Validation of Ultrasound-Based Muscle Elastic Modulus Estimation in a Healthy Human Volunteer Model

Abstract Introduction Measurement of muscle compartment pressure can be important in clinical scenarios such as diagnosing acute compartment syndrome; the gold standard is invasive and costly pressure-transducer measurement. Past studies have shown the promise of ultrasound (US) as a diagnostic adjunct by measuring the elastic modulus of the muscle while it is being compressed by a US probe (compression sonography). However, the reliability of compression sonography in multiple muscle locations has not been previously examined. This study aimed to test the reliability and validity of compression sonography in elastic modulus estimation (EME) in multiple muscle locations. Materials and Methods A phantom model was used to map EME measurement to expected pressure measurement in physiologic and injury states. Ten healthy subjects were evaluated by two investigators using a standard ultrasound probe encased in a multi-sensor shell among the following extremity locations: forearm, arm, lower leg, and thigh. Each extremity location was scanned twice in both relaxed and flexed states of muscle contraction, the latter simulating elevated compartment pressure, for each investigator. Elastic modulus estimation was calculated using force, measured by a load cell, and tissue displacement generated by the US probe, measured by specifically designed software. Institutional institutional review board approval was obtained. Results Using the phantom model, a mean EME above 170.0 kPa was equivalent to pressure states above 20 mmHg. In healthy human volunteers, flexed states reliably produced higher mean EME values (range: 103.0-200.0, SE 5.7-10.5) than relaxed states (range: 49.8-123.0, SE 3.2-5.9) for each extremity location across all ten subjects. The EME difference between flexed and relaxed states was statistically significant for each extremity location (P .001). There was a proportional increase in the EME difference between states as tension increased in the forearm, which was statistically significant (P .001, P .001). Investigators had good agreement for both relaxed (ICC = 0.67, 95% CI 0.49-0.80) and flexed states (ICC = 0.63, 95% CI 0.43-0.77) across subjects and extremity locations. Conclusions Elastic modulus estimation using compression sonography was shown to have good inter- and intra-rater reliability in multiple muscle sites. The human volunteer model reliably produced higher EME values among flexed states versus relaxed, with the flexed states EME mapping to pressure states seen in acute compartment syndrome. This study demonstrates the potential utility of a low-cost ultrasound-based device to aid in detecting elevated muscle compartment pressures.