The critical zone of the rotator cuff is an area approximately 8-15 mm from the insertion of the rotator cuff tendons onto the greater tubercle of the humeral head, mainly within the supraspinatus tendon. This is a watershed zone between the anterior and posterior circumflex humeral, thoracoacromial, and suprahumeral arteries.
It was thought for many years (mainly concluded from cadaver investigations) that this area is relatively hypovascular and hence more susceptible to age-related tendon attritional degeneration. But more recent studies in symptomatic patients with impingement syndrome showed that actually these areas are hypervascular which seems to be due to neovascularisation secondary to the rotator cuff disease and chronic mechanical impingement.
In fact, it seems that the high frequency of tears in this region is multifactorial and related to chronic degeneration impingement and ischaemia.