Fig. (1) Glenoid track, engaging and none engaging HSLs. The upper and most left image shows the shoulder joint in neutral and then, on the right, in abduction and external rotation. The shaded area on the humerii shows the glenoid track. The lesion within the humeral head in the top image can be seen to engage the glenoid as it moves parallel to it as it is more medial than the glenoid track. The lower image demonstrates how this is much less likely to occur in a more vertical orientation and with the lesion sitting within the track i.e. a none-engaging HSL.