Diffuse Optical Tomography in a Spinal Cage: Monte Carlo Simulation and in-Vitro Studies

Vertebral arthrodesis supported by cage devices is a successful surgical option for treating degenerative disorders of the spine. Despite the favourable results reported, the process of formation and maintenance of bone tissue is not well understood. Improving knowledge on these multifactorial biological mechanisms is crucial to advance the technique. Diffuse optical tomography (DOT), implemented with microfabricated optodes, is a promising way of monitoring bone growth in the spinal cage. In this paper, we present a preliminary study on the feasibility and value of such a system. The direct problem has been addressed numerically and through in-vitro measurements. Two scenarios of interbody fusion (creeping substitution and endochondral ossification) have been simulated using Monte Carlo techniques for several optode configurations. A 16times16 photodiode-based DOT system based on fibre-optics has been developed to validate the numerical results in conjunction with an in-vitro model of the biological problem. We have found that the small tissue volumes involved permit a Continuous Wave implementation based on LED´s and photodiodes. The needed levels of optical power are compatible with operation from a telemetry link. Significant contrast has been found for the various stages of spinal fusion in the considered scenarios, which suggests that the inversion problem will be able to differentiate among them.