1. Introduction
Accurately identifying plantar contact and pressure distribution is of great interest to clinicians and researchers in evaluating foot functions, human movement, and posture, which is considered indispensable to understanding its effects on the gait of the human body [
1,
2,
3,
4]. Human gait is considered an essential physiological activity for every individual, to the point of being compared to breathing or sleeping [
5]. Briefly explained, the gait cycle is divided into two phases: the stance and swing phases, representing 60% and 40% of the total human gait, respectively. The stance phase is subdivided into five different stages. When the foot initially has contact with the ground, it is known as the heel strike phase. Once it has made contact, the foot progressively descends towards the ground to have a foot flat, which is the loading response stage. Next is the mid-stance position, where the body tilts forward, with the ankle joint as the pivot and the hip joint on top. Heel rise occurs when the forefoot area comes into contact directly to propel the body. Finally, when part of the toes is the only area in contact with the ground, the end of the stance phase is commonly referred to as toe-off or pre-swing [
6,
7,
8,
9]. The analysis of the distribution of pressure points in the foot under this activity facilitates the understanding of its functionality and the ability of the foot to adapt to different surfaces and conditions. The comprehension of how a foot distributes pressure and adjusts to each step provides valuable insights into a primary data source in gait and posture analysis; foot pressure reveals the otherwise challenging to analyze biomechanical effects occurring at the interface between the foot and the supporting surface, enabling the evaluation of underlying musculoskeletal behavior [
10,
11,
12].
From a biomechanical approach, stresses and strains generated in each position and movement during normal walking set the standard for predicting a possible tendency to develop foot pathologies [
13,
14,
15,
16,
17]. The employment of numerical simulations marks a turning point in analyzing human anatomy behavior in various scenarios and the prosthetic and orthotic design [
18,
19,
20,
21]. Regarding foot healthcare and footwear production, this shift promises a future where efficiency and cost-optimization reign supreme over conventional experimental testing to meet truly personalized insole construction demands [
22,
23]. In addition, an increasing technological trend is guiding the path toward a more sustainable and innovative production of foot orthotics by implementing additive manufacturing [
24,
25,
26].
Foot orthoses have always been a mainstay in podiatric treatment and have conventionally been manufactured using a cast-and-mold approach. However, recent advancements in 3D printing technology offer the potential for patient-specific, customizable insoles. The study and research of foot insole construction through additive manufacturing materials are of great interest to the scientific community to replicate and enhance the mechanical behavior of standard orthoses [
27,
28,
29]. Throughout the literature, different 3D-printable material testing has been conducted to compare the mechanical behavior to reproduce shock-absorbing effects, polymers such as polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), polyamide filaments (NYLON), polyamide (PA), and thermoplastic polyurethane (TPU) are the printing materials most studied and employed for footwear purposes [
30,
31,
32]. An optimized mechanical response along with the highest standard for comfort are the main aims of current 3D-printing technologies for foot insole design; a combination of novel approaches in infill percentages, infill matrix patterns, combined material structures, and numerical analyses [
33,
34,
35,
36,
37,
38] have improved traditional hand-made workflows in orthopedic devices. Another crucial aspect of the recent advancements in 3D-printed patient-specific insoles is 3D scanning technology, which allows the acquisition of more precise anthropometric data, resulting in advantages in time and accuracy to obtain a detailed three-dimensional copy that meets the actual morphology of such a complex geometry of the human body [
39,
40,
41] over traditional foot plaster models and conventional measurement techniques that lack data consistency due to utilizing diverse procedures and different measurement instruments for foot dimension evaluation [
42,
43,
44]. These increasingly technological methods have revolutionized the ergonomics and footwear industry and have significantly impacted clinical applications since they provide cutting-edge advancements in tangible solutions to musculoskeletal disorders [
45,
46,
47,
48]. The capability of 3D modeling and printing of foot orthoses have provided promising prospects for improving patient care of all ages, conditions, and activities. Clinical studies investigating 3D-printed insoles in specific applications are being developed more frequently, with promising results emerging in sports performance enhancement, elderly populations, children’s feet, and pathological feet. These studies have promoted the understanding, diagnosis, and treatment of specific musculoskeletal conditions by applying engineering principles, such as reverse engineering [
49,
50,
51,
52,
53,
54].
Therefore, a comprehensive study that accurately quantifies peak plantar pressure during a normal gait cycle performing all stance phases is crucial to deepening into weight bearing and pressure distribution foot functions, revolutionizing a worldwide approach to foot health management and enabling further precise interventions to individual needs. Consequently, this leads to personalized foot health care flourishment and fully customized foot orthotic devices based on the accurate morphology of the individual [
55,
56,
57]. Through the Finite Element Method, it is possible to obtain and analyze plantar distribution numerically in detail during the different stance phases of the gait cycle. This multifaceted approach promises to significantly advance the understanding of foot pressure points while performing all stance phases during a normal gait, aiming to develop more effective and personalized plantar orthoses.
This study seeks to delve deeper into the interactions among foot soft tissues, pressure points, and orthosis-relieving properties. To employ Finite Element analyses, a 3D anatomical model with high-fidelity detail is generated through established medical imaging techniques [
19]. Such models offer the distinct advantage of capturing the complex geometrical features of human biological systems, enabling detailed analysis and insights not readily achievable through traditional 2D representations. This current research is focused on numerically analyzing stance phases for barefoot plantar pressure validated with experimental testing to design a fully customized foot orthosis for numerically evaluating the biomechanical effects in the sole region utilizing the orthopedic device, where the level of complexity to understanding foot-insole plantar effects challenges experimental testing, which commonly yields to insufficient and poorly explained data for accurate foot function diagnosis. Likewise, this manuscript can provide a different innovative approach to gait patterns, optimize foot health, and enhance the quality of life for individuals requiring plantar support.
4. Discussion
The current research employed a high-complexity biological model of the right foot focused on segmenting soft tissues, intrinsic muscles, and skin. In contrast to most foot Finite Element Analyses that employ bony tissues, this research presents a novel and different approach to analyzing the plantar surface with a yet-defined 3D detailed model development. Both numerical and experimental testing were conducted to biomechanically evaluate the behavior of pressure distribution on the foot sole region during all five stance phases in a normal gait cycle to design a 3D-printable personalized foot insole based on the patient’s unique morphology for evaluating the complex behavior of foot-insole effects presented in the plantar zone. The assignment of loading and boundary conditions presented in the Finite Element Analysis showed an innovative and unconventional method to employ new mechanical considerations to analyze soft tissues; utilizing a displacement as an external agent allowed the development of accurate effects.
Numerical results indicated that the highest concentrations of von Mises stress fields are found in the pre-swing phase in the forefoot, specifically at the hallux, as well as in the values of the experimental analysis. These Finite Element Method results, particularly the stress distribution (ring-shaped), should be interpreted within the context of the model’s construction and considerations, specifically using only soft tissue. The contact between the ground tends to displace the tissue due to its high ductility, resulting in a higher stress concentration in the contour and not in the center. Despite this limitation, the agreement with experimental data regarding pressure spectrum, peak plantar pressure, and average pressure suggests that the model offers valuable insights into soft tissue pressure distribution biomechanical behavior during foot–ground contact. Furthermore, previous research reported this ring-shaped or gap stress distribution field, stating that the mentioned effect occurs only by analyzing soft tissue [
86]. Thus, the plantar pressure distribution showed agreement with the two types of analysis, which validates numerical analyses as both had similar behavior and patterns, having an average error range in the five stance phases of less than 5% in the maximum pressure points of the numerical simulation compared to the experimental one.
On the other hand, most of the analyses showed a uniform stress distribution, commonly green shades in the isochromatic scale, with values between 0.055 and 0.08 MPa, corresponding to the general average pressure. The baropodometric test registered a result of 787 gr/cm
2, equal to 0.0771 MPa. This study evaluated the effects caused on the foot sole for each one of the stance phases from a statically mechanical study, representing the exact moment when the foot is in contact with the ground, disregarding dynamic considerations, which enables this method to optimize and simplify dynamic analysis. The high fidelity of the results obtained in the experimental testing and the first case study indicate that despite any modeling and dynamic simplifications previously mentioned, the principles and parameters utilized in the Finite Element Analysis were accurate and appropriate to simulate the stance phases of a normal gait cycle. Furthermore, the findings of this study are consistent with observations previously reported in the literature of Finite Element Analyses focused on the gait cycle. Despite methodological differences, numerical results align closely with the observations reported by the cited research [
87] for the mid-stance, heel rise, and pre-swing values along the plantar zone; the loading response also shows good agreement in the heel lateral region. Similar stress distribution and values presented in this research for the heel rise phase replicate the findings from different investigation groups [
88,
89]. Results obtained in the hallux when evaluating the pre-swing phase are in solid concordance with results reported in a numerical study in the first ray [
90]. The total elastic strain values in
Appendix A tables show consistent behavior with an analysis of the strain effects in the plantar region skin [
91].
The comprehension of the biomechanical behavior of the model under gait cycle stance phases provided essential insights to design a 3D-printable patient-specific insole. Numerical results for the second case study supported the insole’s material selection and the accurate parametric design to considerably attenuate peak plantar pressure, specifically in critical stance phases, such as heel strike and pre-swing. The prediction of pressure points during the performance of daily activities through Finite Element analyses facilitates novel approaches that seek high-biofidelity methods to analyze and understand dynamics in real-life biomechanics, aiming to enhance current customization principles for orthotic and prosthetic devices, contributing innovative scientific solutions to the medical field.
The presented research is categorized as a presentation of a method for presenting a distinctive approach with innovative mechanical considerations to analyze pressure points in a healthy patient, which also brings certain limitations for the methods implemented. The need to employ the proposed methods in various healthy patients before testing them in pathological foot cases is worth mentioning. Methods employed have a relevant impact in providing proper knowledge and general guidelines in the study of pressure points and their re-distribution towards designing fully customized foot orthopedic devices. Further approaches in pathological and specific-condition cases can potentially be achieved using the described methods; for example, future research focused on the elderly, children, or athletes.
5. Limitations
The interpretation of the findings presented in the current study and their applicability requires careful consideration due to the simplifications employed, the modeling approach conducted, and the need for specific patient data to conduct numerical simulations with a significant degree of fidelity. A clear example of these limitations is the non-consideration of bony tissue (neglecting cortical and trabecular structures) that simplified the construction of biological elements in the model; since the 3D model was only focused on foot soft tissues, the model is not adequate to evaluate whole foot structural conditions; along with the utilized boundary conditions where they were based on specific situations, moments, or instants during stance phases of a normal gait, simulating a quasi-static analysis disregarding dynamic considerations; results estimations cannot be directly and precisely compared to experimental data for the simplified assumptions previously stated. Even though similar behavior patterns are provided, clinical assessment by a professional is necessary for any decision-making procedure. Likewise, constant advancements in medical imaging facilitate the development of increasingly sophisticated three-dimensional biological models. This progress allows the incorporation of detailed representations of the complex foot musculature (intrinsic and extrinsic muscles); furthermore, assigning material properties that more accurately reflect biological tissues is required, as only linear elastic, homogeneous, and isotropic properties were assumed for this research work. In addition, the Finite Element model was constructed based on a specific patient approach and not targeted to a vast population; thus, further investigation is needed to be developed in different population groups. In recognition of the manuscript’s limitations, the results were primarily intended to provide a qualitative analysis of the biomechanical pressure distribution effects on the plantar region from a mechanical-computational perspective. This approach acknowledges that the results presented may not represent the exact whole-foot behavior but estimate a suitable prediction of the effects generated in the plantar zone.
6. Conclusions
The current research has provided feasible results predicting plantar pressure points during the different stance phases, even under quasi-static considerations disregarding dynamic conditions, which demonstrates the impact of the Finite Element Method as a powerful tool for analyzing the human body. Applying an innovative and unconventional way to evaluate the foot sole when performing a gait cycle provided a valuable medical-validated database for clinicians to deepen their understanding regarding foot structural behavior. The reconstruction of biological three-dimensional models combined with numerical simulations is remarkably successful in being the short-term assistive methodology for medical procedures, such as surgical planning, prescription of orthopedic devices, rehabilitation therapies, and more knowledgeable biomechanical principles for education. The emerging techniques to design and develop high-performance customized orthopedics focus on 3D-printable materials that are often numerically evaluated before being printed and further used. Likewise, the use of 3D-printing technologies has increasingly been recognized as a standard for orthopedics design and reconstruction due to their high performance, which are advantageous techniques for being time-efficient and affordable compared to traditional procedures. Precisely, the unique patient-specific needs for plantar supports are successfully being achieved by the methods presented in the manuscript, from the 3D patient morphology modeling to numerical simulations that analyze the accuracy of the insole design and cushioning properties assigned to suitably re-distribute excessive pressure points. Thus, all methods described in the current research align with recent advances in foot biomechanics, which aim to revolutionize the footwear and prosthetic lower limb industry, contributing to enhancing rehabilitation treatments and people’s life quality through optimized foot orthotics with the primary goal of achieving specific individual needs.