Abstract
Epidural anesthesia is a pain relief method achieved by injecting anesthetic into the epidural space using a medical catheter inserted in an epidural needle. To reach the epidural space, the needle penetrates through subcutaneous fat, interspinous ligament, intraspinous ligament and ligamentum flavum. Existing manual methods for epidural space identification are based on the decrease of pressure applied to the needle when it penetrates from the dense ligamentum flavum to the soft epidural space. The failure rate of the manual techniques is high. This work proposes to measure the strain using a single-mode optical fiber, mounted externally on the needle. By the use of an optical backscattering reflectometer, it is possible to achieve a distributed strain sensing over the whole length of the needle. The spectral intensity distribution of the scattered light changes when the optical fiber is exposed to strain variations, so a strain pattern can be retrieved by correlating the unperturbed spectrum to the perturbed one. According to the strain pattern, obtained during the needle insertion into a custom made phantom, which mimics the spinal anatomy, when the needle penetrates the ligamentum flavum, a significant strain increase is determined. When the needle pierces the soft epidural space, the strain slightly drops. The proposed design brings the following advantages: optical fiber embedded externally on the needle does not obstruct the flow of anesthetic fluid; the whole needle acts as a sensor which allows the operator to discriminate tissue layers as well as to define the epidural space.