Sacral Slanting in Adolescent Idiopathic Scoliosis: Definition, Pathomechanism, and Surgical Implications: A Narrative Review.

Sacral slanting-defined as the oblique tilt of the upper sacral endplate measurable on standing anteroposterior whole-spine radiographs-is a distinctive and clinically underappreciated finding in adolescent idiopathic scoliosis (AIS). Its etiology is multifactorial and incidence among AIS patients reaches 40%. The direction and degree of sacral slanting interact with the distal lumbar curve type (L4-left vs. L4-right) to influence the pattern of postoperative radiographic changes at the distal spine and shoulder. In patients with L4-left curves and left-sided slanting, stopping fusion at L3 generally prevents coronal decompensation. Furthermore, 5-year data show that the disc wedge angle below the lowermost instrumented vertebra (LIV) does not progress when LIV is L3, and patient-reported outcomes are preserved without revision surgery. When fusion extends to L4 in patients with high sacral slanting, coronal decompensation is more frequent, yet functional outcomes remain comparable and no revision has been required in published series to date, indicating that the decompensation observed is radiographically significant but clinically tolerable within the medium term. In Lenke 1A patients with right-sided sacral slanting (L4-right type), the slanting lowers the last touching vertebra and may facilitate distal adding-on. In Lenke 2A patients with right-sided slanting, distal adding-on serves as a natural shoulder-rebalancing mechanism and may be reasonably monitored conservatively rather than reflexively treated, pending further validation in independent cohorts, unless progressive or symptomatic. Across all scenarios, the optimal distal fusion level remains an unresolved dilemma requiring further long-term prospective study. This review synthesizes the available literature to characterize sacral slanting as a promising radiographic parameter that may serve as a potentially useful adjunct in distal fusion level planning, while highlighting the need for prospective multicenter validation before its routine incorporation into surgical decision-making in AIS.