Octagonal fronts change where the hinge sits compared with a round or square frame, and the Ray-Ban RB3556N's eight-sided front places it at a sharper angle than either. This compatible pair is cut to the 3556N's 145mm metal-arm spec with that hinge angle accounted for, so it seats the way the model's own arm would.

Angular fronts like this one put more repeated stress on the joint where the arm meets the hinge, since the arm has to open past a sharper corner than it would on a rounder frame. That joint is usually the first thing to loosen, well before the plating or the length of the arm shows any issue. If tightening the hinge screw doesn't fix a wobble, the joint itself has likely worn out, and the arm is the part to replace.