A common method of tracking humeral motion involves securing a thermoplastic cuff to the humerus with an electromagnetic sensor attached. The data on the accuracy of this technique are limited. This study addressed two questions: (a) How similar are surface and bone-fixed measurements of 3-D humeral rotations? (b) How similar are surface and bone-fixed measurements of 3-D humeral translations? Electromagnetic motion sensors were secured to a bone-fixed external humeral fixator, a surface humeral cuff, and the skin over the sternum and scapular acromion process. The 3-D data were collected during successive slow velocity (10–20°/second) repetitions of humeral active-assisted scapular plane abduction, sagittal plane flexion, and internal/external rotation with the arm adducted. Root mean square errors of surface measures compared to bone-fixed angular and translational values were calculated, and paired t-tests were computed between the two methods. Root mean square errors for humeral rotations ranged from 1° (1%) for humeral elevation during scapular plane abduction to 7.5° (9%) for humeral internal/external rotation. Peak errors were under-representations of 5.7° for internal/external rotation during scapular plane abduction and 15.6° for internal rotation with the arm adducted at the side. Average translation errors ranged from 0.1 to 2.1 mm. Data from this study suggest that dynamic measurement of humeral motion with a surface humeral cuff sensor can be performed for certain slow velocity motions with root mean square errors less than 8°. Caution is called for when interpreting internal/external rotation values, which were underrepresented. Results may vary with one’s age, weight, or general physical condition, with different velocities of movement, or with different movements.
Paula M. Ludewig, Thomas M. Cook, and Richard K. Shields
Meena Makhija, Jasobanta Sethi, Chitra Kataria, Harpreet Singh, Paula M. Ludewig, and Vandana Phadke
Two-dimensional fluoroscopic imaging allows measurement of small magnitude humeral head translations that are prone to errors due to optical distortion, out-of-plane imaging, repeated manual identification of landmarks, and magnification. This article presents results from in vivo and in vitro fluoroscopy-based experiments that measure the errors and variability in estimating the humeral head translated position in true scapular plane and axillary views. The errors were expressed as bias and accuracy. The variability with repeated digitization was calculated using the intraclass correlation coefficient (ICC) and the standard error of measurement. Optical distortion caused underestimation of linear distances. The accuracy was 0.11 and 0.43 mm for in vitro and in vivo experiments, respectively, for optical distortion. The intrarater reliability was excellent for both views (ICC = .94 and .93), and interrater reliability was excellent (ICC = .95) for true scapular view but moderate (ICC = .74) for axillary views. The standard error of measurement ranged from 0.27 to 0.58 mm. The accuracy for the humeral head position in 10° out of true scapular plane images ranged from 0.80 to 0.87 mm. The current study quantifies the magnitude of error. The results suggest that suitable measures could be incorporated to minimize errors and variability for the measurement of glenohumeral parameters.
Kristin E. Meyer, Erin E. Saether, Emily K. Soiney, Meegan S. Shebeck, Keith L. Paddock, and Paula M. Ludewig
Proper scapular motion is crucial for normal shoulder mechanics. Scapular motion affects glenohumeral joint function during throwing, yet little is known about this dynamic activity. Asymptomatic subjects (10 male and 10 female), ages 21 to 45, were analyzed. Electromagnetic surface sensors on the sternum, acromion, and humerus were used to collect 3-D motion data during three trials of low-velocity throwing. Scapular angular position data were described for five predetermined events throughout the throw corresponding with classic descriptions of throwing phases, and trial-to-trial reliability was determined. ANOVA compared scapular angles across events. Subjects demonstrated good to excellent reliability between trials of the throw (ICC 0.74–0.98). The scapula demonstrated a pattern of external rotation, upward rotation (peak of approx. 40°), and posterior tilting during the initial phases of the throw, progressing into internal rotation after maximum humeral horizontal abduction. During the arm acceleration phase, the scapula moved toward greater internal rotation and began anteriorly tilting. At maximum humeral internal rotation, the scapula ended in internal rotation (55°), upward rotation (20°), and anterior tilting (3°).