Adjacent segment degeneration at a minimum 2-year follow-up after posterior lumbar interbody fusion: the impact of sagittal spinal proportion: a retrospective case series.

Study design: A retrospective cohort study.

Purpose: To investigate the sagittal plane configuration of the entire spine and its association with the risk of adjacent segment degeneration (ASD) after posterior lumbar interbody fusion (PLIF).

Overview of literature: Although PLIF has demonstrated satisfactory clinical outcomes, it is associated with ASD. However, the geometric mechanical changes that contribute to the occurrence of ASD are not well characterized.

Methods: Radiological parameters were extracted from the whole lateral radiographs. Patients were divided into two groups: the ASD group (segmental kyphosis of ≥10º, and/or a ≥50% loss of disc height, and/or ≥3 mm of anteroposterior translation) and the non-ASD group.

Results: All 112 included patients underwent PLIF for lumbar degenerative diseases. The minimum follow-up period was 2 years, with an average follow-up time of 63.6 months. Fifty-two patients (46.4%) were classified into the ASD group and of these, 13 required reoperation due to failure of conservative treatment. Patients with ASD exhibited significantly more caudal and posterior inflection vertebrae (IV), while the lumbar apical vertebra was significantly more caudal immediately after surgery. The IV position was identified as a significant risk factor for ASD, and the ASD incidence was significantly higher in the group where IV ≤5 (L1 vertebral body) than in the group where IV ≥5.5 (T12-L1 disc) (69.0% vs. 38.6%).

Conclusions: The IV position is a significant risk factor for ASD development. Although it is difficult to control intraoperative IV levels, we note a high risk of ASD in patients with IV lower than T12-L1.