The most important finding of this present study demonstrated that unadjusted pelvic retroversion (insufficient decreased PT), C7-SVA (body lean forward) and PI-LL mismatch were the risk factors for the LBP after OLIF. Therefore, the insufficient decreased PT implied that pelvic still keep retroversion, could not correct the body lean forward and PI-LL mismatch, which might be the reason for the LBP.
Recently, OLIF has become a popular method of treating lumbar degenerative disease with the advantage of minimizing iatrogenic injury on the posterior vertebral structures compared to the posterior lumbar surgery. Theoretically, the indirect neural decompression could be performed by the restore the intervertebral height.[19.21] Abbasi et al performed 303 OLIF procedures on 568 levels and reported OLIF was a safe and efficacious procedure for the degenerative disease. Lin et al found OLIF could achieve equivalent clinical and radiologic outcomes by indirect decompression compared to other posterior lumbar surgery. And it also have better restoration of DH and less blood loss. Chang et al showed favorable clinical outcomes after OLIF for the lumbar spinal stenosis. Similar to their studies, we analyzed 2 groups ( Non-LBP and LBP groups) comparably matched in terms of demographic data and clinical outcomes who were treated with OLIF after a minimum 2 year follow-up. Both groups have significant improved.
Previous studies have suggested that restoring sagittal spinopelvic parameters may play a significant role in improving the quality of life after surgery. Therefore, it is particularly important to identify and restore adequate sagittal spinopelvic alignment when performing fusion. The authors’ hypothesis was that sagittal malalignment was a risk factor strongly correlated with LBP in patients after surgery. It is well known that pathogenesis and development of degenerative lumbar disease was biomechanical changes caused by sagittal unbalance.[25, 26] The PI was an anatomic parameter which played a fundamental role in the sagittal balance and spinal degeneration. As a result, a higher PI implied a higher SS and LL, which might lead to higher shear forces at the lumbosacral junction. That is one of the reason for developing spondylolisthesis.[27–29] According to the posterior lumbar surgery, many studies had suggested that increased SS and LL may lead to better clinical outcomes and less LBP. Failure to reach proper sagittal balance can result in compensatory mechanisms such as decreased SL and LL, increased PT, which have adverse effects on back muscle and eventually lead to LBP[4, 29, 30]. Recently, Liow et al reviewed 63 patients who underwent short-segment lumbar fusion surgery and found patients with higher SS (SS ≥ 30°) experienced less LBP. In their opinion, increased LL and SS indicating better clinical outcomes and sagittal balance.
During the surgery, the larger cage placed at both side of the endplate and located anteriorly vertebral body. In spite of OLIF has effective procedure to indirectly spinal canal decompression and increased SS, there were still parts of patients experienced residual LBP after surgery. From the current study, The statistical data showed that SS in the Non-LBP group at the last follow-up (31.7°±6.9°) significantly improved compared to preoperative values. Nevertheless, SS in the LBP group (26.9°±6.9°)was significantly lower than the other group. After the multiple logistic regression analysis, SS was not the risk factor for LBP after OLIF. As is known to everyone, increased PT means represents pelvic retroversion, which compensates for sagittal spinal imbalance. PT༜20°is recommend to correct the sagittal imbalance and relieve symptoms. In this study, PT in the Non-LBP and LBP groups were 22.3°±10.8° and 15.1°±7.3°at the final follow up, it found significant difference compared to the preoperative value. (P = 0.000) The results suggested degree of the decreased PT in LBP group was not enough to compensated the sagittal imbalance and associated with residual back pain.
In addition, many researches has reported the increased LL and SL correlate with improved clinical outcomes.[32–34] Our results showed that there is a significantly improved SL achieved by single-level OLIF in both groups. although the SL in Non-LBP group were slightly higher than that in LBP group, our results do not show a statistically significant difference between the two groups. However, the LL in Non-LBP group was significant higher than that in LBP group.This suggested that the impact of the interbody fusion is not enough to altering overall alignment.The C7-SVA has been reported to be an important index to assess sagittal imbalance. In our study, C7-SVA significantly decreased in both groups at the final follow-up. The changes of C7-SVA in Non-LBP group was more than that in the LBP groups. Additionally, PI-LL of less than 10° was used to indicate whether sagittal reconstruction has been achieved in the Non-LBP group. we found that OLIF could improve the LL and correct the PI-LL mismatch. Furthermore, the decreased C7-SVA was as evidenced by adjustment of LL. Saadeh et al reported single-level lateral lumbar interbody fusion achieved greater improvements in regional lordosis. In their study, global lordosis was not impacted by the single-level intervention.  Schwab et al showed the postoperative PI-LL mismatch causes greater residual LBP and proposed SVA, PT, and PI-LL were most closely related to poor clinical outcomes and LBP. 
In the perspective of the parameters, although surgery improved the DH, LL,SL,PI-LL match and C7-SVA, ideal sagittal balance could not be achieved in LBP group. OLIF could only limitedly restore sagittal balance with the help of increased the intervertebral height by placing the larger interbody cage anteriorly within the wider distraction of intervertebral space. On the one hand, deficient vertebral distraction is not sufficient for spinal decompression, but also affects the correction of sagittal imbalance. On the other hand, excessive vertebral distraction cause overlarge interbody cage which may increase the risk of subsidence into the endplate and reduce fusion rates. It significantly increased mechanical stress on the adjacent discs. Furthermore, the position of the interbody cage also affected the recovery of the intervertebral height, which indirectly affected the restoration of LL and SL. Therefore, a larger intervertebral cage at the anterior middle third column would be improvement the balance of sagittal spinopelvic alignment. However,with regard to the SL, each level contributes a different and limited magnitude to LL.Therefore, we considered that restoration the intervertebral space height by cage insertion which might not enough alter the mechanical dynamics of the spine.
This study has several limitations. First, this is a mall sample size in a single institution retrospective study with relatively short follow-up period. Future study will need a larger cohort followed for a longer period. Second, in the current study, one patients have received second-stage posterior fixation, most patients did not received posterior fixation, whether necessary of posterior fixation for all patients is still need longer follow-up. Third, although the patients with minor sagittal unbalance in this study had better restored the DH, and corrected the LL after surgery, whether OLIF could correct patients with degenerative scoliosis or major unbalance is still unknown. Furthermore, it is still a challenge to determine that how to correct sagittal spinopelvic alignment and in order to maintain optimal postoperative sagittal balance.