11.1 INTRODUCTION

Over the past few years, the requirement for implants and medical devices in orthopedics has undertaken rapid growth due to some important factors including growing elderly population, technology developments, the rise in chronic diseases, and improved healthcare facilities in developing countries. Currently, there are more than 30 million people worldwide having amputations [1] and most of them involve the lower limb at the transtibial level [2]. With the help of prosthesis, amputees can improve the quality of life. The amputation rate in developing countries, including India, is about 45% of diabetic foot problems, with an estimated 50,000 amputations occurring per year [3]. The prosthetic socket act as a critical interface between amputation and residual limb, which is designed and developed in an iterative process by the prosthetist.

Regardless of the use of advanced technologies in socket manufacturing, definite stump-socket interaction takes place. This interaction results in excessive stresses, patient discomfort in wearing the prosthesis, pain, skin irritation, pistoning, and stump soft tissue damage [4]. It is accepted that the superiority of the socket fit is directly related to the pressure distribution produced at the residual limb-socket interface. The quality of the socket fit is considered to be good if the total load is supported by the pressure tolerant areas (e.g., areas of thick tissue) of the limb. The pressure-sensitive areas (e.g., areas where the bone is close to the surface) should be non-load bearing for a better comfort. Currently, it is forecasted that 20 present of prosthetic sockets fabricated are either discarded due to improper fitting or significantly require modification to ensure satisfactory ‘fit,’ signifying knowledge gap in procedures. The amputee discomfort and pain occurs due to high-pressure interaction at the stump-socket interface, which is one of the important factors to be considered in the area of prosthetics and orthotics. For the reason above, the determination of pressure distribution is useful in effective socket design and development. Additionally, the determination of pressure distribution at the stump-socket interface proves to be an effective parameter to enhance the quality of the socket fit [5].

The transducers have been used for pressure assessment purposes since late 1970 [6]. Several researchers have studied and investigated about force, pressure, displacement, strain, normal, and shear stress at the stump-socket interface [4, 7] using a diaphragm strain gauge, piston-type strain gauge, capacitive, piezoresistive based sensors [8, 9]. Most of the investigators prefer to use piezoresistive sensors such as force sensing resistors (FSR) due to their eminent features including the small size with a simple structure, thin construction, adequate flexibility, good sensitivity, and ease of use [10, 11]. In comparison to other sensors that could be either positioned within prosthetic sockets or mounted on socket wall, all piezoresistive sensors are very thin sheets; ideal to be positioned in-situ inside the prosthetic socket. The accurate pressure measurement required a suitable measurement technique, appropriate sensor; correct positioning of the sensor at the stump-socket interface. A suitable pressure measurement system should be able to produce actual results without changing the initial stump-socket interface condition. Pressure measurement help in the realization of the intricate problems confronted during a socket fitting.

Presently researchers are more focused on determining novel bio-materials for socket manufacturing and further integration of advanced tools like reverse engineering, CAD, and FEM to produce accurate virtual model development and pressure measurement [12–14]. The past research concluded that transducers are applicable for gathering pressure data in limited regions within the prosthetic socket. FEM requires detailed information of the patient’s limb and socket geometry and material properties that is not available for each patient. The aforementioned studies have provided results that deal with the aid in understanding the critical issues faced in socket fitting. It is also found that advanced tools are needed to overcome the limitations of the traditional socket fitting process, providing practical results helpful for clinical significance [15, 16].

In presently available methods of pressure measurement, the pressure can be measured either by sensor introduction into the socket/limb interface, which affects the results collected or by altering the socket to insert the pressure transducer, leading to difficulties in daily use. For overcoming these drawbacks, a computational approach will be beneficial in predicting the pressure at the socket interface. For this reason, a fuzzy logic-based artificial intelligence model have been proposed for the determination of pressure under different conditions, i.e., static, and dynamic. In the field of prosthetics and orthotics, no work is available on applying the fuzzy logic model in pressure measurement and evaluation at the socket interface.

Soft computing techniques have the ability to describe non-trivial complex problems where input and output relations are non-linear. Soft computing techniques such as Fuzzy logic, ANN, evolutionary, and nature-inspired algorithms provide an adequate solution to the variety of complex problems, while acknowledging the uncertainty involved in the problem [17]. Fuzzy logic is effective when an accurate mathematical model is not available, can work with imprecise inputs, and at last can handle the non-linearity with ease. Therefore, in the present study, the authors have selected fuzzy logic technique to model pressure measured at different specific regions in terms of amputee physical parameters. The capability to model the relations amid the different loading conditions and pressure effects on the residual limb would provide an enhanced tool for examination and aid clinicians to analyze socket discomfort issues. It would also enable research on socket interface design and material and will help clinicians in advising involvements for amputees with complex residual limbs. Determining the influence of amputees’ physical parameters on real-life pressure values at the socket will aid the prosthesis to better design ensuring the comfort of the amputees.

11.2 MATERIALS AND METHODS

Ten unilateral below-knee male amputees have been selected for the acquisition of pressure data for this investigation. The details of the amputees are given in Table 11.1. This study considered clinically in different cases. The selected amputees regularly used a prosthesis patella tendon bearing (PTB) socket with a uniform thickness of 5 mm with cotton liner. They had been using exo-skeletal transtibial prosthesis from the last 3 to 21 years with PTB socket manufacturing in Bhagwan Mahaveer Viklang Sahayata Samiti (BMVSS), Jaipur, India. The measurements have been carried out using the Flexi-Force sensor as shown in Figure 11.1.

TABLE 11.1 General Information about Patients

FIGURE 11.1 Flexi-force pressure sensor.

The characteristics of the flexi-force sensor are listed in Table 11.2. The sensor is of small thickness, flexible printed circuit, lightweight custom shape, and size. It can measure the force between any two contacting surfaces and is durable enough to stand up in most environments with a force range (0 to 445 N). These sensors can be easily integrated into the stump-socket interface. The sensor measure both static and dynamic forces between stump and socket. It is constructed from two layers of the substrate; this substrate is composed of polyester film (or polyimide in the case of the high-temperature sensors). On each layer, a conductive material (silver) is applied, followed by a layer of pressure-sensitive ink sensor. The adhesive is then used to laminate the two layers of substrate together to form the sensor. The sensor acts as a variable resistor in an electrical circuit. When the sensor is unloaded, its resistance is very high (greater than 5 MQ); when a force is applied to the sensor, the resistance decreases.

11.3 EXPERIMENTATION AND DATA ACQUISITION (DAQ)

The experiments have been carried out on the amputees using an experimental setup as shown in Figure 11.2. One step-down transformer is used to convert an AC voltage of 220 V to 9–0–9 V. Analogue to digital converter (ADC) is used to converts an analog signal to digital. The result is a sequence of digital values that have been converted from a continuous-time and continuous-amplitude analog signal to a discrete-time and discrete-amplitude digital signal.

FIGURE 11.2 Flexi-force set up with a national instrument system.

One capacitor is used to stabilize the DC voltage. Resistance is used to drop the voltage. Then the experimental setup is connected to data acquisition (DAQ) block/card (DAQ-9171), four channels NI 9234 chassis (national instruments) by BNC probe cable to the virtual instrumentation software (Lab-view) for DAQ. DAQ is the process of measuring an electrical or physical phenomenon such as voltage, current, temperature, pressure, or sound with a computer. A DAQ system consists of sensors, DAQ measurement hardware, and a computer with programmable software. Compared to traditional measurement systems, PC-based DAQ systems exploit the processing power, productivity, display, and connectivity capabilities of industry-standard computers providing a more powerful, flexible, and cost-effective measurement solution. A Flexi-Force sensor is attached to the curved portion of the plastic frame, and the accelerometer board is mounted on its base of the oscillating mass. Flexible force sensor (FFS) system is ready to plug-n-play. The FFS works like any other bridge transducer, by converting non-linear resistance changes to a linear output voltage proportional to force. The calibration of FFS is performed by using the dead weights following standard calibration procedure. The accuracy and repeatability of measurements of FFS were analyzed. The static and dynamic loading tests were performed to obtain pressure sensing parameters. The linearity, repeatability, and hysteresis were found to be +/–4.5%, +/–4.1%, +/–6.1%. The drift was found to less than 5.6%.

In the current study, eight specific regions, as shown in Figure 11.3, have been identified to measure pressure at different loading conditions (half, full, and walking). The regions are Lateral Tibia (P1), Gastrocnemius (P2), PTB (P3), Kick Point (P4), Medial Tibia (P5), Medial Gastrocnemius (P6), Popliteal Depression (P7), and Lateral Gastrocnemius (P8). The interface pressure values have been recorded for two standing, viz. half body weight, and full body weight, for 50 seconds. For walking conditions, the amputee is asked to walk for 12 meters distance. Before initiating a measurement, all hardware components of the FFS system (socket, connecting the cable, converter) must be appropriately connected. The FFS is placed between the liner and socket at eight specific regions. Further, a pressure measurement at all eight regions can be viewed simultaneously in real-time using the software on a laptop/PC screen, and the measurement can be repeated if required.

FIGURE 11.3 The pressure points (Left) and fitting of the sensor (Right) on the limb.

The measured values of pressure at the eight specific regions are presented in Table 11.3. For this study, three trials were performed, and an average of the pressure data is selected and reported. The maximum pressure at all the three (walking, full, and half) conditions are shown in bold. From Table 11.3, it has observed that the strongest impact of maximum pressure between stump-socket interfaces is on the PTB.

TABLE 11.3 Static and Dynamic Pressure (KPa) Data Using Flexi-Force Sensor