Introduction
Spinal correction and fusion surgery are recommended for patients with adolescent idiopathic scoliosis (AIS) with a curve >50° to prevent the progression of deformity [1–3]. However, several postoperative radiographic issues arise in patients with AIS, including postoperative shoulder imbalance (PSI), distal adding-on (DA) in the coronal plane, and distal junctional kyphosis (DJK) in the sagittal plane [3–8].
Several risk factors are associated with these radiographic complications. Factors associated with the upper instrumented vertebra (UIV), such as UIV selection, Lenke type 2 AIS, preoperative shoulder imbalance, preoperative T1 tilt, and upper end vertebra (UEV) at T1, have been identified as risk factors for PSI [5,9–13]. In contrast, factors related to the lower instrumented vertebra (LIV) have been identified as risk factors for DA and DJK. The selected LIV cranial to the last touching vertebra has been identified as a risk factor for DA in the coronal plane, whereas the selected LIV cranial to the sagittal stable vertebrae (SSV) has been identified as a risk factor for DJK in the sagittal plane [3,8,14–17].
Among researchers comparing PSI and DA, Cao et al. [18] found no case of PSI in patients with Lenke type 2 AIS having DA, suggesting that DA may serve as a compensatory mechanism for PSI. However, the correlation between PSI and DJK has not been thoroughly examined. Therefore, the present study aimed to evaluate the correlation between PSI and DJK, including DA, in patients with type 2 AIS.
Materials and Methods
Study design and participants
This retrospective study was conducted between 2007 and 2019 at Keio University Hospital. Ethical approval was obtained from the Institutional Review Board (IRB) of Keio University (IRB approvsl no., 20110142). Informed consent was obtained from all individual participants included in the study.
Data from 523 patients with AIS who underwent posterior correction and fusion surgeries during the study period, including patients with Lenke type 2 AIS, were analyzed. Patients with a follow-up period of at least 2 years and the UIV at the T2 level were included, which eliminates the effect of UIV selection. However, only patients who underwent thoracic fusion surgery were analyzed; thus, those patients in whom the lumbar curve was fixed were excluded. Patients who had undergone spinal surgery, anterior spinal fusion surgery, or combined anterior and posterior spinal fusion surgeries were also excluded.
The enrolled patients were categorized into two groups based on their radiographic shoulder height (RSH) 2 years after surgery. Patients with RSH >10 mm were designated to the PSI group according to previous reports [2,13] and the remaining to the non-PSI group. To ascertain the correlation between the PSI and radiographic complications, including DA and DJK, demographic data, radiographic data (including the presence of postoperative DA and DJK), and clinical outcomes were compared between the two groups. In addition, clinical outcomes were evaluated between patients with PSI and those with DJK.
Variables
The following demographic and radiographic details were documented for each patient: height, body mass index (BMI), proximal and main thoracic Cobb angles, RSH, thoracic kyphosis (TK, T5–12), lumbar lordosis (LL), pelvic incidence (PI), L4 tilt, clavicle chest cage angle difference (CCAD) [19], Risser grade, lumbar modifier, sagittal thoracic modifier, preoperative UEV of the proximal thoracic curve, lower end vertebra (LEV) of the main thoracic curve, SSV, UIV, LIV, and postoperative DA and DJK. SSV was defined as the vertebral level at which 50% of the vertebral body lies anterior to the posterior sacral vertical line on an upright standing lateral radiograph [20]. DA referred to an increase of ≥5° in the angulation of the first disc below the instrumentation, measured 2 after surgery [21]. DJK was defined as either focal kyphosis at LIV-LIV+1 of ≥10° or ≥5° of the kyphotic change in the sagittal disc angle between LIV and LIV+1, measured 2 after surgery [3,7].
To evaluate clinical outcomes, the Scoliosis Research Society (SRS) 22 scores were recorded preoperatively and 2 years postoperatively. These variables were compared between the two groups and patients with PSI and DJK using unpaired t-tests or Pearson’s chi-square tests. SRS-22 scores were also compared between patients with PSI or DA or DJK and patients without these radiographic complications.
Surgical technique
All patients underwent posterior correction surgery using a segmental pedicle screw construct. The last touching vertebra was chosen for LIV. Using the ball tip technique, pedicle screws were placed on both the concave and convex sides [22]. To achieve maximum correction, Ponte osteotomies were performed on the proximal thoracic curve, and inferior facetectomies were executed at every level within the fusion area. To correct scoliosis and create TK, a 5.5-mm diameter titanium alloy rod, designed to match the TK, was first placed on the concave side of the main thoracic curve and then rotated. An under-bent 5.5-mm diameter titanium alloy rod was placed on the convex side of the main thoracic curve for segmental distraction and compression, facilitating the correction of the proximal thoracic curve and prevention of PSI.
Statistical analysis
Unpaired t-tests, Pearson’s chi-square tests, or Fisher’s exact test were used to compare differences between the PSI and non-PSI groups. Cutoff values were defined in accordance with the clinical objectives and outcomes of the univariate analysis. In the univariate analysis, variables with p-values <0.05 and a 95% confidence interval were regarded as significant.
Radiographic parameters on the coronal plane were also evaluated using multiple regression analysis; these parameters included the proximal and main thoracic curve Cobb angles, L4 tilt, and CCAD affecting the RSH preoperatively and at the final follow-up. These statistical analyses were performed with IBM SPSS Statistics ver. 25.0 (IBM Corp., Armonk, NY, USA). To evaluate the comparison with a group of small sample size, the post hoc analysis was calculated using G*Power ver. 3.1.9.6 (Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany; http://www.gpower.hhu.de/).
Results
Descriptive statistics
A total of 62 patients (mean age, 14.1±2.0; range, 10–19 years) were included in this study. Two years after surgery, the mean Cobb angles for the proximal and main thoracic curves exhibited significant correction (p<0.01). The mean RSH increased significantly (p<0.01), and the mean TK decreased significantly (p<0.01). The preoperative UEV was located at T1 in 41 (66%) patients and T2 in 21 (34%). The preoperative LEV was located at T11 in 1 (2%) patient, T12 in 34 (55%), L1 in 14 (23%), L2 in 11 (18%), and L3 in 2 (3%). The preoperative SSV was located at T5 in 1 (2%) patient, T6 in 2 (3%), T7 in 3 (5%), T9 in 5 (8%), T10 in 5 (8%), T11 in 16 (26%), T12 in 13 (21%), L1 in 11 (18%), and L2 in 6 (10%). The selected LIV was at T10 in 1 (2%) patient, T11 in 3 (5%), T12 in 19 (31%), L1 in 15 (24%), L2 in 18 (29%), and L3 in 6 (10%) (Table 1).
In total, 28 patients (45%) were categorized into the PSI group and 34 (55%) into the non-PSI group. The LIV-LEV was higher in the PSI group than in the non-PSI group (PSI, 0.8±0.8; non-PSI, 0.1±0.8; p<0.01), and no significant differences in LIV-SSV were observed between the two groups (PSI, 2.2±2.2; non-PSI, 1.4±2.7; p=0.24). In the PSI group, 3 (11%) patients had DA, whereas in the non-PSI group, 10 (29%) had DA and 4 (12%) had DJK. The LIV-SSV values for the four DJK cases were −3 in two cases, −1 in one, and 0 in one. Among the four patients with DJK, none in either group underwent revision surgery (Table 2). No significant differences were observed in the height, weight, BMI, preoperative and final proximal thoracic Cobb angles, main thoracic Cobb angle, TK, LL, PI, L4 tilt, CCAD, Risser grade, sagittal thoracic modifier, preoperative UEV, preoperative LEV, SSV, and LIV between the two groups; however, the patients in the PSI group were older than those in the non-PSI group, and preoperative SRS-22 self-image score was lower in the PSI group than in the non-PSI group (Tables 2, 3).
In the multiple regression analysis, although the proximal thoracic curve Cobb angle, L4 tilt, and CCAD significantly affected RSH preoperatively, only the proximal thoracic curve Cobb angle significantly affected RSH at the final follow-up (Table 4).
In both groups, no significant differences were found between patients with DA and without DA for preoperative and final thoracic Cobb angle (Table 5).
In the comparison of SRS-22 scores between patients with PSI, DA, or DJK and those without radiographic complications, no significant differences were observed, except for the preoperative self-image of the PSI group (Table 6).
In the comparison between the PSI and non-PSI groups of only patients whose LIV was distal to the LEV, no significant differences were noted in the preoperative and final radiographic data (Table 7). The statistical power values for the comparisons of patients without PSI, DA, or DJK were 0.61, 0.43, and 0.21, respectively.
Case presentations
PSI group
A 16-year-old female teen diagnosed with Lenke type 2 AIS underwent posterior correction surgery using a segmental pedicle screw construct, and postoperative shoulder imbalance was evident. Unlike the former case, DA was not observed at the final follow-up, and postoperative shoulder imbalance persisted (Fig. 1).
Non-PSI group
A 13-year-old female teen diagnosed with Lenke type 2 AIS underwent a similar surgical procedure. Her preoperative RSH was −12.8 mm, and her postoperative RSH was 19.3 mm. DA was evident at the final follow-up, and the PSI was compensated (Fig. 2).
Discussion
This study revealed that PSI was associated with a more distal selection of LIV, whereas DA and DJK were associated with a more proximal selection. Cao et al. [18] reported that patients with Lenke type 2 and DA do not experience PSI, maintaining spinal balance even in cases with stable DA in the coronal plane. The present study demonstrated a similar trend for DJK in the sagittal plane, as observed for DA, where appropriate shoulder balance was achieved, and patients with DJK did not require revision surgery, including three patients with both DA and DJK. Therefore, although selecting a more proximal LIV may contribute to achieving stable DA and DJK, LIV selection should not be extended distally to prevent DA and DJK because favorable shoulder balance and clinical outcomes can still be achieved.
Berjano et al. [23] reported that a larger sagittal distance between the LIV and the gravity line of the body segment above the LIV increases the flexion moment arm, resulting in increased mechanical stress and high risk of DJK. Several reports have demonstrated an increased risk of DJK when the selected LIV was proximal to the preoperative SSV [8,15–17]. In the present study, although no significant differences in LIV−SSV values were observed between the two groups, the LIV−SSV values in patients with DJK were −3 in two, −1 in one, and 0 in one. These results are consistent with the findings of previous studies. In addition, the preoperative SSV displayed a wide distribution, ranging from T5 to L2. In cases with a preoperative SSV located at the middle thoracic level, the LIV−SSV values were too large, which might have affected the statistical results. Thus, selecting an LIV one vertebra caudal to the SSV may be sufficient to prevent DJK. However, given the limited number of DJK cases in this study and the absence of DJK in other cases with the LIV cranial to the SSV, further studies are needed to determine the relationship between SSV and DJK and identify other potential risk factors for DJK.
Regarding radiographic parameters, the L4 tilt and CCAD did not correlate with RSH at the final follow-up. The reason may be that this study included only patients with AIS, for whom the surgical correction was highly effective, resulting in minimal residual trunk deformity that could influence shoulder balance. However, these factors significantly affected the RSH in preoperative cases with an average main thoracic curve Cobb angle >60°. Therefore, in severe cases where the postoperative main thoracic curve Cobb angle remains >60°, the influence of the L4 tilt and CCAD must be considered to achieve a well-balanced spine.
The effect of postoperative DJK on patients with AIS, as assessed by SRS-22 scores, remains a subject of debate. Lowe et al. [3] reported that DJK can be a source of pain, imbalance, and poor cosmesis and may induce mechanical stress on adjacent segments, resulting in adjacent segment degenerative disc disease. Conversely, Segal et al. [17] reported that DJK development does not appear to adversely affect SRS-22 scores or the revision rate at a minimum of 5-year follow-up. Marciano et al. [16] demonstrated that patients with shorter fusions are more likely to have improvements in pain scores, as measured using the SRS-22, only when they do not develop DJK. However, they did not mention the severity of DJK. In the present study, no significant differences in the SRS-22 scores were observed between patients with PSI and those with DJK, and DJK did not require revision surgery. Therefore, shorter fusion segments may be considered to achieve better clinical outcomes in stable DJK cases.
In this study, no significant differences were observed in the SRS-22 outcome scores at the final follow-up between patients with PSI, DA, or DJK and those without these radiographic complications. Previous studies have demonstrated a moderate correlation between the Cobb angle or surface asymmetry and SRS self-image [24,25]. In contrast, Matamalas et al. [26] reported that shoulder imbalance did not correlate with SRS self-image, whereas Stone et al. [27] reported that SRS self-image scores changed from 1 to 10 years after surgery without any radiographic measures [26,27]. Consequently, conditions such as PSI, DA, and DJK, which do not require revision surgery, may exert minimal influence on SRS self-image. However, since the longitudinal data of both radiograph and self-image assessments are still unknown, a prolonged evaluation might be required.
This study has several limitations. The sample size, particularly in the PSI group and among patients with DJK, was relatively small, which may have limited the generalizability of the study findings as indicated by the power analysis. Clinical outcome data of patients with both DA and DJK were insufficient. Additionally, the retrospective design may have introduced selection bias and unmeasured confounders. Thus, future prospective trials with larger sample sizes are warranted to corroborate and expand the findings of this study.
Conclusions
Although more proximal LIV selection might lead to stable DA and DJK, LIV selection should not be extended distally to prevent DA and DJK because favorable shoulder balance and clinical outcomes can still be achieved.
Key Points
This study investigated 62 patients with Lenke type 2 adolescent idiopathic scoliosis (AIS) who underwent posterior correction and fusion surgeries to evaluate the correlation between postoperative shoulder imbalance and distal adding-on (DA) and distal junctional kyphosis (DJK).
More proximal lower instrumented vertebra selection might lead to stable DA and DJK.
Favorable shoulder balance and clinical outcome can still be achieved in patients with stable DA or DJK.
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