Hypothesis: The primary objective of this study was to assess intraobserver repeatability and interobserver reproducibility of the 3-dimensional (3D) coordinates of a set of scapula anatomical landmarks obtained by manual positioning on conventional supine computed tomography (CT) scan-based scapula surface models. The secondary objective was to assess intraobserver repeatability and interobserver reproducibility of the resulting 3D scapular orientation. It was hypothesized that the 3D scapular orientation reliability would be better or similar than established reliability of intraoperative baseplate positioning (ie, version and inclination) in reverse total shoulder arthroplasty.

Methods: Three anatomical landmarks, ie, acromial angle, inferior angle, and trigonum spinae, were manually positioned on 81 scapula surface models using updated landmark definitions. These models were obtained by the segmentation of CT scan images acquired from patients undergoing elective reverse total shoulder arthroplasty procedures at the Department of Surgery at Geneva University Hospitals between May 2022 and December 2023. This procedure was repeated 3 times per scapula by 3 independent observers. A set of parameters corresponding to the 3D landmark coordinates (expressed in an average scapula coordinate system) and the 3D scapular orientation (expressed as 3 angles related to the retraction/protraction, lateral/medial rotation, and internal/external rotation) were computed. Intraobserver repeatability and interobserver reproducibility were assessed for each of these parameters using an intraclass correlation coefficient (ICC) as a relative reliability metric. Standard error of measurement (SEM) was also reported as an absolute reliability metric.

Results: Intraobserver ICC and interobserver ICC ranged from poor to moderate for acromial angle, poor to excellent for inferior angle, and poor to excellent for trigonum spinae. However, low SEM, ranging from 0.4 mm to 2.4 mm for intraobserver repeatability and from 0.2 mm to 1.8 mm for interobserver reproducibility, was obtained for every coordinate of each anatomical landmark. This results in poor to moderate intraobserver ICC and interobserver ICC for scapula angular orientation. Again, low SEM, ranging from 0.4° to 0.8° for intraobserver repeatability and 0.4° to 0.6° for interobserver reproducibility, was obtained, arguing for reliable measurements.

Discussion: This study demonstrates that manual positioning of the 3 scapula anatomical landmarks recommended by the International Society of Biomechanics can be performed in a reliable manner across measures and observers on surface bone models obtained from CT scan images. However, a clear and standardized definition of these landmarks is needed to ensure consistency across measurements.