Kohta Ito

Parameter identification of hyperelastic material properties of the heel pad based on an analytical contact mechanics model of a spherical indentation

Accurate identification of the material properties of the plantar soft tissue is important for computer-aided analysis of foot pathologies and design of therapeutic footwear interventions based on subject-specific models of the foot. However, parameter identification of the hyperelastic material properties of plantar soft tissues usually requires an inverse finite element analysis due to the lack of a practical contact model of the indentation test. In the present study, we derive an analytical contact model of a spherical indentation test in order to directly estimate the material properties of the plantar soft tissue. Force-displacement curves of the heel pads are obtained through an indentation experiment. The experimental data are fit to the analytical stress-strain solution of the spherical indentation in order to obtain the parameters. A spherical indentation approach successfully predicted the non-linear material properties of the heel pad without iterative finite element calculation. The force-displacement curve obtained in the present study was found to be situated lower than those identified in previous studies. The proposed framework for identifying the hyperelastic material parameters may facilitate the development of subject-specific FE modeling of the foot for possible clinical and ergonomic applications.

Three-dimensional innate mobility of the human foot bones under axial loading using biplane X-ray fluoroscopy

The anatomical design of the human foot is considered to facilitate generation of bipedal walking. However, how the morphology and structure of the human foot actually contribute to generation of bipedal walking remains unclear. In the present study, we investigated the three-dimensional kinematics of the foot bones under a weight-bearing condition using cadaver specimens, to characterize the innate mobility of the human foot inherently prescribed in its morphology and structure. Five cadaver feet were axially loaded up to 588 N (60 kgf), and radiographic images were captured using a biplane X-ray fluoroscopy system. The present study demonstrated that the talus is medioinferiorly translated and internally rotated as the calcaneus is everted owing to axial loading, causing internal rotation of the tibia and flattening of the medial longitudinal arch in the foot. Furthermore, as the talus is internally rotated, the talar head moves medially with respect to the navicular, inducing external rotation of the navicular and metatarsals. Under axial loading, the cuboid is everted simultaneously with the calcaneus owing to the osseous locking mechanism in the calcaneocuboid joint. Such detailed descriptions about the innate mobility of the human foot will contribute to clarifying functional adaptation and pathogenic mechanisms of the human foot.

Dynamic measurement of surface strain distribution on the foot during walking

To clarify the mechanism underlying the development of foot disorders such as diabetic ulcers and deformities, it is important to understand how the foot surface elongates and contracts during gait. Such information is also helpful for improving the prevention and treatment of foot disorders. We therefore measured temporal changes in the strain distribution on the foot surface during human walking. Five adult male participants walked across a glass platform placed over an angled mirror set in a wooden walkway at a self-selected speed and the dorsolateral and plantar surfaces of the foot were filmed using two pairs of synchronized high-speed cameras. Three-dimensional (3D) digital image correlation was used to quantify the spatial strain distribution on the foot surface with respect to that during quiet standing. Using the proposed method, we observed the 3D patterns of foot surface strain distribution during walking. Large strain was generated around the ball on the plantar surface of the foot throughout the entire stance phase, due to the windlass mechanism. The dorsal surface around the cuboid was stretched in the late stance phase, possibly due to lateral protruding movement of the cuboid. It may be possible to use this technique to non-invasively estimate movements of the foot bones under the skin using the surface strain distribution. The proposed technique may be an effective tool with which to analyze foot deformation in the fields of diabetology, clinical orthopedics, and ergonomics.

Effect of tibial coronal inclination on hindfoot kinematics: A biomechanical simulation study

Tibial coronal inclination is often recognized in everyday life, but the compensatory kinematic changes to maintain plantigrade of the hindfoot in response to tibial coronal inclination remain unclear. Lower legs and foot specimens obtained from seven human cadavers were loaded vertically with traction of Achilles’ tendon in different tibial inclinations: 0° (neutral), 5°, and 10° medial inclination, and 5° and 10° lateral inclination. The orientations of the tibia, talus, and calcaneus were recorded under vertical load by a three-dimensional digitizing stylus. The angular changes of the talocrural and subtalar joints in the tibial inclinations from neutral were analyzed. The heights of the origins of the talus and calcaneus were also recorded. As the tibia was medially inclined from neutral, the talocrural joint was significantly more dorsiflexed. The subtalar joint was significantly more inverted, plantarflexed, and internally rotated. However, such significant changes in the joint angles were not observed when the tibia was laterally inclined. The height of the talus decreased as the tibia was medially inclined, but it was vice versa when laterally inclined. The compensatory motions of the hindfoot to tibial medial inclination involved coupled movement of both the talocrural and subtalar joints; such motions flatten the medial foot by decreasing the height of the talus. However, such compensatory capacities of the hindfoot to tibial lateral inclinations were limited. Tibial medial inclination under axial loading affects the kinematics of the hindfoot, and this is an important factor to consider in the treatment of flatfoot as well as foot orthotic/footwear intervention.

Direct assessment of foot kinematics during human gait using a dynamic cadaver simulator and a biplane X-ray fluoroscopy

Direct measurement of detailed kinematics of individual anatomical structures in the foot during human locomotion is crucial for understanding morphofunctional roles of the human foot structure that mechanically interacts with the ground in a favourable manner to maintain stable gait. In the present study, we constructed a dynamic gait simulator to load and mobilize the cadaver foot and directly measured the three-dimensional kinematics of foot bones using a biplane X-ray fluoroscopy. A robotic gait simulator was developed to load and mobilize the cadaver foot in a manner similar to the way human foot actually moves and interacts with the ground during walking. The simulator has three legs, fore, middle and hind legs, with the cadaver foot fixed to the middle leg (Figure ​(Figure1A).1A). After the middle cadaver foot contacts the ground, the hind leg departs from the ground. The fore leg replicates the foot-contact with the ground in the next step and toe-off of the middle cadaver foot follows afterward. Tendons of tibialis anterior and soleus were connected to pneumatic actuators to apply forces at the appropriate moment to reproduce how the foot would function during walking. Figure 1 A robotic gait simulator to load and mobilize the cadaver foot in a manner similar to the way human foot moves and interacts with the ground during walking (A). Automatic method to register bone surface models with the two fluoroscopic images recorded ... A biplanar dynamic X-ray fluoroscopic system was developed with Shimadzu Corporation, Kyoto, Japan. The system consists of two sets of x-ray sources and flat panels with a resolution of 2688 x 2208 pixels. We recorded biplanar X-ray videos of foot movement using the system using the dynamic cadaver model at 15 fps. For direct measurement of 3D kinematics of the foot bones, we developed an automatic method to register bone surface models with the two fluoroscopic images (Figure ​(Figure1B).1B). Specifically, the 3D surface models of the foot were generated based on computed tomography (CT), and a similarity measure between occluding contours of the bone surface models with edge-enhanced fluoroscopic images was evaluated to reconstruct the position and orientation of each bone model in a 3D space. Collisions among the reconstructed bones were also evaluated to avoid penetration. Using the biplanar X-ray fluoroscopic images and the proposed reconstruction methodology based on CT, we reconstructed 3D movements of the calcaneus, talus, navicular and cuboid when a human cadaveric foot walked on a flat surface using the simulator. The surface models of the four bones were successfully matched with the corresponding fluoroscopic images and the joint movements were quantified and visualized. The present methodology must be undergone further evaluation, but the proposed framework may serve as an effective tool for understanding the morphofunctional roles of the human foot structure during walking.

Quantification of vertical free moment induced by the human foot–ankle complex during axial loading

Axial loading of the human cadaver lower leg is known to generate eversion of the calcaneus and internal rotation of the tibia if the plantar surface of the foot does not slide on the floor. Such kinematic coupling between calcaneal eversion and internal tibial rotation has been described previously, but no studies have actually quantified the innate ability of the human foot to generate ground reaction moment around the vertical axis of the floor (vertical free moment) due to axial loading of the human cadaver lower leg. This study investigated the vertical free moment generated by eight cadaveric lower leg specimens loaded vertically with traction of the Achilles’ tendon using a six-component force plate. The vertical free moments in all specimens were oriented toward the direction of internal rotation, and the mean magnitude of the vertical free moments was −1.66 N m when an axial load of 450 N was applied. A relatively large ground reaction moment can be applied to the body during walking due to the innate structural mobility of the foot. The structurally embedded capacity of the human foot to generate the vertical free moment may facilitate compensation of the moment generated around the vertical axis of the body during walking due to trunk rotation and leg swing.

The use of shear thickening polymer as a hip protecter

External hip protectors are used by the elderly in preventing hip fracture due to sideway falls. There are some commercial hip protectors which has both energy absorbing and energy shunting properties. In this study, a novel hip protector using shear thickening polymer (STP) is studied. The purpose of this work is to determine the optimal thickness of STP needed for maximum force attenuation. A mechanical test rig to simulate a person falling with sufficient impact energy to fracture the greater trochanter if unprotected was used together with biofidelic femur model which simulates the layer of flesh with skin. 8mm of STP together with 5mm foam gives the best force attenuation. When comparing the overall thickness with commercial hip protectors, STP hip protectors tested have much less thickness. Reduced thickness increases the compliance and comfort of STP hip protectors.

Data set for figure 5-8 from Three-dimensional innate mobility of the human foot bones under axial loading using biplane X-ray fluoroscopy

Changes of the foot dimensions and the 3D movement of each foot bone model due to axial loading shown in figures 5-8

In-vivo viscous properties of the heel pad by stress-relaxation experiment based on a spherical indentation

Identifying the viscous properties of the plantar soft tissue is crucial not only for understanding the dynamic interaction of the foot with the ground during locomotion, but also for development of improved footwear products and therapeutic footwear interventions. In the present study, the viscous and hyperelastic material properties of the plantar soft tissue were experimentally identified using a spherical indentation test and an analytical contact model of the spherical indentation test. Force-relaxation curves of the heel pads were obtained from the indentation experiment. The curves were fit to the contact model incorporating a five-element Maxwell model to identify the viscous material parameters. The finite element method with the experimentally identified viscoelastic parameters could successfully reproduce the measured force-relaxation curves, indicating the material parameters were correctly estimated using the proposed method. Although there are some methodological limitations, the proposed framework to identify the viscous material properties may facilitate the development of subject-specific finite element modeling of the foot and other biological materials.

Three-dimensional measurement of the human cadaver foot bone kinematics under axial loading condition using biplane X-ray fluoroscopy

The anatomical design of the human foot is considered to facilitate generation of bipedal walking. To clarify the morphofunctional mechanisms of the human foot, efforts have been made to directly capture the three-dimensional (3D) movements of the foot bones during walking using bone pins (Lundgren et al., 2008) and X-ray fluoroscopy (Wang et al., 2016). However, such observed foot bone movements during walking were generated as a consequence of the neural control of forces generated by the extrinsic and intrinsic muscles of the foot. Therefore, it is difficult to infer the innate mobility of the foot from the in vivo kinematics of the foot bone during walking. On the other hand, if we use cadaver feet, such factors can be better controlled, possibly leading to a deeper understanding of the innate patterns of the foot bone and hence the causal relationship between foot bone kinematics and the functional adaptations of the foot bone morphology and structure for generation of bipedal locomotion.