A review on the use of additive manufacturing to produce lower limb orthoses
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Orthoses (exoskeletons and fracture fixation devices) enhance users’ ability to function and improve their quality of life by supporting alignment correction, restoring mobility, providing protection, immobilisation and stabilisation. Ideally, these devices should be personalised to each patient to improve comfort and performance. Production costs have been one of the main constraints for the production of personalised orthoses. However, customisation and personalisation of orthoses are now possible through the use of additive manufacturing. This paper presents the current state of the art of additive manufacturing for the fabrication of orthoses, providing several examples, and discusses key research challenges to be addressed to further develop this field.
KeywordsAdditive manufacturing Exoskeleton External fixation Orthoses Personalisation
According to a report from the Centre for Disease Control and Prevention, only in The United States (USA), more than 17 million adults present locomotion problems . These problems are mostly caused by falls, ageing-related diseases or accidents. One of the common methods in treating orthopaedic leg injuries is wearing orthopaedic devices . Orthoses are orthopaedic devices designed to help patients with difficulties to walk or semi-paralysed due to spinal cord injuries (SCI) or stroke [3, 4]. These devices are designed to provide support, stabilisation and immobilisation. Generally, there are two main groups of orthotic devices: exoskeletons and fracture fixation devices. The main difference between these two groups lies on the purposes of using them. Exoskeleton devices are mainly designed to restore/reinforce the human performance [5, 6], whereas the fracture fixation devices (e.g. Ilizarov, splints, casts) are designed for immobilisation/stabilisation of the fractured bones and correction of specific deformities [7, 8].
Exoskeletons are being used for medical (e.g. rehabilitation) , military (e.g. carrying heavy weapons) [10, 11] and industrial (e.g. handling cargo) applications [11, 12, 13, 14]. External fixation devices were developed for the treatment of different bone fractures, limb deformity and soft tissue pathologies, playing a critical role in preventing amputation [15, 16]. The concept was introduced as an alternative to immobilisation in a plaster cast, internal fixation and traction, providing support to a limb using rings and/or wires secured to external scaffolding [17, 18]. These devices can be used for temporary treatment, providing provisional alignment stability, or for permanent treatment in cases such as pelvic fractures, open long bone fractures and periarticular fractures [17, 19].
The Ilizarov is mainly used to hold broken bones together, correct bone deformities, lengthen the shortened limb and address soft tissue atrophy. However, one of the main limitations of this device is the risk of infections as a result of the use of pins and wires. Additionally, this device causes patient discomfort, requires prolonged treatment and a manual lengthening process .
Orthoses are a good example of personalised products. To be effective, they must be designed considering the anatomic characteristics of the user and they must fit the corresponding applications (e.g. rehabilitation or supporting activities). However, due to technological limitations and associated costs, personalisation was not explored before. This paper discusses the implementation of mass personalisation to produce orthotic devices, focusing on the emerging use of additive manufacturing. The most commonly used techniques are discussed, and examples provided. Finally, some research challenges are presented.
2 Mass personalisation of orthoses
Vat photo-polymerisation: an additive manufacturing method in which a liquid polymer contained in a vat is selectively cured using UV radiation from a laser or a lamp.
Material jetting: an additive manufacturing process in which droplets of build material are selectively deposited through a nozzle.
Material extrusion: an additive manufacturing process in which melted material is selectively dispensed through a nozzle.
Powder bed fusion: an additive manufacturing in which thermal energy (laser or electron beam) selectively fuses regions of a powder bed.
Binder jetting: an additive manufacturing process in which a liquid binder is selectively deposited to join powder materials.
Sheet lamination: an additive manufacturing process in which sheets of materials are bonded to form a 3D component.
Direct energy deposition: an additive manufacturing process in which thermal energy is used to fuse materials by melting them as the material is being deposited.
However, among these technologies, only vat photo-polymerisation, powder bed fusion and material extrusion have been explored to produce orthoses.
3 3D-printed orthoses
3.1 Vat photo-polymerisation
Vat photo-polymerisation uses photo-curable polymers which are relatively expensive compared to other polymer-based techniques. Therefore, the use of vat phot-polymerisation for the fabrication of orthoses is limited. Mavroidis et al.  compared AFOs produced through different techniques. A conventional casting process was used to produce PolyPropylene (PP) AFO. Vat photo-polymerisation (Viper SLA machine) was also used to produce AFOs using both Accura 40 and Somos 9120. Gait analyses were carried out on one subject at the Spaulding Rehabilitation Hospital, Boston, with the aid of a motion capture system. Results showed that the performance of additive manufactured custom-fit ankle–foot orthoses is similar to the standard orthosis in terms of controlling the kinematics and kinetics of the ankle with an equivalent walking speed and the step length for all ankle–foot orthoses . No differences in terms of performance were observed between the two additive manufactured orthoses.
The QF fixator was used to treat three male patients with tibial fractures due to traffic accidents. This study shows good results in terms of accuracy of reduction and operation time. In addition, no pin loosening, pin site infection or any other complications were observed. Moreover, the QF presents other advantages including easy assembly and cleaner X-ray images due to the reduced imaged scattering .
3.2 Material extrusion
Chen et al.  used CNC machining and extrusion-based additive manufacturing to produce an AFO. Conventionally manufactured orthosis was in PP, while additive manufactured orthoses were made in Polycarbonate (PC)-ABS and ULTEM. A finite element model was used to calculate the static and dynamic loading during the gait cycle supporting the design phase. Results showed that the additive manufactured AFOs present lower strain during the gait cycle than conventionally manufactured ones. Additionally, the ULTEM ankle–foot orthosis has the lowest strain among the three orthoses. This work also shows that material extrusion can be used to fabricate orthoses with sufficient strength and stiffness .
Vijayaragavan et al.  used the extrusion process to produce a corrective orthosis for the treatment of clubfoot in children. The authors used CT data for both the internal and external definition of the foot. The internal data were used for the definition of the bone structure, while the external data comprised the skin providing the geometrical volume of the foot.
Blaya et al.  designed and fabricated novel splints for the partial rupture of Achilles tendon. The designed splint was produced using both FilaFlex and Polycarbonate materials that guarantee comfort and resistance at the same time. In addition, the authors performed material optimisation studies to reduce the weight of the splint and manufacturing costs.
Jin et al.  investigated the effect of different processing conditions to produce Ankle–Foot Orthoses with improved mechanical properties. The study focused on the following issues: (1) optimal orientation in the working platform, (2) support generation, (3) slicing and (4) tool path generation. Optimal orientation was selected based on the improvement of the build time, part strength and surface finish, and minimising support structures. Adaptive slicing strategies were considered to reduce fabrication time and to improve surface quality. Finally, a contour-parallel tool-path strategy was adopted for the device fabrication.
Turk et al.  combined extrusion-based additive manufacturing with Carbon Fibre-Reinforced Polymers (CFRP) in an autoclave pre-impregnated process for the development of complex-shaped hybrid AM-CFRP structures. Powder bed fusion and extrusion-based additive manufacturing were also used to create titanium functional parts and ST-130 polymeric parts, respectively. The printed components were assembled and over-laminated with a carbon fibre-reinforced polymer and consolidated in an autoclave. A significant weight reduction (28%) was achieved compared to commercial available devices, without compromising mechanical performance.
3.3 Powder bed fusion
4 Challenges and conclusions
Commercially available orthoses are not fully customised devices. They are produced using conventional machining and/or casting process, which does not have the capability to produce small and intricate features. Ideally, orthoses must be fully personalised to be efficient for the treatment of an individual patient with different diseases and injuries. Technological limitations were the main constraint for mass customisation and personalisation. The emergence of additive manufacturing technologies allows the fabrication of custom-made orthoses in a cost-effective way. The combination of additive manufacturing and individual anatomic data allows the fabrication of complex and more comfortable devices reducing cost and development time.
Among the different additive manufacturing techniques currently available, only vat photo-polymerisation, material extrusion and powder bed fusion have been explored. Material extrusion is the most affordable one but limited to the use of polymers.
Orthotic devices can be produced through the use of a wide range of materials such as plastics (thermoplastics and thermosets), metals, synthetic fabrics and combinations of these materials. The most commonly used additive manufacturing materials are ABS, PLA and PA as they can be easily processed and provide adequate mechanical properties. These materials can also be combined with soft natural polymers (hydrogels) able to absorb moisture, reduce friction, reduce skin irritation and increase patient’s comfort.
Most additive manufacturing machines have a working volume smaller than the dimensions of the exoskeleton. In this case, different components must be considered, printed and finally assembled. This increases labour time and cost.
Additive manufacturing has been used to produce small-scale passive orthoses or components for large-scale passive orthoses. The use of additive manufacturing for the fabrication of active exoskeletons is an important challenge requiring not only in printing the built material but also to embed sensors and actuators during the fabrication process.
Additive manufacturing allows freedom of design. In the case of the design of orthoses, this means that new functionality can be considered. The combination of the shape or topology optimisation tool with additive manufacturing, for example, allows the fabrication of lightweight structure without compromising the mechanical performance. Therefore, the design of orthoses to be produced through additive manufacturing must also take into consideration the characteristics and constraints of each technique (design for additive manufacturing).
The first author acknowledges the support received by The King Saud University to conduct his PhD studies.
This research was funded by Saudi Arabian government.
Compliance with ethical standards
Conflict of interest
No potential conflict of interest was reported by the authors.
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