Alignment Factors Associated with the Need for Revision Extension Surgery to the Sacrum After Previous Lumbar Spinal Fusion.

Abstract

Retrospective cohort study from a single academic institution. To identify clinical and radiographic predictors for sacral extension (SE) during revision lumbar fusion. Lumbar fusion is common, with revision rates up to 25.9% within two years. When planning a revision of lumbar fusion, surgeons may extend constructs from L5 to the sacrum to improve stability, decompression, or alignment, but sacral extension alters biomechanics and increases risks such as pseudoarthrosis, adjacent segment disease, and proximal junctional kyphosis. Predictors for sacral extension during revision remain poorly defined. Adult patients undergoing anterior or transforaminal lumbar interbody fusion (ALIF or TLIF) between 2017-2022 at a single academic institution, and those referred for revision with sacral extension, were reviewed. Eligible patients had an index fusion spanning L1-L4 to L5 or above. Sacral extension was defined as instrumentation to S1 or the pelvis within two years. Demographics, frailty indices, radiographic parameters, and complications were collected. Operative notes were reviewed to identify indications. Analyses included t-tests, chi-square, and multivariable logistic regression. Of 181 patients, 50 (27.6%) underwent SE and 131 (72.4%) remained fused between L1-L5. SE patients had higher frailty scores (MFI-5, P=0.018) and lower L4-L5 lordosis (P=0.020). Independent predictors included increased frailty (OR 7.015, P=0.032), greater fusion length (OR 1.796, P=0.012), and reduced L4-S1 lordosis (OR 1.137, P=0.007). Closer alignment of L1PA to ideal was protective (OR 0.81 per degree, P=0.009). Common indications were distal junctional degeneration (58%), foraminal stenosis (40%), and pseudoarthrosis (38%). Frailty, longer constructs, and inadequate caudal lordosis independently predicted sacral extension during revision, while optimal L1PA alignment was protective. The most common indications were distal junctional degeneration, pseudoarthrosis, foraminal stenosis, and spondylolisthesis. These findings may aid preoperative risk stratification and surgical planning.