Alister Hart, Vasiliki Panagiotopoulou and Johann Henckel describe the technologies and evidence that have enabled 3D printing and can collectively be defined as a new field – personalised orthopaedics.
There has been recent increased interest in the subject of personalised medicine, tailor-made treatments based on genetic testing [1-2]. In orthopaedics we have also seen a wave of personalised treatments: tailor-made implants for the knee  and hip joint replacement surgery . We have called this “personalised orthopaedics”.
Personalising orthopaedics began with computer-aided design and computer-aided manufacturing (CADCAM) orthopaedic implants in the 1980s. However, recent technological developments have stimulated a new wave of planning of the individual patient’s surgery (Figure 1); producing patient-specific instrumentation (Figure 2); teaching surgeons using models made directly from patient anatomy (Figure 3); and 3D printing of a customised implant (Figure 4).
These new technologies involve several steps in the pathway to production of a tailor-made solution for patients. First, new imaging protocols, and computer processing of the images, have allowed the capture of three-dimensional bony anatomy with precision and accuracy, and with a low radiation dose . Secondly, there are new software solutions for planning the design and 3D position of the implant. Lastly, 3D printing techniques allow us to generate customised Titanium (Ti) implants with porous surfaces to allow bony ingrowth and solid parts to provide structural support. Together these technologies create tailor-made titanium implants, which are particularly useful for patients with massive acetabular defects (Figure 1).
This review describes the technologies that have enabled 3D printing and can collectively be defined as a new field called personalised orthopaedics.
The enabling technologies for personalised orthopaedics
In this section, we will focus in the developments of the technologies, which enabled the development of personalised orthopaedics.
New imaging protocols, and computer processing of the images
Computed tomography (CT) is a useful tool for hip and knee problems, as it offers 3D representation of the local anatomy and allows for assessment and measurement of the position and orientation of hip and knee implants . However, the radiation of a conventional CT is high (10mSv) which is equivalent to about 4.5 years of background radiation . For the last twenty years, strenuous research has led to significant reduction to the radiation dose of CTs without compromising a good image quality in the knee [5, 8] and hip area [9, 10].
Magnetic Resonance Imaging (MRI) has the advantage of no radiation exposure but has the disadvantage of poorer 3D visualization of bone and the image can be severely affected by the presence of metal artefacts due to implanted orthopaedic devices. Introducing new imaging protocols for metal artefacts reduction sequences (MARS) has resulted in improvement of image quality, even in the presence of hip replacements [10, 11].
New software methods of planning the design and 3D position of the implant
Advances in imaging processing, including automatic segmentation of bone-muscle interface , has resulted in improvement of the 3D image quality. There is commercial grade available software to manipulate these virtual 3D models, in order to design and plan the position of implants . These software packages allow the user to generate 3D imaging data and virtual positioning of the implants pre-operatively. Also, improvement in software has helped the 3D planning of the design of customised implants.
3D printing techniques
3D printing, originally known as stereolithography, was invented in the 1980s by Charles Hull . It is a manufacturing technique of multilayer deposition in order to produce a three-dimensional object. The tools and implants produced by these methods are comparable to similar products manufactured using traditional techniques. In medicine, the main use of 3D printing is to make medical models to aid the surgeon’s understanding of the anatomy and for training purposes . Currently, 3D printing of implants uses digital data from CT or MRI patient scans in order to design a patient-specific implant that fits perfectly to the patient’s bone.
There are several 3D printing technologies, the most commonly used in orthopaedics is powder fusion. These systems rely on an energy source (laser or electron beam) to guide melting or sintering a layer of metal or polymer powder. These techniques are called Selective Laser Sintering (SLS) and Electron Beam Melting (EBM). Printing can be in Cobalt Chrome (CoCr) or Ti alloys. Various 3D shapes can be created with pores small enough for bony cells to grow onto and into the implant.
The clinical applications of 3D printing for orthopaedics
Two main products can be made from the 3D designs: models and implants. 3D printed models of our patients can help us plan surgery, deliver surgery and teach surgery. 3D printed implants can be made from porous titanium, which is ideal for reconstruction of massive, unclassifiable acetabular defects.
3D printed models for pre-operative planning
3D printing is becoming increasingly available to help with visualising and planning of surgical cases with complex anatomy and pathology. This technology has the potential to help the surgeon study and physically review challenging cases for the purposes of education, as it has in other surgical specialities [16, 17]. An example case is shown in Figure 1, an intra-pelvic pseudotumour that is compressing the neurovascular structures [18, 19]. The models help the surgeon understand the 3D anatomy and the position of the pseudotumour and neurovascular structures so that this inflammatory mass can be more easily located, more completely excised and with minimal damage to neurovascular structures.
3D printed models to help precise placement of implants
3D printed instrumentation models, also known as patient-specific instrumentation (PSI), have become widely used for knee replacement , neurosurgery and dentistry [21, 22]. Up to one-third of knee replacements in the US are carried out this way. The technology is also available for hip [23, 24], shoulder [25, 26] and ankle replacement surgery . The rationale for its use in the hip is simple; to accurately position the implants (Figure 2).
In the relatively near term a number of orthopaedic implant companies will have the ability to use a combination of two-dimensional X-rays together with CT or MRI catalogs of thousands of patients to generate 3D PSI.
3D models aid technical teaching
The reduction in the hours worked by trainee surgeons has led to a great interest in the use of novel, simulation-based methods for the training of orthopaedic surgeons [28, 29]. So far, these methods have been employed in the fields of arthroscopy, arthroplasty and trauma surgery [30, 31]. However, it is challenging to recreate complex deformities, which are often encountered in the setting of complex primary or revision arthroplasty surgery. This is a particular problem as such cases are becoming more common , and it is important to devise methods of simulation-based training that incorporate the challenges faced by surgeons in the operating theatre – as close to “real life” as possible (Figure 3).
3D printed porous titanium implants are used for reconstruction of massive, unclassifiable acetabular defects
These technologies come together to aid the reconstruction of massive acetabular defects using 3D printed porous titanium implants. New 3D planning and implant printing technology has enabled a radical change in how surgeons reconstruct massive acetabular defects and pelvic discontinuity in revision hip surgery (Figure 4). Previously, options available to patients with severe bone loss were limited to off-the-shelf jumbo cups, bone graft, metal augments, and mildly bespoke tri flange cups and cages, with joint excision the end of the road. Additive manufacturing has enabled printing of patient specific implants in titanium to treat this unsolved clinical problem.
Additive manufacturing (3D printing) has enabled personalised orthopaedics. From teaching surgeons how to deal with complex anatomy using 3D models and generated from the CT scans of patients, to producing 3D printed metal implants for personalised orthopaedics. Especially in the latter case, 3D printed, porous titanium implants provide personalised solutions to complex problems which otherwise are not possible to solve.
A number of hospitals have purchased 3D printers for on site use: it is easy to contemplate 3D plans that surgeons can use on site to rapidly produce instruments and implants. What currently takes a week or two will in the future become possible within an hour or two. There are potential applications in spine surgeries, fractures, corrective osteotomies and other orthopedic applications to make us more effective.
New pre, intra and post-operative methods are required to help implement the successful implantation of reconstruction of 3D printed titanium implants into patients with unclassifiable acetabular bony defects.
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Professor Alister Hart is a consultant orthopaedic surgeon, specializing in hip and knee problems, at the Royal National Orthopaedic surgeon (RNOH) NHS Trust in Stanmore, London, and holds the chair of academic clinical orthopaedics at University College London (UCL). He has performed more than 3000 operations, including 1000 primary or revision hip and knee replacements. He has won awards from the British Hip Society, American Academy of Orthopaedic Surgeons, Radiological Society of North America and University College London. His research interests focus on how to achieve lifelong function for patients that undergo hip and knee replacements.