Glenoid of the Scapula Act as a Dynamic Stabilizer of Glenohumeral Joint
-2nd Report-

Abstract

The scapula rotates upwardly during shoulder elevation. Given that the center of the rotation was the humeral head, not on the scapula, the movement of the glenoid would be regarded as the rotation along the humeral head. We have reported this concept of “functional glenoid” at the previous annual meeting of Japan Shoulder Society. The purpose of this study was to provide more accurate findings regarding this concept. Patients with habitual shoulder dislocation (unstable group; n=12) and normal control (n=12) were included in the study. Two radiographs were taken of each subject (anteroposterior in internal rotation; IR1, zero position). Two straight lines were drawn from superior and inferior tubercle to the center of the humeral head in IR1 image, and the angle of lines was measured (α angle). The difference of the glenoid inclination angles between IR1 and Zero position (β angle) were then measured, and the glenoid extension ratio (α+β/α) was calculated and statistically analyzed with t-test. β angle was significantly lower in the unstable group than in the control group (p=0.023). The glenoid extension ratio was smaller in the unstable group than in the control group but did not show a significant difference (p=0.078). Interobserver reliability was analyzed with inter-class correlation coefficient and showed good correlation for α angle (r=0.805) and β angle (r=0.711), respectively. Intraobserver reliability was also analyzed with the same method and showed good correlation for α angle (r=0.842) and β angle (r=0.823), respectively. These results may suggest that reduction of “functional glenoid” is associated with dynamic instability of the glenohumeral joint in habitual shoulder dislocation. The methodology we employed was useful in terms of inter/intraobserver reliability.