O2082: LIV Selection in 'Tweener' Patients Treated with MCGR vs. PSF

Background: Selection of the lowest instrumented vertebrae (LIV) is a foundational principle guiding the management of spinal deformity. Equipoise exists regarding the surgical strategy for ‘tweener’ patients where both magnetically controlled growing rods (MCGR) and posterior spinal fusion (PSF) could be employed. There are no studies comparing the LIV for patients treated with MCGR vs PSF. The purpose of this study was to compare the LIV selection in ‘tweener’ patients treated with MCGR or PSF. We hypothesized that surgical strategy would inform LIV selection with the LIV being more caudal in MCGR when compared to PSF.

Methods: A multicenter pediatric spine database was queried for ambulatory patients ages 8-11 years treated by MCGR or posterior spinal fusion with at least 2-year follow up. The relationship between the LIV and preoperative spinal height, curve magnitude, and implant type were assessed. The relationship between the last substantially touched vertebrae (LSTV), the stable vertebrae (SV), and the LIV were also evaluated.

Results: One hundred and fifty-nine patients met inclusion criteria including 82 MCGR and 77 PSF patients. Preoperative curve magnitude was similar between groups (MCGR 68±19.0° vs PSF 66±17.2°, p=0.6), but age (MCGR 9.0±1.0 vs. PSF 10.2±0.8, p< 0.0001) and T1-T12 spinal height (MCGR 194.5±29.8mm vs. PSF 206.4±31.7mm, p=0.041) were different. Neither age (p=0.07) or height (p=0.27) was associated with LIV selection. In contrast, curve magnitude was associated with LIV as larger curves were associated with a more caudal LIV (p=0.004). Distribution of the LIV was more varied in PSF compared to MCGR (Figure 1, p=0.05). L3 was the LIV in 43% of MCGR patients compared to 27% of PSF patients and 29% of PSF patients had an LIV of L1 or above compared to 17% of MCGR patients. The LIV was cephalad to the SV in 68% of PSF compared to 48% of MCGR patients (p=0.02).

Conclusion: The majority of LIV selection in ‘tweener’ patients was at L3 or below regardless of surgical strategy, likely driven by curve magnitude. However, ‘tweener’ patients treated with PSF had more cephalad LIV selections compared to patients treated with MCGR. Potential LIV differences should be considered when selecting MCGR vs. PSF in ‘tweener’ patients.