Abstract
Rehabilitation orthoses is currently a research hotspot in the field of 3D printing. Compared with traditional manufacturing methods, orthoses manufactured by 3D printing can meet the needs of patients in terms of structure and performance. This paper mainly introduces the research progress, main problems, and challenges of 3D printing rehabilitation orthoses in the medical field. At the same time, different 3D printing processes, types of medical materials, and contour extraction methods of the body surface area are compared. Its purpose is to provide a reference for the additive manufacturing of rehabilitation orthoses and their printing methods in medical research.
1. Introduction
Rehabilitation orthoses refer to external appliances assembled in the limbs, trunk, and other human body parts. Its purpose is to prevent or correct the limbs, treat neuromuscular diseases, and compensate for their function. Common products are upper limb orthoses, lower limb orthoses, and spinal orthoses. According to statistics, by the end of 2010, the number of disabled people in China had reached 85.02 million, accounting for 6.34% of the total national population. More than 50 million people were in need of rehabilitation. By 2015, the elderly population over 60 had reached 222 million, among which about 70 million required rehabilitation treatment. Therefore, China has the world’s largest demand and market potential for rehabilitation orthoses [1].
Nowadays, the manufacture of rehabilitation orthoses has become an important problem limiting technological development. The production process of traditional orthoses is complicated, and the operation level of orthopedists is high. At the same time, the wearer will have skin ulceration and other complications, resulting in reduced treatment dependence of patients and affecting the treatment effect [2]. Figure 1 shows the manufacturing process of traditional orthoses. Additive manufacturing (AM), commonly known as 3D printing, is a group of emerging innovative techniques that create objects based on digital models through a layer-by-layer accumulation approach [3]. Compared with traditional manufacturing methods, 3D printing greatly reduces the waste of materials, shortens the manufacturing times, and solves many problems caused by traditional manufacturing processes [4]. This review aims to discuss the research progress and development trend of 3D printing technology for manufacturing rehabilitation orthoses. At the same time, the 3D printing technology, material types, and body surface contour data extraction methods are compared.
2. Printing Process of 3D Printing Rehabilitation Orthoses
Currently, 3D printing rehabilitation orthoses materials are mainly made of polymer materials, replacing traditional materials such as plaster, metal, and leather [5]. Therefore, the research on 3D printing technology for manufacturing rehabilitation orthoses mainly aims at the process methods of nonmetallic materials. 3D printing technology can be divided into various manufacturing methods according to different printing principles. At the same time, different printing methods have different process characteristics and appropriate materials [6]. Considering the cost, material, time, and other factors of various printing methods, 3D printing processes used in the research of rehabilitation orthoses manufacturing mainly include fused deposition modeling (FDM), selective laser sintering (SLS), stereolithography (SL), PolyJet, and multi jet fusion (MJF) [7, 8].
2.1. FDM (Fused Deposition Modeling)
FDM (Figure 2) is a printing technology that heats, melts, and extrudes hot-melt filamentary materials through a high-temperature nozzle. Then, the materials are printed layer-by-layer and solidifies into the desired shape. FDM printing is the most commonly used type of 3D printing rehabilitation orthoses because the technology is matured. At the same time, there are many kinds of raw materials, low prices, and convenient use of instruments. Due to the reason for layer-by-layer stacking, support structures should be added to the prominent areas of the model. Currently, the materials commonly used in FDM printing technology are ABS plastic and PLA, mainly used to manufacture upper and lower limb rehabilitation orthoses.
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In terms of upper limb orthoses, Blaya et al. [9] used FDM technology to make arm orthoses for fracture patients (Figure 3). Compared with traditional splints, these orthoses have lower cost and can be used for recycling, which reduces the burden of patients and achieves the purpose of orthoses. Ayiguli et al. [10] compared the traditional fixation method for finger flexor tendon injury, combined digital medicine, CAD, reverse engineering, and FDM technology. They studied the design and preparation of a customized brace for finger flexor tendon injury (Figure 4). Simultaneously, they found that the brace had better mechanical properties than a plaster bandage, and the shape was accurate enough to meet the patient’s needs. FFF (fused filament fabrication) is a 3D printing technology similar to FDM. Kim et al. [11] used this method to create a personalized wrist brace for scanning 3D images (Figure 5), in which the wrist joint was slightly dorsiflexion. Regarding lower extremity orthoses, some researchers believed that patients with foot drop who wear FFF-printed ankle-foot orthoses are more stable and walk faster than those who wear traditional orthoses [12].
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2.2. SLS (Selective Laser Sintering)
In the SLS technique (Figure 6), laser is used to move along a set path under the program’s control. Then, the low melting point powder is sintered and printed layer-by-layer. Finally, the desired model is printed. This printing technology has high precision and high material utilization and can be quickly produced for parts with a complex thin wall structure. The disadvantages are high printing cost, low surface quality, and easy model warping. In the field of rehabilitation orthoses, nylon and other thermoplastic materials are often used to make lower limb and spinal orthoses.
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In terms of lower limb orthoses, Creylman et al. [13] evaluated the clinical manifestations of AFO (ankle-foot orthosis) printed by SLS and traditional AFO (Figure 7) during gait analysis (Table 1). They found significant beneficial effects of both AFO on spatiotemporal gait parameters and ankle kinematic parameters. Considering that AFO printed with SLS technology can shorten the production time, it is feasible to use SLS technology instead of the traditional way to make AFO. In terms of spinal orthoses, the SLS technology eliminates the need for any support structures when printing large orthoses. Lian et al. [14] used SLS technology, nylon, and finite element analysis software to make 3D-printed scoliosis orthoses for patients. These orthoses use local hollow-out structures with 3 mm thickness, 9 mm radius, and 23 mm spacing (Figure 8), which makes them lighter and more breathable.
2.3. SL (Stereolithography)
SL technology (Figure 9) uses computer-controlled ultraviolet light to scan the surface of photosensitive material in the liquid tank point-by-point so that slurry is cured and formed. The advantages of this technology are fast forming speed, smooth surface, and high precision. However, the cost of materials is high, the variety is less, and postprocessing is needed in the printing process.
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Due to the high production cost of SL, there is little research in the orthoses field. In terms of process, comparing the AFO printed by SLS and FDM, Lu et al. [15] found that SL technology was the best choice in terms of time and material cost by selecting the three most common 3D printing materials (PLA, ABS-like photosensitive resin, and nylon) (Table 2). In terms of cost, some scholars [16] believed that SL technology and printing materials were the most suitable for 3D printing AFO because they were most likely to meet the requirements of clinicians and patients. However, reducing the amount of the printing material used for clinical applications is a problem that must be solved to reduce the cost.
2.4. MJF (Multi Jet Fusion)
MJF, emerging in 2014, is a proprietary technology of Hewlett–Packard (HP) Inc. In contrast to SLS using a laser as the heat source, MJF utilizes an array of infrared lamps as the energy source to fuse the area of interest jetted with a fusing agent that can absorb infrared radiation energy. The fusing agent is deposited by inkjet nozzles installed in a carriage to the designated regions of the powder bed on the voxel level. Meanwhile, a water-based detailing agent is jetted around the contours of the printed parts to inhibit the fusion of powder near the part edges and improve part resolution, as illustrated in Figure 10. This technology has the characteristics of high precision characteristics, fast speed, and good quality. It can add pigments in the printing process to realize color 3D printing. However, it also has the disadvantages of less suitable materials, low hardness, and strength of the manufactured model, so it is rarely used in the field of orthoses.
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Due to the good accuracy and quality of products printed by this technology, some scholars [17] used SL and MJF technologies combined with materials to print knee orthoses (Figure 11). In other words, MJF technology and TPU materials were used to make the inner part of the orthoses where the body is in contact. FDM technology and ABS materials were used to make the outer support and fixation part, which solved the patient’s need for comfort. By comparing the printing effects of four materials (Table 3), Liu [18] found that AFO printed with MJF technology and PA12 could improve the walking ability and activities of daily living for patients with acute stroke.
2.5. PolyJet
PolyJet (Figure 12) is used as a carriage with four or more inkjet heads, and ultraviolet (UV) lamps traverse the workspace, depositing tiny droplets of photopolymers, materials that solidify when exposed to UV light. After printing a thin layer of the material, the process repeats until a complete 3D object is formed. PolyJet features the ability to spray different types of photosensitive resins to generate the part’s body and support structure, respectively, in order to facilitate the processing of parts after printing. However, this kind of printing technology also has the disadvantage of poor strength and hardness of manufacturing models, so there are only a few applications in the field of orthoses.
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Because the products printed by this technology are not strong, most of them are wrist orthoses. Cazon et al. [19] compared the 3D printing multimaterial wrist orthoses with traditional orthoses, and they found that the 3D printing wrist orthoses were stronger than traditional orthoses in terms of mobility and mechanics (Figure 13). Some scholars [20] divided the wrist orthoses into two parts. The internal structure was made by PolyJet printing, and the external structure was made by traditional injection moulding. At the same time, the internal structure was fixed on the external structure, which solved the problems of poor ventilation and insufficient hardness.
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2.6. Comparison of 3D Printing Process Features
In conclusion, the working principle, advantages, disadvantages, and research status of 3D printing technology commonly used in rehabilitation orthoses. The following will further analyze the characteristics, processing accuracy, materials, and other contents of various printing technologies.
In terms of printing speed, SL technology has a speed advantage over FDM and SLS technology because it solidifies the material by laser irradiation and does not need to heat the material during the printing process. To accelerate powder sintering, MJF technology sprays different fusing agents on the material’s surface. It is also a printing process with a faster printing speed. Regarding production cost, FDM technology has a simple technical principle and a wide variety of materials. It is one of the lowest production costs in the 3D printing process. However, SL and PloyJet technologies use photosensitive resin curing technology, which leads to higher material costs. In terms of surface accuracy of printed parts, SL and PloyJet technology can achieve high surface accuracy of printed parts because of the good fluidity of printed materials. Table 4 shows the comprehensive comparison of each printing process:
3. Types of Materials for 3D Printing Rehabilitation Orthoses
According to the published literature in the field of rehabilitation orthoses, [21–23] 3D printing is mainly used to make new rehabilitation orthoses for the upper limbs, ankles, feet, spine, and other parts of the human body. Commonly used materials are divided into rigid materials and flexible materials. Some experts and scholars found that the structural integrity, stiffness, low thickness, and patient comfort of orthoses made of additives and composite materials were improved [24].
3.1. Rigid Materials
At present, the most widely used materials in the 3D printing process are thermoplastic materials and photosensitive resins. The materials commonly used in FDM and SLS are mainly thermoplastic, while SL and PloyJet technologies are mainly photosensitive resins.
ABS, PA, and PLA are the most widely used thermoplastic materials in 3D printing rehabilitation orthoses. ABS is a noncrystalline thermoplastic polymer material with high strength, toughness, and easy processing. It is the most common and widely used thermoplastic material in life. At the same time, it is also the most commonly used consumable material for FDM and SLS technology. Due to its good mechanical properties, Polish graduate students in bioengineering used ABS materials and FDM printing technology to make customized finger orthoses [25] that can easily grasp objects with fingers for a patient. PLA is a new environment-friendly degradable material made from renewable raw materials. It has the properties of rapid degradation, machinability, biocompatibility, and heat resistance (Table 5). Compared with ABS, this material has high stiffness, low melting point, and low forming temperature, which is conducive to saving energy. Gorski et al. [26] designed a wrist orthosis with high outer hardness (PLA) and good inner softness (ABS) (Figure 14) to improve the pressure resistance of the orthoses. Because of the properties of PLA material, PLA is often mixed with other materials to obtain better material properties. Tao et al. [27] used the tensile test, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and powder X-ray diffraction to determine the weight of polymer materials as follows: TPU/PLA: 0%/100% (TP0), 25%/75% (TP25), and 50%/50% (TP50) for mechanical properties, thermal properties, and structural analysis (Figure 15), they found that the finger orthoses designed with the ratio of 25%/75% mixed materials had the best flexibility. Because of its good mechanical properties, heat resistance, and corrosion resistance [28], PA is not only the material with a good air permeability of the model obtained by the SLS printing process but also the main material for MJF printing technology. Li and Gao [29] compared the comprehensive advantages of orthoses with different 3D printing processes (Figure 16); they found that the orthoses made of MJF process and PA11 material had the best mechanical performance.
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Photosensitive resins are liquid resins that solidify when exposed to UV light. They are mainly used in SL and PloyJet printing processes. Photosensitive resins can mimic the different properties of other materials because of their various properties. SL printing process can mimic the characteristics of common thermoplastic materials (such as ABS and PA) when using a single photosensitive resin in different forms. The PloyJet printing process uses different colors of materials to print parts with a full gamut of color properties. For rehabilitation orthoses, photosensitive resin materials can not only meet the rigid and flexible requirements but also make the surface of the orthoses have higher smoothness. However, the photosensitive resin material is very sensitive to UV light, so the orthoses can easily harden and become brittle, affecting their durability.
3.2. Flexible Materials
TPU is a flexible elastomer used in FDM, SLS, and MJF printing processes. It has the advantages of good wear resistance and tear resistance. It is also an environmentally friendly material that can be recycled and decomposed [30]. Due to its good elasticity and flexibility, Wei et al. [31] treated 33 patients with cervical spondylosis as the research object. The cervical correction pillow was made through the cervical lateral X-ray films of patients, three-dimensional body surface information of the neck and individual disease differences (Figure 17). Through the comparison before and after 24 weeks of treatment, they found that the use of 3D printing personalized vertebral correction pillow can effectively correct the physiological curvature of the cervical spine. It is an effective method for the treatment of patients with cervical spondylosis. At the same time, the orthoses made of TPU and PLA material have good performance, low price, and avoid patient complications. Liu et al. [32] compared 3D-printed wrist orthoses with polymer plaster splints. They found wrist orthoses (Figure 18) made of TPU and PLA seem more effective.
3.3. Material Comparison
Table 6 is a comparison of the applicable technology, material cost, and material properties of rigid materials and flexible materials that are currently suitable for the design of rehabilitation orthoses:
4. Extraction of Body Surface Contour Data for 3D Printing Rehabilitation Orthoses
Extraction of body surface contour data plays an important role in whether 3D-printed rehabilitation orthoses are effective for patients. An accurate digital model can be obtained only when the body surface data of the patient’s limb is accurately obtained. If body surface contour data extraction is not accurate, it is difficult to generate rehabilitation orthoses that accurately match patients. Currently, there is no standard technology for extracting body surface contour data. However, the commonly used 3D body surface contour data acquisition techniques in clinical practice mainly include human tomography data reconstruction (CT or MR) and new 3D optical scanning technology [33] (Figure 19).
4.1. Human Tomographic Data Reconstruction (CT or MR)
The data acquisition methods of CT tomography and MR scanning are obtained by nonoptical measurement, which has the advantages of fast inspection speed and high resolution. Moreover, both methods have been confirmed by a large number of clinical studies, and they can collect accurate 3D model data. Liao et al. [34] reconstructed the body surface contour by importing the CT scan data of the patient’s arm into Mimics and Geomagic Studio software. Then, they designed and manufactured a 3D-printed wrist orthosis that met the patient’s requirements (Figure 20). Because a CT scan carries a certain amount of radiation, to reduce unnecessary harm and protect patients, researchers generally use MR scans to obtain structural information. Chen et al. [35] of Southern Medical University studied an MR imaging reconstruction method for upper limb fracture orthoses (Figure 21), which improved the contour extraction accuracy of the upper limb body surface. However, MR scanning takes long time, and data extraction is tedious, which requires a certain understanding of human anatomy and digital operation. At the same time, patients are required to maintain a fixed posture during the scanning process, local skin compression may easily lead to inaccurate data extraction. But, this does not mean that CT and MR scans can be replaced because these two scans allow biomechanical finite element analysis of the area where orthoses are in contact with the limb to optimize the orthoses’ structural design [36].
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4.2. New 3D Optical Scanning Technology
The new 3D optical scanning technology can quickly and accurately obtain 3D information about the body surface contour structure without touching the patient. According to different scanning methods, 3D optical scanning technology is divided into three scanning strategies: point, line, and plane. Although point and line scanning have high scanning accuracy, the operation is complicated, and the scanning time is long. Surface scanning can obtain patients’ 3D body surface information by quickly scanning the contour of one side of the patient. Although the point cloud density of this scanning method is large and fast, the accuracy is far less than the point line scanning method. Due to the manufacture of rehabilitation orthoses only needs the contour information of the body surface, the handheld device has the characteristics of simple operation and fast data collection speed. Therefore, it has been widely used in the design of rehabilitation orthoses [37]. Meng et al. [38] scanned the patient’s ankle joint with an optical scanner, imported the scan data, and constructed the ankle joint model through Geomagic Studio modeling for visual imaging (Figure 22). Finally, the protective semirigid ankle brace was prepared by three-dimensional clipping method.
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5. Biomechanics of 3D Printing Rehabilitation Orthoses
Biomechanical performance evaluation of 3D printing rehabilitation orthoses is mainly reflected in gait analysis, modification of orthoses shape design parameters, and analysis of human pressure distribution. Some scholars explored the changes in the stiffness of different support shafts in AFO. They found that these changes had little impact on gait [39]. However, when changing the lateral shape of the posterior foot of the foot orthoses, the muscle group activity above the knee and the plantar pressure will be changed [40]. Xu et al. [41] conducted a plantar pressure test on the 3D-printed insole. They found that the printed insole could reduce the damage related to plantar lesions and effectively improve the comfort of patients with plantar fasciitis.
The application of finite element analysis plays an important role in the virtual design and correction effects simulation of orthoses [42]. Some scholars added finite elements in the CAD/CAM process to optimize the design of 3D printing orthoses and simulated the correction effect after wearing to design more portable, breathable, and comfortable orthoses [43]. Li et al. [44] applied parameterization (Figure 23) and finite element (Figure 24) to the insole design. They proposed the parameterized design method, which greatly reduced the workload and could easily adjust the insole design to improve comfort. Ali et al. [45] used finite element analysis to check biomechanical performance of metamaterial intramedullary (IM) nails for bone union. Thus, they designed metamaterial IM nails for different fracture gap sizes according to patient-specific needs. In addition, finite elements can also simulate the mechanical properties of material filling at different porosity to print the bone plate with optimal performance and the best healing effect [46].
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6. Challenges and Future Prospects of 3D Printing Rehabilitation Orthoses
6.1. Challenges
3D printing orthoses have been widely used in clinical practice due to their advantages, such as lightweight, personalized, and fast production. But, 3D printing has not been widely used in the field of medical rehabilitation in real life, mainly for three reasons: firstly, in the field of 3D printing, although the design software of various parts is relatively mature, there is a lack of professional 3D design software for the rehabilitation orthoses industry. Secondly, the postprocessing of 3D printing rehabilitation orthoses is complicated, and the process needs to be improved. For example, the FDM printing process has low dimensional accuracy, so the surface quality of a printed model is rough. Thirdly, the cost and materials of 3D-printed rehabilitation orthoses need to be improved. For example, the orthoses printed with nylon material have a hard texture and rough surface.
6.2. Future Prospects
With the new demand for the function and performance of rehabilitation orthoses in the new era, 3D printing rehabilitation orthoses will show new development prospects in terms of design and manufacturing, as well as materials and functions, as shown below: Firstly, cross-regional data collection of rehabilitation orthoses design effectively simplify the modeling process so that ordinary users enjoy more convenience. Secondly, the biomass materials of rehabilitation orthoses are environmentally friendly and skin-friendly. With the enhancement of human environmental awareness, adding biomass materials to 3D printing thermoplastic materials can improve the moulding and mechanical properties of materials so that orthoses appliances have good environmental protection and skin affinity. Thirdly, the intelligence and personalization of rehabilitation orthosis’ function; for example, adding sensors to receive user data in real-time through the mobile terminal. It can support further optimization design and process improvement and treatment effect of orthoses.
7. Conclusion
3D printing technology has laid a good foundation for manufacturing rehabilitation orthoses. Its rapid and highly integrated characteristics greatly shorten orthoses’ research and development cycle. This paper reviews the process principles and research status of different 3D printing technologies in rehabilitation orthoses. At the same time, the common methods of body surface contour data extraction and biomechanical finite element analysis are introduced. It is believed that with the proposal and implementation of the “Made in China 2025” plan, 3D printing rehabilitation orthoses will have incomparable advantages and development prospects in the future with the traditional production methods. At the same time, more and more patients will experience unprecedented improvements in the function and quality of life.
Data Availability
The original data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The work reported in this paper was financially supported by the Dongguan Sci-tech Commissioner (No. 20211800500102). This fund comes from this project “Research on Digital Design and Intelligent Manufacturing of Complex Orthopedic Rehabilitation AIDS based on 3D printing.”