Miniature fiber optic pressure sensor with composite polymer-metal diaphragm for intradiscal pressure measurements

Abstract

Lower back pain, which continues to be a leading cause of disability in people of all ages, has been associated with degenerative disc diseases. Some studies on humans and animals have found that the mechanical stress, especially the stress-induced fluid pressure within the intervertebral disc, is one of the main factors and indicators of the health of a disc.^{1, 2} However, a quantitative study remains a challenge due to the constrained space inside a disc and the lack of miniature sensors. A simplified diagram of the healthy intervertebral disc is illustrated in Fig. 1 . The disc is composed of an annulus fibrosus and a nucleus pulposus. The annulus fibrosus is a strong radial tirelike structure made up of lamellae, which are concentric sheets of collagen fibers connected to the vertebral end plates. The nucleus pulposus is enclosed by the annulus fibrosus. Although the nucleus pulposus is composed of water, collagen, and proteoglycans (PGs), the dominant portion of it is the fluid (water and PGs). The amount and pressure of water in the nucleus vary from time to time depending on physical activity and state of health. To measure the interstitial fluid pressure, a pressure sensor has to be inserted into the nucleus as shown in Fig. 1. Traditionally, commercially available miniature sensors are inserted via a needle through the annulus into the nucleus. Due to the relatively large size (in millimeters) of the sensors, the experimental studies are limited to humans, pigs, and rabbits that have large discs.^{3, 4, 5, 6}