Development and application of in-fibre Bragg grating contact force sensors for application to the human hip

This dissertation presents contact force sensors that are based on an emerging fibre-optic sensing technology, the in-fibre Bragg grating (FBG), for contact force measurements between cartilage surfaces in the human hip. There are two main motivations for force measurement in hips (and other joints). First, there is clinical evidence that suggests excessive force magnitude and duration can cause painful degeneration of joints. Second, insights from ex vivo force measurements during simulated physiologic loading are the basis of the rationale for corrective surgeries meant to halt degeneration and restore proper joint function by restoring natural joint mechanics. The current standard tools for force measurements in joints are force/stress sensitive films.

There are problems associated with inserting these films into joints that affect the force/stress measurements. To insert the films, the joint must be dissected of surrounding soft tissues and, ultimately, the joint must be taken apart (disarticulated). Following disarticulation, films are fixed to cartilage surfaces, and the joint is re-assembled so that physiologic loads can be applied. The negative consequence of dissection and disarticulation is that the natural mechanics of the intact joint are permanently lost and, therefore, film measurements do not indicate the actual joint mechanics. Moreover, covering cartilage surfaces with rigid films alters the natural contact mechanics of the joint.

The force sensors presented in this dissertation are designed for local force measurement over the region of the optical fibre containing the FBG and address limitations of force/stress sensitive films. The FBG force sensors are extremely small (major diameters ranging from 0.165 mm to 0.24 mm) and can be inserted into joint spaces without dissection of soft tissues and disarticulation thereby allowing the joint to remain intact. Theoretical and experimental results indicate that FBG sensor measurements are less affected by the mechanical properties of cartilage than are film sensors.

The sensors presented in this dissertation also address limitations with previous FBG based force sensors and are the first application of FBGs in intact human hips. The sensors are smaller, and therefore less invasive, and insensitive to orientation, axial strain and temperature, unlike other FBG sensors presented in the literature.