Immediate efficacy of low-intensity focused ultrasound versus planar ultrasound in patients with myofascial pain syndrome of upper trapezius: a randomized controlled clinical trial

Myofascial pain syndrome (MPS), a chronic musculoskeletal disorder characterized by myofascial trigger points (MTrPs), causes localized pain and dysfunction. The objective is to compare the immediate therapeutic effects of low-intensity focused ultrasound (LIFU) versus low-intensity planar ultrasound (LIPU) on pain and function in upper trapezius MPS. This study was designed as a single-blind, randomized controlled trial. The patients (20 ≤ aged ≤ 70) diagnosed with MPS of the upper trapezius were randomly allocated to either LIFU (study group; n = 20; 9 men, 11 women) or LIPU (control group; n = 20; 15 men, 5 women) treatments (1.5 W/cm2, 1 MHz, 5 min, once a day, 3 days). The primary outcome was pain severity, assessed immediately post-treatment using the Visual Analog Scale (VAS; 0 ~ 10 cm), and served as the central efficacy endpoint. Secondary outcomes comprised supplemental assessments including: (1) multidimensional pain characterization via the Short-Form McGill Pain Questionnaire (SF-MPQ); (2) functional disability quantification through the Neck Disability Index (NDI); and (3) neuromuscular biomarkers measured by surface electromyography (sEMG) parameters (root mean square RMS, median frequency MF). The outcomes were assessed before treatment and 10 min, 72 h after completing the third and final session of treatment. Pain intensity, as measured by the VAS, decreased significantly immediately after treatment in both the LIFU group (median difference MD = -2.0 cm; 95% confidence interval CI: -2.5 to -1.5 cm; p = 0.002) and the LIPU group (MD = -1.0 cm; 95% CI: -1.5 to -0.5 cm; p = 0.001), with these reductions maintained at 72 h post-treatment. Both SF-MPQ and NDI scores showed significant improvements following treatment in both groups. A significant reduction in RMS parameters was observed in the LIPU group immediately after treatment (MD = -15.3 µV; 95% CI: -20.0 to -10.5 µV; p = 0.028). No significant differences were observed between the groups in terms of pain alleviation, functional improvement, and alterations in neuromuscular electrophysiological activity (p > 0.05); however, moderate to large effect sizes suggest possible clinical relevance. No adverse events were reported. Both LIFU and LIPU can be considered effective therapeutic options, offering immediate symptom relief in patients with upper trapezius MPS. The protocol was registered at the Chinese Clinical Trial Register (ChiCTR2500097431) as a clinical trial on 19/02/2025 http://www.chictr.org.cn/ .