This study demonstrated that unadjusted pelvic retroversion (i.e., insufficiently decreased PT), leaning forward position of the body (i.e., insufficiently decreased C7-SVA), and PI-LL mismatch were independent risk factors for LBP after OLIF. An insufficient decrease in PT after OLIF implied that the pelvis continued to be retroverted and could not correct the forward leaning position of the body and the PI-LL mismatch, which might be the cause of LBP.
It is well known that biomechanical changes caused by sagittal imbalance are involved in the pathogenesis of degenerative lumbar disease. The PI is an anatomic parameter that plays a fundamental role in sagittal balance and spinal degeneration. A higher PI indicates a higher SS and LL, which might lead to higher shear forces at the lumbosacral junction, and is one of the causes of spondylolisthesis.[22-24] Therefore, restoration of the sagittal spinopelvic parameters is essential for improving patients’ quality of life after surgery. It is particularly important to restore an adequate sagittal spinopelvic alignment when performing spinal fusion surgery. It has been hypothesized that sagittal malalignment is a risk factor strongly correlated with LBP in patients after surgery.[25, 26] Many studies have reported that increased SS and LL after posterior lumbar surgery may lead to better clinical outcomes and less LBP. Failure to achieve proper sagittal balance results in compensatory mechanisms such as decreased SL and LL, and increased PT, which have adverse effects on the back muscles and eventually lead to LBP.[7,24,27] Recently, Liow et al. reviewed 63 patients who underwent short-segment lumbar fusion surgery and found that patients with higher SS (SS ≥ 30°) experienced less LBP; in their opinion, increased LL and SS indicated better clinical outcomes and sagittal balance.[28]
Recently, OLIF has become a popular method of treating lumbar degenerative disease, as it has the advantage of minimizing iatrogenic injury to the posterior vertebral structures when compared with posterior lumbar surgery. Theoretically, indirect neural decompression can be achieved by restoring the intervertebral height. Abbasi et al. performed 303 OLIF procedures on 568 levels, and reported that OLIF was a safe and efficacious procedure for lumbar degenerative disease.[29] Lin et al. found that OLIF could achieve equivalent clinical and radiologic outcomes by indirect decompression, as compared to other posterior lumbar surgeries, while achieving better restoration of DH and causing less blood loss.[30] Chang et al. also obtained favorable clinical outcomes after OLIF for lumbar spinal stenosis.[31] Consistent with the above studies, we observed significant improvement in clinical outcomes after OLIF and a minimum follow-up of 2 years in both our study groups (non-LBP and LBP groups), which were comparably matched in terms of demographic data and clinical outcomes.
Although OLIF can effectively lead to indirect spinal canal decompression and increased SS, some patients experienced residual LBP after the surgery. The current study showed that in the non-LBP group, the SS at the last follow-up (31.7° ± 6.9°) had significantly improved compared to the preoperative value. In contrast, in the LBP group, the SS at the final follow-up (26.9° ± 6.9°) was significantly lower than the corresponding value in the non-LBP group. However, multiple logistic regression analysis showed that SS was not a risk factor for LBP after OLIF. It is known that increased PT indicates pelvic retroversion, which compensates for sagittal spinal imbalance. A PT of <20° is recommended to correct sagittal imbalance and relieve symptoms.[24] In this study, the PT at the final follow-up was 22.3° ± 10.8° and 15.1° ± 7.3° in the LBP and non-LBP groups, respectively, and these values significantly differed from the preoperative values (P = 0.000). These results suggest that the degree of the decrease in PT in the LBP group was not enough to compensate for the sagittal imbalance and was associated with residual back pain.
In addition, many research studies have reported that increased LL and SL are correlated with improved clinical outcomes.[32-34] Our results showed that SL was significantly improved after single-level OLIF in both groups. Although the SL in the non-LBP group was slightly higher than that in the LBP group, the difference was not statistically significant. However, the LL in the non-LBP group was significantly higher than that in the LBP group. This suggested that the impact of the interbody fusion was not enough to alter the overall spinal sagittal alignment, despite the placement of a large cage on both sides of the endplate and anterior to the vertebral body during OLIF.
The C7-SVA has been reported to be an important index of sagittal imbalance.[3,6,7] In our study, the C7-SVA had significantly decreased in both groups at the final follow-up. The change in C7-SVA was greater in the non-LBP group than in the LBP group. Additionally, a PI-LL mismatch of <10° was used to indicate whether sagittal reconstruction had been achieved in the non-LBP group. We found that OLIF could improve LL and correct PI-LL mismatch. Furthermore, the decreased C7-SVA was as evidenced by adjustment of LL. Saadeh et al. reported that single-level lateral lumbar interbody fusion greatly improved regional lordosis, but global lordosis was not impacted by the single-level intervention.[35] Schwab et al. showed that postoperative PI-LL mismatch causes greater residual LBP and proposed that SVA, PT, and PI-LL mismatch were most closely related to poor clinical outcomes and LBP.[36]
Although surgery improved DH, LL, SL, PI-LL mismatch, and C7-SVA, an ideal sagittal balance could not be achieved in the LBP group. OLIF could only partially restore sagittal balance by increasing the intervertebral height through the placement of a large interbody cage anteriorly within the wider distraction of the intervertebral space. On the one hand, deficient vertebral distraction is insufficient for spinal decompression and affects the correction of sagittal imbalance. On the other hand, excessive vertebral distraction necessitates the use of an overlarge interbody cage, which increases the risk of subsidence into the endplate, reduces fusion rates, and significantly increases mechanical stress on adjacent discs. Furthermore, the position of the interbody cage affected the recovery of the intervertebral height, which indirectly affected the restoration of LL and SL. Therefore, the placement of a larger intervertebral cage in the anterior- or middle-third of the spinal column would improve the sagittal spinopelvic alignment. However, with regard to the SL, each spinal level contributes a different and limited magnitude to the LL. Therefore, we considered that restoration of the intervertebral height by cage insertion might be insufficient to alter the mechanical dynamics of the spine.
To our knowledge, this is the first study to investigate the incidence of LBP and impact of sagittal spinopelvic alignment on patients after OLIF. It is particularly important to identify and restore sagittal spinopelvic alignment when performing this procedure. However, this study has several limitations. First, this was a single-center, retrospective study with a small sample size and a relatively short follow-up period. Further studies with larger cohorts followed up for longer periods are needed. Second, in the current study, one patient underwent second-stage posterior fixation; most patients did not undergo posterior fixation; whether posterior fixation is required for all patients needs to be determined using a longer follow-up period. Third, although OLIF restored DH and corrected LL in patients with minor sagittal imbalance, it is unclear whether OLIF will result in similar corrections in patients with degenerative scoliosis or excessive imbalance. Furthermore, the ideal method for correcting sagittal spinopelvic alignment to maintain optimal postoperative sagittal balance is difficult to determine.