In patients with degenerative spondylolisthesis, anterior displacement of inferior articular processes and osteophyte formation at superior articular process lead to lateral recess stenosis, while the displacement of the disc and thickening of ligamentum flavum cause central canal stenosis[3]. Choosing lumbar fusion for patients with symptomatic degenerative spondylolisthesis was challenged[12, 13], especially for degenerative spondylolisthesis without segmental instability. If lumbar fusion was necessary with the evidence of segmental instability, using mini-open oblique lateral approach, OLIF allows for large cage insertion which can result in reduction of disc bulging, the elongation of ligamentum flavum and thus enlarging lumbar spinal canal[6, 14, 15]. That’s why indirect neural decompression could be achieved through this method.
The slip reduced by OLIF
The degree of slip negatively correlates with canal size and patients’ quality of life preoperatively[16, 17]. Although attempt for complete reduction of slip was not necessary in direct decompression procedure in terms of improving patient-reported outcomes[18], complete reduction of slip and restoring the normal anatomy of this segment can increase the canal size, meanwhile it can achieve indirect decompression in OLIF procedure with percutaneous pedicle screw fixation.
Sato et al[15] compared axial canal diameter, sagittal canal diameter and spinal canal cross-sectional area before and after OLIF procedures with posterior fixation for degenerative spondylolisthesis. Slip ratio was reduced from 14% preoperatively to 5% postoperatively and all those parameters of canal size were increased with slip reduction. As a result, reducing slip as much as possible can decompress the nerve impingement to the greatest extent, particularly necessary for indirect decompression in OLIF procedures.
Inserting large-size cage raises the disc height, hence stretches the ligamentous structures around the slipped level and reduces the slip. In this study, following cage insertion, disc height was improved from 8.2mm to 11.8mm, while the slip ratio was improved from 11.1% to 8.3% and slip reduction ratio was 25.6% on average, which meant one quarter of slip were reduced by stand-alone cage insertion. These findings supported the mechanism of slip reduction by cage insertion alone. However, residual slip of 8.3% limits the stand-alone technique in terms of capacity of indirect nerve decompression.
In this study, following posterior fixation, the slip was further reduced to 2.1%, slip reduction ratio was 57.9% on average, which meant greater than half of slip reduction was achieved by posterior fixation. Therefore, additional posterior fixation and reduction maneuver could reduce the slip to larger extent than stand-alone technique could.
Segmental lordosis improved by OLIF
Segmental lordosis can be improved by insertion of cage with lordotic angle design[19]. The magnitude of improvement correlates with preoperative segmental lordosis and anteroposterior position of cage[20]. This current study, however didn’t show increase the segmental lordosis by cage insertion alone (8.7°to 8.3°). The likely cause was that the large preoperative segmental lordosis (8.7°) limited the capacity of anterior realignment due to tightness of anterior longitudinal ligament. Additional posterior fixation shortened the posterior column and further increased the segmental lordosis (10.7°) in this study, which indicated that posterior fixation could further improve segmental lordosis even if anterior realignment reached its limit.
Stand-alone versus additional posterior fixation
Stand-alone cage insertion of OLIF procedure without posterior fixation is advocated by some surgeons[7, 8]. Several clinical results favoring standalone technique were reported in the literature[7, 21], and those favorable outcomes depend on inserting a large cage which can both achieve indirect decompression effect and provide instant stability by axial loading[6]. However, some drawbacks of stand-alone technique were shown during follow-ups. Cage subsidence and subsequent loss of correction may occur without posterior fixation[8].The effect of indirect decompression was also decreased during follow-up in some patients undergoing stand-alone techniques[10]. A recent meta-analysis showed the reoperation rate and occurrence of cage migration was higher for standalone technique[9].
OLIF with posterior fixation can enhance the segmental stability, decrease the rate of cage subsidence and migration, and maintain the instant indirect decompression effect by cage insertion[9]. Additionally, as this study revealed, posterior fixation for patients with degenerative spondylolisthesis can further reduce the slip that maximizes the effect of indirect decompression, together with improvement of segmental lordosis. Therefore, OLIF with additional posterior fixation was recommended for patients with degenerative spondylolisthesis.
Limitations
Although this study allows to demonstrate the changes of segmental alignment within the same procedure setting, this retrospective observational study has some limitations. Firstly, all the slip were Grade I spondylolisthesis (slip ratio: 3.5% to 23%) with most slipped levels located at the L4/5 level, even if the inclusion criteria included Grade I and II slips, which may constrain drawing conclusion for Grade II slip or other segments. Secondly, additional fixation did improve the segmental alignment together with favorable symptoms and disability improvements in short-term, however, whether the improved segmental alignment or the superiority of posterior fixation can be maintained is still uncertain in long-term. Thirdly, the result of sagittal alignment changes was derived from combined analysis of open and percutaneous pedicle screw fixation due to relatively small sample size. Open and percutaneous fixation probably result in difference alignment changes. However, separate analysis showed consistent outcome between two groups, making the combined analysis reasonable.