Detection of Loosening of the Humeral Component of Elbow Prostheses

INTRODUCTION: Total elbow arthroplasty (TEA) is an effective treatment option for painful arthritis and severe traumatic fractures, providing relief from pain and restoring elbow function. TEA has continued to evolve over the past four decades. However, due to the distinctive anatomy and biomechanical properties of the elbow joint, the complication rates of TEA are 20% to 40% higher as compared to total hip or knee arthroplasty [1]. In the literature the leading cause of failure is aseptic loosening, associated with more than 52% of all revisions, with the humerus component being involved [2]. Identifying early signs of aseptic loosening still remains a challenge and symptoms of loosening are quite nonspecific. Conventionally, the diagnostic methods which are used to assess the performance and detect implant loosening involves clinical measures and imaging techniques that require radiation exposure to patients. These techniques have low sensitivity, specificity and their accuracy for detecting early aseptic loosening is extremely low. The goal of this study was to design and evaluate a new diagnostic technique which has the ability to non-invasively measure real time implant performance, providing invaluable feedback to clinicians and patients and has the capability for early detection of aseptic loosening without the need for radiographic imaging.
METHODS: For in-vitro implant micro-motion measurement, a magnetic measuring system was used which included a magnetic sensor and a magnet. To obtain the correlation between the magnetic field and displacement, an algorithm was formulated to identify the migration parameters, namely static and dynamic. For the calibration of the sensor, a mechanical testing system (TA, Electro Force 3300, Boston, USA) was used to provide input migration of the implant via its two motorized stages i.e. linear and torsion with the resolution of 0.5 μm linearly, 0.01 degrees angularly (torsion) and 0.01 Hz frequency. To provide a repeatable simulation of implant migration, we designed and fabricated an adjustable fixture for holding the sensor and magnet. This fixture was attached with the electro force and provided 2 axis movement (x and y direction) for linear displacement and also for torsion. The linear axis was moved from 1 mm to 12 mm in x/y axis with the step size ranging from 0.1- 0.5 mm having frequency from 0.1- 2 Hz, while the torsion was rotated from 1 degree to 10 degrees in x/y axis with a step size of 0.1- 0.5 degree having frequency from 0.1- 2 Hz. The manual position stage was used to adjust the distance between sensor and magnet in z-axis. The moving least square method was used to reduce the noise level of the signal and two types of tests were carried out i.e. linear displacement detection and angular displacement detection.
RESULTS SECTION: The working range of the sensor was calculated as 3 mm to 12 mm in z axis and 0.3 mm to 10 mm in x/y axis linearly with a resolution of 0.3 mm in the x/y axis. The angle range was up to 6.0 degrees in the x/y plane with a resolution of 0.1 degrees at a radius of 42 mm. The resolution depended upon the operating condition. When operated in the calibrated range the maximum detected distance between the source and sensor was approximately 10.5 mm, while the maximum displaced distance was approximately 6mm linearly and 3.5 degree angularly in x/y axis. The repeatability and stability of the calibrated sensor showed an excellent result (0.05 mm Standard deviation for 150 cycles). Figure 1A shows the filtered signal when the implant was migrated statically between -1 to 1 mm with a step size of 0.5 mm and figure 1B shows the comparison of filtered data with standard data. The system was able to detect accurate result when frequency was between 0.1 to 0.5 Hz.
DISCUSSION: This study demonstrates how a magnetic measuring system can be used to assess early migration of the implant and has many advantages such as low cost, radiation free, electrical stability, high accuracy and robustness. The limitation to this system is that it currently operates at low frequency between 0.1 to 0.5 Hz. Greater than this frequency the sensor sensed the change but could not assessed the accurate distance. Furthermore, any implant tilting beyond 4 degrees results in a non-accurate reading however this error can be compensated by adding another sensor, which is currently in progress.
SIGNIFICANCE/CLINICAL RELEVANCE: Currently, signs and symptoms of loosening may not be clinically apparent until late stages of failure due to the lack of accurate and sensitive early diagnostic tools. Rapid and accurate tools which could detect the micro-motion of the implant are increasingly needed. Using these types of sensors to develop a smart system for better understanding the performance of implant position will not only help patients but will have a high impact in the advancement of biomedical tools which will save time for clinicians and reduce healthcare cost.
REFERENCES:
1. Voloshin, I., et al., Complications of total elbow replacement: A systematic review. Journal of Shoulder and Elbow Surgery, 2011. 20(1): p. 158-168.
2. Registry, N.J., NJR'S 12TH ANNUAL REPORT 2015. 2015.