The Differentiation of Muscle Containing Myofascial Trigger Points from Healthy Muscle Using Shear Wave Elastography

Abstract

Myofascial trigger points (MTrPs) are palpable, painful nodules that develop in skele- tal muscle and are a characteristic finding in myofascial pain syndrome (MPS), a common chronic pain condition. Symptomatic MTrPs are frequently the target of therapeutic treat- ment for the management of MPS. However, the pathophysiology of MTrPs is poorly under- stood and the diagnostic criteria remain inconsistent. Based on previous investigations into MTrPs using ultrasound imaging, we hypothesized that muscles containing MTrPs have 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 wave speed with muscle fiber orientation as a method of determining anisotropy in muscle fibers. We developed a standardized method of acquiring shear wave elastography images at different transducer orientations using a custom transducer holder with a fixed imaging window. The holder was fitted over palpable MTrPs in the upper trapezius of patients suf- fering from chronic neck pain (>3 months). The transducer holder allowed for registration of the palpable MTrPs to any collected images. 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. The results of this study showed an increase in muscle fiber asymmetry with increasingly symptomatic muscle, with normal muscle having the lowest asymmetry (6.64 ± 7.23 °, N = 11), then latent MTrPs (11.51 ± 9.86 °, N = 22), and finally active MTrPs (14.19 ± 11.76 °, N = 40). A statistically significant difference was found between the normal and active MTrPs groups (p<0.014). This data suggests that disruption of muscle fiber architecture may be an important feature that distinguished active MTrPs from normal muscle tissue.