INTRODUCTION
Adult spinal deformity (ASD) is a debilitating condition associated with sagittal malalignment causing substantial pain and functional disability [1-4]. It is well known that increased sagittal deformity leads to worse health-related quality of life [5-7]. Therefore, the optimal restoration of spinopelvic malalignment has been a cornerstone of surgical management for ASD for achieving good clinical outcomes [8,9]. Several authors have suggested the optimal surgical targets, including Scoliosis Research Society (SRS)-Schwab classification, age-adjusted sagittal alignment goals, and Global Alignment and Proportion (GAP) score [10-12]. Although these systems have their own correction targets, the common determinant factors are patient’s age and pelvic incidence (PI). Therefore, the current guidelines will propose the same surgical target without considering of the severity of baseline sagittal imbalance in patients of the same age and PI.
ASD is a disease entity with a wide spectrum of severity. For patients with mild sagittal deformity, only a small gap exists between the current sagittal imbalance status and the surgical target. Therefore, less-morbid surgery may be sufficient to achieve optimal sagittal correction. In contrast, patients with severe sagittal imbalance will have a larger gap to the desired correction target from the current deformity status, frequently necessitating more complicated surgery, thereby increasing the perioperative surgical burden such as operation time, perioperative morbidity, and length of hospital stay [13]. However, it is undetermined how the effect of severity of baseline sagittal imbalance on the clinical outcomes after corrective surgery for ASD remained undetermined. We hypothesized the clinical outcomes would not be inferior, even in patients with severe baseline sagittal imbalance, if the correction was performed successfully. In the current study, we aimed to determine the clinical impact of the baseline sagittal imbalance severity by comparing various perioperative and postoperative outcomes among the patients with mild, moderate, and severe baseline sagittal imbalances.
MATERIALS AND METHODS
This study was approved by the Institutional Review Board of Samsung Medical Center (2024-03-027). The requirement for informed consent was waived due to the retrospective nature of this study.
1. Study Cohort
This was a retrospective case series study based on records retrieved from a prospective ASD database at Samsung Medical Center. The study cohort included consecutive patients who underwent surgery for degenerative-type ASD between 2012 and 2021. Patient inclusion criteria were as follows: ≥ 60 years of age; ASD radiographically defined by C7 sagittal vertical axis (SVA) ≥ 50 mm, PI–lumbar lordosis (LL) mismatch ≥ 10°, or pelvic tilt (PT) ≥ 25° or coronal Cobb angle ≥ 30°; and ≥ 5 fused vertebral levels from the sacrum, all including the pelvis with iliac fixation. The severity of baseline sagittal imbalance was determined based on the SRS-Schwab classification. The SRS-Schwab classification consists of 3 sagittal modifiers of PI–LL mismatch, SVA, and PT [12]. Each sagittal modifier was graded as 0 (< 10°), + (10°–20°), ++ (> 20°) for PI–LL mismatch, 0 (< 40 mm), + (40–95 mm), ++ (> 95 mm) for SVA, and 0 (< 20°), + (20°–30°), ++ (> 30°) for PT. Scores were assigned to each item of the sagittal modifiers, for example, 0 points for grade 0; 1 point for grade +; and 2 points for grade ++. By modifying the previously reported categorization of baseline sagittal imbalance using the SRS-Schwab classification [14,15], patients were classified into 3 groups: mild (score: 1 or 2 points), moderate (score: 3 or 4 points), and severe (score: 5 or 6 points). No patients had a total score of 0 point.
More than 2-years of follow-up with complete radiographic, and patient-reported outcome measure (PROM) data were required for inclusion. Patients were excluded if they lacked appropriate radiographs; had not completed the PROM questionnaire at the final follow-up; had undergone previous thoracic or lumbar fusion surgery; or had syndromic, neuromuscular, inflammatory, or other pathological, rather than degenerative, conditions.
2. Collected Data
The demographic data included age, sex, body mass index (BMI), T score, and American Society of Anesthesiologists (ASA) physical status classification grade. Variables related to the surgical technique included total fusion level, oblique lumbar interbody fusion (OLIF), anterior column realignment (ACR), usage of additional rods, cement augmentation in uppermost instrumented vertebra (UIV), and 3-column osteotomy. The perioperative variables included operation time, estimated blood loss, number of red blood cells (RBCs) transfused, intensive care unit (ICU) admission, length of hospital stay, incidence and causes of return to the operating room during the hospital stay, and postoperative medical complications.
Standing posteroanterior and lateral whole-spine radiographs were analyzed at baseline and immediately after surgery (approximately 1 week postoperatively) to measure the following radiographic parameters: PI, LL, PI–LL mismatch, sacral slope (SS), PT, thoracic kyphosis (TK), T1 pelvic angle (TPA), and SVA. For the posteroanterior and lateral whole-spine radiographs, all patients positioned their hands on their shoulders. In addition to postoperative comparison of absolute values of sagittal parameters, the appropriateness of surgical correction was evaluated with regard to how much the postoperative sagittal alignment met the correction target of the legacy systems such as SRS-Schwab classification, age-adjusted sagittal alignment goals, and GAP score [10-12]. SRS-Schwab classification was previously described in study cohort section. The ideal age-adjusted PI–LL was calculated using a previously reported formula: PI–LL= (age–55 years)/2+3 [16]. Then, based on the offset value between actual PI–LL and ideal PI–LL values, the patients were divided into the following 3 groups: undercorrection (offset > 10°), matched correction (offset within± 10°), and overcorrection (offset < -10°). Finally, the GAP score is expressed as the total score of relative pelvic version, relative lumbar, lordosis distribution index, relative spinopelvic alignment, and age, ranging from 0 to 13 points [10]. Three groups were created according to the total score as follows: proportioned (score, 0−2), moderately disproportioned (score, 3−6), and severely disproportioned (score, ≥ 7).
Mechanical failures such as proximal junctional complications and rod fractures were recorded. Proximal junctional kyphosis (PJK) was defined as a proximal junctional angle (PJA) of ≥ 10° and increase of PJA ≥ 10° compared to preoperative PJA [17]. Proximal junctional failure (PJF) indicated fracture at the UIV or UIV+1, failure of UIV fixation, myelopathy, or any reasons of revision surgery [17].
Clinical outcomes were compared using 3 PROM questionnaires, namely, the visual analogue scale (VAS) for the back pain, Oswestry Disability Index (ODI), and the SRS-22 questionnaire (SRS-22) scores. Preoperative and final PROM questionnaires were used for analysis. In addition, we compared the proportion of patients achieving minimal clinically important difference (MCID) in VAS, ODI, and SRS-22 at the last follow-up. The MCID values used in the current study were 1.2 for VAS, 12.8 for ODI [18]. For SRS-22, the MCID values were 1.05 for function, 0.85 for pain, 1.05 for appearance, 0.70 for mental, and 1.05 for subtotal, respectively [19].
3. Statistical Analysis
Data are presented as frequencies with percentages for categorical variables and as means with standard deviations for continuous variables. Comparisons of variables among the 3 groups were performed using chi-square or Fisher exact tests for categorical variables and analysis of variance with a post hoc test (Tukey test) for continuous variables. Statistical analyses were conducted by professional statisticians using IBM SPSS Statistics ver. 27.0 (IBM Co., Armonk, NY, USA). A p-value < 0.05 was considered statistically significant.
RESULTS
A total of 259 patients met the inclusion criteria and were included in the study cohort. The mean age was 69.0 years and 225 patients (86.9%) were female. There were 42, 62, and 155 patients in the mild, moderate, and severe groups, respectively (Table 1). There were more female patients, and the T score was significantly less, in the severe group. There were no differences in age, BMI, or ASA physical status classification grade among the 3 groups. With regard to operative variables, the number of fusion levels differed significantly among the 3 groups (6.1, 7.2, and 7.5, respectively; p= 0.001). Significantly more patients underwent OLIF surgery at L5–S1, ACR, cement augmentation in UIV, and 3-column osteotomy as the severity of baseline sagittal imbalance increased (p= 0.048, p< 0.001, p= 0.041, and p= 0.001, respectively). The operation time, total number of RBC transfusion, and number of patients requiring ICU care were significantly greater in the severe group (p = 0.004, p = 0.031, and p= 0.027, respectively). The length of hospital stay was significantly longer in the severe group than in the mild group (p= 0.049). None of the patients in the mild group required revision surgery during their hospital stay; however, the inpatient revision rate was not statistically significant. There were no cases of revision surgery due delayed complication other than PJF or rod fracture after discharge in both groups. Moreover, there were no significant differences in postoperative medical complications among the 3 groups.
With regard to radiographic parameters, the PI was significantly smaller in the mild group than in the severe group (50.9° vs. 55.2°, p = 0.023) (Table 2). Other preoperative sagittal parameters showed significant differences among the 3 groups in terms of LL, PI–LL, SS, PT, TK, TPA, and SVA. There were no significant differences in the postoperative LL, SS, PT, TPA, or SVA. However, the postoperative PI–LL mismatch was significantly greater in the severe group (2.6°, 4.9°, and 8.4°, respectively; p= 0.003), and the postoperative TK was the smallest in the severe group. Postoperative changes in all sagittal parameters were significantly greater in the severe group. With regard to the SRS-Schwab classification, significantly more patients achieved a sagittal modifier grade 0 of PI–LL mismatch in the mild group than the other groups (p= 0.033) (Fig. 1). However, there were no differences in number of patients with regard to sagittal modifier grades of PT or SVA. There were more patients with undercorrection relative to age-adjusted PI–LL targets in the severe group (Fig. 2). No significant differences were found in patient distribution relative to the GAP score (Fig. 3).
There was a trend of increasing PJK and PJF as the baseline severity increased (Table 3). However, no statistically significant differences were found among the 3 groups in terms of PJK and PJF development or revision surgery for PJF (p= 0.270, p= 0.162, and p= 0.799, respectively). The incidence of rod fractures, as well as the revision rate for rod fractures, did not differ among the 3 groups (p= 0.569 and p= 0.265, respectively).
There were no significant differences in the preoperative VAS scores for back pain, ODI, or SRS-22 scores among the 3 groups (Table 4). There were also no differences in the scores at the last follow-up or their postoperative changes in the VAS scores for back pain and SRS-22 scores However, the ODI score at the last follow-up was significantly lower in the mild group than in the severe group (29.6 vs. 37.0, p= 0.019). ODI improvement was also higher in the mild group then in the other groups (28.5, 17.8, and 20.4, respectively; p = 0.029). Regarding the MCID, there were no significant differences in the number of patients to achieve MCID in VAS, ODI, and all components of SRS-22 such as activity, pain, appearance, mental, and subtotal domains (Table 5). In subgroup analyses, a significantly higher proportion of patients in the mild group achieved the MCID in the ODI compared to the moderate group (78.6% vs. 56.5%, p=0.02). Similarly, a greater percentage of patients in the severe group reached MCID in the appearance score of the SRS-22 questionnaire than in the moderate group (78.1% vs. 56.5%, p= 0.025). In the severe group without rod fracture, patients with undercorrection exhibited a higher ODI score at the last follow-up than those with matched or overcorrection; however, this difference was not statistically significant, likely due to the small sample size (41.8 vs. 34.0, p= 0.063), and the SRS-22 total score at the last follow-up was significantly lower in patients with undercorrection compared to those with matched or overcorrection (2.8 vs. 3.5, p= 0.013).
DISCUSSION
Given that a positive sagittal imbalance leads to poor clinical outcome [6,7], optimal restoration of spinopelvic malalignment is a key factor in achieving good clinical outcomes. Considering that the alignment target is largely determined by the patient’s PI and age, patients with a more severe sagittal imbalance may require a more aggressive surgical strategy to reach the desired surgical target. In the current study, we observed that patients in the severe group had a higher probability of undergoing more invasive surgeries, such as OLIF at L5–S1, ACR, and 3-column osteotomy. Neuman et al. [13] reported a surgical invasiveness threshold to predict the odds of major complication. They found that surgical variables, such as 3-column osteotomy, anterior interbody fusion (vs. posterior interbody fusion), iliac fixation, and revision surgery, significantly increased the risk of surgical and medical complications. Samuel et al. [2]0 conducted a similar study to investigate perioperative morbidity. They observed that a longer operative time was a better predictor of inpatient complications than surgical invasiveness itself. Song et al. [21] also reported that operation time was associated with a higher rate of 30-day morbidity and blood transfusion. In the current study, the operation time was significantly longer in the severe group than in the mild group, but the gap between groups was not large with just 1.6 hours. In the current study, there were no cases of return to the operating room due to inpatient surgical complications in the mild group. Although approximately 5% of patients in the moderate and severe groups required revision surgery during their hospital stay, all complications were treated successfully, leaving no permanent deficit. There were no significant differences in medical complications between the groups. Therefore, although surgery in the severe group increased the surgical burden with regard to surgical invasiveness and inpatient morbidity, the complication rate and its treatability were within acceptable ranges.
In the current study, the severity of baseline sagittal imbalance and postoperative changes of all sagittal parameters were clearly distinguished among the 3 groups. We observed that PJK and PJF developed less frequently in the mild group than in the other groups. However, no statistical significance was found for the development of PJK and PJF or revision surgery. The severity of baseline sagittal imbalance and subsequent postoperative greater change in sagittal deformity are known risk factors for PJK and PJF development [22-25]. However, the appropriateness of postoperative sagittal correction is equally crucial. Our findings indicate no significant differences in achieving matched correction postoperatively among mild, moderate, and severe groups. Furthermore, the severe group had a lower incidence of overcorrection, compared to mild and moderate groups. Considering that overcorrection increases PJF risk [9,11,25], the lower rate of postoperative overcorrection in severe group could have decreased the incidence of PJF. However, several studies have published contradictory results showing that the amount of correction or the final sagittal alignment did not affect PJK or PJF development [26,27]. Further follow-up studies are required to clarify this discrepancy. The incidence of rod fractures showed a trend similar to that of PJK and PJF. The incidence of rod fracture and the revision rate were lowest in the mild group, but these results did not reach statistical significance in the Fisher exact test or in the intergroup subanalyses. It is currently understood that mechanical failure after ASD surgery is closely associated with the shape of sagittal alignment such as the GAP score, rather than the absolute value of radiographic parameters [10,28,29]. In the current study, we found that GAP score categories did not differ among the groups; therefore, our findings can explain the negative findings of mechanical complication occurrence among the 3 groups.
We observed the greatest postoperative PI–LL value, the fewest patients achieving grade 0 in the SRS-Schwab PI–LL modifier, and the more patients with undercorrection relative to the age-adjusted PI–LL in the severe group. It is well known that undercorrection has been associated with poor clinical outcomes in ASD surgery [8,9,30]. Our findings indicate that a higher proportion of patients in the severe group experienced undercorrection after surgery and had significantly elevated ODI scores and SRS-22 total scores at the final follow-up, consistent with previous studies. The optimal restoration of sagittal malalignment is crucial for success after ASD surgery. Lee et al. [8] reported that the strict correction relative to all sagittal modifiers of SRS-Schwab classification ensures better SRS-22 scores even in a long-term follow-up of 90.3 months. Park et al. [9,25] also demonstrated that matched correction relative to the age-adjusted PI–LL target is necessary to achieve good clinical outcomes and to reduce PJK development. Patients with severe sagittal imbalance are likely to be undercorrected compared to those with mild and moderate sagittal imbalance. Therefore, greater efforts are required to achieve adequate correction as the severity of baseline sagittal imbalance increases.
This study has a few limitations. First, an inherent limitation of this study is the retrospective nature, which allows for the possibility of selection bias. Second, the results of this study may lack the generalizability considering the heterogeneous nature of patients with ASD because we only included patients with degenerative-type ASD. However, we applied strict inclusion criteria, such as narrow age group (≥ 60 years), main preoperative diagnosis with sagittal imbalance, and pelvic fixation in all cases, during patient selection to reduce such heterogeneity. Third, there was no investigation regarding the history of lower limb joint replacement surgery. Severe knee arthritis and similar conditions necessitating lower limb joint replacement can impact compensatory mechanisms in patients with ASD. However, we routinely check the range of motion of hip or knee, and patients with severe flexion contracture in these joints, irrespective of undergoing total joint surgery, were not included. Finally, we adopted the SRS-Schwab classification to group patients according to the baseline severity of sagittal malalignment. Different results may be obtained if other criteria, such as an age-adjusted alignment target or GAP score, are applied. However, the SRS-Schwab classification is currently the most popular tool in the current literatures for defining the severity of sagittal deformity [31-34].
CONCLUSION
Patients with more severe sagittal imbalance were treated with more invasive surgical methods, with an increased perioperative surgical burden. Regardless to severity of baseline sagittal imbalance, all patients exhibited significant radiological and clinical improvements after surgery. However, in term of ODI, the severe group demonstrated slightly worse clinical outcomes compared to the other groups, probably due to relatively higher proportion of undercorrection. Therefore, more rigorous correction is necessary to achieve optimal sagittal alignment specifically in patients with severe baseline sagittal imbalance.
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