Comparative Efficacy of 3D-Printed Insoles in Managing Common Foot Conditions: A Review

Introduction

Medical orthotic insoles are crucial assistive devices inserted into footwear to support the foot’s structure and enhance stability; thus, they are widely utilized for improving gait and functional movement [1,2]. These insoles contribute to maintaining biomechanical balance by supporting the arch, ball, or heel of the foot, helping to distribute body weight evenly across the foot, thereby reducing localized pressure [3]. Moreover, medical orthotic insoles are highly effective in alleviating pressure, absorbing shocks, and minimizing damage, particularly for patients with difficulty bearing weight due to foot injuries or ulcers [4]. These functions relieve foot pain and fatigue, improve overall posture and gait performance, and enhance the patient’s physical capabilities and quality of life [5]. Currently, insoles are employed for therapeutic purposes across various patient populations. For instance, in diabetic foot ulcer patients, insoles are used to reduce pressure on the ulcerated areas, promoting wound healing and decreasing the likelihood of recurrence [6]. Furthermore, insoles play a critical role in biomechanical correction, such as for flat feet, plantar fasciitis, and bunions, where structural abnormalities or chronic pain are present, in addition to alleviating pain and restoring foot balance [7]. Thus, orthotic insoles have evolved in managing foot health and preventing gait-related disorders from simple, supportive devices to essential therapeutic tools [8].

Three-dimensional (3D) printing technology is an advanced manufacturing technique that creates 3D structures layer by layer, and it has garnered significant attention across various industries, including healthcare, where it is driving innovative changes [9]. In particular, 3D printing is regarded as an ideal technology for producing precise, customized orthotics tailored to the anatomical structure or specific pathological characteristics of an individual [10,11]. Traditional insole manufacturing methods rely on manual labor, assembling multiple components and finely adjusting shapes and angles to meet patient-specific needs [12,13]. Subsequently, this process requires skilled technicians, is time-consuming, and presents challenges in post-production modifications [14]. Conversely, 3D printing technology offers numerous advantages that overcome these limitations. Firstly, it allows for the easy creation of complex and intricate geometric structures, enabling designs that would be difficult to achieve through traditional methods [15]. Furthermore, the rapid production speed and the ability to design, validate, and modify the product based on digital data make it particularly suitable for quality control and the production of patient-specific orthotics [15]. These advantages have positioned 3D printing as a more efficient and precise method for manufacturing medical orthotic insoles.

The technological advancement for producing medical orthotic insoles directly impacts patient treatment outcomes and quality of life, with 3D-printed insoles increasingly recognized for their potential as therapeutic tools. Therefore, this review aims to compare the clinical effectiveness and practicality of 3D-printed insoles with traditionally manufactured insoles and discuss their potential applications and limitations in future healthcare settings.

Search Strategy

We searched the MEDLINE, Embase, Scopus, and the Cochrane Library databases for relevant studies published until May 13, 2024. The keywords used in the search were: “3D printing”, “3D printed”, “3D print*”, “3D-printing”, “3D-printed”, “3D-print*”, and “insole”.

The inclusion criteria were as follows: (1) studies evaluating the effectiveness of 3D-printed insoles, (2) studies comparing 3D-printed insoles with insoles manufactured using non-3D printing technologies or barefoot conditions, and (3) reports written in English. There were no restrictions on the study design or target diseases.

The exclusion criteria were as follows: (1) healthy individuals, (2) technical development reports, and (3) reviews, case reports, letters, conference presentations, or other undefined formats.

Literature Search Results

A total of 764 studies were initially considered, and 68 duplicate studies were excluded. After reviewing titles and abstracts, 685 studies were further excluded. The remaining 11 articles were assessed, of which 1 did not have a control group, 2 compared 3D printing, 1 involved 3D printing only for some components of the insole, and 2 were technical development reports. One further study was excluded as it focused on healthy individuals. Ultimately, 4 studies were included in this review [16–19].

Application of 3D-printed Insoles in Diabetes

In 2017, Telfer et al [16] conducted a study investigating the effectiveness of 3D-printed insoles in 18 patients with type 2 diabetes and the risk of foot ulcers. The insoles were custom-made using foam boxes to cast the feet, and a series of measurements related to the internal forefoot anatomy were obtained using ultrasound imaging (MyLab 70, Esaote, Genoa, Italy). Three types of custom insoles were produced for each patient: (1) standard shape-based, directly milled insoles; (2) virtually optimized, directly milled insoles; and (3) virtually optimized, 3D-printed insoles. The conventional method for producing insoles typically involved using foam boxes to cast the feet and a standard prescription form. The materials used included ethylene vinyl acetate (EVA) with a Shore A hardness of 40 and a top cover. The 2 virtually optimized insoles were designed using 3D scanning of the foam boxes through computer-aided design software (Rhino V5, McNeel and Associates, WA). The virtually optimized direct-milled insoles used EVA, while the 3D-printed insoles were produced using polylactic acid (PLA) with an Airwolf HD2x printer (Airwolf 3D, Costa Mesa, CA). All participants wore 3 types of insoles and standard shoes (Marsden, Peacocks Medical Group Ltd., UK) in a randomly assigned order and walked. The performance of the insoles in the pressure distribution on the forefoot at risk of ulceration was evaluated, and in contrast to standard insoles, virtually optimized milled insoles exhibited a mean maximum pressure that was 41.3 kPa lower. In comparison, 3D-printed insoles showed a mean maximum pressure of 40.5 kPa lower. Hence, no significant difference was observed in the forefoot loading performance between the virtually optimized milled and 3D-printed insoles.

Application of 3D-printed Insoles in Plantar Fasciitis

In 2019, Xu et al [17] investigated the impact of prefabricated and 3D-printed insoles on the pressure distribution and comfort of 60 patients with bilateral plantar fasciitis. This study included traditional premade sponge corrective shoe insoles and customized 3D-printed EVA insoles. All patients wore flat-soled shoes purchased 1 month before the experiment. Customized insoles were devised via a computer-aided design and fabricated using computer-aided manufacturing. The model of the patient’s foot was obtained using a plantar pressure collection device. The insole design was based on the data collected from the patient’s gait using a Footscan 3D gait analysis system (RSscan International, Olen, Belgium). The insoles were fabricated using a Bodyarch X1 printer. Patients were randomly assigned to receive 1 of the 2 insole types, and assessments were conducted during weeks 0 and 8 using the 10-meter walk test. Plantar pressure was analyzed using the Footscan® system, and comfort was measured using a visual analog scale (VAS; 0=no discomfort, 10=extremely uncomfortable). When using 3D-printed insoles, a significant increase in the maximum pressure in the hallux and first metatarsal areas was observed when compared with prefabricated insoles. However, in the mid-heel and lateral foot areas, the maximum pressure was significantly lower when 3D-printed insoles were used than the prefabricated soles. Regarding comfort evaluation, patients using 3D-printed insoles experienced an average decrease of 4.22 points in the VAS score. In contrast, patients using prefabricated insoles experienced an average reduction of 3.47 points, indicating that 3D-printed insoles provided greater comfort to patients with plantar fasciitis compared to prefabricated soles.

Application of 3D-printed Insoles in Flat Feet

In 2019, Xu et al [18] compared the effects of 3D-printed and prefabricated insoles on plantar pressure and comfort in 80 patients with flexible flat feet. The patients were randomly assigned to either a group wearing standardized shoes and customized EVA 3D-printed insoles or a group wearing standardized shoes and prefabricated insoles. To create the customized EVA 3D-printed insoles, individual foot models were obtained using a plantar pressure plate–pressure collection device and 3D-printed insoles were produced using the Bodyarch X1® printer. The patients underwent evaluations for pressure on their soles and comfort before receiving the insoles and 8 weeks later. A 10-meter walk test was conducted using the Footscan® 7 gait second-generation system (2096×472×18 mm; RSscan International, Olen, Belgium) to measure foot pressure and comfort using the VAS. In both the 0- and 8-week assessments, patients wearing 3D-printed insoles had significantly lower peak pressures on the first to fourth metatarsals than the pressure experienced by those wearing prefabricated insoles. However, the peak pressure in the mid-foot region was significantly higher. Regarding comfort assessment, patients using 3D-printed insoles experienced an average decrease of 5.49 points in the VAS score compared with their pre-insole scores. In contrast, patients using prefabricated insoles had an average reduction of 2.2 points. In the comfort category, the 3D-printed insoles were significantly superior to prefabricated insoles. The aforementioned results indicate that 3D-printed insoles can help distribute pressure from the metatarsals to the mid-foot region, reduce the risk of injury in individuals with symptomatic flat feet, and provide greater overall comfort than prefabricated insoles.

Furthermore, Cheng et al [19] evaluated the performance of 3D-printed insoles in 10 patients with flexible flat feet in 2021. In this study, the foot shapes of the participants were captured using a standard foam impression box while sitting to customize the insoles. Then, the insoles were designed using the computer-aided design software isoleCAD (Nmotion Orthotic Lab, Knoxville, TN, USA). Three types of 3D-printed insoles were produced: (1) reinforced and undercut arch supports (R+U+), (2) reinforced without undercut arch supports (R+U−), and (3) without reinforced and undercut arch supports (R-U+). The insoles were printed using a fusion deposition modeling 3D printer (iSun3D Flx2, eSUN Industrial Co. Ltd., Shenzhen, China) with thermoplastic polyurethane (TPU; eTPU-95A, Shenzhen Esun Industrial Co. Ltd., Shenzhen, China). All 3D-printed insoles had an 18-mm heel cup, and an additional 3 mm was added to the R+U+ condition. The forefoot of all insoles was printed with a thickness of 0.4 mm, and the infill rate for TPU was set at 30%. Under the R+ condition, the arch area infill rate increased by 50%. The pressure exerted on the soles of the feet during walking was evaluated for the 3 insole categories and those not wearing any insoles. All participants underwent experiments under 4 conditions in a randomly assigned sequence, with a 5-minute rest period between each condition. All 3 types of 3D-printed insoles reduced the maximum pressure on the medial hindfoot compared to no insoles while increasing the maximum pressure in the medial mid-foot region. Furthermore, in the R+U− condition, the maximum pressure in the medial mid-foot and medial hindfoot were significantly higher than in the R+U+ and R-U+ conditions. On the lateral side, participants in the R+U+ group had a significantly lower maximum pressure at the forefoot and a significantly higher maximum pressure at the mid-foot than those in the R-U+ and R+U− groups. Thus, wearing 3D-printed insoles was more effective at distributing pressure than walking without insoles. Furthermore, these results provide reference information for arch reinforcement and undercutting that can be used to produce 3D-printed insoles for flexible flat feet.

Materials Used in the Production of 3D-Printed Insoles

The materials used for the 3D-printed insoles were EVA, PLA, and TPU. EVA has excellent flexibility and durability, even at low temperatures. Moreover, EVA is lightweight and resistant to chemicals and contaminants, making it a commonly used material for producing athletic shoes and bags [20,21]. PLA is another lightweight material, with low melting temperature and minimal warping, making it a popular choice for 3D printing. However, PLA loses most of its rigidity and strength at high temperatures, and it is less durable than plastics [22]. TPU can maintain flexibility even at high temperatures, has excellent elasticity, and is easy to process. In addition, TPU exhibits high wear resistance and durability, making it widely used in producing shoes, sports equipment, and medical devices [23,24]. However, TPU undergoes excessive shrinkage during printing, which can affect the print quality, making the process challenging [25]. Traditional insoles are composed of polyurethanes, which exhibit excellent shock absorption properties, low density, and outstanding mechanical characteristics. However, returning them to their initial state can be challenging if excessive loads lead to deformation. Furthermore, polyurethane is highly influenced by temperature and humidity [26].

Thermoplastics, such as TPU and PLA, are commonly utilized in 3D printing for insole fabrication, and extensive research is underway to combine these materials to achieve optimal performance and to adjust key properties such as flexibility, durability, and comfort [27,28]. Additionally, since insoles are subject to continuous loading and deformation, the ability to absorb elastic energy is critical. Consequently, ongoing research focuses on developing novel materials and advanced printing technologies to enhance performance and durability under these stress-loading conditions [29,30].

The Potential of 3D-Printed Insoles

The 4 articles [16–19] included in this review demonstrated the effectiveness of using 3D-printed insoles in the management of flexible flat feet, diabetic feet at risk for foot ulcers, and plantar fasciitis. Moreover, 3D-printed insoles can provide plantar pressure distribution and comfort similar to or better than conventional treatment methods. Several factors in the 3D-printed insole production process are believed to contribute to the improved results. (1) Advances in shape acquisition technology have improved accuracy, enabling precise digitization of individual patients’ physical characteristics [31] – this method can provide more accurate results than direct manual measurements of body dimensions. (2) Traditional insoles typically employ a technique in which the patient’s foot is cast in a plaster model in the supine or prone position to acquire the shape [14]. In contrast, to produce 3D-printed insoles, an impression foam box was used to obtain the shape of the plantar surface under a weight-bearing load [16,19]. This approach significantly reduces the time required to acquire the shape of the foot and offers a convenient and stable method for replicating a patient’s foot shape [19]. Furthermore, the foam box scanning method provides excellent reliability for measuring the width and length of the foot, allowing precise customization [32]. (3) Various designs can be tested before 3D printing through computer-aided design [33]. Additionally, this process enables accurate customization of the insole height, length, width, relief for sensitive areas, and reinforcement in areas requiring support to match an individual patient’s condition. These factors contribute to the effectiveness of 3D-printed insoles, making them promising options for individuals with various foot conditions.

Future Directions

The application of 3D printing technology in the production of personalized medical devices has the potential to transform the paradigm of medical orthotic insole manufacturing. This shift could significantly enhance the feasibility of patient-centered treatment, thereby improving the quality and efficiency of healthcare services. However, the practical applicability and limitations of 3D printing technology in medical settings should be carefully considered.

Ultimately, 3D printing could create custom insoles that precisely reflect the unique characteristics of individual patients. This capability may allow for fulfilling each patient’s biomechanical needs, potentially maximizing treatment outcomes and addressing issues that may not be easily resolved with standardized insoles. Furthermore, compared to traditional manufacturing methods, 3D printing could offer faster production speeds, and the design validation and modification processes could be conducted in a digital environment, potentially reducing unnecessary tasks. While the initial investment in 3D printers, scanners, and trained personnel may be high, it is possible that this approach could lead to cost savings and more efficient production in the long term. Moreover, data-driven treatments could be supported by digitally collecting patient foot data and incorporating the data into 3D modeling. This might create a feedback loop between data and design, fostering a continuous improvement environment and increasing the potential for better outcomes as the data accumulate.

However, in practice, technological barriers may pose challenges. For 3D printing technology to be widely adopted in medical settings, further improvements in the stability and precision of hardware and software may be necessary. In particular, developing materials capable of meeting the high mechanical strength, elasticity, and durability requirements for insole production remains a critical challenge. Additionally, the high costs associated with initial equipment acquisition and software development may make it difficult for smaller medical institutions to access the technology. Moreover, to effectively utilize medical 3D printing, specialized professionals with technical expertise and clinical experience may be required. However, the current pool of experts in this field is limited. Therefore, the time and cost associated with training and education for technology adoption and operation should be carefully considered. Furthermore, clinical research on the long-term therapeutic effects and durability of 3D-printed insoles remains limited; meanwhile, additional studies are required to validate the efficacy of 3D printing technology fully. Additionally, the need for clear international regulations or standards regarding the quality and safety of 3D-printed insoles presents challenges in ensuring product reliability and clinical consistency. Thus, collaboration and research in areas such as materials science and regulatory frameworks are essential.

Conclusions

This study investigated the application of 3D printing technology in medical orthotic insoles. The results suggest that 3D-printed insoles have the potential to provide better plantar pressure distribution and comfort for patients with conditions such as flexible flat feet, diabetic foot, and plantar fasciitis. Further, 3D printing technology holds promise for insole production and appears to set a new standard for personalized medical devices, which could lead to more precise, patient-centered treatments and improved healthcare efficiency. However, efforts must be made to overcome technical, economic, and regulatory challenges. Ultimately, while 3D-printed insoles have the potential to transform foot health management and treatment, realizing this innovation will require the harmonious development of 3D printing technology and related regulatory frameworks.