Wiltse et al.  classified lumbar spondylolisthesis into dysplasia, spondylolysis, degeneration, trauma, and pathology based on the cause of the disease. In previous studies, it has been observed that degenerative and isthmic spondylolisthesis are more common in double-level spondylolisthesis; degenerative double-level spondylolisthesis is more common compared with isthmic spondylolisthesis [3, 4]. Although there is no consensus on the pathogenesis of lumbar spondylolisthesis, some scholars believe that multilevel degenerative lumbar spondylolisthesis may be associated with factors such as advanced age, high BMI, changes in the direction of the lumbar facet joints, degenerative changes in the intervertebral discs, ligament and paravertebral muscle dysfunction and high PI, and high PI, LL, and SS will bring greater shear forces to the lumbosacral junction and make isthmus stress greater, so high PI is considered to be a predictor of double-level lumbar spondylolisthesis [12,13,14]. In double-level degenerative spondylolisthesis, there were more females than males, and it was mainly considered that it may be related to pregnancy, systemic joint laxity, and hormones; and the affected segment was mainly L3/L4 spondylolisthesis, which was mainly considered to be related to the denser lumbosacral ligaments . In double-level isthmic spondylolisthesis, there were more males than females, and most of them had related factors such as strenuous exercise and heavy physical work, and L4/L5 spondylolisthesis was predominant in the affected segments, which were mainly considered to be associated with chronic fatigue fractures in the lumbosacral region as a stress concentration in the lumbar spine [1, 9]. In this study, there were 21 cases of double-level spondylolisthesis at L3/L4, including 18 cases of degenerative spondylolisthesis and 15 cases of double-level spondylolisthesis at L4/L5, including 10 cases of isthmic spondylolisthesis, which were consistent with the results of previous studies.
When viewed from the lateral side, there are five physiological curves in the human spine, connected superiorly to the skull base and inferiorly to the pelvis, which together constitute the sagittal sequence of the spine. Normal sagittal sequences allow the human body to maintain an upright state through minimal energy expenditure and load, but this sagittal balance will be broken as the spine changes retrograde. Ferrero et al.  compared the characteristics of sagittal parameters in single-level versus double-level degenerative spondylolisthesis and concluded that PI, PT, and C7 inclination angles were significantly higher in double-level spondylolisthesis compared with single-level spondylolisthesis, and lower lumbar lordosis loss was more significant in the double-level spondylolisthesis group. Du et al.  proposed that although there are differences in the pathogenesis between double-level isthmic spondylolisthesis and double-level degenerative spondylolisthesis, from the nature of double-level spondylolisthesis, patients with continuous double-level isthmic spondylolisthesis have lower disc height and more significant forward slippage, which may also be the main reason for their sagittal imbalance. Therefore, compared with single-level lumbar spondylolisthesis, either degenerative or isthmic double-level lumbar spondylolisthesis presents with significant sagittal imbalance, including loss of lower lumbar lordosis, anteversion of the trunk, and compensatory flexion of the hip and knee joints, and this sagittal imbalance seriously affects the quality of life of patients [7, 8]. Schwab et al.  showed that in adult patients with spinal deformity, ODI scores were higher when PT > 22°, SVA > 47 mm, and PI-LL > 11°, and mismatch between PI and LL was significantly associated with patient quality of life. Lafage et al.  also proposed that an increase in PT is closely related to a worsening of the patient's quality of life and that a combination of SVA and PT is needed to assess sagittal imbalance. Because there is no comprehensive assessment method for sagittal imbalance, Schwab classification is the main method for assessing sagittal imbalance, and TPA, T1-SPi, and TLA global sagittal parameters are added to assess the changes of imaging parameters before, after, and at the last follow-up in double-level lumbar spondylolisthesis. In a geometric relationship, TPA = PT + T1-SPi, reflects both pelvic rotation and spinal tilt, although there is no agreement on the orthopedic target value of TPA in adult patients with spinal deformity, TPA values have been identified to be closely related to the patient's quality of life, and the advantage of TPA is that it is not changed by the patient's postural changes, and its TPA values remain unchanged regardless of whether there are compensatory changes in the patient's pelvis or hip and knee joints [18, 19]; TLA mainly responds to segmental compensatory changes in degenerative diseases from proximal to distal segments, thereby assessing thoracolumbar segmental changes . The results of this study showed that compared with those before surgery, TPA and T1-SPi in L3/L4 and L4/L5 spondylolisthesis groups were significantly improved after surgery and at the last follow-up compared with those before surgery, and there was statistical significance; while TLA was improved from 13.77 ± 8.75° preoperatively to 10.38 ± 6.06° postoperatively in L3/L4 spondylolisthesis group, and from 17.51 ± 8.80° preoperatively to 13.06 ± 8.11° postoperatively in L4/L5 spondylolisthesis group, and there was no statistical difference. The authors concluded that postoperative SVA, TPA, T1-SPi, and PT recovered well, trunk anteversion and pelvic retroversion recovered, but there was no significant change between thoracolumbar segments, and thus concluded that preoperative two-level spondylolisthesis showed a state of instability locally in the lumbar spine, compensatory changes occurred through trunk anteversion superiorly and pelvic retroversion inferiorly, while thoracolumbar segmental compensation was less, so although TLA improved postoperatively, it was not statistically significant, and the above parameter changes may also be related to the compensatory mechanism of the elderly during degeneration. For an in-depth understanding of imaging parameters, we can provide corresponding guidance during orthopedic procedures .
Interbody fusion is widely accepted due to the greater severity of spinal stenosis and poor intervertebral stability in double-level lumbar spondylolisthesis [22, 23]. Common interbody fusion methods include the anterior/trapezius approach (ALIF/OLIF) and posterior approach (PLIF/TLIF). The advantages of ALIF/OLIF are that it can maintain the tension of posterior spinal structures by preserving the posterior ligamentous complex, and it is easier to completely remove the intervertebral disc and thus place a larger cage; however, most complications of ALIF/OLIF are also related to its approaches, such as potential visceral injury, retrograde ejaculation and sympathetic dysfunction . Compared with ALIF/OLIF, PLIF/TLIF paravertebral muscle dissection can anatomically bring greater iatrogenic trauma, but it can provide a wider and direct vision of the surgical field and provide conditions for thorough nerve root decompression. Compared with PLIF, the transforaminal approach makes intraoperative traction between the dural sac and nerve roots less necessary, which can avoid the possibility of dural sac and nerve root injury to some extent . TLIF requires resection of at least one facet joint, and can directly decompress the lateral recess stenosis, achieve interbody dynamic distraction reduction through pedicle screws and cages, and better restore lumbar lordosis and sagittal spinal sequences . Considering the sagittal features of double-level lumbar spondylolisthesis, the surgical emphasis includes not only the management of spondylolisthesis but also the correction of the overall sagittal spinal sequence. It is prudent to consider how distraction reduction of localized spondylolisthesis impacts the overall sagittal spinal sequence and to define an appropriate surgical target for the patient before surgery.
Given that patients with double-level spondylolisthesis are generally older, have more comorbidities, and have limited tolerance to the degree of surgery, our surgical strategy was not developed to target ideal sagittal parameters for peers. For spondylolisthesis, we recommend that reduction be accomplished as far as possible rather than pursuing a perfect anatomical reduction because forced reduction may result in relative displacement of the nerve roots causing injury. The results of this study showed that the spondylolisthesis parameters (SD, SA, SP) were significantly improved after TLIF in both L3/L4 spondylolisthesis and L4/L5 spondylolisthesis, and the spondylolisthesis parameters were well maintained at the last follow-up, indicating that double-level TLIF was effective in the appropriate reduction and intervertebral distraction of double-level spondylolisthesis. Schwab et al.  suggested that adult patients with spinal deformity should be targeted at LL = PI ± 9°, PT < 20°, and SVA < 50 mm, while Soroceanu et al.  also proposed that complications such as adjacent segment degeneration and internal fixation device loosening may result if the postoperative sagittal parameters fail to meet the revised classification of SRS-Schwab classification. The results of this study showed that SVA, TPA, and T1-SPI were significantly improved after TLIF in the L3/L4 and L4/L5 spondylolisthesis groups, indicating that TLIF significantly improved the state of trunk inclination based on correcting the spondylolisthesis distance and angle; while PI-LL improved from 17.11 ± 7.04 preoperatively to 14.34 ± 5.17 postoperatively in the L3/L4 spondylolisthesis group; and from 20.35 ± 8.26 preoperatively to 16.90 ± 7.83 postoperatively in the L4/L5 group. Among them, PI-LL did not meet the SRS-Schwab modified classification, but the overall sagittal parameters achieved satisfactory results. The authors analyzed that the degree of PI and LL matching is specific and cannot be measured by the same criterion, which is basically consistent with the findings of Diebo et al. . After fixed fusion and appropriate restoration of LL, the sagittal parameters (SVA, TPA, T1-SPi) were also significantly improved, and the author concluded that the sagittal parameters were dynamic changes, and the instability of the lower lumbar spine or lumbosacral intervertebral space was improved by surgery, and the lordosis and sagittal sequences were also restored, so we believed that appropriate restoration of lumbar lordosis according to the Schwab modification classification could obtain satisfactory sagittal changes. At the same time, compared with preoperative, the VAS score, JOA score, and ODI index of the lumbar region and lower extremities of the patients were significantly improved after surgery and at the last follow-up, indicating that two-level TLIF is effective in reconstructing lumbar lordosis, restoring sagittal sequence, improving patient function, and improving patient quality of life.
Our experience suggests that most patients with double-level spondylolisthesis have previously been diagnosed because various factors have not been paid attention to, and lumbar and lower limb symptoms further worsen with the progression of the disease, showing more significant facet joint hyperplasia, spinal canal or lateral recess stenosis and significant sagittal imbalance on imaging, while TLIF can directly remove the facet joints and decompress the lateral recess while restoring the lumbar lordosis angle. The authors believe that the change of spondylolisthesis parameters is the basis of the change of sagittal parameters, and by correcting the local vertebral slippage, the recovery of intervertebral space height and angle, that is, improving the sagittal compensation of the spine, the sagittal sequence balance of the spine can be better restored. We recommend that reconstruction of lumbar lordosis be accomplished by placing CAGE of appropriate vertebral body size to distract the anterior vertebral space and placing pedicle screws. Global sagittal sequence balance was restored by changes in local sagittal parameters.
However, this study is a single-center institutional review, the sample size is small, there is still a lack of control of different surgical procedures, there are still defects, and the shortcomings can be further improved in the future, to clarify the clinical efficacy of TLIF in the treatment of double-level spondylolisthesis.