Abstract
BACKGROUND AND OBJECTIVES:
The Scoliosis Research Society (SRS)-Schwab system does not include a pelvic compensation (PC) subtype, potentially contributing to gaps in clinical characteristics and treatment strategy for deformity correction. It also remains uncertain as to whether PC has differing roles in sagittal balance (SB) or imbalance (SI) status. To compare radiological parameters and SRS-22r domains between patients with failed pelvic compensation (FPC) and successful pelvic compensation (SPC) based on preoperative SB and SI.
METHODS:
A total of 145 adult spinal deformity patients who received deformity correction were analyzed. Radiographic and clinical outcomes were collected for statistical analysis. Patients were classified into 4 groups based on the median value of PT/PI ratio (PTr) and the cutoff value of SB. Patients with low PTr and high PTr were defined as FPC and SPC, respectively. Radiographic and clinical characteristics of different groups were compared.
RESULTS:
Patients with SPC exhibited significantly greater improvements in lumbar lordosis, pelvic tilt, PTr, and T1 pelvic angle as compared to patients with FPC, irrespective of SB or SI. No apparent differences in any of SRS-22r domains were observed at follow-up when comparing the SB-FPC and SB-SPC patients. However, patients with SI-SPC exhibited significantly better function, self-image, satisfaction, and subtotal domains at follow-up relative to those with SI-FPC. When SI-FPC and SI-SPC patients were subdivided further based on the degree of PI-LL by adjusting for age, the postoperative function and self-image domains were significantly better in the group with overcorrection of PI-LL than undercorrection of PI-LL in SI-FPC patients. However, no differences in these SRS-22r scores were observed when comparing the subgroups in SI-SPC patients.
CONCLUSION:
Flexible pelvic rotation is associated with benefits to the correction of sagittal parameters, irrespective of preoperative SB or SI status. However, PC is only significantly associated with clinical outcomes under SI. Patients with SI-FPC exhibit poorer postoperative clinical outcomes, which should be recommended to minimize PI-LL.
KEY WORDS: Adult spinal deformity, Sagittal balance and imbalance, Failed pelvic compensation, Successful pelvic compensation
ABBREVIATIONS:
- ANOVA
analysis of variance
- ASD
adult spinal deformity
- FPC
failed PC
- LL
lumbar lordosis
- MCID
minimum clinically important difference
- PC
pelvic compensation
- PCC
pelvic compensatory capacity
- PI
pelvic incidence
- PROs
patient-reported outcomes
- PT
pelvic tilt
- PTr
PT/PI ratio
- SB
sagittal balance
- SI
sagittal imbalance
- SPC
successful PC
- SRS
Scoliosis Research Society
- SVA
sagittal vertical axis
- TK
thoracic kyphosis
- TPA
T1 pelvic angle.
For adult spinal deformity (ASD), correction efforts center on achieving an optimal sagittal balance (SB).1,2 Restoring an ideal sagittal alignment following corrective surgery is vital given that such alignment is closely tied to patient-reported outcomes (PROs).3-5 The Scoliosis Research Society (SRS)-Schwab classification system offers surgeons with guidance regarding corrective treatments.6,7 Recent work has increasingly highlighted the importance of comprehensively assessing whether pelvic compensation (PC) occurs during corrective surgery.8-11 However, PC is not taken into consideration in the SRS-Schwab system.
Early studies primarily described PC based on pelvic tilt (PT).8,12 However, as PT is proportional to constant pelvic incidence (PI), larger PI levels may inevitably coincide with a larger PT. To overcome the limitations inherent in this definition of PT, Kwon et al13 proposed describing failed pelvic compensation (FPC) based upon a low PT/PI ratio (PTr), whereas they defined successful PC (SPC) by a high PTr. However, these prior studies did not employ an exclusive system for the evaluation of spinal deformities, nor did they provide corresponding corrective strategies for specific subgroups. These past studies have also only assessed pelvic compensatory capacity (PCC) in the context of sagittal imbalance (SI). For ASD patients exhibiting SB, how PCC impacts postoperative spinal alignment and PROs also remains uncertain.
Given the persistent uncertainties regarding the clinical and radiographic characteristics associated with SB-FPC and -SPC and the absence of defined corrective targets for SI-FPC and -SPC, this study was developed to compare radiological parameters and SRS-22r domains among patients classified according to their SB/SI and FPC/SPC status, and to explore appropriate corrective strategies.
METHODS
Patient Selection
The Institutional Review Board of our institution (approval number: 2018014) permitted this study. All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee. All patients provided written informed consent to participate. Eligible patients were those individuals who had been diagnosed with ASD, were 18+ years of age, and exhibited one or more of the following: Cobb angle ≥20°, SVA ≥5 cm, PT ≥ 25°, and/or thoracic kyphosis (TK) ≥ 60°. A retrospective review of 280 consecutive patients who underwent the thoracolumbar fusion treatment of more than 4 spinal levels for ASD management at our institution between September 2018 and June 2021 was performed, and all patients for whom full-length standing radiographs and completed PRO data from before and 2 years after the operation were available were included in this study. Patients were excluded if their spinal deformities were associated with malignancies, sepsis, infections, musculoskeletal diseases, or a neuromuscular etiology.
Radiographic Measurement
Full-length freestanding posteroanterior and lateral spine radiographs were obtained for all patients. These scans were analyzed, and the mapping and identification of various anatomical landmarks were achieved using validated software11 (SpineView, ENSAM Laboratory of Biomechanics), enabling the calculation of the following parameters: TK, lumbar lordosis (LL), PT, PI, sagittal vertical axis (SVA), T1 pelvic angle (TPA), and Cobb angle (Figure 1A-1C),14 for which the respective interobserver correlation coefficients were 0.79, 0.76, 0.80, 0.78, 0.88, 0.81 and 0.78.
FIGURE 1.
Radiographic measurements of sagittal parameters. A, TK, LL, PT, and PI. B, SVA and TPA. C, Cobb angle. LL, lumbar lordosis; PI, pelvic incidence; PT, pelvic tilt; SVA, sagittal vertical axis; TK, thoracic kyphosis; TPA, T1 pelvic angle.
Patient Classification
Patients were classified into 4 groups according to their PTr values for evaluating PCC, and their SVA values for evaluating balance.13,15 FPC was defined by a PTr <0.53, which was the median value for patient cohort. SI was defined by an SVA >50 mm, which was the cutoff for imbalance. Representative cases from each are shown in Figure 2A-2D.
FIGURE 2.
Characteristics of full-length freestanding lateral spine radiographs of patients with adult spinal deformity at baseline and follow-up period. A, Group SB-FPC: low PT/PI ratio (PTr, <0.53) and low SVA (<50 mm). B, Group SB-SPC: high PTr (>0.53) and low SVA (<50 mm). C, Group SI-FPC: low PTr (<0.53) and high SVA (>50 mm). D, Group SI-SPC: high PTr (>0.53) and high SVA (>50 mm). FPC, failed pelvic compensation; PI, pelvic incidence; PT, pelvic tilt; PTr, PT/PI ratio; SB, sagittal balance; SI, sagittal imbalance; SPC, successful pelvic compensation; SVA, sagittal vertical axis.
Assessment of PROs
The 22-item SRS-22r questionnaire is specific to scoliosis-related PROs, and consists of 6 domains: function, pain, self-image, mental health, satisfaction, and subtotal,16 with each domain being scored from 1 to 5 where higher scores correspond to better patient outcomes.17,18 The minimum clinically important difference (MCID) values for the SRS-22r based on data from a Japanese cohort have previously been reported as follows: function = 0.90, pain = 0.85, self-image = 1.05, mental health = 0.70, and subtotal = 1.05.19 The rate of MCID achievement for all SRS-22r domains at 2 years post-surgery was calculated.20
Surgical Procedures
All corrective surgical procedures were performed by 2 experienced surgeons (SBL and CK) via an open approach, and all patients received >4-level fusion treatment. Corrective procedures employed: (1) a posterior approach that combines Smith-Peterson osteotomy with or without three-column osteotomy, or (2) a combined anterior and posterior approach that includes oblique lumbar interbody fusion and Smith-Peterson osteotomy with or without three-column osteotomy. Deformity correction efforts aimed to reduce the PI-LL to <10°. The sacrum or pelvis was the lowest instrumented vertebra, while the T9-L2 segments were the uppermost instrumented vertebrae. Interbody cage fusion was commonly performed for the lower lumbar segment. Deformity correction was conducted using bilateral 5.5-mm dual titanium rods.
Statistical Analysis
Data are reported as means ± SD (SD) or n (%), and continuous variables were compared using independent sample t-tests or one-way analysis of variance (ANOVA) with Tukey's or Dunnett's post hoc tests, as appropriate. Categorical variables were compared using χ2 tests. SPSS 20.0 (Inc) was used for all statistical testing, and P < .05 was the threshold of significance.
RESULTS
Baseline Patient Characteristics
Table 1 presents patient characteristics, including radiographic parameters and PROs. One hundred forty five satisfied the inclusion criteria, including 113 females and 32 males. The average age of patients was 71.03 years and the average body mass index was 24.87. Three-column osteotomy was performed in 55 patients (37.9%). Forty two patients (29.0%) had postoperative complications. Revision surgeries were performed in 17 patients (11.7%). Patients showed sagittal malalignment at baseline, with a mean TK of 28.08°, LL of 25.54°, PT of 26.27°, PI of 48.76°, PTr of 0.54, PI-LL of 23.21°, SVA of 71.28 mm, and TPA of 25.53°. Patients had worse SRS-22r domain scores preoperatively, with a mean function score of 2.84, pain score of 3.00, self-image score of 2.61, mental health score of 2.69, and a subtotal score of 2.79.
TABLE 1.
Patient Demographics
| Characteristic | Value |
|---|---|
| No. of patients | 145 |
| Age | 71.03 ± 8.58 |
| No. of female | 113 (77.9%) |
| BMI (kg/m2) | 24.87 ± 3.26 |
| Three-column osteotomy | 55 (37.9%) |
| Complications | 42 (29.0%) |
| Revision surgery | 17 (11.7%) |
| Radiographic parameters | |
| Baseline TK (°) | 28.08 ± 17.39 |
| Baseline LL (°) | 25.54 ± 18.30 |
| Baseline PT (°) | 26.27 ± 10.16 |
| Baseline PI (°) | 48.76 ± 10.01 |
| Baseline PTr | 0.54 ± 0.19 |
| Baseline PI−LL (°) | 23.21 ± 18.18 |
| Baseline SVA (mm) | 71.28 ± 66.94 |
| Baseline TPA (°) | 25.53 ± 11.92 |
| Patient-reported outcomes | |
| SRS-22r function | 2.84 ± 0.79 |
| SRS-22r pain | 3.00 ± 0.75 |
| SRS-22r self-image | 2.61 ± 0.85 |
| SRS-22r mental health | 2.69 ± 0.75 |
| SRS-22r subtotal | 2.79 ± 0.65 |
BMI, body mass index; LL, lumbar lordosis; PI, pelvic incidence; PT, pelvic tilt; PTr, PT/PI ratio; SRS-22r, Scoliosis Research Society-22r; SVA, sagittal vertical axis; TK, thoracic kyphosis; TPA, T1 pelvic angle.
Data are presented as the mean ± SD or as n (%).
Analyses of Radiological Parameters
The radiographic parameters for these patients at baseline and at follow-up were next analyzed using ANOVAs, as were the changes in these parameters from baseline (Table 2). At baseline, patients in the SB-FPC group had the highest LL and Cobb values as well as the lowest PI-LL relative to the 3 other groups. The SI-SPC group presented with the lowest LL and the highest PI-LL, PTr, SVA, and TPA values as compared to the other 3 groups. At follow-up, SVA values in the SB- and SI-FPC groups were markedly higher than those in the SB- and SI-SPC groups (SB-FPC vs SB-SPC, 23.26 vs 15.00 mm; SI-FPC vs SI-SPC, 52.80 vs 31.29 mm), respectively. Significantly better improvements in LL, PT, PTr, and TPA were also observed in the SB- and SI-SPC groups relative to the SB- and SI-FPC groups (SB-SPC vs SB-FPC, LL: 6.38° vs 1.89°; PT: 7.32° vs 0.32°; PTr: 16% vs 1%; TPA: 5.93° vs −0.29°; SI-SPC vs SI-FPC, LL: 22.87° vs 6.94°; PT: 9.43° vs 2.88°; PTr: 20% vs 6%; TPA: 16.22° vs 5.29°). Cobb angle values were reduced most dramatically in the SB-FPC group relative to the other groups, and both LL and PT improved most significantly in the SI-SPC group as compared to the other groups.
TABLE 2.
Comparison of Radiological Parameters Among Groups
| Variables | Low SVA | High SVA | Post hoc | ||
|---|---|---|---|---|---|
| Low PTr | High PTr | Low PTr | High PTr | ||
| SB-FPC (N = 39) | SB-SPC (N = 23) | SI-FPC (N = 39) | SI-SPC (N = 44) | ||
| Baseline | |||||
| TK (°) | 29.87 ± 16.05 | 30.54 ± 20.09 | 30.12 ± 16.00 | 23.38 ± 17.89 | |
| LL (°) | 41.46 ± 12.45 | 26.96 ± 15.26 | 27.89 ± 13.15 | 8.62 ± 13.57† | a***, b***, c***, e***, f*** |
| PI (°) | 49.83 ± 10.94 | 47.43 ± 8.73 | 48.28 ± 8.68 | 48.92 ± 11.04 | |
| PTr | 0.37 ± 0.12 | 0.64 ± 0.11 | 0.45 ± 0.13 | 0.71 ± 0.14† | a***, b*, c***, d***, f*** |
| PI−LL (°) | 8.37 ± 12.68 | 20.47 ± 16.26 | 20.39 ± 12.03 | 40.30 ± 13.82† | a**, b**, c***, e***, f*** |
| SVA (mm) | 12.64 ± 26.54 | 12.93 ± 26.13 | 97.41 ± 45.00 | 130.58 ± 55.55† | b***, c***, d***, e***, f** |
| TPA (°) | 14.75 ± 6.48 | 23.62 ± 7.51 | 23.80 ± 6.53 | 37.61 ± 10.64† | a***, b***, c***, e***, f*** |
| Cobb (°) | 26.32 ± 9.95 | 14.75 ± 11.78 | 14.86 ± 11.31 | 17.93 ± 11.34† | a**, b***, c** |
| Final follow-up | |||||
| TK (°) | 32.44 ± 14.18 | 28.44 ± 11.94 | 31.31 ± 12.29 | 30.08 ± 11.63 | |
| LL (°) | 43.35 ± 10.62 | 33.33 ± 8.69 | 34.82 ± 12.08 | 31.48 ± 10.62† | a**, b**, c*** |
| PT (°) | 18.66 ± 7.95 | 23.18 ± 8.57 | 18.92 ± 6.95 | 25.00 ± 7.50† | c**, f** |
| PTr | 0.36 ± 0.13 | 0.48 ± 0.15 | 0.39 ± 0.12 | 0.51 ± 0.12† | a**, c***, d*, f*** |
| PI−LL (°) | 6.59 ± 12.33 | 13.74 ± 10.86 | 13.55 ± 11.02 | 17.70 ± 10.30† | a*, b*, c*** |
| SVA (mm) | 23.26 ± 38.04 | 15.00 ± 34.66 | 52.80 ± 47.28 | 31.29 ± 37.77† | a*, b**, d**, f* |
| TPA (°) | 15.04 ± 6.99 | 17.70 ± 6.94 | 18.51 ± 8.57 | 21.39 ± 8.16† | c** |
| Cobb (°) | 9.99 ± 6.52 | 9.21 ± 7.19 | 8.28 ± 7.41 | 8.57 ± 6.71 | |
| Change from baseline | |||||
| TK (°) | 2.57 ± 7.59 | −2.10 ± 11.55 | 1.18 ± 10.49 | 6.69 ± 11.80† | e** |
| LL (°) | 1.89 ± 7.07 | 6.38 ± 14.17 | 6.94 ± 9.02 | 22.87 ± 12.18† | a*, c***, e***, f*** |
| PT (°) | −0.32 ± 5.25 | −7.43 ± 6.55 | −2.88 ± 4.75 | −9.43 ± 6.78† | a***, c***, d*, f*** |
| PTr | −0.01 ± 0.11 | −0.16 ± 0.15 | −0.06 ± 0.12 | −0.20 ± 0.15† | a***, c***, d*, f*** |
| SVA (mm) | 10.62 ± 37.59 | 2.07 ± 35.10 | −44.61 ± 49.65 | −99.29 ± 54.66† | b***, c***, d**, e***, f*** |
| TPA (°) | 0.29 ± 4.74 | −5.93 ± 7.65 | −5.29 ± 6.84 | −16.22 ± 9.04† | a**, b**, c***, e***, f*** |
| Cobb (°) | 16.33 ± 8.90 | 5.54 ± 6.82 | 6.57 ± 6.86 | 9.36 ± 8.09† | a***, b***, c*** |
FPC, failed pelvic compensation; LL, lumbar lordosis; PI, pelvic incidence; PT, pelvic tilt; PTr, PT/PI ratio; SB, sagittal balance; SI, sagittal imbalance; SPC, successful pelvic compensation; SVA, sagittal vertical axis; TK, thoracic kyphosis; TPA, T1 pelvic angle.
P < .05.
P < .01.
P < .001.
Indicate P (ANOVA) < .05.
Multiple post hoc comparison a: Group SB-FPC vs SB-SPC, b: Group SB-FPC vs SI-FPC, c: Group SB-FPC vs SI-SPC, d: Group SB-SPC vs SI-FPC, e: Group SB-SPC vs SI-SPC, f: Group SI-FPC vs SI-SPC.
Data are presented as the mean ± SD.
Analyses of PROs
SRS-22r domains at baseline and follow-up were analyzed using ANOVAs, as were the rates at which MCID was achieved for the various SRS-22r domains (Table 3). Patients in the SI-FPC and -SPC groups exhibited worse function, self-image, and subtotal scores relative to those in the SB-FPC and -SPC groups before surgery. No significant differences in any SRS-22r domains were observed at follow-up when comparing the SB-FPC and -SPC groups, although the function, self-image, satisfaction, and subtotal domain scores in the SI-SPC group were significantly better than those in the SI-FPC group. With respect to the rates of MCID, the SI-SPC and -FPC groups respectively exhibited the highest and lowest proportions of patients that achieved MCID for the function, self-image, and subtotal domains. However, patients with SB-FPC and -SPC performed similarly with respect to these domains.
TABLE 3.
Comparison of Patient-Reported Outcomes Among Groups
| SRS-22r Domains | Low SVA | High SVA | Post hoc | ||
|---|---|---|---|---|---|
| Low PTr | High PTr | Low PTr | High PTr | ||
| SB-FPC | SB-SPC | SI-FPC | SI-SPC | ||
| Baseline | |||||
| Function | 3.19 ± 0.61 | 3.21 ± 0.52 | 2.55 ± 0.83 | 2.60 ± 0.83† | b**, c**, d**, e** |
| Pain | 3.19 ± 0.71 | 3.13 ± 0.65 | 2.97 ± 0.75 | 2.81 ± 0.81 | |
| Self-image | 2.99 ± 0.61 | 3.06 ± 0.66 | 2.36 ± 0.88 | 2.27 ± 0.86† | b**, c***, d**, e** |
| Mental health | 2.85 ± 0.70 | 2.76 ± 0.65 | 2.65 ± 0.76 | 2.56 ± 0.83 | |
| Subtotal | 3.05 ± 0.56 | 3.04 ± 0.48 | 2.63 ± 0.68 | 2.56 ± 0.66† | b*, c**, d*, e* |
| Final follow-up | |||||
| Function | 3.52 ± 0.79 | 3.51 ± 0.73 | 3.05 ± 0.79 | 3.48 ± 0.74† | b*, d*, f* |
| Pain | 3.96 ± 0.50 | 3.84 ± 0.39 | 3.68 ± 0.55 | 3.77 ± 0.51 | |
| Self-image | 3.94 ± 0.53 | 4.07 ± 0.47 | 3.21 ± 0.75 | 3.74 ± 0.56† | b***, d***, f*** |
| Mental health | 3.48 ± 0.55 | 3.34 ± 0.63 | 3.25 ± 0.61 | 3.26 ± 0.77 | |
| Satisfaction | 3.67 ± 0.80 | 3.80 ± 0.69 | 3.50 ± 0.66 | 3.93 ± 0.59† | f* |
| Subtotal | 3.73 ± 0.43 | 3.69 ± 0.31 | 3.29 ± 0.50 | 3.56 ± 0.49† | b***, d**, f* |
| MCID achieved | P | ||||
| Function | 14 (35.9%) | 9 (39.1%) | 12 (30.8%) | 27 (61.4%) | .025 |
| Pain | 19 (48.7%) | 11 (47.8%) | 17 (43.6%) | 28 (63.6%) | .282 |
| Self-image | 24 (61.5%) | 15 (65.2%) | 12 (30.8%) | 32 (72.7%) | .001 |
| Mental health | 23 (59.0%) | 13 (56.5%) | 17 (43.6%) | 30 (68.2%) | .159 |
| Subtotal | 13 (33.3%) | 7 (30.4%) | 10 (25.6%) | 26 (59.1%) | .009 |
FPC, failed pelvic compensation; MCID, minimal clinically important difference; PTr, PT/PI ratio; SB, sagittal balance; SPC, successful pelvic compensation; SRS-22r, Scoliosis Research Society-22r; SVA, sagittal vertical axis.
P < .05.
P < .01.
P < .001.
Indicate P (ANOVA) < .05.
Multiple post hoc comparison a: Group SB-FPC vs SB-SPC, b: Group SB-FPC vs SI-FPC, c: Group SB-FPC vs SI-SPC, d: Group SB-SPC vs SI-FPC, e: Group SB-SPC vs SI-SPC, f: Group SI-FPC vs SI-SPC.
Bold P values indicate statistical significance.
Data are presented as the mean ± SD or as n (%).
Age-Adjusted PI-LL-Based Analyses of Differences in Radiological Parameters and PROs
The SI-FPC and -SPC groups were further subdivided based on the degree of LL correction relative to PI by adjusting for age, which uses following formula: (age-adjusted ideal PI-LL)-(postoperative PI-LL).4,21,22 The 3 groups were as follows: group U (undercorrection; <−10°), group I (ideal correction; 0° ± 10°), and group O (overcorrection; >10°). The radiological parameters, SRS-22r subdomains and the rates at which MCID was achieved for these different subgroups were then compared (Table 4). Most of the analyzed radiological parameters differed markedly between the subsets of patients in both the SI-FPC and SI-SPC groups. Notably, significant differences in the scores for the function and self-image domains also differed significantly among the 3 subgroups within the SI-FPC group. According to post hoc analyses, group U showed significantly poorer clinical outcomes than group O. However, no differences in these SRS-22r scores were observed when comparing the 3 patient subgroups in the SI-SPC group.
TABLE 4.
Comparison of Radiological Parameters and Patient-Reported Outcomes According to the Degree of LL Correction Relative to PI by Adjusting for Age
| Variables | SI-FPC | P | SI-SPC | P | ||||
|---|---|---|---|---|---|---|---|---|
| Group Ua (N = 11) | Group Ia (N = 20) | Group Oa (N = 8) | Group Ua (N = 15) | Group Ia (N = 19) | Group Oa (N = 10) | |||
| Radiological parameters | ||||||||
| TK (°) | 20.77 ± 7.74 | 33.27 ± 11.17 | 40.89 ± 10.24 | <.001 | 26.34 ± 10.81 | 30.12 ± 9.58 | 36.15 ± 15.40 | .157 |
| LL (°) | 23.51 ± 12.40 | 36.56 ± 5.68 | 46.05 ± 11.47 | <.001 | 25.22 ± 9.16 | 32.80 ± 11.17 | 39.00 ± 4.60 | .009 |
| PT (°) | 22.82 ± 5.00 | 18.77 ± 5.82 | 13.93 ± 9.09 | .018 | 30.31 ± 6.81 | 24.42 ± 5.44 | 17.41 ± 5.72 | .001 |
| PTr | 0.46 ± 0.08 | 0.38 ± 0.10 | 0.30 ± 0.15 | .009 | 0.58 ± 0.08 | 0.51 ± 0.11 | 0.41 ± 0.13 | .004 |
| PI−LL (°) | 26.42 ± 6.31 | 12.13 ± 5.05 | −0.59 ± 6.22 | <.001 | 28.57 ± 7.03 | 16.66 ± 4.20 | −2.54 ± 5.54 | <.001 |
| SVA (mm) | 65.20 ± 50.48 | 56.99 ± 43.65 | 25.27 ± 46.54 | .165 | 44.54 ± 44.86 | 28.16 ± 36.88 | 16.18 ± 17.43 | .105 |
| TPA (°) | 24.54 ± 7.59 | 17.57 ± 7.00 | 12.57 ± 9.14 | .006 | 28.19 ± 7.26 | 20.18 ± 5.73 | 12.74 ± 3.60 | <.001 |
| SRS-22r Domains | ||||||||
| Function | 2.67 ± 0.81 | 3.09 ± 0.64 | 3.48 ± 0.84 | .032 | 3.33 ± 0.83 | 3.57 ± 0.71 | 3.50 ± 0.69 | .442 |
| Pain | 3.50 ± 0.48 | 3.63 ± 0.58 | 4.03 ± 0.47 | .112 | 3.72 ± 0.43 | 3.73 ± 0.52 | 3.86 ± 0.63 | .465 |
| Self-image | 2.99 ± 0.73 | 3.23 ± 0.69 | 3.50 ± 0.75 | .047 | 3.53 ± 0.54 | 3.87 ± 0.59 | 3.80 ± 0.41 | .139 |
| Mental health | 3.11 ± 0.63 | 3.26 ± 0.60 | 3.43 ± 0.65 | .549 | 3.47 ± 0.65 | 3.14 ± 0.86 | 3.16 ± 0.77 | .604 |
| Satisfaction | 3.30 ± 0.70 | 3.45 ± 0.62 | 3.75 ± 0.71 | .490 | 4.00 ± 0.62 | 3.85 ± 0.67 | 3.99 ± 0.50 | .923 |
| Subtotal | 3.07 ± 0.50 | 3.30 ± 0.44 | 3.59 ± 0.52 | .069 | 3.51 ± 0.43 | 3.57 ± 0.58 | 3.58 ± 0.42 | .706 |
| MCID achieved | ||||||||
| Function | 2 (18.2%) | 6 (30%) | 4 (50%) | 10 (66.7%) | 13 (68.4%) | 4 (40%) | ||
| Pain | 4 (36.4%) | 7 (35%) | 6 (75%) | 9 (60%) | 14 (73.7%) | 5 (50%) | ||
| Self-image | 3 (27.3%) | 5 (25.0%) | 4 (50%) | 10 (66.7%) | 16 (84.2%) | 6 (60%) | ||
| Mental health | 4 (36.4%) | 9 (45%) | 4 (50%) | 10 (66.7%) | 15 (78.9%) | 5 (50%) | ||
| Subtotal | 2 (18.2%) | 4 (20%) | 4 (50%) | 9 (60%) | 13 (68.4%) | 4 (40%) | ||
FPC, failed pelvic compensation; LL, lumbar lordosis; MCID, minimal clinically important difference; PI, pelvic incidence; PT, pelvic tilt; PTr, PT/PI ratio; SI, sagittal imbalance; SPC, successful pelvic compensation; SRS-22r, Scoliosis Research Society-22r; SVA, sagittal vertical axis; TK, thoracic kyphosis; TPA, T1 pelvic angle.
Group U means offset value between the actual and age-adjusted ideal PI−LL < −10°, group I means offset value within ± 10°, and group O means offset value > 10°.
Bold P values indicate statistical significance.
Data are presented as the mean ± SD.
DISCUSSION
The results of this study indicated that patients with SPC were able to achieve better postoperative sagittal correction than patients with FPC irrespective of their SB or SI status. There were no apparent differences at follow-up for any SRS-22r domains when comparing patients with SB-SPC and -FPC. Nevertheless, in the condition of SI, PC was found to play a more important role in shaping operative outcomes. SI-FPC patients exhibited the lowest MCID achievement rates of patients in any group for the SRS-22r domains, which should be recommended to minimize PI-LL to the greatest extent possible when completing corrective procedures for these patients. Comparatively, SI-SPC patients had the highest proportion of achieved MCID in terms of function, self-image, and subtotal scores among 4 groups. These patients are thus the most suitable candidates for surgical correction, and are also better able to tolerate less stringent standards for PI-LL correction. These results offer insight into the characteristics of patients in the SB/SI-FPC and SB/SI-SPC categories, emphasizing differences in PCC in the context of SB and SI, while also providing valuable insight regarding viable operative strategies that can aid in the development of individualized treatment plans.
PTr and SVA Value-Based Classification
PT has been used to describe PCC in prior studies.8,12 As PT is a parameter that is posture-dependent, however, it is proportional to PI, which is a morphologically fixed and constant parameter. Those patients with a large PI may thus inevitably exhibit a larger PT. This suggests that effectively evaluating PC initiation necessitates simultaneous attention to both PT and PI. In order to mitigate these individual differences, PTr may thus represent a more effective parameter for use in distinguishing between subsets of patients. In prior reports, PCC has only been discussed in the context of SI.8,12 Among patients exhibiting preoperative SB, in contrast, the role of PCC has yet to be established. The present study thus assessed the radiographic parameters and PROs of ASD patients classified into subgroups according to their PTr and SVA values. The PTr cutoff used herein was 0.53, which was the median value for this cohort and was higher than the 25th percentile value of 0.4 reported by Ferrero et al,15 although it was below the median value of 0.68 proposed by Kwon et al.13 Prior studies assessing sagittal parameters in asymptomatic populations have demonstrated the gradual retroversion of the pelvis over the course of aging (Supplemental Digital Content 1, http://links.lww.com/NEU/E287).23 The PTr cutoff of 0.53 used in this study was higher than the maximum PTr of 0.36 observed among asymptomatic individuals with an average age of over 75 years, supporting the rationale for the use of a PTr >0.53 as a cut-off to define PC.
Characteristics of SB-FPC and -SPC
Based on our categorization, individuals in the SB-FPC group were patients with limited PCC even if their SVA values were within the normal range who had been included in this study owing to local coronal or mild sagittal deformities. These patients exhibited the greatest improvements in coronal Cobb angles but the least substantial improvements in sagittal parameters, with particularly small changes in PTr (1%), suggesting that PC was not initiated in these patients and that surgery was performed with the goal of remediating coronal deformities. Individuals in the SB-SPC group were patients who had developed PC to compensate for potential spinal malalignment. Relative to patients with SB-FPC, patients with SB-SPC exhibited greater recovery on their postoperative spino-pelvic alignment, despite their more severe preoperative sagittal deformity. Notably, patients in both of these groups exhibited similar performance for the SRS-22r domains at baseline and follow-up, and achieved MCID at similar rates, suggesting that the initiation of PC for individuals in SB did not have any significant effect on postoperative clinical outcomes.
Characteristics of SI-FPC and Treatment Planning
Kwon et al13 found that the postoperative Oswestry Disability Index and SVA were respectively inferior and larger among SI-FPC patients, and similar conclusions were also reported in studies of SI-FPC patients by Lafage et al12 and Smith et al.24 These findings also align well with the present results, as SI-FPC patients exhibited the largest postoperative SVA and the worst postoperative SRS-22r domains among patients in these 4 groups. No significant improvement in the spinal alignment of SI-FPC patients following ASD surgery were observed with respect to either regional or global sagittal parameters, much as was observed in the SB-FPC group. When SI-FPC patients were further subgrouped based on the degree of LL correction relative to PI by adjusting for age (Figure 3A-3C), relative to those patients with undercorrection of PI-LL, patients with overcorrection of PI-LL tended to experience more favorable clinical outcomes. Robust correction of PI-LL mismatch to the greatest extent possible may thus be necessary, potentially owing to the limited pelvic rotation ability in these patients such that SB restoration was only possible through LL over-correction. It is also vital to explore why these patients exhibit limited PCC in order to define the most optimal treatment strategy, as the underlying reasons are likely to be multifactorial. Prior studies have reported that patients with hip pathology, including hip osteoarthritis and dislocation, tend to exhibit a limited pelvic retroversion range.25,26 Other reports have found that neuromuscular disease and soft tissue degeneration have the potential to compromise PCC.27,28 Future studies should thus focus on further elucidating the mechanisms that underlie this limited PCC.
FIGURE 3.
A, Group U: A 57-year-old woman with SI-FPC underwent undercorrection of PI-LL (offset: −11.7°). Preoperative and 2-year postoperative whole-spine radiographs are shown. B, Group I: A 71-year-old woman with SI-FPC underwent ideal correction of PI-LL (offset: −4.6°). Preoperative and 2-year postoperative whole-spine radiographs are shown. C, Group O: A 80-year-old woman with SI-FPC underwent overcorrection of PI-LL (offset: 12.21°). Preoperative and 2-year postoperative whole-spine radiographs are shown. FPC, failed pelvic compensation; LL, lumbar lordosis; PI, pelvic incidence; SI, sagittal imbalance.
Characteristics of SI-SPC and Treatment Planning
Regarding changes in radiological parameters, Kwon et al13 first described SPC patients and indicated that they tend to exhibit better PT changes and lower postoperative SVA values relative to patients with FPC, in line with the present results irrespective of SB/SI status. Among the 4 groups in this study, SI-SPC patients also exhibited the most significant improvements in LL and TPA. This suggests that SI-SPC patients have the greatest potential for improved sagittal alignment. With respect to PROs, MCID analyses are often used as a stand-in for clinically significant PRO changes, with achieving postoperative MCID thus being a key metric for clinical efficacy.29 In this study, SI-SPC patients exhibited the highest rates of postoperative MCID achievement for the function, pain, and subtotal SRS-22r domains, suggesting that these patients exhibited the greatest odds of experiencing pronounced clinical improvements after surgery. While baseline SRS-22r domain scores in the SI-SPC group were associated with a relatively low point of departure, flexible pelvic retroversion provides a foundation for achieving superior postoperative clinical outcomes. This may result in a greater potential to achieve MCID for SRS-22r domains, in line with the conclusions published by Arima et al30 who found that patients with severe ASD are likely to achieve MCID for SRS-22r domains such that they should be prioritized when selecting the most appropriate therapeutic strategy. In line with SI-FPC, SI-SPC patients were also stratified according to the age-adjusted corrective PI-LL (Figure 4A-4C). No significant differences in any SRS-22r domains were observed irrespective of whether these patients exhibited undercorrection, ideal correction, or overcorrection of PI-LL. SI-SPC patients thus were also better able to tolerate the restoration of sagittal alignment such that less stringent correction standards, which were not associated with worse clinical outcomes.
FIGURE 4.
A, Group U: A 78-year-old woman with SI-SPC underwent undercorrection of PI-LL (offset: −11.16°). Preoperative and 2-year postoperative whole-spine radiographs are shown. B, Group I: A 77-year-old woman with SI-SPC underwent ideal correction of PI-LL (offset: 2.97°). Preoperative and 2-year postoperative whole-spine radiographs are shown. C, Group O: A 84-year-old man with SI-SPC underwent overcorrection of PI-LL (offset: 10.77°). Preoperative and 2-year postoperative whole-spine radiographs are shown. LL, lumbar lordosis; PI, pelvic incidence; SI, sagittal imbalance; SPC, successful pelvic compensation.
Limitations
This study has some limitations. Firstly, it is a retrospective design. Larger sample sizes and longer follow-up intervals will be important in future studies. Secondly, the pathophysiological causes and potential mechanisms of FPC were not examined at length. The weakness of the hip extensor muscles, sacroiliac joint degeneration, and motor neuron deterioration may be contributing factors responsible for FPC incidence,31 emphasizing a need for further research focused on this topic. Third, as PC initiation was associated with SB, this study excluded patients with severe coronal deformities, with all patients exhibiting a baseline coronal Cobb angle of less than 40°. Finally, utilizing an SVA threshold of 50 mm in our single-center study with a small sample size and specific racial demographic provides a more universal and practical approach for diverse settings. However, the consensus is now that SVA should be adjusted for age. Assessing SVA across various age groups allows for a more accurate representation of SI. A more standard cutoff value of PTr is also required to investigate whether PC occurs. Future research should consider these factors, exploring the uses of different age-adjusted SVA thresholds and normative PTr.
CONCLUSION
Flexible pelvic rotation was associated with better correction of radiographic outcomes irrespective of SB or SI. PC initiation may have no impact on clinical outcomes in patients exhibiting SB, whereas it played a more important role in the context of SI. Patients with SI-FPC experienced worse postoperative clinical outcomes, which should be recommended to minimize PI-LL during corrective surgery. However, SI-SPC patients have a greater likelihood of benefitting from surgical treatment and are better able to tolerate less stringent sagittal malalignment restoration.
Acknowledgments
Author Contributions: Shibao Lu and Wei Wang conceived and designed the study; Zheng Wang, Dongfan Wang, Sitao Zhang and Weiguo Zhu collected; Zheng Wang, Xinli Hu, Haojie Zhang, and Chao Kong analyzed and interpreted the patient data. Zheng Wang and Xiaolong Chen wrote the paper. All authors read and approved the final manuscript. The article described has not been submitted elsewhere for publication, in whole or in part. All authors read and approved the final manuscript.
Footnotes
Supplemental digital content is available for this article at neurosurgery-online.com.
Zheng Wang and Xiaolong Chen contributed equally to this work.
Contributor Information
Zheng Wang, Email: 626174106@qq.com.
Xiaolong Chen, Email: chensmalldragon@163.com.
Xinli Hu, Email: huxinli123@qq.com.
Haojie Zhang, Email: haojiezhang7@163.com.
Weiguo Zhu, Email: zwgspine@163.com.
Dongfan Wang, Email: wdfdoctor@126.com.
Sitao Zhang, Email: zhangsitao116@sina.com.
Chao Kong, Email: kong988500@163.com.
Funding
This study was funded by the National Key Research and Development Program of China (No. 2020YFC2004900), National Natural Science Foundation of China (H0608), Natural Science Foundation of China (No. 81672201, 81871794) and Beijing Hospitals Authority's Ascent Plan (Grant No. DFL20190802).
Disclosures
The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.
SUPPLEMENTAL DIGITAL CONTENT
Supplemental Digital Content 1. Table. The Mean Value of Pelvic Parameters by Age.
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Associated Data
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Supplementary Materials
Supplemental Digital Content 1. Table. The Mean Value of Pelvic Parameters by Age.