Spatially distributed circumferential strain measurement in an artificial spinal disc using fibre Bragg gratings

Artificial disc replacements (ADRs) are orthopaedic implants designed to replace degenerated intervertebral discs, a common aetiology of chronic back pain. ADRs which emulate the biomechanics of the natural disc promote the natural physiological kinematics, which is important for minimising postoperative deterioration of surrounding structures. By quantifying and comparing the displacements and strains within the ADR and the natural disc, target values can be established for future biomimetic ADR designs. However, measuring internal strain in these devices can be challenging. To this end, this study presents a novel application of fibre optic strain sensing through the use of fibre Bragg gratings. In this proof-of-principle study, an ADR model was fabricated with an embedded fibre optic sensor and subjected to physiological loads. The ADR model consisted of an elastomeric ring reinforced with Kevlar fibre. This simulated the annulus fibrosus (AF) of the natural disc and was left either filled or unfilled with an elastomeric material which simulated the nucleus pulposus (NP) of the natural disc. Fixation of the disc to artificial bone was achieved using three pins on the superior face and three pins on the inferior face of the ADR. Using wavelength division multiplexing of the fibre Bragg gratings, the hoop strain within the AF was monitored with a spatial resolution of 10 mm. Measurements revealed local strain concentrations around the fixation points and that the nucleated configuration increased circumferential strain in the AF by over 10 times for a given axial displacement. The measured strain asymmetry was correlated with lateral bending angle with an accuracy of ±0.5 degrees, which could be applied to tracking changes to the range of motion in load-controlled bending tests, for example. With further development, this technology has the ability to assess future designs during pre-clinical testing.