Investigations into the fat pads of the sole of the foot: anatomy and histology

Changes in functional characteristics of heel fat pad with age

BACKGROUND

This study aimed to investigate age-related changes in the heel fat pad's microchamber and macrochamber layers, particularly focusing on load-induced alterations. Understanding these changes is crucial for elucidating age-related differences in foot mechanics and their potential implications for mobility and comfort.

METHODS

Fifty-five healthy individuals were divided into three age groups: young adults (≤29 years), middle-aged adults (30-44 years), and elderly individuals (≥45 years). Ultrasonic imaging was utilized to measure the thickness of the heel fat pad's microchamber and macrochamber layers under varying load conditions. Thickness, percentage changes, and ratios of load-induced thickness changes were calculated to assess age-related differences.

FINDINGS

Under no-load conditions, both microchamber and macrochamber layers of the heel fat pad were significantly thicker in middle-aged and elderly individuals than in young adults. When load was applied middle-aged and elderly participants exhibited smaller changes in the macrochamber layer and larger changes in the microchamber layer compared to young adults.

INTERPRETATION

Our findings suggest that age influences the structural characteristics and response of the heel fat pad to mechanical loading. Thicker heel fat pad layers in middle-aged and elderly individuals under no-load conditions may reflect age-related changes in fat distribution or composition. Moreover, differences in load-induced thickness changes indicate altered mechanical properties with age, potentially affecting shock absorption and overall foot function. Understanding these age-related variations can help develop interventions aimed at preserving foot health and mobility across the lifespan.

A narrative review of the measurement methods for biomechanical properties of plantar soft tissue in patients with diabetic foot

This article provides an overview of the development history and advantages and disadvantages of measurement methods for soft tissue properties of the plantar foot. The measurement of soft tissue properties is essential for understanding the biomechanical characteristics and function of the foot, as well as for designing and evaluating orthotic devices and footwear. Various methods have been developed to measure the properties of plantar soft tissues, including ultrasound imaging, indentation testing, magnetic resonance elastography, and shear wave elastography. Each method has its own strengths and limitations, and choosing the most appropriate method depends on the specific research or clinical objectives. This review aims to assist researchers and clinicians in selecting the most suitable measurement method for their specific needs.

Mechanical Behaviour of Plantar Adipose Tissue: From Experimental Tests to Constitutive Analysis

Plantar adipose tissue is a connective tissue whose structural configuration changes according to the foot region (rare or forefoot) and is related to its mechanical role, providing a damping system able to adsorb foot impact and bear the body weight. Considering this, the present work aims at fully describing the plantar adipose tissue's behaviour and developing a proper constitutive formulation. Unconfined compression tests and indentation tests have been performed on samples harvested from human donors and cadavers. Experimental results provided the initial/final elastic modulus for each specimen and assessed the non-linear and time-dependent behaviour of the tissue. The different foot regions were investigated, and the main differences were observed when comparing the elastic moduli, especially the final elastic ones. It resulted in a higher level for the medial region (89 ± 77 MPa) compared to the others (from 51 ± 29 MPa for the heel pad to 11 ± 7 for the metatarsal). Finally, results have been used to define a visco-hyperelastic constitutive model, whose hyperelastic component, which describes tissue non-linear behaviour, was described using an Ogden formulation. The identified and validated tissue constitutive parameters could serve, in the early future, for the computational model of the healthy foot.

Effects of extreme cyclic loading on the cushioning performance of human heel pads under engineering test condition

Human heel pads commonly undergo cyclic loading during daily activities. Low cyclic loadings such as daily human walking tend to have less effect on the mechanical properties of heel pads. However, the impact of cyclic loading on cushion performance, a vital biomechanical property of heel pads, under engineering test condition remains unexplored. Herein, dynamic mechanical measurements and finite element (FE) simulations were employed to explore this phenomenon. It was found that the wavy collagen fibers in the heel pad will be straightened under cycle compression loading, which resulted in increased stiffness of the heel pad. The stiffness of the heel pads demonstrated an inclination to escalate over a span of 50,000 loading cycles, consequently resulting in a corresponding increase in peak impact force over the same loading cycles. Sustained cyclic loading has the potential to result in the fracturing of the straightened collagen fibers, this collagen breakage may diminish the stiffness of the heel pad, leading to a reduction in peak impact force. This work enhances understanding of the biomechanical functions of human heel pad and may provide potential inspirations for the innovative development of healthcare devices for foot complex.

The effect of diabetes and tissue depth on adipose chamber size and plantar soft tissue features

BACKGROUND

Plantar ulceration is a serious complication of diabetes. However, the mechanism of injury initiating ulceration remains unclear. The unique structure of the plantar soft tissue includes superficial and deep layers of adipocytes contained in septal chambers, however, the size of these chambers has not been quantified in diabetic or non-diabetic tissue. Computer-aided methods can be leveraged to guide microstructural measurements and differences with disease status.

METHODS

Adipose chambers in whole slide images of diabetic and non-diabetic plantar soft tissue were segmented with a pre-trained U-Net and area, perimeter, and minimum and maximum diameter of adipose chambers were measured. Whole slide images were classified as diabetic or non-diabetic using the Axial-DeepLab network, and the attention layer was overlaid on the input image for interpretation.

RESULTS

Non-diabetic deep chambers were 90 %, 41 %, 34 %, and 39 % larger in area (26,954 ± 2428 µm2 vs 14,157 ± 1153 µm2), maximum (277 ± 13 µm vs 197 ± 8 µm) and minimum (140 ± 6 µm vs 104 ± 4 µm) diameter, and perimeter (405 ± 19 µm vs 291 ± 12 µm), respectively, than the superficial (p < 0.001). However, there was no significant difference in these parameters in diabetic specimens (area 18,695 ± 2576 µm2 vs 16627 ± 130 µm2, maximum diameter 221 ± 16 µm vs 210 ± 14 µm, minimum diameter 121 ± 8 µm vs 114 ± 7 µm, perimeter 341 ± 24 µm vs 320 ± 21 µm). Between diabetic and non-diabetic chambers, only the maximum diameter of the deep chambers differed (221 ± 16 µm vs 277 ± 13 µm). The attention network achieved 82 % accuracy on validation, but the attention resolution was too coarse to identify meaningful additional measurements.

CONCLUSIONS

Adipose chamber size differences may provide a basis for plantar soft tissue mechanical changes with diabetes. Attention networks are promising tools for classification, but additional care is required when designing networks for identifying novel features.

DATA AVAILABILITY

All images, analysis code, data, and/or other resources required to replicate this work are available from the corresponding author upon reasonable request.

Description of the Pliocene marsupial Ambulator keanei gen. nov. (Marsupialia: Diprotodontidae) from inland Australia and its locomotory adaptations

Diprotodontids were the largest marsupials to exist and an integral part of Australian terrestrial ecosystems until the last members of the group became extinct approximately 40 000 years ago. Despite the frequency with which diprotodontid remains are encountered, key aspects of their morphology, systematics, ecology and evolutionary history remain poorly understood. Here we describe new skeletal remains of the Pliocene taxon Zygomaturus keanei from northern South Australia. This is only the third partial skeleton of a late Cenozoic diprotodontid described in the last century, and the first displaying soft tissue structures associated with footpad impressions. Whereas it is difficult to distinguish Z. keanei and the type species of the genus, Z. trilobus, on dental grounds, the marked cranial and postcranial differences suggest that Z. keanei warrants genus-level distinction. Accordingly, we place it in the monotypic Ambulator gen. nov. We, also recognize the late Miocene Z. gilli as a nomen dubium. Features of the forelimb, manus and pes reveal that Ambulator keanei was more graviportal with greater adaptation to quadrupedal walking than earlier diprotodontids. These adaptations may have been driven by a need to travel longer distances to obtain resources as open habitats expanded in the late Pliocene of inland Australia.

Optimization Design of the Inner Structure for a Bioinspired Heel Pad with Distinct Cushioning Property

In the existing research on prosthetic footplates, rehabilitation insoles, and robot feet, the cushioning parts are basically based on simple mechanisms and elastic pads. Most of them are unable to provide adequate impact resistance especially during contact with the ground. This paper developed a bioinspired heel pad by optimizing the inner structures inspired from human heel pad which has great cushioning performance. The distinct structures of the human heel pad were determined through magnetic resonance imaging (MRI) technology and related literatures. Five-layer pads with and without inner structures by using two materials (soft rubber and resin) were obtained, resulting in four bionic heel pads. Three finite element simulations (static, impact, and walking) were conducted to compare the cushioning effects in terms of deformations, ground reactions, and principal stress. The optimal pad with bionic structures and soft rubber material reduced 28.0% peak vertical ground reaction force (GRF) during walking compared with the unstructured resin pad. Human walking tests by a healthy subject wearing the 3D printed bionic pads also showed similar findings, with an almost 20% decrease in peak vertical GRF at normal speed. The soft rubber heel pad with bionic structures has the best cushioning performance, while the unstructured resin pad depicts the poorest. This study proves that with proper design of the inner structures and materials, the bionic pads will demonstrate distinct cushioning properties, which could be applied to the engineering fields, including lower limb prosthesis, robotics, and rehabilitations.

Effect of loading history on material properties of human heel pad: an in-vivo pilot investigation during gait

BACKGROUND

This study was aimed to develop a novel dynamic measurement technique for testing the material properties and investigating the effect of continuous compression load on the structural and mechanical properties of human heel pad during actual gait.

METHODS

The dual fluoroscopic imaging system (DFIS) and dynamic foot-ground contact pressure-test plate were used for measuring the material properties, including primary thickness, peak strain, peak stress, elastic modulus, viscous modulus and energy dissipation rate (EDR), both at time zero and following continuous loading. Ten healthy pilot subjects, aged from 23 to 72 (average: 46.5 ± 17.6), were enrolled. A "three-step gait cycle" is performed for all subjects, with the second step striking at a marked position on the force plate with the heel to maintain the location of the tested foot to be in the view of fluoroscopes. The subjects were measured at both relaxed (time-zero group) and fatigue (continuous-loading group) statuses, and the left and right heels were measured using the identical procedures.

RESULTS

The peak strain, peak stress, elastic modulus, and EDR are similar before and after continuous load, while the viscous modulus was significantly decreased (median: 43.9 vs. 20.37 kPa•s; p < 0.001) as well as primary thicknesses (median: 15.99 vs. 15.72 mm; p < 0.001). Age is demonstrated to be moderately correlated with the primary thicknesses both at time zero (R = -0.507) and following continuous load (R = -0.607). The peak stress was significantly correlated with the elastic modulus before (R = 0.741) and after continuous load (R = 0.802). The peak strain was correlated with the elastic modulus before (R = -0.765) and after continuous load (R = -0.801). The correlations between the viscous modulus and peak stress/ peak strain are similar to above(R = 0.643, 0.577, - 0.586 and - 0.717 respectively). The viscous modulus is positively correlated with the elastic modulus before (R = 0.821) and after continuous load (R = 0.784).

CONCLUSIONS

By using dynamic fluoroscopy combined with the plantar pressure plate, the in vivo viscoelastic properties and other data of the heel pad in the actual gait can be obtained. Age was negatively correlated with the primary thickness of heel pad and peak strain, and was positively correlated with viscous modulus. Repetitive loading could decrease the primary thickness of heel pad and viscous modulus.

Diabetes Status is Associated With Plantar Soft Tissue Stiffness Measured Using Ultrasound Reverberant Shear Wave Elastography Approach

INTRODUCTION

The purpose of this study was to investigate the association between the mechanical properties of plantar soft tissue and diabetes status.

METHOD

51 (M/F: 21/30) participants with prediabetes onset (fasting blood sugar [FBS] level > 100 mg/dL), age >18 years, and no lower limb amputation were recruited after ethical approval was granted from Pontificia Universidad Catolica del Peru ethical review board. Ultrasound reverberant shear wave elastography was used to assess the soft tissue stiffness at the 1st metatarsal head (MTH), 3rd MTH, and the heel at both feet.

RESULTS

Spearman's rank-order correlation (rho) test indicated a significant (P < .05) positive correlations between FBS level and the plantar soft tissue shear wave speed at the 1st MTH: rho = 0.402 (@400 Hz), rho = 0.373 (@450 Hz), rho = 0.474 (@500 Hz), rho= 0.395 (@550 Hz), and rho = 0.326 (@600 Hz) in the left foot and rho = 0.364 (@450 Hz) in the right foot. Mann-Whitney U test indicated a significantly (P < .05) higher shear wave speed in the plantar soft tissue with the following effect sizes (r) at the 1st MTH of the left foot at all tested frequencies: r = 0.297 (@450 Hz), r = 0.345 (@500 Hz), r = 0.322 (@550 Hz), and r = 0.275 (@600 Hz), and at the 1st MTH of right foot r = 0.286 (@400 Hz) in diabetes as compared with the age and body mass index matched prediabetes group.

CONCLUSION

An association between fasting blood sugar level and the stiffness of the plantar soft tissue with higher values of shear wave speed in diabetes versus prediabetes group was observed. This indicated that the proposed approach can improve the assessment of the severity of diabetic foot complications with potential implications in patient stratification.

Comparison of material properties of heel pad between adults with and without type 2 diabetes history: An in-vivo investigation during gait

OBJECTIVE

This study was aimed to compare the material properties of heel pad between diabetes patients and healthy adults, and investigate the impact of compressive loading history and length of diabetes course on the material properties of heel pad.

METHODS

The dual fluoroscopic imaging system (DFIS) and dynamic foot-ground contact pressure-test plate were used for measuring the material properties, including primary thickness, peak strain, peak stress, stiffness, viscous modulus and energy dissipation ratio (EDR), both at time zero and following continuous loading. Material properties between healthy adults and DM patients were compared both at time zero and following continuous weight bearing. After then, comparison between time-zero material properties and properties following continuous loading was performed to identify the loading history-dependent biomechanical behaviour of heel pad. Subgroup-based sensitivity analysis was then conducted to investigate the diabetes course (<10 years vs. ≥10 years) on the material properties of heel pad.

RESULTS

Ten type II DM subjects (20 legs), aged from 59 to 73 (average: 67.8 ± 4.9), and 10 age-matched healthy adults (20 legs), aged from 59 to 72 (average: 64.4 ± 3.4), were enrolled. Diabetes history was demonstrated to be associated with significantly lower primary thickness (t=3.18, p=0.003**), higher peak strain (t=2.41, p=0.021*), lower stiffness (w=283, p=0.024*) and lower viscous modulus (w=331, p<0.001***) at time zero, and significantly lower primary thickness (t=3.30, p=0.002**), higher peak strain (w=120, p=0.031*) and lower viscous modulus (t=3.42, p=0.002**) following continuous loading. The continuous loading was found to be associated with significantly lower primary thickness (paired-w=204, p<0.001***) and viscous modulus (paired-t=5.45, p<0.001***) in healthy adults, and significantly lower primary thickness (paired-w=206, p<0.001***) and viscous modulus (paired-t=7.47, p<0.001***) in diabetes group. No any significant difference was found when conducting the subgroup analysis based on length of diabetes course (<10 years vs. ≥10 years), but the regression analysis showed that the length of diabetes history was positively associated with the peak strain, at time zero (r=0.506, p<0.050) and following continuous loading (r=0.584, p<0.010).

CONCLUSIONS

Diabetes patients were found to be associated with decreased primary thickness and viscous modulus, and increased peak strain, which may contribute to the vulnerability of heel pad to injury and ulceration. Pre-compression history-dependent behaviour is observable in soft tissue of heel pad, with lowered primary thickness and viscous modulus.

Stepping Back to Minimal Footwear: Applications Across the Lifespan

Minimal footwear has existed for tens of thousands of years and was originally designed to protect the sole of the foot. Over the past 50 yr, most footwear has become increasingly more cushioned and supportive. Here, we review evidence that minimal shoes are a better match to our feet, which may result in a lower risk of musculoskeletal injury.

Can plantar fibroblast implantation protect amputees from skin injury? A recipe for skin augmentation

Skin injuries remain a persistent problem for users of lower-limb prostheses despite sustained progress in prosthesis design. One factor limiting the prevention of skin injuries is that skin on the residual limb is not suited to bear the mechanical loads of ambulation. One part of the body that is suited to this task is the sole of the foot. Here, we propose a novel strategy to actively augment skin's tolerance to load, increasing its resistance to mechanically induced injuries. We hypothesise that the load tolerance of skin can be augmented by autologous transplantation of plantar fibroblasts into the residual limb dermis. We expect that introducing plantar fibroblasts will induce the overlying keratinocytes to express plantar-specific keratins leading to a tougher epidermis. Using a computational finite element model of a weight-bearing residual limb, we estimate that skin deformation (a key driver of pressure ulcer injuries) could be halved by reprogramming skin to a plantar-like phenotype. We believe this strategy could yield new progress in pressure ulcer prevention for amputees, facilitating rehabilitation and improving quality of life for patients.

A Step in the Right Direction: A Prospective Randomized, Controlled Crossover Trial of Autologous Fat Grafting for Rejuvenation of the Heel

BACKGROUND

The shock-absorbing soft tissues of the heel are composed of dermis and specialized fat pads. Heel fat pad atrophy is common and can be painful and debilitating. In our previous work, autologous fat grafting was effective for treating pain from forefoot fat pad atrophy.

OBJECTIVES

The authors hypothesized that autologous fat grafting to the heel would relieve pain and improve function in patients with heel fat pad atrophy.

METHODS

Patients with heel fat pad atrophy and associated pain were recruited and randomized into 2 groups. Group 1 received autologous fat grafting on enrollment and was followed for 2 years. Group 2 received offloading and activity modification for 1 year, then crossed over, underwent autologous fat grafting, and was followed for 1 year afterward. Outcome measures included ultrasound-measured fat pad and dermal thickness; pedobarograph-measured foot pressures and forces; and patient-reported outcomes as measured by the Manchester Foot Pain and Disability Index.

RESULTS

Thirteen patients met the inclusion criteria and completed the study. Seven (12 affected feet) were randomized into Group 1; and 6 (9 affected feet) were randomized into Group 2. The average age was 55 years and BMI was 30.5 kg/m2. Demographics did not significantly differ between groups. Heel fat pad thickness increased after autologous fat grafting but returned to baseline at 6 months. However, autologous fat grafting increased dermal thickness significantly and also increased fat pad thickness under a compressive load compared with controls at 6 and 12 months. Foot pain, function, and appearance were also significantly improved compared with controls at 6 and 12 months.

CONCLUSIONS

Autologous fat grafting improved patient-reported foot pain, function, and appearance and may rejuvenate local soft tissues in patients with heel fat pad atrophy.

LEVEL OF EVIDENCE: 3

Variations in plantar pressure variables across elliptical trainers in older adults

BACKGROUND

Ellipticals are used to address walking and cardiorespiratory training goals of older adults, some of whom are at risk for foot injuries. Variations in joint kinematics and muscle demands when using different ellipticals could lead to plantar pressure differences. This study explored plantar pressure variables during gait and use of four ellipticals.

METHODS

Plantar pressures were recorded while 10 adults [68.1 (4.5) years] walked and used the True, Octane, Life Fitness, and SportsArt ellipticals. Repeated-measures ANOVAs (5 × 1) identified forefoot and heel differences across conditions.

FINDINGS

Maximum forefoot forces and peak pressures were significantly lower than walking for each elliptical condition with one exception (Life Fitness peak pressure). However, sustained elliptical pedal contact time contributed to forefoot pressure-time integrals and dosages (i.e., cumulative pressure during one minute of activity) not varying significantly amongst elliptical and walking conditions. Heel maximum forces and peak pressures were significantly lower than walking during all elliptical conditions except SportsArt. Heel contact time on SportsArt and Octane exceeded walking, and SportsArt heel contact time exceeded Life Fitness. Heel pressure-time integral was greater on SportsArt compared to walking, Life Fitness, and True. Sports Art heel dosage exceeded Life Fitness and True.

INTERPRETATION

While elliptical training's sustained double limb support diminished maximal forces and peak pressures under the forefoot and heel compared to walking, each ellipticals' pressure-time integral and dosage were not significantly lower than walking. These findings point to the importance of carefully initiating elliptical training programs to minimize tissue injury, particularly if sensory neuropathy is present.

Medial Plantar Artery Flap for Wound Coverage of the Weight-Bearing Surface of the Heel

Covering soft tissue defects of the heel is particularly challenging because of the highly specialized functional units unique to the plantar fat pad and the shear and compressive forces experienced in this area. The medial plantar artery fasciocutaneous flap provides the unique ability to restore both sensation and the functional units to the plantar heel by taking tissue similar to that which was lost and relocating it from a non-weight-bearing portion of the foot, while maintaining minimal host morbidity. This provides a lasting solution and may prevent future ulcerations from occurring.

Biomechanical insights into the role of foot pads during locomotion in camelid species

From the camel’s toes to the horse’s hooves, the diversity in foot morphology among mammals is striking. One distinguishing feature is the presence of fat pads, which may play a role in reducing foot pressures, or may be related to habitat specialization. The camelid family provides a useful paradigm to explore this as within this phylogenetically constrained group we see prominent (camels) and greatly reduced (alpacas) fat pads. We found similar scaling of vertical ground reaction force with body mass, but camels had larger foot contact areas, which increased with velocity, unlike alpacas, meaning camels had relatively lower foot pressures. Further, variation between specific regions under the foot was greater in alpacas than camels. Together, these results provide strong evidence for the role of fat pads in reducing relative peak locomotor foot pressures, suggesting that the fat pad role in habitat specialization remains difficult to disentangle.

Identification of potential plantar ulceration among diabetes patients using plantar soft tissue stiffness

This study investigates the relationship between plantar tissue stiffness and selected parameters, including age, diabetes mellitus (DM) duration, body mass index (BMI), and HbA_1c level. 70 diabetes patients with no foot problems were recruited. The plantar soft tissue at the 2nd sub-metatarsal head (MTH) pad was examined using the novel indentation system developed. The stiffness constant, K, was used to describe the tissue stiffness. The four factors (age, DM duration, BMI, and HbA_1c level) were plotted against the plantar tissue stiffness. The scatter plots revealed that a higher plantar tissue stiffness was usually associated with (1) BMI>25  kgm^-2, (2) HbA_1c score >10% (86 mmol/mol), and (3) DM duration >10 years. The three risk criteria were further evaluated using the binary classification test. The predictions were reported to be fairly accurate and reliable in detecting stiffened tissues. The study has successfully identified the strong association of BMI, HbA_1c, and DM duration with the plantar tissue properties. Special attention should be given to the high risk group with BMI>25 kgm^-2, HbA_1c score >10% (86 mmol/mol), and DM duration >10 years. The high diagnostic odds ratio attained suggests its potential usefulness in helping clinicians to diagnose diabetic foot more efficiently.

The thickness of heel fat-pad in patients with plantar fasciitis

Objective

The aim of this study was to investigate the thickness of heel fat pad (THP) and to detect the relationship between the plantar fasciitis (PF) and age, occupation, BMI, longitudinal arch, the thickness of heel fat-pad in the patients with PF.

Methods

A total of 50 patients (29 women and 21 men; mean age: 46.5 years (range: 22–70)) that were diagnosed with PF were included to this study. Patients' affected side were compared with the healthy opposite side with the angle of medial arch (AMA) and first metatarsophalangeal angle (FMTPA) on the foot radiograms, and THP and thickness of first metatarsal fat pad (TFMFP) using ultrasonography (USG) of both feet.

Results

The mean AMAs of feet with pain and without pain were 122.56° and 120.60°, respectively. The mean FMTPAs of feet with pain and without pain were 14.72° and 14.40°, respectively. The mean THPs of feet with pain at the point of the medial calcaneal tubercle and the mean TFMFPs of the feet with pain at the point of the first metatarsal head were 19.45 mm and 6.75 mm, respectively. The mean THPs of feet without pain at the point of the medial calcaneal tubercle and the mean TFMFPs of the feet without pain at the point of the first metatarsal head were 19.94 mm and 6.75 mm, respectively. It was observed that the mean AMA in the heels with pain was significantly higher than that of the heel without pain (p < 0.05) and the mean THP in the heels with pain was significantly thinner than that of the heel without pain (p < 0.05).

Conclusion

The results indicate that USG is an accurate and reliable imaging technique for the measurement of THP in the diagnosis of plantar fasciitis and the heel pad was thinner in the painful heels of patients with plantar fasciitis.

Level of evidence

Level III, Diagnostic Study.

Morphology and composition play distinct and complementary roles in the tolerance of plantar skin to mechanical load

Plantar skin on the soles of the feet has a distinct morphology and composition that is thought to enhance its tolerance to mechanical loads, although the individual contributions of morphology and composition have never been quantified. Here, we combine multiscale mechanical testing and computational models of load bearing to quantify the mechanical environment of both plantar and nonplantar skin under load. We find that morphology and composition play distinct and complementary roles in plantar skin's load tolerance. More specifically, the thick stratum corneum provides protection from stress-based injuries such as skin tears and blisters, while epidermal and dermal compositions provide protection from deformation-based injuries such as pressure ulcers. This work provides insights into the roles of skin morphology and composition more generally and will inform the design of engineered skin substitutes as well as the etiology of skin injury.

Comprehensive Biomechanism of Impact Resistance in the Cat's Paw Pad

Cats are able to jump from a high-rise without any sign of injury, which is attributed in large part to their impact-resistant paw pads. The biomechanical study of paw pads may therefore contribute to improving the impact resistance of specific biomimetic materials. The present study is aimed at investigating the mechanics of the paw pads, revealing their impact-resistant biomechanism from macro- and microscopic perspectives. Histological and micro-CT scanning methods were exploited to analyze the microstructure of the pads, and mechanical testing was conducted to observe the macroscopic mechanical properties at different loading frequencies. Numerical micromodels of the ellipsoidal and cylindrical adipose compartments were developed to evaluate the mechanical functionality as compressive actions. The results show that the stiffness of the pad increases roughly in proportion to strain and mechanical properties are almost impervious to strain rate. Furthermore, the adipose compartment, which comprises adipose tissue enclosed within collagen septa, in the subcutaneous tissue presents an ellipsoid-like structure, with a decreasing area from the middle to the two ends. Additionally, the finite element results show that the ellipsoidal structure has larger displacement in the early stage of impact, which can absorb more energy and prevent instability at touchdown, while the cylindrical structure is more resistant to deformation. Moreover, the Von Mises of the ellipsoidal compartment decrease gradually from both ends to the middle, making it change to a cylindrical shape, and this may be the reason why the macroscopic stiffness increases with increasing time after contact. This preliminary investigation represents the basis for biomechanical interpretation and can accordingly provide new inspirations of shock-absorbing composite materials in engineering.

In Vivo Measurement of Plantar Tissue Characteristics and Its Indication for Foot Modeling

Plantar heel pain is one of the most common musculoskeletal disorders and generally causing long term discomfort of the patients. The objective of the present study is to combine in vivo experimental measurements and finite element modelling of the foot to investigate the influences of stiffness and thickness variation of individual plantar tissues especially the heel pad on deformation behaviours of the human foot. The stiffness and thickness variance of individuals were measured through supersonic shear wave elastography considering detailed heel pad layers refered to in literature as: dermis, stiffer micro-chamber layer, softer macro-chamber layer. A corresponding foot model with separated heel pad layers was established and used to a sensitivity analysis related to the variance of above-mentioned tissue characteristics. The experimental results show that the average stiffness of the micro-chamber layer ranged from 24.7 (SD 2.4) kPa to 18.8 (SD 3.5) kPa with the age group increasing from 20-29 years old to 60-69 years old, while the average macro-chamber stiffness is 10.6 (SD 1.5) kPa that appears to slightly decrease with the increasing age. Both plantar soft tissue stiffness and thickness of male were generally larger than that of female. The numerical simulation results show that the variance of heel pad strain level can reach 27.5% due to the effects of stiffness and thickness change of the plantar tissues. Their influences on the calcaneus stress and plantar pressure were also significant. This indicates that the most appreciate way to establish a personalized foot model needs to consider the difference of both individual foot anatomic geometry and plantar soft tissue material properties.

FEM analysis in excellent cushion characteristic of ostrich (Struthio camelus) toe pads

African ostrich (Struthio camelus) is the largest and fastest extent bipedal animal. The ostrich mainly relies on the 3rd toe to support the entire body under high-speed motion. The short and severe impact concentrated on the limited area would produce tremendous momentary internal stress and strain, which may contribute to the phalanges disloaction, soft tissue damage and the like. The vibration and excessive negative acceleration caused by the ground reaction force also affect the stability of the touching process. Therefore, ostrich toe pads are required to have excellent cushion characteristics. However, current studies often explains the cushion properties by analyzing the macro-microscopic structure of the pad organism, and there is a paucity of research on its biomechanical behaviour. Consequently, from the perspective of multi-layer structure and biomaterial assembly, this study aims to explain the biomechanical characteristics of the ostrich toe pads by FEM (Finite Element Method) analysis. Based on results, we deem that the ostrich toe pad could absorb energy and reduce vibration effectively. Firstly, the multi-layer structure of the pads make the stress and strain decay from outside to inside. Secondly, the minimal response frequency of the pad is 164.22 Hz, making it effectively avoid resonance phenomenon. Finally, the composite material model has the best performance in decreasing the negative acceleration peak value, the impact force peak value and the maximal equivalent stress value at velocities of 0.669 m/s and 1.339 m/s. These results help to further understand the buffer mechanism of the ostrich toe pad, and also have important inter-species reference value for the pathogenesis of human foot soft tissue injury.

Compressed Fixation Combined with Vacuum-Assisted Closure for Treating Acute Injury of the Heel Fat Pad

Background

Treating acute injury of the heel fat pad is different from treating common soft tissue damage. Due to the paucity of literature on the topic, we described our initial experience treating acute injury of the heel fat pad to determine the ideal treatment method.

Material/Methods

A total of 53 patients with acute injury of the heel fat pad admitted to our hospital were selected for the study and were randomly divided into 2 groups: the compressed fixation combined with vacuum-assisted closure group and the only reimplanted and sewn group. Twenty-seven of the heel fat pads were compressed and fixed using a flat, hard piece of plastic and hollow screws; then, they were covered with a vacuum-assisted closure device. The other 27 were only sewn without tension. The clinical results were evaluated according to the American Orthopedic Foot and Ankle Society hindfoot score and the British Medical Research Council function evaluation criteria

Results

In the compressed fixation combined with vacuum-assisted closure group, flaps of 12 feet with retrograde avulsion injury survived successfully. Partial flap necrosis occurred in 8 feet. Seven feet underwent repair using the neurocutaneous vascular resupinated island flap. Results were excellent or good for 74% of patients according to the AOFS. However, in the only reimplanted and sewn group, results were excellent or good for 44% of patients according to the AOFS.

Conclusions

Compressed fixation with vacuum-assisted closure is effective for treating acute injury of the heel fat pad, with high success rates and good utility.

Histomorphological and biochemical properties of plantar soft tissue in diabetes

BACKGROUND

Diabetes results in pathophysiological changes, leading to tissue that is unable to withstand and adapt to the same loads, resulting in breakdown. Certain locations are more susceptible to breakdown, yet differences between locations are largely not well understood. The authors performed a histological and biochemical analysis of isolated plantar adipose tissue at six relevant locations.

METHODS

Tissue from six plantar locations (hallux, first, third and fifth metatarsal heads, lateral midfoot and calcaneus) was taken from fresh cadaveric feet of older diabetic and older non-diabetic intact donors. Histomorphological and biochemical analysis of isolated plantar tissue from both diabetic and non-diabetic feet at six relevant locations was performed.

RESULTS

The main differences found between diabetic and non-diabetic tissue were in the thickness of the septal walls and the elastin content. Diabetic tissue had significantly thicker septal walls and an increased elastin concentration. When comparing the calcaneus to other locations, although there were no differences found in the thickness of the septal walls of diabetic tissue, elastin content was lower in the calcaneous tissue compared to the non-calcaneus sites.

CONCLUSIONS

Modifications in the structural and biochemical properties could translate to changes in the mechanical properties. This information could lead to an understanding of how the structural and biochemical changes result in an increase in susceptibility of tissue to breakdown with load at the different locations of the foot.

A comparison of axial circumference between superficial circumflex iliac artery perforator flap and other workhorse flaps in dorsal foot reconstruction

BACKGROUND

Management and reconstruction of dorsal foot defects present various difficulties and challenges. The main purpose of this article is to discuss experiences of using superficial circumflex iliac perforator (SCIP) flap in various defects of the dorsal foot region.

METHODS

From August 2012 to February 2015, a total of 13 patients - nine males and four females - received a reconstruction operation for dorsal foot defects using SCIP flaps (the SCIP group). The defects were caused by trauma (n = 9), diabetes (n = 3), and malignancy (n = 1). The mean age was 43.07 years, which ranged between 19-70 years. Additionally, 19 other patients, who underwent reconstruction operation of the foot dorsum using ALT and TDAP flaps during the same study period, were recruited for comparison. The axial circumference of the operated foot and unoperated contralateral foot was measured, and the difference were compared between all patients, as well as between the two groups.

RESULTS

All of the flaps survived after the operation, but secondary procedures were performed in two cases due to partial skin necrosis. The mean follow-up period was 13 months. The mean axial circumference discrepancy of the SCIP group (12.08 ± 2.96 mm) was significantly lower compared with ALT (25.21 ± 3.16 mm) and TDAP (29.88 ± 1.55 mm) groups (p < 0.01).

CONCLUSIONS

The authors experienced good results with using the SCIP flap in dorsal foot reconstruction. Reconstruction with the SCIP flap can be a good surgical option, with better postoperative symmetry and minimal donor sequelae, compared to conventional workhorse flaps.

The association between mechanical and biochemical/histological characteristics in diabetic and non-diabetic plantar soft tissue

Diabetes, and the subsequent complication of lower limb ulcers leading to potential amputation, remains an important health care problem in United States, even with declining amputation rates. It has been well documented that diabetes can alter the mechanical properties (i.e., increased stiffness) of the plantar soft tissue, although this finding is not universal. Similarly, biochemical, and histological changes have been found in the plantar soft tissue, but, as with the mechanical changes, these findings are not consistent across all studies. Our group׳s work has demonstrated that diabetes increases plantar soft tissue modulus and increases elastic septal thickness. The purpose of the current study was to explore the association between mechanical, biochemical and histological properties. Using previously collected data, a linear mixed effects regression was conducted. The correlations were weak; of the 32 that were tested, only 3 (modulus to septal thickness when location was accounted for, energy loss to total collagen, and energy loss to collagen/elastin ratio) were statistically significant, none with an R2 greater than 0.10. The main differences in the means were increased tissue stiffness and increased septal wall thickness, both trends were supported in the literature. However, as the correlations were weak, it is likely that another unexamined biochemical factor (perhaps collagen crosslinking) is associated with the mechanical tissue changes.

Material properties of the heel fat pad across strain rates

The complex structural and material behaviour of the human heel fat pad determines the transmission of plantar loading to the lower limb across a wide range of loading scenarios; from locomotion to injurious incidents. The aim of this study was to quantify the hyper-viscoelastic material properties of the human heel fat pad across strains and strain rates. An inverse finite element (FE) optimisation algorithm was developed and used, in conjunction with quasi-static and dynamic tests performed to five cadaveric heel specimens, to derive specimen-specific and mean hyper-viscoelastic material models able to predict accurately the response of the tissue at compressive loading of strain rates up to 150 s⁻¹. The mean behaviour was expressed by the quasi-linear viscoelastic (QLV) material formulation, combining the Yeoh material model (C10=0.1MPa, C30=7MPa, K=2GPa) and Prony׳s terms (A1=0.06, A2=0.77, A3=0.02 for τ1=1ms, τ2=10ms, τ3=10s). These new data help to understand better the functional anatomy and pathophysiology of the foot and ankle, develop biomimetic materials for tissue reconstruction, design of shoe, insole, and foot and ankle orthoses, and improve the predictive ability of computational models of the foot and ankle used to simulate daily activities or predict injuries at high rate injurious incidents such as road traffic accidents and underbody blast.

Differences in the mechanical characteristics of plantar soft tissue between ulcerated and non-ulcerated foot

AIMS

The purpose of this study was to investigate the differences in mechanical properties of the plantar soft tissue between the ulcerated and non-ulcerated feet in patients with diabetic neuropathy.

METHODS

Thirty nine patients who met the inclusion criteria participated in this study. Ten out of 39 participants had an active ulcer at a site other than the plantar heel and the first metatarsal head. Real time ultrasound elastography was performed to measure the soft tissue thickness and stiffness of the heel pad and sub-metatarsal fat pad. To account for the qualitative nature of conventional real time elastography, relative tissue stiffness was assessed against that of a standardised ultrasound standoff material.

RESULTS

The results indicated that the ulcerated group had a significantly lower heel pad relative stiffness (t (37)=2.559, P=0.015, η2=0.150) in the left foot.

CONCLUSIONS

The observed difference in the stiffness of the heel pad between the ulcerated and non-ulcerated feet indicates a possible link between tissue mechanics and ulceration. Further analysis of the data proposed in this study provided a quantitative assessment of plantar fat pad deformability which can contribute to understanding the role of tissue biomechanics in ulceration.

Heel Pad Stiffness in Plantar Heel Pain by Shear Wave Elastography

The goal of the study was to evaluate the reliability of supersonic shear wave elastography in measuring heel pad stiffness and the change in heel pad stiffness in patients with plantar heel pain. In the reliability test involving 12 normal participants, each heel pad was tested six times in succession, and adequate reliability was reflected in the intraclass correlation coefficients (0.95, 0.93 and 0.96 for the microchambers, macrochambers and bulk heel pad, respectively). In the clinical assessment involving 20 normal participants and 16 unilateral plantar heel pain patients, diseased heel pads (86.8 ± 22.9, 36.8 ± 7.7 and 46.6 ± 10.9 kPa for the microchambers, macrochambers and bulk heel pad, respectively) were significantly stiffer than unaffected heel pads (66.8 ± 14.1, 25.2 ± 5.7, 34.2 ± 6.6 kPa) and those of normal participants (60.9 ± 11.4, 26.3 ± 6.1, 31.8 ± 6.3 kPa), suggesting that the heel pad with plantar heel pain was associated with loss of elasticity.

Minimum indentation depth for characterization of 2nd sub-metatarsal head and heel pad tissue properties

Most in-vivo indentation techniques are limited by the lack of adequate indentation into the plantar soft tissue. The purpose of this study is therefore to assess the effect of deformation depth on plantar soft tissue behavior and to establish a guideline for the minimum indentation depth that is sufficient to quantify critical plantar soft tissue behavior. Twenty young subjects (20-25 years) participated in this study. The test was conducted with equal weight borne on each of the participants׳ feet to mimic the static stance of the gait cycle. During the experiment, the indenter probed the 2nd sub-metatarsal head (MTH) and heel pad tissue at a constant rate of 12.3 mm/s. The maximum tissue deformation induced was varied from 1.2 mm to 6.0 mm, in steps of 1.2 mm. The tissue stiffness obtained from the tissue response curves was compared and fitted to the proposed viscoelastic model. As the probe tip indents deeper into the plantar soft tissue beyond a threshold depth, Xs, the force gradient increases notably. The absolute value of Xs was approximately 2.23 mm and 2.14 mm at the heel and 2nd sub-MTH respectively. Indentation depths which were less than this threshold depth might not be representative of the nature of plantar soft tissue nor reflect the critical deformation it experiences during physical activities that expose the tissue to risk of ulceration. Our study indicated the necessity to induce a minimum tissue indentation depth in order to describe its actual characteristics. By doing so, additional useful parameters can be obtained to identify potentially abnormal soft tissue.

Plantar loading reflects ulceration risks of diabetic foot with toe deformation

Diabetes has been one of the most common chronic diseases all over the world. The purpose of this study was to quantitatively assess the foot loading characteristics of diabetic patients with fifth-toe deformity through a comparative analysis with diabetic patients with healthy and normal feet. Six neuropathic diabetic female subjects with the fifth-toe deformation and six age-matched neuropathic diabetic controls without any feet deformities participated in the walking test. Dynamic barefoot plantar pressure was measured with Novel EMED force plate. Peak pressure and pressure-time integral for all 7 foot regions (rearfoot, midfoot, lateral forefoot, central forefoot, medial forefoot, great toe, and other toes) were collected. Peak pressure was significantly higher in the patients with toe deformity in rearfoot, central forefoot, and great toe regions compared with the control group. Meanwhile, loading sustaining period extended longer in great toe region of deformed group than in that of the control group, and the center of pressure was nearly in the big toe region during toe offstage. Diabetic patients with fifth-toe deformity could have plantar contact area reduction in the other toes part and increased loading to the great toe part. The result showed that fifth-toe deformity was associated with potential ulceration risk especially in hallux region.

Increased substance P expression in the trochanteric bursa of patients with greater trochanteric pain syndrome

Greater trochanteric pain syndrome (GTPS) is a pathology that can involve the trochanteric bursa or the tendons which attach to the greater trochanter. To clarify the potential importance of bursa versus tendon pathology and of substance P (SP) in contributing to pain in this condition tendon and bursa tissue biopsies were obtained from 34 patients with GTPS and 29 control subjects. Specimens were evaluated via light microscopy for histopathological and morphological differences, as well as using immunohistochemistry for macrophages (CD68), inflammatory cells (CD45) and SP. Bursa [stroma score, mean (SD): 4.18 (1.65) vs. 2.53 (1.61), p = 0.051] and tendon [Bonar score, mean (SD): GTPS mean (SD) 12.65 (2.0), control (10.43 (4.84), p = 0.04] from subjects with GTPS demonstrated more extensive signs of pathology than specimens from control subjects. There was a significantly greater presence of SP in the bursa (frequency: 9/12 vs. 6/16, p = 0.047), but not in the tendon (8/12 vs. 8/15, p = 0.484) of subjects with GTPS compared to controls. An increased presence of SP in the trochanteric bursa may be related to the pain associated with GTPS.

High heritability of hallux valgus and lesser toe deformities in adult men and women

OBJECTIVE

To estimate the heritability of 3 common disorders affecting the forefoot, i.e., hallux valgus, lesser toe deformities, and plantar forefoot soft tissue atrophy, in white adult men and women.

METHODS

Between 2002 and 2008, a trained examiner used a validated foot examination to document the presence of hallux valgus, lesser toe deformities, and plantar soft tissue atrophy in 2,446 adults from the Framingham Foot Study. Among these, 1,370 participants with an available pedigree structure were included. Heritability was estimated using pedigree structures by the Sequential Oligogenic Linkage Analysis Routines package. Results were adjusted for age, sex, and body mass index.

RESULTS

The mean age of the participants was 66 years (range 39-99 years) and 57% were women. The prevalence of hallux valgus, lesser toe deformities, and plantar soft tissue atrophy was 31%, 29.6%, and 28.4%, respectively. Significant heritability was found for hallux valgus (range 0.29-0.89, depending on age and sex) and lesser toe deformity (range 0.49-0.90, depending on age and sex). The heritability for lesser toe deformity in men and women ages >70 years was 0.65 (P = 9 × 10(-7)). Significant heritability was found for plantar soft tissue atrophy in men and women ages >70 years (H(2) = 0.37, P = 3.8 × 10(-3)).

CONCLUSION

To our knowledge, these are the first findings of heritability of foot disorders in humans, and they confirm the widely-held view that hallux valgus and lesser toe deformities are highly heritable in white men and women of European descent, underscoring the importance of future work to identify genetic determinants of the underlying genetic susceptibility to these common foot disorders.

The plantar fat pad and the diabetic foot--a review

There has been much debate concerning the pathologic consequences of diabetes on the plantar fat pad and its subsequent association with the development of a foot ulcer. This review article documents two theories regarding pathophysiology in diabetic foot ulcer formation as they are related to the plantar fat pad and discusses current treatment options for this pathophysiological phenomenon. Traditionally, fat pad atrophy in diabetic patients was thought to result as an irregular arrangement of collagen fibrils within the septal walls as a result of glycation as well as diminishing adipocyte size due to thickened septal walls. Contrary to this traditional theory, a model depicting distal fat pad migration from under the metatarsal heads has been described in the diabetic patient. Such pad migration renders the metatarsal heads vulnerable to increased pressure, which, in turn, predisposes to foot ulceration. This migratory fat pad theory plays a significant role in approaches to the prevention of diabetic foot ulceration and subsequent amputation. Various methods of fat pad supplementation and claw toe management are impacted by the pathophysiological changes described and new avenues of therapy may be based on these changes.

A stereologic study of the plantar fat pad in young and aged rats

Plantar fat pad (PFP) is a tissue structure that absorbs the initial impact of walking and running and ultimately bears body weight at standing. This study was designed to quantify the histomorphological changes of the PFP in aged rats. The most medial PFP was dissected from the hind feet of young rats (4 months old, n = 6) and aged rats (24 months old, n = 6). Histological structure and cellular senescence of PFP were analyzed stereologically and histomorphometrically. Immunohistochemistry of matrix metalloproteinase 9 (MMP9) was also performed on PFP tissue sections. Compared with young rats, the thickness of epidermis, dermis and septa of the PFP were significantly reduced in the aged rats. The total volume of adipose tissue in the PFP of aged rats was only about 65% of that in the young rats. The microvascular density and the number of fat pad units (FPU), a cluster of adipocytes enclosed by elastin septa, in the PFP were unchanged in the aged rats. In the aged rats, the number of adipocytes per FPU was reduced but the number of simple adipocyte clusters, without surrounding septa, was increased. The shift of the types of adipocyte clusters in the aged PFP was accompanied by degradation of elastin fibers and increased expression of MMP9. In conclusion, the PFP, particularly the elastic septa, degenerates significantly in aged rats and this may contribute to the pathology of PFP-related diseases.

Quantitative evaluation of the viscoelastic properties of the ankle joint complex in patients suffering from ankle sprain by the anterior drawer test

PURPOSE

Biological tissues such as ligaments exhibit viscoelastic behaviours. Injury to the ligament may induce changes of these viscoelastic properties, and these changes could serve as biomarkers to detect the injury. In the present study, a novel instrument was developed to non-invasive quantify the viscoelastic properties of the ankle in vivo by the anterior drawer test. The purpose of the study was to investigate the reliability of the instrument and to compare the viscoelastic properties of the ankle between patients suffering from ankle sprain and controls.

METHODS

Eight patients and eight controls participated in the present study. The reliability test was performed on three randomly chosen subjects. In patient and control test, both ankles of each subject were tested to evaluate the viscoelastic properties of the ankle. The viscosity index was defined for quantitatively evaluating the viscosity of the ankle. Greater viscosity index was associated with lower viscosity. Injured and uninjured ankles of patient and both ankles of controls were compared.

RESULTS

The instrument exhibited excellent test-retest reliability (r > 0.9). Injured ankles exhibited significantly less viscosity than uninjured ankles, since injured ankles of patients had significantly higher viscosity index (8,148 ± 5,266) compared with uninjured ankles of patients (948 ± 617; p = 0.008) and controls (1,326 ± 613; p < 0.001).

CONCLUSIONS

The study revealed that the viscoelastic properties of the ankle can serve as sensitive and useful clinical biomarkers to differentiate between injured and uninjured ankles. The method may provide a clinical examination for objectively evaluating lateral ankle ligament injuries.

The mangled foot and ankle: results from a 2-year prospective study

OBJECTIVE

To determine the outcome of the mangled foot and ankle undergoing limb salvage surgery that required free tissue flaps for wound closure compared with a similar patient foot and ankle injury group that underwent early below knee amputation (BKA).

DESIGN

: Prospective longitudinal study.

SETTING

: Eight level 1 trauma centers.

PATIENTS/PARTICIPANTS

LEAP (Lower Extremity Assessment Project) study. One hundred seventy-four open severely injured hindfoot or ankle injuries (116 had salvage; 58 had a BKA).

INTERVENTION

Patients either required immediate amputation or salvage was attempted.

MAIN OUTCOME MEASUREMENTS

The Sickness Impact Profile (SIP) was the principal measure of outcome (higher SIP scores equal greater disability). Secondary outcomes included walking speed, number of rehospitalizations for injury-related complications, time to full weight-bearing, the visual analog pain scale, and return to work at 2 years.

RESULTS

When compared to patients treated with standard BKA, salvage patients who required free flaps and/or ankle arthrodesis had significantly worse 2-year outcomes. They had overall SIP scores that were 2.5 points higher and psychosocial SIP scores that were 8.4 points higher at 24 months (P = 0.014 and P = 0.013, respectively). Physical SIP scores were 3.7 points higher in the free flap and/or arthrodesis group but only approached statistical significance (P = 0.10). After adjusting for the need for free flap and/or arthrodesis, the salvage pathway had clinically, but not statistically, significantly better overall and psychosocial SIP scores than the standard BKA patients (P = 0.34 and P = 0.20, respectively).

CONCLUSIONS

: Patients with severe foot and ankle injuries who require free tissue transfer or ankle fusion have SIP outcomes that are significantly worse than BKA with typical skin flap design closure.

LEVEL OF EVIDENCE

Therapeutic Level II. See Instructions for Authors for a complete description of levels of evidence.

Investigations on the viscoelastic behaviour of a human healthy heel pad: In vivo compression tests and numerical analysis

The aim of this study was to investigate the viscoelastic behaviour of the human heel pad by comparing the stress–relaxation curves obtained from a compression device used on an in vivo heel pad with those obtained from a three-dimensional computer-based subject-specific heel pad model subjected to external compression. The three-dimensional model was based on the anatomy revealed by magnetic resonance imaging of a 31-year-old healthy female. The calcaneal fat pad tissue was described with a viscohyperelastic model, while a fibre-reinforced hyperelastic model was formulated for the skin. All numerical analyses were performed to interpret the mechanical response of heel tissues, with loading conditions and displacement rate in agreement with experimental tests. The heel tissues showed a non-linear, viscoelastic behaviour described by characteristic hysteretic curves, stress–relaxation and viscous recovery phenomena. The reliability of the investigations was validated by the interpretation of the mechanical response of heel tissues under the application of three pistons with diameter of 15, 20 and 40 mm, at the same displacement rate of about 1.7 mm/s. The maximum and minimum relative errors were found to be less than 0.95 and 0.064, respectively.

Cooling reduces the cutaneous afferent firing response to vibratory stimuli in glabrous skin of the human foot sole

Skin on the foot sole plays an important role in postural control. Cooling the skin of the foot is often used to induce anesthesia to determine the role of skin in motor and balance control. The effect of cooling on the four classes of mechanoreceptor in the skin is largely unknown, and thus the aim of the present study was to characterize the effects of cooling on individual skin receptors in the foot sole. Such insight will better isolate individual receptor contributions to balance control. Using microneurography, we recorded 39 single nerve afferents innervating mechanoreceptors in the skin of the foot sole in humans. Afferents were identified as fast-adapting (FA) or slowly adapting (SA) type I or II (FA I n = 16, FA II n = 7, SA I n = 6, SA II n = 11). Receptor response to vibration was compared before and after cooling of the receptive field (2-20 min). Overall, firing response was abolished in 30% of all receptors, and this was equally distributed across receptor type (P = 0.69). Longer cooling times were more likely to reduce firing response below 50% of baseline; however, some afferent responses were abolished with shorter cooling times (2-5 min). Skin temperature was not a reliable indicator of the level of receptor activation and often became uncoupled from receptor response levels, suggesting caution in the use of this parameter as an indicator of anesthesia. When cooled, receptors preferentially coded lower frequencies in response to vibration. In response to a sustained indentation, SA receptors responded more like FA receptors, primarily coding "on-off" events.

Determination of the augmentation effects of hyaluronic acid on different heel structures in amputated lower limbs of diabetic patients using ultrasound elastography

This study measured tissue properties of different anatomies of heels in amputated lower limbs of diabetic patients before and after hyaluronic acid (HA) or normal saline (NS) injections. Seven amputated lower limbs from six diabetic patients constituted the experimental group and one amputated lower limb from a diabetic patient served as the control. The limbs were placed in a fixation platform. A 5-12 MHz linear-array ultrasound transducer controlled by a stepping motor was used to load and unload tested heels. The loading-unloading velocity was 6 mm/s and the maximum loading stress was 178 kPa. Loading-unloading tests were performed before and after 1 mL HA injections into heels in the experimental group. The control limb underwent the same test before and after 1 mL NS injection. The unloaded thickness and Young's modulus of the macrochambers, microchambers and heel pads were determined before and after the interventions. The unloaded thickness of the macrochambers and the heel pad increased significantly (p = 0.012) after HA injection. The Young's modulus of the macrochambers decreased nonsignificantly after HA injections. Similar thickness and tissue stiffness changes were observed in the control limb. The baseline heel-pad energy dissipation ratio (EDR(hp)) was 81.3 ± 1.3% and decreased significantly (p = 0.012) to 73.1 ± 1.7% after HA injections. The EDR(hp) in the control increased after NS injection. Histologic examinations revealed localized HA accumulation in the macrochambers with an extension into the adjacent fibrous septa. Injection of HA can increase tissue thickness and enhance heel-pad tissue resilience.