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Article

The Design and Application of an Advanced System for the Diagnosis and Treatment of Flatfoot Based on Infrared Thermography and a Smart-Memory-Alloy-Reinforced Insole

by
Ali F. Abdulkareem
1,
Auns Q. Al-Neami
1,
Tariq J. Mohammed
2 and
Hayder R. Al-Omairi
3,4,*
1
Biomedical Engineering Department, College of Engineering, Al-Nahrain University, Baghdad 64040, Iraq
2
Alkarkh General Hospital, Alkarkh Health Directorate, Iraqi MOH, Baghdad 10011, Iraq
3
Department of Biomedical Engineering, University of Technology—Iraq, Baghdad 10066, Iraq
4
Applied Neurocognitive Psychology Lab, Carl von Ossietzky Universität Oldenburg, 26129 Oldenburg, Germany
*
Author to whom correspondence should be addressed.
Prosthesis 2024, 6(6), 1491-1509; https://doi.org/10.3390/prosthesis6060108
Submission received: 30 August 2024 / Revised: 24 November 2024 / Accepted: 26 November 2024 / Published: 9 December 2024
(This article belongs to the Special Issue Recent Advances in Foot Prosthesis and Orthosis)
Browse Figures
Versions Notes

Abstract

:
Background: Flatfoot deformity is a common condition in children and teenagers that may increase the risk of knee, hip, and back pain. Most of the insoles suggested to treat flatfoot symptoms are not designed to adapt to foot temperature during walking, and they are either too soft to provide support or hard enough to be uncomfortable. Purpose: This study aims to develop an advanced solution to diagnose and treat flexible flatfoot (FFT) using infrared thermography measurements and a hybrid insole reinforced by nitinol (NiTiCu) smart-memory-alloy wires (SMAWs), this super-elastic alloy can return back to its pre-deformed shape when heated, which helps to reduce the local high-temperature points caused by the uneven pressure of FFT. This approach achieves a more uniform thermal distribution across the foot, which makes the hybrid insole more comfortable. Methods: The study involved 16 subjects, divided into two groups of eight flat-footed and eight normal. The procedure includes two parts, namely, designing a prototype insole with SMAW properties based on thermography measurement by using SolidWorks, and evaluating this design using Ansys. Second, a hybrid insole reinforced with SMAWs is customized for flatfoot subjects. The thermography measurement differences between the medial and lateral sides of the metatarsophalangeal line are compared for the normal and flatfoot groups before and after wearing the suggested design. Results: The results show that our approach safely diagnosed FFT and significantly improved the thermal distribution in FFT subjects by more than 80% after wearing the suggested design. A paired t-test reported significant (p-value > 0.001) thermal decreases in the high-temperature points after using the SMAW insole, which was closely approximated to the normal subjects. Conclusions: the SMAW-reinforced insole is comfortable and suitable for treating FFT deformity, and infrared thermography is an effective tool to evaluate FFT deformity.

1. Introduction

Flat feet, or pes planus, represent a prevalent foot deformity characterized by a depressed arch, thereby making the sole partially or completely in contact with the ground and causing the internal rotation of the tibia and femur, as well as an anterior pelvic tilt [1,2,3]. Flatfoot is a familiar bone deformity that can be diagnosed in childhood, as young as six years old [4]. This deformity is associated with global health concerns, such as lower back pain [5,6]. Also, it affects the kinematics and kinetics of the lower extremities, ground reaction forces, muscle movement, and overall gait, which have been related to various clinical measures, including symptoms (with a focus on pain in particular), radiography, and quality of life. Typically, flat feet distort the normal gait and usually require therapy to correct them; in severe cases, where the pain becomes unbearable, surgery may be required [7,8,9,10]. The shoe, barefoot, and insole groups were significantly different in the kinematics of lower limb joints, as Kulcu et al. found, evaluating immediate changes in gait with the use of bilateral silicone insoles and hypothesizing that silicone insoles would have improved joint kinematics and kinetics [11,12,13]. Traditional insoles designed to treat flatfoot symptoms often fall short, especially for children. Many conventional options are either too soft, providing inadequate arch support, or too rigid, causing discomfort and restricting natural foot movement. These shortcomings can lead to persistent foot pain and discomfort, especially during activities that require prolonged standing or walking. As a result, children may be less inclined to engage in physical activity, further aggravating their condition. This highlights the need for innovative solutions such as our hybrid insole reinforced with a nitinol smart memory alloy (SMA). By combining comfort with the necessary structural support, our approach aims to effectively mitigate the challenges associated with flatfoot and provide a more suitable option for young patients. Thus, with most traditional insoles, including customized insoles, there are still shortcomings that need to be optimized [13,14,15,16,17]. The majority of flatfoot patients can also experience relief from their symptoms and a delay in the disease’s progression when using traditional orthopedic insoles. However, there are still restrictions: while most sarapus exhibit some consistency as a result of the arch collapsing, there are still significant variations in the distribution of plantar pressure amongst sarapus. Orthopedic insoles may even harm lower limbs or exacerbate foot deformities if they are not able to properly fit the wearer’s unique foot characteristics [18]. Flat feet can be classified as either flexible or rigid arches, which may or may not reform in non-weight-bearing postures [5]. Flexible flatfoot (FFT) is the most common posture deformity among children [18]. The arch of the FFT subjects appears normal during sitting and toeing, but would collapse while standing, e.g., in the case of weight bearing. However, the arch remains flat in the rigid flatfoot position despite the foot having repositioned [19,20]. Partial or complete collapse of the medial longitudinal arch (MLA) of the foot, together with the following three-dimensional foot deformities: eversion of the hindfoot, adduction, and supination of the forefoot; lateral subluxation of the navicular; and valgus deformity of the heel, are postural deformities associated with flatfoot [3].
Since pediatric flatfoot usually first appears in childhood and can last into adulthood, early identification is crucial. Pediatric patients’ formerly supple feet will eventually become more rigid, and it is thought to induce foot tiredness and pain. This could happen in one’s early adolescent or early adult years. The hindfoot will unavoidably undergo adaptive modifications that affect how it interacts with the forefoot. As the calcaneus shifts into the valgus and the hindfoot everts, the forefoot must be supinated to maintain plantigrade plantation. There is a greater chance of a gastrocnemius–soleus contracture, because the Achilles tendon shifts laterally with the calcaneus and the axis of stress on the subtalar joint shifts. Rigidity rises as a result of these structural alterations, making therapy more difficult [21,22,23]. Researchers discovered that the rate of anterior knee discomfort and intermittent back pain was almost twice as high in teenage patients with moderate to severe flatfoot [24].
Currently, most flatfoot assessments focus on the medial longitudinal arch (MLA), using common techniques including traditional arch height, navicular drop [24], footprint index [1,25], and radiographs [26]. However, the arch height can be calculated from the differences between the ground and the highest point of the soft tissue on the medial arch’s margin [27]. The navicular drop test has a moderate to good intra-reproducibility of measurement and was originally described as a way to characterize the height change in the navicular bone under weight bearing and non-weight bearing conditions [28]. The footprint index classification is a popular technique that has historically been carried out using an ink-type instrument [29]. Recently, the plantar pressure system or the planter scanning system has been used to measure and determine the footprint [30,31]. Based on X-rays, researchers have employed the arch index to identify flatfoots; nevertheless, determining the arch type is expensive and time-consuming and also requires a high dose to complete the test, but it is a reliable process [3]. Even though the X-ray test is reliable, the human foot is a complex structure with an intricate skeletal framework (bones) and diverse tissue components (soft tissue). From that perspective, functional and anatomical diagnostics are required at the same time, and due to this complex foot structure, no validated technique can accurately assess the functional and anatomical capabilities of the foot [32]. Some of the techniques used inertial measurement units (IMUs) for walking analysis, because IMUs were developed recently in the medical field, utilized especially in the analysis of movement [33], for example, foot clearance [34].
Modern methods of diagnosing flatfoot require a combination of functional and anatomical diagnostics. For example, infrared thermography has been suggested as a tool to evaluate the anatomical and functional condition of the human foot, such as in cases of Platypodia [32,35]. The objectives of this study are, first, to use infrared thermography to determine the thermal distribution on the medial and lateral sides of the metatarsophalangeal line (MTPL) of the normal subjects and FFT clinically diagnosed subjects to confirm the efficiency of these measurements. Second, to design and test an interactive hybrid insole to treat flatfoot symptoms in children from six to eighteen years old, aiming for final treatment in the future. These insoles incorporated advanced materials of nitinol smart memory alloy (SMA) aimed at mitigating flatfoot deformities in the future, alleviating pain, and enhancing comfort. However, traditional insoles were found to be ineffective in reducing pain, especially in children. In contrast, the hybrid insole with SMA was more comfortable than the traditional insole without SMA. We evaluated the performance of our suggested approach on the empirical data and compared the thermal distribution on the medial and lateral sides of MTPL of the FFT subjects before and after wearing the NiTiCu reinforced insole with normal subjects. The study involved eight clinically diagnosed flatfoot subjects and eight normal subjects, with data used to assess the quantitative and qualitative aspects of our suggested design and application. Statistical analysis, including a paired t-test with effect size (Cohen’s D), was performed after and before wearing the hybrid insole to follow up on the significant differences in the performance of our suggested design and application.

2. Materials and Method

2.1. Subjects

Sixteen subjects participated in our study, divided into two groups (eight normal subjects and eight FFT subjects). The FFT subjects are clinically diagnosed by physical examination [36,37,38]. The physical examination was used to analyze the foot mechanism of the FFT subjects compared to the normal subjects. The examination was performed during walking; the examiner asked the subject about the source or location of the pain, and assessed the alignment of the foot and ankle, which helps in evaluating the strength of the surrounding ligaments and tendons [38]. Normal subjects had no history of postural stability issues, gait abnormalities, or cognitive impairment in the past 12 months. The IRB of Al-Nahrain University, College of Engineering, Biomedical Engineering Department approved the study protocol under the code (N. BME/24/25/1). Subjects were excluded if they had sustained lower extremity injuries within six months before participation and still experienced pain, had undergone surgery on their hip, knee, or ankle joints, or could not walk or stand due to pain in the lower extremities or back. All of the subjects’ information is shown in Table 1.
The age of the subjects who participated in this study was (7–16) years old, which is parallel with the recent studies, such as (7–15) years old [38], and (7–14) years old [39]. Most studies chose a subject of an average age of around 11 years old, similar to our study.

2.2. Equipment and Materials

Several pieces of equipment and materials have been used in this study, including:

2.2.1. Processing Computer

The computer used for software design is an HP desktop with an Intel® Core™ (Santa Clara, CA, USA) i7-12700 processor (2.1 GHz up to 4.9 GHz, 25 MB L3 cache, 12 cores, 20 threads), 1 TB hard memory, 64 GB DDR5-3600 MHz RAM (2 × 32 GB), and a NVIDIA® GeForce RTX™ (3050 (Santa Clara, CA, USA) 4 GB GDDR6 dedicated) graphics card.

2.2.2. Nitinol Smart-Memory-Alloy Wires (SMAWs)

In this study, we employed 10 m nitinol smart-memory-alloy wires (NiTiCu-20-5 (SMAWs) AF = 20 °C (Ø = 0.5 mm, Heiko Engelhardt company, Offenburg, Germany) due to their biocompatibility, shape-memory effect, and super-elasticity [40]. The wires used in this study have the following properties, as specified in the manufactured guide: reshaped wire with a transformation temperature of 20 °C and a diameter of 0.5 mm. When cooled to temperatures of approximately 10 °C, the material becomes plastically deformable. Conversely, when heated to temperatures of approximately 20 °C, the material undergoes a phase transformation, whereby it “remembers” its original shape. This material is designated as a shape-memory alloy (SMA) due to its unique ability to undergo shape changes when subjected to specific temperature conditions. At normal ambient temperatures (approximately 20 °C), the alloy exhibits super-elastic behavior. The alloy’s copper content (NiTiCu) contributes to narrow hysteresis compared to binary nickel titanium. The wire has a “straight” embossed shape. The material is readily deformable at temperatures below that of a typical room. It can also be easily manipulated in this condition. It is possible to extend the wire by up to 8% without causing any damage. Upon heating above the conversion temperature (for example, by the application of warm water or an electric current), the material returns to its embossed shape and generates significant forces.

2.2.3. Evaluation of the NiTiCu Wires Insole

Testing the nitinol wires and Ethylene Vinyl Acetate (EVA) insole matrix: This step was important to obtain estimated knowledge about the forces of the insole and to obtain information important for the Ansys simulation. Several tests were carried out for both materials as follows: Tensile tests for nitinol wires were performed, while the other material properties were considered from the manufacturer labels. A tensile test, compressibility test, hardness test, and thermal conductivity test for the EVA material were performed, while the other material properties were considered from the manufacturer labels. A tensile test was carried out for both NiTiCu wires and EVA materials; for more details, see Supplementary Materials Figures S1 (a and b for NiTiCu wires) and S2 (a and b for EVA sheet).
The hardness test was carried out manually using a shore harness tester (TH200). For more details, see Supplementary Materials Table S1 and the chart curve in Figure S3, which show the results of an electrical compression machine. The thermal conductivity results are shown in Supplementary Material Table S2.

2.2.4. Polymer Plates from Ethylene Vinyl Acetate (EVA)

The EVA used in this study was evaluated based on several tests, such as the tensile test, compressibility test, hardness test, and thermal conductivity test, to ensure no mechanical errors occur during the customization of our suggested insole. The material properties are shown in Supplementary Materials Table S3.

2.2.5. Infrared Thermography Detector

The infrared thermography detector used in this study was from the sensor technology model (iHA417W) with a detecting range of 8–14 microns. The detector properties are ≤25.4 × 25.4 × 30.3 with a 9.1 mm lens, a typical NETD of <40 mk, a detect distance of 0.5 m/5 m; it is lightweight, at 32.2 ± 3 g, and has a superior temperature measurement accuracy of ≤±0.5 °C. This detector is registered according to ROHS/REACH standards.

2.2.6. Silicone Rubber

We used a silicone rubber insole with a Poisson’s ratio of 0.49 and a Young’s modulus 104 N/m2 as an insole matrix to fix nitinol wire inside it for the SolidWorks premium 2021 SP3 and Ansys 2023 (v231) simulation softwares.

2.3. Experimental Procedure and Study Design

As shown in Figure 1, the first step involved measuring subject characteristics (e.g., weight, shoe size, etc.). Then, we measured the thermal distribution on the medial and lateral sides of both normal and FFT subjects using an IR detector. Please note that a consultant physician uses a physical examination of the foot and on whom it is performed to analyze the foot mechanics. Additionally, we must mention that if the pain is severe, doctors may recommend imaging tests, such as X-rays, CT scans, MRIs, or ultrasounds, to detect the nature and degree of deformity, but this is not the case for the flexible flatfooted subjects who participated in this study. Subjects with normal feet and FFT were asked to walk for 10 min and then stand on one foot for 40 s to ensure a good heat transfer between the insole and feet; in the meantime, their feet were thermally photographed using an IR detector. From these thermal measurements, we calculate the average temperature on the medial and lateral sides of MTPL for each flatfoot subject. Based on these calculations, a foot trace for an individual flatfooted subject was designed with twenty NiTiCu wires spread based on the thermal footprint and inserted into the traces using SolidWorks premium 2021 SP3 software. These designs were imported into Ansys 2023 (v231) software to simulate the effect of the wires, applying a hypothetical force of 500 newtons to represent the subject weight of a 14-year-old child [41] to evaluate insole enhancement. After evaluating the simulation design, we customized the hybrid NiTiCu insole with 12 NiTiCu wires, inserted the insole in their shoe, and asked the subjects with FFT to wear their shoe with the NiTiCu hybrid insole on one foot, walk for 10 min, and repeat the one-foot stand for thermal imaging and temperature measures to compare the average temperature on the medial and lateral sides of MTPL of the foot before and after processing. Size differences between the medial and lateral sides of the foot were evaluated by positioning the feet on a glass board, tracing them, and manually measuring differences. The results indicated that the lateral side of the metatarsophalangeal line is about 10% larger than the medial side. Comfortable foot positioning, while seated, was found to be at an average angle of 25° from the vertical line parallel to the foot.

2.4. Software System Design

In order to minimize temperature variation, the subject places his or her foot in the box while continuous infrared heat monitoring is activated. The person then immediately takes off their shoe after standing on one foot, and a picture slide is taken. If any delay occurs in the thermography procedure, the entire procedure is repeated from the start.
We used SolidWorks Premium 2021 SP3 [30,42,43] and Ansys Workbench 2023 (v231) software to design a custom insole for each subject and initial results evaluation, which have been widely used in recent studies [44,45]. SolidWorks Premium 2021 SP3.0 release software was used to import the foot model of the normal subject to the SolidWorks workbench, then draw a cube larger than that foot model and insert the model in it. By removing the foot from the cube, a foot trace was confirmed, as shown in Figure 2. The extra surrounding area of the cube was removed around the foot trace, and an insole model was confirmed. Afterwards, 20 wires shaped as normal arch, 16 cm in length, were inserted into the model. Finally, the insole model was finished and saved using the IGS file type, so that it would be accepted by the Ansys software.
Ansys “release 2023R1 Inc\v231” software simulation was carried out by importing the insole model created by SolidWorks in (IGS form) to the Ansys working bench. All of the Ansys analysis is based on the mechanical properties of the EVA material and nitinol wires. It is assumed that the average weight of the subjects in this study was around 50.9 kg, which corresponds to a force of approximately 500 Newton, as they were between the ages of 7 and 18 years old [41]. The wires are assumed to exert a force of 5 newtons at the start of the simulation and a maximum force of 20 newtons at the end of the simulation According to the 0.5 mm diameter NiTiCu force property supplied by the manufacturer for 10 m: If it is heated over the conversion temperature (e.g., in warm water or by an electric current), it returns to the embossed shape and develops considerable forces. It can heat up to approx. 50 °C at a diameter of 0.5 mm and a weight of 6.5 kg vertically. This corresponds to a force of 65 N, i.e., for 16 cm of wire length used in this simulation, a maximum of 1 N will be assumed for each reshaped wire (i.e., 20 N for 20 wire) in the center of the wires, decaying to zero at both ends of the wires. The forces of the wire decay one Newton per centimeter to the lateral side of the wires, starting from the center. The simulation consists of five steps, from zero force to 20 newtons for 20 wires, to mimic heat transfer from the foot to the proposed insole. It must be mentioned that some references mentioned another NiTi SMA exerting the maximum output force of the SMA micro-coil, which was approximately 720 mN at 105 °C [40,46].

2.5. Hardware System Design

In this study, we modified the traditional EVA insole by inserting 12 of our suggested NiTiCu wires into the medial side of the insole. Each wire was the same length, used to reinforce the insole for the proposed design. Finally, the insole will be glued using silicone to close the incision opening. The NiTiCu wires were reshaped using a graphite template carved by a CNC carving machine. The design of the graphite template was established using SolidWorks, as shown in Figure 3 by tracing a normal human subject’s foot medial arch. Then, the template containing 12 NiTiCu wires was heated in an electric oven at 450 °C for 10 min, as recommended by the manufacturer. Figure 4, Figure 5 and Figure 6 below show the template and the reshaped wires.

2.6. SMA Placement Design

Twelve NiTiCu wires of 12 cm length each were inserted in the incision made in the medial side of the EVA insole plate along its length from the center of the insole, spread transversely 5 mm from its medial rim to about 12 mm, i.e., 1 mm between each two adjacent wires. Figure 6 shows the final manufactured NiTiCu SMA wire-reinforced insole used for testing. This design was established to mimic normal foot medial arch action in normal human subjects, so that the pressure forces were redistributed, normalizing the pressure forces on the soles of flatfeet subjects.

2.7. Data Analysis

We use an IR detector to measure the average temperature across the medial and lateral sides of the metatarsophalangeal line for subjects with a normal foot as the first group, subjects with flat feet before wearing the proposed insole as the second group, and subjects with flat feet after wearing the proposed insole as the third group. Then, the difference between the average temperature on the medial and lateral sides is calculated for each subject by subtraction. The results in this investigation targeted only a limited population of flatfoot patients, children with flexible flatfoot without any other complications, so all thermal measurements from all subjects were considered and no results were ignored. After calculating the differences in temperature for all subjects, an average temperature difference for each group was calculated. Finally, a qualitative comparison was made between the thermal measurements of normal subjects and FFT subjects before and after wearing the proposed insole.
Statistically, we applied a paired t-test with a bootstrap sample size of N = 1000 and calculated the effect size (Cohen’s D) to investigate the significant thermal differences across MTPL between normal and flatfoot subjects and between flatfoot subjects before and after wearing the hybrid proposed insole.
Every participant was asked to rate their feelings after using the hybrid insole from 1 to 10, in comparison to how they felt wearing conventional flatfoot therapy insoles and how they felt when they wore no insole at all.

3. Results

3.1. Qualitative Comparison of Normal and Flatfoot Subjects Before and After Testing

In this section, we briefly provide a qualitative comparison of the average temperature across the medial and lateral sides of MTPL of the foot between the normal subjects, and the FFT subjects before and after wearing the NiTiCu insole design. The experimental results are based on two components averaged across subjects: the IR thermographic results and the software results. The results showed that there was an average temperature difference between the medial and lateral sides of the metatarsophalangeal line of 0.016875 °C for normal subjects, 1.07125 °C for flatfooted children, and 0.2725 °C after wearing the hybrid insole, which is closer to normal subjects. The data obtained from the analysis of thermal distribution differences were imported into IBM SPSS Bootstrapping 22 version software, which was then used to calculate the results of the t-test, Cohen’s D, and bootstrap. The thermographic results demonstrated a significant (p-value < 0.001, Cohen’s D = 3.827) decrease in high-temperature points for the flatfoot subjects after wearing our suggested NiTiCu insole in comparison with the flatfoot subjects’ group before wearing the NiTiCu insole. All comparisons are still significant (p-value < 0.001) after the bootstrap test with an n = 1000 sample. However, there are significant differences between the normal subjects and FFT subjects before wearing the NiTiCu-reinforced insole, but insignificant differences were reported after wearing the NiTiCu insole, as shown in Figure 7.
The results demonstrated that the highest difference in average temperature between the medial and lateral lines across the MTPL was 1.31 °C before wearing the NiTiCu insole, decreasing to 0.32 °C after wearing the NiTiCu insole, which is an improvement ratio of 14:1. However, the lowest difference in average temperature between the medial and lateral lines across MTPL was 1.0 °C before wearing the NiTiCu insole, decreasing to 0.75 °C after wearing the NiTiCu insole, which is an improvement ratio of 4:1. The comparison of the normal and subjects with FFT is shown in Table 2 below. There is a significant difference between the normal and FFT subjects before wearing the NiTiCu insole, but there is an insignificant difference between them after wearing the NiTiCu insole, as shown in Table 2 and Table 3. In addition, Table 3 shows the effect of the hybrid insole in improving thermal distribution on flatfoot subjects across MTPL to be closer to that of normal subjects, by interactively supporting the medial arch of the foot. Through long-term use, this technique aims to change the bone and muscle structure of the foot in a way that reshapes it closer to the natural foot.
An illustrative example (see Figure 8) was used to demonstrate the average temperature distribution on the medial and lateral sides of MTPL of a FFT subject before and after wearing the NiTiCu insole, as well as for a normal subject, showing how the temperature became approximately equal between the medial and lateral sides of MTPL after wearing the NiTiCu insole. The figure also shows the thermal distribution of a normal subject, demonstrating that the average temperature on the medial line of MTPL is approximately equal compared to the average temperature on the lateral line of MTPL on the left and right foot.
After using our proposed hybrid insole, the FFT subjects rated its comfort on a scale of 1 to 10 based on a questionnaire administered by the examiner. Their responses are shown in Table 4.

3.2. SolidWorks Results

We used the SolidWorks premium 2021 SP3 to design the suggested insole with and without NiTiCu wires to investigate the effect of the NiTiCu wires on the pressure distribution of the flatfoot subjects. Figure 9 is an example of a flatfoot subject with a weight of 59 kg and a shoe size of 41.

3.3. Ansys Simulation Results

By using Ansys simulation, the results show that there is a suitable effect on the foot, with a total maximum vertically upward deformation of 1.3078 mm in the center of the medial arch of the foot, as shown in Figure 10a,b, and Von-mises equivalent stress of 16.383 MPa, as shown in Figure 11, which gave an estimated value of the efficiency of using SMA nitinol wires in reinforcing the traditional insole.
To clarify the efficiency in comparison with the traditional insole, an imaginary traditional insole (not reinforced using nitinol SMA wires) was used in the Ansys simulation under the same conditions, and the results show that there was 0.016646 MPa downward equivalent Von-mises stress and a downward deformation for the insole (i.e., the insole compressed under body weight) of 0.061132 mm, as shown in Figure 12 and Figure 13 below.

4. Discussion

Flatfeet deformity is a widespread condition impacting a lot of people, especially in their childhood. Primarily, flat feet disrupt the natural reflexes of the sole, which impacts posture and gait [47]. Both posture and gait are crucial for balance and perception. When natural posture is affected, it limits the ability to maintain an upright stance, while an altered gait leads to unstable movements. If flat feet are not treated promptly, various complications can arise, including painful stress, muscle weakness, and significant functional disabilities, all of which can severely impact an individual’s quality of life [32].
The aim of this study is to develop a model to detect FFT deformity based on thermal technology (IR imaging) by measuring the temperature differences between the medial and lateral sides of the MTPL of the foot, who are then clinically diagnosed by physical examination as flatfooted people and compared with those of normal people. Based on these measurements, we designed a simulation prototype of a hybrid insole to treat this deformity using SolidWorks, and tested this design using Ansys software. According to these simulation results, a hybrid insole reinforced with NiTiCu smart-memory-alloy wires to improve the thermal distribution on the medial and lateral sides of the MTPL of the foot was designed. The innovative design of the SMA-reinforced insoles not only improves comfort and heat distribution for children with FFT, but also offers a personalized treatment solution that can adapt to individual foot dynamics, potentially leading to improved clinical outcomes. By effectively addressing localized high-temperature points and redistributing weight, this approach can significantly reduce associated pain and discomfort, enabling improved mobility and overall quality of life for children. In addition, the integration of infrared thermography as a diagnostic tool enhances the ability to monitor treatment progress and make data-driven decisions, which could ultimately streamline clinical workflows and improve patient management strategies. The results show that our proposed approach improves the thermal balance on the medial and lateral sides and provides a more comfortable insole for the FFT subjects. Figure 7 shows a significant reduction in the high-temperature points on the medial or lateral side after wearing our proposed insole, and there are insignificant differences between the normal subjects and the FFT subjects after treatment.
In this study, nitinol SMA wires formed in the shape of a normal foot medial arch were used to reinforce the insole to mimic normal foot medial arch action in normal human subjects in an interactive pattern relative to foot temperature change during normal activity, so that the pressure forces could be redistributed to normalize the pressure forces on the soles of flatfeet subjects.
In contrast to A. Urakov, D. Nikityuk, A. Kasatkin, and I. Lukoyanov’s study, we found that the average temperature differences across MTPL was 0 °C and 0.3 °C larger on the medial side than lateral side [48], for normal subjects and flatfooted subjects, respectively; our approximation finds that the average of the temperature differences across MTPL was 0.016875 °C and 1.07125 °C larger in the medial side than lateral side, for normal subjects and flatfooted subjects, respectively [34]. There could be two possible explanations for the differences in findings between their study and ours. First, they employed an uncontrolled imaging design in which the patient is positioned 1.5 m from the infrared detector without a shield to protect them from ambient infrared radiation. The tiny sample size (only five normal and five flatfooted samples were examined) may be the second explanation. Important data loss could result from either of these causes. The size of the data set was the main factor affecting the overall performance. This is because the images collected for this research are quite small. However, we used a bootstrap test with N = 1000 to increase the sample size. After the bootstrap test, the differences were still significant. From the illustrative example in Figure 8, we can clearly see how the high-temperature point on the medial side decreased to be approximately equal to the temperature on the lateral and medial sides and near that of the normal subject, which indicates that our proposed approach provides the best solution to treat the FFT subjects, and according to the survey, the hybrid insole reduced the pain. The SolidWorks insole design was very appropriate and was close to the real insole shape with 3D measurements, making it suitable for Ansys simulation. The Ansys simulation results show a high deformation effect, which means high lifting forces will affect the patient’s foot, making pressure distribution correction applicable, enough to achieve the purpose of the study and treat flatfoot deformity. The Ansys simulation results, which indicate that the forces exerted by the SMA wires can reach up to 20 Newtons, are clinically significant, as they demonstrate the insole’s ability to provide adequate support to the medial longitudinal arch during weight-bearing activities, thereby improving stability and comfort for children with flexible flatfoot. In addition, the deformation patterns observed suggest an effective distribution of load across the foot, which is critical for reducing localized high-temperature areas and alleviating pain associated with flatfoot deformities. In a future work, a long-term follow-up of all subjects who participated in this study will be carried out, so that we can determine the effect of the proposed insole on reducing FFT deformity.
Furthermore, our approach offers a comprehensive, low-cost method for both verifying the clinical diagnosis of FFT using non-ionizing noninvasive IR thermography and immediately treating the symptoms of this deformity with an interactive hybrid insole that made use of nitinol SMAW characteristics. The primary constraint of this study is that, in order to obtain the necessary thermal image at the required position at MTPL, the process of thermal imaging for the foot needs to be more appropriately designed. Additionally, the manufacturing process of the hybrid insole takes a long time, because it is necessary to create a mold and use special software for designing customized insoles.
Our method of creating hybrid insoles typically costs between USD 30 and USD 40 for research purposes, which includes fees for materials, molds, and design. Because of the interactive insole’s increased comfort and interactive pressure impact, it is considered inexpensive. This can be compared to alternative insole construction techniques, like 3D-printing insoles, which are considered a passive treatment intended to raise the medial midfoot’s peak pressure and pressure–time integral, indicating support on the medial longitudinal arch, along with an offloading on the hindfoot and greater ankle dorsiflexion, costing roughly USD 22 [9,47].
On the other hand, a traditional insole is more cost-effective and could be adopted, but its support of the foot is imprecise. Plantar pressure redistribution insoles (PPRIs) are another type of insole, which are individually customized plantar pressure-based insoles that help users change the abnormally distributed pressure on the pelma. They are more adequate than traditional insoles, but they cost more, and they are still considered a passive treatment method [48,49].
One of the commonly used techniques for flatfoot diagnostics is radiographic imaging, or X-ray, but this technique is time-consuming, costly, and carries the risk of radiation exposure; furthermore, this technique does not provide a functional image of the foot [1,50,51]. Another widely used technique is the footprint index, usually using a podoscope, which is correlated with age, gender, and BMI. The indices under investigation are appropriate for identifying adult flatfoot, particularly Clarke’s angle, which has a very high diagnostic accuracy for this population [52]. Given that radiography is rather costly and causes biohazards, and the footprint index is not recommended for children, neither of these techniques can offer an interactive functional anatomy diagnosis. However, our approach is cost-effective (it only requires an IR detector to confirm and follow-up the diagnosis for many patients) and provides a functional anatomical thermal image.
This study recommends more significant thermographic design procedures in the future, if possible. It would be significant if a researcher were to find a highly IR-transparent material that can carry the subject’s weight, so that the subject stands on that material and is thermally imaged using an IR detector located under this material, i.e., a weight-bearing thermal image. If that is not possible, our approach could be modified to automatically detect the flatfoot deformity using image-processing software that would minimize the processing time as much as possible.
Future investigations should assess the long-term durability of the proposed insole and its sustained effect on the foot’s structure and function. Additionally, future studies are required to compare our proposed approach with traditional techniques, such as biomechanical measurements, to evaluate their relative effectiveness.
The significance of the temperature differences between the medial and lateral sides of the foot lies in the clinical relevance of thermal distribution. In a normal foot, there is typically an equilibrium in temperature distribution, reflecting a balance in the load distribution across the foot. However, the high-temperature point is a source of pain in the feet. Moreover, in cases of FFT, the medial side often experiences increased load, leading to higher localized temperatures. These temperature variations are indicative of biomechanical imbalances. By comparing the thermal profiles of flatfoot and normal feet, we can assess the severity of deformity and monitor the effectiveness of interventions, such as the use of corrective insoles, in restoring a more balanced temperature distribution.
One limitation of our approach is that we did not compare our method with traditional biomechanical measurements or other established techniques for flatfoot diagnosis. Although our thermography-based technique is promising, further research is needed to validate its accuracy against these conventional methods.
In addition, while our insole design showed significant improvements in thermal distribution for flatfoot subjects, the long-term durability of the SMA-reinforced insole remains unexplored. Future studies should investigate how the prolonged use of this design affects foot structure and function over time.
In addition, we recommend the investigation of more advanced thermographic designs. For example, the development of a highly IR-transparent material that can support the subject’s weight would allow for weight-bearing thermal imaging, providing a more realistic and direct measurement of thermal distribution under weight-bearing conditions. If such a material is not available, an alternative could be to design a foot fixation mechanism that minimizes thermal data loss and does not interfere with the natural foot temperature during imaging. This would ensure more accurate thermal data collection while maintaining accuracy.

5. Conclusions

The current study concludes that by improving thermal distribution on the flatfoot and significantly reducing high-temperature points, our approach provides a comprehensive and novel solution for the diagnosis and treatment of FFT. Using infrared thermography, we effectively measured temperature differences between the medial and lateral sides of the foot to confirm our diagnosis and evaluate treatment efficacy. The customized hybrid insole reinforced with NiTiCu smart-memory-alloy wires (SMAWs) showed significant improvements in comfort and functionality for the flatfoot subjects compared with the normal subjects.

Key Findings and Implications

Our findings suggest that this SMAW-reinforced insole can reduce localized high-temperature points caused by unequal pressure distribution and achieve a more uniform distribution of heat across the foot. This innovative approach not only addresses the immediate symptoms of FFT, but also has the potential to improve long-term outcomes by minimizing discomfort and improving overall foot health. By integrating advanced materials and thermographic technology, our research offers a promising pathway for future applications in the treatment of children’s flat feet and highlights the need for personalized solutions in the management of this common condition. A limitation of our study is that our thermographic-based method was not compared with conventional biomechanical measurements or other established methods of flatfoot diagnosis, which necessitates further validation of our approach. In addition, although our SMA-reinforced insole showed significant improvements in thermal distribution, its long-term durability and effects on foot structure and function remain to be investigated. Future research should also explore advanced thermographic designs, such as the development of a highly IR-transparent material for weight-bearing thermal imaging, or the implementation of a foot fixation mechanism that minimizes the loss of thermal data without altering the natural temperature of the foot.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/prosthesis6060108/s1, Figure S1a: Tensile test results for nitinol SMA wires (Test results values); Figure S1b: Tensile test results for nitinol SMA wires (The ratio between y axis Force and x axis elongation); Figure S2a: Tensile test results for EVA sheet (Load–extension curve); Figure S2b: Tensile test results for EVA sheet (Stress-strain curve); Figure S3: Compressibility curve for EVA; Table S1. Compressibility test results; Table S2. Conductivity test results; Table S3. EVA Mechanical Properties.

Author Contributions

Conceptualization, A.F.A., A.Q.A.-N. and T.J.M.; methodology, A.F.A.; software, A.F.A.; validation, A.F.A., A.Q.A.-N., T.J.M. and H.R.A.-O.; formal analysis, A.F.A. and H.R.A.-O.; investigation, A.F.A.; resources, A.F.A.; data curation, A.F.A., A.Q.A.-N. and H.R.A.-O.; writing—original draft preparation, A.F.A.; writing—review and editing, A.F.A., A.Q.A.-N., T.J.M. and H.R.A.-O.; visualization, A.F.A., A.Q.A.-N. and T.J.M.; supervision, A.Q.A.-N. and T.J.M.; project administration, A.F.A. and A.Q.A.-N.; funding acquisition, A.F.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

The study was conducted in accordance with the Declaration of Helsinki, and approved by the IRB of the Biomedical engineering department, College of Engineering, Al-Nahrain University under the code (N.BME/24/25/1) (approval date: 15 Jan 2024).

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study. Written informed consent has been obtained from the patient(s) to publish this paper.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

We thank the Biomedical, Mechanical, and Prosthetics and Orthotics Engineering departments. We thank the Ministry of Health/Technical Efforts Directorate. We also thank Hussain Faisal and Fadhil Kareem Farhan for their technical support.

Conflicts of Interest

The authors declare no conflicts of interest.

Correction Statement

This article has been republished with a minor correction to the Institutional Review Board Statement. This change does not affect the scientific content of the article.

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Figure 1. Experimental procedure and study design.
Figure 1. Experimental procedure and study design.
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Figure 2. SolidWorks insole trace.
Figure 2. SolidWorks insole trace.
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Figure 3. SolidWorks template design.
Figure 3. SolidWorks template design.
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Figure 4. CNC-carved graphite template with NiTiCu wires fixed inside it, so that it can reshape and take the medial arch foot shape.
Figure 4. CNC-carved graphite template with NiTiCu wires fixed inside it, so that it can reshape and take the medial arch foot shape.
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Figure 5. Reshaped NiTiCu SMA wires.
Figure 5. Reshaped NiTiCu SMA wires.
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Figure 6. Manufactured insole reinforced by NiTiCu SMA wires.
Figure 6. Manufactured insole reinforced by NiTiCu SMA wires.
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Figure 7. The subjects’ results of the temperature differences between the medial and lateral lines. (A) The blue bar represents the flatfoot group before wearing the NiTiCu insole; (B) the red bar represents the flatfoot group after wearing the hybrid insole; and (C) the green bar represents the normal subjects. Three stars (***) indicate a significance level of p-value < 0.001, which was reported by comparing the flatfoot group before and after wearing the NiTiCu insole, and p-value < 0.001 when comparing the flatfoot group without the NiTiCu insole with normal subjects.
Figure 7. The subjects’ results of the temperature differences between the medial and lateral lines. (A) The blue bar represents the flatfoot group before wearing the NiTiCu insole; (B) the red bar represents the flatfoot group after wearing the hybrid insole; and (C) the green bar represents the normal subjects. Three stars (***) indicate a significance level of p-value < 0.001, which was reported by comparing the flatfoot group before and after wearing the NiTiCu insole, and p-value < 0.001 when comparing the flatfoot group without the NiTiCu insole with normal subjects.
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Figure 8. The thermal distribution of a (A) FFT subject before wearing the NiTiCu insole, the average temperature difference between the medial and lateral sides is 1.25 °C; (B) the same flatfoot subject after wearing the NiTiCu insole, the temperature difference between the medial and lateral sides is 0.09 °C; and (C) the temperature difference of a normal subject between the medial and lateral sides is 0.06 °C.
Figure 8. The thermal distribution of a (A) FFT subject before wearing the NiTiCu insole, the average temperature difference between the medial and lateral sides is 1.25 °C; (B) the same flatfoot subject after wearing the NiTiCu insole, the temperature difference between the medial and lateral sides is 0.09 °C; and (C) the temperature difference of a normal subject between the medial and lateral sides is 0.06 °C.
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Figure 9. (A) Side and (B) top view of the imaginary insole.
Figure 9. (A) Side and (B) top view of the imaginary insole.
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Figure 10. (a,b) show the total upward vertical deformation using Ansys software (max: 1.3078 mm).
Figure 10. (a,b) show the total upward vertical deformation using Ansys software (max: 1.3078 mm).
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Figure 11. Equivalent upward vertical Von-mises stress (max: 16.383 MPa).
Figure 11. Equivalent upward vertical Von-mises stress (max: 16.383 MPa).
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Figure 12. Equivalent downward vertical Von-mises stress (max: 0.016646 MPa).
Figure 12. Equivalent downward vertical Von-mises stress (max: 0.016646 MPa).
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Figure 13. Total downward vertical deformation (max: −0.061132 mm).
Figure 13. Total downward vertical deformation (max: −0.061132 mm).
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Table 1. Subjects’ information.
Table 1. Subjects’ information.
Subjects’ InformationNormal Foot (No. = 8)FFT (No. = 8)
Mean±StdMean±Std
Age (year)12.51.772811.6253.1139
Weight (kg)49.7513.2648.23718.722
Height (cm)155.68.484146.515.042
Average shoe size (Euro)39.252.187637.1253.907
Table 2. The temperature distribution differences on both sides of the metatarsophalangeal line for all eight normal foot subjects for each subject’s left and right foot; it also includes each subject’s data (age, gender, weight, length, and foot size).
Table 2. The temperature distribution differences on both sides of the metatarsophalangeal line for all eight normal foot subjects for each subject’s left and right foot; it also includes each subject’s data (age, gender, weight, length, and foot size).
S. No.Age, Years/GenderWeight, kg/
Length, cm		Shoe Size, EUTemperature Medial TM − Lateral TL, °C for Both
R.F./L.F.
SN111/Male55/15940L5 − L3 = 33.41 − 33.23 = 0.18
L5 − L6 = 33.66 − 33.56 = 0.06
SN214/Male45/16042L5 − L3 = 33.73 − 33.66 = 0.07
L3 − L5 = 33.35 − 33.16 = 0.19
SN312/Male42/14540L5 − L3 = 32.1 − 31.93 = 0.17
L3 − L5 = 31.91 − 32.04 = −0.13
SN49/Male53/14339L5 − L3 = 31.66 − 31.54 = 0.12
L3 − L5 = 30.48 − 30.06 = −0.12
SN514/Female40/15538L5 − L3 = 37.16 − 37.29 = −0.13
L3 − L5 = 31.65 − 31.6 = 0.05
SN613/Male78/16942L2 − L1 = 33.23 − 33.29 = −0.06
L2 − L1 = 33.23 − 33.29 = −0.06
SN714/Male50/15438L5 − L3 = 29.73 − 29.6 = 0.13
L3 − L5 = 29.41 − 29.73 = −0.32
SN813/Female35/16037L5 − L3 = 28.23 − 28.30 = −0.07
L3 − L5 = 28.85 − 28.66 = 0.19
Average Temperature, °C0.016875 °C
Table 3. The temperature distribution differences on both sides of the metatarsophalangeal line before and after wearing the NiTiCu wire-reinforced EVA insole for all eight flatfoot child subjects; it also includes each subject’s data (age, gender, weight, length, and foot size).
Table 3. The temperature distribution differences on both sides of the metatarsophalangeal line before and after wearing the NiTiCu wire-reinforced EVA insole for all eight flatfoot child subjects; it also includes each subject’s data (age, gender, weight, length, and foot size).
S. No.Age, Years/GenderWeight, kg/Length, cmShoe Size, EUTemperature Medial TM − Lateral TL Before Wearing Insole, °CTemperature Medial TM − Lateral TL After Wearing Insole, °C
S19/Female39/13335L2M30.54 − L1L29.23 = 1.31L2M30.41 − L1L30.73 = −0.32
S211/Male27.3/13534L2M27.1 − L1L28.29 = 1.19L1L28.29 − L2M29.6 = −0.31
S37/Male24.6/12631L5M27.16 − L3L25.91 = 1.25L5M26.48 − L3L27.48 = −1
S415/Female45/14836L4M20.23 − L2L19.23 = 1L3M22.29 − L5L22.16 = 0.13
S510/Male71/16039L5M27.41 − L3L26.16 = 1.25L5M25.35 − L3L25.6 = −0.25
S614/Female45/15038L3M27.04 − L5L26.16 = 0.88L3M27.54 − L5L27.54 = 0
S716/Male75/17243L4M28.85 − L2L27.85 = 1L4M28.23 − L2L28.96 = −0.75
S811/Male59/14841L4M31.73 − L5L31.04 = 0.69L3M26.98 − L5L26.66 = 0.32
Average Temperatures1.071250.28375
Table 4. Rate of FFT subjects’ comfortability to the hybrid insole.
Table 4. Rate of FFT subjects’ comfortability to the hybrid insole.
S. No.Age, Years/GenderWeight, kg/Length, cmShoe Size, EUTemperature Medial TM − Lateral TL After Wearing Insole, °CSatisfaction Rate
S19/Female39/13335L2M30.41 − L1L30.73 = −0.328
S211/Male27.3/13534L1L28.29 − L2M29.6 = −0.319
S37/Male24.6/12631L5M26.48 − L3L27.48 = −15
S415/Female45/14836L3M22.29 − L5L22.16 = 0.1310
S510/Male71/16039L5M25.35 − L3L25.6 = −0.259
S614/Female45/15038L3M27.54 − L5L27.54 = 010
S716/Male75/17243L4M28.23 − L2L28.96 = −0.755
S811/Male59/14841L3M26.98 − L5L26.66 = 0.328
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MDPI and ACS Style

Abdulkareem, A.F.; Al-Neami, A.Q.; Mohammed, T.J.; Al-Omairi, H.R. The Design and Application of an Advanced System for the Diagnosis and Treatment of Flatfoot Based on Infrared Thermography and a Smart-Memory-Alloy-Reinforced Insole. Prosthesis 2024, 6, 1491-1509. https://doi.org/10.3390/prosthesis6060108

AMA Style

Abdulkareem AF, Al-Neami AQ, Mohammed TJ, Al-Omairi HR. The Design and Application of an Advanced System for the Diagnosis and Treatment of Flatfoot Based on Infrared Thermography and a Smart-Memory-Alloy-Reinforced Insole. Prosthesis. 2024; 6(6):1491-1509. https://doi.org/10.3390/prosthesis6060108

Chicago/Turabian Style

Abdulkareem, Ali F., Auns Q. Al-Neami, Tariq J. Mohammed, and Hayder R. Al-Omairi. 2024. "The Design and Application of an Advanced System for the Diagnosis and Treatment of Flatfoot Based on Infrared Thermography and a Smart-Memory-Alloy-Reinforced Insole" Prosthesis 6, no. 6: 1491-1509. https://doi.org/10.3390/prosthesis6060108

APA Style

Abdulkareem, A. F., Al-Neami, A. Q., Mohammed, T. J., & Al-Omairi, H. R. (2024). The Design and Application of an Advanced System for the Diagnosis and Treatment of Flatfoot Based on Infrared Thermography and a Smart-Memory-Alloy-Reinforced Insole. Prosthesis, 6(6), 1491-1509. https://doi.org/10.3390/prosthesis6060108

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