An instrumented tissue tester for measuring soft tissue property under the metatarsal heads in relation to metatarsophalangeal joint angle

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.

Muscle Contributions to Take-Off Velocity in the Long Jump

PURPOSE

A key determinant of long jump performance is the ability to increase the vertical velocity of the center of mass (COM) while minimizing the loss in forward velocity (running speed) during the take-off phase, but exactly how this occurs is not fully understood. We combined a three-dimensional musculoskeletal model of the body with dynamic optimization theory to simulate the biomechanics of the long jump take-off and determine the contributions of the individual leg muscles to jump performance.

METHODS

The body was modeled as a 29-degree-of-freedom skeleton actuated by a combination of muscles and net joint torques. A dynamic optimization problem was solved to reproduce full-body motion and ground-force data recorded from experienced subelite jumpers. The optimization solution then was analyzed to determine each muscle's contribution to the ground-force impulse and hence the change in velocity of the COM during the take-off phase.

RESULTS

The hip, knee, and ankle extensors dominated the change in velocity of the COM during take-off. Vasti (VAS) generated the highest support impulse and contributed one-third (33%) of the increase in vertical COM velocity generated by all the muscles. Soleus (SOL) and gluteus maximus (GMAX) also developed substantial support impulses and contributed 24% and 16% of the increase in vertical COM velocity, respectively. VAS also generated the highest braking impulse and contributed approximately one-half (55%) of the loss in forward COM velocity generated by all the muscles, whereas SOL and GMAX made much smaller contributions (12% and 7%, respectively).

CONCLUSIONS

VAS, SOL, and GMAX contributed nearly three-quarters (73%) of the increase in vertical COM velocity at take-off, suggesting that these muscles ought to be prioritized in strength training programs aimed at improving long jump performance.

Finite element stress analysis of the bearing component and bone resected surfaces for total ankle replacement with different implant material combinations

Background

A proper combination of implant materials for Total Ankle Replacement (TAR) may reduce stress at the bearing component and the resected surfaces of the tibia and talus, thus avoiding implant failure of the bearing component or aseptic loosening at the bone-implant interface.

Methods

A comprehensive finite element foot model implanted with the INBONE II implant system was created and the loading at the second peak of ground reaction force was simulated. Twelve material combinations including four materials for tibial and talar components (Ceramic, CoCrMo, Ti6Al4V, CFR-PEEK) and three materials for bearing components (CFR-PEEK, PEEK, and UHMWPE) were analyzed. Von Mises stress at the top and articular surfaces of the bearing component and the resected surfaces of the tibia and talus were recorded.

Results

The stress at both the top and articular surfaces of the bearing component could be greatly reduced with more compliant bearing materials (44.76 to 72.77% difference of peak stress value), and to a lesser extent with more compliant materials for the tibial and talar components (0.94 to 28.09% difference of peak stress value). Peak stresses at both the tibial and talar bone-implant interface could be reduced more strongly by using tibial and talar component materials with smaller material stiffness (7.31 to 66.95% difference of peak stress value) compared with bearing materials with smaller material stiffness (1.11 to 24.77% difference of peak stress value).

Conclusions

Implant components with smaller material stiffness provided a stress reduction at the bearing component and resected surfaces of the tibia and talus. The selection of CFR-PEEK as the material of tibial and talar components and UHMWPE as the material of the bearing component seemed to be a promising material combination for TAR implants. Wear testing and long-term failure analysis of TAR implants with these materials should be included in future studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-021-04982-3.

In vivo soft tissue compressive properties of the human hand

Background/Purpose

Falls onto outstretched hands are the second most common sports injury and one of the leading causes of upper extremity injury. Injury risk and severity depends on forces being transmitted through the palmar surface to the upper extremity. Although the magnitude and distribution of forces depend on the soft tissue response of the palm, the in vivo properties of palmar tissue have not been characterized. The purpose of this study was to characterize the large deformation palmar soft tissue properties.

Methods

In vivo dynamic indentations were conducted on 15 young adults (21–29 years) to quantify the soft tissue characteristics of over the trapezium. The effects of loading rate, joint position, tissue thickness and sex on soft tissue responses were assessed.

Results

Energy absorbed by the soft tissue and peak force were affected by loading rate and joint angle. Energy absorbed was 1.7–2.8 times higher and the peak force was 2–2.75 times higher at high rate loading than quasistatic rates. Males had greater energy absorbed than females but not at all wrist positions. Damping characteristics were the highest in the group with the thickest soft tissue while damping characteristics were the lowest in group with the thinnest soft tissues.

Conclusion

Palmar tissue response changes with joint position, loading rate, sex, and tissue thickness. Accurately capturing these tissue responses is important for developing effective simulations of fall and injury biomechanics and assessing the effectiveness of injury prevention strategies.

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.

Investigation of the relationship between localized cumulative stress and plantar tissue stiffness in healthy individuals using the in-vivo indentation technique

This study was conducted to determine whether prolonged and repetitive exercise stiffens the plantar soft tissue. Healthy female subjects in their early 20s with a similar body mass index but different majors (13 engineers (controls) and 13 ballet dancers) were recruited. Tissue thickness was measured using ultrasound, while peak stress, stress distribution, and center of pressure were obtained Zebris^® pressure mat. Stiffness was evaluated using a custom-made tissue indentation system. F-test and independent sample T-test were used to determine significant differences between the two groups. No significance was found in the thickness of the second sub-metatarsal head (MTH) and heel between the two groups. In the second sub-MTH, the ballet group showed higher peak stress, loading rate, and stiffness than the control group. Conversely, in the heel region, all the results were higher for the control group. The results of this study quantify the impact of exercise on the stiffness of plantar soft tissue and confirm that even healthy individuals who do prolonged and repetitive exercise have stiffer plantar soft tissue.

Foot internal stress distribution during impact in barefoot running as function of the strike pattern

The aim of the present study is to examine the impact absorption mechanism of the foot for different strike patterns (rearfoot, midfoot and forefoot) using a continuum mechanics approach. A three-dimensional finite element model of the foot was employed to estimate the stress distribution in the foot at the moment of impact during barefoot running. The effects of stress attenuating factors such as the landing angle and the surface stiffness were also analyzed. We characterized rear and forefoot plantar sole behavior in an experimental test, which allowed for refined modeling of plantar pressures for the different strike patterns. Modeling results on the internal stress distributions allow predictions of the susceptibility to injury for particular anatomical structures in the foot.

The influence of sex, body mass and body mass index on plantar soft-tissue stiffness in healthy people in their 60s

Foot abnormality has become a public health concern. Early detection of pathological soft tissue is therefore an important preventive measure, especially in older people who generally have a higher risk of foot pathology. However, the interpretation of plantar tissue stiffness data - whether to normalize the data or to separate the data on the basis of sex- remains questionable. The objective of this study was to assess the influence of sex and physical attributes such as body mass (BM) and body mass index (BMI) on plantar soft-tissue stiffness, and to evaluate whether it is necessary to isolate the differences in sex, BM and BMI in the data analysis. One hundred healthy subjects in their 60s were recruited for the experiment. Localized force response was obtained underneath the second metatarsal head (MTH) pad at three different dorsiflexion angles of 0°, 20°, 40° and the hallux and heel at 0°. No significant relationship was found between the independent variables and plantar stiffness. From the experimental results, it can be deduced that BM and BMI are weakly associated with plantar tissue stiffness, and that there is no significant difference in stiffness between male and female participants. No difference was found between left and right foot measurements. This suggests that normalizing of plantar tissue stiffness by BM and BMI is not necessary in healthy people in their 60s. The data can be pooled and treated equally regardless of sex.

Prediction of plantar soft tissue stiffness based on sex, age, bodyweight, height and body mass index

15% of Diabetes Mellitus (DM) patients suffer high risk of ulceration and 85% of the amputation involving DM population is caused by non-healing ulcers. These findings elucidate the fact that foot ulcer can result in major amputation especially to the DM and elderly population. Therefore, early diagnosis of abnormally stiffened plantar soft tissue is needed to prevent the catastrophic tissue damage. In order to differentiate between normal and pathological tissues, a threshold reference value that defines healthy tissue is required. The objective of this study is to perform a multivariate analysis to estimate the healthy plantar tissue stiffness values based on the individuals physical attributes such as bodyweight (BW), height and body mass index (BMI) as well as their age and sex. 100 healthy subjects were recruited. Indentation was performed on 2nd metatarsal head pad at 3 different dorsiflexion angles of 0°, 20°, 40° and the hallux and heel at 0°. The results showed the important influences of BW, height and BMI in determining the plantar tissue stiffness. On the other hand, age and sex only play minimal roles. The study can be further extended to increase the reliability and accuracy of the proposed predictive model by evaluating several other related parameters such as body fat content, footwear usage, frequency of sports participation, etc.

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.

Constitutive formulation and numerical analysis of the biomechanical behaviour of forefoot plantar soft tissue

The aim of this work is to provide a numerical approach for the investigation of the mechanical behaviour of the forefoot soft tissues. The development of reliable numerical models of biological structures requires the definition of constitutive formulations that actually interpret the mechanical response of the constituent biological tissues and their structural arrangement. A specific visco-hyperelastic constitutive model is provided to account for the typical features of soft plantar tissue mechanics, as geometric and material non-linearity, almost-incompressible behaviour and time-dependent phenomena. Constitutive parameters are evaluated by the analysis of experimental data from compression and stress relaxation tests on tissue samples. A three-dimensional finite element model of the forefoot region is developed starting from the analysis of biomedical images, leading to the evaluation of overall structural response. The reliability of model and analyses is assessed by the comparison of experimental and numerical results pertaining to indentation tests. The numerical model developed allows to evaluate the mechanical response of plantar soft tissue in terms of stress and strain distribution.

The in vivo plantar soft tissue mechanical property under the metatarsal head: implications of tissues׳ joint-angle dependent response in foot finite element modeling

Material properties of the plantar soft tissue have not been well quantified in vivo (i.e., from life subjects) nor for areas other than the heel pad. This study explored an in vivo investigation of the plantar soft tissue material behavior under the metatarsal head (MTH). We used a novel device collecting indentation data at controlled metatarsophalangeal joint angles. Combined with inverse analysis, tissues׳ joint-angle dependent material properties were identified. The results showed that the soft tissue under MTH exhibited joint-angle dependent material responses, and the computed parameters using the Ogden material model were 51.3% and 30.9% larger in the dorsiflexed than in the neutral positions, respectively. Using derived parameters in subject-specific foot finite element models revealed only those models that used tissues׳ joint-dependent responses could reproduce the known plantar pressure pattern under the MTH. It is suggested that, to further improve specificity of the personalized foot finite element models, quantitative mechanical properties of the tissue inclusive of the effects of metatarsophalangeal joint dorsiflexion are needed.

Quantification of plantar soft tissue changes due to aging in various metatarsophalangeal joint angles with realistic tissue deformation

The nonlinearity of plantar soft tissue is seldom examined because of the small extent of deformation induced during indentation for measurement purposes. Furthermore, in most indentation experiments, the metatarsophalangeal joint (MTPJ) angle is not well controlled, although it has been proven to have a significant stiffening effect on sub-metatarsal head (MTH) pads. Hence, the study aims to quantify changes in the mechanical properties of plantar soft tissue due to aging under an experimental condition which is similar to walking. This is done by subjecting the tissue to an appropriate level of deformation at various MTPJ angles. A custom-made in vivo tissue indenter was used to measure directly the force-indentation response of the plantar tissue of two healthy groups: "Young" (n=25, mean age 22) and "Elderly" (n=25, mean age 67) subjects. Tests were performed on the 2nd sub-MTH pad at angles of 0°, 20°, 40° MTPJ dorsiflexion, as well as at the hallux and heel pad at 0° MTPJ angle. At all three plantar sites tested, elderly subjects showed significantly higher tissue stiffness than the young (p<0.05). However, the stiffening effect of MTPJ angle was not notably influenced by aging. In this work, tissue stiffness is quantified in stiffness constant (K) based on the proposed indentation technique. It is hypothesized that the increase in stiffness with age observed is probably due to compositional change in the plantar soft tissue.

Investigation of the biomechanical effect of variable stiffness shoe on external knee adduction moment in various dynamic exercises

Background

The growing ageing population and high prevalence of knee osteoarthritis (OA) in athletes across nations have created a strong demand for improved non-invasive therapeutic alternatives for knee OA. The aim of this study is to investigate the effect of the variable stiffness shoe (VSS), a new non-invasive therapeutic approach, on external knee adduction moment (EKAM) in various dynamic exercises. EKAM is believed to have positive correlation with the progression and development of knee OA.

Methods

Thirty young participants (16 male and 14 female; age 22.6 ± 1.9 years) from National University of Singapore were enrolled in this study. The tested activities were walking, running, drop-landing, and lateral hopping. All the dynamic exercises were recorded simultaneously by the 8-camera VICON Motion Systems (Oxford Metric, UK) with a sampling rate of 100 Hz.

Results

The results showed that the EKAM was reduced in all the dynamic exercises with the use of VSS. The VSS produced significant reductions in the peak EKAM during walking (4.97%, p = 0.039), running (11.15%, p = 0.011), drop-landing (11.18%, p = 0.038) and lateral hopping (17.34%, p = 0.023) as compared to the control shoe.

Conclusions

The reduction of EKAM with the use of VSS in various dynamic exercises demonstrates its potential in delaying the onset and the progression of knee OA in early stage of knee OA patients.