Shear wave elastography for characterizing muscle tissue in myofascial pain syndrome

Abstract

Myofascial pain syndrome (MPS) affects 85% of chronic pain sufferers in a specialty pain center. Neck and low-back are commonly affected by MPS. Myofascial trigger points (MTrPs) are characteristic findings of MPS and are palpable tender nodules in the muscles of symptomatic subjects. Mechanical characterization of MTrPs and surrounding tissue can offer important insight about the pathophysiology of the MPS, which is currently poorly understood. In this study, we propose an inexpensive technique, based on ultrasound shear wave elastography, to objectively measure mechanical properties of MTrPs and surrounding tissue in the upper trapezius. In an ongoing clinical study, we recruited 34 subjects: 12 healthy controls, 10 with not spontaneously painful MTrPs (latent) and 12 with symptomatic chronic neck pain (>3 months) and active (spontaneously painful) MTrPs. Shear wave elastography was performed on the upper trapezius of all subjects using the Ultrasonix RP system and an external vibrator. Voigt’ s model was used to estimate shear modulus G and viscosity μ of the interrogated tissue. Preliminary analysis demonstrates that symptomatic muscle tissue in subjects with neck pain is stiffer (G = 8.40 ± 8.31 kPa, mean ± standard deviation) compared to muscle in control subjects (G = 2.86 ± 2.48 kPa) (p < 0.05), and that active MTrPs are more viscous (μ = 17.10 ± 9.46 Pa*s) than surrounding tissue (μ = 10.59 ± 5.96 Pa*s). Latent MTrPs (μ = 24.31 ± 12.72 Pa*s) and surrounding tissue (μ = 20.09 ± 7.48 Pa*s) are more viscous than normal tissue (μ = 10.96 ± 4.17 Pa*s).