Fig. 2.

Two types of impingement interactions coexist; it could correspond to a friction of the polyethylene against the bone (A and B). These repetitive frictions might lead with time to progressive bony abrasion. These phenomena are probably the cause of a rapid apparition of scapular notching. They are the results of multiple motions (adduction, rotations, extension) and not the consequence of a simple contact with the pillar in adduction with the arm at the side as previously believed. Contrarily, some impingements are related to an abutment with no possibilities for either component to continue the movement (C and D).

Source: From Lädermann et al,¹⁵ with permission.