Special Poster Session 51st International Society for the Study of the Lumbar Spine Annual Meeting 2025

Introduction
The etiology of curve progression in adolescent idiopathic scoliosis (AIS) remains unclear, and limited literature examines its structural characteristics. Previous studies have addressed asymmetry in intervertebral disc and vertebral body height or facet joint degeneration, but research focusing on facet joint asymmetry is scarce. Additionally, while inferior articular process resection is the most common and effective technique in posterior correction surgery for AIS, the resection width has not been quantified. This study aims to evaluate facet joint size and asymmetry in AIS, investigate its association with scoliosis progression, and quantify the width of inferior articular process resection.

Methods
We retrospectively analyzed 200 AIS patients with Lenke type 1 curves, excluding cases with congenital or neuromuscular scoliosis. Radiographic evaluations included measurements of the major thoracic curve (MTC), thoracolumbar kyphosis (TLK), lumbar lordosis (LL), lumbar scoliosis (LS), side-bending, and traction radiography. Facet joint size was assessed as a ratio relative to the vertebral arch size. The X-axis of the facet joint was measured as a/b, and the Y-axis as d/c (Figure 1). Facet joint asymmetry was defined as an X-axis or Y-axis R/L ratio exceeding 80%. We examined correlations between facet asymmetry and scoliosis. Patients were also classified into two groups based on MTC severity: L group (40–45°) and H group (≥55°). Statistical analyses, including Pearson's correlation and Student’s t-test, examined the associations between facet asymmetry and scoliosis.

Results
Facet Joint Size: The right facet joint was significantly larger than the left up to T6, after which the left facet predominated from T7 onward. Along the Y-axis, facet joint size progressively decreased towards the lower thoracic vertebrae (Figure 2,3).

Facet Joint Asymmetry: A notable switch in facet joint asymmetry was observed between the vertebra above the apex and the apex itself (Table 1), with a significant correlation between MTC angle and facet asymmetry (r=0.35).

Group Comparison: Demographics showed no significant differences between the L and H groups. However, radiographic assessments revealed significantly greater facet asymmetry in the H group compared to the L group (H group vs. L group; 4.3±1.7 vs. 3.0±1.86, p<0.05), with a higher incidence of asymmetry in caudal vertebrae relative to the apex (Table 2,3).

Conclusion
This study identified the vertebral levels at which facet joint size and asymmetry switch, providing a basis for quantifying facet osteotomy width in AIS surgery. The findings suggest a potential association between facet asymmetry and scoliosis progression. While previous studies have reported consistently larger right facets in normal thoracic vertebrae, in AIS patients, we observed a switch in facet sizes, suggesting that facet asymmetry may influence scoliosis development and progression. These results imply that AIS may not be entirely idiopathic; instead, congenital abnormalities in facet joint morphology could be contributing factors.