Radiographic Assessment of Segmental Correction and Subsidence Following Posterior Lumbar Fusion with Expandable Interbody Cages: A 12-Month Retrospective Cohort Study

Background: Expandable interbody cages have gained popularity in posterior lumbar fusion due to their ability to restore disc height and segmental alignment with minimal surgical exposure. However, concerns remain regarding their long-term durability, particularly the risk of implant subsidence. This study evaluates radiographic outcomes and subsidence rates over a 12-month period following posterior lumbar fusion with a novel expandable cage. Methods: We conducted a retrospective cohort study of 40 consecutive adult patients who underwent posterior lumbar fusion using a posteriorly placed expandable cage (Calibrate PSX) at a single academic institution. Radiographic parameters—including anterior disc height, posterior disc height, segmental lordosis, and global lumbar lordosis (L1–S1)—were measured preoperatively, immediately postoperatively, and at 3 and 12 months. Subsidence was graded according to the classification proposed by Levy et al. (2024). Generalized estimating equations were used for longitudinal analysis, with statistical significance set at p < 0.05. Results: Among 44 implanted levels, 16 levels (36.4%) demonstrated subsidence at 3 months, increasing to 20 (45.5%) by 12 months. No surgical revisions were required at 12 months. Anterior disc height increased significantly in the immediate postoperative period (+8.6 mm, p < 0.0001) but declined over time, with greater loss in the subsidence group (–7.1 mm vs. –1.9 mm, p = 0.013). Posterior disc height followed a similar trend. However, segmental and global lumbar lordosis remained stable, with no significant differences between subsidence and non-subsidence groups. Minor intraoperative complications occurred in 6 patients (13.6%) and resolved without sequelae. Conclusions: Expandable interbody cages provide substantial immediate postoperative improvements in disc height and alignment. While subsidence can occur over time, segmental and global lordosis are generally maintained. The overall subsidence rate was modest and not associated with adverse clinical outcomes. These findings support the cautious use of expandable cages in posterior lumbar fusion, with attention to patient selection, surgical technique and implant selection to mitigate the risk of subsidence.