Determining Lowest Instrumental Vertebrae (LIV) on Prone X-ray Can Save Fusion Levels with Good Correction and Balance in AIS Patients Compared to Traditional Methods

Background: Minimizing the fusion levels in PSF for AIS is important. Previous studies have shown good results utilizing TV as the LIV. TV is the ‘touched’ vertebra determined by central sacral vertical line on standing AP XRs (TVS). We have found that TV moves proximally on supine/prone XRs. Thus utilizing TV on prone XRs (TVP) in LIV decision making may allow even shorter fusion.

Methods: There were three groups. Group I: patients where TVP was used to determine LIV. Group II: patients where TVS was used to determine LIV. Group III: non-operative AIS (Risser 4/5, Cobb <30) to determine ‘acceptable’ end vertebra tilt and disc wedging. Patients with only thoracic fusion were excluded. Cobb angle, coronal balance (CB) , LIV tilt angle and translation, and disc wedging were collected at preop and postop.Median values and interquartile were collected for the subsets.

Results: The control group had 132 patients with a median (IQR) Cobb of 20° (16-26), age of 16 (14.8-17), coronal balance 1.4 (0.5-2.2), disc wedging of 4° (2-5), and LIV tilt of 10° (7-13).
In Group I (n=102), median preoperative Cobb was 53.8° and coronal balance was 1.8. Final Cobb was 12.4° and coronal balance was 0.9. Compared to controls, Group I patients had significantly less coronal imbalance (p =0.023), lower disc wedging (p>0.001) and LIV tilt (p < 0.001).
In Group II (n=26), preoperative Cobb was 53.5° and coronal balance was 2. Final Cobb was 20° and coronal balance was 0.7. Group II patients could have saved an average 2.24 levels, if fused to TVP.
Preoperative Cobb angle, coronal balance, LIV tilt, disc angle, and LIV translation were similar between Group I and Group II. While final coronal balance was not significantly different between Group I and Group II, final Cobb angle (p < 0.001), disc angle (p < 0.001), and LIV translation (p=0.002) were all significantly smaller for Group I. Group II fused significantly fewer levels (p = 0.005), and had significantly more patients with final disc angle > 5° (p < 0.001).

Conclusion: In AIS, using TVP to determine LIV allows for shorter fusion. LIV tilt and disc wedging is also within ‘acceptable’ levels determined on controls. TVP is an effective and better way to determine the lowest instrumented vertebra.