The clinical and radiographic outcomes of stand-alone Oblique Lateral Interbody Fusion in Treatment of Adult Degenerative Scoliosis

Background The minimally invasive treatment for adult degenerative scoliosis has become more and more popular. The purpose of this study was to evaluate the efficiency of stand-alone oblique lateral interbody fusion for the treatment of adult degenerative scoliosis in terms of clinical and radiological outcomes. Methods A total of 18 patients with ADS who underwent stand-alone OLIF in our hospital from July 2017 to May 2018 were enrolled in the study. Clinical evaluations were performed with visual analogue scale (VAS) and Oswestry Disability Index (ODI). Radiographic outcomes were recorded in terms of coronal Cobb angle and lumbar lordosis. Results Mean patient age was 62.4 years, 50% of patients were female. Average follow up was 18.4 months. The average operative duration was 87.4 minutes, whilst the mean estimated blood loss was 45.6 ml. Mean coronal Cobb angle corrected from preoperative 15.2° to the final follow-up 6.8° (p < 0.05); and mean lumbar lordosis improved from preoperative 30.0° to 39.4° (p < 0.05). Mean disc height increased from preoperative 0.7 cm to 1.1 cm at final follow-up (p < 0.05). Mean VAS improved from 5.5 to 2.2 (p < 0.05). The mean preoperative and the final follow-up Oswestry Disability Indices were 27.8% and 13.1% respectively (p < 0.05). Conclusions Stand-alone OLIF could be regarded as an efficient and safe option the ADS careful selected patients.


Background
Adult degenerative scoliosis (ADS) is a common clinical disease characterized by a Cobb angle more than 10° in the coronal plane, which affects approximately 6% of people over 50 years old [1]. The disease was mainly caused by the asymmetrical degeneration and hyperplasia of the 3 facet joint, leading to the pathological changes such as spinal instability and deformity [2]. It is widely accepted that surgeries should be performed for patients with severe low back pain and progressive exacerbation of neurogenic symptoms which seriously affects quality of life, or imaging abnormalities of lumbar lordosis, thoracolumbar kyphosis and lateral sliding, and rapid development of spinal deformity [3][4]. At present, the surgical treatment for ADS include posterior lumbar interbody fusion (PLIF), transforaminal lumbar interbody fusion (TLIF) and anterior lumbar interbody fusion (ALIF). As for the PLIF and TLIF, the surgery has more neurological complications and paraspinal muscle stripping and dural adhesion may seriously affect postoperative outcomes [5]. The ALIF process may increase the risk of injury to the vascular and superior hypogastric plexus near the aortic bifurcation which may lead to excessive blood loss and retrograde ejaculation [6][7]. During the last decade, the extreme/direct lateral interbody fusion (D/XLIF) has attracted more and more attention in the appliaction of deformity correction in ADS. The previous studies has found that XLIF/DLIF could effectively restore coronal balance [8][9]. However, XLIF/DLIF achieved the intervertebral disc with a lateral approach passing through psoas major, which will cause damage to the psoas muscle during operation resulting in postoperative pain and paresthesia of the lower limbs segment [10]. In 2012, Silvestre [11] first reported the oblique lumbar interbody fusion (OLIF), which is through the anterior peritoneal approach between the front of the psoas muscle and the vascular sheath, thus avoiding direct damage to the psoas major and lumbar plexus.

4
Although OLIF technology has been applied to the treatment of a serious kinds of lumbar degenerative diseases, the efficiency of OLIF technology for the treatment of ADS has not been thoroughly studied by now. Beng evaluated the effect of interbody distraction by OLIF with cortical bone trajectory screws fixation for the treatment of adult spinal deformity of 28 patients and found OLIF technique had the potential to correct misalignment in adult spinal deformity [12]. Besides, Nitin Agarwal [13] considered stand-alone LLIF was a safe and effective surgical procedure by investigating 55 patients with a 10-year follow-up. Herein, we analyze the clinical outcomes and radiographic correction of coronal deformity and lumbar lordosis in patients with ADS using stand-alone OLIF technology.

Patient population
We retrospectively reviewed the files of all patients with ADS who were treated with stand-alone OLIF technology from July 2017 to May 2018, and their clinical and radiographic results were summarized and analyzed.
Inclusion criteria included the following: 1) the degenerative scoliosis Was mainly located in L1-L5; 2) Lenke-Silva grade was I ~ V and there was no need for posterior osteotomy surgery; 3) patients who experienced chronic low back pain with or without lower limb radiative pain and conservative treatments had no effect after at least 3 months; 4) the follow-up was more than one year. Exclusion criteria included:patients combined with severe osteoporosis with bone density T values less than -2.5; 2) patients who were diagnosed with scoliosis for inflammation, tumor or trauma; 3) previous lumbar fusion surgery .

Surgical procedure
The patient underwent general anesthesia and no neuromonitoring was required during the procedure. A transverse skin incision of 4 cm was made at the left lateral abdomen, which was in the same horizontal plane as the target intervertebral disc. For multilevel cases, the incision was centered between the surgical levels. The abdominal wall muscles were are bluntly separated. The retroperitoneum was entered by blunt separation with the fingers, then the psoas was retracted posteriorly and the abdominal vessels were retracted anteriorly; a guidewire was inserted in the middle of the target intervertebral disc with the help of a C-arm. Sequential dilators were placed over the guidewire, then a lighted retractor was placed over the dilatorsand fixed to the vertebral body with a pin, and the operation field was exposed; The annulus fibrosus and the nucleus pulposus was removed with the nucleus pulposus clamp; then, the cartilage endplates were resected for exposure of the bony endplates; a wide and lordotic intervertebral fusion cage (Medtronic Clydesdale, Memphis, Tennessee) packed with allograft bone was inserted into the target disc with the guidance of C-arm. Then, the incision was closed layer by layer. All patients were not received posterior internal fixation and no drainage tube was placed after surgery.

Assessment of clinical and radiographic outcomes
The operation time, the amount of intraoperative blood loss, hospital stay and complications were recorded. Moreover, the coronal Cobb angle, disc height (DH = mean value of the leading and trailing edge height of the intervertebral disc) and the lumbar lordosis (LL) of the patient before the operation and the final follow-up were measured.
The Visual analog scale (VAS) score and the Oswestry disability index (ODI) score of the patients before the operation and the final follow-up were compared to evaluate the 6 clinical efficacy.

Statistical analysis
Statistical analysis was performed using SPSS18.0 software (IBM Corp, Armonk, NY). All data were expressed as mean ± standard deviation (SD). Paired t-test were performed to compare outcome. P-value less than 0.05 was considered statistical significance in all analyses.

Patient Demographics and Operative Data
Eighteen patients (9 females, 50% ) with the average age 62.4 years (range 35-75 years) were enrolled in the study. A total of 25 lumbar levels were treated including 11 single level and 7 double levels. The mean operation time was (87.4±19.7) min per surgery (range 50-120 min) and 63.0 minutes per interbody fusion level. The intraoperative blood loss was estimated from 30 ml to 60 ml, with an average of (45.6±9.1) ml, while every each segment of OLIF averaged 32.8 mL The length of hospital stay was 6-14 days, with an average of (9.3 ± 2.2) days.

Clinical outcomes
The 18 patients were followed up for 12-21 months, with an average follow-up time of (18.4±2.4) months. The VAS score (2.2±0.5 points) and ODI index (13.1%±2.5%) of the final follow-up were both significantly lower compared to those observed before the surgery (VAS score: 5.5±0.7 points; ODI: 27.8%±6.2%) (Figure 1 ). The difference between the groups was statistically significant (P<0.05).

Complications
Among all the patients, there were no intraoperative complications such as abdominal organ injury, vascular injury and ureteral damage. Two of the patients had postoperative pain and numbness in front of the left thigh. And the symptoms were relieved after one 7 week of the treatment with drugs. Four patients was found to have the cage subsidence, which is the most common complication of stand-alone OLIF technology. The evaluation criteria for the subsidence of the cage are as described by Abdala [14]. The degree of cage subsidence was classified from grade 0 to grade III based on loss of postoperative disc height using the following scale: Grade 0 was defined as 0-24% loss of postoperative disc height; Grade I was 25-49%; Grade II was 50-74%; and Grade III was 75-100% in lateral Xray of the lumbar spine. In this study, the cage subsidence of the four patients was regarded as grade 0, and no posterior pedicle screw internal fixation and decompression was performed in the second stage. Besides, those patients were successfully treated after conservative therapy during the follow-up.

Spinal deformity correction
The 14.3°(range 5°-61.5°) (Figure 3). The difference in LL was statistically significant (P < 0.05). Immediately after surgery, Mean disc height was significantly increased from 0.7±0.3 mm preoperatively to 1.3±0.2 mm immediately postoperatively and 1.1±0.2 mm at final follow-up (Figure 4). Although the height of the intervertebral disc was decreased at final follow-up, it was still higher compared with preoperatively. The difference was statistically significant (P<0.05). 8

Discussion
As we know, most cases of ADS present in the elderly who often have osteoporosis and underly multiple medical comorbidities, which maybe better to receive a minimally invasive operation. At present, minimally invasive surgery such as XLIF/DLIF with or without posterior internal fixation has become a widely accepted surgical solution for the treatment of ADS and prohibited satisfactory efficiency [15][16]. However, the incidence of postoperative complications such as lumbar plexus nerve and psoas muscle injury is 10% to 20% which led to abnormalities, thigh pain, and gluteal motor weakness after DLIF/XLIF operation even if with the help of intraoperative electrophysiological monitoring during the surgery [17]. In the contrast, the OLIF approaches the intervertebral disc via a corridor between anterior border of the psoas muscle and abdominal major vessels [18], which could effectively reduce the complications of postoperative thigh pain, paresthesia and other lumbar plexus injury. Anatomically, OLIF technology could be regarded as a unique advantage in the treatment of ADS because the ideal exposure space for OLIF technology is L2-L5 where ADS mainly occur. Coronal Cobb angles in ADS patients are usually mild [19]. For example, Paterakis [20]  was also seen in Jin's study [21], which showed that a significant increase insegmental lordosis and disc height with OLIF.
Biomechanical studies have shown that the recovery of LL was beneficial to increase the tension of the anterior longitudinal ligament, which could improve the intervertebral fusion rate and reduce the rate of degeneration of adjacent segments [22]. Another study [23] also found that the key to the recovery of the sagittal balance of the lumbar spine was to restore LL. The recovery of the sagittal balance can reduce the adjacent segmental degeneration caused by the compensated sagittal imbalance and improve the patient's symptoms.
Up to now, it has been a major controversy whether it is necessary for posterior internal fixation for OLIF technology in the treatment of ADS or not. Some scholars recommend the use of posterior screw internal fixation immediately, and some scholars suggested single-stage internal fixation [24]. They concerned that the absence of posterior pedicle screw fixation may cause the cage subsidence, which in turn affects clinical outcomes due to the loss of disc height. In my opinion, OLIF technology maintained the spinal stability without damaging the paraspinal muscles, anterior and posterior longitudinal ligaments and facet joints compared with traditional posterior approach such as PLIF and TLIF. In addition, lateral cage bone graft window is much bigger than the traditional PLIF/TLIF which could improve the fusion rate. After the large-sized cage was implanted, it could be tightly fixed to the epiphysis ring by the help of the tightened anterior and posterior longitudinal ligaments. Steffen [25] has shown that a wider cage placed in the periphery of the endplate has lower incidence to occur implant subsidence compared to a narrower implant owing to its efficiency in providing segmenal stability. Moreover, There were also reports of stand-lone ALIF and stand-alone LLIF, which showed good clinical efficacy. For instance, Rao [26] reported an overall clinical success rate of 93% for 27 patients diagnosed as low grade lumbar spondylolisthesis with standalone ALIF procedure. Ahmadian [27] evaluated the efficacy and clinical outcomes of 59 patients who underwent stand-alone lateral interbody fusion, 18 patients with ADS included. The results showed that stand-alone MIS-LIF was an available choice for carefully selected patients. Our data showed that DH was higher than postoperatively immediately after surgery, although DH decreased at the final follow-up, it was still higher than preoperative DH. In addition, it was also believed that the presence of radiographic subsidence does not directly correlate to clinical outcomes [28]. The VAS and ODI scores were statistically lower relative to preoperatively, which indicated that patients were received significant improvement of life quality in the current study. Zhang[29] also found that the cage subsidence in a total of 15 fused segments of the patients who underwent stand-alone OLIF, but symptoms in these patients had been alleviated during follow-up. However, it is worth noting that stand-alone has its own indications. We believe that bone density is an important factor that should be considered. Tempel [30] found that the incidence of cage subsidence in patients with bone density T values from -1.0 to -2.4 was significantly higher than the patients with bone density T values > -1.0, so they suggested that patients whose bone density T value less than -1.0 should be treated with posterior pedicle screw fixation. We also do not recommend stand-alone OLIF technology for patients who suffered from osteoporosis (T < 2.5) because the incidence of cage subsidence increases significantly. Besides, it is necessary to carry out adequate communication with the patients before stand-alone OLIF surgery that posterior decompression and internal fixation may be performed once clinical symptoms occurred again after surgery.
There are also some limitations in the study. The study was retrospectively reviewed and the results confirmed that OLIF technology can achieve good clinical results in patients with ADS, but there is still a lack of randomized controlled trials with XLIF/DLIF and PLIF.
Secondly, the number of patients enrolled in this study is small, and the follow-up time is short, so its long-term efficacy and complications remain to be further observed and summarized.

Conclusions
The goal of this study was to evaluate the clinical and radiographic outcomes of standalone OLIF in treatment of ADS.
Significant improvement of disc height, VAS scores,reductions in ODI, the sagittal and coronal balance were observed at latest follow-up compared to preoperative scores. In our experience, stand-alone OLIF could be seen as a safe and efficient technique in the treatment of ADS patients with mild coronal plane deformity by indirect decompression and arrest curve progression.

Availability of data and materials
The final dataset will be available from the corresponding author.

Ethics approval and consent to participate
All the patient in the study were informed that data would be submitted for publication and the provided written consent. All procedures performed in the study were approved by the ethics committee of Wannan Medical college affiliated Yijishan Hospital.

Consent for publication
The patient was informed that data concerning the case would be submitted for publication, and he provided consent. The coronal cobb angle was corrected significantly at the final follow-up.
19 Figure 3 The lumber lordosis improved significantly at the final follow-up.
20 Figure 4 The mean DH increased from 0.7cm preoperatively to 1.3cm postoperatively, and decreased to 1.1 cm at the final follow-up.