Designing Hand Orthoses: Advances and Challenges in Material Extrusion

Abstract

The intricate structure of human hands requires personalized orthotic treatments, especially with the growing aging population’s demand for accessible care. While traditional orthoses are effective, they face challenges of cost, customization time, and accessibility. Additive manufacturing, particularly material extrusion (MEX) techniques, can effectively address challenges in orthotic device production by enabling automated, complex, and cost-effective solutions. This work aims to provide engineers with a comprehensive set of design considerations for developing hand orthoses using MEX technology, focusing on applying design for additive manufacturing principles, to enhance rehabilitation outcomes. This objective is achieved by establishing design requirements for hand orthoses, reviewing design choices and methodologies across conventional and state-of-the-art MEX-based devices, and proposing an innovative approach to orthotic design. Hand orthosis design requirements were gathered through workshops with occupational therapists and categorized into engineer-, medical-, and patient-specific needs. A review of 3D-printed hand orthoses using MEX analyzes various design approaches, providing insights into existing solutions. The study introduces a modular design concept aimed at improving rehabilitation by enhancing customizability and functionality. It highlights the potential of MEX for creating personalized, cost-effective orthoses and offers recommendations for future research, to optimize designs and improve patient outcomes.

1. Introduction

Healthy human hands are incredibly versatile, enabling us to hold, grasp, manipulate objects, interact with the environment, protect ourselves, and communicate. This versatility stems from the hand’s complex structure, which, however, complicates healthcare due to the multitude of conditions requiring diverse treatments, primarily rehabilitation. The need for accessibility of medical care keeps increasing due to the growing aging population, who are often more vulnerable to various types of health conditions, leading to the desire to find new solutions for improved treatment [1].

Orthoses are a commonly used device for the treatment of hand conditions. Orthoses can be prescribed for short-term use during acute recovery phases or for long-term wear to manage chronic conditions. However, the effective use of orthoses requires individualization to achieve better therapy outcomes. The need for customization poses significant challenges in traditional fabrication, including high costs, lengthy customization times, the requirement for skilled clinicians, and limited access to advanced technologies [2].

One potential solution to these challenges is additive manufacturing (AM), commonly known as 3D printing. AM addresses these issues by enabling cost-effective production, reducing manual labor through automation, and allowing for the creation of complex structures tailored to individual needs. This technology not only overcomes the traditional barriers to orthosis fabrication but also provides a similar rehabilitation effectiveness, making it a promising alternative in both acute and chronic care settings [3]. It is possible thanks to layer-wise object fabrication from a digital model. There are several types of AM processes, including stereolithography (SLA), selective laser sintering (SLS), multi-jet fusion (MJF), and several material extrusion (MEX) methods, with the widely used fused filament fabrication (FFF).

SLA uses UV light to cure photopolymer resins, producing detailed, smooth parts, but requiring extensive post-processing and being limited to single-material production. SLS fuses powder with a laser, creating high-quality parts without support structures, but sharing similar limitations, with the added drawback of a high hardware cost.

Commercial 3D-printed devices usually utilize MJF technology, which binds powder particles with a binder. This process allows the production of large quantities of pieces at a low cost and in a short time, while offering good and reliable quality. However, this technology faces limitations, such as a narrower range of compatible materials and higher equipment costs [4]. Consequently, alternatives need to be explored. Conversely, despite its infrequent commercial use in this field, MEX technology stands out for its accessibility, affordability, and versatility, allowing for the use of various thermoplastic materials, as well as multi-material settings and multi-axis manufacturing possibilities, making it suitable for the production of cost-effective, personalized medical products directly within clinics and whenever required [5]. At the same time, 3D-printed orthoses are comparable to conventional products and often provide additional advantages [2]. However, there have not yet been many long-term studies proving their effectiveness [6].

However, integrating MEX methods into orthopedic practice is still an ongoing development process, due to the need for further research to optimize the design, material selection, and manufacturing techniques for orthotic devices, as well as compliance with regulatory requirements to ensure the safety and quality of these devices [7,8]. To overcome these challenges, there is a growing need to utilize design for additive manufacturing (DfAM) principles, which focus on designing products specifically for the AM process, thereby enhancing their performance, functionality, and manufacturability [9]. The design process cannot be separated from the technology, because the intricate structures required for effective orthoses are inherently dependent on the capabilities and limitations of MEX techniques. Therefore, a deep understanding of these techniques is essential to produce functional and optimized products.

The goal of this work is to provide engineers with key design considerations for developing customizable and accessible hand orthoses using MEX technology. By offering in-depth analyses of materials, design principles, and innovative approaches, the study aims to equip engineers with a broad understanding of the factors influencing the functionality and manufacturability of orthoses, leading to more practical applications. Furthermore, it introduces a modular design concept aimed at enhancing rehabilitation outcomes and highlights opportunities for future research to optimize orthotic design.

2. Method

The development of effective hand orthoses requires a deep integration of design principles with the specific capabilities and limitations of the chosen production technology. This section outlines the methods used to gather and analyze data, aiming to provide design considerations engineers should take into account when developing hand orthoses using MEX technology. It details the process of collecting requirements through workshops with occupational therapists, and the narrative review methodology employed to assess the existing literature on MEX-printed hand orthoses. Section 3 introduces the manufacturing of hand orthoses and presents their requirements, followed by an analysis of conventional designs, to familiarize readers with design choices. Section 4 provides an overview of various approaches in hand orthotics using MEX printing, and concludes with a discussion on an innovative application of this technology—the modular design concept.

2.1. Requirement Collection

The requirements were gathered during a series of five workshops, attended by a total of six occupational therapists specialized in hand splinting. For each workshop, typically, three or four therapists from the group were present. The workshops primarily focused on 3D-printed hand orthoses. Discussions covered reviewing various design ideas, co-designing, and presenting commercial methods for producing personalized orthoses. The described requirements for hand orthosis were extracted across all discussions.

The collected requirements were categorized into main and sub-categories. This work proposes dividing the requirements into three categories: engineer-specific, medical-specific, and patient-specific, each containing several sub-categories with specific examples of concern. These examples are further supplemented with data from the literature.

Finally, several examples of commercial and commonly used customized devices are provided, to help the reader understand the application of these general requirements in specific cases, as well as to present existing solutions in designs that achieve certain therapeutic goals.

2.2. Narrative Review

A narrative review methodology was selected, utilizing the Google Scholar online database to capture various technologies related to hand orthoses produced via MEX printing. The search employed various combinations of keywords, including: ‘additive manufacturing’, ‘3D printing’, ‘fused filament fabrication’, ‘FFF’, ‘FDM’, ‘material extrusion’, ‘hand’, ‘wrist’, ‘finger’, ‘thumb’, ‘upper limb’, ‘upper extremity’, ‘orthosis’, ‘splint’, ‘device’, ‘assistive’, ‘passive’, ‘active’, ‘static’, ‘dynamic’, and ‘exoskeleton’.

After retrieving the search results, specific inclusion criteria were applied to refine the literature search. The inclusion criteria were as follows:

• The paper is accessible to the authors and is a scientific article written in English.
• The described device is a hand orthosis produced via MEX printing.
• The device is intended for medical purposes.
• The study presents the mechanical aspects of the designed hand orthosis.
• The study includes a figure illustrating the designed device.
• The study provides sufficient information on the design choices.

The search and selection process were guided by the authors comprehensive understanding and experience in hand orthosis design and MEX technology. The aim was to provide a broad overview of different design approaches in the field, by including studies representing both common and less common design choices, showcasing the variety of and rationale behind different approaches. Consequently, a diverse set of hand orthoses were selected, to demonstrate distinct design features and to reflect the range of possibilities in this area.

Numerous 3D-printed devices aimed at assisting other joints of the body in ADL were not included, due to the focus on the hand. Therefore, devices that did not involve the wrist or finger joints, or were not at least partially produced via MEX technology, were excluded. The review concentrated on design approaches in this field, hence not all repeated design choices were included.

The framework was developed to review and analyze the substantial number of studies identified in the literature search. While the review does not attempt to be exhaustive, it aims to provide a representative overview of the various design types and their applications. By categorizing devices as static or dynamic, the review highlights differences and similarities, providing insights into the rationale for selecting certain designs. Grouping the different aspects of design choices was intended to facilitate an understanding of the variety within the field and to assist future designers in recognizing established trends and design alternatives. The authors expertise in the field facilitated a qualitative synthesis of the most relevant and significant developments. The review does not cover every design choice made for each device, but instead highlights specific, key, and previously unmentioned design choices.

Recognizing the advantages of a narrative approach, which allows for a focused exploration of the most relevant works, the authors acknowledge that this methodology may have introduced some bias in the selection and interpretation of the literature. However, every effort has been made to present a balanced and comprehensive overview that accurately reflects the diversity of design approaches within hand orthoses produced via MEX printing, drawing on the authors’ expertise and deep understanding of the field.

3. Hand Orthosis Development

Hand orthoses, also known as splints, are wearable devices that are an integral part of rehabilitation and are indicated for diseases and injuries in the traumatological, orthopedic, rheumatological, and neurological fields, as well as for congenital deformities of the upper extremities. They fulfill several roles, such as protecting joints and bones after trauma, reducing pain and inflammation, correcting deformities, and supporting people in activities of daily living (ADL). They can be divided into static and dynamic [10,11,12]. Static splints are usually used to stabilize a joint [10,12,13]; however, their function extends beyond stabilization. They are also employed to protect the joint during ADL or at night [14], and to correct improper posture or deformities [11,15]. In contrast, dynamic splints are designed to mobilize one or more joints, while controlling the range of motion, providing stabilization and supporting specific movements [12,14]. These splints consist of a static base and at least one movable part [16,17]. These devices can be passively actuated using springs or other elastic elements, or actively actuated, often referred to as exoskeletons, which are powered, for example, by electrical motors.

The classification of orthoses is not limited to static and dynamic categories [10,15,16,17,18]. For instance, static progressive splints apply constant torque to a joint for gradual mobilization, which can be adjusted progressively. To ensure splinting is an effective treatment, it must be tailored to the specific condition being addressed [19].

Hand splints serve various roles in rehabilitation. In arthritis they are used for pain relief [20], while in neurological disorders like stroke or multiple sclerosis, they compensate for muscle weakness [21] and prevent contractures, to reduce the risk of muscle spasticity [15]. Splints are also used to correct joint misalignment in trauma [22] and stabilize post-surgery structures [23]. Dynamic splints promote mobility in stiff joints or after tendon injuries [21].

During the workshops, it was identified that discussions between technicians and therapists may lead to differing perspectives on splinting. The technical approach to splint fitting typically involves selecting a specific splint model customized for a particular clinical condition. However, it is important to recognize that multiple options for splinting are usually available in a therapeutic setting. A single splint with the same technical and design features can be used for various clinical conditions, while different types of splints can achieve the desired outcome for the same clinical condition. Additionally, combinations of static, dynamic, and redressing splints can greatly benefit the patient.

As the diverse functions and classifications of hand orthoses have been established, it is important to consider the methods by which these devices are created. The manufacturing process influences the effectiveness and comfort of the orthoses, directly impacting patient outcomes.

3.1. Orthosis Manufacturing

Beyond functional classification, orthoses can also be distinguished by their method of production: off-the-shelf orthoses and custom-made orthoses. Off-the-shelf orthoses are prefabricated splints available in various sizes, designed for general use. However, these industrially manufactured models do not account for the unique characteristics and specific needs of each patient’s hand. In contrast, customized orthoses are tailored to meet the individual anatomical characteristics and specific requirements of each patient.

In the past, creating customized orthoses often required using cast molds, due to the high temperatures of the materials during the process, which made direct application to the patient’s hand impossible [24]. Therefore, this process was labor-intensive and imprecise. The introduction of new thermoplastic materials, such as polypropylene, polyethylene, and Kydex, has significantly improved the customization process. These materials are low-thermoplastic materials that can be processed at temperatures between 50° and 70° Celsius, and therefore can be molded directly onto the patient’s hand at lower temperatures, resulting in a more accurate and comfortable fit [24].

Thermoplastic materials have diverse properties that impact various aspects of orthosis design, including patient comfort, meeting the functional requirements of the splint, and accommodating the therapist’s processing preferences [19]. These advancements have revolutionized orthosis manufacturing, enabling faster production times, greater precision, and improved patient outcomes. The whole process is not limited to material shaping but also involves the incorporation of various mechanical elements, such as joints, elastic components, and padding. Despite these advancements, the process remains largely dependent on manual craftsmanship, which can lead to variability in the final product’s quality. Consequently, there is a pressing need for the development of more advanced solutions to streamline and improve these processes.

The range of materials used in hand splinting extends far beyond thermoplastics. Neoprene provides soft, flexible support for dynamic splints, while aluminum adds strength and durability [17]. Silicone, along with EVA and Plastazote, is commonly used for padding in pressure-sensitive areas, enhancing comfort [25,26]. Although less common today, leather is a sturdy and breathable option [17]. Carbon fiber composites, though more expensive, offer advantages for lightweight and strong dynamic or sports orthoses [27]. Material selection depends on the patient’s needs, balancing between stability, flexibility, and comfort. Many modern splints combine materials to optimize therapeutic outcomes.

According to Jacobs and Austin [28], to ensure the optimal fit of a splint, it is crucial to make careful considerations before creating it. This process should involve answering a set of questions to understand the context in which the splint will be used.

• Which anatomical structures are impacted, and how will the splint address them?
• What therapeutic objectives should the splint achieve?
• What is the recommended duration and schedule for wearing the splint?
• When and under what conditions should the splint be adjusted?
• What key factors must be considered during the fitting process to ensure optimal function and comfort?
• What is the correct positioning of the splint to achieve the desired therapeutic outcome?
• Are there any contraindications or potential risks associated with this splint that need to be addressed?
• What monitoring and maintenance protocols are required for the splint, including who is responsible and the frequency of checks?

During the workshops, it was identified that the context of use should be further explored by understanding the habits and preferences of the individuals. By providing solutions that fit their expectations and involve their favorite activities, higher involvement in the rehabilitation process can be achieved. Building on this understanding of the context of use, the next sub-section will explore the essential requirements for designing orthoses, which have been categorized and are comprehensively explained.

3.2. Design Requirements

This sub-section explores the essential requirements for designing hand orthoses, which are crucial for ensuring their effectiveness, comfort, and usability. These requirements were collected during a series of workshops and are categorized, as presented in Figure 1. Each category addresses different aspects necessary for the successful creation of a hand orthosis. The following sub-section describes these requirements in detail, providing a comprehensive overview of the design process.

3.2.1. Engineer-Specific

When designing hand orthoses, it is important to find a balance between robustness and weight. The orthosis must withstand the wear and tear of daily use, ensuring long-term reliability and reducing the need for frequent replacements. At the same time, it should remain as light as possible, to prevent user fatigue and discomfort during prolonged use. Employing lightweight and durable materials, like advanced polymers and metals, helps to maintain the orthosis’s structural integrity over time and provide consistent support to the user. Furthermore, this also enables a more minimalistic design, which avoids interference with the ADL, allowing the user to perform tasks without unnecessary hindrance. This can be achieved by designing the orthosis to cover only essential areas or by utilizing lattice structures [29]. For optimal comfort, the maximum recommended weight for hand orthoses is 200 g [30], with commonly used materials typically having a thickness of up to 3.2 mm.

Designers must consider technological limitations, to ensure that the design is feasible within current manufacturing capabilities and utilizes practical processes. This includes considering the technologies available for creating the orthosis and ensuring that the design can be produced at a reasonable cost and time. This consideration helps in the practical implementation and scaling of the design. This practicality is also important for making the orthosis widely accessible, considering global variations in resources and manufacturing infrastructure, to ensure affordability for patients. Balancing the cost with the quality of materials, design, and technological processes is important for making the orthosis accessible to a broader group.

The orthosis should be designed for sustainability, focusing on reuse, adjustability, and recyclability [31]. Incorporating green manufacturing techniques, modular designs, and adjustable components, as well as enabling repairs or modifications without the need for full device replacement, minimizes waste and the environmental footprint [31,32]. Materials should also be selected considering recyclability, ensuring the device can be effectively managed at the end of its life.

3.2.2. Medical-Specific

From a medical perspective, the orthosis must be safe to use and support the rehabilitation goals. It should provide a non-invasive fit that does not restrict blood flow, scratch the skin, or irritate wounds. Proper pressure distribution is crucial, to avoid causing new injuries or discomfort. Achieving a non-invasive fit involves customizing the orthosis to the patient’s unique anatomy and using soft, flexible materials in areas that contact the skin. Use of blunt edges can prevent cuts. The materials used should not cause skin irritation, promote fungus or bacteria growth, and should be hypoallergenic. This is vital for preventing infections and allergic reactions, ensuring the user’s skin remains healthy. Biocompatible materials like medical-grade silicones and hypoallergenic fabrics are commonly used to meet this requirement.

Additionally, the orthosis must provide stability and support by stabilizing joints in required directions, while protecting against external forces. Proper fixation and support enable effective rehabilitation and ADL performance, without unnecessary hindrance, and provide stabilization in the required joint position. In case of dynamic orthoses, they should prevent hyperextension, contractures, and unwanted movements, while supporting the expected range of motion (RoM). This helps in rehabilitation by guiding proper joint movements and preventing further injuries. Adjustable elements in the orthosis can help tailor the RoM to the user’s current needs, as they may change over time or even within the same day.

3.2.3. Patient-Specific

Patient comfort is a key factor influencing adherence to orthosis use. The orthosis should be designed to match the patient’s anatomical features and include a soft cushioning layer to prevent pressure sores and discomfort. Ensuring comfort can involve using padded straps, ergonomic shapes, and well-selected materials. Ventilation helps maintain skin health by preventing heat build-up and sweating, thereby reducing the risk of skin irritation and infection. This can be achieved through features that allow airflow, such as perforated materials or breathable fabrics. Additionally, the orthosis should be easy to clean with water, soap, or other commonly accessible detergents to maintain hygiene, especially for devices worn over long periods or shared among multiple patients. Materials that are resistant to bacteria and fungi growth and designs that allow easy disassembly for cleaning are important considerations.

Ease of use is another important aspect. The orthosis should be user-friendly, accommodating different levels of independence and mobility. It should be easy to put on and take off with one hand, and allow for quick adjustments. One-hand donning can be achieved by utilizing Velcro bands with a loop-through fastening system or the Boa Fit system [33]. Moreover, the orthosis should be adjustable to meet the specific needs of each patient. This includes accommodating changes over time, such as swelling, recovery stages, or different activities. Features like adjustable straps, modular components, and flexible materials can enhance customizability.

During the design process, aesthetic considerations must also be taken into account. The appearance of the orthosis can impact patient morale and willingness to use the device. An aesthetically pleasing design can reduce stigma and increase motivation [34]. This can be achieved through stylish design, customizable colors, and a modern, sleek appearance. Moreover, each patient has unique comfort levels and expectations regarding a device. It is essential to consider individual preferences and lifestyle factors, as what is acceptable for one patient may not be for another. Personalized fittings and involving patients in the design process can help in achieving a positive patient-subjective opinion about the orthosis.

This overview emphasizes the most important aspects to consider when designing splints, noting that the requirements often intertwine and influence one another. Moreover, there are other considerations that, while less critical, may also be relevant. Although the focus is on hand orthoses, these points can be extended to the design of all types of orthoses. The list is universal and not tied to any specific technology, ensuring that the principles outlined can be applied across different manufacturing methods.

3.3. Diverse Designs of Hand Orthoses

This sub-section provides an overview of several existing hand orthosis designs, highlighting their purposes, design principles, and the specific needs they address (

Table 1. Examples of different commercial and customized hand orthoses with a description of their application and explanation of different aspects of the design.

Orthosis Application	Design Explanation
[35] Stabilization of the wrist after fracture such as a distal radial fracture, as well as for immobilization in the case of carpal tunnel syndrome or other signs of overuse in the wrist area.	Thermoplastic, customized rigid structure on the ventral side of a forearm and a hand. Velcro fasteners make it easier to put on and take off the splint independently. The recess around the MCP joint of the thumb and fingers 2–5 allows movement of a large part of the thumb and finger movements, but the orthosis restricts the overall mobility of the wrist. Stability is achieved by fixation over a half-length of the forearm.
[36] Stabilization of the wrist and thumb after traumatic injuries of the thumb and arthrosis of the thumb saddle joint	Pre-fabricated splint which immobilizes the thumb up to the end joint. The rigid grid structure enables better ventilation, while padding improves wearer comfort.
[37] Limiting hyperextension in the case of a swan neck deformity of the fingers.	Minimalistic design based on three support points with a small area covered by the orthosis. It allows sliding of the orthosis on the finger.
[38] Improved finger alignment in ulnar deviation from inflammatory and degenerative conditions.	Minimalist design that requires no additional fasteners thanks to wrapping around the hand. The bars between the fingers place the fingers in a physiological position.
[39] Stabilization of the movement of the wrist in flexion and extension. Support of finger extension for people with joint contractures problems.	A chain mechanism, mounted on the dorsal side of the fingers, uses rubber bands to support finger extension without producing axial force on the finger joints. The wrist joint allows for wrist flexion and extension. This type of joint can be used to limit the RoM of the hand joint. The torsion spring in this joint can be activated to support extension, and deactivated when donning the orthosis. The position of the wrist joint can be adjusted using slotted holes placed on the forearm part.
[40] Stabilization of the wrist with slight extension of the wrist. Support in finger extension after peripheral lesions of the radial nerve and associated radial nerve palsy.	A dorsal (exercise) splint that fixes the wrist in a slightly extended position to improve grasping and uses rubber or spring tension, which are placed on an outrigger to support extension of the fingers in the MCPs and support a physiological gripping function, without elastic elements gliding across the skin and creating an uncomfortable compression feeling due to occurring force.
[41] Prevents hyperextension of the 4th and 5th MCPs after ulnar nerve injury.	Liver-shaped palm design allows free tissue movement and full RoM of fingers and thumb. The palm arc shape holds the splint firmly, with leather loops and elastic bands on the palm side maintaining physiologically favorable MCP positions for the 4th and 5th fingers.
[42] Active support of thumb, middle finger, and ring finger flexion for people with central nervous system disorders.	Soft glove with integrated wire system to support hand closing. Wires go on the sides of the fingers, so they are not inhibited when grasping. The close-fitting textile design allows for a light device and natural haptic feeling. The motor units can be placed outside the hand, reducing the carried weight. Closed tips of the fingers allow the force to be transferred without sliding the glove across the finger and placing sensors. Pressure sensors are used to detect the intention of hand closing.
[33] A robotic glove for the support of hand function after strokes and hand injuries. The fingers can be mobilized in flexion and extension.	Rigid glove with actuated joints placed on the dorsal side, leaving the palmar side free for grasping. The splint is attached to the forearm and the fingertips on the hand. Boa system used to facilitate donning/doffing the device with one hand. Actuators integrated on the forearm part to shift the weight and volume from the hand. They are controlled by an sEMG signal, with electrodes placed on the forearm. Soft elements used on the fingers to increase comfort. Wrist is fixed, which simplifies device control.

). By examining these examples, readers will gain a first insight into the diverse approaches and solutions in the field of hand orthotic devices. Each example is accompanied by a figure with a reference to manufacturer product information, a detailed description of its application, and the underlying design considerations, illustrating the functionality and thought process behind these medical devices.

Implementing all the requirements for hand orthoses can be challenging and is often not possible. A common simplification involves restricting the movement of the wrist and metacarpophalangeal (MCP) joints to a single plane. Splinting is particularly demanding, as it must cater to a wide variety of needs and conditions, often requiring careful compromises. Especially for patients with severe impairments, achieving the perfect balance between functionality, comfort, and aesthetic appeal can be difficult.

In recent years, advancements in 3D printing technology have opened new possibilities for the design and customization of hand orthoses. This approach offers the potential to create more personalized and adaptable solutions, addressing the unique needs of each patient with greater precision.

4. Current Approaches in 3D-Printed Hand Orthotics

The initial advantage of 3D printing for hand orthosis lies in its ability to digitize and automate the production process, resulting in a customized product. The standard workflow for such devices includes 3D scanning of the limb, then the user designs a device in semi-automated manner, optionally using finite element analysis. Finally, the model is 3D-printed and post-processed, if needed. However, this process still demands expert labor, which constitutes the majority of the total cost [43].

Another significant advantage of 3D printing is the ability to create intricate designs and geometries that are unattainable with other technologies, while still maintaining simplicity and cost-effectiveness in production. However, AM technologies are not without their design limitations, which is why DfAM methods should be employed to achieve the optimal results [44]. Failing to consider the manufacturing process and lacking knowledge of DfAM early in the design phase can severely limit the design possibilities [45]. This section focuses on the current trends and approaches in MEX-printed hand orthotics, examining their features and limitations.

4.1. Static Orthoses

Many MEX-printed splints take advantage of the ability to print complex shapes, integrating cut-outs and lattice structures for ventilation, weight reduction, and increased aesthetic appeal [46,47]. Typically, these orthoses are printed vertically, for time efficiency and a better surface quality compared to horizontal printing, as well as being self-supporting during the process [48]. However, this comes at the cost of robustness, due to the weaker build direction [48]. The MEX process is relatively slow, and the surface exhibits visible layer lines that may require post-processing to meet standards for skin-contact applications [49]. The time efficiency of the process is especially compromised in simple and small orthoses, such as the one in the third row of Table 1, used for finger immobilization [50]. However, for more complex orthoses, even if the manufacturing process takes several hours [46], it can still offer advantages [3,49].

One approach to overcoming some of MEX technology’s limitations, first proposed by Choi et al. [50] and further investigated by Popescu et al. [51], involves printing the orthosis flat using low-temperature thermoplastics, similarly to materials used in commercial orthoses, and then thermoforming it on the user’s hand (Figure 2A). Barros et al. [52] further developed this idea by proposing a method to produce an origami-shaped orthosis that can be printed flat in its unfolded form and then folded, gaining robustness.

Three-dimensional-printed orthoses are not only limited to rigid structures, commonly utilizing materials like polylactic acid (PLA) or acrylonitrile butadiene styrene (ABS) [2], but can also utilize soft and flexible materials, such as thermoplastic polyurethane (TPU) [2], to omit fasteners and increase the comfort of the device, without the need for very precise modeling and providing sufficient support to the limb [53]. For higher comfort, Górski et al. [54] introduced dual-material orthoses, consisting of a rigid and robust outer shell, with soft padding. However, this approach requires a dual extrusion printer and good bonding between materials, as well as longer printing times, due to material switching. Zhou et al. [55] used the multi-material concept to produce a more comfortable design, easing the donning and doffing of the orthosis and maintaining cleanliness by eliminating Velcro straps and replacing them with a flexible section on the side of the splint, similarly to in the comparable solution introduced by Paterson et al. [56]. However, this orthosis was produced using two different methods of 3D printing, and the parts were glued together, hindering its potential implementation. To overcome some of the challenges of multi-material printing, Popescu et al. [57] proposed using a flexible material with variable mechanical properties to deliver an orthosis that offers enough immobilization, while being comfortable; with the whole process using a simple printer with one nozzle, thereby saving time and simplifying the process.

Research has also focused on adding versatile functionalities to 3D-printed orthoses. Lee et al. [58] proposed an orthosis with changeable elements, fixed by a magnet, to support different ADL. Jones et al. [59] integrated sixteen multi-axial soft load-sensing nodes into a thumb splint to measure the forces between the hand and the splint in ADL. To measure these forces, Tan et al. [60] developed a 3D-printed soft sensor with a fish-scale structure, which they placed under the orthosis. Two further devices, one proposed by Barmouz et al. [61] and another by Cheng et al. [62], used shape memory polymers and a spiral-form design for easily donable orthoses, without the need for additional fasteners (Figure 2B). Shape memory properties are used in the first orthosis to slowly extend the hand, while allowing finger flexion, thanks to a three-support-point structure around the fingers, and in the second design to self-tighten the orthosis around the hand.

4.2. Dynamic Orthoses

Dynamic orthoses employ various actuation types and joint mechanisms to facilitate hand movement [63]. Their complex mechanical structure makes them difficult to customize and don [64]. Nonetheless, several approaches have utilized 3D printing for dynamic orthoses. Merchant et al. [65] developed a device for the entire arm, excluding fingers, covering two degrees of freedom of the wrist joint. Motors are directly placed on the rotary joints of the orthosis, aligned with biological joints. Such an approach has several drawbacks, such as the bulkiness and hard alignment of the device. On the other hand, it applies pure torque to move a joint, making it effective and comfortable. However, in such solutions, the point of force application must be carefully chosen to not cause discomfort.

A commonly used joint mechanism is the four-bar linkage [3,66,67,68], which provides joint stability and allows for complex trajectories, while utilizing a single actuator, keeping the device lightweight. This mechanism is often employed for thumb opposition in grasp performance (Figure 2C). Cui et al. [69] introduced 8- and 10-bar linkage mechanisms to guide the three phalanges of each finger. Esposito et al. [70] achieved a similar movement with their bar linkage mechanism. Dragusanu et al. [71] designed an exoskeleton using two four-bar linkage mechanisms on two fingers, coupled to a single actuator via a gear-based differential mechanism. This 3D-printed device allows each finger to move independently when encountering obstacles, reducing the weight, while effectively supporting object grasping.

Chen et al. [72] introduced a modular design with an exchangeable thumb mechanism. This design allows the four-bar linkage mechanism supporting opposition pinch to be replaced with a rotational bar supporting spherical grasp, by inserting the component into a slot. Both joints are actuated by a gear-adjustable spring. Risangtuni et al. [73] proposed another modular idea: a four-bar linkage mechanism that can be coupled with a 3D-printed Pneu-Net (pneumatic actuator) (Figure 2D) to support wrist extension. This actuator uses compressed air for bi-directional bending movement and is applicable in hand orthotics. This combination achieves a complementary effect, as the soft material actuator lacks the necessary stability, which is effectively provided by the rigid four-bar mechanism. Ang and Yeow [74,75] developed finger exoskeletons almost entirely from these actuators, creating lightweight, soft devices that are safe and comfortable.

Despite their widespread use, four-bar mechanisms and their variations have drawbacks: they are bulky, complex to design and fit, and use rigid materials that decrease comfort. Moreover, they have one degree of freedom, which, while allowing for single-point actuation, often hinders natural joint movement. Although, this feature can be desirable in some cases.

Another type of joint in dynamic orthoses is based on elastic elements, such as Bowden cables or metal straps, also simply referred to as springs. These elements are not 3D-printed but are embedded into 3D-printed structures that guide their movement, such as a three-layered sliding spring mechanism (Figure 2E), which contains metal (or other elastic, yet robust) straps connected to 3D-printed segments [76,77,78]. Some springs are fixed to the segments, while others slide, resulting in translating linear movement into joint bending and enabling rotational motions of the finger joints. This compact design transfers a high level of force, without harming the user due to misalignment and without causing discomfort from finger joint compression. It supports bidirectional movement and is only placed on the dorsal side of the hand. Li et al. [79] introduced a similar design, but reduced the number of springs to one by implementing a multi-segment mechanism attached to a fabric glove with Velcro, mechanically limiting the maximal extension and enhancing the afety.

Bowden cables can also create actuation in both flexion and extension, while restricting other directional movements less than springs. However, they create undesired axial force on the joint. Araujo et al. [80] developed a hand exoskeleton, with 3D-printed segments on a textile and cables sliding through them to perform finger movements. The Bowden cable connects to the fingertips, resulting in a compact, lightweight design, though its robustness and safety in a medical context are questionable. Haarman et al. [81] combined Bowden cables and metal straps to create a joint that does not transfer axial force to the finger joints. This is achieved with a concertina-like mechanism fixing flexible segments on the strap, creating a rotation point for the joint, while the cable controls its position by adjusting the segment proximity. The joint mechanically limits the maximal finger extension by locking the segments, enhancing safety, and it is easily mounted on the hand using magnets. However, it can only be mounted on the hand’s side, activating either one or a group of fingers together. Bagneschi et al. [82] integrated Bowden cables into a glove, connecting them to elastic rings easily placed on the fingers. Similarly, in [83], the cables also ran through the dorsal and palmar sides to support both finger flexion and extension. However, these designs cover the palmar side, making it difficult to grip, and they compress the finger joints. Moreover, glove-based designs present donning challenges.

Another group of joints use elastic cords or strings connected to springs or motors. These strings support longitudinal movement, while allowing freedom in other directions, which is useful against misalignment. The next four orthoses use a similar concept to the one in the sixth row of Table 1, but with some variations.

Dudley et al. [84] used the same design concept, but they used 3D printing only for customization of static parts. Michalec and Faller [85] added sensors to such an orthosis, incorporating a 3D-printed linear encoder to read finger positions. Yang et al. [86] designed an orthosis where strings limit the maximal finger flexion, while allowing full extension, used to stretch cramped spastic fingers. The position can be adjusted over time by rotating a pulley on the forearm’s dorsal part. Ates et al. [87] iteratively improved dynamic orthoses by implementing purchased sensors and a wrist support. Initially, they identified inefficiencies in force transmission for high finger flexion and conceptualized devices with a bendable outrigger or direct torque transmission via a bar mechanism. However, for a higher sensor accuracy, they compromised these designs into a stiff rotational outrigger with a short string connected to thimbles. The thumb joint has an additional unactuated degree of freedom, allowing thumb articulations. Additionally, they implemented a four-bar linkage mechanism for wrist movement, supporting flexion and extension, while blocking abduction and adduction.

Park et al. [88,89] proposed designs using a twisting cable connected to a bendable beam to move all fingers together. In the first design, it is placed on the palmar side; in the second, on the dorsal side. These designs allow bi-directional actuation with a single actuator. In the second orthosis, by adjusting the cable path, the movement is sequential, starting with the MCP joint, and followed by the proximal interphalangeal (PIP) joint.

There have also been attempts to place actuators directly on the joint. Bos et al. [90,91] connected hydraulic cylinders over the MCP and PIP joints, using a compliant structure on the finger sides to absorb potential shear forces. However, this approach increases the overall bulk of the device and shifts its weight to a more external point on the hand.

The use of MEX printing in dynamic hand orthotics has been limited. The literature features various designs using other AM technologies, including shape memory polymers [5] and complex four-bar linkage mechanisms for finger movement [92]. However, to the best of our knowledge, MEX printing has been not yet utilized in such designs.

4.3. Introduction of Modular Design

Dynamic orthoses, although complex and challenging to customize, have benefited from 3D printing technologies, enabling more precise and adaptable designs. Innovations like modular components and elastic element-based joints have improved the versatility of usage and customization. Despite advancements in this field, they are still far from being implemented in commercial usage. Future research should focus on developing lighter, more versatile and adaptable devices that can better serve people in rehabilitation and ADL. This sub-section endeavors to bridge this gap by introducing a concept for customizable dynamic orthoses capable of addressing a wide array of conditions and facilitating enhanced hand rehabilitation.

The concept was developed by thoroughly understanding the requirements and capabilities of the MEX method, coupled with insights gained from workshops with experts. While AM is primarily employed to tailor the geometry of the orthosis to the patient’s hand, this technology also holds significant potential for adapting the functional aspects to specific health conditions. Therefore, the proposed orthosis emphasizes the utilization of this functional adaptation concept.

The envisioned orthosis comprises multiple modules that can be interconnected to craft tailored devices for specific conditions—a modular design. This approach capitalizes on universal design principles and identifies commonalities among various orthotic types. The design consists of a base and two module types: an articulating module, and a sensing module (Figure 3).

The base serves as the orthosis’s structure, enveloping the body part to provide support for the other components. It may consist of one or more parts affixed to the body, featuring both a rigid layer for load transmission and stabilization, and a soft layer for enhanced comfort, which can also function independently under lower loads.

Printing the base flat allows printing with continuous fibers and enables subsequent shaping of the structure to match the user’s hand after hot water bathing; or if using soft materials exclusively, this step is unnecessary. This process can be repeated multiple times when needed. Moreover, flat printing enables printing with a partially filled infill, thereby incorporating uniformly distributed perforations, which not only promote ventilation but also reduce the print time. Moreover, it simplifies the integration of additional components such as straps, joints, electronics, or actuators.

The first module category encompasses articulating joints, which mimic, restrict, or strengthen the movement of specific body joints as necessary. These joints can be subdivided into three types: those limiting RoM, those providing passive support, and those exerting active force (exoskeletons). Ideally, this is achieved with compliant joints that limit the axial force acting on the joint.

The second module category focuses on integrating sensors, enabling the articulating joint to monitor the body joint position, track progress, provide user and therapist feedback, or enhance engagement through gamification. Notably, technologies like those developed by Michalec and Faller [85] offer 3D-printed sensors that streamline electronics assembly for medical professionals.

Tailoring the configuration of modules to individual needs and directly 3D printing them in clinics enhances the sustainability and accessibility, reducing reliance on lengthy transportation. Such a device can mimic various functions of existing devices, from basic splinting using only a rigid base, to comfortable exoskeletons integrating all module categories. This approach not only provides patients with cost-effective solutions tailored to their needs, but also streamlines the work of medical professionals, reducing manual labor and development time through an automated workflow.

By focusing on both geometrical and functional adaptation, modular design allows for the quick reconfiguration of the orthosis over time. This flexibility enables adjustments when less support is needed or when corrections are required for a better fit, and the orthosis can be tailored by users for specific activities throughout the day. Often, a particular condition necessitates various rehabilitation tasks, all of which can be addressed with a single orthosis adapted for each session and enhanced by the ability to collect data. Such a design also reduces the need for a large inventory to suit different patients, and could become a treatment standard supported by a single best-practice guide. Moreover, this design can be built upon over time as new solutions become accessible. Finally, it can be personalized to meet aesthetic needs, an important factor in preventing feelings of stigmatization. Utilization of MEX printing further enhances these possibilities.

The primary challenge lies in developing compact and versatile articulating joints that can be easily adapted to support different joint movements. In addition, small and lightweight connectors for joining modules together with freedom of placement are seen as a big challenge to overcome. MEX technology can still targeted these challenge by utilization of compliant structures, which allow for flexibility without complex joints, reducing the size and weight. Moreover, utilization of reinforced materials could provide the necessary strength, while multi-material printing could enhance versatility by offering varied mechanical properties within a single joint.

While other printing technologies might seem appealing, due to their higher precision and resolution, combining multiple manufacturing methods would increase process complexity and require new hardware, raising costs. Therefore, sticking to a single manufacturing technology, when possible, is more practical.

5. Conclusions

This work aimed to enhance the design of MEX-printed orthoses by identifying and analyzing the design factors and requirements involved in developing different types of orthoses. It began by offering a glance at requirements and standard orthosis design, and then analyzed the state-of-the-art in MEX-printed orthoses. This approach underscored the significant evolution from traditional orthotic solutions to advanced, customized designs made possible by MEX technology.

The research highlighted that many current designs are suboptimal, largely due to a lack of viable alternatives. This underscores the critical need for clear, accessible design considerations to improve design quality from the outset. By carefully evaluating these considerations, designers can develop orthoses that are not only functionally effective, but also manufacturable within the constraints of MEX technology.

It is essential to recognize that designing orthoses extends beyond meeting medical and functional requirements. Designers must also consider the limitations inherent in the manufacturing process. MEX technology, while offering significant advantages, imposes specific constraints that must be carefully navigated to achieve the desired outcomes.

One limitation of this review is its exclusive focus on MEX technology. While MEX has considerable promise, other additive manufacturing technologies present a broader range of design possibilities. While some of these alternative designs could potentially be adapted for MEX, research and experimentation are required to determine their feasibility.

Hand orthotics pose unique challenges when using MEX, such us achieving the necessary strength and a good surface quality, as well as printing small, intricate components or very elastic parts. Despite these challenges, the potential of material extrusion methods to influence orthopedic manufacturing is undeniable. By enabling the production of customized, cost-effective solutions, MEX technology can drive continued innovation and adoption in the field. However, realizing this potential will require the implementation of specific technological solutions to address current limitations.

The proposed modular design represents a promising approach that leverages the strengths of 3D printing. Modular designs could offer flexible, adaptable solutions that better meet individual patient needs, enhancing the overall effectiveness of hand orthotics, all achieved in the framework of a single solution. However, this solution remains conceptual, thus significant work remains to validate and refine these concepts. The design phase is merely the first step; the next crucial phase involves development of prototypes and evaluation of their safety and effectiveness.

By aligning with DfAM principles and collaborating closely with medical and engineering experts, it is possible to unlock the potential of MEX in creating personalized, high-quality hand orthotics that meet both medical standards and practical usability, thereby enhancing patient outcomes.