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

Highlights

• DLS with a kyphotic configuration is associated with lumbar instability.

• FE underestimates pathological segmental mobility in patients with kyphotic DLS.

• US is superior to FE in measuring translational motion for kyphotic DLS.

• Instability was detected in 84.6% of kyphotic DLS on US but only in 11.5% on FE.

Abstract

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 L_4/5 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.

Introduction

The combinationof disc degeneration, facet joint hypertrophy, ligament thickening and segmental instability in degenerative lumbar spondylolisthesis (DLS) often results in back pain and neurogenic claudication, which frequently requires surgical intervention [1,2]. Although there are multiple modalities of surgical treatment for symptomatic DLS, including decompression, fusion with or without instrumentation, and interbody fusion, controversy persists for determining the optimal surgical option for DLS in certain clinical scenarios [3]. According to recent surveys on surgical intervention for DLS, segmental instability is a key factor in determining the treatment algorithm, which is largely based on concerns about iatrogenic destabilization of the olisthetic segment leading to poor long-term results [[4], [5], [6]].

From a radiographic view, segmental instability of DLS is defined as either the abnormal alignment of spinal segments or, in a broader definition, excessive mobility and angulation at the olisthetic segment [7]. Because degenerative changes of the disc and facet joint may hinder angular motion, abnormal angulation is a poor method to reveal segmental instability [2]. Therefore, the criterion of instability is commonly set as a translational motion >3 mm and/or >8% of the width of the above adjacent vertebra [2,8]. Various positions of patients have been measured to estimate the instability on roentgenography, among which lateral dynamic flexion and extension radiography (FE) is typically used to evaluate intersegmental hypermobility [9]. Additionally, a combination of upright lateral X-ray and supine MR images has been reported to yield results at least comparable to those of FE in evaluating instability of DLS, which is an easily available, alternative diagnostic modality to FE [8].

However, controversy remains as to whether disc degeneration or facet joint arthrosis is the initiating cause of DLS. Moreover, disc degeneration is a common pathway in pathology of DLS, which results in decreases in disc height [10]. In terms of slip configuration at the olisthetic level, two distinct subtypes separated by slip angle and kyphotic and non-kyphotic angulation were identified in clinical practice. A kyphotic configuration is primarily characterized by a kyphotic slip angle on a neutral lateral radiograph and a posterior disc height higher than the anterior disc height, whereas a non-kyphotic slip angle has a lordotic slip angle with a higher anterior disc height [5].

Recently, increased concern has arisen regarding the subset of DLS patients with a kyphotic configuration [11]. This kyphotic subset and those a with decreased segmental lordosis need to be identified as a separate group of DLS, as discussed in recent classification schemes of DLS [11,12]. However, there remains a paucity of data on the stability of DLS patients with different slip configurations. Furthermore, the modalities that can provide a more accurate assessment of stability in DLS with different slip configurations are unclear. Therefore, the current study was performed to determine the segmental stability and motion characteristics of kyphotic versus non-kyphotic spondylolisthesis through FE and US modalities and to determine whether US or FE is better able to identify instability.