Optical Coherence Tomography Guided Laser Cochleostomy: Towards the Accuracy on Tens of Micrometer Scale

Lasers have been proven to be precise tools for bone ablation.
Applying no mechanical stress to the patient, they are potentially
very suitable for microsurgery on fragile structures such as the
inner ear. However, it remains challenging to control the
laser-bone ablation without injuring embedded soft tissue. In this
work, we demonstrate a closed-loop control of a short-pulsed CO2
laser to perform laser cochleostomy under the monitoring of an
optical coherence tomography (OCT) system. A foresighted detection
of the bone-endosteum-perilymph boundary several hundred
micrometers before its exposure has been realized. Position and
duration of the laser pulses are planned based on the residual bone
thickness distribution. OCT itself is also used as a highly
accurate tracking system for motion compensation between the target
area and the optics. During ex vivo experimental evaluation on
fresh porcine cochleae, the ablation process terminated
automatically when the thickness of the residual tissue layer
uniformly reached a predefined value. The shape of the resulting
channel bottom converged to the natural curvature of the endosteal
layer without injuring the critical structure. Preliminary
measurements in OCT scans indicated that the mean absolute accuracy
of the shape approximation was only around 20 mu m.