Prototype Design and Workflow for a Scoliosis Brace Monitoring and Evaluation System

Abstract

While bracing has been proven to be an effective treatment strategy for adolescent idiopathic scoliosis, its effectiveness is often found to be inconsistent in practice. One of the main reasons for this is that the forces applied by the brace may be subject to constant change, while its required effect is expected to change over time as well. Brace treatment becomes even more complicated when considering that both the patient and the practitioner are provided with limited to no information on how this relation may be changing over time. While measurements of the pressure distribution may be used for evaluation of the brace, most commercially available devices are too expensive or unsuitable to be integrated in a brace. This study therefore sought to investigate the possibility of designing a cost effective monitoring system based on force sensitive resistors (FSR) that can be implemented in the brace to evaluate brace performance.

Several metrics were defined to evaluate different functional aspects of the brace. This simultaneously served as a basis for the system requirements, which focused on finding a cost effective solution for evaluating brace performance, while no restrictions on dimensions and mass of the prototype were set at this time. The hardware of the system was based on an Arduino Mega microcontroller connected to custom designed PCBs for voltage regulation and signal conditioning, which were integrated in a purposely designed belt. The belt was connected to a sensor pad with a total of 15 integrated FSRs (FSR402 by Interlink Electronics) and was designed to fit the pad located in the lumbar area of the brace. The FSRs in the sensor pad were calibrated in the 0 - 26kPa range using a model developed for creep and hysteresis compensation. When tested with a consistent actuation system, a random error of 4% was found. The model also showed to yield a decent approximation of the hysteresis behavior of the FSRs using a parametric third order polynomial function, but further research is required for this to be validated.

In order to evaluate the three dimensional effect of the brace forces on the spinal column, the spatial geometry of the spine and the sensor pad had to be reconstructed. A generative design algorithm was developed for doing this using only a surface model of the torso, an AP radiograph of the spine, standard vertebra dimensions and the geometry of the sensor pad including the relative two dimensional locations of the integrated FSRs. The method yields a low resolution reconstruction of the required geometry and serves as a proof of concept for acquiring data that normally would require sophisticated equipment which may not always be available.

Validation showed that the system could effectively be used to measure pressure distribution inside a scoliosis brace. Differences were measured related to posture and respiratory movement, which was found to have an effect on the mean pressure and the pressure distribution in the lumbar area. Combining the measurements with the spatial geometry allowed for computation of equivalent bending moments, which seemed to be in line with the expected function of the brace.

Although this research showed promising results for an FSR based pressure measurement system for scoliosis braces, further research is required to optimize the different aspects of its design.