The Effect of L5 Morphology on Prevention of L5-S1 Degeneration Following Floating Fusion for Degenerative Spine Disorders.

Study design: Retrospective Study.

Objectives: The selection of floating fusion or lumbosacral fusion arises when treating patients with instability or stenosis of the lower lumbar spine concomitantly radiographic degeneration of L5-S1. This study aimed to investigate the preoperative anatomical or morphological factors affecting the survivorship of the L5-S1 after floating fusion.

Methods: This study included 77 patients who had undergone floating fusion surgery through the TLIF approach. Preoperative radiographic parameters were evaluated using anteroposterior and lateral lumbar spine radiographs. The patients were divided into two groups according to the Modified Pfirrmann Grading and total endplate score. Multivariable regression analysis was performed to explore the relationships between the anatomical or morphological characteristics of L5 and the degeneration of L5-S1.

Results: The disc degeneration group exhibited a smaller height ratio of the iliac crest (H_i) and a less L5 deep position. Furthermore, the right/left height of L4 (H_r/H_l) and the right/left width of transverse process of L5 (C_Rt/C_Lt) were significantly higher in the disc and endplate degeneration groups. Multiple logistic regression analysis revealed that H_i and C_Rt were independently associated with L5-S1 disc degeneration, whereas H_r was a significant risk factor for L5-S1 endplate degeneration.

Conclusion: Anatomical and morphological characteristics of L5, such as smaller H_i, higher C_Rt and H_r, were associated with an increased risk of L5-S1 degeneration in patients after floating fusion. These findings may indicate the fusion level when addressing lower lumbar degenerative diseases and the concurrent radiographic degeneration of L5-S1.