recovery.

To minimise re-constructive surgery time, the cutting guides can also enable pilot holes to be drilled at the correct position on the bone sections in readiness for the final fixation screws. These are shown in the harvested bone sections in the right hand image, above.

Removal and preparation of the damaged jaw section also requires cutting and drilling guides, attached to the healthy parts of the mandible. Once again, the guides assist the surgeon to pre-drill pilot holes for the mandibular plate implant. The image below shows cutting guides and the resulting jaw bone structure ready for re-construction.

To reconstruct the jaw, the harvested sections of fibula bone and tissue are connected to the remaining healthy jaw sections and the mandibular plate implant is fixed in place to hold the new mandible together (shown below left). Soft tissue from the fibula replaces the removed soft tissue from the jaw. The pre-drilled holes enable rapid fixation, whilst anatomy-conforming location features on the mandibular plate ensure that everything is located correctly during fixation.

Restorative dental surgery with implants and bridges can be performed after the new jaw bones have knitted together (see right hand image, above).

Refer to 3D modelling and printing - saving theatre time and providing excellent patient outcome for a case study that explores this topic in detail.

Veterinary re-constructive surgery

These new techniques are also making an impact in veterinary surgery. As is the case with people, dogs commonly develop tumours, some of which require surgical removal followed by re-construction. Digital workflows and PSIs can offer similar benefits to the veterinary practice and to animals requiring medical treatment. As you might expect, with the anatomical variation within dogs being significantly more pronounced than within people (think Great Dane compared with a Chihuahua), achieving an excellent implant fit with an off-the-shelf device is even more difficult.  

In the case of a dog with a tumour on its maxilla (upper jaw), the procedure requires total excision followed by reconstruction. Following a revision maxillectomy, a complex custom titanium implant is used to reconstruct the dog's bone structure, designed by Voxelmed in Germany.

Rapid production of the implant using AM is also beneficial to enable surgery before the tumour grows any further. In this case, the implant was manufactured within two weeks of the diagnosis at the Additive Design in Surgical Solutions (ADEISS) centre in London, Ontario, Canada.

In such a complex anatomical region, maintaining the animal's bone structure is vital to its quality of life following the procedure. In this case, preserving the shape and functionality of the nasal and oral cavities helps the dog to breathe and eat easily.

Refer to Additive manufacturing in veterinary surgery - saving a well-loved member of the family for more details of this pioneering case study in canine reconstructive surgery.

Summary

Surgeons in both human and veterinary medicine are increasingly turning to pre-planned procedures using 3D-printed PSIs and custom surgical tools.

Underpinned by a digital workflow supported by streamlined software tools, personalised orthopedic surgery delivers superior outcomes for patients, whilst reducing costs for health service providers.

Next steps

Renishaw manufactures PSIs at its Healthcare Centre of Excellence, located in Miskin, Wales. The facility enables the manufacture of custom medical devices under an ISO 13485 quality management system, as well as education and training for the life sciences community.

Image above - floor-plan of the Renishaw Healthcare Centre of Excellence, providing production of dental frameworks, craniomaxillofacial patient-specific implants, jigs and guides.

Clinicians in North America can explore how to deploy PSIs with the Additive Design in Surgical Solutions (ADEISS) Centre. Located on the Western University campus in London, Ontario, ADEISS is a new development facility that enables researchers, clinicians and industry to design and fabricate working versions of medical devices, implants, surgical tools and intra-operative guides, using Renishaw metal additive manufacturing equipment.

ADEISS has a Medical Device Establishment License which allows it to supply class 1 medical devices into Canada. Compliance with ISO 13485, FDA and Health Canada requirements will be achieved in 2018.

For more details and further case studies, see Metal 3D printing for healthcare