Assessment of approaches to estimate scapular orientation in children with brachial plexus birth injury

Background: Challenges in measuring dynamic scapular orientation limit assessment of scapulothoracic and glenohumeral contributions to shoulder function in children with brachial plexus birth injury (BPBI). Double calibration acromion marker cluster (D-AMC) and linear model approaches have been validated to estimate scapular motion in healthy adults, but neither has been evaluated in BPBI. Research question: Are the linear model and D-AMC approaches able to accurately estimate scapular orientation in children with BPBI at functional arm postures? Methods: Seventeen children with BPBI positioned their affected limbs in 11 static positions while their segment orientations were measured with motion capture. Linear model and D-AMC estimates of scapular orientation were compared against palpation at six of the static positions with functional relevance to BPBI using a three-way repeat measures ANOVA and a comparison of root mean square errors (RMSE) against literature AMC values for healthy adults. Results: The D-AMC was similar to palpation across all positions and scapular axes while the linear model differed from palpation in a few instances. RMSEs of the D-AMC (3.7–14.8°) and particularly the linear model (4.6–24.8°) were generally at or beyond the upper range of past AMC analyses on healthy adults (1.6–14.2°), especially for more complex, multiplanar arm postures. Despite the D-AMC outperforming the linear model, this approach still produced clinically meaningful (>10°) errors for roughly (12.7–22.5%) of subjects. Significance: Current methods for estimating dynamic scapular orientation remain less than ideal for BPBI. Use of the D-AMC may be appropriate to gain broad insights into general dynamic scapulothoracic and glenohumeral function; however, given their potential for producing clinically meaningful errors, the D-AMC and linear model are not recommended for diagnostic purposes or outcomes assessment on an individual patient basis unless their patient-specific accuracy has been evaluated and confirmed prior to use.