Rule breakers achieve successful shoulder balance: unraveling the myth of upper instrumented vertebrae selection criteria.

Purpose: This study compared shoulder balance outcomes in "rule breakers" (RB) vs. "rule followers" (RF) based on commonly utilized upper instrumented vertebrae (UIV) selection guidelines.

Methods: Adolescent idiopathic scoliosis (AIS) patients (Lenke 1-4) who underwent posterior spine fusion (PSF) with minimum 2-year follow-up had radiographic measurement of shoulder balance including first rib angle (FRA), T1 tilt, coracoid process height difference (CPHD), and clavicle angle (CA) at preop, postop, 6-month, 1-year, and 2-year timepoints. Postoperative outcomes were compared between RB and RF groups defined based on the UIV selection guidelines of Rose and Lenke.

Results: Among 88 patients (43 RF, 45 RB), age, gender, preoperative T1 tilt, FRA, CA, and CPHD were not significantly different between groups (p > 0.05). Immediately post-surgery, the RF group had more balanced shoulders (CPHD: 11.6 mm vs. 15.7 mm, p = 0.033; CA: 2.8° vs. 3.6°, p = 0.045; FRA: 3.4° vs. 5.1°, p = 0.009; T1 tilt: 4.7° vs. 6.1°, p = 0.045). At 2 years, no difference was observed between RF vs. RB in CA (2.3 vs. 2.2°, p = 0.857) and CPHD (8.5 vs. 8.1 mm, p = 0.791). FRA and T1 tilt were higher in RB vs. RF (FRA: 4.6 vs. 2.9°, p = 0.002; T1 tilt: 5.6 vs. 3.9, p = 0.008). Shoulder balance (CPHD < 1 cm) was achieved in 73.1% of RB and 69.9% of RF at 2-year follow-up (p = 0.216).

Conclusion: Adherence to commonly accepted UIV selection guidelines did not predict better shoulder balance. The RB group had worse shoulder balance immediately post-surgery, but also improved more over time. These results suggest the need to refine current UIV selection and management.

Level of evidence: III.