Myofascial trigger points (MTrPs) are common in soft tissue musculoskeletal pain conditions. It is believed that MTrPs are local contractures within the extrafusal fibers of skeletal muscles. Further characterizing muscle pathophysiology is desirable to design effective treatments. This study was designed to measure muscle fiber architecture in the neighbourhood of MTrPs using ultrasound Shear Wave Elastography (SWE), a novel approach for quantitatively assessing muscle fiber. We hypothesized that muscle containing active (symptomatic) MTrPs has disorganized fiber architecture.

Twenty-four participants (14 women, ages 20–60 years) met criteria for chronic myofascial pain affecting the neck region. All underwent a clinical history and physical examination of the neck and upper trapezius to identify MTrPs and pain self-report. A Supersonic Aixplorer ultrasound imaging system with an L10-4 ultrasound transducer was used to image the upper trapezius muscle. A custom transducer holder (Fig. 1) enabled imaging through a 20mm diameter window placed over the palpable MTrP or normal tissue, and rotation of the imaging plane to precise angles from 0° (along fibers) to 90° (perpendicular to fibers). Peak shear wave velocity (SWV) is observed longitudinally along muscle fibers (0°), and is minimum across fibers (90°). Muscle fiber anisotropy (a measure of fiber alignment and organization) was quantified as the asymmetry (degrees) in the shear-wave velocity profile around the peak SWV.

Muscles with active (symptomatic) MTrPs (n=24) exhibited greater anisotropy (14.25±11.01°), compared to normal, asymptomatic muscle tissue (n=12), which is more isotropic (7.16±6.70°) (P<0.05) (Fig. 2). A positive correlation was found between the maximum anisotropy and average pain (Pearson's r=0.54).

Muscle containing active MTrPs is anisotropic compared to normal muscle tissue, suggesting muscle contractures are in regions of anisotropy. The degree of anisotropy correlates with pain levels.