The association between scapular orientation and spinopelvic parameters

The scapula plays a crucial role in upper limb movement and stability. Studies have reported that scapular orientation including scapular internal rotation (SIR), scapular protraction (PRO), and scapular upward rotation (SUR) should be integrated into the planning of reverse total shoulder arthroplasty. While previous studies assessed scapular orientation in relation to the spinal parameters, they stop at thoracic kyphosis (TK) without examination of spinopelvic parameters such as lumbar lordosis, pelvic incidence, pelvic tilt, or sacral slope. Therefore, this study aims to examine the association between spinopelvic parameters and scapular orientation.

This is a retrospective single-center study including adult patients undergoing positron emission tomography scans for nonorthopedic indications, with no spinal deformity or upper extremity pathology affecting scapular positioning. Scapular orientation and spinopelvic parameters were measured for each patient. Patients were grouped into 3 posture groups (A, B, and C) based on their SIR as defined by previous studies. Correlations between scapular and spinopelvic parameters were assessed using Pearson correlation coefficients, and group differences were evaluated using analysis of variance with post-hoc analysis.

One-hundred patients were included with a mean age of 49.8 ± 9.8 years, and 70% being females. The mean values of the scapular orientation parameters were 41.6 ± 9.8° for SIR, 87.2 ± 4.9° for PRO, and 12.4 ± 4.1° for SUR. SIR positively correlated with TK (coefficient = 0.20, P = .046) and Barrey ratio (coefficient = 0.21, P = .04); PRO negatively correlated with T1 pelvic angle (coefficient = -0.21, P = .03), T1 slope (coefficient = -0.21, P = .04), and C7 slope (coefficient = -0.20, P = .048); and SUR positively correlated with lumbar lordosis (coefficient = 0.20, P = .04). Group C had a lower pelvic tilt (P = .02) and a lower T1 pelvic angle (P = .04) compared to group B. In addition, group C had a higher Barrey ratio compared to group A (P = .03). As for scapular orientation parameters, group C had a higher PRO compared to group B (P = .001) and a higher SUR compared to group B (P < .001) and group A (P = .001).

Spinopelvic parameters are associated with scapular orientation, supporting the concept of a global biomechanical relationship between the spine, pelvis, and shoulder girdle. These findings suggest that global sagittal alignment, beyond TK alone, may influence scapular positioning and might need to be considered in future studies and clinical applications.