Mechanical Design Optimization of Intraoperative Spinal Load Sensing during Scoliosis Correction Surgery

Technologies used in corrective scoliosis surgery do not provide accurate, validated measurements of applied loading on the spine. Proposing a solution to optimize intra-operative load sensing to enhance surgical outcomes, mechanical factors of a capacitive load sensor are examined. Using ASTM D3574-17, experimental methods were performed to verify material homogeneity and validity, to identify critical factors in maximizing compressive strength, and to understand preliminary fatigue behavior for reliability measures. In leveraging the Design of Experiment (DOE) methodology to decrease device variability, the mechanical factors explored were: sensor thickness, diameter ratio of conductive foam, density, and surface hardness. Multiple iterations DOEs identified high thickness and low diameter ratios as significant factors which increase the output response of compressive strength. After identifying the optimal factor combination for the sensor it was found that the maximum experimental load range was 15.57N-16.9lbf. Fatigue testing was then performed on the highest performing factor combination group from the compression results. From the two rounds that were tested on sensor specimen, no significant difference was found between the two groups' rates of changes in thickness per compression. Each round of foam testing resulted in similar thickness values, which suggests that the sensor has potential to perform consistently during a 6-8 hour surgery if a material with improved elasticity and mechanical strength is used. Thus, the experimental procedures fulfill proof-of-concept tests to indicate feasibility of compressive strength and reliability of the sensor's mechanical features. Future experimentations will involve using a different dielectric material in place of the foam, such as a conductive thermoset or thermoplastic elastomer. Additional levels for each factor will be test to test the behavior of the material to yield a higher compressive strength and certainty of reliability. Overall, this study was useful in identifying significant factors for achieving compressive strength, while also providing evidence of the device's potential for reliability during scoliosis surgeries.