LAUSR

OBJECTIVE We investigated the thoracic segment corresponding to the inferior margin of the rhomboid major muscle (RMM) using ultrasound (US) to evaluate its potential as a reliable anatomic landmark for segment identification. DESIGN A prospective observational study. SETTING An operating room. SUBJECTS Patients who underwent procedures around the thoracic spine. METHODS 400 segments corresponding to the RMM's inferior margin were identified using paravertebral sagittal US and confirmed by fluoroscopy in 100 participants in the prone position with upward and downward shoulder rotation, comprising 4 datasets (up-right, up-left, down-right, and down-left). The US identification of the RMM's inferior margin was dichotomously scored (clear vs. ambiguous). Each dataset was divided into two groups (dominant segment group vs. remaining segments group) and compared. Factors relevant to the dominant segment associated with the RMM's inferior border were determined using univariable analyses. RESULTS The T6 segment was observed most commonly (59.5%) along the RMM's inferior border on paravertebral sagittal US acquired in the prone position, followed by T5 (25.0%), T7 (12.8%), and T4 (2.7%), respectively. The segments corresponding to the RMM remained unchanged based on shoulder posture in most participants (N = 74, 74%). The RMM's inferior border was clearly distinguishable in 330 cases (82.5%). When the RMM's inferior border was clearly identified, the corresponding segment was likely to match T6 in all dataset with their ORs ranged from 3.24 to 6.2. CONCLUSIONS The RMM's inferior border over the transverse process corresponded to T6 most frequently on paravertebral sagittal US and its deep fascia was clearly visible in most cases.