In-vivo detection of patellar tendon creep using a fibre-optic sensor

Abstract

Tendon creep causes an increase in longitudinal and negative transverse strain (Poisson effect), resulting in a pressure increase on an implanted optic fibre. An optic fibre was inserted through a volunteer's right patellar tendon, after hysteresis and measurement sensitivity were characterised. The volunteer performed three isokinetic knee-extensions and one isometric knee-extension on a dynamometer. In-vivo calibrations of quadriceps torque versus sensor output were obtained and used to predict the torque delivered during the isometric exercise. Optic fibre hysteresis induced an 18% full-scale root mean square error, and in-vivo calibrations were less dependent on loading rate than in-vitro calibrations. Dynamometer isometric torque readings decreased by 3.7% over 180 seconds while sensor predictions indicated a 5.9% rise. This rise can be ascribed to the increase in transverse strain (resulting from creep). The fibre-optic sensor was influenced by tendon creep. Knowledge of tendons' time dependent behaviour are essential in medical engineering modelling studies.