Characteristics analysis of Segmental and Regional Lumbar Spontaneous Compensation Post Selective Thoracic Fusion in Lenke 1 and 2 Adolescent Idiopathic Scoliosis

post selective fusion


Abstract Objective
To explore the characteristics of compensation of unfused lumbar region post selective thoracic fusion in Lenke 1 and 2 adolescent idiopathic scoliosis Background Preserving lumbar mobility in the compensation is signi cant in controlling pain and maintaining its functions. The spontaneous correction of the distal unfused lumbar curve after STF has been widely reported, but previous study has not concentrated on the characteristics of compensation of unfused lumbar region post selective thoracic fusion.

Method
A total of 51 Lenke 1 and2 AIS patients were included, whose lowest instrumented vertebrae was L1 from January 2013 to December 2019. For further analysis, demographic data and coronal radiographic lms were collected before surgery, at immediate erect postoperatively and nal follow-up. The wedge angles of each unfused distal lumbar segments were measured, and the variations in each disc segment were calculated at the immediate postoperative review and nal follow-up. Meanwhile, the unfused lumbar curve was divided into upper and lower parts, and calculated their curve angles and compensations.

Conclusion
When choosing L1 as the lowest instrumented vertebrae, the distal unfused lumbar segments' compensation showed a decreasing trend from the proximal end to the distal end. The adjacent L1/2 and L2/3 discs signi cantly contributed to this compensation. Background Lenke  In addition, the spontaneous lumbar curve correction (SLCC) can be achieved by correcting the main thoracic curve. In spinal fusion, it is believed that preserving lumbar mobility is advantageous in controlling pain and maintaining its functions. However, when planning the surgical treatments, more attention should be paid to the patients' unfused lumbar curve compensation ability due to its importance to the coronal balance.
Otherwise, the compensation characteristics of the spontaneous distal lumbar curve remain unclear even though it has been mentioned in some articles.
Bachmann et al. (Bachmann et al., 2020) validated that STF mainly produced changes in the upper half of the lumbar curve, leaving the lower half and the lumbosacral takeoff angle with little change. Mason et al. (DE & P, 1991) proposed that most lumbar coronal corrections could occur in the proximal region above the lumbar apex post STF. They explained that the proximal lumbar coronal curve could be more signi cantly corrected than the distal lumbar area because the proximal lumbar curve would become more lordotic in the sagittal plane immediately after surgery(KH, J, KY, & NY, 2007). Meanwhile, with a more distal lowest instrumented vertebrae (LIV), there is increased disc pressure and segmental motion at the adjacent level, followed by an overall reduction in lumbar activity and an increased risk of disc degeneration. Meric et al. (Enercan et al., 2015) conducted a retrospective study of AIS patients who received STF treatment, with at least ten years follow-up demonstrated a moderate rise in disc degeneration in the unfused segments. Facet joint degeneration was signi cant at the upper two levels adjacent to the lowest instrumented vertebra.
Although the spontaneous correction of the distal unfused lumbar curve after STF has been widely . However, all these studies regarded the unfused distal segments as an ensemble. Till, no further research has been reported on the impact of segmental or regional disc variation.
Our study focused on the distribution of distal unfused lumbar disc variation and explored the characteristics of compensation of unfused lumbar region post selective thoracic fusion in Lenke 1 and 2 adolescent idiopathic scoliosis.

Patients populations
A total of 51 consecutive patients were enrolled in this study between January 2013 and December 2019 met the inclusion and exclusion criteria. The inclusion criteria were: 1). 10 ≤ Age ≤ 18 years old; 2). According to Lenke classi cation, patients were diagnosed with Lenke 1 and 2 AIS and received a one-stage posterior correction surgery with pedicle screw; 3). LIV was L1 vertebrae; 4). The total follow-up time exceeded 24 months. The exclusion criteria were: 1). Other types of AIS or spine deformity; 2). LIV was above or below L1 vertebrae. In addition, patients without adequate radiological materials were also excluded. This current study was approved by the institutional review board of our hospital, and the patients in our study provided written informed consent for the study.

Data Collection
The demographic data, including age, gender, height, weight, BMI and Lenke type were recorded. Surgeryrelated information was recorded, such as UIV (upper instrumented vertebrae), fusion segments, and pedicle screws. All patients provided full spine standing posterior-anterior X-ray before surgery, at the immediate postoperative follow-up and nal follow-up. The Risser sign was calculated according to the preoperative pelvic X-ray. Other radiographic parameters were measured, such as proximal thoracic Cobb angles, main thoracic Cobb angles, thoracolumbar/lumbar Cobb angles, translation of thoracic apex (TAVT, the distance between the apex vertebra of the main thoracic curve and the cervical 7 vertebrae plumb line (C7PL)), translation of thoracolumbar/lumbar apex (LAVT, the distance between the apex vertebra of the thoracolumbar/lumbar curve and the center sacral vertical line (CSVL)) and coronal balance (the horizontal distance between the CSVL drawn from C7PL). The disc wedge angle was measured as the angle between the lines along the inferior endplate of the upper and the superior endplate of the lower vertebra in a segment, L1/2, L2/3, L3/4, L4/5 and L5/S1 disc were measured, respectively ( Fig. 1). Each segment's variation of disc wedge angle was calculated at immediate postoperative follow-up and nal follow-up reviews. As for the analysis of integral distal lumbar compensation, upper coronal lumbar curve (the Cobb angle between L1 and L4) and lower coronal lumbar curve (the Cobb angle between L4 and S1) were measured, and their compensation ability was also calculated at each follow-up. Radiographic parameters were measured by two experienced attending doctors of spine deformity, and the average value was adopted for further analysis.

Statistical analysis
Statistical analysis was performed using SPSS 19.0 statistics software (SPSS Inc, Chicago, IL). Descriptive statistics were presented in the form of mean ± standard deviation (SD). ANOVA analysis was used to assess the quantitative data among different periods, and the SNK method was used for pairwise comparison. Pair t-test analysis was utilized to assess the compensation ability of the upper and lower coronal lumbar curve.
Correlation analysis was also adopted to clarify the composition and compensation ability of each segment in the whole unfused lumbar region. P < 0.05 was considered statistical signi cance.  Radiographic parameters were shown in Table 2. The average main thoracic Cobb angle was 44.1 ± 7.7°, the mean thoracolumbar/lumbar Cobb angle was 24.1 ± 9.3°, and the mean coronal balance (C7PL-CSVL) was 11.2 ± 7.9mm, preoperatively. According to ANOVA analysis, there was a signi cant difference in proximal thoracic Cobb angle, main thoracic Cobb angle, thoracolumbar/lumbar Cobb angle, and thoracic AVT when comparing the preoperative X-ray with immediate postoperative erect X-ray or in preoperative X-ray and nal follow-up X-ray. No signi cant difference was found in the parameters when comparing the immediate postoperative erect and nal follow-up review X-rays. With arthrodesis, the main thoracic curve's correction was approximately 30° and remained stable until the nal follow-up. The thoracolumbar/lumbar curve was spontaneously compensated with a correction rate of more than 70%. The preoperative mean thoracic AVT was 33.0 ± 9.1 mm and was signi cantly improved at immediate erect postoperatively (P < 0.001) and at nal follow-up (P < 0.001). Additionally, no signi cant difference was observed in either lumbar AVT or coronal balance.   The preoperative disc wedge angles of L1/2, L2/3, L3/4, L4/5 and L5/S1 were − 2.86 ± 4.32°, -3.53 ± 4.22°, -2.71 ± 4.55°, -0.36 ± 3.40° and 2.03 ± 2.57°, respectively. At the nal follow-up, the disc wedge angle was approximately zero with spontaneous correction of the lumbar curve. However, as for L4/5 disc and L5/S1 disc level, a signi cant difference between preoperative and postoperative immediate X-rays was detected in each distal unfused disc wedge angle. A signi cant difference was found at nal follow-up in L1/2 disc, L2/3 disc, L3/4 disc, and L5/S1 disc level compared to preoperative X-ray. When considered upper and lower coronal lumbar curve as integral, the upper integral showed signi cance whether in the postoperative period or at nal follow-up. The preoperative upper and lower coronal lumbar curve were 15.87 ± 6.64° and 5.08 ± 3.93°, which were changed to 10 As shown in Table. 4, a strong association was found between the following disc wedge angle and the TL/L Cobb angle at nal follow-up using Pearson correlation statistics: L1/2 wedge angle (r = 0.518, p < 0.001) and L2/3 wedge angle (r = 0.468, p = 0.001). Moreover, the correlation of disc compensation and spontaneous lumbar correction at nal follow-up showed a similar tendency (Table 5): L1/2 disc compensation (r = 0.542, p < 0.001) and L2/3 disc compensation (r = 0.437, p = 0.001). As for the preoperative TL/L Cobb angle, the correlation was not signi cant in the preoperative L3/4 wedge angle (r = 0.205, p = 0.148).  ) pointed out that optimal postoperative outcomes for STF should include a lumbar Cobb angle less than 26 °, coronal balance 2 cm or less, deformity-exibility quotient less than 4, lumbar correction more than 37%, and trunk shift less than 1.5 cm. However, these studies only focus on overall compensation behavior and aim to improve clinical strategies.

Discussion
In this study, we calculated each disc wedge variation of distal unfused lumbar segments to further elucidate the characteristics of spontaneous compensation of the lumbar curve after STF. The results showed that the proximal two segments at level L1/2 and L2/3 accounted for most total compensation. The distal unfused lumbar segments provided the more distal the segment, the less compensation. Furthermore, we found that total disc compensation consisted of less than half of the total postoperative lumbar curve compensation.
This phenomenon may indicate that the lumbar curvature is often affected and includes the thoracic vertebrae, such as T10, T11 and T12. However, since all of our cases chose L1 as LIV, our study did not further investigate the fused thoracic discs, we focused attention on the unfused lumbar segments. As shown in Fig. 2, the compensation ability of the lumbar segments showed a decreasing tendency, with a major role being played by the proximal adjacent lumbar curve. Moreover, our integral analysis indicated that the upper coronal lumbar curve was responsible for most of the compensation, which was consistent with the opinion of Na et al. (Na, Ha, Harms, & Choi, 2010). They were the rst to divide the lumbar curve into the proximal and distal curves by their respective lumbar apex and concluded that looking at the proximal lumbar curve exibility might be an alternative indicator for measuring the lumbar exibility in MT-AIS patients treated by STF. We believe that the characteristics of residual lumbar curve after STF may be closely associated with the adding-on phenomenon and may provide evidence when choosing the correct LIV.
Then, what is the reason for the non-uniformity of unfused distal segment compensation? We believed that the exibility of the distal unfused segments might be different. Zhao et al. (J et al., 2018) analyzed the characteristics of cobb angle distribution in the Lenke 5C AIS patients. They found that the disc angles had symmetric distribution in the main thoracolumbar/lumbar curve, while the distal segment is more exible.
The thoracolumbar/lumbar curve's apex was often L1 or L2 vertebrae, and its distal segments may correspond to the L1/2 and L2/3 segments that were consistent with our study results. Na et al. (Na et al., 2010) also found that the lumbar apex of 28 main thoracic curve patients was between L2 and L3, and Even though our study focused on the residual lumbar curve segmental characteristics in Lenke 1 and 2 AIS patients who were performed STF, several limitations should be considered. First, we only included patients whose LIV was L1 vertebrae for the homogeneity analysis of disc compensation. Further researches on other LIV selection and comparison should be performed. Second, only coronal position data were studied in our research but not a sagittal plane, and there was no speci c analysis of related complications. Finally, this was a single-center study, and multi-centric research should be conducted to further validate the results.

Conclusion
The residual lumbar curve can be corrected spontaneously with the thoracic curve correction after posterior selective thoracic fusion in Lenke 1 and 2 AIS patients. When selecting L1 as the lowest instrumented vertebrae, the compensation of distal unfused lumbar segments showed a declining tendency to contribute to the compensation; with the immediately adjacent L1/2 and L2/3 disc contributed most in this compensation. This study was approved for the current study protocol (including surgery and X-ray scanning) by the ethics committee of our university (Local Ethics Committee of Changhai Hospital, SMMU, No. CHEC20170163). All subjects in our study provided written informed consent for the study and all methods were carried out in accordance with relevant guidelines and regulations.

Consent for publication
Not applicable

Availability of data and materials
The data that support the ndings of this study are available from Changhai Hospital, China but restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available. Data are however available from the authors upon reasonable request and with permission of Changhai Hospital, China.

Competing interests
The author(s) declared no potential con icts of interest with respect to the research, authorship, and/or publication of this article.

Funding
This study was supported by National Natural Science Fund of China (namely Nos.81972035).

Authors' contributions
Kai Chen, Xiao Zhai, Tianjunke Zhou, Yu Deng contributed equally to this paper, and were the Co-rst author, and have drafted the work or substantively revised it. Shaofeng Chen, Changwei Yang and Ming Li have checked, analyzed and edited the work.  The demonstration of disc wedge angle measurement of each lumbar segments.