Microsensor for Intramuscular Pressure Measurement

Project Coordinator: Duane Morrow:
morrow.duane@mayo.edu

Currently, the integrated electromyogram (EMG) is the standard used as an indirect indicator of the timing and intensity of muscle contraction. However, the relationship between EMG and muscle tension is unclear. There is a need for a reliable measure of muscle tension under dynamic conditions. This need may be filled through the measurement of intramuscular pressure (IMP). The overall objective of this project is to provide a useful clinical tool for in-vivo quantification of muscle force. The specific aims of this proposed project will be to further develop a fiber optic microsensor for monitoring intramuscular pressure and validate this technology by animal testing, theoretical modeling, and in-vivo evaluation of research subjects and patients with muscle disorders.
A prototype optical fiber micropressure sensor for in-vivo muscle pressure (IMP) measurement (Figure 59) has been developed through a collaboration between the Mayo Clinic and Luna Innovations. The diameter of the prototype sensor is 500_m. A patent application has been filed regarding this design.

In the past year, progress has been made to examine the performance characteristics and biocompatability of the microsensor. We have also continued to further develop mathematical modeling of muscle performance in order to be able to compare experimentally collected results to theoretical models of muscle function. Additionally, our collaborators at the University of California, San Diego have tested the microsensor in an animal model to further examine its ability to sense both active and passive muscle tension.

The performance characteristics of the microsensors are evaluated in a calibration chamber. The microsensor had an accuracy, repeatability, and linearity better than 2% full-scale output (FSO) and hysteresis slightly higher than 4.5% FSO over a range of 0-250 mmHg.

The transducer was evaluated for biocompatibility using the International Organization for Standardization (ISO) Standard 10993-6: “Biological evaluation of medical devices- Part 6: Tests for local effects after implantation.” The test compared the biological response of the test sensor to a control material made of inert silica glass. The test and control materials were implanted in the paraspinal muscles of the adult New Zealand white rabbits while the animals were anesthetized. This study demonstrated that the pressure microsensor is biocompatible.

A mechanical muscle model for simulating muscle mechanics was developed based on the finite element method. Nonlinear continuum mechanics were used to study the contractile active and passive properties of skeletal muscle. This model was used to predict intramuscular pressure. (Figure 60).

A relationship between intramuscular pressure and active and passive muscle tension was determined using the isolated tibialis anterior (TA) of a New Zealand white rabbit. The knee was fixed in a custom jig, and the distal tendon of the TA was attached to a servomotor. A fiberoptic pressure transducer was inserted into the TA. The peroneal nerve was stimulated to define optimal length (L0) and the length-tension curve was created. The shape of this curve presumably represents a scaled and distorted version of the sarcomere length-tension curve previously published. Passive muscle tension increased in a fairly exponential fashion at length beyond optimal length. The length-pressure relationship (Figure 61) generally mimicked the shape of the length-tension curve (Figure 62). These data indicate that IMP measurement provides a fairly accurate index of relative muscle tension.

Figure 59: Intramuscular Pressure Microsensor

Figure 60: Comparison of experimental and simulated muscle pressure and tension.	Figure 62: Muscle length-tension relationship for active and passive tension.