Arc of motion and socket depth in reverse shoulder implants.

BACKGROUND

Reverse shoulder arthroplasty relies on its congruent ball/socket joint to restore shoulder function. For a simple ball/socket joint, as shown in total hip arthroplasty, range of motion decreases with the increase of articular constraint. We challenge here that this intuitive concept might not be held in reverse shoulder arthroplasty because of the effect of multiple concurrent factors.

METHODS

Abduction impingement-free arc of motion in reverse shoulder arthroplasty was examined with a virtual computer model. Six articular constraints, defined by normalized socket depths, were simulated. Four concurrent factors: glenosphere diameter, lateral offset of glenosphere from the glenoid surface, humeral neck-shaft angles, and locations of the glenosphere on the glenoid surface, were also studied, which composed a total of 81 combinations and 486 individual conditions.

FINDINGS

Three distinct classes of arc of motion relative to the articular constraint were revealed: I--arc of motion decreased with increased constraint (57%), II--arc of motion with a complex relationship to constraint (37%), and III--arc of motion increased with increased constraint (6%).

INTERPRETATION

Classes II and III were counter-intuitive which could be caused by impingement on the acromion associated primarily with superior positioning. Surgeons may need to be aware of it when the glenoid component has to be placed superiorly. The detailed motion/constraint relationship will further help engineers improve the design in reverse shoulder arthroplasty.