Total Knee Arthroplasty is an extremely common procedure carried out across the United States. However, despite the extensive research and testing leading to new methods of surgery and improved implant designs, approximately 20% of patients are dissatisfied with their knee replacements. Like any system, there are multiple factors that can lead to failure. These include wear and loosening, which can be caused by a misalignment during surgery or unbalanced ligaments. In order to detect loosening, there have been several attempts to utilize passive sensors, such as piezoelectric transducers and strain gauges, installed in artificial knee replacements to detect a shift in the proper alignment of the implant. There has also been recent work reported that utilized the active electromechanical impedance (EMI) sensing method, which uses a single PZT in order to detect a change in the state of the monitored system, to monitor knee replacements. However, the study isolated the system so that there was no external force applied to the PZTs while the testing was performed. This work is intended to evaluate the reliability of the EMI method for monitoring of total knee replacements under an applied force in order to determine whether or not this method can be utilized in-vivo to evaluate if a replacement has failed before it becomes hazardous to the patient. This work utilizes a rectangular block of artificial bone, a rectangular block of a similar metallic alloy used in tibial trays, and a polyethylene block made of material similar to the polymer used in bearings in order to simulate the tibia, tibial tray, and bearing, respectively. The artificial bone and metallic alloy components are bonded together with bone cement, and a PZT transducer is bonded to the alloy component using superglue. The polymer component is placed on top of the PZT transducer. The system is tested under static load to monitor several stages of artificial damage. This work is intended to serve as a foundation for further in-vivo and intra-operative studies.
Total Knee Arthroplasty (TKA) continues to be a common and important orthopedic procedure for many in the United States. Despite recent medical advancements and increasing knowledge in the orthopedic community, it has been determined that 20% of TKA patients are still dissatisfied with their knee replacements. Causes of this failure include septic loosening and wear on the bearing component of the implant. Another cause of failure that has received specific attention from the mechanical community is aseptic loosening, which has been attributed to unbalanced ligaments or misalignment of the implant components. Previous efforts have been made to detect loosening by using passive force sensors such as piezoelectric transducers or strain gauges to detect misalignment. An alternative to this is to perform active sensing or structural health monitoring to evaluate possible loosening before it becomes a critical concern to the patient. One method of structural health monitoring, called the electromechanical impedance (EMI) method, is particularly attractive as it can use a single, compact piezoelectric transducer to determine the state of the host structure. This work is intended to evaluate the ability of the EMI method in sensing loosening between the cement and bone of a TKA tibial tray. This work will utilize real tibial trays implanted into synthetic bone (Sawbone) to evaluate the feasibility of detecting loosening using the EMI method. The intention of this work is to serve as a foundation for further in-vivo and intraoperative studies.
For the past century, developing an understanding of human locomotion has been instrumental in advancing orthopedic medical knowledge and technology. Historically, the field of human gait analysis has relied on force plates to investigate the forces occurring between feet and contacted surfaces. A new thrust in recent years has been to investigate foot contact forces by using specialized force sensing insoles. The medical community has already benefited from initial force sensing insole designs. Despite this technological advancement, the currently existing force sensing insoles are largely “one size fits all.” This presents a challenge for the medical community as an accurate and ergonomic measurement system is not available for patients with special orthopedic needs such as those with flat feet or diabetic ulcers. Introduced here is the potential solution of using soft 3D printed material, called NinjaFlex, to develop custom, ergonomic insoles which possess embedded force sensors for plantar pressure detection. In this paper, groundwork for developing such a custom force sensing insole is laid by investigating the ability to use force sensors embedded into a geometrically simplistic 3D printed structure to detect forces applied to the overall system. Three different force sensors are investigated and their ability to accurately detect force in this configuration is compared. Additionally, a simple model relating sensed force to force applied to the system is developed. The intentions of this work are to verify the feasibility of a custom force sensing insole which further benefits the medical community.
KEYWORDS: Transducers, Sensors, Ferroelectric materials, Surgery, Prototyping, Signal generators, In vivo imaging, Signal processing, MATLAB, Data acquisition
Total knee arthroplasty, as one of the most common surgeries in the United States, has been widely used to help restore the functionality of damaged knee joints. Alignment of the knee joint during surgery is an extremely important factor to achieve a successful operation. Several methods have been used to quantify the alignment and to provide surgeons with a repeatable method of surgery. However, lack of in vivo information has hindered establishment of correlation between intra- and postoperative knee conditions. In this work, the application of multiple piezoelectric transducers encapsulated inside the ultra high molecular weight polyethylene knee bearing for collecting in vivo data is suggested. The piezoelectric elements display the ability to sense and harvest energy from the joint during daily activity. As a sensor, piezoelectric transducers are designed to measure the compartmental forces as well as the location of the contact points between the femoral and tibial components of the knee implant. Initially, finite element analysis is performed to investigate the sensing performance of the system. In addition, a prototype instrumented bearing is fabricated and the performance of the system in measuring the forces and locations is investigated experimentally. In the experiments, the voltage signals generated by the piezoelectrics are obtained and processed to measure two components of force as well as two different contact points, one each on the medial and lateral compartments of the knee bearing. On the other hand, the actual force profile and the location of contact areas are recorded using the load frame’s built in load cell, and pressure-sensitive films, respectively, and compared to the measured data from the piezoelectrics. The result of FE simulation showed a maximum error of about 1.5% in force sensing and a maximum deviation of about 0.5 mm in the measured location of the contact points. The experimental results also showed that the measured force and location by the piezoelectric sensors match the actual quantities measured from load frame and pressure film fairly well.
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