Design of a Flexible Bionic Ankle Prosthesis Based on ...

2.1

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Human Ankle–foot Reconstruction

Ankle–foot bone images of the healthy (left) side of a below-knee amputee (age: 27 years, weight: 77 kg; height: 182 cm; no history of lower extremity diseases on the healthy side) were obtained by a GE Revolution CT scanner (General Electric, USA) at 0.624 mm slice intervals. The subject provided prior written informed consent before participating in the CT scanning and gait measurements. Each image had dimensions of 512 × 512 pixels and represented a slice in the sagittal plane of the human body (Fig. 1a). A surface model was then rebuilt and smoothed in Mimics (Materialize NV, Belgium) and mirrored to a solid right ankle–foot model (Fig. 1a) in SolidWorks (Dassault System, USA). This study was approved by the Ethics Committee of the Second Hospital of Jilin University (Log# 2,021,072), in accordance with the Declaration of Helsinki (2013) and Biomedical Research Involving Human Subjects International Code of Ethics for Research (2002).

Design of the ankle prosthesis inspired by the human ankle–foot complex. a Acquisition of the ankle–foot model. b Three soft material inclusions. c Prosthetic ankle design. d 3D rendering of the prosthesis

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2.2

Design of the Bionic Ankle Prosthesis

In this study, three types of soft material inclusions (Fig. 1b) were designed to imitate the soft tissues surrounding the human ankle, providing stability and adaptability during walking. To facilitate comparative analysis and eliminate uncertain factors, the internal structure designs of the three inclusions were consistent. The specific method of modeling was to carry out regular processing according to the well-positioned foot bone parts, set each part of the foot bone according to the ellipse, and pull out the mold to stretch out the entity to form the internal configuration of inclusion. For the external shape, according to the characteristics of the human leg, the shape of the type 1 inclusion was assumed as a regular configuration (cylindrical). Type 2 inclusion was divided into upper and lower parts in which the shape of the internal and external condyles was the boundary (approximately cylindrical). As the upper half considered as the main part to perform relatively large movements of plantar flexion and dorsiflexion, the outer profiles were thickened. Meanwhile, the thickness of the regular configuration in the lower half was reduced. For the type 3 inclusion, considering the better supporting deformation of the soft materials, the thickness of the type 1 inclusion was too large. As a result, according to the human bone profiles, the outer profiles of the type 3 inclusion were obtained by removing the extra area (by a cylindrical cut). Polyurethane rubber was selected as the soft material for all three types because of its favorable mechanical properties and stable chemical structure [20, 22].

For the rigid parts of the ankle prosthesis, talus and calcaneus were kept as original, tibia and fibula were constructed as a whole (i.e., tibiofibular), leg tube connectors were placed at the top of the integrated tibiofibular part, and all parts of the ankle bones were scaled in proportion (Fig. 1c). The ankle model parts processed were then imported into a 3D printer, Stratasys J850 (Stratasys, USA), to produce the physical prototype of talus, calcaneus, tibiofibular, and tube connectors. The foot plate was derived from the profiles of human foot bones and simplified as a 4-mm curved plate. The human foot arch has great mechanical characteristics in terms of shock absorption and stability [23]. To better simulate the gait movement of the human body, the foot plate in this study takes the role of the human arch into consideration. The foot plate was made of commercial carbon fiber to provide adequate strength and reduce the overall weight. To suitably limit the movements of the soft material inclusions and make the connection with the foot plate more stable, the prosthetic ankle was covered by a metal shell (Fig. 1d).

2.3

Finite Element Analysis

Three types of FE simulations were carried out in this study to select the appropriate type of soft material inclusions, analyze the 3D ankle stiffness of the prosthesis under different working conditions, and calculate the rotation axis of the prosthetic ankle during a normal walking gait. A 3D FE model (Fig. 1c and d) containing tibiofibular, talus, calcaneus, leg connectors, soft material inclusions, and foot plate was developed in Abaqus CAE (Simulia, USA). All parts were modeled using high-quality tetrahedral mesh elements (C3D4), and corresponding material parameters (elastic modulus and Poisson's ratio) [24] were assigned to different components (Table 1). It should be noted that the rigid part here is the metal shell surrounding the bionic prosthetic ankle (not leg connector), which is used to fix and wrap the polyurethane materials. Tie constraints were applied to connect the tibiofibular to the leg connector and form the calcaneus, or the soft inclusion into the foot plate. Frictionless sliding contacts were defined between the interface of bone to bone and bone to soft material inclusions.

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To select the proper type of soft inclusions with minimum principal stress, a kinematic simulation was conducted during the stance phase of walking gait for all three types of designs. The stance phase was divided into 70 time instants, and the kinematic simulation was conducted at each instant. The ground reactions at each instant were defined according to the literature [17]. To investigate the 3D ankle stiffness of the prosthesis, 12 different working conditions were defined based on the compression test setups [25]. A vertical force of 400 N was applied to obtain the stress and displacement values under each condition. The deformation of the prosthetic ankle and the moment around the ankle were then calculated to quantify the ankle stiffness at each condition. To calculate the rotation axis of the prosthetic ankle during walking, the local bone coordinate system was defined for the tibiofibular, talus and calcaneus. The relative 3D transformations between these bones were calculated for all the 70 time instant stance phases of normal walking. The input parameters for each simulation mainly included the angle between the foot and the ground as well as the ground reactions, which were derived from the literature [17, 26].

2.4

Joint Axis Calculation

To verify that the bionic ankle could restore the natural movements of a healthy human ankle, the walking gait of the prosthesis was simulated in Abaqus. The rotation axis of the talocrural joint and the subtalar joint during walking was calculated using a helical axis approach [27] in the FE simulation. The joint axis was defined as the axis that best represented the rotation occurring between two articulating bone-like components during a defined time period from heel strike to toe off (talocrural: talus and tibia; subtalar: calcaneus and talus). The landmarks used to define the bone frame are shown in Table 2, and their coordinates at different time instants were obtained from the FE analysis. The talocrural or subtalar joint axis was described by a position vector and an orientation vector in the talus bone frame and then transformed to be described in the global foot frame. The orientation angles of the talocrural and subtalar joint axes, including deviation and inclination, were then calculated for comparison with the literature. All calculations were conducted by using MATLAB (MathWorks).

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2.5

Gait Measurements

The subject was the amputee who participated in the human ankle–foot reconstruction by CT scanning. In the early stage, the experimental subjects were briefly introduced to the process and trained accordingly, which ensured the smooth progress of the experiment and the collection of relatively stable data. Wearing the flexible bionic ankle prosthesis, the subject walked on a 10-m sidewalk at three different self-selected speeds: slow (1.25 ± 0.27 m/s), normal (1.51 ± 0.32 m/s), and fast (1.82 ± 0.36 m/s). Each speed was measured 10 times to ensure representative walking data were recorded and used for all analyses. Kinematic data were collected at 100 Hz using a seven-infrared camera motion capture system (Vicon, UK), and ground reaction force/moment data were recorded at 1000 Hz using a three force-plate array (Kistler, Switzerland). To ensure experiment safety, handrails were placed on both sides of the passages of the three force measuring plates (the handrails were not reflective).

A group of specially designed thermoplastic plates [28] were attached to the six body segments (feet, shanks, thighs), each with a cluster of four reflective markers. The plastic plates reduced the relative movement between the markers on a segment, thereby improving the accuracy of the measured data [29, 30]. The anatomical landmarks were located from a series of static calibration procedures by using a calibration wand and reflective markers. The calibration markers were then removed before walking tests according to the calibrated anatomical system technique [31]. The joint centers were defined based on anatomical landmarks. Measured data were processed using customized MATLAB codes for 3D kinematic and kinetic analysis of general biomechanical multibody systems. The marker data were filtered using a low-pass zero-lag fourth-order Butterworth digital filter with a cutoff frequency of 6.0 Hz.

2.6

Statistical Analysis

The results of gait measurement are shown as the mean ± standard deviation (n = 10). Statistical significance was tested using ANOVA (single factor) by SPSS 25.0 software (IBM, USA). Probability values of p < 0.05 were considered statistically significant, and all data are presented at a p < 0.05 significance level unless otherwise stated.

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