Fiber Bragg Grating Sensors for Clinical Measurement of the First Metatarsophalangeal Joint Quasi-Stiffness

Assessing the mobility of the first metatarsophalangeal joint (MTPJ) of a human foot is useful in clinical practice, but there are no standard methods of measurement. The present study developed a new instrumentation using Fiber Bragg grating (FBG) sensor and load cell to quantify the first MTPJ quasi-stiffness in a clinical setting. This system is portable, lightweight, and allows quantification of quasi-stiffness over different ranges of motion in both loading and unloading directions. The laboratory setting validation results showed that FBG sensors could measure MTPJ angular displacement with reasonably good accuracy. The proposed system was successfully trialed in a hospital setting operated by a clinician on eight human subjects. Non-linear torque-angular displacement relationship was observed in both loading and unloading phases, with varying MTPJ quasi-stiffness in the early [loading 6.30 (2.62) Nmm/° unloading 8.46 (2.29) Nmm/°], middle [loading 7.13 (2.17) Nmm/°, unloading 11.11 (2.94) Nmm/°], and late [loading 24.54 (7.14) Nmm/°, unloading 14.50 (4.77) Nmm/°] ranges of motion. The new method for measuring the first MTPJ quasi-stiffness established in the present study serves as a reference and opens up opportunities for future clinical investigations.