Radiographic and clinical outcome of lumbar lateral interbody fusion for extreme lumbar spinal stenosis of Schizas grade D: a retrospective study

Background: Extreme lumbar spinal stenosis was thought to be a relative contraindication for lumbar lateral interbody fusion (LLIF) and was excluded in most studies. This is a retrospective study to analyze the radiographic and clinical outcome of LLIF for extreme lumbar spinal stenosis of Schizas grade D. Methods: For radiographic analysis, we included 202 segments from 124 patients who underwent LLIF between June 2017 and December 2018. Lumbar spinal stenosis was graded according to Schizas’ classification. Anterior and posterior disc heights, disc angle, foramen height, spinal canal diameter and central canal area were measured on CT and MRI. For clinical analysis, 18 patients with at least one segment of grade D were included. Visual analogue scale (VAS) and Oswestry disability index (ODI) scores were used to evaluate clinical outcome. Continuous variables were confronted by using Student's t-test, obtaining a statistically significant difference for values inferior to 0.05. Results: Among the 202 segments included for radiological evaluation, there were 42 grade A segments, 41 grade B segments, 101 grade C segments and 18 grade D segments. Postoperatively, the average change of midsagittal canal diameter of grade D was significantly greater than that of grade A, and not significantly different compared to grades B and C. As to the average changes of disc height, bilateral foraminal height, disc angle and central canal area (CCA), grade D was not significantly different from the others. The average postoperative CCA of grade D was significantly smaller than the average preoperative CCA of grade C. Eighteen patients with grade D stenosis were followed up for an average of 19.61 ± 6.32 months. Clinical evaluation revealed an average improvement in the ODI and VAS scores for back and leg pain by 20.77%, 3.67 and 4.15 points, respectively. Sixteen of 18 segments with grade D underwent posterior decompression. Conclusion: The radiographic decompression effect of LLIF for Schizas grade D segments were comparable with that of other grades. Posterior decompression was necessary for LLIF to achieve a satisfactory clinical outcome for extreme lumbar spinal stenosis of Schizas grade D.


Background
As a minimally-invasive technique, lateral lumbar interbody fusion (LLIF) has become the first choice of many spine surgeons in recent years. LLIF is capable of restoring foraminal and intervertebral height, thecal sac area, and alignment while with less trauma and lower approach-related morbidity compared with traditional open decompression techniques (1,2), making it especially suitable for elderly patients, patients with multi-level lumbar spine diseases and patients who cannot tolerate large operations.
LLIF, as an indirect decompression technique, does not directly remove a disc or osteophyte protruding into the spinal canal, and its decompression effect is not as thorough as traditional posterior decompression surgery. Radiographic studies have shown that improvement of the crosssectional area of the spinal canal is significantly smaller after LLIF than after minimally-invasive transforaminal lumbar interbody fusion (3,4). Generally, extreme central canal stenosis, defined by a complete loss of cerebrospinal fluid signal on preoperative magnetic resonance imaging (MRI), was thought to be a relative contraindication for LLIF. According to Schizas' classification(5), Grade D stenosis is defined as extreme stenosis, in which, in addition to no rootlets being recognizable, there is no epidural fat posterior to the dural sac (Fig. 1). Since patients with extreme stenosis (grade D) were excluded in most studies, the clinical and radiographic outcomes of LLIF for extreme lumbar spinal stenosis remain unknown. However, extreme lumbar spinal stenosis is common in clinical practice, especially in patients with multi-level lumbar degenerative disease. For the sake of less invasiveness, it is reasonable to perform LLIF for those patients instead of traditional open surgery, although additional posterior decompression is sometimes needed. The current study was the first to evaluate the indirect neural decompression effect in patients with extreme lumbar spinal stenosis. In current study, we compared the radiographic outcomes of LLIF for stenosis of Schizas grades A, B, C and D. Then, clinical outcomes of LLIF for a series of cases with stenosis of Schizas grade D were retrospectively evaluated.

Methods Patients
Retrospectively, patients with the main diagnosis of degenerative lumbar spinal stenosis who underwent crenel lateral interbody fusion (CLIF) (6, 7), a modified extreme lateral interbody fusion technique, by our surgical group between June 2017 and December 2018 were reviewed. Patients who suffered from significant lumbar scoliosis, grade 2 spondylolisthesis, lumbar fracture or had undergone prior lumbar surgery were excluded from this study. All the segments were grouped according to Schizas' lumbar stenosis classification(5). Grade A stenosis is the mildest, with abundant cerebrospinal fluid inside the dural sac. In grade B stenosis, the rootlets occupy the whole of the dural sac, but they can still be individualized. In grade C, no rootlets can be recognized but epidural fat can be visualized posteriorly. In grade D, in addition to no rootlets being recognizable there is no epidural fat posteriorly.

Radiological and clinical assessments
Standing lateral plain radiographs, MRI, and CT scans were obtained for all patients preoperatively and postoperatively. We measured the imaging data before and after the stage I CLIF (before the stage Ⅱ posterior internal fixation). All radiographic parameters were measured using measurement tools on PACS. The main measurement indexes included: the disc angle (DA), the anterior and posterior disk height (ADH and PDH), the bilateral intervertebral foramen height (IFH) on CT, and midsagittal canal diameter (CD) and the axial central canal area (CCA) on MRI (Fig.2).
A total of 18 patients with at least one level with grade D stenosis who were followed for at least 6 months were clinically reviewed. The patients comprised seven males and eight females with a followup time of 18.53 ± 6.39 months (range: 9 -26 months) (Tab.1). Clinical outcomes were assessed by an experienced clinical research coordinator using a visual analogue scale (VAS) for back and leg pain and Oswestry Disability Index (ODI). The minimal clinically important difference for the ODI was 10 points(8). These data were compared between before surgery and at the last follow-up. In addition, perioperative data and complications were recorded.

Surgical techniques
The CLIF technique is a modified technique of lateral lumbar interbody fusion, aimed to minimize the approach-related complications of the traditional transpsoas approach (XLIF)(6). There are some unique features different from traditional XLIF. The psoas muscle working window was selected according to safe working zone on axial MRI of the target intervertebral space, the sagittal central line of the working zone should be located at least 1 cm anterior to nerve root. The psoas muscle was split longitudinally along the muscle fiber until the lateral intervertebral space was visualized. The genitofemoral nerve inside the psoas muscle might be found and was gently mobilized to posterior with a small amount of muscle fiber. A novel designed retractor was positioned in longitudinal direction to maintain the working window of psoas muscle. Two vertebral screws were used to fix the retractors to the vertebral body as close as possible to the endplate, and then assemble the retractors to the fixed ring. The intervertebral space preparation and implant placement were consistent with the traditional LLIF.
During the second stage, usually one week after the first stage, additional direct posterior decompression was performed due to inadequate resolution of stenotic symptoms and the surgeons' preference. If direct decompression was required, open pedicle screws were applied, otherwise bilateral percutaneous screws were used.

Statistics
Descriptive data are represented as means ± standard deviation (SD). Continuous variables were analyzed by 2-sample t test and paired t test. The data collected were processed using PASW Statistics 18.0 (SPSS Inc., Chicago, IL, USA). Values of P <.05 were considered to indicate statistical significance.

Comparison of radiographic outcomes with other grades
Among the 202 segments included in this study, there were 42 (20.79%) segments of grade A, 41 (20.30%) segments of grade B, 101 (50%) segments of grade C and 18 (8.91%) segments of grade D.
Overall, both the average ADH and PDH were significantly increased ( Table 2). Since the average preoperative PDH was significantly smaller than the ADH, the average change rate of PDH (58.80 ± 67.49%) was significantly larger than that of the ADH (34.44 ± 4.17%). The average increase of DA was 1.23 ± 3.81° (P < 0.001), which is small, partly attributed to the greater improvement rate of PDH than ADH. Both the average left and right IFH were significantly increased. The average change and change rate of right IFH were insignificantly greater than the left. The average midsagittal CD and axial CCA on MRI were significantly increased. The average change rate of midsagittal CD was 35.86 ± 64.01%. The average change rate of axial CCA was 25.97 ± 26.06%.
With regard to the average change of midsagittal CD, the change in grade D was significantly greater than that in grade A, but did not differ significantly from grades B or C. Interestingly, the average change rate of midsagittal CD increased from grade A to D, peaking at 79.69 ± 86.23%. As to the change of axial CCA, grade D did not differ significantly from the others. Likewise, the average change rate of axial CCA increased from grade A to D, peaking at 52.91 ± 34.41% (Table 3 and Fig. 3).
With regard to the average change of ADH, PDH, DA and both sides IFH, grade D showed no significant difference compared with the others (Table 4).

Clinical Outcome Of Patients With Extreme Degenerative Lumbar Stenosis
Eighteen patients with at least one level of grade D who underwent CLIF were clinically reviewed. All of them were retrospectively followed-up, with a mean follow-up time of 19.61 ± 6.32 months. The clinical follow-up analysis revealed a statistically-significant improvement of established outcome scores. The mean ODI improved from 43.33 ± 7.32% preoperatively to 22.56 ± 8.63% at the last follow-up (P < 0.005). In a similar manner, the VAS for back decreased from 6.06 ± 1.35 to 2.39 ± 0.78 (P < 0.005), while the VAS for leg decreased from 5.39 ± 1.24 to 1.89 ± 1.02 (P < 0.005). Sixteen of 18 segments (88.89%) with grade D underwent posterior decompression (Figs. 4 and 5). Only one patient presented with worsening back pain and neurological function at 3 months after surgery, who received a stand-alone CLIF surgery. However, he refused to undergo a posterior decompression.
In this group, seven patients (38.89%) presented with surgery-related complications. A total of 11 complications occurred in seven patients. Pain in the front of the thigh was reported in five cases, and numbness was observed in three cases. Muscle weakness of the psoas major muscle was decreased in two cases, one of them suffered from psoas hematoma and was relieved after conservative treatment (Fig. 6). One patient suffered from deep venous thrombosis and interventional therapy was performed. There were no complications such as knee extension weakness, vascular injury, sympathetic nerve injury, visceral injury or ureteral injury in this series.

Discussion
The current study shows that the radiographic decompression effect of LLIF for Schizas grade D segments was comparable with the effect on other grades. However, patients with extreme lumbar spinal stenosis are not good candidates for LLIF alone. Stand-alone LLIF is not suggested for such patients, but with concomitant posterior decompression, LLIF can achieve a satisfactory clinical outcome for extreme lumbar spinal stenosis.
Although the indirect neural decompression effect of LLIF for lumbar stenosis has been addressed in previous studies (3,4,(9)(10)(11)(12)(13), the current study was the first to evaluate the indirect neural In the current group, seven out of eight patients with extreme lumbar stenosis who underwent single-level LLIF received second-stage posterior decompression. One patient who was treated with singlelevel stand-alone LLIF reported worse of back pain and neurological deterioration at the last follow-up.
Lack of posterior supplemental fixation may lead to a loss of acquired indirect decompression after the operation. Thus, we do not suggest stand-alone surgery for patients with extreme spinal stenosis.
Posterior lumbar interbody fusion may be a better surgical option for patients with single-level extreme lumbar spinal stenosis.
There are some limitations to this study including the retrospective nature of the study, the limited follow-up, and the small sample size of grade D. Since CT and MRI were not performed during followup in most cases, we could not show the radiographic changes during follow-up. The clinical outcomes of patients with extreme lumbar stenosis were not compared with those with mild lumbar stenosis.

Conclusions
The radiographic decompression effect of LLIF for Schizas grade D segments were comparable with that of other grades. Patients with extreme lumbar spinal stenosis are not good candidates for LLIF alone. Posterior decompression was necessary for LLIF to achieve a satisfactory clinical outcome for extreme lumbar spinal stenosis of Schizas grade D.

Availability of data and materials
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Ethics approval and consent to participate
Ethics approval was obtained from the Second Affiliated Hospital, School of Medicine, Zhejiang University Human Research Ethics Committee (Reference:2019-527).

Consent for publication
Not applicable.   This P value is the result of comparison between before and after surgery. This P value is the result of comparison of change value with grade D.      A 66-year-old woman suffered from contralateral iliopsoas hematoma (arrow).