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Development of a Master Socket for optimised design of prosthetic socket for lower limb amputees

Periodic Reporting for period 1 - SocketMaster (Development of a Master Socket for optimised design of prosthetic socket for lower limb amputees)

Reporting period: 2015-02-01 to 2016-07-31

In developed countries, more than 90% of limb amputees achieve their mobility through the use of prostheses. Nowadays, there is nothing more frustrating to the prosthetic limb users than to be told to stop using their prosthesis because of medical problems. Since the amputees will have to wear a prosthetic limb for the rest of their life, the comfort of a prosthetic limb is the primary consideration for both manufacturer and service providers, as they are keen to help the prosthetic limb users to regain a good quality of life.

In general, the walking energy consumed by an above knee amputee is approximately 80% more than a healthy person. This is because of the movement complexity associated with the human knee and ankle. A significant part of the additional energy is “consumed” between prosthetic socket and stump which exacerbates soft tissue and skin problems.

Pressure sores normally develop on the skin where mechanical pressure is applied locally over a bony prominence. An inappropriately fitted socket creates focal pressure associated with shearing force applied at the amputation site. It initiates irritation and inflammation of the superficial layers of the skin. Continuous weight-bearing degrades the integrity of the skin further. Discomfort will develop when the pressure sore gets into the subcutaneous layers of the fascia. In a worst case, it leads to an infection.

In order to be able to design a proper load bearing characteristic for a specific residual limb, one has to have prolonged training before becoming an experienced prosthetist. Nevertheless, the current design approach of prosthetists is highly subjective, and sockets are made without access to comprehensive information related to the comfort of an amputee such as friction, pressure etc.

The SocketMaster project aims to develop a new technique to address the problems described above. This will be done by integrating micro sensors into a SocketMaster tool which can help prosthetists to achieve fast customised design and manufacturing of prosthetic sockets for lower limb (trans-femoral and trans-tibial) amputees. The project has the following main objectives:

(1) To develop micro sensors able to measure pressure, friction, temperature and other parameters relevant to the comfort of patients within the interface between the residual limb and the socket in both static and dynamic situations. These sensors will be miniaturised such that they can be integrated in a SocketMaster tool.

(2) To develop a SocketMaster framework by assembling the sensor system in a rigid hosting socket in such a way that the sensors' positions can be adjusted to achieve comfortable configuration for the patient. The pressure distributions, friction information and loading etc. during walking tests at will be acquired and processed to achieve optimised socket design. The design process will be completed within 2 hours after activity testing, and the resultant digital 3D data will allow manufacture by a rapid prototyping machine for fast fabrication of tailored sockets for a specific patient.

(3) To perform clinical trials with 50 patients to validate the SocketMaster technique and procedures.
The project officially started on February 1st, 2015. During the 18 months of the reporting period from February 1st to July 31st, 2016, work has been carried out mostly according to the original plan. Details of the work and the results are as follows.

Work Package 1: System specification and preparatory research

A further literature review and open discussions have been carried out at the beginning of the project. All partners have provided specifications for the SocketMaster system based on their own experience and expertise associated with prosthetic socket design and manufacturing. This included the work conditions in which the SocketMaster system will operate, the material types and structures of mechanical and electronic framework of the SocketMaster tool, the materials and layout of the pressure, friction and temperature sensors, and the number of patients for clinical validation trial. The result is a specification document that has been agreed by all partners. This specification includes the parameters on the characters of gaits, the geometries, dimensions of the SocketMaster tool that can fit most patients but still maintain sufficient stability, the characters of pressure/friction/etc. between residual limb and socket, and other clinical requirements of the system to be developed.

As the clinical condition of above knee amputees is complex, it is not possible to determine all the parameters in the specification at this stage. Therefore, only recommendations using a range for the parameters were provided, and the functional specification serves as a guide for other tasks of the project. This specification document will be updated when new information is available.

Work package 2: Sensor development

Extensive work on sensor development has been carried out. This started with a comprehensive modelling and analysis of the MEMS piezoresistive force sensors upon mechanical normal and shear forces. During the modelling work, the main physical parameters that will be implemented and optimised in the final design of the sensors were analysed. The first experimental prototype for technological evaluation tests of the system architecture will be based on 5 MEMS force sensors, covered by a polymeric dome (Cover cap) for force distributions. The sensors are mounted on a PCB board with integrated electronics for the sensors read-out and data transmission by I2C bus.

The second version of the system that will be used for the final tests will consist of 5 force sensors integrated on the same silicon chip for functional tests and it will include:
• Optimization of both design/fabrication process (e.g. Circular diaphragms, vertical etching, and backside contacts);
• Optimization of the polymeric dome (Cover cap);
• The board is mounted on a PCB with bus interconnections.

The layout of a piezoresistive MEMS–based sensor prototype has been finalised and the MEMS fabrication process has started. Key features of the sensors include:
• SOI process on 6” Wafer
• 109 DEVICES/wafer
• 256 TEST CHIPS/wafer
• 64 TEST STRIPS/wafer

As an essential part of sensor integration, the first version of the sensors system architecture (chip on board) has also been established. The PCB board has 4 layers with a dimension of28x43 mm. the smallest line width is 100μm. The read-out circuit has 5 independent signals from each Wheatstone bridge, with A/D conversion, I2C data transmission protocol, and 10-bit address space. The bus speed is 100 kbit/s with standard mode.

The prototype sensors were experimentally tested. They were mounted on a first test prototype of PCB board with integrated electronics for the sensors read-out and data transmission by I2C bus. The pressure sensor was tested in the dedicated setup under both pressure gradient and temperature variation. Tests showed that the behaviour of the sensor is linear for all temperatures, and the linear coefficients can be parametrized as a function of temperature obt
The expected final results are:

1) Multifunctional micro sensor system: With this sensor system, the distribution of pressure, temperature, moisture and friction of the residual limb at a specific point of gait cycle can be quantified.

2) A SocketMaster medical tool: The SocketMaster tool will be able to fit patients with varying sizes of residual limb. The data collected from the SocketMaster tool will be fed into a prosthetic socket design system, so that an optimised prosthetic socket can be designed. The integrated system will help a prosthetist to achieve rapid socket design and manufacturing with optimised comfort for the patient.

3) Residual limb evaluation model: The residual limb of a patient will be evaluated by a hand held device. Comprehensive finite element simulations will be carried out, which will help develop a biomechanical model that can be used to turn the dynamic data collected by the SocketMaster tool during walking tests into an optimised socket for the patient.

4) Data processing software: Various data from the multifunctional sensors will be processed so that an optimal socket design can be achieved.

5) Clinical validation report: This is one of the most important outcomes of the project. It will demonstrate the effectiveness of the SocketMaster system.

6) New socket design procedure: With the novel Master Socket technique, a new clinical procedure for prosthetic socket design will be formulated.

Potential impact and use

Currently, there are three main causes leading to limb loss; (a) trauma, civilian and military (45%), (b) vascular diseases (54%), including diabetes and peripheral arterial disease and (c) cancer (less than 2%). Diabetes is the major cause leading to amputation and according to the International Center for Limb Salvage, patients with diabetes have 70% more probability to develop gangrene and require amputation, compared to normal population. In the US the estimated amputees population is nearly 2 million people and this population is estimated to double by the year 2050 to 3.6 million people. Globally, there are more than one million limb amputations annually –one every 30 seconds- and this number is expected to increase since the International Diabetes Federation predicts that diabetes will burgeon from 336 million in 2011 to 552 million people by 2030.

Existing method for socket design is similar to the process of a tailor making a suit. The first step involves taking the necessary measurements for a good fit; hence it can be understood that the development of the socket significantly depends on the skills of the prosthetist. The socket significantly determines patient’s comfort, as it is responsible for distributing the loads from the residual limb to the prosthetic component. The SocketMaster system targets the development of sockets customised to the residual limb pressure distribution of each individual user while also ensuring an ideal fit to the stump.

The SocketMaster project is expected to contribute to
(1) Secure and reinforce European leadership in the microsystem sector, expanding its share in smart systems for medical applications.
(2) Seize new opportunities in addressing societal challenges in health and well-being.

Firstly, the SocketMaster project targets reinforcement of Europe’s position in the field of medical technology. According to a report published on orthopedic devices raked in $34.8 billion worldwide in 2014, which is around €30bn. Assuming 10% is for prosthetics, which equates €3bn. Also assuming that the SocketMaster product will capture an overall 1% of the world prosthetics market, this will equate to €30million. This increased market share will allow the European medical technology companies, 95% of which are Small and Medium Sized Enterprises, to strengthen their position in the worldwide market. Assuming that one job is created per €150,000 worth of sales, the SocketMaster product will create approximate
First design of a piezoresistive MEMS – based sensor prototype