Early Compensatory Segmental Angle Changes at L3-L4 and L4-L5 After a L5-S1 Interbody Fusion for a Grade 1 Spondylolisthesis

STUDY DESIGN: The retrospective study included patients who underwent an L5-S1 anterior (ALIF) or transforaminal (TLIF) lumbar interbody fusion with posterior pedicle screw instrumentation for grade 1 spondylolisthesis from 2018 to 2022. OBJECTIVE: To compare early reciprocal changes at the L3-L4 and L4-L5 adjacent levels 6 months after ALIF or TLIF at L5-S1. BACKGROUND: Degenerative and chronic isthmic spondylolistheses often result in decreased segmental lordosis (SL) at L5-S1. This can lead to lordotic overcompensation at adjacent levels to maintain spinopelvic balance. However, the fate of adjacent angles after interbody fusion is not well understood. PATIENTS AND METHODS: Preoperative and 6-month postoperative measurements of SL (L3-L4, L4-L5, and L5-S1), lumbar lordosis, and pelvic incidence (PI) were obtained from sagittal standing radiographs. Preliminary t tests were performed for descriptive purposes, and multiple regression was used for hypothesis testing. RESULTS: Ninety-eight patients met the inclusion criteria (50 ALIF and 48 TLIF). A greater amount of lordosis achieved at L5-S1 was significantly associated with a greater reduction of SL at L4-L5 ( r = -0.65, P < 0.001) or L3-L4 ( r = -0.46, P < 0.001). A greater preoperative PI was associated with a greater reduction of SL at L4-L5 ( r = -0.42, P < 0.001) and at L3-L4 ( r = -0.44, P < 0.001). CONCLUSION: At 6 months after a lumbar interbody fusion at L5-S1, greater compensatory changes with lordosis reduction are observed at the supra-adjacent L4-L5 and L3-L4 levels in patients achieving greater L5-S1 SL. In addition, preoperative PI played a role in influencing lordotic correction.