Does kyphotic configuration on upright lateral radiograph correlate with instability in patients with degenerative lumbar spondylolisthesis?

OBJECTIVE: To investigate the segmental instability of degenerative lumbar spondylolisthesis (DLS) with a kyphotic configuration at the involved segment, and to determine the most useful diagnostic modalities in the evaluation of instability.

PATIENTS AND METHODS: This study reviewed a consecutive series of patients with L4/5 DLS between July 2010 and May 2016. The enrolled patients were divided into two groups based on preoperative neutral radiographs: the kyphotic group (Group K) and non-kyphotic group (Group NK). Translational and angular motion was determined by comparing upright lateral radiograph (U) with a supine sagittal MR image(S) (Combined, US) or flexion/extension radiographs (FE).

RESULTS: There were 26 and 201 patients in Groups K and NK, respectively. In comparison to Group NK, Group K demonstrated significantly higher translational motion (12.4% vs. 7.0%, P < 0.001) on US analysis, but significantly lower translational motion (4.2% vs. 6.4%, P < 0.001) on FE analysis. Angular motion was detected to be significantly lower in US versus FE in Group NK (1.2° vs. 7.8°, P < 0.001), while of no difference in Group K (P > 0.05). In Group K, "instability" was recognized in 84.6% of patients using US versus 11.5% patients using FE (P < 0.001); While in Group NK, no significant difference was observed in the incidence of "instability" between FE and US (31.3% vs. 27.8%, P = 0.444). Overall, Group K had a significantly higher incidence of instability than Group NK (84.6% vs. 31.3%, P < 0.001).

CONCLUSION: DLS with a kyphotic configuration is a distinct subgroup associated with segmental instability. The modality of US is shown to be superior to traditional FE in measuring translational motion and identifying instability for DLS patients with a kyphotic configuration.