Clinical and Radiological Evaluation of Cage Subsidence Following Oblique Lumbar Interbody Fusion

Abstract

Background

Cage subsidence was previously reported as one of the most common complications following oblique lumbar interbody fusion (OLIF). We aimed to assess the impacts of CS on surgical results following OLIF, and determine its radiological characteristics and related risk factors.

Methods

Two hundred and forty-two patients underwent OLIF at L4-5 and with a minimum 12 months follow-up were reviewed. Patients were divided into three groups according to the extent disk height (DH) decrease during the follow-up: no CS (DH decrease ≤ 2 mm), mild CS (2mm < DH decrease ≤ 4 mm) and severe CS (DH decrease > 4mm). The clinical and radiological results were compared between groups to evaluate the radiological features, clinical effects and risk factors of CS.

Results

CS was identified in 79 (32.6%) patients, including 48 (19.8%) with mild CS and 31 (11.8%) with severe CS. CS mainly identified within 1 month postoperatively and did not progress after 3 months postoperatively, and more noted in the caudal endplate (44, 55.7%). In terms of clinical results, patients in the mild CS group were significantly worse than those in the no CS group, and patients in the severe CS group were significantly worse than those in the mild CS group. There was no significant difference in fusion rate between no CS (92.6%, 151/163) and mild CS (83.3%, 40/48) groups. However, significant lower fusion rate was observed in severe CS group (64.5%, 20/31) compared to no CS group. CS related risk factors included osteoporosis (OR = 5.976), DH overdistraction (OR = 1.175), flat disk space (OR = 3.309) and endplate injury (OR = 6.135).

Conclusion

CS following OLIF was an early postoperative complication. Higher magnitudes of CS were associated with worse clinical improvements and lower intervertebral fusion. Osteoporosis and endplate injury were significant risk factor for CS. Additionally, flat disk space and DH overdistraction were also correlated with the increased probability of CS.

Introduction

Oblique lumbar interbody fusion (OLIF) is an effective treatment for patients with degenerative lumbar disc disease. Indirect decompression of neural elements could achieve through distracting the reduced intervertebral space by using an enlarged interbody cage, thus alleviating neurogenic intermittent claudication [1–2].

Maintaining the restored intervertebral space is the one of the inherent requirements following OLIF [3]. However, as a complication believed to result in losing of the intervertebral space. cage subsidence (CS) has been reported as common event following OLIF, with an incidence fluctuated between 10.0%-40.0% [2–3].

Currently, variability exists in the impacts of CS on surgical outcomes after traditional lumbar interbody fusion (LIF) surgery, with reported as an innocent occurrence in some studies [4–5], while an adverse event that causes pain and even failure of surgery in other reports [6–7]. However, to our knowledge, no early studies have thoroughly addressed whether CS affects surgical outcomes following OLIF. We launched this study to determine the impacts of CS on clinical and radiological results following OLIF, and to further study its occurrence characteristics and related risk factors, so as to provide some suggestions for preventing CS.

Methods

This was a retrospective study that was approved by the institutional review board in our hospital. Patients who underwent OLIF combined with anterolateral fixation between October 2017 and December 2019 at our institution were retrospectively reviewed, and waived the requirements for informed patients consent because of its retrospective nature. The inclusion criteria were patients who diagnosed with mild spinal stenosis (Schizas grade A or B [8]) and degenerative instability at L4-5. We excluded patients who underwent surgery at multi-level, who diagnosed with severe stenosis (Schizas grade C or D) or stenosis caused by extruded herniated disc, calcified disc or bony spur formation, who diagnosed with isthmic spondylolisthesis or severe degenerative spondylolisthesis (Meyerding grade Ⅲ-Ⅳ). Patients who follow-up less than 12 months were also excluded.

Surgical procedure

Surgical procedures were performed as described previously [9]. Anterolateral fixation was performed to exempt the posterior structures of the lumbar spine.

Radiological and clinical evaluation

Lumbar 3d-CT and X-ray were taken at preoperatively, 1 day, 1, 3, 6, and 12 months postoperatively. Disk height (DH) was measured as the vertical distance between the midpoint of the cranial endplate and the caudal endplate on the 3D-CT midsagittal plane (Figure 1a). After surgery, patients with DH decrease ≤ 2 mm were classified into the no CS (NCS) group, those with 2mm < DH decrease ≤ 4 mm were classified into the mild CS (MCS) group, and those with DH decrease > 4 mm were classified into the severe CS (SCS) group [6]. The demographics analysed included sex, age, diagnosis, bone mineral density (BMD), and body mass index (BMI). We utilized the minimum T score obtained from the hip using dual-energy X-ray absorptiometry (DEXA) scans, as lumbar spine DEXA information is often inaccurate in patients with lumbar degenerative pathology [11]. The radiological parameters analysed included DH distraction, cage position, endplate sclerosis or injury, disk space morphology, and fusion rate. DH distraction was calculated as the increment of DH at 1 day postoperatively compared with preoperatively. The cage position was measured as the percentage of the distance between the anterior metal marker and the leading edge of the caudal endplate to the length of caudal endplate using X-ray taken at 1 day postoperatively. Disk space morphology was classified on MRI as flat, concave, or irregular according to the criterion described by Pappou et al [12]. The clinical outcomes analysed included visual analogue scale (VAS) pain scores of the lower back and leg, and the Oswestry Disability Index (ODI), which were recorded at preoperatively and 1, 3 and 12 months postoperatively. Fusion was evaluated using 3D-CT taken at 12 months postoperatively according to the criteria described by Bridwell et al. [13]. The clinical outcomes and fusion rates were compared between patients in the three groups. The demographic and radiological parameters were also compared between patients with and without CS.

Statistical analysis

SPSS 22.0 (IBM Corp., Armonk, New York, USA) software was used for analysis. Chi-squared analysis was performed for categorical variables, and one-way analysis of variance was performed for continuous variables. Significance was set at P < 0.05. Univariate binary logistic regression (UBLR) was used to adjust for confounding variables, variables with P<0.15 were allowed to enter the multivariate binary logistic regression analysis (MBLR), and P<0.05 was considered statistically significant for MBLR.

Results

A total of 242 patients were finally enrolled in the study. CS was identified in 79 (32.6%) patients, including 48 (19.8%) in the MCS group and 31 (12.8%) in the SCS group. The remaining 163 (67.4%) patients were assigned to the NCS group. There were no significant differences in demographic parameters between the three groups (Table 1).

Characteristics Of Subsidence

The significant DH decrease of the subsidence segments occurred within 3 months postoperatively, from 11.0 ± 1.7 mm 1 day postoperatively to 8.5 ± 2.0 mm 1 month postoperatively (P < 0.001) and continued to 7.6 ± 1.9 mm at 3 months postoperatively (P = 0.003). Compared with 3 months postoperatively, the DH only slightly decreased to 7.3 ± 1.9 mm (P = 0.291) and 7.1 ± 1.8 mm (P = 0.084) at 6 and 12 months postoperatively (Fig. 2). At 1 month postoperatively, a total of 61 patients were identified with CS including 51 with MCS and 10 with SCS. This number increased to 79 at 3 months postoperatively, including 55 with MCS and 24 with SCS, indicating that significantly decreased in the ratio of MCS to SCS (P = 0.042). After 3 months postoperatively, no additional CS occurred. Only 5 and 2 segments developed from MCS to SCS at 6 and 12 months postoperatively, without no significantly changes in the ratio of MCS to SCS segments (P = 0.250, 0.156) compared to the 3 months postoperatively. (Fig. 3). A total of 44 CS occurred in the caudal endplate, 30 occurred in both caudal and cranial endplates, and the remaining 5 only occurred in the cranial endplate. Typical radiological datum is shown in Fig. 1.

Clinical And Radiological Results

The clinical outcomes of three groups are presented in Table 2 and Fig. 4. No significant differences in the clinical score between the three groups preoperatively (P = 0.069, 0.085, 0.094). At 1 month postoperatively, ODI and VAS scores of lower backs in MCS group were significantly higher than those in NCS group (P < 0.001), but significantly lower than those in SCS group (P < 0.001). There were no significant differences in VAS scores of legs between NCS and MCS groups 1 month postoperatively (P = 0.057), and both of them were significantly lower than those in SCS group (P < 0.001). At 3 months postoperatively, there were no significantly difference in all clinical results between NCS and MCS groups (P = 0.064, 0.836, 0.180), and both significantly lower than those in SCS group (P < 0.001). At 12 months postoperatively, ODI and VAS score of lower back in NCS and MCS groups were significantly lower than those of SCS groups (P < 0.001), while VAS score of leg were comparable between three groups (P_(NCS−SCS) = 0.775, P_(MCS−SCS) = 0.724).

The fusion rates were 92.6% (151/163), 83.3% (40/48) and 64.5% (20/31) in NCS, MCS and SCS groups, respectively. No significant differences in fusion rates between NCS and MCS groups (P = 0.053) or MCS and SCS groups were observed (P = 0.056). However, fusion rate in the SCS group was significantly lower than that in NCS group (P < 0.001)

Risk Factors

In UBLR analysis, BMD (P < 0.001), distraction of DH (P = 0.019), disc space morphology (P = 0.044), endplate sclerosis (P = 0.075), and endplate injury (P = 0.001) were significantly associated with CS, while age, sex, BMI, diagnosis, and cage position were not (Table 3). In the further MBLR analysis (Fig. 5), osteoporosis and endplate injury showed as significant risk factor for CS (P < 0.001), the adjusted odds ratio (OR) were 5.976 (2.636–13.548) and 6.135 (2.337–16.105), respectively. In addition, the flat disk space and the DH overdistraction were also significantly correlated with the increased probability of CS (P < 0.01), the adjusted OR were presented as 3.309 (1.670–6.558) and 1.775 (1.360–2.316), respectively. In contrast, endplate sclerosis showed as a significant protective factor for CS (P = 0.019), with the adjusted OR of 0.120 (0.020–0.703).

Discussion

CS is a progressive process that manifests as cages sinking into vertebrae through adjacent endplates prior to complete fusion [7]. Currently, large variations were reported in the process of CS after LIF technique [14–15]. Chen et al. [10] presented CS as a late event which was identified at 3 months postoperatively and continuously progressed until 2 years after lateral LIF (LLIF) technique. In contrast, Marchi et al. [7] argued that the CS occurred mainly within 6 weeks and without significant progress after 3 months following LLIF. A similar trend was found in this present study, our results suggested that CS following OLIF should be classified as an early complication that occurred primarily at 1 month postoperatively and did not progress significantly after 3 months postoperatively. Therefore, the early postoperative stage should be considered a vital period to address CS after OLIF surgery.

CS has great significance for LIF technique, as it makes some surgical goals may not be met. Various studies have compared CS to the surgical results following LIF technique, and a clear relationship was not found [4–7]. Our results indicated that CS were related to surgical results following OLIF. Higher magnitudes of CS were associated with worse surgical improvements. Marchi et al [8] proposed that low grade CS (DH reduction less than 25%) was the results of endplate remodelling due to the natural curvature of endplate, and does not interfere with subsequent fusion. Similarly, we noted that mild CS yielded a comparable fusion rate compared to no CS group. But it caused a transient poor clinical improvement. We speculated that this poor clinical improvement may be the result of transient local bony changes such as endplate inflammation, and may abate over time after CS stabilizes and correct to similarly improved clinical outcomes. In contrast, we found that severe CS caused not only poor clinical achievements, but also significantly reduced the fusion rate. On the one hand, we inferred that severe CS may aggravate and prolong this bone change, thus causing aggravated and constant poor clinical improvements. On the other hand, with respect to intervertebral fusion, it requires a stable biomechanical environment stable to promote trabecular connections. We considered that the remarkable reduction in the height of the intervertebral space due to severe CS may result in the re-relaxation of the ligamentous structure at the index level and thus fail to provide a stable biomechanical environment necessary for the fusion process, and eventually leading to reduced fusion rate [17]. Based on our aforementioned results, as CS following OLIF was associated with poor surgical improvements, it was helpful to identify the related risk factors so that CS can be prevented.

Risk factors related to CS are multifactorial. Generally speaking, the endplate stiffness and the interfacial load between the implant and the endplate are the basic factors affecting the occurrence of CS [18]. Oxland et al. [19] proposed that the endplate stiffness decreased by approximately 33% after injury, thus inducing CS. In our study, we found that endplate injury significantly increased the occurrence of CS, therefore, we suggest that attention should be given to avoiding endplate injury intraoperatively, which may be beneficial in reducing CS. The endplate stiffness also varies with the anatomic region [20]. Hou et al. [20] demonstrated that the failure loading required for CS was maximum when the cage was placed posterolaterally on the endplate with the strongest stiffness. Kim et al. [14] also reported that anterior cage position was a risk factor for CS following transforaminal LIF (TLIF) surgery. However, we failed to find a clear relationship between CS and cage position. We speculate that the position of the cage placed through the oblique channel was overall anterior and the range of anteroposterior position was narrow [21], so it is may not enough to reflect the stiffness discrepancy at different endplate regions. In addition, early study concluded that the cranial endplate is 40% stiffer than the caudal endplate [22], we confirmed this conclusion in the present study as we found that CS occurred more frequently in the caudal endplate.

BMD was also considered to be a vital factor affecting endplate stiffness. Hou et al. [20] found that decreased BMD resulted in lower endplate stiffness and lower failure loading for CS. Tempel et al. [11] presented that the sensitivity and specificity of a DEXA T score of -1.0 or less for predicting CS following LLIF were 78.3% and 63.2%, respectively. Park et al. [23] calculated that osteoporosis increased the CS risk following TLIF by 4.8-fold. We calculated that the osteoporosis increased the by 6.0-fold following OLIF, which was slightly higher than the risk coefficient of CS following TLIF. This finding indicated that higher requirements may be asked for bone conditions to prevent CS following OLIF.

Increased compressive forces is another basic mechanism which drive to endplate fracture and CS [24]. DH over-distraction is widely accepted as risk factor for CS in reports covering cervical fusion surgery [25]. In our study, we found that the increase in DH distraction was significantly correlated with CS following OLIF. Therefore, selecting appropriate cage height and avoiding excessive DH distraction may be beneficial for CS prevention. At present, the correct methods to select the appropriate cage height are still controversial. Some studies have suggested that the height should be determined according to the DH measured preoperatively [26], while others recommend should be determined by the compressive and distractive force generated by cage implantation [24].

The impacts of disc space morphology on CS have been preliminarily mentioned. Park et al. [23] presented that pear-shaped disc space was more likely to induce CS, interpreted as the smaller contact between the endplate and cage causes a stress concentration, thus inducing CS. Similarly, we found the CS risk significantly increased in cases with flat disc space compared to concave space. Therefore, customizing a specific shape cage according to the disc space shape to increase the match between cage and endplate maybe helpful to reduce CS.

Limitations

We acknowledge that the limitation of this study concerns is its retrospective design. In addition, other factors that may affect CS, such as cage length and lumbar lordosis [4, 6], were not involved in this study and would be further investigated. Finally, we were limited to examining the short-term impacts of CS on surgical outcomes, and the long-term results need to be traced further.

Conclusion

This investigation reviewed the impacts of CS on OLIF surgery, and further investigated its characteristics and related risk factors. We concluded that CS following OLIF surgery should be considered an early postoperative complication. Higher magnitudes of CS were associated with worse clinical improvements and lower intervertebral fusion. Osteoporosis and endplate injury were significant risk factor for CS. In addition, a flat disk space, and DH overdistraction were also correlated with the CS. Attention should closely paid to eliminate related risk factors to avoid CS when performed OLIF surgery.

List Of Abbreviations

LIF, Lumbar interbody fusion

OLIF, Oblique lumbar interbody fusion

LLIF, Lateral lumbar interbody fusion

TLIF, Transforaminal interbody fusion

CS, Cage subsidence

NCS, No cage subsidence

MCS, Mild cage subsidence

SCS, Severe cage subsidence

DH, Disk height

BMD, Bone mineral density

BMI, Body mass index

DEXA, Dual-energy X-ray absorptiometry

VAS, Visual analogue scale

ODI, Oswestry Disability Index

UBLR, Univariate binary logistic regression

MBLR, Multivariate binary logistic regression

Declarations

Ethics approval and consent to participate: The study was approved by Ethics Committee of West China Hospital, Sichuan University. Informed consent was waived because of its retrospective nature.

Consent for publication: Not Applicable

Availability of data and materials: Available via the corresponding author's email when the manuscript is received:

Competing interests: The authors have no conflicts of interest to declare.

Funding: No fundings

Authors' contributions: Long Zhao: Conceptualization, Writing- Original draft preparation. Tianhang Xie: Methodology. Xiandi Wang: Data curation. Zhiqiang Yang: Data curation. Xingxiao Pu: Software. Yufei Lu: Statistics. Jiancheng Zeng: Supervision, Validation.

Acknowledgements: We would like to thank AJE (www.aje.com) for English language editing.

The authors declare that all methods were performed in accordance with the relevant guidelines and regulations

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