The glenoid track refers to the area of contact between the humeral head and glenoid and is defined as ~83% of glenoid width. A Hill-Sachs defect that is smaller than the track ("on-track") will maintain contact and is at lower risk of engagement and instability during full range shoulder movement especially abduction. Conversely, a Hill-Sachs defect that is larger than the glenoid track ("off-track") will be at increased risk of engagement and instability (i.e. is an "engaging Hill-Sachs defect"). 

Evaluation of the glenoid track status should be done routinely in the cross-sectional imaging assessment of the shoulder joint in cases of anterior instability. Determination if the lesion is on-track or off-track can greatly affect management including different operative approaches if surgery is required ¹.