Author: Farzaneh Mohammadbeigi
Mohammadbeigi, Farzaneh, 2023 Design and implementation of a passive wireless sensor to monitor bone fracture healing, Flinders University, College of Science and Engineering
Terms of Use: This electronic version is (or will be) made publicly available by Flinders University in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. You may use this material for uses permitted under the Copyright Act 1968. If you are the owner of any included third party copyright material and/or you believe that any material has been made available without permission of the copyright owner please contact copyright@flinders.edu.au with the details.
Bone fractures are a common orthopaedic injury that can result in significant morbidity and prolonged recovery. Timely and accurate monitoring of fracture healing is critical for determining the effectiveness of treatments, diagnosis of nonunion in early stages of healing, and optimizing patient outcomes. Current methods of assessing fracture healing, such as imaging measures and clinical evaluations, are subjective and may not provide enough information about the progression of healing. Considering that the primary goal of treatment is to restore the mechanical function of the fractured bone, it is essential to have a method for quantifying the mechanical conditions or structural stability of the healing of the bone defect. Mechanical evaluation methods can provide real-time information on the mechanical environment, including induced stresses and strains on the healing bone. Smart implants can be a good solution for a quantitative approach to mechanical assessment. With a smart implant, sensors can be incorporated into the implant to measure parameters such as load and strain, which can provide more objective and quantitative data on the healing process. Additionally, smart implants can provide real-time feedback, allowing for earlier intervention if any issues arise during the healing process.
For this study, the intramedullary nail was chosen as the implant of choice. The intramedullary nail has been widely used in the treatment of long bone fractures due to its biomechanical advantages and ability to provide stable internal fixation. Intramedullary nails offer several advantages over plates for designing a smart implant to monitor bone fracture healing, including load sharing and biomechanical stability.
The first step in this process was to define the induced strain range and the pattern that is induced on the intramedullary nail throughout the healing course. This was accomplished through a series of biomechanical tests that involved inserting an optical fibre containing 29 FBG sensors into the nail. The sensors detected the induced strain over the length of the nail, allowing accurate measurement and monitoring of the forces acting on the implant during the healing process. The instrumented nail was tested in a Sawbone femur with a 3 mm gap created along the femur shaft's centreline to simulate a transverse diaphysis fracture.
With a focus on early diagnosis of bone healing complications, the physical properties of bone callus tissues during the early stages of the healing process were replicated using polymer materials. This was achieved through the creation of four stages of callus simulation, each with a range of young modulus values from 0 to 180 MPa. In biomechanical testing, the instrumented nail and bone complex were subjected to a load of 150 N, which is considered safe for application a week after a trauma. In axial compression tests, all four models showed similar strain patterns. The strain readings with FBG from the posterior plane of the nail were measured, processed, and
6
analysed. According to the findings, the required sensitivity was about 20 μɛ with maximum strain of 381 μɛ.
In passive wireless applications, the sensor must gather energy from an internal or external source to do the measurement and transmit the data. Therefore, minimal power consumption is desired in this technology, which applies to a passive wireless sensor used during the course of bone fracture healing.
This thesis proposed using an InterDigital Capacitor (IDC) as a strain sensor for this application due to their high linearity, low hysteresis, and low energy consumption.
The sensitivity of the sensor was characterized through a cantilever test. The gauge factor was found to be in the range of 3, which is higher than those of commercially available strain gauges. The test sensor showed excellent linearity across a test range of applied strains up to 400 μɛ. The gauge factor was determined for an in vivo temperature and humidity range, and it was shown to be stable and independent of temperature and humidity. The preliminary results suggest that it is feasible to achieve an appropriate reproducibility of the sensor gauge factor when using multiple sensors with the same parameters.
To create a passive wireless design for a smart implant with a capacitive strain sensor, identifying the energy harvesting method is crucial as the sensor requires power for wireless data transmission. Therefore, identifying the energy harvesting method first ensures an efficient and effective wireless system design. RFID Inductive coupling was selected as an appropriate method for powering the sensor and transmitting sensor data from the wireless power transfer approaches. An RFID passive wireless system was created to integrate the capacitive sensor intended for measuring the strain of an intramedullary nail. This wireless system utilized an air-core circular antenna for data and power transmission, allowing for wireless monitoring of the nail's strain. The selected antenna was chosen from several proposed and evaluated coils and was found to have a power and data transmission distance of about 8 cm with tissue material interposed. The data acquired from a wired setup with a commercial evaluation board showed good agreement with the developed wireless system. The implemented system demonstrated high performance in terms of data and energy transfer for the implanted sensor. Therefore, the developed system is potentially deployable for intramedullary nail implantation.
Keywords: Bone Fracture Healing, Mechanical Assessment, Smart Implant, Instrumented Nail, InterDigital Capacitor (IDC), Wireless Power Transfer, Passive Wireless, FBG Sensors, Passive RFID, Capacitive strain Sensor
Subject: Engineering thesis
Thesis type: Doctor of Philosophy
Completed: 2023
School: College of Science and Engineering
Supervisor: Prof. Karen Reynolds