Design Considerations of a Fiber Optic Pressure Sensor Protective Housing for Intramuscular Pressure Measurements

Abstract

Intramuscular pressure (IMP), defined as skeletal muscle interstitial fluid pressure, reflects changes in individual muscle tension and may provide crucial insight into musculoskeletal biomechanics and pathologies. IMP may be measured using fiber-optic fluid pressure sensors, provided the sensor is adequately anchored to and shielded from surrounding muscle tissue. Ineffective anchoring enables sensor motion and inadequate shielding facilitates direct sensor-tissue interaction, which result in measurement artifacts and force-IMP dissociation. The purpose of this study was to compare the effectiveness of polyimide and nitinol protective housing designs to anchor pressure sensors to muscle tissue, prevent IMP measurement artifacts, and optimize the force-IMP correlation. Anchoring capacity was quantified as force required to dislodge sensors from muscle tissue. Force-IMP correlations and non-physiological measurement artifacts were quantified during isometric muscle activations of the rabbit tibialis anterior. Housing structural integrity was assessed after both anchoring and activation testing. Although there was no statistically significant difference in anchoring capacity, nitinol housings demonstrated greater structural integrity and superior force-IMP correlations. Further design improvements are needed to prevent tissue accumulation in the housing recess associated with artificially high IMP measurements. These findings emphasize fundamental protective housing design elements crucial for achieving reliable IMP measurements.

Keywords: Force; Isometric activation; Microsensor; Skeletal muscle; Tibialis anterior.

FIGURE 1

(a) A representative image of a pressure microsensor with no housing. (b) Detailed assembly drawing of the polyimide housing sensor. A barbed polyimide housing sensor (c) before and (d) after isometric activations of a rabbit tibialis anterior. (e) Detailed assembly drawing of the nitinol housing sensor. A nitinol housing sensor (f) before and (g) after isometric activations. White arrowheads indicate barbs. Inspection of nitinol housings after testing revealing (h) an unobstructed recess and visible sensor diaphragm and (i) tissue accumulation in the recess. Polyimide housing barbs were damaged after muscle activations. In contrast, nitinol housing barbs remained intact.

FIGURE 2

Comparison of pullout forces of sensors with a single barbed polyimide (P) and four-barbed nitinol (N) housings inserted into the rabbit tibialis anterior either parallel (//) or perpendicular (⊥) to the direction of fiber shortening. Barbed polyimide housings required a significantly greater pullout force compared to barbless polyimide housings (p < 0.001). No significant difference was found between nitinol housings and barbed polyimide housings.

FIGURE 3

Representative force (solid black) and intramuscular pressure (IMP; dashed grey) measured using a sensor with a (a) polyimide (P) housing and a (b) nitinol (N) housing during isometric activation of a rabbit tibialis anterior at optimal length. Black arrowheads indicate negative pressure artifacts. Box plots showing the (c) coefficient of determination (COD) between force and IMP, (d) minimum IMP (IMP_min) and (e) maximum IMP (IMP_max) measured using four polyimide housing sensors (n = 46 trials) and seven nitinol housing sensors (n = 47 trials). Sub-analyses of nitinol housing sensor on the effect of (f) insertion direction relative to fiber shortening direction (perpendicular, ⊥, n = 31 trials; parallel, //, n = 16 trials) on the COD and (g) the presence (tissue, T; n = 23 trials) or absence (no tissue, nT, n = 24 trials) of tissue accumulation in the recess on the IMP_max. Sensors with nitinol housings had a superior force-IMP correlation compared to polyimide housings and eliminated negative pressure artifacts. Parallel insertion resulted in a significantly greater COD compared to perpendicular insertion. The presence of tissue resulted in a significantly greater IMP_max compared to when tissue was not present in the recess. *p < 0.001, **p < 0.05.