Intraoperative functional zero-position is associated with superior outcomes after reverse total shoulder...

Optimal implant positioning in reverse total shoulder arthroplasty (rTSA) remains challenging because of the lack of standardized intraoperative criteria. The "zero-position," defined as alignment of the humeral axis with the scapular spine, represents a balanced state in native shoulders. In rTSA, a similar alignment, termed the functional zero-position, may reflect balanced glenohumeral motion. This study evaluated its association with post-operative outcomes in patients with cuff tear arthropathy (CTA).

A retrospective review of 45 shoulders (43 patients) with CTA and ≥2-year follow-up was performed. Patients were grouped based on intraoperative achievement of the functional zero-position under fluoroscopic guidance during passive scaption. Range of motion and Constant Murley scores were compared. Fisher exact test, Mann-Whitney U test, Friedman test, and two-way repeated-measures analysis of variance were used (P< .05).

The functional zero-position was achieved in 35 shoulders. Both groups showed significant improvement in flexion and abduction from 6 months onward. A significant group-by-time interaction was observed for elevation (flexion, P= .0024; abduction, P= .0005). Flexion was significantly greater in the functional zero-position group from 6 months, and abduction after 1 year. External rotation improved only in the functional zero-position group, with significant between-group differences beginning at 6 months. At final follow-up, elevation, external rotation, and Constant Murley scores were higher in the functional zero-position group.

In CTA, intraoperative achievement of the functional zero-position was associated with superior range of motion, favorable recovery, and improved function, suggesting that it serves as a practical intraoperative reference for optimizing soft-tissue balance in rTSA.