Comparison of the clinical ecacy of two xation methods combined with OLIF in the treatment of lumbar spondylolisthesis

Background To observe the clinical ecacy of an anterior single rob-screw xation (ASRSF) combined with the oblique lumbar intervertebral fusion (OLIF) approach compared with a posterior percutaneous screw xation(PPSF) combined with OLIF in the treatment of lumbar spondylolisthesis. This is a retrospective case-control study. Patients with lumbar spondylolisthesis treated with either ASRSF combined with OLIF or PPSF combined with OLIF from January 2016 to January 2018 were enrolled in this study. None of the patients had posterior decompression. The visual analog scale (VAS) and Oswestry dysfunction index (ODI) were used for clinical ecacy assessment. The pre- and post-operational disc height, height of foramen, subsidence and migration of cages, fusion rate, and surgery-related complications were compared between the two groups.


Abstract Background
To observe the clinical e cacy of an anterior single rob-screw xation (ASRSF) combined with the oblique lumbar intervertebral fusion (OLIF) approach compared with a posterior percutaneous screw xation(PPSF) combined with OLIF in the treatment of lumbar spondylolisthesis.

Methods
This is a retrospective case-control study. Patients with lumbar spondylolisthesis treated with either ASRSF combined with OLIF or PPSF combined with OLIF from January 2016 to January 2018 were enrolled in this study. None of the patients had posterior decompression. The visual analog scale (VAS) and Oswestry dysfunction index (ODI) were used for clinical e cacy assessment. The pre-and postoperational disc height, height of foramen, subsidence and migration of cages, fusion rate, and surgeryrelated complications were compared between the two groups.

Results
Fifty-three patients were included in this single-center study. According to the xation methods, patients were divided into the ASRSF group (group A, 25 cases) and the PPSF group (group B, 28 cases). There was no statistical difference in surgery-related complications between groups. There was a signi cant difference in the VAS score at 1 week post-surgery(2.3 ± 0.5Vs3.5 ± 0.4, P = 0.01), and 3 months postoperation (2.2 ± 0.3VS 3.0 ± 0.3, P = 0.01). Comparison of post-operative imaging data showed that there was a signi cant difference in the height of the foramen between groups at 3 months post-surgery(18.1 ± 2.3 mm Vs 16.9 ± 1.9 mm, P = 0.04). At 24 months post-surgery, the ODI was 12.65 ± 3.6 in group A and 19.1 ± 3.4 in group B (P = 0.01). Twelve months after surgery, the fusion rate in group A was 72.0% and78.6% in group B (not statistically signi cant, P = 0.75). Fusions were identi ed in all patients at 24 months post-surgery.

Conclusion
Compared to PPSF, ASRSF combined with OLIF for lumbar spondylolisthesis can reduce post-operative low back pain in the early stages, maintain the height of the foramen superiorly, and improve the performance of lumbar function.

Background
With the aging population, degenerative spinal disease (DSD) has become an important pathological cause of immobility and incapacitation in elder adults. Globally, the incidence of DSD exceeds 43.1%, of which 35.3% have severe dysfunction [1][2][3][4]. The incidence of DSD in low-and middle-income classes is 4 times higher than that of the high-income class. DSD has increasingly become a social issue that seriously affects the quality of life of many people [2,3,5]. Among DSD patients, the incidence of lumbar spondylolisthesis is more than 12.5% [2,3].
After the failure of conservative treatments, patients with lumbar spondylolisthesis often require surgery.
The primary surgical goal is decompression and long-term stability in the target segments. Fusion is typically the most e cient treatment in this type of surgery. Currently, commonly used fusion methods include transforaminal lumbar interbody fusion (TLIF), anterior lumbar interbody fusion (ALIF), posterior lumbar interbody fusion (PLIF), and oblique lumbar intervertebral fusion (OLIF). OLIF technology has attracted attention from surgeons due to its advantages in reducing trauma, improving safety, and restoring lumbar lordosis. In recent years, with the application of the standalone cage, the surgical procedure was simpli ed but had uncertain post-operative stability [6]. Considering early post-operative stability, OLIF often requires combined posterior xation with iatrogenic soft tissue impairment. From the perspective of soft tissue protection, OLIF combined with posterior percutaneous screws has become a more mainstreams procedure having superior stability and better clinical outcomes [7][8][9][10]. However, due to the lack of anatomical landmarks, the risk of screw misplacement and nerve damage is relatively high [7,8].
Anterior single screw xation is commonly used in anterior surgery for deformity correction and debridement. This treatment has shown excellent performance in maintaining stability and reducing iatrogenic trauma. It was speculated whether this treatment could provide additional stability in the OLIF procedure. Thus, the present study aims to analyze the clinical e cacy of the anterior single screw rod combined with OLIF compared to using a posterior percutaneous pedicle screw combined with OLIF in the treatment of lumbar spondylolisthesis.

Study design
This retrospective case-control study included patients who underwent OLIF surgery for lumbar spondylolisthesis from January 2016 to January 2018 in Xi'an Jiaotong University A liated Honghui hospital medical center. This study was approved by the Ethics Committee of Xi'an Jiaotong University (approval number: 201606012). Given the retrospective nature of the study, patients' informed consents were not necessary.
The diagnosis of spondylolisthesis was based on pre-operative X-ray examination. Inclusion criteria include: imaging con rmed lumbar spondylolisthesis located from lumbar 2 to lumbar 4, clinical symptoms related to spondylolisthesis, such as low back pain and claudication, 4-6 weeks of conservative treatments including brace xation and medications yet without symptoms alleviation, degree of slippage classi ed as Meyerding Grade I or II, and patient follow-up for more than 24 months. Exclusion criteria were: previous history of lumbar spine surgery,post-operative residual symptoms that required secondary direct decompression, incomplete medical data records.

Surgical procedures
All operations were performed by the corresponding author. The left side approaches were used in all cases. The presence of scoliosis did not affect the side of the surgical approach. After a 5-cm skin incision was made, 6-10cm anterior to the mid-portion of the marked disc, the surgeon approached the retroperitoneal space by blunt dissection and mobilization of the peritoneum anteriorly to expose the anatomical oblique lateral corridor. The soft tissue was expanded, then the working channel was placed, and if necessary, the segmental blood vessels were ligated. After discectomy and endplate preparation, Non-steroidal anti-in ammatory drugs combined with muscle relaxants were used for post-operative analgesia. The back muscle function was exercised by swimming and gymnastics named "skydiver to superman to swimmer,"as recommended by the North American Spine Society (NASS).

Data collection
Demographic data including gender, age, bone density (BMD), body mass index (BMI), and surgical segments were collected. VAS was used to rate low back and lower extremity pain.The ODI index was used to evaluate pre-and post-operative lumbar function. Follow-ups were performed at 1 week, 3 months, 12 months, and 24 months after surgery.
All patients underwent routine pre-and post-operative standing anteroposterior and lateral plain X-ray and exion-extension plain X-ray to assess inter-segmental stability. CT scan was used to evaluate the presence of migration or subsidence of cages, and to identify bone fusion. Subsidence was de ned as a cage sinking into an adjacent vertebral body by > 2mm,based on comparisons with previous CT images. Cage migration was de ned as a posterior movement of the cage by ≥ 3mm compared with the immediately post-operative image. CT scan was also used to measure the height of intervertebral space and foramen before and after the operation. The imaging measurement was independently performed by two independent radiographers who did not participate in the study. The intra-class correlation coe cients of all variables were greater than 0.85. CT images were sliced 2 mm thick. The height of intervertebral space was de ned as the vertical distance between the tangent lines of the upper and lower endplate dome. The height of the foramen was de ned as the distance from the lower position of the upper pedicle to the upper position of the lower pedicle in the target segment. The intervertebral fusion was evaluated with the Bridwell standard, in which grade 1 and 2 were considered to be clinical fusion.

Data analysis
Statistical analysis was performed using SPSS 18.0 for Windows (IBM, Armonk, NY, USA). Normally distributed continuous variables were presented as means ± standard deviation, and were analyzed with the Student's t test. Non-normally distributed continuous variables were presented as medians (range) and they were analyzed with the Wilcoxon test. Categorical variables were presented as frequencies, and were analyzed with the Pearson Chi-Square test or the Fisher's exact test, as appropriate. All tests were two-tailed, and P values < 0.05 were considered statistically signi cant.

Results
Demographic data for the patients in this study are shown in Table 1  ASRSF, anterior single rod-screw xation; PPSF, percutaneous pedicle screw xation.
No signi cant difference was found in pathological segment distribution (P = 0.38) or the degree of slippage(P = 0.78) between the two groups ( Table 1). The pre-operative lumbar lordosis was 34.6 ± 4.1° in group A and 35.5 ± 3.8° in group B, and the difference was not signi cant(P = 0.41). The pre-operative intervertebral space height was 10.8 ± 2.4 mm in group A and 10.3 ± 2.1 mm in group B without signi cant difference (P = 0.46). The height of the pre-operative foramen was 12.5 ± 1.6 mm in group A and 12.5 ± 1.4 mm in group B, without signi cant difference between groups (P = 0.92).
The post-operative clinical results were also compared. The VAS score of leg pain at 3 months after the operation was 1.1 ± 0.7 in group A and 1.3 ± 0.8 in group B, without signi cant difference (P = 0.71). One week after surgery, the VAS score of low back pain in group A was signi cantly lower than in group B(2.3 ± 0.5 vs 3.5 ± 0.4, P = 0.01). Three months post-surgery, the reported low back pain in group A was still signi cantly lower than in group B (2.2 ± 0.3 vs 3.0 ± 0.3, P = 0.01). The superiority of group A in reducing low back pain vanished at 12 months after surgery (Table 2). At 24 months post-surgery, the postoperative ODI was 12.65 ± 3.6 in group A and 19.1 ± 3.4 in group B, with a signi cant difference (P = 0.01).
There was no signi cant difference in the ODI between groups at 1 week, 3 months, and 12 months postsurgery. Three months post-surgery, the foramen height was 18.1 ± 2.3 mm in group A and 16.9 ± 1.9 mm in group B, showing a statistical difference (P = 0.04). Twelve months after the operation, there was no signi cant difference concerning the change in foramen height. No statistical difference was found in the height of the intervertebral space between the two groups at any follow-up time (Table 3). There was a signi cant difference between the pre-and 12 months post-surgeryimaging parameters between the two groups ( Table 4).
The Bridwell method was applied in the fusion assessment with grade I and II considered as successful fusion. Twelve months after the operation, 18 cases achieved fusion in group A (fusion rate 72%), and 22 in group B (fusion rate 78.6%). There was no signi cant difference between the two groups (χ 2 = 0.31, P = 0.75). All patients achieved fusion at 24 months post-surgery. No subsidence was found in any patients during follow-ups.
In terms of complications, one patient in group A encountered a limited cage migration at 3 months postsurgery. There were four cases of post-operative abdominal distension, two cases of dysuria, and two cases of transient numbness at the anterolateral portion of the thigh in group A. No instrument complication occurred. In group B, there was one case of posterior subcutaneous hematoma, two cases of screw misplacement (no neurological complications), one case of super cial skin infection, three cases of abdominal distension, and one case of transient thigh numbness. There were no statistical differences in complications between the two groups(χ 2 = 4.71, P = 0.31).A typical case is shown in Fig. 1.

Discussion
Due to advantages in reducing soft tissue damage, improving lumbar lordosis, and reducing nerve tissue disturbance, OLIF has attracted recent attention from spine surgeons. OLIF has been found to increase the cross-sectional area of the dural sac by a median of 30.2%, and increase the neural foramen area by an average of 30.0% [11][12][13][14]. Compared with traditional posterior TLIF surgery, the probability of nerve root injury was about 1.3% in OLIF the procedure [12,14]. No patients in this study experienced postoperative nerve root edema or direct nerve root injury. This indicated that OLIF was a superior technique in protecting nerve tissue.
Since OLIF does not involve iatrogenic damage to the posterior structure, the impairment of spinal stability is limited. There has been some controversy about whether additional xation is necessary [6,14]. However, due to the different elasticity modulus between the cage and the endplate, there is a risk of subsidence when the cage is used alone [14,15]. Therefore, the insertion of pedicle screws is necessary for patients with endplate damage, osteoporosis, or post-operative residual radicular symptoms that require posterior surgeries [13,15]. Lin et al. [16] evaluated 52 patients who underwent OLIF without posterior instrumentation, and reported a fusion rate of 81.9% at 24 months after surgery as assessed by CT scan imaging. Kim et al. [17] reported a 12month fusion rate of 92.9% in 29 OLIF patients with posterior pedicle screw xation as assessed with CT. In the present study, no cage subsidence nor nonfusion were found within 24 months post-surgery, which further con rmed that limited internal xation can reduce the risk of cage subsidence and promote intervertebral space fusion.
In this study, the intervertebral space fusion rate at 12 months post-surgery was 72% in group A and 78.6% in group B, which were relatively low. We presume the major negative factor for the inferior fusion rate was the material of the bone graft. Compared with iliac crest or bone morphogenic protein, using allogeneic bone could result in a lower fusion rate [11,13].
Posterior percutaneous screw xation could reduce paraspinal muscle damage where iatrogenic impairment to the paravertebral soft tissues is still unavoidable.This could lead to muscular atrophy and low back pain. Secondarily, given the lack of anatomical reference markers, percutaneous pedicle screw implantation has a relatively higher incidence of screw misplacement compared with open surgery [7,8].
Utilizing a corridor of OLIF for segmental xation can effectively reduce the risk of screw misplacement for the direct procedure of implantation and the massive docking area for screws.
In this study, when compared with PPSF, ASRSF resulted in lower back pain at 1 week and 3 months after surgery. The ODI index was also lower in the ASRSF group at 24 months post-surgery, which con rmed the superiority of ASRSF in relieving pain and improving lumbar function. This may be due to evidence of paraspinal muscle protection.
The height of the intervertebral foramen was comparatively better maintained at 3 months post-surgery in the ASRSF group compared with the PPSF group. It was presumed this might originate from the relatively close position to the central axis of the spine in the anterior xation group, which diminishes the "selflocking" phenomenon in the posterior xation that could result in a reduction of the intervertebral foramen area [18]. However, this superiority disappeared at 12 months post-surgery, which might be related to a possible increase in lumbar lordosis.
ASRSF showed acceptable stability in the debridement of intervertebral space and anterior vertebrectomy, which con rmed the single screw rod could provide stability given that the posterior column was intact [19][20][21]. In this study, there was no internal xation migration in the anterior group, supporting the stability for anterior xation solely. The possible reason comes rst from the inaction of the posterior column in the OLIF procedure. Second, the average age of the patients in this study was 43.3 years and the pre-operative BMD was around − 1.3, which equally contributed to maintaining stability and the excellent skeletal condition of the participants.
In summary, ASRSF combined with OLIF for lumbar spondylolisthes could reduced early post-operative low back pain more effectively, increased the area of foramen, and improved post-surgerylumbar function compared with the clinical manifestation of PPSF with OLIF. In addition, it provided su cient stability for intervertebral space fusion.
The present study had some limitations. First, this study was a retrospective one. Due to the lack of case selection, there might be a selection bias. Second, this study was a single-center study with a relatively