Kinematic coupling of the glenohumeral and scapulothoracic joints generates humeral axial rotation

Abstract

Glenohumeral and scapulothoracic motion combine to generate humerothoracic motion, but their discrete contributions towards humerothoracic axial rotation have not been investigated. Understanding their contributions to axial rotation is important to judge the effects of pathology, surgical intervention, and physiotherapy. Therefore, the purpose of this study was to investigate the kinematic coupling between glenohumeral and scapulothoracic motion and determine their relative contributions towards axial rotation. Twenty healthy subjects (10 M/10F, ages 22-66) were previously recorded using biplane fluoroscopy while performing arm elevation in the coronal, scapular, and sagittal planes, and external rotation in 0° and 90° of abduction. Glenohumeral and scapulothoracic contributions towards axial rotation were computed by integrating the projection of glenohumeral and scapulothoracic angular velocity onto the humeral longitudinal axis, and analyzed using one dimensional statistical parametric mapping and linear regression. During arm elevation, scapulothoracic motion supplied 13-20° (76-94%) of axial rotation, mainly via scapulothoracic upward rotation. The contribution of scapulothoracic motion towards axial rotation was strongly correlated with glenohumeral plane of elevation during arm elevation. During external rotation, scapulothoracic motion contributed 10° (8%) towards axial rotation in 0° of abduction and 15° (15%) in 90° of abduction. The contribution of scapulothoracic motion towards humerothoracic axial rotation could explain the simultaneous changes in glenohumeral plane of elevation and axial rotation associated with some pathologies and surgeries. Understanding how humerothoracic motion results from the functional coupling of scapulothoracic and glenohumeral motions may inform diagnostic and treatment strategies by targeting the source of movement impairments in clinical populations.

Keywords: Axial rotation; Biplane fluoroscopy; Glenohumeral; Kinematic coupling; Scapulothoracic.

Figure 1:

Illustration of how the scapulothoracic motion contributes strictly humerothoracic elevation when the glenohumeral (GH) plane of elevation (PoE) is 0° (A-C) but contributes strictly to humerothoracic axial rotation when the glenohumeral PoE is 90° (D-F). Both motions are shown at discrete points of scapulothoracic upward rotation (0°, 45°, 90°). The black bar represents the direction approximating the forearm axis of a flexed elbow. When the anteroposterior scapular axis and the humerus’ longitudinal axis are perpendicular (A-C), the scapulothoracic joint does not generate humeral axial rotation. However, when they are aligned (D-F) every degree of scapulothoracic upward rotation results in one degree of humeral axial rotation. When the glenohumeral PoE is positive, scapulothoracic upward rotation contributes to internal humerothoracic axial rotation; when the glenohumeral PoE is negative, it contributes to external axial rotation. The axial rotation generated from one degree of scapulothoracic upward rotation is determined by the cosine of the angle between the scapulothoracic upward rotation axis and the humerus’ longitudinal axis (at 60°, GH PoE=30° → 0.5°; at 45°, GH PoE=45° → 0.71°).

Figure 2:

Graphical depiction of external rotation in the transverse plane (A, External Rotation in Adduction, ER-ADD) and external rotation in the sagittal plane (B, External Rotation at 90° of Abduction, ER-ABD). For ER-ADD trials, subjects were instructed to maintain the elbow by their torso with the hand on the abdomen and thumb pointing up, and to laterally rotate to their full ROM at ~45°/sec. For ER-ABD trials, subjects were instructed to point their elbow towards the side of the room while allowing the hand to hang naturally due to its weight, and laterally rotate up to their full ROM at ~45°/sec. Green lines denote the starting position of the forearm axis, and red lines denote the ending position of the forearm axis.

Figure 3:

Illustration of the elevation-generating (red) and axial rotation (green) axes for projection of angular velocity. The axial rotation axis is coincident with the longitudinal axis of the humerus. However, the elevation-generating axis is not coincident with any anatomical axis. It always lies on the transverse plane and an infinitesimal rotation about it causes the humerus to elevate along the superoinferior axis. In contrast, the glenohumeral elevation axis of rotation (orange) does not strictly cause the humerus to elevate along the superoinferior axis because of scapular tilt (~30° in this illustration). Furthermore, the scapulothoracic upward rotation axis (Euler-based, yellow) and the glenohumeral elevation axis are not co-aligned. Therefore, traditional SHR compares angles of rotation about two different axes of rotation. In contrast, coordinated SHR compares the relative rotations of the glenohumeral and scapulothoracic joints about the same elevation-generating axis. The illustration shows two different orientations of the humerus (but just one for the scapula for visual clarity) to emphasize that all axes of rotation depend on the orientation of the humerus and scapula.

Figure 4:

Comparison of glenohumeral (GH) and scapulothoracic (ST) contributions to humerothoracic (HT) axial rotation for (A) coronal plane abduction (CA), (B) scapular plane abduction (SA), (C) forward elevation (FE), (D) external rotation in adduction (ER-ADD), and (E) external rotation in 90° abduction (ER-ABD) motions. The singular data points for CA, SA, and FE indicate axial rotation contributions at maximum humerothoracic elevation (differs by subject). The error bars around the singular data point and the shaded regions indicate ±1 standard deviation. The orange line at the top of arm elevation plots indicates regions where SPM1D found that scapulothoracic-contributed axial rotation was NOT statistically different than humerothoracic axial rotation (indicated by ~), while the green line indicates the same for glenohumeral axial rotation. This highlights the influence of scapulothoracic-contributed and glenohumeral axial rotation towards humerothoracic axial rotation during different phases of arm elevation. In all other regions scapulothoracic-generated and glenohumeral axial rotation were statistically different from humerothoracic axial rotation (p<0.001). The black line at the top of ER-ABD and ER-ADD plots indicates regions where SPM1D found that scapulothoracic-contributed axial rotation was statistically different than 10% of humerothoracic axial rotation (p<0.001).

Figure 5:

Contributions of scapulothoracic upward rotation (Upward Rot), re/protraction (RePro), and tilt to scapulothoracic-contributed (ST-contributed) axial rotation for (A) coronal plane abduction (CA), (B) scapular plane abduction (SA), (C) forward elevation (FE), (D) external rotation in adduction (ER-ADD), and (E) external rotation in 90° of abduction (ER-ABD) motions. The singular data points for CA, SA, and FE indicate axial rotation contributions at maximum humerothoracic elevation (differs by subject). The error bars around the singular data point and the shaded regions indicate ±1 standard deviation. The solid black line at the top of plots indicates regions where SPM1D found significant differences between contribution components. For elevation and ER-ABD trials, scapulothoracic upward rotation contribution was compared to the contributions of re/protraction and tilt. For ER-ADD trials scapulothoracic re/protraction contribution was compared to the contributions of upward rotation and tilt. The following suprathreshold events exceeded p≤0.001: SA, Upward Rot vs RePro (p=0.020); SA, Upward Rot vs Tilt (p=0.010).

Figure 6:

Correlation between scapulothoracic-contributed (ST-contributed) axial rotation and glenohumeral (GH) plane of elevation (PoE). (A) Glenohumeral PoE and (B) scapulothoracic-contributed axial rotation are shown by arm elevation activity. The singular data points for coronal plane abduction (CA), scapular plane abduction (SA), and forward elevation (FE) indicate glenohumeral PoE and scapulothoracic-contributed axial rotation at maximum humerothoracic elevation (differs by subject). The error bars around the singular data point and the shaded regions indicate ±1 standard deviation. (C) Scapulothoracic-contributed axial rotation was moderately correlated with the mean glenohumeral PoE for CA (R=0.65, p=0.003) and SA (R=0.67, p=0.002), and strongly correlated for FE (R=0.72, p<0.001) and when considering all elevation trials (R=0.94, p<0.001).

Fig. 7:

Comparison of traditional (Euler) scapulohumeral rhythm (SHR) and coordinated SHR for (A) coronal plane abduction (CA), (B) scapular plane abduction (SA) and (C) forward elevation (FE) motions. Because subjects had different resting humerothoracic elevation angles, each trial was interpolated between resting humerothoracic elevation angle (0%) to maximum humerothoracic elevation (100%). The shaded regions indicate ±1 standard deviation. The black line at the top of each plot indicates regions where SPM1D found differences between coordinated SHR and traditional SHR. Coordinated SHR was higher than traditional SHR for CA and SA, especially during the first 20% of arm elevation. No statistically significant differences were found for FE.