Comparison of scapular local coordinate systems

Background: Our purposes were to compare between the original and current recommended standard methods of three-dimensional scapular rotation descriptions and to examine the prevalence of gimbal-lock for scapular motion during scapular plane abduction. Additionally we compared these standards to an alternative method and a glenoid based description. Methods: Eleven asymptomatic subjects were studied using electromagnetic sensors secured to bone-fixed pins in the scapula and humerus during two repetitions of scapular plane abduction. Anatomical landmarks defined scapular axes. Scapular angular data were analyzed at humerothoracic elevation angles from initial to maximum elevation. Repeated measures ANOVAs were performed for each variable with a significance level of P < 0.05. An anatomical model was used to compare the standards to the alternative and glenoid methods. Findings: For scapular upward rotation and tilting, larger differences occurred between standards at higher angles of elevation. The current standard measured 12.4° less upward rotation and 6.1° greater posterior tilting at maximum elevation as compared to the original. The current standard measured 11.6° less scapular internal rotation across all elevation angles. Using the original landmarks, six subjects attained a mean end-range humerothoracic elevation of 147.4° (SD 12.1°), with a mean end-range scapular upward rotation of 54.4°. The alternative method was more closely aligned to the glenoid method than the current standard. Interpretation: Significant differences were found between the two standards. The current standard interprets the same scapular motion with less internal rotation and upward rotation, and more posterior tilting than the original. No subjects reached upward rotation positions nearing gimbal-lock. Axis orientations also affect clinical interpretation. The alternative method appears worthy of further consideration as shoulder kinematic measurement further evolves.