The first FOS used to sense IMP was the Camino sensor [191,192]. Basically, it consisted of a catheter tip with a diameter of 1.35 mm enclosed in a saline-filled sheath with side holes and a 2.1-mm outer diameter (Fig. 10.14). A pressure-sensitive diaphragm caused the mirror distance from the fiber tip to vary, changing the intensity of the reflected light [6].

The sensor was validated for IMP measurements with a slit catheter [191] and experiments were done to study muscle force during isometric and concentric exercises [191], to correlate IMP with muscle depth [193], to understand compartment muscle syndrome following prolonged pelvic surgery [194], and to analyze the muscles' contribution during gait [195]. However, prolonged measurements resulted in patient discomfort, which was associated with the large diameter and rigidity of the polyethylene sheath [191]. To accomplish the demands of sensor miniaturization Kaufman et al. [196] proposed a new FOS with a diameter of only 360 μm based on an extrinsic Fabry–Perot (FP) air cavity between a polished end fiber and a reflective membrane [196,197] (Luna Innovations, Blacksburg, VA, USA). The sensor's accuracy, repeatability, and linearity were better than 2% FSO, with a hysteresis of 4.5% FSO and sampling frequency of 66 Hz (∼10 Hz with eight channels). Its accuracy was better than that of most of the fluid-filled systems (between 1% and 18%), but lower than that of electronic transducer-tipped catheters (0.2% accuracy) [198]. Its biocompatibility was tested using ISO Standard 10993-6:2007 (Tests for Local Effects After Implantation) [199]. Nevertheless, it was tested only for in vivo intracardiac pressures [200]. However, a second-generation sensor (Luna Innovations) with a smaller diameter (250–280 μm), similar accuracy (1.45 ± 0.32%) and repeatability (1.5 ± 0.81%), but lower hysteresis (0.60% FSO) and higher sampling frequency (960 Hz, ∼240 Hz with four channels) was used to study IMP in anesthetized rabbits [201,202]. In 2015, a commercial FOS (Samba 420 LP, Samba Sensors AB, Göteborg, Sweden) was also used to measure IMP to investigate the relation between the amplitude of pulse-synchronous IMP oscillations and the absolute level of IMP in a human experimental model of abnormally elevated IMP (simulated compartment syndrome) of the leg [203].

The IVD is a fibrocartilaginous structure found between adjacent vertebrae of the spine. It has two main components, a central nucleus pulposus (NP) and, surrounding it, the annulus fibrosus. In the mechanical view the NP acts as a cushion, increasing the intradiscal pressure linearly under compressive load [210]. This pressure was measured through FOSs in several studies. An FBG incorporated into a stainless steel hypodermic tube, with a diameter of 1.5 mm and the tip sealed with a silicone membrane, was used by Dennison et al. [16,17]. The sensor mean sensitivity to pressure was −2.7 ± 1.5 × 10⁻⁵ mV/MPa. A good agreement was obtained with SG sensors, yet the mean relative difference in disc response to load between the FBG sensor and the SG sensor was 9.39% and ranged from 0.424% to 33.2% [16]. The relative difference between FEA and experimental results for the corresponding sensitivities was 11.1% [17].

Seeking to take advantage of the geometric properties of the OFs, Hsieh et al. [211] and Nesson et al. [19,212] proposed an FP air cavity sensor of only 0.366 mm. The sensor exhibited a linear response to the applied pressure over the range of 0–70 kPa, a sensitivity of 0.0206 μm/kPa, and a resolution of 0.17 kPa. Despite being attractive for in vivo and clinical practice it was used only in vitro for measurements of intradiscal pressure in rodent tail discs [19,211–213].

Signal processing of phase and wavelength modulation of light is more complex and more expensive than intensity modulation. Taking this in mind Huang et al. [214] proposed a low-cost and easy-to-interrogate sensor based on Fresnel reflection equations. The sensor was sealed in a 25-gauge needle (500 μm outer diameter) and coated with a thin film on the tip of the sensor. The developed system is capable of measuring pressure up to 1 MPa.

With regard to commercial sensors, several solutions have been also presented for physiological pressure measurements. Those from Samba Sensors (Västra Frölunda, Sweden) and Radi Medical Systems (Uppsala, Sweden) have been used to measure intradiscal pressure. In fact, Samba FP sensors are no longer available [215]. Their technical specifications included diameters of 0.36 or 0.42 mm, an accuracy of ±20 mbar with an error of ±2.5% FSO (from −0.1 to 10 bar), and ±3% FSO (from 10 to 17 bar). They also offered the possibility of being coated with radiopaque material for X-ray or fluoroscopic monitoring [215]. Several studies have reported their use to monitor intradiscal pressure in pigs [216,217], sheep [215], rabbits [218], and human cadaveric spines [219]. In the study by Roriz et al. [215] a Samba sensor was used. However, instead of using the Samba interrogation unit, a built-for-purpose interrogation unit based on light-intensity modulation was applied to measure the intradiscal pressure signal of an anesthetized sheep under spontaneous breathing (Fig. 10.15).

In the case of the Radi Medical Systems sensor, it was used to monitor intradiscal pressure in sedated pigs [220] and patients suffering from lumbar back pain [221]. Its technical specifications included a diameter of 0.55 mm, a pressure range from 0 to 800 kPa, a combined nonlinearity and hysteresis less than 0.5% FSO, and a time response lower than 0.2 s [221].