• View in gallery

    Cross section of honeycomb architecture infilling density parts (from left to right): 40%, 36%, 30%, 26%, 20%, and 16%.

  • View in gallery

    The relationship between honeycomb architecture infilling density and Shore A hardness for thermoplastic polyurethane parts (linear regression modeling) (P < .001).

  • 1. 

    Dunn JE: Prevalence of foot and ankle conditions in a multiethnic community sample of older adults. Am J Epidemiol 159: 491, 2004.

  • 2. 

    Hill CL, Gill TK & Menz HB et al.: Prevalence and correlates of foot pain in a population-based study: the North West Adelaide health study. J Foot Ankle Res 1: 2, 2008.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 3. 

    Rome K, Howe T & Haslock I: Risk factors associated with the development of plantar heel pain in athletes. The Foot 11: 119, 2001.

  • 4. 

    Taunton JE: A retrospective case-control analysis of 2002 running injuries. Br J Sports Med 36: 95, 2002.

  • 5. 

    Irving DB, Cook JL & Menz HB: Factors associated with chronic plantar heel pain: a systematic review. J Sci Med Sport 9: 11, 2006.

  • 6. 

    van Leeuwen KDB, Rogers J & Winzenberg T et al.: Higher body mass index is associated with plantar fasciopathy/‘plantar fasciitis': systematic review and meta-analysis of various clinical and imaging risk factors. Br J Sports Med 50: 972, 2016.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 7. 

    Martin RL, Davenport TE & Reischl SF et al.: Heel pain—plantar fasciitis: revision 2014. J Orthop Sports Phys Ther 44: A1, 2014.

  • 8. 

    Bonanno DR, Landorf KB & Menz HB: Pressure-relieving properties of various shoe inserts in older people with plantar heel pain. Gait Posture 33: 385, 2011.

  • 9. 

    Chia JK, Suresh S & Phua JM et al.: Comparative trial of the foot pressure patterns between corrective orthotics, formthotics, bone spur pads and flat insoles in patients with chronic plantar fasciitis. Ann Acad Med Singap 38: 869, 2009.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 10. 

    McMillan A & Payne C: Effect of foot orthoses on lower extremity kinetics during running: a systematic literature review. J Foot Ankle Res 1: 13, 2008.

  • 11. 

    Mills K, Blanch P & Chapman AR et al.: Foot orthoses and gait: a systematic review and meta-analysis of literature pertaining to potential mechanisms. Br J Sports Med 44: 1035, 2010.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 12. 

    Murley GS, Landorf KB & Menz HB et al.: Effect of foot posture, foot orthoses and footwear on lower limb muscle activity during walking and running: a systematic review. Gait Posture 29: 172, 2009.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 13. 

    Bonanno DR, Landorf KB & Munteanu SE et al.: Effectiveness of foot orthoses and shock-absorbing insoles for the prevention of injury: a systematic review and meta-analysis. Br J Sports Med 51: 86, 2017.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 14. 

    Ritchie C, Paterson K & Bryant AL et al.: The effects of enhanced plantar sensory feedback and foot orthoses on midfoot kinematics and lower leg neuromuscular activation. Gait Posture 33: 576, 2011.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 15. 

    Rebouh C: Principaux matériaux utilisés dans les orthèses plantaires. Podologie [published online, 2008; doi: 10.1016/S0292-062X(08)41965-7].

    • Search Google Scholar
    • Export Citation
  • 16. 

    Mavroidis C, Ranky RG & Sivak ML et al.: Patient specific ankle-foot orthoses using rapid prototyping. J Neuroeng Rehabil 8: 1, 2011.

  • 17. 

    Pallari JHP, Dalgarno KW & Woodburn J: Mass customization of foot orthoses for rheumatoid arthritis using selective laser sintering. IEEE Trans Biomed Eng 57: 1750, 2010.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 18. 

    Telfer S, Abbott M & Steultjens MPM et al.: Dose–response effects of customised foot orthoses on lower limb kinematics and kinetics in pronated foot type. J Biomech 46: 1489, 2013.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 19. 

    Telfer S, Gibson KS & Hennessy K et al.: Computer-aided design of customized foot orthoses: reproducibility and effect of method used to obtain foot shape. Arch Phys Med Rehabil 93: 863, 2012.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 20. 

    Telfer S, Pallari J & Munguia J et al.: Embracing additive manufacture: implications for foot and ankle orthosis design. BMC Musculoskelet Disord 13: 84, 2012.

  • 21. 

    Dombroski CE, Balsdon ME & Froats A: The use of a low cost 3D scanning and printing tool in the manufacture of custom-made foot orthoses: a preliminary study. BMC Res Notes 7: 443, 2014.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 22. 

    Ge Q, Sakhaei AH & Lee H et al.: Multimaterial 4D printing with tailorable shape memory polymers. Sci Rep 6: 31110, 2016.

  • 23. 

    Oftadeh R, Haghpanah B & Vella D et al.: Optimal fractal-like hierarchical honeycombs. Phys Rev Lett 113: 104301, 2014.

  • 24. 

    Ajdari A, Jahromi BH & Papadopoulos J et al.: Hierarchical honeycombs with tailorable properties. Int J Solids Struct 49: 1413, 2012.

  • 25. 

    Wang K, Chang Y-H & Chen Y et al.: Designable dual-material auxetic metamaterials using three-dimensional printing. Materials Design 67: 159, 2015.

  • 26. 

    Bates SRG & Farrow IR Trask RS: 3D printed polyurethane honeycombs for repeated tailored energy absorption. Materials Design 112: 172, 2016.

  • 27. 

    Meththananda IM, Parker S & Patel MP et al.: The relationship between Shore hardness of elastomeric dental materials and Young's modulus. Dental Materials 25: 956, 2009.

  • 28. 

    Bassi AC, Casa F & Mendichi R: Shore A hardness and thickness. Polymer Test 7: 165, 1987.

Are Three-Dimensional–Printed Foot Orthoses Able to Cover the Podiatric Physician's Needs?

Relationship Between Shore A Hardness and Infilling Density

Edem Allado MD, MSc, Mathias Poussel MD, PhD, Isabelle Chary-Valckenaere MD, PhD, Clément Potier MSc, Damien Loeuille MD, PhD, Eliane Albuisson MD, PhD, and Bruno Chenuel MD, PhD
View More View Less

Background

Current management of foot pain requires foot orthoses (FOs) with various design features (eg, wedging, height) and specific mechanical properties (eg, hardness, volume). Development of additive manufacturing (three-dimensional [3-D] printing) raises the question of applying its technology to FO manufacturing. Recent studies have demonstrated the physical benefits of FO parts with specific mechanical properties, but none have investigated the relationship between honeycomb architecture (HcA) infilling density and Shore A hardness of thermoplastic polyurethane (TPU) used to make FOs, which is the aim of this study.

Methods

Sixteen different FO samples were made with a 3-D printer using TPU (97 Shore A), with HcA infilling density ranging from 10 to 40. The mean of two Shore A hardness measurements was used in regression analysis.

Results

Interdurometer reproducibility was excellent (intraclass correlation coefficient, 0.91; 95% confidence interval [CI], 0.64–0.98; P < .001) and interprinter reproducibility was excellent/good (intraclass correlation coefficient, 0.84; 95% CI, 0.43–0.96; P < .001). Linear regression showed a positive significant relationship between Shore A hardness and HcA infilling density (R2 = 0.955; P < .001). Concordance between evaluator and durometer was 86.7%.

Conclusions

This study revealed a strong relationship between Shore A hardness and HcA infilling density of TPU parts produced by 3-D printing and highlighted excellent concordance. These results are clinically relevant because 3-D printing can cover Shore A hardness values ranging from 40 to 70, representing most FO production needs. These results could provide important data for 3-D manufacturing of FOs to match the population needs.

Background

Current management of foot pain requires foot orthoses (FOs) with various design features (eg, wedging, height) and specific mechanical properties (eg, hardness, volume). Development of additive manufacturing (three-dimensional [3-D] printing) raises the question of applying its technology to FO manufacturing. Recent studies have demonstrated the physical benefits of FO parts with specific mechanical properties, but none have investigated the relationship between honeycomb architecture (HcA) infilling density and Shore A hardness of thermoplastic polyurethane (TPU) used to make FOs, which is the aim of this study.

Methods

Sixteen different FO samples were made with a 3-D printer using TPU (97 Shore A), with HcA infilling density ranging from 10 to 40. The mean of two Shore A hardness measurements was used in regression analysis.

Results

Interdurometer reproducibility was excellent (intraclass correlation coefficient, 0.91; 95% confidence interval [CI], 0.64–0.98; P < .001) and interprinter reproducibility was excellent/good (intraclass correlation coefficient, 0.84; 95% CI, 0.43–0.96; P < .001). Linear regression showed a positive significant relationship between Shore A hardness and HcA infilling density (R2 = 0.955; P < .001). Concordance between evaluator and durometer was 86.7%.

Conclusions

This study revealed a strong relationship between Shore A hardness and HcA infilling density of TPU parts produced by 3-D printing and highlighted excellent concordance. These results are clinically relevant because 3-D printing can cover Shore A hardness values ranging from 40 to 70, representing most FO production needs. These results could provide important data for 3-D manufacturing of FOs to match the population needs.

As the sole interface with the ground, the foot is a critical component in locomotion, and it encompasses many pathologies, from musculoskeletal disorders, inflammatory rheumatism, and diabetic foot to recurrent falls in the geriatric population.

In occidental countries, the prevalence of plantar pain is estimated to be 4% to 7% in the general population, 8% to 22% in the athlete population, and 12% to 20% in those older than 75 years.1-4 Risk of plantar pain has been shown to increase with obesity and foot morphology.5,6

To manage foot pain and its common consequences, an arsenal of therapeutics already exists: symptomatic medication treatment with different levels of pain relievers and functional management by physiotherapy and foot orthoses (FOs).7 One of the main effects of FOs is a reduction in plantar pressure peaks through better pressure distribution across the plantar sole.8,9 Several studies also described the influence of FOs in fields that directly concern the foot care spectrum: kinetics, kinematics, muscle activity, and proprioception. Current management of plantar pain involves FOs to treat plantar morphological deformities or modify the load on the injured tissues.10-12

This manual manufacturing requires the use of chemicals (glue) and specific machines, thus exposing the podiatric physicians to health risks: grinding dust, fine particles, and hand traumatism. Moreover, this is a time-consuming activity, with results that may be subject to great variability.

To obtain optimal efficiency, specific mechanical properties of FOs must remain stable over time (eg, hardness, resistance).8,13,14 In Europe, podiatric physicians use several types of materials to reproduce the mechanical properties necessary for FO manufacturing. One of the main properties is the elasticity of materials and, more precisely, their hardness, which is evaluated in Shore A ranging from 0 to 100 (Shore A range: ethyl vinyl acetate, 30–70; polyethylene, 36–70).15

Shore A hardness measures the surface resistance of soft plastics, specifically, elastomers, up to a penetration force of a 1-kg punch (9.8 N) (International Organization for Standardization [ISO] 21509). The unit has a maximum of 100 and a minimum of 0. It is evaluated with a step of one (ie, 93, 94, or 95 of Shore A) and used with a step of 10 in daily life (ie, 10, 20, and 30 of Shore A).

Digital and additive three-dimensional (3-D) printer manufacturing have offered new possibilities for musculoskeletal disorder management and particularly for FO production.16-21 Indeed, the use of 3-D printing allows reproducibility and an automated manufacturing process while maintaining customization to the patient's needs. Currently, several multi-material strategies have been described, but none of them has brought an indisputable reliability. The analysis of internal architectures of 3-D porous structures showed potential access to new mechanical properties.15,22

The aim of this study was to analyze the ability of 3-D printing with a single material to reproduce the hardness properties of traditional FOs. We hypothesized that 3-D printing could produce FOs with durometer characteristics similar to those that FO practitioners currently use.

Methods

Material Characteristics (Filament, Printer, Printing Modalities, and Parts)

In this study, we chose a polyurethane filament with a printing temperature ranging from 195°C to 210°C (thermoplastic polyurethane [TPU] Shore A: 97–1.75 mm diameter).

A fused deposition modeling (FDM) printer was used to realize materials from this wire. An FDM printer was chosen to produce the studied volumetric parts. We used the Bizer Dual MK8 3-D extruder (225 × 145 × 155 mm) (Zhuhai CTC Electronic Co, Zhuhai City, China) with an open source driver system (ReplicatorG 0040) for driver.

Printing modalities were as follows: infill density, 0.2 mm by layer; temperature, 205°C; speed, 30 mm/sec; internal architectures, hexagonal honeycomb architecture (HcA) of 30°; and no 3-D printing support structures.

The 10-mm-thick cylindrical part was designed on Autodesk AutoCAD 2016 Mac (Autodesk Inc, San Rafael, California). The volume file in stl format was transcribed in printer numeric control format (gcode) by Slic3r software, Version 1.2.9.

Outcome: Shore A

The ISO 21509 norms of Shore A hardness measurement impose a minimum thickness of 10 mm to avoid the effect of support on which the part is resting. The analysis was performed using an analytical Shore A Sauter durometer (Sauter GmbH, Balingen, Germany) (Association Française de Normalisation ISO standards). A test bench was used for measurements. The measurement requires a flat surface to allow orthogonal application of the durometer punches on the surface of the part. For this study, it was decided to produce standard cylindrical parts 10 mm thick.

Evaluation of Part Reference.

The ISO 21509 norms also refer to a measurement bias that can be induced by the effect of the parts' edges. A height of 10 mm and a constant infilling density of 20% were used. Thus, parts of different cylinder base diameters (100, 90, 80, 70, 60, 50, 40, 30, and 20 mm) were made to determine the minimum diameter for which there was no impact of this method.

Reproducibility.

An intermachine reproducibility analysis was also performed using a second printer (MakerBot Replicator 2x; MakerBot Industries LLC, Brooklyn, New York). We used the same filament in both printers and the same printer characteristics (temperature, printer speed, infill). An interdurometer reproducibility analysis was achieved with a second Shore A durometer (Bareiss Prüfgerätebau GmbH, Oberdischingen, Germany) (ISO 868 ASTM D 2240). This step ensures the absence of any bias directly related to a manufacturing defect of the printer and/or durometer. Group 1 measures were made by Shore A Sauter Durometer; Group 2 measures were made by Bareiss durometer.

Podiatric Physician Evaluation.

Blind Shore evaluation of the printer sample was performed by an independent podiatric physician. A comparison of his reading and the durometer evaluation was performed. For this analysis we used ten ordinal Shore values (10, 20, 30, 40, 50, 60, 70, 80, 90, and 100). We rounded each durometer's results to the nearest ten.

Modeling the Relationship Between Density and Hardness.

For this study, the mean of the two hardness measures was used for each part, with an HcA infilling density varying from 10% to 40%, with 2% increments. The mean was used for the graphical representation and to study the relationship between Shore hardness A and the infilling density of the HcA part.

Statistical Analyses

According to the sample size, quantitative variables are presented as means, medians, and interquartile ranges (IQRs). Median and mean values are very close as the greatest difference observed is equal to 0.22 (group 2). Concerning IQRs, the greatest is equal to 1.0 (group 2 and MakerBot). To determine the minimum diameter of the part and considering the precision of the measurement, a value of ±2 points from the median was chosen as the threshold to determine an edge effect. Correlation and linear regression analyses were performed to describe and study the association between infilling density and hardness. To analyze the reproducibility, the intraclass correlation coefficient (ICC) was calculated with a 95% confidence interval (CI). Analyses were performed using IBM SPSS Statistics for Windows, Version 23.0 (IBM Corp, Armonk, New York), and P < .05 was considered statistically significant.

Results

Evaluation of Part Reference Diameter of Cylinder Base

The safety measurements of the parts with different diameters showed a constant set function (Table 1). The first and second parts of CTC printing represented groups 1 and 2, respectively.

Table 1.

Measurement of the Shore A Part Effect of a 20% Infilling Density Depending on the Printer, the Durometer, and the Part Diameter

Table 1.

From 30 to 100 mm diameter of cylinder base, the hardness measures remained between 55 and 57 Shore A. We observed a 2-point excess gap from the median for the 20 mm diameter only. This effect was reproduced even after a change of machine or durometer. Thus, the 30 mm diameter has been defined as the reference diameter for parts production in this study.

Interdurometer and Interprinter Reproducibility

An evaluation of interdurometer and interprinter reproducibility was performed on cylindrical parts with 20% HcA infilling density, a 10-mm-high TPU, and different diameters (Table 1). The interdurometer reproducibility analysis revealed an ICC of 0.91 (95% CI, 0.64–0.98; P < .001). The evaluation of the FDM interprinter reproducibility analysis revealed an ICC of 0.84 (95% CI, 0.43–0.96; P < .001).

Podiatric Medical Evaluation

On 30 samples, the concordance between the evaluator and the durometer was 86.7%. All of the discordant cases are in the nearest adjacent category (eg, 40 from Shore A for an assessment by the podiatric physician to 50 from Shore A).

Modeling the Relationship Between Density and Hardness

Two Shore A hardness measurements were made on each TPU part, with density varying from 10% to 40% (Fig. 1) at 2% increments (Table 2). The relationship between HcA infilling density (range, 10%–40%) and Shore A hardness is linear with a positive slope (R2 determination coefficient of 0.955; linear regression P < .001) (Fig. 2).

Figure 1.
Figure 1.

Cross section of honeycomb architecture infilling density parts (from left to right): 40%, 36%, 30%, 26%, 20%, and 16%.

Citation: Journal of the American Podiatric Medical Association 111, 5; 10.7547/20-062

Table 2.

Relationship Between Infilling Density and Shore A Hardness

Table 2.
Figure 2.
Figure 2.

The relationship between honeycomb architecture infilling density and Shore A hardness for thermoplastic polyurethane parts (linear regression modeling) (P < .001).

Citation: Journal of the American Podiatric Medical Association 111, 5; 10.7547/20-062

Discussion

To our knowledge, this is the first study to analyze the relationship between Shore A hardness and HcA infilling density ranging from 10% to 40% of TPU parts produced by 3-D printing. The present results showed a strong positive linear relationship between Shore A hardness and HcA infilling density. We studied the ability of 3-D printing on the basis of a single material to produce and reproduce the hardness properties of traditional FOs.

Printing in 3-D is a process providing a lot of possibilities in the field of mechanical properties thanks to precise structure study, structural prioritizing (pattern repetitions at different printing scales), and infilling density variations. The use of 3-D printing allows transfer of manufacturing time to a machine. This also provides protection of the practitioner's health in relation to dust and chemicals (solvent glue). It is also reported that the price of FDM-type 3-D printers has dropped sharply, making the device more largely accessible to the profession.

However, the change in technique imposes a significant learning curve of 3-D printing's modalities: machine management and computer-aided design. To do so it is essential to implement adapted software to the profession to facilitate the integration of this technique into the daily life of the podiatric physician (integration of footprints, scanning feet or form).

In the present study, we chose to use HcA for the economy of raw material, its lightness, and its absorption properties.23-26

For podiatric medical use, there is a need for material stability and the ability to absorb the shoe-related shear forces. The hexagonal HcA was chosen for its mechanical characteristics and its energy-absorbing capacities. Shore A hardness is the unit used by podiatric physicians in daily practice to adapt plastic and composite materials but has been studied only a few times in scientific research.27 The use of Shore A hardness as a unit allows us to keep an approach that reflects practical care. To obtain the best reproducibility, particular attention was given to the measurement conditions: a thickness of 10 mm in accordance with ISO standards and a test bench of 1 kg. In fact, the influence of the thickness on the measurement is one of the main impediments for this unit. There is actually some evidence of thickness influence on hardness. A variability of up to 6 points for a 2 mm thickness has been demonstrated, the whole stabilizing at an 8 mm diameter.28 In practice, podiatric medicine is an artisanal job, especially in Europe. The excellent concordance results confirmed the 3-D printing possibility in usual FO's manufacture.

Conclusions

This pilot study analyzed the Shore A hardness of test parts with varying infilling densities. Although the results are encouraging, we cannot directly transpose it immediately for podiatric medical use. Further work is needed to evaluate the biocompatibility and the resistance to other stresses perceived by a plantar orthosis. In addition, the results cover 75% of the hardness range used by podiatric physicians. This study made it possible to establish the methodological basis for a future clinical study comparing the effects of traditional FOs and 3-D FOs (with the same material and different Shore A hardnesses). To our knowledge, this is the first study to investigate the relationship between Shore A hardness and 3-D printing architecture. These results confirm the technical feasibility of such an approach in the context of daily podiatric medical practice. However, further studies are needed to assess the resistance and durability of these new materials, and also to compare effectiveness in 3-D printed and traditional FOs. Thus, the implementation of 3-D–printed FOs would be a great advent in the field of plantar disorder care by making this treatment highly reproducible, customizable, and accessible to the greatest number of patients.

Financial Disclosure: None reported.

References

  • 1. 

    Dunn JE: Prevalence of foot and ankle conditions in a multiethnic community sample of older adults. Am J Epidemiol 159: 491, 2004.

  • 2. 

    Hill CL, Gill TK & Menz HB et al.: Prevalence and correlates of foot pain in a population-based study: the North West Adelaide health study. J Foot Ankle Res 1: 2, 2008.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 3. 

    Rome K, Howe T & Haslock I: Risk factors associated with the development of plantar heel pain in athletes. The Foot 11: 119, 2001.

  • 4. 

    Taunton JE: A retrospective case-control analysis of 2002 running injuries. Br J Sports Med 36: 95, 2002.

  • 5. 

    Irving DB, Cook JL & Menz HB: Factors associated with chronic plantar heel pain: a systematic review. J Sci Med Sport 9: 11, 2006.

  • 6. 

    van Leeuwen KDB, Rogers J & Winzenberg T et al.: Higher body mass index is associated with plantar fasciopathy/‘plantar fasciitis': systematic review and meta-analysis of various clinical and imaging risk factors. Br J Sports Med 50: 972, 2016.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 7. 

    Martin RL, Davenport TE & Reischl SF et al.: Heel pain—plantar fasciitis: revision 2014. J Orthop Sports Phys Ther 44: A1, 2014.

  • 8. 

    Bonanno DR, Landorf KB & Menz HB: Pressure-relieving properties of various shoe inserts in older people with plantar heel pain. Gait Posture 33: 385, 2011.

  • 9. 

    Chia JK, Suresh S & Phua JM et al.: Comparative trial of the foot pressure patterns between corrective orthotics, formthotics, bone spur pads and flat insoles in patients with chronic plantar fasciitis. Ann Acad Med Singap 38: 869, 2009.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 10. 

    McMillan A & Payne C: Effect of foot orthoses on lower extremity kinetics during running: a systematic literature review. J Foot Ankle Res 1: 13, 2008.

  • 11. 

    Mills K, Blanch P & Chapman AR et al.: Foot orthoses and gait: a systematic review and meta-analysis of literature pertaining to potential mechanisms. Br J Sports Med 44: 1035, 2010.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 12. 

    Murley GS, Landorf KB & Menz HB et al.: Effect of foot posture, foot orthoses and footwear on lower limb muscle activity during walking and running: a systematic review. Gait Posture 29: 172, 2009.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 13. 

    Bonanno DR, Landorf KB & Munteanu SE et al.: Effectiveness of foot orthoses and shock-absorbing insoles for the prevention of injury: a systematic review and meta-analysis. Br J Sports Med 51: 86, 2017.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 14. 

    Ritchie C, Paterson K & Bryant AL et al.: The effects of enhanced plantar sensory feedback and foot orthoses on midfoot kinematics and lower leg neuromuscular activation. Gait Posture 33: 576, 2011.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 15. 

    Rebouh C: Principaux matériaux utilisés dans les orthèses plantaires. Podologie [published online, 2008; doi: 10.1016/S0292-062X(08)41965-7].

    • Search Google Scholar
    • Export Citation
  • 16. 

    Mavroidis C, Ranky RG & Sivak ML et al.: Patient specific ankle-foot orthoses using rapid prototyping. J Neuroeng Rehabil 8: 1, 2011.

  • 17. 

    Pallari JHP, Dalgarno KW & Woodburn J: Mass customization of foot orthoses for rheumatoid arthritis using selective laser sintering. IEEE Trans Biomed Eng 57: 1750, 2010.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 18. 

    Telfer S, Abbott M & Steultjens MPM et al.: Dose–response effects of customised foot orthoses on lower limb kinematics and kinetics in pronated foot type. J Biomech 46: 1489, 2013.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 19. 

    Telfer S, Gibson KS & Hennessy K et al.: Computer-aided design of customized foot orthoses: reproducibility and effect of method used to obtain foot shape. Arch Phys Med Rehabil 93: 863, 2012.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 20. 

    Telfer S, Pallari J & Munguia J et al.: Embracing additive manufacture: implications for foot and ankle orthosis design. BMC Musculoskelet Disord 13: 84, 2012.

  • 21. 

    Dombroski CE, Balsdon ME & Froats A: The use of a low cost 3D scanning and printing tool in the manufacture of custom-made foot orthoses: a preliminary study. BMC Res Notes 7: 443, 2014.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 22. 

    Ge Q, Sakhaei AH & Lee H et al.: Multimaterial 4D printing with tailorable shape memory polymers. Sci Rep 6: 31110, 2016.

  • 23. 

    Oftadeh R, Haghpanah B & Vella D et al.: Optimal fractal-like hierarchical honeycombs. Phys Rev Lett 113: 104301, 2014.

  • 24. 

    Ajdari A, Jahromi BH & Papadopoulos J et al.: Hierarchical honeycombs with tailorable properties. Int J Solids Struct 49: 1413, 2012.

  • 25. 

    Wang K, Chang Y-H & Chen Y et al.: Designable dual-material auxetic metamaterials using three-dimensional printing. Materials Design 67: 159, 2015.

  • 26. 

    Bates SRG & Farrow IR Trask RS: 3D printed polyurethane honeycombs for repeated tailored energy absorption. Materials Design 112: 172, 2016.

  • 27. 

    Meththananda IM, Parker S & Patel MP et al.: The relationship between Shore hardness of elastomeric dental materials and Young's modulus. Dental Materials 25: 956, 2009.

  • 28. 

    Bassi AC, Casa F & Mendichi R: Shore A hardness and thickness. Polymer Test 7: 165, 1987.

University Center of Sports Medicine and Adapted Physical Activity, University Hospital of Nancy, Nancy, France.

Department of Rheumatology, University Hospital of Nancy, Nancy, France.

Development, Adaptation and Disadvantage, Cardiorespiratory Regulations and Motor Control, University of Lorraine, Nancy, France.

Vence Podiatry, Vence, France.

Université de Lorraine, Centre National de la Recherche Scientifique, Institut Élie Cartan de Lorraine, Nancy, France.

Université de Lorraine, Faculté de Médecine, InSciDenS, Nancy, France.

Corresponding author: Edem Allado, MD, University Center of Sports Medicine and Adapted Physical Activity, University Hospital of Nancy, Rue du Morvan - F-54000, Nancy, France. (E-mail: e.allado@chru-nancy.fr)

Conflict of Interest: None reported.