The causes of isthmus spondylolisthesis are complex, including genetic, traumatic, mechanical and hormonal factors [12,13]. Among them, the abnormality of spine and pelvis parameters is an important factor in the occurrence and development of spondylolisthesis . Therefore, for the surgical principles of spondylolisthesis, in addition to decompression, reduction, fixation and fusion, attention should be paid to the reconstruction of lumbosacral sagittal balance. Zhou et al.  found that patients with isthmic spondylolisthesis have larger PI and L5I than asymptomatic healthy people, while patients with multi-segmental spondylolisthesis have larger SVA, PT and smaller LL, L4-S1 SL than single-segmental spondylolisthesis, which means that surgery is more challenging in correcting sagittal deformities of 2-level spondylolisthesis.
Du et al.  treated 58 cases of L4 and L5 spondylolisthesis with TLIF. The postoperative PT, SS, LL, L4-S1 SL, SVA and other parameters were significantly improved as compared with those before operation, it was found that 2-level TLIF was more effective in restoring local and global sagittal balance. Song et al.  treated 32 cases of 2-level isthmus spondylolisthesis with PLIF, the excellent and good rate of JOA score recovery was 84.3%, and the fusion rate was 87.5%, which was similar to our results. In this study, the JOA score, VAS score and sagittal plane parameters were significantly improved compared with those before operation through more than 2 years follow-up, which once again demonstrated the effectiveness of PLIF in the treatment of 2-level spondylolisthesis.
We believe that the advantage of PLIF in the treatment of 2-level spondylolisthesis lies in [13, 15-19]: 1. after spondylolisthesis, the posterior lamina is almost floating, and is usually accompanied by severe articular process hyperplasia and osteophyte formation, removal of posterior structure is beneficial to the reduction; 2. fully enlarge the spinal canal and nerve root foramen to avoid squeezing the nerve root after reduction; 3. laminectomy can obtain more autogenous bone, which avoids the rejection of allogeneic bone and the complications of iliac bone removal, which is beneficial to the interbody fusion. 4. double-segmental interbody fusion is beneficial to the restoration of intervertebral foramen height and lumbosacral sagittal plane; 5. discectomy, intervertebral height and sagittal plane reconstruction, fusion and internal fixation can be completed in one stage.
In this study, the overall improvement rate of JOA after operation was (64.81 ±27.83) %. We divided the patients into two groups according to the threshold of 50% improvement rate of JOA score. Through comparative analysis, it was found that poor reconstruction of L4-S1 SL and LDI was an important factor affecting prognosis. Kuhta et al.  treated 57 cases of single-segment degenerative spondylolisthesis with TLIF and found that insufficient correction of SL during operation would lead to forward shift of center of gravity and affect clinical prognosis. Takahashi et al.  found that the insufficient increase of L3-L5 SL in the treatment of L3 and L4 double-segment degenerative spondylolisthesis by PLIF was related to the poor clinical prognosis. He et al.  believe that in the treatment of degenerative spondylolisthesis with PLIF, the recovery of SL and PT is related to the clinical effect, and the increase of SL and the decrease of PT may play a positive role in relieving low back pain after operation. In this study, Δ L4-S1 SL, L4-S1 SL in group G was higher than that in group P, and positively correlated with the improvement rate of JOA score, indicating the importance of reconstruction of L4-S1 SL in double-segmental spondylolisthesis for improving clinical prognosis. According to the concept of the "core economic cone" , when the body is within the smallest cone, the muscle consumes the least energy, when the radius of the cone increases, the muscles of the back and buttocks must be activated to maintain body balance. L4-S1 is the base of the spine, when it is not properly corrected, it may lead to overall imbalance, to compensate for torso tilt, extension of the remaining segments and pelvic retroversion may be the cause of low back pain.
In order to reconstruct L4-S1 SL, we think the following can be done during the operation: 1. the hip joint should be properly extended before operation; 2. the posterior hyperplastic structures should be removed completely, which is beneficial to the shortening of the rear and the opening of the front during compression; 3. a moderate-sized cage should be anterior placed within disc space[22, 24, 25]; 4. in order to avoid the difficulty of compression, the anterior part of disc space can be grafted first, and then the remaining bone can be implanted after compression; 5. the ideal range of L4 – S1 SL is 2/3 of LL, and LL is related to PI [11,26, 27], so individual bending should be performed; 5. L4 and L5 screws toward the lower endplate is more beneficial to the compression and the recovery of segmental lordosis.
LDI is the ratio of L4-S1 SL to LL, which is an important parameter of lumbosacral. Janik et al.  Considered that the ideal LDI in asymptomatic population should be 2/3. Relevant studies have shown that small LDI in adult spinal deformity may lead to postoperative proximal junctional kyphosis . Zheng et al. found that patients with abnormal LDI after PLIF were more likely to have adjacent segment degeneration (ASD) than those with normal LDI, and patients with low LDI were more likely to have ASD than those with high LDI. In this study, the LDI of the 2 cases of internal fixation failure was less than 50%, indicating that abnormal LDI will increase the incidence of mechanical complications. We found that the mean LDI and proportion of LDI within the normal range in the G group were higher than those in group P, and there was a positive correlation between LDI and JOA score improvement rate, indicating that for double level spondylolisthesis, increasing L4-S1 SL appropriately and paying attention to its matching with LL can improve the clinical prognosis of patients to a certain extent.
The shortcomings of this study are: 1. the limitations of single center retrospective analysis, the incidence rate of 2-level isthmic spondylolisthesis is low, and the number of cases is relatively small, some parameters may not get statistical difference.
Therefore, relevant conclusions need to be verified by prospective, multi-center, long-term follow-up of large sample of cases;2. taking JOA score improvement rate as the basis for grouping has some subjectivity; 3. there are many reasons that affect the prognosis of patients, we only discuss the radiological parameters as the risk factors of clinical prognosis. However, as far as we know, this is one of the few clinical and sagittal follow-up studies of PLIF in the treatment of 2-level isthmic spondylolisthesis, and it is the first time to explore the relationship between ΔL4–S1 SL, L4–S1 SL, LDI and the clinical prognosis of L4 and 5 isthmic spondylolisthesis.