The standing T1–L1 pelvic angle: a useful radiographic predictor of proximal junctional kyphosis in adult spinal deformity

Eiji TakasawaDepartment of Spine and Orthopedic Surgery, Japanese Red Cross Medical Center, Tokyo; and
Department of Orthopedic Surgery, Gunma University Graduate School of Medicine, Gunma, Japan

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Naohiro KawamuraDepartment of Spine and Orthopedic Surgery, Japanese Red Cross Medical Center, Tokyo; and

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Yoichi IizukaDepartment of Orthopedic Surgery, Gunma University Graduate School of Medicine, Gunma, Japan

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Junichi OhyaDepartment of Spine and Orthopedic Surgery, Japanese Red Cross Medical Center, Tokyo; and

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Yuki OnishiDepartment of Spine and Orthopedic Surgery, Japanese Red Cross Medical Center, Tokyo; and

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Junichi KunogiDepartment of Spine and Orthopedic Surgery, Japanese Red Cross Medical Center, Tokyo; and

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Hirotaka ChikudaDepartment of Orthopedic Surgery, Gunma University Graduate School of Medicine, Gunma, Japan

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OBJECTIVE

Proximal junctional kyphosis (PJK), which can worsen a patient’s quality of life, is a common complication following the surgical treatment of adult spinal deformity (ASD). Although various radiographic parameters have been proposed to predict the occurrence of PJK, the optimal method has not been established. The present study aimed to investigate the usefulness of the T1–L1 pelvic angle in the standing position (standing TLPA) for predicting the occurrence of PJK.

METHODS

The authors retrospectively extracted data for patients with ASD who underwent minimum 5-level fusion to the pelvis with upper instrumented vertebra between T8 and L1. In the present study, PJK was defined as ≥ 10° progression of the proximal junctional angle or reoperation due to progressive kyphosis during 1 year of follow-up. The following parameters were analyzed on whole-spine standing radiographs: the T1–pelvic angle, conventional thoracic kyphosis (TK; T4–12), whole-thoracic TK (T1–12), and the standing TLPA (defined as the angle formed by lines extending from the center of T1 and L1 to the femoral head axis). A logistic regression analysis and a receiver operating characteristic curve analysis were performed.

RESULTS

A total of 50 patients with ASD were enrolled (84% female; mean age 74.4 years). PJK occurred in 19 (38%) patients. Preoperatively, the PJK group showed significantly greater T1–pelvic angle (49.2° vs 34.4°), conventional TK (26.6° vs 17.6°), and standing-TLPA (30.0° vs 14.9°) values in comparison to the non-PJK group. There was no significant difference in the whole-thoracic TK between the two groups. A multivariate analysis showed that the standing TLPA and whole-thoracic TK were independent predictors of PJK. The standing TLPA had better accuracy than whole-thoracic TK (AUC 0.86 vs 0.64, p = 0.03). The optimal cutoff value of the standing TLPA was 23.0° (sensitivity 0.79, specificity 0.74). Using this cutoff value, the standing TLPA was the best predictor of PJK (OR 8.4, 95% CI 1.8–39, p = 0.007).

CONCLUSIONS

The preoperative standing TLPA was more closely associated with the occurrence of PJK than other radiographic parameters. These results suggest that this easily measured parameter is useful for the prediction of PJK.

ABBREVIATIONS

ASD = adult spinal deformity ; AUC = area under the curve ; BMI = body mass index ; 3CO = 3-column osteotomy ; PI = pelvic incidence ; PI-LL = pelvic incidence–lumbar lordosis mismatch ; PJK = proximal junctional kyphosis ; PT = pelvic tilt ; ROC = receiver operating characteristic ; SS = sacral slope ; SVA = sagittal vertical axis ; TK = thoracic kyphosis ; TLPA = T1–L1 pelvic angle ; TPA = T1–pelvic angle ; UIV = upper instrumented vertebra .

OBJECTIVE

Proximal junctional kyphosis (PJK), which can worsen a patient’s quality of life, is a common complication following the surgical treatment of adult spinal deformity (ASD). Although various radiographic parameters have been proposed to predict the occurrence of PJK, the optimal method has not been established. The present study aimed to investigate the usefulness of the T1–L1 pelvic angle in the standing position (standing TLPA) for predicting the occurrence of PJK.

METHODS

The authors retrospectively extracted data for patients with ASD who underwent minimum 5-level fusion to the pelvis with upper instrumented vertebra between T8 and L1. In the present study, PJK was defined as ≥ 10° progression of the proximal junctional angle or reoperation due to progressive kyphosis during 1 year of follow-up. The following parameters were analyzed on whole-spine standing radiographs: the T1–pelvic angle, conventional thoracic kyphosis (TK; T4–12), whole-thoracic TK (T1–12), and the standing TLPA (defined as the angle formed by lines extending from the center of T1 and L1 to the femoral head axis). A logistic regression analysis and a receiver operating characteristic curve analysis were performed.

RESULTS

A total of 50 patients with ASD were enrolled (84% female; mean age 74.4 years). PJK occurred in 19 (38%) patients. Preoperatively, the PJK group showed significantly greater T1–pelvic angle (49.2° vs 34.4°), conventional TK (26.6° vs 17.6°), and standing-TLPA (30.0° vs 14.9°) values in comparison to the non-PJK group. There was no significant difference in the whole-thoracic TK between the two groups. A multivariate analysis showed that the standing TLPA and whole-thoracic TK were independent predictors of PJK. The standing TLPA had better accuracy than whole-thoracic TK (AUC 0.86 vs 0.64, p = 0.03). The optimal cutoff value of the standing TLPA was 23.0° (sensitivity 0.79, specificity 0.74). Using this cutoff value, the standing TLPA was the best predictor of PJK (OR 8.4, 95% CI 1.8–39, p = 0.007).

CONCLUSIONS

The preoperative standing TLPA was more closely associated with the occurrence of PJK than other radiographic parameters. These results suggest that this easily measured parameter is useful for the prediction of PJK.

In Brief

Researchers used the preoperative T1–L1 pelvic angle in the standing position to predict proximal junctional kyphosis after long fusion surgery in patients with adult spinal deformity. The preoperative T1–L1 pelvic angle in the standing position was associated more closely with the occurrence of proximal junctional kyphosis after surgery compared with other conventional radiographic parameters. This simple parameter is useful for the prediction of proximal junctional kyphosis. These findings may provide important information for surgical planning, postoperative management, and obtaining informed consent for patients.

Proximal junctional kyphosis (PJK) is a major complication following the surgical treatment of adult spinal deformity (ASD), which can worsen a patient’s quality of life and which is associated with an increased reoperation rate of up to 55%.1–3 Several risk factors for the development of PJK have been identified, including older age, low bone mineral density, magnitude of correction, fusion to pelvis, and selection of upper instrumented vertebra (UIV).1–5

Radiographic parameters that can predict the development of PJK after surgery are of clinical importance, because they may enable surgeons to make a better surgical plan to reduce unfavorable outcomes. To date, various predictive radiographic parameters that can be measured before surgery have been proposed, including large sagittal vertical axis (SVA), preoperative thoracic kyphosis (TK), and T1–pelvic angle (TPA) values. Despite their clinical importance, the usefulness of these parameters is still limited by the low to moderate specificity of the prediction6–10 or cumbersome measurement. Thus, a simple and practical method with better predictivity is critically needed.

In this study, we focused on the T1–L1 pelvic angle in the standing position (standing TLPA), a novel sagittal parameter.11 This study aimed to evaluate the clinical efficacy of the standing TLPA in the prediction of PJK in comparison to conventional sagittal radiographic parameters.

Methods

This retrospective, single-center study included patients with ASD who underwent surgery between January 2015 and May 2018. All patients were enrolled if they met the radiological criteria for ASD (i.e., coronal Cobb angle > 20°, C7 SVA > 50 mm, pelvic tilt [PT] > 25°, or T4–12 thoracic kyphosis [TK] > 60°) on a whole-spine standing radiograph (Fig. 1). The inclusion criteria for this study were a minimum of 1 year of follow-up and a minimum of 5-level fusion, including the pelvis. In this study, we focused on patients with ASD whose UIV was stopped at the lower thoracic and thoracolumbar junction between T8 and L1 to observe the postoperative behavior of the unfused thoracic segment after ASD surgery. The exclusion criteria were as follows: 1) UIV setting at the upper thoracic region > T8; 2) the presence of neuromuscular disease, rheumatoid arthritis, infectious disease, or malignant etiology; and 3) inability to undergo radiographic evaluation or unclear radiographic results. PJK was defined as a proximal junctional angle showing > 10° progression in comparison to the baseline proximal junctional angle,1–5,12 or reoperation due to PJK within 1 year postoperatively. This study was approved by our institutional review board, and informed consent was obtained from all patients before the start of the study or surgery.

FIG. 1.
FIG. 1.

Flow diagram of patient enrollment. LIV = lower instrumented vertebra.

Surgical Technique

In our institution, lumbar and thoracic constructs were separately built as follows: 1) lumbar deformity was corrected by the cantilever technique and fixed from the pelvis to L1 or T12 as one segment (the lumbar construct); and 2) the 2–3 cranial vertebrae from the lumbar construct, including UIV, were fixed by properly contoured short rods (the thoracic construct). Then, the two constructs were connected by lateral connectors. Finally, posterior fusion was performed with an autograft harvested from local and iliac bone. When trying to apply a single rod across long segments against a deformed spine, rod bending is often difficult, and inappropriate rod contours apply excessive force to screws, which may cause loosening or pulling out of screws. In our surgical technique, by creating the thoracic and lumbar constructs separately and connecting the two constructs with lateral connectors, it is easy to bend and apply the thoracic rods in an ideal contour at the proximal end of the thoracic construct. All surgeries were performed by the senior spine surgeon (N.K.).

In our surgical strategy, deformity correction was planned by judging rigidity/flexibility in the lumbar region on preoperative fulcrum backward-bending films. For rigid deformity, multilevel posterior column osteotomy or 3-column osteotomy (3CO) was planned. The UIV was selected as the spinal vertebra beyond the planned thoracolumbar inflection point. The thoracic rods (titanium, diameter 5.5 mm) were bent to the kyphotic contour to avoid rigid fixation and excessive corrective force on the transitional part from the UIV to the unfused thoracic spine, to prevent screw pullout and breakage leading to PJK. Hooks or sublaminar taping (wiring) were not used at the UIV. We considered that the posterior tension band structures, such as the spinous process, supraspinous/interspinous ligament, and ligamentum flavum, play an important role in stabilizing the posterior spine and preventing PJK after surgery. We therefore preserved the posterior tension band structures down to 2–3 levels caudal to the UIV.

Demographic Data Collection

Demographic data were collected, including age, sex, and body mass index (BMI). In addition, surgical characteristics, such as the number of levels fused, the use of 3CO (i.e., pedicle subtraction osteotomy, posterior vertebral column resection), UIV, and revision data, were also recorded.

Radiographic Parameters

The imaging protocol involved patients standing unsupported in a position of comfort with their arms flexed and their fingers touching their clavicles to avoid superimposition of their arms over the spine. The sagittal radiographic spinopelvic parameters included the SVA, PT, sacral slope (SS), pelvic incidence–lumbar lordosis mismatch (PI-LL), conventional TK, whole-thoracic TK, TPA, and the standing TLPA.

The TLPA is formed by a line extending from the center of T1 to the femoral head axis and a line from the femoral heads to the center of L1 (Fig. 2).11 The TLPA was originally proposed by Janjua et al. to assess the thoracic flexibility in a sitting position as patients relax and remove the effect of thoracic muscle strength.11 In this study, the TLPA was measured in a standing radiograph. The preoperative standing TLPA had radiographic variations corresponding to the pathology and stage of ASD (Fig. 3).

FIG. 2.
FIG. 2.

Definition of the TLPA. The TLPA (angle illustrated by red curved solid line) is formed by a line extending from the center of T1 to the femoral head axis and a line from the femoral heads to the center of L1. Figure is available in color online only.

FIG. 3.
FIG. 3.

Variations of the preoperative standing TLPA corresponding to the pathology and stage of ASD. A large standing-TLPA value indicates a pathological decompensated state characterized by abnormal forward truncal inclination (A) and thoracic hyperkyphosis. A small standing-TLPA value can account for preoperative compensatory hypokyphosis of the thoracic segment against the lumbopelvic pathology regardless of the direction of truncal inclination (B shows a forward and C shows a backward truncal inclination). Figure is available in color online only.

Whole-spine standing radiographs were obtained at baseline (before surgery), at 2 weeks after, and at 1 year after surgery. If revision surgery was scheduled for the development of PJK during the follow-up period after initial ASD surgery, the parameters at that time (before reoperation) were recorded as the final outcome.

The radiographic parameters were measured by two independent observers (E.T. and J.O., board-certified trainers of the Japanese Society for Spine Surgery and Related Research). The interrater reliability estimate for the standing TLPA (intraclass correlation coefficient 0.988, 95% CI 0.972–0.995) was excellent.

Statistical Analyses

The Mann-Whitney U-test was used for the comparison of continuous variables. Fisher’s exact probability test was used for the comparison of categorical variables. A multivariate logistic regression analysis was then performed to determine the thoracic compensatory parameters that were associated with the development of PJK. Clinically relevant potentially associated factors were entered into a multivariate logistic regression model (the predefined significance level for inclusion in the regression model was p ≤ 0.20 in the univariate analysis, and a pair of variables with a variance inflation factor > 5 was excluded from the model to handle multicollinearity). The receiver operating characteristic (ROC) curves of the candidate parameters were generated and areas under the curve (AUCs) were calculated to identify and compare the accuracy and reliability of these parameters for predicting the development of PJK. High, moderate, and low accuracies were defined as AUC ≥ 0.9, AUC > 0.7 to < 0.9, and AUC > 0.5 to ≤ 0.7, respectively, whereas an AUC ≤ 0.5 was defined as a chance result. The AUCs of these ROC curves were compared using the DeLong test. All statistical analyses were conducted using the EZR software program (version 1.36, Saitama Medical Center, Jichi Medical University, Saitama, Japan).13 A p value < 0.05 was considered to indicate statistical significance, and the variance was the standard deviation.

Results

Patient Population

A total of 50 patients with ASD were included in this study (mean age 74.4 years [range 59–86 years], 84% female) (Table 1). The UIV levels were as follows: T8 (n = 2), T9 (n = 2), T10 (n = 33), T11 (n = 3), T12 (n = 5), and L1 (n = 5). Twenty patients (40.0%) underwent 3CO, most commonly at L4 (n = 8), followed by L2 (n = 6), L3 (n = 5), and L1 (n = 1).

TABLE 1.

Demographic data and radiographic analysis at baseline in 50 patients with ASD

VariableWhole Cohort, N = 50PJK, n = 19Non-PJK, n = 31p Value
Age, yrs74.4 (6.6)76.2 (7.1)73.1 (6.2)0.12
BMI22.3 (3.0)23.7 (2.5)21.4 (3.0)0.011
Sex, % female84.0%89.5%80.6%0.69
Fused levels7.6 (1.1)7.8 (0.9)7.5 (1.2)0.19
% 3CO40.0%47.4%35.5%0.55
 No. w/ PSO18/209/99/11
 No. w/ PVCR2/200/92/11
C7 SVA, mm135.8 (66.0)157.6 (65.4)122.5 (63.7)0.067
PT, °33.8 (11.1)40.1 (9.5)30.0 (10.3)0.001
SS, °17.8 (13.4)10.4 (11.3)22.3 (12.7)0.002
PI, °51.6 (10.4)50.1 (7.1)52.5 (12.0)0.44
LL, °−9.3 (20.9)−6.0 (20.6)−11.3 (21.2)0.39
PI-LL, °42.3 (20.9)44.1 (18.6)41.2 (22.4)0.64
Conventional TK, °22.2 (16.2)26.6 (16.6)17.6 (14.4)0.012
Whole-thoracic TK, °24.8 (15.1)27.0 (17.7)20.0 (12.5)0.11
TPA, °39.9 (14.5)49.2 (11.8)34.4 (13.2)<0.001
Standing TLPA, °20.6 (12.0)30.0 (8.6)14.9 (10.0)<0.001

PSO = pedicle subtraction osteotomy; PVCR = posterior vertebral column resection.

Data are presented as the mean (SD).

Radiographic Parameters at Baseline

PJK occurred in 19 (38%) of the patients in the whole cohort. Preoperatively, the patients who developed PJK (PJK group) had greater PT, conventional TK, TPA, and standing-TLPA values in comparison to those who did not (non-PJK group) (Table 1). The SVA, PI, PI-LL, and whole-thoracic TK values of the two groups did not differ to a statistically significant extent (all p > 0.05).

Radiographic Outcomes

Patients in both groups showed similarly good sagittal alignment with the restoration of SVA (56.5 ± 45.8 mm vs 48.1 ± 38.0 mm, p = 0.48) and PI-LL (14.5° ± 10.3° vs 11.7° ± 8.5°, p = 0.30) at 2 weeks after surgery (Table 2). Nevertheless, the PJK group showed significantly greater PT, conventional TK, whole-thoracic TK, TPA, and standing-TLPA values (all p < 0.05). During the 1-year follow-up period, 31.6% (6/19) of the PJK cases underwent revision surgery at a mean time point of 7.5 ± 2.9 months: vertebral fracture and pseudarthrosis at the UIV (n = 3), implant breakage (n = 2), and disc failure and myelopathy (n = 1).

TABLE 2.

Radiographic analysis after surgery in 50 patients with ASD

Variable2 Wks Postop1 Yr Postop
PJK, n = 19Non-PJK, n = 31p ValuePJK, n = 19Non-PJK, n = 31p Value
C7 SVA, mm56.5 (45.8)48.1 (38.0)0.4886.6 (48.5)61.7 (40.7)0.057
PI−LL, °14.5 (10.3)11.7 (8.5)0.3017.6 (10.7)12.7 (11.9)0.15
PT, °28.7 (7.1)20.6 (9.3)0.00232.5 (9.6)22.7 (8.2)<0.001
SS, °19.4 (7.3)28.7 (12.7)0.00616.5 (10.1)27.3 (12.5)0.003
Conventional TK, °38.6 (6.8)30.3 (12.7)0.01250.7 (12.4)31.4 (12.8)<0.001
Whole-thoracic TK, °46.0 (9.8)35.0 (9.9)<0.00157.0 (11.9)35.0 (10.5)<0.001
TPA, °28.0 (5.4)19.9 (7.7)<0.00133.4 (7.5)23.2 (7.9)<0.001
Standing TLPA, °18.2 (3.5)9.0 (5.0)<0.00123.1 (4.8)11.1 (5.7)<0.001

Data are presented as the mean (SD).

Risk Factor Analysis

A multiple logistic regression analysis was performed to adjust for age, sex, BMI, conventional TK, whole-thoracic TK, TPA, and standing TLPA (Table 3). The variance inflation factor showed no evidence of a multicollinearity problem among independent variables. After adjustment, the preoperative standing TLPA and whole-thoracic TK remained as independent predictors of PJK (standing TLPA, OR 1.2, 95% CI 1.06–1.44, p = 0.007; whole-thoracic TK, OR 1.1, 95% CI 1.01–1.27, p = 0.029).

TABLE 3.

Logistic regression analysis: independent predictive measure of PJK

VariableOR95% CIp Value
Standing TLPA1.21.06–1.440.007
Whole-thoracic TK1.11.01–1.270.029
TPA1.00.93–1.120.645
Conventional TK0.980.90–1.060.629

We performed an ROC analysis to evaluate the predictive ability of the preoperative standing TLPA and whole-thoracic TK (Fig. 4). The preoperative standing TLPA had a significantly greater AUC value in comparison to whole-thoracic TK (standing TLPA, AUC 0.86, 95% CI 0.76–0.96; whole-thoracic TK, AUC 0.64, 95% CI 0.47–0.81). The ROC curves also demonstrated that the standing TLPA had significantly higher accuracy and reliability as a predictor of PJK in comparison to whole-thoracic TK (DeLong test: p = 0.032). The optimal cutoff value of the standing TLPA was 23.0°; this showed acceptable sensitivity and specificity (sensitivity 0.79, specificity 0.74). With this cutoff value, the preoperative standing TLPA was a strong predictor of PJK (OR 8.4, 95% CI 1.8–38.9, p = 0.007) (Fig. 5).

FIG. 4.
FIG. 4.

The ROC curve analysis to evaluate factors predicting the development of PJK. The graph shows the ROC curves for the preoperative standing TLPA (solid line) and whole-thoracic TK (dotted line). A significant difference was observed between the AUC values of the standing TLPA (0.86) and the whole-thoracic TK (0.64) (DeLong test: p = 0.032).

FIG. 5.
FIG. 5.

Distribution of the standing-TLPA values. The data plots show the distribution of the preoperative standing-TLPA values in the PJK and the non-PJK groups. Horizontal solid lines indicate the median values.

In the additional ROC analysis, the cutoff value of standing TLPA was 30° to predict reoperation due to PJK in our study. This finding was significant (AUC 0.797, 95% CI 0.62–0.98) with both an acceptable sensitivity and specificity (sensitivity 0.67, specificity 0.86).

Discussion

This study of 50 patients with ASD examined the usefulness of preoperative radiographic parameters in the prediction of PJK. We found that the preoperative standing TLPA was more closely associated with the occurrence of PJK in comparison to other radiographic parameters. Our results demonstrated that the optimal cutoff value of the preoperative standing TLPA for the detection of patients with ASD who were at risk of developing PJK was > 23° (OR 8.4).

Is Measurement of Thoracic Angle Alone Enough to Predict PJK?

Thoracic hypokyphosis or even thoracic lordosis is considered a compensatory mechanism for sagittal malalignment,14 whereas thoracic hypokyphosis before surgery is associated with the development of PJK.15 In contrast, preoperative hyperkyphosis is another important risk factor of PJK.2–5 The conflicting data in previous studies suggests that the mere measurement of a thoracic angle is not enough to predict PJK. A plausible explanation is that PJK is interactively caused by various perioperative thoracic profiles, such as thoracic shape16 and inclination,17,18 and thoracic flexibility.11,19,20 In this study we therefore verified the advantages of the preoperative standing TLPA in reflecting such crucial elements of the thoracic spine.

Comparison of the Standing TLPA and Thoracic Angles

A recent report by Lafage et al.16 provided insight into the thoracic morphology and proportions (thoracic shape) among asymptomatic subjects. They suggested that the conventional measurement of T4–12 kyphosis tends to underestimate the maximum kyphosis and that the T1–12 thoracic curve is a more appropriate description of the thoracic shape. The TLPA,11 a thoracic component of the TPA, includes the whole-thoracic segment from T1 to T12, and can therefore be more sensitive for predicting the occurrence of PJK in comparison to conventional TK.

Clinically, patients with ASD present with various degrees of disability and recruit thoracic compensation differently.18,21 Although the consequent global spinal balance and inclination are crucial elements of both the thoracic compensatory mechanism and the development of PJK, the conventional and whole-thoracic TK angles do not reflect these elements. On the other hand, the standing TLPA can account for both the spinal balance and inclination corresponding to lumbar-pelvis obliquity because it reflects the properties of the TPA.22 In this study we showed the variation of the standing TLPA corresponding to three postural categories in patients with ASD: forward, neutral, and backward truncal inclination (Fig. 3). As a result, the standing TLPA can simultaneously evaluate not only the T1–12 whole-thoracic angle (shape) but also its balance and inclination, which are associated with both thoracic compensation and the development of PJK.

Limitations

The present study was associated with some limitations. First, we examined PJK that occurred within 1 year after surgery. Although a longer follow-up period would provide additional information, prior studies reported that the majority of PJK occurs in the first year after surgery.2,23 Second, this was a small, retrospective cohort study, and larger validation studies should be conducted to confirm our findings. Third, we focused only on radiographic parameters, and did not include the age-related physiological status, such as suboptimal bone quality and muscle weakness, which may affect the pathology of PJK.24–26 Fourth, although it is well known that overcorrection can be a PJK risk across every age group (especially in elderly patients), we did not describe the relationship between the perioperative change in parameters and the risk of PJK. However, our study found no significant difference in postoperative PT and SVA values (major sagittal and global parameters) between the PJK and non-PJK group. Furthermore, the postoperative PI-LL in our elderly cohort (mean age > 70 years) was within the age-adjusted alignment goal ranging from 10.5° to 17.0° (> 65 years old) reported by Lafage and colleagues,27,28 suggesting moderate correction in both the PJK and non-PJK groups. Despite these limitations, the application of the standing TLPA was practical and showed a strong predictive ability for PJK. Our study provides relevant information to aid in surgical decision-making, such as the UIV selection and whether or not to extend the fusion beyond lower thoracic levels to prevent future PJK.

Conclusions

The preoperative standing TLPA was associated more closely with the occurrence of PJK after surgery in comparison to other conventional radiographic parameters. Our results suggest that this simple parameter is useful for the prediction of PJK. These findings may provide important information for surgical planning, postoperative management, and obtaining informed consent for patients.

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: Takasawa. Acquisition of data: Takasawa, Kawamura, Ohya, Onishi. Analysis and interpretation of data: Takasawa, Kawamura, Iizuka, Chikuda. Drafting the article: Takasawa, Chikuda. Critically revising the article: all authors. Reviewed submitted version of manuscript: Takasawa, Kawamura, Chikuda. Approved the final version of the manuscript on behalf of all authors: Takasawa. Statistical analysis: Takasawa. Administrative/technical/material support: Takasawa, Kawamura. Study supervision: Chikuda.

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    • Export Citation
  • 12

    Yagi M, Rahm M, Gaines R, Maziad A, Ross T, Kim HJ, 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
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 13

    Kanda Y . Investigation of the freely available easy-to-use software ‘EZR’ for medical statistics. Bone Marrow Transplant. 2013;48(3):452458.

  • 14

    Diebo BG, Gammal I, Ha Y, Yoon SH, Chang JW, Kim B, et al. Role of ethnicity in alignment compensation: propensity matched analysis of differential compensatory mechanism recruitment patterns for sagittal malalignment in 288 ASD patients from Japan, Korea, and United States. Spine (Phila Pa 1976).2017;42(4):E234E240.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 15

    Watanabe K, Lenke LG, Bridwell KH, Kim YJ, Koester L, Hensley M . Proximal junctional vertebral fracture in adults after spinal deformity surgery using pedicle screw constructs: analysis of morphological features. Spine (Phila Pa 1976).2010;35(2):138145.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 16

    Lafage R, Steinberger J, Pesenti S, Assi A, Elysee JC, Iyer S, et al. Understanding thoracic spine morphology, shape, and proportionality. Spine (Phila Pa 1976).2020;45(3):149157.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 17

    Lafage V, Schwab F, Patel A, Hawkinson N, Farcy JP . 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
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 18

    Ferrero E, Liabaud B, Challier V, Lafage R, Diebo BG, Vira S, et al. Role of pelvic translation and lower-extremity compensation to maintain gravity line position in spinal deformity. J Neurosurg Spine. 2016;24(3):436446.

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

    Pierce KE, Horn SR, Jain D, Segreto FA, Bortz C, Vasquez-Montes D, et al. the impact of adult thoracolumbar spinal deformities on standing to sitting regional and segmental reciprocal alignment. Int J Spine Surg. 2019;13(4):308316.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 20

    Zhou S, Li W, Wang W, Zou D, Sun Z, Xu F, et al. Sagittal spinal and pelvic alignment in middle-aged and older men and women in the natural and erect sitting positions: a prospective study in a Chinese population. Med Sci Monit. 2020;26:e919441.

    • Search Google Scholar
    • Export Citation
  • 21

    Ferrero E, Skalli W, Lafage V, Maillot C, Carlier R, Feydy A, et al. Relationships between radiographic parameters and spinopelvic muscles in adult spinal deformity patients. Eur Spine J. 2020;29(6):13281339.

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

    Protopsaltis T, Schwab F, Bronsard N, Smith JS, Klineberg E, Mundis G, et al. The T1 pelvic angle, a novel radiographic measure of global sagittal deformity, accounts for both spinal inclination and pelvic tilt and correlates with health-related quality of life. J Bone Joint Surg Am. 2014;96(19):16311640.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 23

    Daniels AH, Bess S, Line B, Eltorai AEM, Reid DBC, Lafage V, et al. Peak timing for complications after adult spinal deformity surgery. World Neurosurg. 2018;115:e509e515.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 24

    Yagi M, Ohne H, Konomi T, Fujiyoshi K, Kaneko S, Komiyama T, et al. Teriparatide improves volumetric bone mineral density and fine bone structure in the UIV+1 vertebra, and reduces bone failure type PJK after surgery for adult spinal deformity. Osteoporos Int. 2016;27(12):34953502.

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

    Tsuboi H, Nishimura Y, Sakata T, Ohko H, Tanina H, Kouda K, et al. Age-related sex differences in erector spinae muscle endurance using surface electromyographic power spectral analysis in healthy humans. Spine J. 2013;13(12):19281933.

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

    Hyun SJ, Kim YJ, Rhim SC . Patients with proximal junctional kyphosis after stopping at thoracolumbar junction have lower muscularity, fatty degeneration at the thoracolumbar area. Spine J. 2016;16(9):10951101.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 27

    Lafage R, Schwab F, Challier V, Henry JK, Gum J, Smith J, et al. Defining spino-pelvic alignment thresholds: should operative goals in adult spinal deformity surgery account for age?. Spine (Phila Pa 1976).2016;41(1):6268.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 28

    Lafage R, Schwab F, Glassman S, Bess S, Harris B, Sheer J, et al. Age-adjusted alignment goals have the potential to reduce PJK. Spine (Phila Pa 1976).2017;42(17):12751282.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
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Illustration from Levi and Schwab (pp 653–659). Copyright Roberto Suazo. Published with permission.
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    FIG. 1.

    Flow diagram of patient enrollment. LIV = lower instrumented vertebra.

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    FIG. 2.

    Definition of the TLPA. The TLPA (angle illustrated by red curved solid line) is formed by a line extending from the center of T1 to the femoral head axis and a line from the femoral heads to the center of L1. Figure is available in color online only.

  • View in gallery
    FIG. 3.

    Variations of the preoperative standing TLPA corresponding to the pathology and stage of ASD. A large standing-TLPA value indicates a pathological decompensated state characterized by abnormal forward truncal inclination (A) and thoracic hyperkyphosis. A small standing-TLPA value can account for preoperative compensatory hypokyphosis of the thoracic segment against the lumbopelvic pathology regardless of the direction of truncal inclination (B shows a forward and C shows a backward truncal inclination). Figure is available in color online only.

  • View in gallery
    FIG. 4.

    The ROC curve analysis to evaluate factors predicting the development of PJK. The graph shows the ROC curves for the preoperative standing TLPA (solid line) and whole-thoracic TK (dotted line). A significant difference was observed between the AUC values of the standing TLPA (0.86) and the whole-thoracic TK (0.64) (DeLong test: p = 0.032).

  • View in gallery
    FIG. 5.

    Distribution of the standing-TLPA values. The data plots show the distribution of the preoperative standing-TLPA values in the PJK and the non-PJK groups. Horizontal solid lines indicate the median values.

  • 1

    Glattes RC, Bridwell KH, Lenke LG, Kim YJ, Rinella A, Edwards C II . 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
    • PubMed
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  • 2

    Lau D, Clark AJ, Scheer JK, Daubs MD, Coe JD, Paonessa KJ, 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.

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  • 3

    Sardar ZM, Kim Y, Lafage V, Rand F, Lenke L, Klineberg E . State of the art: proximal junctional kyphosis-diagnosis, management and prevention. Spine Deform. 2021;9(3):635644.

    • Crossref
    • PubMed
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  • 4

    Lafage R, Beyer G, Schwab F, Klineberg E, Burton D, Bess S, et al. Risk factor analysis for proximal junctional kyphosis after adult spinal deformity surgery: a new simple scoring system to identify high-risk patients. Global Spine J. 2020;10(7):863870.

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    • PubMed
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  • 5

    Liu FY, Wang T, Yang SD, Wang H, Yang DL, Ding WY . Incidence and risk factors for proximal junctional kyphosis: a meta-analysis. Eur Spine J. 2016;25(8):23762383.

    • Crossref
    • PubMed
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  • 6

    Yagi M, Akilah KB, Boachie-Adjei O . Incidence, risk factors and classification of proximal junctional kyphosis: surgical outcomes review of adult idiopathic scoliosis. Spine (Phila Pa 1976).2011;36(1):E60E68.

    • Crossref
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  • 7

    Maruo K, Ha Y, Inoue S, Samuel S, Okada E, Hu SS, et al. Predictive factors for proximal junctional kyphosis in long fusions to the sacrum in adult spinal deformity. Spine (Phila Pa 1976).2013;38(23):E1469E1476.

    • Crossref
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  • 8

    Kim YJ, Lenke LG, Bridwell KH, Kim J, Cho SK, Cheh G, Yoon J . Proximal junctional kyphosis in adolescent idiopathic scoliosis after 3 different types of posterior segmental spinal instrumentation and fusions: incidence and risk factor analysis of 410 cases. Spine (Phila Pa 1976).2007;32(24):27312738.

    • Crossref
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  • 9

    Sebaaly A, Riouallon G, Obeid I, Grobost P, Rizkallah M, Laouissat F, et al. Proximal junctional kyphosis in adult scoliosis: comparison of four radiological predictor models. Eur Spine J. 2018;27(3):613621.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 10

    Kikuchi K, Miyakoshi N, Abe E, Kobayashi T, Abe T, Kinoshita H, et al. Proximal junctional fracture and kyphosis after long spinopelvic corrective fixation for adult spinal deformity. J Orthop Sci. 2021;26(3):343347.

    • Crossref
    • PubMed
    • Search Google Scholar
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  • 11

    Janjua MB, Tishelman JC, Vasquez-Montes D, Vaynrub M, Errico TJ, Buckland AJ, Protopsaltis T . The value of sitting radiographs: analysis of spine flexibility and its utility in preoperative planning for adult spinal deformity surgery. J Neurosurg Spine. 2018;29(4):414421.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 12

    Yagi M, Rahm M, Gaines R, Maziad A, Ross T, Kim HJ, 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
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 13

    Kanda Y . Investigation of the freely available easy-to-use software ‘EZR’ for medical statistics. Bone Marrow Transplant. 2013;48(3):452458.

  • 14

    Diebo BG, Gammal I, Ha Y, Yoon SH, Chang JW, Kim B, et al. Role of ethnicity in alignment compensation: propensity matched analysis of differential compensatory mechanism recruitment patterns for sagittal malalignment in 288 ASD patients from Japan, Korea, and United States. Spine (Phila Pa 1976).2017;42(4):E234E240.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 15

    Watanabe K, Lenke LG, Bridwell KH, Kim YJ, Koester L, Hensley M . Proximal junctional vertebral fracture in adults after spinal deformity surgery using pedicle screw constructs: analysis of morphological features. Spine (Phila Pa 1976).2010;35(2):138145.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 16

    Lafage R, Steinberger J, Pesenti S, Assi A, Elysee JC, Iyer S, et al. Understanding thoracic spine morphology, shape, and proportionality. Spine (Phila Pa 1976).2020;45(3):149157.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 17

    Lafage V, Schwab F, Patel A, Hawkinson N, Farcy JP . 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
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 18

    Ferrero E, Liabaud B, Challier V, Lafage R, Diebo BG, Vira S, et al. Role of pelvic translation and lower-extremity compensation to maintain gravity line position in spinal deformity. J Neurosurg Spine. 2016;24(3):436446.

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

    Pierce KE, Horn SR, Jain D, Segreto FA, Bortz C, Vasquez-Montes D, et al. the impact of adult thoracolumbar spinal deformities on standing to sitting regional and segmental reciprocal alignment. Int J Spine Surg. 2019;13(4):308316.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 20

    Zhou S, Li W, Wang W, Zou D, Sun Z, Xu F, et al. Sagittal spinal and pelvic alignment in middle-aged and older men and women in the natural and erect sitting positions: a prospective study in a Chinese population. Med Sci Monit. 2020;26:e919441.

    • Search Google Scholar
    • Export Citation
  • 21

    Ferrero E, Skalli W, Lafage V, Maillot C, Carlier R, Feydy A, et al. Relationships between radiographic parameters and spinopelvic muscles in adult spinal deformity patients. Eur Spine J. 2020;29(6):13281339.

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

    Protopsaltis T, Schwab F, Bronsard N, Smith JS, Klineberg E, Mundis G, et al. The T1 pelvic angle, a novel radiographic measure of global sagittal deformity, accounts for both spinal inclination and pelvic tilt and correlates with health-related quality of life. J Bone Joint Surg Am. 2014;96(19):16311640.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 23

    Daniels AH, Bess S, Line B, Eltorai AEM, Reid DBC, Lafage V, et al. Peak timing for complications after adult spinal deformity surgery. World Neurosurg. 2018;115:e509e515.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 24

    Yagi M, Ohne H, Konomi T, Fujiyoshi K, Kaneko S, Komiyama T, et al. Teriparatide improves volumetric bone mineral density and fine bone structure in the UIV+1 vertebra, and reduces bone failure type PJK after surgery for adult spinal deformity. Osteoporos Int. 2016;27(12):34953502.

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

    Tsuboi H, Nishimura Y, Sakata T, Ohko H, Tanina H, Kouda K, et al. Age-related sex differences in erector spinae muscle endurance using surface electromyographic power spectral analysis in healthy humans. Spine J. 2013;13(12):19281933.

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

    Hyun SJ, Kim YJ, Rhim SC . Patients with proximal junctional kyphosis after stopping at thoracolumbar junction have lower muscularity, fatty degeneration at the thoracolumbar area. Spine J. 2016;16(9):10951101.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 27

    Lafage R, Schwab F, Challier V, Henry JK, Gum J, Smith J, et al. Defining spino-pelvic alignment thresholds: should operative goals in adult spinal deformity surgery account for age?. Spine (Phila Pa 1976).2016;41(1):6268.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 28

    Lafage R, Schwab F, Glassman S, Bess S, Harris B, Sheer J, et al. Age-adjusted alignment goals have the potential to reduce PJK. Spine (Phila Pa 1976).2017;42(17):12751282.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation

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