Intraoperative functional zero-position is associated with superior outcomes after reverse total shoulder arthroplasty for cuff tear arthropathy: a retrospective comparative study.

BACKGROUND

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).

METHODS

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).

RESULTS

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.

CONCLUSION

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.