Ultrasound assessment of T7 vertebral kinematics during induced segmental rotation

Clinical assessment of vertebral segment motion in the horizontal plane relies on applying anterior force to the tissues overlying transverse processes (TPs). Although current osteopathic principles suggest that this force induces vertebral rotation around a vertical axis, this mechanism has not been verified.

The objective of this study was to assess whether anterior force applied to the TP of the seventh thoracic vertebra (T7) induced vertebral rotation about a vertical axis.

Ten healthy, adult participants volunteered for our cross-sectional observational study. With participants prone, we utilized ultrasound to identify the lateral portion of the T7 TPs. Over that point, a human-controlled, 3D-printed thumb applied 40 N of force. To evaluate TP motion, we obtained ultrasound recordings from two positions: (1) medial to the applied force with a slight angular shift to assess ipsilateral TP displacement; and (2) directly over the contralateral TP to assess contralateral TP displacement. We performed six pushes on each TP, three recorded by the ultrasound on the ipsilateral side and three on the contralateral side, totaling 12 pushes per participant. We visualized and measured TP displacement secondary to the forces applied during each push on the ultrasound video recordings. A 3D motion capture system tracked reflective markers on the thumb, and the ultrasound probe was utilized to measure vertical displacement and angular shift from baseline (no force) to peak force, providing quality control for manual handling. We utilized a mixed-effects model to compare ipsilateral and contralateral TP displacement and to adjust for confounders such as variability in force and ultrasound probe movement during data capture.

Of the 10 participants, 6 (60 %) were women, mean (SD) age was 25.3 (1.6) years, and mean body mass index (BMI) was 22.6 (1.0) kg/m². The mean (SD) maximum applied force was 40.7 (2.1) N, and vertical displacement of the 3D-printed thumb was 9.7 (2.4) mm. After adjusting for confounders, a significant difference was found between ipsilateral and contralateral TP displacement (p<0.001); the ipsilateral TP moved anteriorly (mean [SE]=4.5 [0.2] mm), and the contralateral TP moved posteriorly (mean [SE]=1.1 [0.2] mm).

Our results indicated that the ipsilateral TP moved approximately 4 times more than the contralateral TP and suggested that the thoracic vertebrae do not rotate on a purely vertical axis when pushing anteriorly on the TP. Instead, the observed vertebral segment motion suggested a mechanism of coupled motion consisting of primarily anterior translation and some rotation rather than a simple rotation around a central axis.