With the ever intensified aging of China's population, the incidence of DS has been rising gradually, which has become a major cause of waist and leg pain among middle-aged and elderly populations. Currently, choice of DS treatment strategy and evaluation of relevant therapeutic efficacy have been the focus of spine surgeons. For patients with severe low back, leg pains and progressive aggravation of DS, which affect their quality of life and are not curable by conservative therapies, receiving active surgical treatment is needed. The aims of surgical treatment are to alleviate clinical symptoms, maintain spinal biomechanical stability and control the incidence of complications . DS patients are characterized by multiple underlying diseases and weak surgical risk resistance since the majority of them are middle-aged and elderly people. Ensuring the surgical safety is necessary while guaranteeing the therapeutic efficacy. Thus, it is important to reduce the surgical trauma and shorten the operation time. Conventional posterior osteotomy and orthopedic internal fixation have problems such as long operative duration, large intraoperative bleeding volume, high technical requirements and slow postoperative recovery [6-9]. Meanwhile, XLIF/DLIF combined with posterior internal fixation may cause damage to the psoas muscle and lumbar plexus, which can lead to rather high neurological complications as well [11-12]. OLIF, which enters obliquely through the natural gap between large abdominal vessels and psoas muscle from the anterior lateral side, dissects the psoas muscle bluntly, removes the intervertebral disc, restores the vertebral height, and decompresses stenotic spinal canal or nerve root canal indirectly, and can thereby correct the lateral kyphosis. Since the beginning of 2016, our hospital has been treating DS by OLIF combined with posterior "O" arm CT-navigated long segment internal fixation, which yielded satisfactory early clinical outcomes. With advantages like minimal invasiveness, accurate navigated screw insertion and high bone fusion rate, the surgical strategy can provide a new option for minimally invasive treatment of DS.
Apical vertebrae in DS are commonly seen in L2/3, L3/4 position, while the segments exposed with OLIF are L2~L5, so OLIF can fully expose the surgical area, which has a unique advantage. With a surgical incision of approximately 3-4 cm, OLIF also has the advantages of small incision and low intraoperative bleeding. Silvestre et al.  analyzed the data of 179 patients who underwent OLIF, and reported a mean bleeding volume of 57 ml for a single segment. Fujibayashi et al . reported a mean blood loss of 17.6 ml in a single segment of OLIF.. In the present study, the 15 enrolled patients exhibited bleeding volume of merely (50.2±10.7) ml during the OLIF operation, so postoperative drainage tube placement was not needed. In the meanwhile, the posterior long segment internal fixation is implemented via the longissimus-multifidus approach to avoid damage to the paravertebral muscles. This procedure also has the small trauma, low bleeding advantages.
DS features degeneration of intervertebral discs in the scoliotic segments. The asymmetrical intervertebral stenosis causes lateral vertebral body slip, joint dislocation and ligamentum flavum hypertrophy, which can induce nerve traction and compression to result in corresponding clinical symptoms. OLIF procedure allows placement of a larger, wider interbody fusion cage than the posterior surgery, which can play an indirect decompression role. Sato et al.  reported a 61% increase in the intervertebral space height, a 21% increase in the left side of intervertebral foramen area and a 39% increase in the right side of intervertebral foramen area after OLIF. Besides, VAS and ODI decreased markedly than before surgery, thus demonstrating a good indirect decompression effect of the surgery. For the patients enrolled in this study, the OLIF interbody fusion cage used was 45-55 mm in length and 12-16 mm in height. Besides, the intervertebral space height was (5.9±2.7) mm before surgery, and was (10.9±1.2) mm at the final follow-up, with an average increase of 84%. The VAS and ODI at the final follow-up also improved remarkably than the preoperative values. Hence, OLIF combined with posterior "O" arm CT-navigated long segment internal fixation is effective in opening the intervertebral space and indirectly decompressing the spinal canal and intervertebral space, without the need for posterior spinal canal decompression, which avoids damage to the spinal dura mater and nerve roots.
Patients with DS often have coronal scoliosis, sagittal lumbar lordosis disappearance or segmental kyphosis, their coronal Cobb angle is not large, and main deformity is reduced lumbar lordosis . OLIF is effective in correcting the coronal scoliosis and the sagittal imbalance. According to a report by Fujibayashi et al . , the lumbar lordosis angle of responsible vertebra increased by 4.5° on average following OLIF as compared to the preoperative value. For the patients enrolled in this study, the interbody fusion cages were inserted in parallel by OLIF, and then the posterior long segment internal fixation was performed. At the final follow-up, the coronal Cobb angle decreased by 14.8°, LL increased by 13.5°, and SVA decreased by 17.2 mm, thus correcting the coronal scoliosis and lumbar lordosis effectively and achieving sagittal balance.
Studies have found that the navigation aid can effectively enhance the accuracy of pedicle screw placement, shorten the screwing time, reduce the bleeding volume and lower the radiation exposure of both surgeons and patients [28-29]. For the patients enrolled in this study, 240 screws were placed via the "O" arm CT navigation. The excellent rate of intraoperative "O" arm 3D scanning was 96%, and no screw insertion-related complications were noted.
OLIF is operated under direct vision through the natural gap anterior to psoas muscle. The psoas muscle is dissected bluntly while avoiding damage to the muscle and lumbar plexus. Damage to the spinal cord and nerve roots can be avoided by not entering the spinal canal. Meanwhile, retention of the anterior and posterior longitudinal ligaments can prevent vertebral instability, which effectively reduces the complications of lumbar plexus injury such as psoas muscle weakness, as well as numbness of inguinal region and anterior medial thigh. Among the 15 enrolled patients in this study, 1 patient developed anterior medial pain of left thigh, and 2 patients had weakness of left hip flexion postoperatively. All of them recovered during the follow-up period. The cause was analyzed to be excessive intraoperative traction. OLIF is capable of managing the responsible intervertebral discs fully, reducing the soft tissue adhesion of endplate and providing a preferable bone fusion environment. Moreover, after placement of the larger and wider interbody fusion cage, the surrounding ligament can be tightened to attain immediate stability, and the contact area of bone graft increases. Osseous fusion of all interbody fusion cages was found on the 6 months postoperative lumbar CT for the 15 patients in this study, with a fusion rate of 100%.
In conclusion, the early clinical outcomes of the OLIF combined with posterior "O" arm CT-navigated long segment internal fixation for treatment of DS are satisfactory, with advantages like minimal invasiveness, accurate navigated screw insertion, high bone fusion rate and few complications, which can thus offer a new option for minimally invasive treatment of DS.
This study is a retrospective review, with small sample size, no control group and limited postoperative follow-up duration. A prolonged follow-up is needed to further observe and summarize the long-term outcomes and complications of the proposed surgical strategy.