Does the position of cage affect the clinical outcome of lateral interbody fusion in lumbar spinal stenosis?

DOI: https://doi.org/10.21203/rs.2.20211/v1

Abstract

Background: Lateral interbody fusion (LLIF) decompress the neural elements indirectly by increasing the height of disc, instead of resecting the disc or osteophytes herniated to the canal. When performing LLIF, the position of interbody cage is quite important for the outcome of decompression. This study aims to identify the ideal cage position in LLIF and to investigate if the posterior instrumentation would affect the indirect decompression.

Methods: This is a retrospective study. Patients underwent 2-stage surgeries: stage I was LLIF and stage II was percutaneous pedicle screws fixation after 1 week. Anterior disc height (ADH), posterior disc height (PDH), left and right foraminal height (FH) and segmental angle (SA) were measured on lateral CT reconstructions. The cross-sectional area of the thecal sac (CSA) was determined by the outlined area of the thecal sac on a T2-weighted axial MRI. The patients were subgroups according to the cage position: the anterior (cage located at the anterior 1/3 of disc space) and posterior groups (cage located at the posterior 2/3 of disc space). P-values <0.05 were considered significant.

Results: This study included 46 patients and 71 surgical levels. After stage I LLIF, significant increase in ADH, PDH, bilateral FH was found in both 2 subgroups, as well as the CSA (all p<0.01). SA increased 2.84±3.2° in anterior group after stage I LLIF and increased 0.81±3.1° in posterior group (p=0.013). After stage II surgery, SA was similar between anterior and posterior group (p=0.20). CSA showed no difference between the 2 groups.

Conclusion: The anteriorly placed cage may provide better improvement of anterior disc height and segmental angle after stand-alone LLIF surgery. After the second stage posterior instrumentation, the cage position would not affect the segmental angle or foraminal height.

Background

Compared with the conventional posterior decompression and instrumentation technique, the lateral interbody fusion (LLIF) has been reported with advantages of less blood loss, less complication, quicker return to work(1, 2). As aging of the population, many elder patients cannot endure more invasive posterior approach, and thus LLIF is an alternative option.

LLIF decompress the neural elements indirectly by increasing disc height, instead of resecting disc and bony structure which lead to stenosis. The increase of the height of foraminal area (FA) and of the cross-sectional area of the thecal sac (CSA) will lead to indirect decompression of the nerve roots and dural sac, which has been reported by previous studies(3, 4).

When performing LLIF, the position of interbody cage is quite vital for the outcome of decompression, fusion rate as well as the subsidence rate. The anteriorly located cage can benefit restoration of the segmental angle (SA), whereas the posteriorly located cage might be favorable for achieving the indirect decompression outcome(5). Thus, it is somehow controversial for the position of the cage in the LLIF surgery. Park et al(4). reported that the cage within the anterior 1/3 of disc space could benefit achieving the restoration of the SA without compromising the indirect neural decompression. While Jin et al(6). found that the middle 1/3 of the disc space was better for improvement of FA and posterior disc height in LLIF. Both studies performed posterior instrumentation followed the LLIF study, which would affect the evaluation of actual surgical outcome of LLIF. On the contrary, Alimi et al(7). found that the position of cage showed no impact on the FA and the CSA. Thus, this study aims to identify the ideal cage position in LLIF and to investigate if the posterior instrumentation would affect the indirect decompression.

Material And Methods

Cohort

This is a retrospective study. The patients underwent LLIF was reviewed from June 2016 to April 2018. The inclusion for this study was as below: 1) degenerative lumbar stenosis patients; 2) failed conservative treatment after at least 6 months; 3) presented with low back pain and claudication; 4) underwent LLIF and posterior instrumentation surgery; 5) at least 6 months follow-up. Patients with cage subsidence during follow-up, with grade 2 spondylolisthesis or more severe, with fusion of the facets, with severe osteoporosis were excluded. Thus, the final study cohort included 46 patients with 71 levels.

 

Surgical procedure

The 2-stage surgeries were performed through mini-open lateral transpsoas approach. The stage I was LLIF. The starting point was determined by the identification of the anterior margin of the disk space by feeling and aimed at the 1/3 point of the disk space. After making the starting hole in the disk space, fluoroscopy was used again throughout all surgical procedures. Cages were filled with an allogenous chip bone graft. Bone morphogenic protein was not used in all cases because bone morphogenic protein has not been approved in our country. After the completion of the anterior procedure, stage II posterior instrumentation was performed for all patients using percutaneous pedicle screws after 1 week.

 

Radiographic measurements

CT scans, magnetic resonance imaging (MRI) and plain X-ray films were taken at baseline, immediate post-LLIF surgery, and immediate after posterior instrumentation. All measurements were performed on PACS system. Anterior disc height (ADH), posterior disc height (PDH), left and right foraminal height (FH) and segmental angle (SA) were measured on lateral CT reconstructions. ADH was defined as the perpendicular distance from the anterior corner of the lower endplate to the upper endplate. PDH was defined as the perpendicular distance from the posterior corner of the upper endplate to the lower endplate. SA was defined as the Cobb’s angle between the upper endplate and lower endplate at the operated level. To determine the cage position, the upper endplate of the caudal vertebra was evenly divided into 3 zones on a lateral plain radiograph. The cage position was determined by the location of cage’s center. The CSA were measured on MR images. CSA was determined by the outlined area of the thecal sac on a T2-weighted axial MRI.

 

Statistics

The patients were subgroups according to the cage position: the anterior (cage located at the anterior 1/3 of disc space) and posterior groups (cage located at the posterior 2/3 of disc space). Paired t tests were used to compare radiographic parameters between the two subgroups. ANOVA was performed to compare the parameters between baseline, post-stage I and post-stage II surgery. Regression analysis was also performed. Statistical analysis was performed using SPSS software (version 20.0.0; SPSS Inc., Chicago, IL). P-values <0.05 were considered significant.

Results

This study included 46 patients (20 male and 26 female). Age averaged 61.45±6.35 years (range: 50–78 years). A total of 71 levels of LLIF was performed: 26 single levels, 15 double levels and 5 triple levels fusion. Among the surgical levels, there were 15 levels at L2/3, 26 at L3/4 and 30 at L4/5. The height of cages was 12mm for 33 levels, 14 mm for 35 levels and 16mm for 3 levels. The width of the cages averaged 47.4±3.4mm (range, 45.0–55.0mm). The anterior cage group has 24 levels and the posterior group has 47 levels (Table 1). The average height of the cages was 13.25mm±1.3mm in anterior group and 13.11±1.2mm in posterior group (p = 0.26). At baseline, no significant difference was found between anterior and posterior groups in terms of ADH, PDH, FH (all p>0.05, Table 2)

As shown in Table 2, after stage I LLIF, significant increase in ADH, PDH, bilateral FH was found in both 2 subgroups, as well as the CSA (all p<0.01). Comparing to posterior group, the increase of ADH was significantly larger in anterior group (3.41±3.25mm vs. 1.92±3.05mm, p = 0.022), while the increase of PDH was similar between the 2 subgroups (p = 0.66). SA increased 2.84±3.2° in anterior group after stage I LLIF and increased 0.81±3.1° in posterior group (p = 0.013). The increase of bilateral FH and the CSA showed no difference between the 2 groups (p>0.05, Table 2).

After stage II posterior instrumentation, ADH, PDH and bilateral FH showed no significant change compared to those after stage I LLIF surgery. ADH was still significantly larger in anterior group compared to posterior group. Slightly increased SA was found in the posterior group while SA showed no change in anterior group (Table 3). After stage II surgery, SA was similar between anterior and posterior group (p = 0.20). CSA was not calculated after posterior instrumentation due to metal artifacts.

To identify the factors affecting the increase in CSA, regression analysis was performed (Table 4). Multivariate analysis revealed no significant factors that affect the change ratio of CSA.

Discussion

Lateral lumbar interbody fusion is a surgical technique that achieves indirect decompression and restoration of lumbar alignment with the insertion of a large interbody cage into the intervertebral space, which distracts the annular fibrous and ligaments(8). The main aim of LLIF surgery is to restore disc space height which would lead to reduction of pain and improvement in disability. The minimally invasive LLIF approach has been reported to reduce tissue trauma, operative time and recovery time. The effects of successful indirect decompression on the neural element have been reported in a number of studies(9, 10). It also has been reported that the SA increased by a mean of 2.8°–9.0° per level through LLIF(4). However, the indirect decompression and increase of SA may not be able to be achieved at the same time since more increase of SA may lead to smaller posterior disc height. Therefore, this study analyzed the impact of cage position on decompression and change of SA. The results revealed that the cage position within the anterior 1/3 of disc space would benefit the anterior disc height as well as SA after LLIF surgery, but SA was similar after stage II posterior instrumentations.

The benefit of LLIF on SA has been reported in many studies. Park et al(4). reported greater increase in anterior disc height and SA when the cage was placed in the anterior 1/3 area than in the middle 1/3 area. Kepler et al(11). also demonstrated that anterior cage position resulted in the largest SA increase (7.4°) whereas posterior position reduced the angle by a mean of –1.2°. Their study was conducted based on patients with both LLIF and posterior fixation. Our results showed larger ADH and SA in anterior group after stage I LLIF surgery, while SA showed no difference after stage II posterior instrumentation. The difference of SA after stage I and stage II surgery may be due to the pressure during posterior instrumentation. When performing percutaneous pedicle screw fixation, the rod was pre-contoured. The surgeons need to press to install the rod. Since the disc had been removed and the facets was not fused according to inclusion criteria, the pressure from posterior side would increase SA, especially in posterior group where the cage (like a hinge) was closer to posterior edge of vertebra. Our results also observed slightly decreased PDH after stage II fixation even if the difference was not statistically significant. Melikian et al.(12) showed in a biomechanical study that none of the cages, including the 30° lordotic cage, caused a decrease in posterior disk height suggesting hyperlordotic cages do not cause foraminal stenosis.

The sagittal alignment has been emphasized in spinal degenerative deformity and degenerative diseases in the recent decades(13). The restoration of lumbar lordosis was correlated to better clinical outcome and less incidence of adjacent segmental degeneration (ASD), even in short-level fusion surgery. Kim et al(14) reported that the restoration of the SA is important to increase pelvic tilt and to achieve good clinical outcomes in L4–L5 degenerative spondylolisthesis. Recently Tian et al(15). showed that improved lumbar lordosis was correlated to a lower incidence of ASD, and that proper disc height and segmental lordosis restoration were essential for prevention of ASD. According to our results, both anterior and posterior placed cage could be used to improvement SA after LLIF surgery.

Regarding to indirect decompression, significant increase in CSA and bilateral FH were observed in this study after LLIF. Our results of CSA and FH change was comparable to previous reports. Rao et al(16). reported that anterior lumbar interbody fusion resulted in significant indirect foraminal decompression and that PDH was a significant factor in the restoration of the FH. Oliveira et al(17). showed an increase of average disc height (41.9%), FH (13.5%), foraminal area (24.7%), and central canal diameter (33.1%) after LLIF surgery. However, our study failed to find the independent factors for CSA increase based on regression analysis. Park et al. demonstrated that preoperative CSA was the only independent factor which correlated to the increase of CSA and that the cage position did not affect the increase of CSA. Thus, the cage position and the cage size would not affect the indirect decompression in a LLIF surgery.

Based on our results, we may postulate that the place of cage may depend on different situations. In patients with lumbar hypo-lordosis or even kyphosis, old age, osteoporosis and in those who may have endplate injury during surgery, we recommend placing the cage in the anterior 1/3 of disc space to avoid potential cage subsidence and to improvement segmental lordosis. For those with severe foraminal stenosis, the cage could be placed at the posterior 2/3 of the disc space since our results showed slightly more increase of bilateral FH (without statistical significance).

Limitations of the current study include its relatively small sample size and the lack of long-term follow-up data. These longer-term data are necessary for determining pseudarthrosis rates regarding cage subsidence between anterior and posterior cage group. In addition, this study did not include the evaluation of clinical outcomes because the study time was immediate after surgery. However, it is well-documented in other reports that successful indirect neural decompression resulted in good clinical outcomes.

Conclusion

The anteriorly placed cage may provide better improvement of anterior disc height and segmental angle after stand-alone LLIF surgery. After the second stage posterior instrumentation, the cage position would not affect the segmental angle or foraminal height. The indirect decompression, presented by CSA, would not be affected by the cage position.

Abbreviations

LLIF: Lateral lumbar interbody fusion; ADH: Anterior disc height; PDH: Posterior disc height; FH: Foraminal height; SA: Segmental angle; CSA: Cross-sectional area of the thecal sac; FA: Foraminal area.

Declarations

Acknowledgments

We would like to thank all the participants who took part in this research for their time and help.

Funding

No funding.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Authors’ contributions

GQ, XW, JL and BYue were responsible for the design, conducting the study, data analysis. MG and BYang were responsible for the writing process. All authors read and approved the final manuscript.

Ethics approval and consent to participate

Written informed consent was obtained from all participants in accordance with protocol approved by the Affiliated Hospital of Qingdao University Research Ethics Board.

Consent for Publication

Not applicable.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

Reference

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Tables

Table 1: Surgical levels for the anterior and posterior subgroups

 

Anterior group

Posterior group

p

Cage height (mm)

13.25±1.3

13.11±1.2

0.26

Surgical levels

L2/3

4

11

0.71

L3/4

9

17

L4/5

11

19

Total

24

47

 

 

 

Table 2: Comparison of radiographic parameters between anterior and posterior subgroups

 

 

Anterior group

Posterior group

p

ADH (mm)

Baseline

13.41±4.02

14.13±3.25

0.33

Post stage I LLIF

16.82±3.12

15.05±2.97

0.51

Post stage II fixation

17.05±3.22

15.62±3.04

0.25

PDH (mm)

Baseline

8.65±2.74

7.43±2.62

0.68

Post-LLIF

9.83±1.77

9.27±2.11

0.54

Post stage II fixation

9.92±1.85

9.31±2.42

0.33

SA (°)

Baseline

8.24±4.28

8.41±5.33

0.65

Post-LLIF

11.08±3.87

9.22±4.38

0.04

Post stage II fixation

10.23±3.65

9.35±4.18

0.49

Left FH (mm)

Baseline

18.31±2.45

17.84±2.62

0.84

Post-LLIF

19.64±1.46

19.11±1.74

0.72

Post stage II fixation

19.52±1.33

19.27±1.69

0.76

Right FH (mm)

Baseline

17.85±2.66

16.96±2.48

0.59

Post-LLIF

19.03±1.84

18.72±1.49

0.47

Post stage II fixation

18.84±1.62

18.92±1.33

0.68

CSA (mm2)

Baseline

92.56±49.89

83.68±57.66

0.18

Post-LLIF

124.82±47.36

116.39±52.44

0.31

 

Table 3: Multivariate regression for increase of CSA

Parameters

β±SE

p

Cage position

-0.14

0.42

Change of SA

0.07

0.38

Surgical levels

-

-

Change of ADH

-

-

Change of PDH

-

-