Ultrasound features of myofascial trigger points: a multimodal study integrating preliminary histological findings from the upper trapezius.

This study investigated the ultrasound and histological characteristics of myofascial trigger points (MTrPs) in the upper trapezius (UT) following dry needling (DN) therapy. Forty-nine participants with active MTrPs in one UT were included, with the contralateral healthy side serving as control. Ultrasound assessments (UT thickness, Young's modulus, vascular parameters including peak systolic velocity [PSV], resistance index [RI], and vascularity index [VI]) were performed: (1) pre-intervention, (2) 1-hour after DN. Pain levels were measured using the visual analog scale (VAS). Key findings showed affected UTs were initially thicker and stiffer than the healthy side (11.24 ± 1.81 mm, 32.28 ± 9.70 kPa vs. 10.00 ± 1.46 mm, 19.16 ± 6.39 kPa; p < 0.05), with greater deep fascia thickness (1.08 ± 0.14 mm vs. 0.92 ± 0.14 mm). Post-DN, the thickness and stiffness of the affected UTs decreased (9.64 ± 1.74 mm, 24.14 ± 5.31 kPa, P < 0.05). Perfusion improved, evidenced by a lower RI (0.78 ± 0.05 vs. 0.86 ± 0.03) and higher VI (12.90 ± 6.35 vs. 2.53 ± 1.69), while PSV remained unchanged. Eventually, VAS scores dropped from 7.86 ± 0.90 to 5.00 ± 1.29 (p < 0.05). Prior to DN, histological analysis (n = 5 biopsies) revealed characteristic MTrPs remodeling (muscle fiber atrophy, adipose tissue hyperplasia) without contraction knots. DN alleviates pain by reducing thickness and stiffness of muscle and enhancing perfusion, providing a mechanistic basis for its therapeutic effects in myofascial pain management.