Characterization of local muscle fiber anisotropy using shear wave elastography in patients with chronic myofascial pain

Myofascial trigger points (MTrPs) are palpable painful nodules that can develop in skeletal muscle and are associated with myofascial pain syndrome (MPS), a common chronic pain condition. Symptomatic MTrPs are often a therapeutic target for management of MPS, but the pathophysiology of MTrPs are poorly understood, and the diagnostic criteria lack reliability. Based on the results of our previous studies involving ultrasound imaging of MTrPs, we hypothesized that muscle containing MTrPs has heterogeneous fiber orientation within the affected zone and surrounding areas compared to palpably normal muscle. For this study, we sought to utilize the known variation of shear speed with muscle fiber orientation as a method for determining anisotropy in muscle fibers. We developed a standardized method to acquire shear wave elastography images at different transducer orientations in the upper trapezius muscle of subjects with chronic neck pain (>3 months). The images were aligned with palpable MTrPs using a custom transducer holder with a fixed imaging window. The variation of the shear wave speed of the muscle as a function of the transducer angle was used as a surrogate for muscle fiber orientation. The asymmetry of the fiber orientation was then determined and compared between active (spontaneously-painful) MTrPs, latent (not spontaneously-painful) MTrPs, and normal muscle tissue. Preliminary results from this study showed a statistically significant increase in asymmetry of fiber orientation between active (12.36 ± 7.96° N=14) and normal sites (5.75 ± 4.92° N=8; p<;0.027). Our data suggest that disruption of fiber architecture may be an important feature that distinguishes active MTrPs from normal muscle tissue.