Pilot study of the microprocessor-controlled prosthetic knee with a novel hydraulic damper

Abstract

BACKGROUND:

A prosthetic knee is the key component of the transfemoral prosthesis. The performance of the prosthetic knee determines the walking ability of transfemoral amputees.

OBJECTIVE:

This study proposes a microprocessor-controlled prosthetic knee with a novel hydraulic damper and evaluates the performance of the prosthetic knee by gait symmetry index.

METHODS:

The homotaxial knee joint with electrical-controlled hydraulic cylinder which adjusts knee flexion and extension damping independently and continuously by single motor was designed. Gait symmetry tests under different walking speeds (0.6 m/s, 1.1 m/s and 1.6 m/s) were conducted to evaluate the performance of the proposed microprocessor-controlled prosthetic knee.

RESULTS:

The symmetry index values indicated that the stance phase was more asymmetry than swing phase. In the swing phase, the knee angle symmetry was observed in different speeds. The number values of symmetry index were smaller than 15% in swing phase.

CONCLUSIONS:

The proposed microprocessor-controlled prosthetic knee could meet the demands of walking.

Keywords

Microprocessor-controlled prosthetic knee hydraulic damper transfemoral amputee gait symmetry walking ability

1. Introduction

There are more than 30 million amputees in the world [1], and lower limb loss in China is about 1.58 million [2]. Many transfemoral amputees benefit from advances in lower limb prosthetic technology. The basic function needs of walking ability has been addressed. However, the quality of life of transfemoral amputees still leaves significant room for improvement [3]. The focus of improving the mobility of transfemoral amputees was to develop a prosthetic knee with higher performance [4].

Microprocessor-controlled prosthetic knees can be divided into two types: powered or active prosthetic knees and variable-damping or semi-active prosthetic knees [5]. Powered prosthetic knees are able to inject power into the knee. The most common actuation system is an electrical drive with a transmission gear or a belt drive with ball screw transmission. Serial elastic elements, parallel spring and pneumatic actuated muscles are also used as actuators [6]. Variable-damping knee cannot provide active knee torque. The power source is the lower residual limb of the thigh. The damping is usually adjusted by fluid control. Hydraulic, pneumatic and magnetorheological dampers are the most common choice [7]. For the functional benefit and affordable weight and size, variable-damping knee is the research focus of laboratories and the industry.

The aim of this work was to propose a microprocessor-controlled prosthetic knee with a novel hydraulic damper and evaluate the performance of the prosthetic knee by gait symmetry index. The electrical-controlled hydraulic cylinder which adjusts knee flexion and extension damping independently and continuously by single motor was designed. Gait symmetry under different walking speeds were tested to evaluate the performance of the proposed microprocessor-controlled prosthetic knee.

Figure 1.

Microprocessor-controlled prosthetic knee.

2. Methods

2.1 Mechanical structure and sensor system

The prosthetic knee (Fig. 1a) has a homotaxial joint design with an electrically controlled hydraulic damper (Fig. 1b). In the electrically controlled hydraulic damper (Fig. 1c), the upper push rod of the piston was pivotally connected to the upper leg segment of the prosthetic knee and the lower end of the cylinder was pivotally connected to the lower leg segment. The microprocessor reacted at various transition points in the gait by activating a motor which adjusted the butterfly valve in the damper. The butterfly valve was able to vary flow port areas and fluid flow rates to thereby vary resistance to knee joint rotation in either flexion or extension at the same time. The footswitches and gyroscope were suitable for obtaining on/off information. However, the location of the force sensors was not appropriate for the change of shoes. To deal with this problem, a special ankle pylon that adapted many pressure sensors on the market was presented in Fig. 2. The design included upper connection, bolted connection, tube adapter and pressure sensors. The fabrication and assembly of the ankle pylon was easy. The placement was done in such a manner that the change of the artificial foot had no effect on the gait detection. The angle sensor (WOA-C-RS485, MIRAN), gyroscope (HI219, HiPNUC) and force sensitive resistors (FSR402, INTERLINKELECTRONICS) were chosen to establish the sensor system of the prosthetic knee.

Figure 2.

Special ankle pylon.

2.2 Gait symmetry tests

The gait symmetry compared the kinematics of the healthy leg and prosthetic leg. The gait symmetry represented the adaptability to speed changes of the microprocessor-controlled prosthetic knee. It was a key index of the performance of the prosthetic knee [8]. The gait symmetry quantification used three indices introduced by Karaharju-Huisman et al. [9].

(1)
Symmetry Index (SI)

$\textit{SI}=\frac{2(X_{R}-X_{L})}{X_{R}+X_{L}}\times 100\%$

Where $X_{R}$ was the right limb data and $X_{L}$ was the corresponding data for the left limb. The prosthetic leg side was $X_{R}$ and healthy side was $X_{L}$ in this work. $\textit{SI}=$ 0 represented perfect symmetry and $\pm$ showing limb dominance.
(2)
Ratio I

$R_{I}=\frac{X_{R}}{X_{L}}$

Where $R_{I}=$ 1 represented perfect symmetry.
(3)
Ratio II

$R_{II}=\frac{(X_{R}-X_{L})}{\max(X_{R},X_{L})}$

Where $R_{II}=$ 0 represented perfect symmetry and $\pm$ showing limb dominance.

The simulation and evaluation prototype of microprocessor-controlled prosthetic knee in the authors’ previous work [10] was used to conduct the gait symmetry test under different walking speeds. Stance and swing phase were detected by the proposed sensor system. The symmetry indices between prosthetic and intact knee angle under different speeds were analyzed. The angles of intact and prosthetic knee in the same phase were translated into percent of gait to show the repeatability.

Table 1
Symmetry indices data calculated under different speeds

Speed(m/s) Gait phase SI $R_{I}$ $R_{II}$

0.6 Stance 25.3% 1.29 2.29

Swing $-$ 4.6% 0.95 $-$ 0.50

1.1 Stance 24.5% 1.28 2.25

Swing 11.8% 1.13 1.36

1.6 Stance 24.7% 1.67 3.16

Swing 13.1% 1.14 1.55

3. Results

Speed(m/s)	Gait phase	SI	$R_{I}$	$R_{II}$
0.6	Stance	25.3%	1.29	2.29
	Swing	$-$ 4.6%	0.95	$-$ 0.50
1.1	Stance	24.5%	1.28	2.25
	Swing	11.8%	1.13	1.36
1.6	Stance	24.7%	1.67	3.16
	Swing	13.1%	1.14	1.55

The symmetry indices data calculated under different walking speeds are shown in Table 1. The knee angle symmetry differed by gait phase. There was a significant difference in symmetry between stance and swing. The symmetry index values indicated that the stance phase was more asymmetry than swing phase. This is because the stance pre-flexion was smaller than the normal gait. The stance phase kinematic symmetry, in most cases, depended on differences in the knee position during early stance and mid stance. In the swing phase, the symmetrical kinematics was observed in different speeds. The number values of SI were smaller than 15% in swing phase.

4. Conclusions

The work proposed a microprocessor-controlled prosthetic knee with a novel hydraulic damper. The homotaxial knee joint with electrical-controlled hydraulic cylinder which adjusts knee flexion and extension damping independently and continuously by a single motor was designed. Compared to the existing knee prosthesis, it reduced the weight by using a single motor. The sensors were placed in such a manner that the change of the artificial foot had no effect on the gait detection. Gait symmetry tests under different walking speeds were conducted to evaluate the performance of the proposed microprocessor-controlled prosthetic knee. The symmetry index results indicated that the symmetry was acceptable. The proposed microprocessor-controlled prosthetic knee could meet the demands of walking.

Footnotes

Acknowledgments

This work was supported by the National Key R&D Program of China (number: 2018YFB1307303) and the Natural Science Foundation of China (number: 61473193).

Conflict of interest

None to report.

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