Two different types of postoperative sagittal imbalance after long instrumented fusion to the sacrum for degenerative sagittal imbalance

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  • 1 Department of Orthopedic Surgery, Eunpyeong St. Mary’s Hospital, College of Medicine, The Catholic University of Korea, Seoul;
  • | 2 Department of Orthopedic Surgery, Seoul St. Mary’s Hospital, College of Medicine, The Catholic University of Korea, Seoul; and
  • | 3 Department of Orthopedic Surgery, Kyung Hee University Hospital at Gangdong, Seoul, South Korea
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OBJECTIVE

Few studies have addressed that dynamic sagittal imbalance can develop distal to the spinal fusion and cause sagittal malalignment, unlike proximal junctional kyphosis (PJK) in the proximal portion. The purpose of this study was to investigate risk factors between the 2 different types of postoperative sagittal imbalance after long fusion to the sacrum for the treatment of degenerative sagittal imbalance (DSI).

METHODS

Eighty patients who had undergone surgical correction for DSI were included. Radiographic measurements included spinopelvic parameters on whole-spine plain radiographs and degeneration of paravertebral muscles on MRI. Univariate and multivariate analyses for clinical and radiological factors were conducted for respective risk factors. In subgroup analyses, the 2 different types of postoperative sagittal imbalance were directly compared.

RESULTS

Forty patients (50%) developed postoperative sagittal imbalance; of these patients, 22 (55.0%) developed static proximal kyphosis from PJK, and 18 patients (45.0%) developed dynamic sagittal imbalance without PJK. The independent risk factors in proximal kyphosis were greater postoperative pelvic tilt (HR 1.11) and less change in sacral slope (SS) (HR 1.09), whereas there were more fusion levels (HR 3.11), less change in SS (HR 1.28), and less change in thoracic kyphosis (HR 1.26) in dynamic sagittal imbalance. Directly compared with the proximal kyphosis group, dynamic sagittal imbalance was more commonly found in patients who had less correction of sagittal parameters as well as fatty atrophy of the paravertebral muscles. Clinical outcomes in the dynamic sagittal imbalance group were superior to those in the proximal kyphosis group.

CONCLUSIONS

Optimal correction of sagittal alignment should be considered in long instrumented fusion for DSI, because insufficient correction might cause one of 2 different types of postoperative sagittal imbalance at different sites of decompression. Dynamic sagittal imbalance compared with proximal kyphosis was significantly associated with less correction of sagittal alignment, in conjunction with more fusion levels and degeneration of the paravertebral muscles.

ABBREVIATIONS

ASD = adult spinal deformity; CSA = cross-sectional area; DSI = degenerative sagittal imbalance; LL = lumbar lordosis; ODI = Oswestry Disability Index; PI = pelvic incidence; PJA = proximal junctional angle; PJF = proximal junctional failure; PJK = proximal junctional kyphosis; PSO = pedicle subtraction osteotomy; PT = pelvic tilt; SS = sacral slope; SVA = sagittal vertical axis; TK = thoracic kyphosis; UIV = upper instrumented vertebra; VAS = visual analog scale.

OBJECTIVE

Few studies have addressed that dynamic sagittal imbalance can develop distal to the spinal fusion and cause sagittal malalignment, unlike proximal junctional kyphosis (PJK) in the proximal portion. The purpose of this study was to investigate risk factors between the 2 different types of postoperative sagittal imbalance after long fusion to the sacrum for the treatment of degenerative sagittal imbalance (DSI).

METHODS

Eighty patients who had undergone surgical correction for DSI were included. Radiographic measurements included spinopelvic parameters on whole-spine plain radiographs and degeneration of paravertebral muscles on MRI. Univariate and multivariate analyses for clinical and radiological factors were conducted for respective risk factors. In subgroup analyses, the 2 different types of postoperative sagittal imbalance were directly compared.

RESULTS

Forty patients (50%) developed postoperative sagittal imbalance; of these patients, 22 (55.0%) developed static proximal kyphosis from PJK, and 18 patients (45.0%) developed dynamic sagittal imbalance without PJK. The independent risk factors in proximal kyphosis were greater postoperative pelvic tilt (HR 1.11) and less change in sacral slope (SS) (HR 1.09), whereas there were more fusion levels (HR 3.11), less change in SS (HR 1.28), and less change in thoracic kyphosis (HR 1.26) in dynamic sagittal imbalance. Directly compared with the proximal kyphosis group, dynamic sagittal imbalance was more commonly found in patients who had less correction of sagittal parameters as well as fatty atrophy of the paravertebral muscles. Clinical outcomes in the dynamic sagittal imbalance group were superior to those in the proximal kyphosis group.

CONCLUSIONS

Optimal correction of sagittal alignment should be considered in long instrumented fusion for DSI, because insufficient correction might cause one of 2 different types of postoperative sagittal imbalance at different sites of decompression. Dynamic sagittal imbalance compared with proximal kyphosis was significantly associated with less correction of sagittal alignment, in conjunction with more fusion levels and degeneration of the paravertebral muscles.

In Brief

The authors investigated risk factors between 2 types of postoperative sagittal imbalance—proximal kyphosis and dynamic sagittal balance—after long fusion to the sacrum for the treatment of degenerative sagittal imbalance (DSI). Dynamic sagittal imbalance might develop in the form of progressive decompensation through the hip joints due to weakness of hip and back muscles in patients with less correction of sagittal alignment, more fusion levels, and atrophy of the paravertebral muscles. Clinical outcomes in the dynamic sagittal imbalance group were superior to those in the proximal kyphosis group. Optimal correction of sagittal alignment should be considered in long instrumented fusion for DSI.

Restoration of normal sagittal balance, leading to improvement in the quality of life of individuals afflicted with adult spinal deformity (ASD), is a critically important goal when spinal surgery is performed.1,2 Because long fusions with instrumentation are frequently required for degenerative sagittal imbalance (DSI), these surgeries have been associated with many postoperative complications and often lead to revision.3,4

Postoperative sagittal imbalance is a complication seen after long fusion and can occur at the proximal, intraconstruct, or distal portion, depending on the location. Sagittal malalignment from proximal junction to spinal fusion includes proximal junctional kyphosis (PJK) or proximal junctional failure (PJF), developing as segmental kyphosis above the fusion level.5–11 Pseudarthrosis and rod fracture within fusion levels can also cause postoperative sagittal imbalance.12 Finally, lumbosacral disc degeneration in fusions ending at L5, failure of distal fixation, and pelvic fracture have been associated with a risk of development of positive sagittal malalignment from the distal junction.13,14

However, except for the previously mentioned risk factors, dynamic sagittal imbalance can develop, whereby whole spinal fusion segments tilt forward and the hip joints appear to function as a center of rotation.15 Progressive postoperative malalignment included static PJK developing at a proximal portion or dynamic sagittal imbalance at a distal portion after excluding treated patients with pseudarthrosis and construct failure. This study was designed to investigate the risk factors and determine the mechanism for the 2 different types of postoperative sagittal imbalance in patients treated with long instrumented fusion to sacrum with sacropelvic fixation for DSI.

Methods

A clinical and radiological database from 2006 to 2014 was retrospectively reviewed at a single institution. This study was approved by our institutional review board. We included patients who were older than 60 years at the time of index surgery and had subsequently undergone correction surgery for DSI with the following criteria: sagittal vertical axis (SVA) ≥ 5 cm, > 4 levels of fusion to the sacrum with bilateral iliac screw fixation, and a minimum of 24 months of follow-up. We screened 148 patients who had undergone reconstructive surgery for ASD and included 80 patients who met these criteria in this study.

Whole-spine plain radiography was performed after patients walked for 10 minutes to induce fatigue of hip and back extensor muscles at the last follow-up. Progressive postoperative sagittal imbalance was defined as a > 5-cm difference between the postoperative and last follow-up SVAs.16,17 Postoperative sagittal imbalances due to pseudarthrosis and construct failure were excluded, because radiological outcomes might be affected by loss of correction. Four patients with pseudarthrosis and 2 patients undergoing revision surgery for rod failure and screw loosening were excluded from our study.

The patients were divided into 3 groups. Patients without postoperative sagittal imbalance were included in the balanced group. Postoperative sagittal imbalances were divided into 2 groups: patients with radiological evidence of PJK at the proximal portion of the spinal fusion were included in the proximal kyphosis group (Fig. 1),5 and those without a change in the proximal junctional angle (PJA) were included in the dynamic sagittal imbalance group (Fig. 2).

FIG. 1.
FIG. 1.

Lateral radiographs obtained in a 70-year-old woman in the proximal kyphosis group. A: Preoperative radiograph revealing DSI. B: Postoperative radiograph revealing sagittal correction. C: Subsequent radiograph revealing sagittal imbalance with PJK. D: Last follow-up radiograph revealing sagittal correction after the revision surgery.

FIG. 2.
FIG. 2.

Lateral radiographs obtained in a 67-year-old woman in the dynamic sagittal imbalance group. A: Preoperative radiograph revealing DSI. B: Postoperative radiograph revealing sagittal correction. C: Last follow-up radiograph revealing dynamic sagittal imbalance without PJK.

Clinical Parameters

Patient demographics were assessed according to age, sex, BMI (kg/m2), bone mineral density, and the American Society of Anesthesiologists physical status classification. Surgical factors included the number of fusion levels, upper instrumented vertebra (UIV) level, surgical approach (anterior-posterior approach vs posterior alone), and pedicle subtraction osteotomy (PSO). Clinical outcomes included the Oswestry Disability Index (ODI) and a 10-point visual analog scale (VAS) of back and leg pain preoperatively and at the last follow-up. Surgical outcomes, specifically postoperative major complications requiring revision surgery, were compared.

Radiological Measurements

Radiographic parameters, including pelvic incidence (PI), sacral slope (SS), pelvic tilt (PT), lumbar lordosis (LL), thoracic kyphosis (TK), and SVA, were measured with the picture archiving and communication system (PACS) preoperatively and at 3 months postoperatively as previously described.18,19 In addition, optimal correction was defined as PT < 20°, PI-LL mismatch within 10°, and SVA < 50 mm.20 The cross-sectional area (CSA) of paravertebral muscles, including the erector spinae and multifidus muscles, was manually contoured using the software at L3–4 from preoperative MRI.21 The CSA measurement at the L3–4 disc level was selected because the CSA of the paravertebral muscles has been found to be the largest overall at this level and on the right side.22,23 The total CSA of the paravertebral muscles and ratio of the CSA-paravertebral muscle/CSA-disc were assessed. The amount of fatty change was measured as the ratio of the average mean signal intensities of the muscle and subcutaneous fat (Fig. 3).21,24,25 Additionally, degenerative changes of the intervertebral disc adjacent to the UIV were rated according to Pfirrmann et al.26

FIG. 3.
FIG. 3.

A: Axial T1-weighted MR image showing the measurement of the CSA of the paravertebral muscle and intervertebral disc on the right side at L3–4. B: Axial T2-weighted MR image showing the measurement of the amount of fatty change as the ratio between the mean signal intensity of the paravertebral muscle and subcutaneous fat at L3–4.

Statistical Analysis

Perioperative continuous variables were compared using unpaired Student t-tests between each group and paired t-tests within each group. Categorical variables were compared using Fisher’s exact test or Pearson’s chi-square test, depending on the size of the sample. Variables with p < 0.10 on the univariate analysis were included for multivariate analysis using logistic regression testing. Analyses of variance with post hoc analysis were used to compare the parameters between the 3 groups: balanced group, dynamic sagittal imbalance group, and proximal kyphosis group. Statistical analyses were done using IBM SPSS Statistics for Windows (version 24.0; IBM Corp.) with a level of statistical significance of 0.05.

Results

The patients included in this study consisted of 69 women and 11 men with a mean age of 67.6 ± 6.8 years. Forty-nine patients were diagnosed with degenerative lumbar kyphoscoliosis and 31 with degenerative lumbar kyphosis. The mean number of fusion levels was 6.0 ± 1.6 (range 4–10 levels), and fusions ending at the thoracolumbar junction (T11–L1) were done in 53 patients compared with above the thoracolumbar junction in 27 patients; 26 patients underwent combined anterior-posterior surgery, and PSO was performed in 22 patients (27.5%) (Table 1). The mean (± SD) follow-up period was 40 ± 35.6 months (range 24–136 months).

TABLE 1.

Univariate analysis of patient demographics and surgical factors for proximal kyphosis and dynamic sagittal imbalance

CharacteristicBalanced GroupProximal Kyphosis Groupp Value*Dynamic Sagittal Imbalance Groupp ValueTotal
No. of patients40221880
Mean age, yrs67.2 ± 7.069.1 ± 8.00.96468.8 ± 4.70.39967.6 ± 6.8
Sex0.0810.290
 Female34221369
 Male60511
Diagnosis0.4990.584
 DLKS27111149
 DLK1311731
Mean BMD, T score−3.0 ± 1.2−3.0 ± 0.70.977−3.0 ± 0.90.999−3.0 ± 1.0
Mean BMI, kg/m223.6 ± 3.525.8 ± 3.90.02724.4 ± 3.90.46024.4 ± 3.8
No. of current smokers2 (5)1 (4.5)>0.990 (0)>0.993 (3.8)
ASA class>0.99>0.99
 I or II38211776
 III or IV2114
Mean no. of fusion levels5.5 ± 1.46.1 ± 1.80.1496.9 ± 1.30.00016.0 ± 1.6
UIV0.7600.001
 At TL junction3116653
 Above TL junction961227
Surgical approach0.1140.920
 Anterior-posterior154726
 Posterior only25181154
No. of patients w/ PSO14 (35.0)4 (18.2)0.1634 (22.2)0.33022 (27.5)

ASA = American Society of Anesthesiologists; BMD = bone mineral density; DLK = degenerative lumbar kyphosis; DLKS = degenerative lumbar kyphoscoliosis; TL = thoracolumbar (T11–L1).

Values are presented as the number of patients (%) unless stated otherwise. Mean values are reported as mean ± SD. Boldface type indicates statistical significance.

Univariate analysis of the proximal kyphosis group compared with the balanced group.

Univariate analysis of the dynamic sagittal imbalance group compared with the balanced group.

Forty patients (50%) developed postoperative sagittal imbalance. Of these patients, 22 (55.0%) developed static proximal kyphosis from PJK, and 18 patients (45.0%) developed dynamic sagittal imbalance without PJK. The patients without postoperative sagittal imbalance at the most recent follow-up were compared with the patients in the proximal kyphosis group (n = 22) and dynamic sagittal imbalance group (n = 18). In this study, patients in the dynamic sagittal imbalance group did not develop PJK during the follow-up period.

Risk Factor Analysis

Univariate analysis revealed higher BMI as a risk factor for proximal kyphosis (25.8 ± 3.9 vs 23.6 ± 3.5 kg/m2, p = 0.027), whereas the risk factors for dynamic sagittal imbalance were more fusion levels (6.9 ± 1.3 vs 5.5 ± 1.4 levels, p = 0.0001) and fusions ending above the thoracolumbar junction (p = 0.001) (Table 1).

Univariate analysis of radiological factors for proximal kyphosis demonstrated that greater postoperative PT (26.4° ± 9.2° vs 21.4° ± 8.9°, p = 0.042), less change in SS (4.6° ± 8.2° vs 9.7° ± 9.6°, p = 0.041), and amount of fatty changes (38.9% ± 11.2% vs 32.6% ± 8.5%, p = 0.05) were significant risk factors. Meanwhile, the univariate analysis of radiological factors showed that less change in LL (14.8° ± 11.0° vs 26.2° ± 20.2°, p = 0.029), less change in SS (2.8° ± 6.1° vs 9.7° ± 9.6°, p = 0.007), less change in PT (−4.1° ± 6.3° vs −11.1° ± 9.4°, p = 0.005), and less change in TK (5.6° ± 8.7° vs 12.1° ± 9.7°, p = 0.020) were the significant risk factors for dynamic sagittal imbalance. In addition, a lower CSA-muscle/CSA-disc ratio (76.2% ± 16.1% vs 89.9% ± 21.3%, p = 0.035) and amount of fatty changes (40.5% ± 14.9% vs 32.6% ± 8.5%, p = 0.047) were also significant risk factors for dynamic sagittal imbalance (Table 2).

TABLE 2.

Radiological parameters between the balanced, proximal kyphosis, and dynamic sagittal imbalance groups

Balanced Group (n = 40)Proximal Kyphosis Group (n = 22)p Value*Dynamic Sagittal Imbalance Group (n = 18)p Value
Preop
 PI, °53.5 ± 12.559.3 ± 11.40.07957.7 ± 12.90.277
 LL, °10.0 ± 20.819.1 ± 21.10.10820.4 ± 17.60.069
 PI-LL, °43.5 ± 19.540.2 ± 19.70.52938.1 ± 18.00.319
 SS, °21.1 ± 26.226.7 ± 11.70.05826.2 ± 12.40.111
 PT, °32.5 ± 9.334.0 ± 11.10.58931.2 ± 10.40.619
 TK, °8.2 ± 18.313.8 ± 10.40.20216.0 ± 14.00.115
 LL-TK, °2.8 ± 17.34.3 ± 16.70.7502.5 ± 15.60.731
 SVA, mm55.5 ± 65.687.8 ± 74.80.596102.7 ± 82.10.232
Postop
 LL, °36.2 ± 14.840.1 ± 12.10.28935.2 ± 14.10.818
 PI-LL, °15.6 ± 13.917.6 ± 12.30.57220.6 ± 13.50.208
 SS, °30.8 ± 11.531.4 ± 10.50.85829.0 ± 10.70.577
 PT, °21.4 ± 8.926.4 ± 9.20.04227.1 ± 12.80.057
 TK, °20.1 ± 12.325.0 ± 15.80.17821.6 ± 12.20.666
 LL-TK, °16.1 ± 15.815.1 ± 17.60.82313.6 ± 13.10.564
 SVA, mm27.7 ± 38.132.7 ± 33.40.61239.9 ± 43.60.297
Postop changes
 LL, °26.2 ± 20.221.1 ± 17.80.32614.8 ± 11.00.029
 SS, °9.7 ± 9.64.6 ± 8.20.0412.8 ± 6.10.007
 PT, °−11.1 ± 9.4−7.6 ± 8.40.143−4.1 ± 6.30.005
 TK, °12.1 ± 9.79.4 ± 13.20.8955.6 ± 8.70.020
 SVA, mm−51.6 ± 59.4−36.2 ± 70.20.819−62.8 ± 78.10.561
CSA of paravertebral muscles, mm21778 ± 3501776 ± 3320.5571566 ± 3500.058
CSA-muscle/CSA-disc ratio89.9 ± 21.395.8 ± 23.30.98376.2 ± 16.10.035
Amount of fatty change, %32.6 ± 8.538.9 ± 11.20.0540.5 ± 14.90.047
Pfirrmann grade2.8 ± 0.83.0 ± 0.90.5053.0 ± 0.80.396

Mean values are reported as mean ± SD. Boldface type indicates statistical significance.

Univariate analysis of the proximal kyphosis group compared with the balanced group.

Univariate analysis of the dynamic sagittal imbalance group compared with the balanced group.

Parameters with p < 0.10 in the univariate analysis were analyzed using multivariate analysis. Greater postoperative PT (HR 1.11, p = 0.026) and less change in SS (HR 1.09, p = 0.036) were independent risk factors for proximal kyphosis, whereas more fusion levels (HR 3.11, p = 0.044), less change in SS (HR 1.28, p = 0.03), and less change in TK (HR 1.26, p = 0.049) were the independent risk factors for dynamic sagittal imbalance (Table 3).

TABLE 3.

Multivariate analysis of risk factors for proximal kyphosis and dynamic sagittal imbalance

Proximal Kyphosis Group (n = 22)Dynamic Sagittal Imbalance Group (n = 18)
HR95% CIp Value*HR95% CIp Value
BMI1.180.98–1.420.085
No. of fusion levels3.111.03–9.360.044
UIV above TL junction7.960.297–2120.250
Postop PT, °1.111.01–1.210.026
Less postop change, °
 LL1.040.87–1.280.652
 SS1.091.01–1.180.0361.281.02–1.600.03
 PT1.110.84–1.450.470
 TK1.261.00–1.850.049
CSA-muscle/CSA-disc ratio1.090.98–1.210.101
Amount of fatty change1.031.01–1.210.6991.080.98–1.200.07

Boldface type indicates statistical significance.

Multivariate analysis in the proximal kyphosis group.

Multivariate analysis in the dynamic sagittal imbalance group.

Comparison Between the 2 Sagittal Imbalance Groups

In the subgroup analyses, the distribution based on sex differed significantly between the 2 groups. All patients in the proximal kyphosis group were women, while 13 patients (72.2%) in the dynamic sagittal imbalance group were women (p = 0.013). The UIV was at the thoracolumbar junction in 16 patients (72.7%) in the proximal kyphosis group and in 6 patients (33.3%) in the dynamic sagittal imbalance group (p = 0.013). Dynamic sagittal imbalance was observed in patients with undercorrection of their deformity, such as changes in LL < 30° (63.6% vs 94.4%, p = 0.02), postoperative PI-LL > 10° (45.5% vs 77.8%, p = 0.038), and postoperative SVA ≥ 50 mm (10.0% vs 41.2%, p = 0.028). Moreover, the prevalence of optimal correction (postoperative PT < 20°, PI-LL ≤ 10°, SVA < 50 mm) was significantly higher in the proximal kyphosis group, compared with the dynamic sagittal imbalance group (50% vs 17.6%, p = 0.04). Regarding paravertebral muscles, the CSA was significantly greater in the proximal kyphosis group than in the dynamic sagittal imbalance group (1776 ± 332 mm2 vs 1566 ± 350 mm2, p = 0.05). The CSA-muscle/CSA-disc ratio was also greater in the proximal kyphosis group (95.8% ± 23.3% vs 76.2% ± 16.1%, p = 0.005) (Table 4).

TABLE 4.

Direct comparison of radiological parameters between proximal kyphosis and dynamic sagittal imbalance

Proximal Kyphosis Group (n = 22)Dynamic Sagittal Imbalance Group (n = 18)p Value
Mean age, yrs69.1 ± 8.068.8 ± 4.80.254
Sex0.013
 Female22 (100)13 (72.2)
 Male0 (0)5 (27.8)
UIV0.013
 At TL junction16 (72.7)6 (33.3)
 Above TL junction6 (27.3)12 (66.7)
Preop PT, °0.069
 <306 (27.3)10 (55.6)
 ≥3016 (72.7)8 (44.4)
Preop PI-LL, °0.412
 <307 (31.8)8 (44.4)
 ≥3015 (68.2)10 (55.6)
Preop SVA, mm*0.942
 <10012 (60)10 (58.8)
 ≥1008 (40)7 (41.2)
Change in SVA, mm*>0.99
 <5010 (50)8 (47.1)
 ≥5010 (50)9 (52.9)
Change in LL, °0.020
 <3014 (63.6)17 (94.4)
 ≥308 (36.4)1 (5.6)
Postop PI-LL, °0.038
 ≤ ±1012 (54.5)4 (22.2)
 > ±1010 (45.5)14 (77.8)
Postop SVA, mm*0.028
 <5018 (90)10 (58.8)
 ≥502 (10)7 (41.2)
Optimal correction*0.04
 No10 (50)14 (82.4)
 Yes10 (50)3 (17.6)
Mean CSA of paravertebral muscles, mm21776 ± 3321566 ± 3500.05
Mean CSA-muscle/CSA-disc ratio95.8 ± 23.376.2 ± 16.10.005
Mean % fatty changes38.9 ± 11.240.5 ± 14.90.698
Mean Pfirrmann grade3.0 ± 0.93.0 ± 0.80.852

Values are presented as the number of patients (%) unless stated otherwise. Mean values are reported as mean ± SD. Boldface type indicates statistical significance.

Two patients in the proximal kyphosis group and 1 patient in the dynamic sagittal imbalance group did not undergo whole-spine radiography.

Optimal correction is defined as PT < 20°, PI-LL ≤ ±10°, SVA < 50 mm.

Clinical Outcomes

Regarding clinical outcomes, patients in the balanced group reported significant improvement in their back VAS score (4.1 ± 2.8 vs 7.1 ± 2.3, p = 0.0001), leg VAS score (3.3 ± 3.2 vs 6.4 ± 3.0, p = 0.001), and ODI (42.5% ± 22.3% vs 57.3% ± 19.7%, p = 0.002), while significant improvement of back VAS score (5.3 ± 2.8 vs 7.5 ± 2.6, p = 0.022) was only documented in the dynamic sagittal imbalance group. No significant improvement was reported by patients in the proximal kyphosis group at last follow-up (Table 5). When the clinical outcomes were compared between the 3 groups, significant differences of back VAS were found at last follow-up. The back VAS score was highest in the proximal kyphosis group, followed by the dynamic sagittal imbalance group and the balanced group (6.2 ± 2.7 vs 5.3 ± 2.8 vs 4.1 ± 2.8 points, p = 0.021). In direct comparison with the proximal kyphosis group, patients in the dynamic sagittal imbalance group reported significant ODI improvement at last follow-up (−10.1% ± 26.8% vs 1.2% ± 21.2%, p = 0.038).

TABLE 5.

Clinical outcomes between the balanced, proximal kyphosis, and dynamic sagittal imbalance groups

Balanced GroupProximal Kyphosis GroupDynamic Sagittal Imbalance Group
Mean ± SDp ValueMean ± SDp ValueMean ± SDp Value
Back VAS score
 Initial7.08 ± 2.280.00017.15 ± 2.740.2207.47 ± 2.580.022
 Last follow-up4.11 ± 2.756.20 ± 2.655.29 ± 2.76
Leg VAS score
 Initial6.39 ± 3.000.0015.60 ± 3.710.1516.53 ± 3.04
 Last follow-up3.34 ± 3.154.30 ± 3.254.65 ± 3.52
ODI, %
 Initial57.3 ± 19.70.00248.0 ± 20.80.81155.6 ± 22.50.06
 Last follow-up42.5 ± 22.349.1 ± 21.545.5 ± 21.9
Postop complicationsPTE (n = 1); epidural hematoma (n = 1); hardware irritation (n = 1)UIV fracture (n = 3); instrumentation failure (n = 1); wound infection (n = 1)Wound infection (n = 1)
Incidence of complications3/40 patients (7.5%)5/22 patients (22.7%)1/18 patients (5.6%)
 p value*0.1190.787

PTE = pulmonary thromboembolism.

Boldface type indicates statistical significance.

Compared with the balanced group.

Postoperative complications were not significantly different between the groups (Table 5). However, 4 patients (18.2%) in the proximal kyphosis group had fusion extension because of UIV fracture (3 patients) and proximal pedicle screw pullout (1 patient) within 1 year after index surgery, whereas revision surgery was not done in the balanced or dynamic sagittal imbalance groups.

Discussion

The causes of postoperative sagittal imbalance could be multifactorial, and many studies have reported that advanced age, osteoporosis, suboptimal correction of the deformity, fusion level, paravertebral muscle and hip extensor muscle weakness, and fusion stopping at L5 represent viable risk factors.6–8,10,14,27 Dynamic sagittal imbalance could also be developed at the portion distal to the spinal fusion and cause sagittal malalignment. However, few reports have reported dynamic sagittal imbalance as being different from static PJK after long instrumented fusion.15,27 To the best of our knowledge, no study has investigated the risk factors for these 2 different types of postoperative sagittal imbalance. Therefore, the purpose of this study was to investigate the mechanism of different types of sagittal imbalance that can develop after surgical correction for DSI.

Multivariate analysis in this study revealed that greater postoperative PT (HR 1.11) and less change in SS (HR 1.09) were independent risk factors in the proximal kyphosis group. More fusion levels (HR 3.11), less change in SS (HR 1.28), and less change in TK (HR 1.26) were the independent risk factors for dynamic sagittal imbalance. These risk factors were not the result of a compensatory mechanism, because radiological parameters had been measured before the postoperative sagittal imbalance developed.

This study found that, in comparison with the balanced group, insufficient correction of sagittal alignment was associated with postoperative sagittal imbalance. This finding is consistent with those of several previous studies that demonstrated that insufficient correction of sagittal alignment represented a significant risk factor for postoperative sagittal imbalance. Berjano et al.12 reported that insufficient correction was observed in all patients undergoing revision surgery for postoperative sagittal malalignment. Interestingly, Sebaaly et al.28 reported that moderate PJK (10° < PJA < 20°) was observed with undercorrection of the sagittal balance and severe PJK (PJA ≥ 20°) with overcorrection. According to their findings, most PJK in our study was moderate, and insufficient correction might cause postoperative sagittal imbalance in the form of moderate PJK or dynamic sagittal imbalance rather than severe PJK or PJF.

What, then, determines the difference between proximal kyphosis and dynamic sagittal imbalance? Direct comparison between the 2 groups showed that proximal kyphosis was more commonly observed in female patients who had fusion ending at the thoracolumbar junction (T11–L1). Dynamic sagittal imbalance was observed in patients with undercorrection of a deformity, such as changes in LL of < 30°, postoperative PI-LL > 10°, and postoperative SVA ≥ 50 mm. Moreover, the degeneration of paravertebral muscles, including decreased total CSA and lower CSA-muscle/CSA-disc ratio, was significantly greater in the dynamic sagittal imbalance group than in the proximal kyphosis group. Based on our findings, we suggest that less correction of sagittal alignment, combined with more fusion levels and the degeneration of paravertebral muscles, resulted in progressive sagittal imbalance in the form of dynamic sagittal imbalance rather than proximal kyphosis.

Regarding the possible mechanism, long instrumented fusion with sacropelvic fixation eliminates any possibility of compensation in the fused area. However, insufficient correction of sagittal alignment might result in 2 different types of postoperative sagittal imbalance as a result of decompensation. The site of decompensation is different between static proximal kyphosis and dynamic sagittal imbalance: above the fusion level or through the hip joints, affected by degree of deformity correction and muscle weakness. Proximal kyphosis resulting from PJK might develop as segmental kyphosis could cause forward decompensation of segments above the fusion level in static form. However, in dynamic sagittal imbalance, the fused spine bends together with the pelvis, with the hip joints as the center of rotation (Fig. 4).

FIG. 4.
FIG. 4.

Two different types of postoperative sagittal imbalance. A: Static proximal kyphosis as segmental kyphosis above the fusion level. B: Dynamic sagittal imbalance without PJK, with the hip joint serving as the center of rotation. Figure is available in color online only.

Compared with proximal kyphosis, dynamic sagittal imbalance with less correction of sagittal alignment and more fusion levels increases the mechanical demands of pelvis extension to compensate for sagittal imbalance. Failure of compensatory mechanisms because of hip and back extensor muscle fatigue and weakness may cause forward leaning in the form of dynamic sagittal imbalance before the development of PJK.29 In this regard, PJK did not develop during follow-up in patients with dynamic sagittal imbalance in this study. Progressive decompensation for insufficient correction because of weakness in the hip and back muscles is a key mechanism for dynamic sagittal imbalance. Lee et al.15 also reported that postoperative sagittal imbalance different from PJK developed after PSO for DSI in 20% of patients, resulting from extensor muscle weakness. In our study, the weakness of the paravertebral muscles was significantly greater in dynamic sagittal imbalance than in proximal kyphosis. This finding was consistent with previous reports that paravertebral muscles were significantly associated with dynamic parameters of spinopelvic and lower-limb joints obtained by 3D gait analysis.16,30

Clinical outcomes were also different between the 3 groups. Patients in the balanced group reported that leg VAS, back VAS, and ODI scores were significantly improved at the last follow-up. However, only the back VAS score was significantly improved in the dynamic sagittal imbalance group (5.3 vs 7.5), with no clinical improvement in the proximal kyphosis group. The back VAS score was highest in the proximal kyphosis group, followed by the dynamic sagittal imbalance group and the balanced group (6.2 ± 2.7 vs 5.3 ± 2.8 vs 4.1 ± 2.8, respectively). Direct comparison between the 2 different kyphoses showed significant ODI improvement in the dynamic sagittal imbalance group compared with the proximal kyphosis group at last follow-up (−10.1% ± 26.8% vs 1.2% ± 21.2%, p = 0.038). Hassanzadeh et al.6 also reported that patients with PJK had significantly higher ODI than did those without PJK (43.4% vs 20.4%, p < 0.01).

Moreover, 4 patients in the proximal kyphosis group required extension of posterior spinal fusion within 1 year after the index surgery, whereas no revision surgery was done in the balanced or dynamic sagittal imbalance groups. Previous studies also demonstrated that PJK resulted in worse clinical outcomes, including local pain and revision surgery.7,31 In our study, clinical outcomes in the dynamic sagittal imbalance group were superior to those in the proximal kyphosis group. This disparity may arise because dynamic sagittal imbalance developed as malalignment due to weakness of the hip and back extensor muscles, whereas proximal kyphosis worsened the clinical outcomes as local disruption and failure developed in the form of PJK.

This study had some limitations. First, longitudinal evaluation of radiological parameters was not done in this study, although we did measure the parameters, including the paravertebral muscles, on preoperative MRI. Because progressive decompensation due to weakness of the hip and back muscles is a key mechanism for dynamic sagittal imbalance, a prospective study with longitudinal evaluation would have been helpful in understanding the mechanism of the 2 different types of postoperative sagittal imbalance. Second, this study was a retrospective review, the cohorts included were relatively small, and selection bias might have influenced the results. Finally, we found that sufficient correction in long instrumented fusion for DSI was important to prevent 2 different types of postoperative sagittal imbalance. However, optimal correction of deformity is still debated. Further prospective studies with long-term follow-up are needed to validate our findings and to provide the criteria for optimal correction by deformity surgery. Despite these limitations, the strength of this study is that it is the first to evaluate spinopelvic parameters and paravertebral muscles as risk factors for 2 different sagittal imbalances.

Conclusions

Insufficient correction of sagittal alignment caused 2 different types of postoperative sagittal imbalance as a result of decompensation. Compared with proximal kyphosis, dynamic sagittal imbalance might develop in the form of progressive decompensation through the hip joints due to weakness of the hip and back muscles in patients who had less correction of sagittal alignment, in conjunction with more fusion levels and atrophy of the paravertebral muscles. Clinical outcomes in the dynamic sagittal imbalance group were superior to those in the proximal kyphosis group. Optimal correction considering the risk factors of the 2 different types of postoperative sagittal imbalance should be conducted in long instrumented fusion to the sacrum for DSI.

Acknowledgments

This work was supported by a Small Grant for Exploratory Research (SGER) through the Ministry of Education of the Republic of Korea and the Catholic University of Korea Songeui (2018R1D1A1A02049202).

Disclosures

The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

Author Contributions

Conception and design: Ha. Acquisition of data: Park, SI Kim, Han. Analysis and interpretation of data: Park, SI Kim, Han. Drafting the article: Park. Critically revising the article: Ha, YH Kim. Reviewed submitted version of manuscript: Ha, Park, YH Kim. Approved the final version of the manuscript on behalf of all authors: Ha. Statistical analysis: Park, SI Kim. Study supervision: Ha, YH Kim.

References

  • 1

    Glassman SD, Bridwell K, Dimar JR, et al. The impact of positive sagittal balance in adult spinal deformity. Spine (Phila Pa 1976). 2005;30(18):20242029.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 2

    Lafage V, Schwab F, Patel A, et al. Pelvic tilt and truncal inclination: two key radiographic parameters in the setting of adults with spinal deformity. Spine (Phila Pa 1976). 2009;34(17):E599E606.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 3

    Chang DG, Ha KY, Kim YH, Lee EW. Spinopelvic alignment by different surgical methods in the treatment of degenerative sagittal imbalance of the lumbar spine. Clin Spine Surg. 2017;30(4):E390E397.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 4

    Kim HJ, Iyer S. Proximal junctional kyphosis. J Am Acad Orthop Surg. 2016;24(5):318326.

  • 5

    Glattes RC, Bridwell KH, Lenke LG, et al. Proximal junctional kyphosis in adult spinal deformity following long instrumented posterior spinal fusion: incidence, outcomes, and risk factor analysis. Spine (Phila Pa 1976). 2005;30(14):16431649.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 6

    Hassanzadeh H, Gupta S, Jain A, et al. Type of anchor at the proximal fusion level has a significant effect on the incidence of proximal junctional kyphosis and outcome in adults after long posterior spinal fusion. Spine Deform. 2013;1(4):299305.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 7

    Kim HJ, Bridwell KH, Lenke LG, et al. Proximal junctional kyphosis results in inferior SRS pain subscores in adult deformity patients. Spine (Phila Pa 1976). 2013;38(11):896901.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 8

    Kim YJ, Bridwell KH, Lenke LG, et al. Proximal junctional kyphosis in adult spinal deformity after segmental posterior spinal instrumentation and fusion: minimum five-year follow-up. Spine (Phila Pa 1976). 2008;33(20):21792184.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 9

    Lau D, Clark AJ, Scheer JK, et al. Proximal junctional kyphosis and failure after spinal deformity surgery: a systematic review of the literature as a background to classification development. Spine (Phila Pa 1976). 2014;39(25):20932102.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 10

    Lee JH, Kim JU, Jang JS, Lee SH. Analysis of the incidence and risk factors for the progression of proximal junctional kyphosis following surgical treatment for lumbar degenerative kyphosis: minimum 2-year follow-up. Br J Neurosurg. 2014;28(2):252258.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 11

    Yagi M, Rahm M, Gaines R, et al. Characterization and surgical outcomes of proximal junctional failure in surgically treated patients with adult spinal deformity. Spine (Phila Pa 1976). 2014;39(10):E607E614.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 12

    Berjano P, Bassani R, Casero G, et al. Failures and revisions in surgery for sagittal imbalance: analysis of factors influencing failure. Eur Spine J. 2013;22(suppl 6):S853S858.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13

    Cho KJ, Suk SI, Park SR, et al. Risk factors of sagittal decompensation after long posterior instrumentation and fusion for degenerative lumbar scoliosis. Spine (Phila Pa 1976). 2010;35(17):15951601.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 14

    Kuhns CA, Bridwell KH, Lenke LG, et al. Thoracolumbar deformity arthrodesis stopping at L5: fate of the L5-S1 disc, minimum 5-year follow-up. Spine (Phila Pa 1976). 2007;32(24):27712776.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 15

    Lee CS, Lee CK, Kim YT, et al. Dynamic sagittal imbalance of the spine in degenerative flat back: significance of pelvic tilt in surgical treatment. Spine (Phila Pa 1976). 2001;26(18):20292035.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 16

    Arima H, Yamato Y, Hasegawa T, et al. Discrepancy between standing posture and sagittal balance during walking in adult spinal deformity patients. Spine (Phila Pa 1976). 2017;42(1):E25E30.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 17

    Son SM, Shin JK, Goh TS, et al. Predictive findings of the presence of stooping in patients with lumbar degenerative kyphosis by upright whole spine lateral radiography. Spine (Phila Pa 1976). 2018;43(8):571577.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 18

    Moon MS, Lee H, Kim ST, et al. Spinopelvic orientation on radiographs in various body postures: upright standing, chair sitting, Japanese style kneel sitting, and Korean style cross-legged sitting. Clin Orthop Surg. 2018;10(3):322327.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 19

    Schwab F, Lafage V, Patel A, Farcy JP. Sagittal plane considerations and the pelvis in the adult patient. Spine (Phila Pa 1976). 2009;34(17):18281833.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 20

    Schwab F, Patel A, Ungar B, et al. Adult spinal deformity-postoperative standing imbalance: how much can you tolerate? An overview of key parameters in assessing alignment and planning corrective surgery. Spine (Phila Pa 1976). 2010;35(25):22242231.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 21

    Ntilikina Y, Bahlau D, Garnon J, et al. Open versus percutaneous instrumentation in thoracolumbar fractures: magnetic resonance imaging comparison of paravertebral muscles after implant removal. J Neurosurg Spine. 2017;27(2):235241.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 22

    Käser L, Mannion AF, Rhyner A, et al. Active therapy for chronic low back pain: part 2. Effects on paraspinal muscle cross-sectional area, fiber type size, and distribution. Spine (Phila Pa 1976). 2001;26(8):909919.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 23

    Marras WS, Jorgensen MJ, Granata KP, Wiand B. Female and male trunk geometry: size and prediction of the spine loading trunk muscles derived from MRI. Clin Biomech (Bristol, Avon). 2001;16(1):3846.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 24

    Hyun SJ, Bae CW, Lee SH, Rhim SC. Fatty degeneration of the paraspinal muscle in patients with degenerative lumbar kyphosis: a new evaluation method of quantitative digital analysis using MRI and CT scan. Clin Spine Surg. 2016;29(10):441447.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 25

    Kim CY, Lee SM, Lim SA, Choi YS. Impact of fat infiltration in cervical extensor muscles on cervical lordosis and neck pain: a cross-sectional study. Clin Orthop Surg. 2018;10(2):197203.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 26

    Pfirrmann CW, Metzdorf A, Zanetti M, et al. Magnetic resonance classification of lumbar intervertebral disc degeneration. Spine (Phila Pa 1976). 2001;26(17):18731878.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 27

    Cho KJ, Kim KT, Kim WJ, et al. Pedicle subtraction osteotomy in elderly patients with degenerative sagittal imbalance. Spine (Phila Pa 1976). 2013;38(24):E1561E1566.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 28

    Sebaaly A, Sylvestre C, El Quehtani Y, et al. Incidence and risk factors for proximal junctional kyphosis: results of a multicentric study of adult scoliosis. Clin Spine Surg. 2018;31(3):E178E183.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 29

    Ha KY, Kim SI, Kim YH, et al. Jack-knife posture after correction surgery for degenerative sagittal imbalance—does spinopelvic parameter always matter in preventing stooping posture? Spine Deform. 2018;6(6):771780.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 30

    Lee JH, Lee SH. Does lumbar paraspinal muscles improve after corrective fusion surgery in degenerative flat black? Indian J Orthop. 2017;51(2):147154.

  • 31

    Yagi M, King AB, Boachie-Adjei O. Incidence, risk factors, and natural course of proximal junctional kyphosis: surgical outcomes review of adult idiopathic scoliosis. Minimum 5 years of follow-up. Spine (Phila Pa 1976). 2012;37(17):14791489.

    • Crossref
    • Search Google Scholar
    • Export Citation

Illustration of S1 pedicle subtraction osteotomy for the treatment of sacral fractures and high-grade spondylolisthesis (spondy). Copyright University of California, San Francisco, Department of Neurosurgery. Published with permission. See the article by Lau et al. (pp 577–587).

  • View in gallery

    Lateral radiographs obtained in a 70-year-old woman in the proximal kyphosis group. A: Preoperative radiograph revealing DSI. B: Postoperative radiograph revealing sagittal correction. C: Subsequent radiograph revealing sagittal imbalance with PJK. D: Last follow-up radiograph revealing sagittal correction after the revision surgery.

  • View in gallery

    Lateral radiographs obtained in a 67-year-old woman in the dynamic sagittal imbalance group. A: Preoperative radiograph revealing DSI. B: Postoperative radiograph revealing sagittal correction. C: Last follow-up radiograph revealing dynamic sagittal imbalance without PJK.

  • View in gallery

    A: Axial T1-weighted MR image showing the measurement of the CSA of the paravertebral muscle and intervertebral disc on the right side at L3–4. B: Axial T2-weighted MR image showing the measurement of the amount of fatty change as the ratio between the mean signal intensity of the paravertebral muscle and subcutaneous fat at L3–4.

  • View in gallery

    Two different types of postoperative sagittal imbalance. A: Static proximal kyphosis as segmental kyphosis above the fusion level. B: Dynamic sagittal imbalance without PJK, with the hip joint serving as the center of rotation. Figure is available in color online only.

  • 1

    Glassman SD, Bridwell K, Dimar JR, et al. The impact of positive sagittal balance in adult spinal deformity. Spine (Phila Pa 1976). 2005;30(18):20242029.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 2

    Lafage V, Schwab F, Patel A, et al. Pelvic tilt and truncal inclination: two key radiographic parameters in the setting of adults with spinal deformity. Spine (Phila Pa 1976). 2009;34(17):E599E606.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 3

    Chang DG, Ha KY, Kim YH, Lee EW. Spinopelvic alignment by different surgical methods in the treatment of degenerative sagittal imbalance of the lumbar spine. Clin Spine Surg. 2017;30(4):E390E397.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 4

    Kim HJ, Iyer S. Proximal junctional kyphosis. J Am Acad Orthop Surg. 2016;24(5):318326.

  • 5

    Glattes RC, Bridwell KH, Lenke LG, et al. Proximal junctional kyphosis in adult spinal deformity following long instrumented posterior spinal fusion: incidence, outcomes, and risk factor analysis. Spine (Phila Pa 1976). 2005;30(14):16431649.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 6

    Hassanzadeh H, Gupta S, Jain A, et al. Type of anchor at the proximal fusion level has a significant effect on the incidence of proximal junctional kyphosis and outcome in adults after long posterior spinal fusion. Spine Deform. 2013;1(4):299305.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 7

    Kim HJ, Bridwell KH, Lenke LG, et al. Proximal junctional kyphosis results in inferior SRS pain subscores in adult deformity patients. Spine (Phila Pa 1976). 2013;38(11):896901.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 8

    Kim YJ, Bridwell KH, Lenke LG, et al. Proximal junctional kyphosis in adult spinal deformity after segmental posterior spinal instrumentation and fusion: minimum five-year follow-up. Spine (Phila Pa 1976). 2008;33(20):21792184.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 9

    Lau D, Clark AJ, Scheer JK, et al. Proximal junctional kyphosis and failure after spinal deformity surgery: a systematic review of the literature as a background to classification development. Spine (Phila Pa 1976). 2014;39(25):20932102.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 10

    Lee JH, Kim JU, Jang JS, Lee SH. Analysis of the incidence and risk factors for the progression of proximal junctional kyphosis following surgical treatment for lumbar degenerative kyphosis: minimum 2-year follow-up. Br J Neurosurg. 2014;28(2):252258.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 11

    Yagi M, Rahm M, Gaines R, et al. Characterization and surgical outcomes of proximal junctional failure in surgically treated patients with adult spinal deformity. Spine (Phila Pa 1976). 2014;39(10):E607E614.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 12

    Berjano P, Bassani R, Casero G, et al. Failures and revisions in surgery for sagittal imbalance: analysis of factors influencing failure. Eur Spine J. 2013;22(suppl 6):S853S858.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 13

    Cho KJ, Suk SI, Park SR, et al. Risk factors of sagittal decompensation after long posterior instrumentation and fusion for degenerative lumbar scoliosis. Spine (Phila Pa 1976). 2010;35(17):15951601.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 14

    Kuhns CA, Bridwell KH, Lenke LG, et al. Thoracolumbar deformity arthrodesis stopping at L5: fate of the L5-S1 disc, minimum 5-year follow-up. Spine (Phila Pa 1976). 2007;32(24):27712776.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 15

    Lee CS, Lee CK, Kim YT, et al. Dynamic sagittal imbalance of the spine in degenerative flat back: significance of pelvic tilt in surgical treatment. Spine (Phila Pa 1976). 2001;26(18):20292035.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 16

    Arima H, Yamato Y, Hasegawa T, et al. Discrepancy between standing posture and sagittal balance during walking in adult spinal deformity patients. Spine (Phila Pa 1976). 2017;42(1):E25E30.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 17

    Son SM, Shin JK, Goh TS, et al. Predictive findings of the presence of stooping in patients with lumbar degenerative kyphosis by upright whole spine lateral radiography. Spine (Phila Pa 1976). 2018;43(8):571577.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 18

    Moon MS, Lee H, Kim ST, et al. Spinopelvic orientation on radiographs in various body postures: upright standing, chair sitting, Japanese style kneel sitting, and Korean style cross-legged sitting. Clin Orthop Surg. 2018;10(3):322327.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 19

    Schwab F, Lafage V, Patel A, Farcy JP. Sagittal plane considerations and the pelvis in the adult patient. Spine (Phila Pa 1976). 2009;34(17):18281833.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 20

    Schwab F, Patel A, Ungar B, et al. Adult spinal deformity-postoperative standing imbalance: how much can you tolerate? An overview of key parameters in assessing alignment and planning corrective surgery. Spine (Phila Pa 1976). 2010;35(25):22242231.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 21

    Ntilikina Y, Bahlau D, Garnon J, et al. Open versus percutaneous instrumentation in thoracolumbar fractures: magnetic resonance imaging comparison of paravertebral muscles after implant removal. J Neurosurg Spine. 2017;27(2):235241.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 22

    Käser L, Mannion AF, Rhyner A, et al. Active therapy for chronic low back pain: part 2. Effects on paraspinal muscle cross-sectional area, fiber type size, and distribution. Spine (Phila Pa 1976). 2001;26(8):909919.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 23

    Marras WS, Jorgensen MJ, Granata KP, Wiand B. Female and male trunk geometry: size and prediction of the spine loading trunk muscles derived from MRI. Clin Biomech (Bristol, Avon). 2001;16(1):3846.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 24

    Hyun SJ, Bae CW, Lee SH, Rhim SC. Fatty degeneration of the paraspinal muscle in patients with degenerative lumbar kyphosis: a new evaluation method of quantitative digital analysis using MRI and CT scan. Clin Spine Surg. 2016;29(10):441447.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 25

    Kim CY, Lee SM, Lim SA, Choi YS. Impact of fat infiltration in cervical extensor muscles on cervical lordosis and neck pain: a cross-sectional study. Clin Orthop Surg. 2018;10(2):197203.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 26

    Pfirrmann CW, Metzdorf A, Zanetti M, et al. Magnetic resonance classification of lumbar intervertebral disc degeneration. Spine (Phila Pa 1976). 2001;26(17):18731878.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 27

    Cho KJ, Kim KT, Kim WJ, et al. Pedicle subtraction osteotomy in elderly patients with degenerative sagittal imbalance. Spine (Phila Pa 1976). 2013;38(24):E1561E1566.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 28

    Sebaaly A, Sylvestre C, El Quehtani Y, et al. Incidence and risk factors for proximal junctional kyphosis: results of a multicentric study of adult scoliosis. Clin Spine Surg. 2018;31(3):E178E183.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 29

    Ha KY, Kim SI, Kim YH, et al. Jack-knife posture after correction surgery for degenerative sagittal imbalance—does spinopelvic parameter always matter in preventing stooping posture? Spine Deform. 2018;6(6):771780.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 30

    Lee JH, Lee SH. Does lumbar paraspinal muscles improve after corrective fusion surgery in degenerative flat black? Indian J Orthop. 2017;51(2):147154.

  • 31

    Yagi M, King AB, Boachie-Adjei O. Incidence, risk factors, and natural course of proximal junctional kyphosis: surgical outcomes review of adult idiopathic scoliosis. Minimum 5 years of follow-up. Spine (Phila Pa 1976). 2012;37(17):14791489.

    • Crossref
    • Search Google Scholar
    • Export Citation

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