Fig. 1

The head-shaft angle (HSA) measurement method. A vertical line is drawn from the superior to the inferior border of the articular surface (Line 1) and a line perpendicular to this line is then drawn to run through the centre of the humeral head (Line 2). The angle between the latter line and a line parallel to the long axis of the humeral shaft (Line 3) is the humeral head-shaft angle. We found that this angle (angle B) was equal to the angle between the vertical and the long axes (angle A) plus 90°