
==== Front
Spine (Phila Pa 1976)
Spine (Phila Pa 1976)
BRS
Spine
0362-2436
1528-1159
Lippincott Williams & Wilkins Hagerstown, MD

38747237
SPINE167375
10.1097/BRS.0000000000005043
00007
3
Clinical Case Series
Lumbar Apex Position as an Independent Risk Factor for Adjacent Segment Diseases in Patients Undergoing Short-Level Transforaminal Lumbar Interbody Fusion
Wang Muyi MD ab18795885536@163.com

Wang Xin MD ab747996263@qq.com

Wu Jingbin MD ab2861446597@qq.com

Shen Yifei MD ab1640987810@qq.com

Qiu Yong MD cwangbin_spine@163.com

Sun Xu MD yuyang_spine@163.com
c
Zhou Dong MD adzhoudong1012@163.com

Jiang Yuqing MD abjyqbaba@126.com

a Department of Orthopedics, Affiliated Changzhou Second People's Hospital of Nanjing Medical University, Changzhou, Jiangsu, China
b Changzhou Medical Center, Nanjing Medical University, Changzhou, Jiangsu, China
c Division of Spine Surgery, Department of Orthopedic Surgery, Affiliated Drum Tower Hospital, Medical School of Nanjing University, Nanjing, Jiangsu, China
d Department of Orthopedics, Affiliated Changzhou Children's Hospital of Nantong University, Changzhou, Jiangsu, China
Address correspondence and reprint requests to Dong Zhou, MD; E-mail: zhoudong1012@163.com​, and Yuqing Jiang, MD; E-mail: jyqbaba@126.com, Department of Orthopedics, The Affiliated Changzhou Second People's Hospital of Nanjing Medical University, Gehu Middle Road 68, Changzhou 213003, China.
15 10 2024
15 5 2024
49 20 14351444
25 3 2024
30 4 2024
Copyright © 2024 The Author(s). Published by Wolters Kluwer Health, Inc.
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article distributed under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives License 4.0 (CCBY-NC-ND), where it is permissible to download and share the work provided it is properly cited. The work cannot be changed in any way or used commercially without permission from the journal. http://creativecommons.org/licenses/by-nc-nd/4.0/

Study Design.

Retrospective study.

Objective:

To investigate whether lumbar apex position had an impact on the development of adjacent segment disease (ASD) following transforaminal lumbar interbody fusion (TLIF).

Summary of Background Data.

Previous studies have demonstrated that solely concentrating on lumbar lordosis value is not suitable, and neglecting the significance of lumbar apex can lead to mechanical complications. However, the relationship between lumbar apex and ASD is still not well understood.

Methods.

In this retrospective study, 234 consecutive patients who underwent L3-5 or L4-5 TLIF for degenerative diseases were reviewed. The study evaluated the associations between sagittal parameters and pelvic incidence (PI). Patients were labeled “matched” when lumbar apex position aligned with the theoretical target, and “mismatched” when it did not. Multivariate analysis was applied to find the independent risk factors of ASD. In addition, a focused subanalysis was performed based on the lumbar apex position (ideal match, cranial from ideal, and caudal from ideal).

Results.

After an average follow-up period of 70.6 months, 68 cases were identified as having ASD. Postoperatively, 64.7% (44 of 68) of the patients with ASD exhibited a mismatched lumbar apex, compared with 41% (68 of 166) of those without ASD (P < 0.001). PI correlated significantly with proximal lordosis (PL) and lordosis distribution index (LDI) but not with distal lordosis (DL). Multivariate analysis identified age, L3–5 fusion, postoperative DL, and postoperative mismatched lumbar apex as independent risk factors of ASD. Upon the subanalysis, it was discovered that there were unique compensatory strategies in the cranial and caudal groups, with notable variations in postoperative DL, PL, and LDI among three groups (all P value of <0.05).

Conclusion.

Lumbar apex position significantly influenced the risk of ASD. To restore the lumbar apex to its ideal position, a proper value and distribution of DL should be attained.

Level of Evidence.

4.

Key words:

adjacent segment disease
spinopelvic sagittal alignment
lumbar apex
proximal lordosis
distal lordosis
OPEN-ACCESSTRUE
==== Body
pmcIn recent years, transforaminal lumbar interbody fusion (TLIF) has gradually become a common surgical approach for treating refractory lumbar degenerative diseases. Although TLIF is a highly effective surgical approach, it can also lead to some postoperative complications. Adjacent segment disease (ASD) is a common issue following TLIF surgery, and frequently leads to compromised clinical outcome.1 Therefore, it is of great practical significance for surgeons to predict the risk of ASD development in the patients who underwent TLIF. Recent researches underscore the significant influence of sagittal malalignment on the incidence of ASD. Pelvic incidence-lumbar lordosis (PI-LL) mismatch has been recognized as a significant risk factor of ASD, but this formula is not without its limitations.2–5

LL represents the measurement of the angle formed by the L1–S1 endplates, and individuals with identical LL measurements may exhibit varying sagittal lumbar configurations.6 Thus, Roussouly et al. 7 identified four distinct spinal profiles in the healthy population, categorized according to variations in the shape of lordosis. Our previous research confirmed that Roussouly classification could be used to predict ASD.4 However, when using Roussouly classification to assess whether the current shape matches the theoretical type, numerous parameters need to be evaluated. This evaluation process is tedious and complex, which significantly reduces the reliability and reproducibility of the results, making it impractical for direct clinical application. Consequently, Roussouly classification should be simplified when it comes to evaluating patients with lumbar degenerative diseases.

The lumbar apex is a critical component originally introduced in the Roussouly classification and can significantly impact the shape of the lumbar spine.8–10 Recent studies have shown a significant relationship between PI and lumbar apex, indicating that pelvic morphology may influence lumbar shape.6,8 Subsequent studies have revealed that focusing exclusively on the value of LL without considering the importance of the lumbar apex position is inadequate for the surgical management of adult spinal deformities.6,11,12 However, the relationship between lumbar apex position and ASD remains unclear. Hence, this study aimed to assess how the position of the lumbar apex affected the development of ASD after short-level TLIF surgery and investigate the relationship between spinopelvic alignment and lumbar apex.

MATERIAL AND METHODS

Patients

Approval from the institutional review board was granted for this study, with each participant providing informed consent. A review was performed on individuals who had L4–5 or L3–5 TLIF and posterior screw fixation for lumbar degenerative conditions at our centers from January 2009 to January 2018. This study only included patients who were within the age range of 40 to 80 years at the time of their initial surgery. The criteria for exclusion were outlined as: (1) had incomplete clinical and radiographic data, (2) follow-up duration of less than 5 years, (3) had a history of lumbar spine surgery, (4) the Cobb angle of the lumbar or thoracolumbar curve greater than 10° on the coronal plane, and (5) had a diagnosis of acute or delayed deep surgical site infection (SSI) after the primary surgery. Preoperative imaging included standing posteroanterior and lateral radiographs, as well as computed tomography (CT) scans and magnetic resonance imaging (MRI). These radiographs were also taken at each subsequent follow-up visit. In addition, at the final follow-up, flexion-extension lateral radiographs and MRI were taken.

Radiographic Evaluation

Preoperative degeneration degree of disc and facet joint at the cranial adjacent segment were assessed according to previously established criteria.13,14 Before surgery and at the 3-month postoperative follow-up, the following parameters were recorded (Fig. 1): (1) PI, (2) pelvic tilt (PT), (3) sacral slope (SS), 4) LL, (5) proximal lordosis (PL), (6) distal lordosis (DL), (7) sagittal vertical axis (SVA), (8) lordosis distribution index (LDI), which is the proportion of DL that contributes to LL, and (9) segmental lordosis (SL, not shown in Fig. 1), the lordosis measured between the upper and lower instrumented vertebrae. All parameters were independently assessed by two observers, with the average values being used for analysis.

Figure 1 Spinopelvic parameters measured on upright x-ray images.

Building on the researches of Pizones et al., 12,15 patients were sorted into four “theoretical” categories based on their PI values: type 1 (PI <45° and lumbar apex at L5), type 2 (PI <45° and lumbar apex at the L4–L5 disk), type 3 (45° ≤ PI ≤ 60° and lumbar apex at L4), and type 4 (PI >60° and lumbar apex at the L3–4 disk or above). The lumbar sagittal shape was also recorded to distinguish between type 1 and type 2 profiles. Type 1 is characterized by a nonharmonious back with long thoracolumbar kyphosis and short hyperlordosis, whereas type 2 tends to have a harmonious flat back with a longer but flat LL.16 Patients were labeled “matched” when lumbar apex position aligned with the theoretical target, and “mismatched” when it did not.

ASD Definition

Radiological degeneration was diagnosed based on radiographic and MRI evidence of new pathologies at the segment immediately adjacent to the fusion site, which were absent before surgery.4,5 ASD was characterized as either the development or exacerbation of radiological degeneration near the fused segments that led to recurrent clinical symptoms during the follow-up period.17–19 All patients with ASD included in this study required revision surgery to alleviate symptoms, but not everyone was able to undergo the procedure. Some patients refused surgery and preferred to try conservative treatment. In addition, some patients of older age with multiple comorbidities faced a significantly high surgical risk and, ultimately, chose conservative treatment as well.

Statistical Analyses

Statistical analyses were performed using SPSS version 25.0 (IBM Corp.). For continuous data, differences were assessed using the unpaired t test. For categorical data, analysis was performed using the chi-square test or Fisher's exact test when appropriate. A P value threshold of less than 0.05 was established as the criterion for statistical significance. In the univariate analysis, baseline characteristics and postoperative spinopelvic parameters that exhibited a P value of less than 0.1 were selected for inclusion in the multivariate analysis. A forward stepwise regression method was then applied to evaluate the adjusted relationships between potential predictors and the development of ASD.

The study also analyzed the correlations between PI and postoperative lumbar sagittal parameters utilizing either the Pearson's or Spearman's correlation analysis, depending on the data distribution. Simple linear regression was further performed to explore the relationship between PI and PL. In addition, in a focused subanalysis, patients were categorized based on the postoperative position of the lumbar apex relative to an ideal location: ideal match, cranial from ideal, and caudal from ideal. The clinical characteristics and spinopelvic parameters among groups were assessed using one-way analysis of variance test and least significance difference post hoc test.

RESULTS

Patients

A total of 527 patients were reviewed. Of these, 165 had insufficient follow-up duration, 70 did not have full imaging available for analysis, 32 had a lumbar curve exceeding 10°, 21 had a history of lumbar spine surgery, and 5 were diagnosed as acute deep SSI after surgery, leaving 234 patients who met inclusion criteria. Over an average follow-up period of 70.6 months (ranging from 60 to 121 mo), 68 cases were diagnosed as ASD. All cases of ASD were found in the cranial adjacent segment, and there were no cases of ASD occurring in the caudal adjacent segment or in both the cranial and caudal adjacent segments. Up to the present, 31 patients have undergone revision surgery. Table 1 showed significant differences between two groups regarding age at the time of the initial surgery, the level of fusion, underlying etiology, and the presence of osteoporosis (all with P values of less than 0.05).

TABLE 1 Baseline Characteristics of the Non-ASD and ASD Groups

	Non-ASD	ASD	P	
No. patients	166	68	—	
Age at surgery, y	58.5±10.5	64.0±7.6	<0.001	
Sex	
 Male	66 (39.8%)	36 (52.9%)	0.065	
 Female	100 (60.2%)	32 (47.1%)		
Fusion level	
 L4–5 (1 level)	102 (61.4%)	16 (23.5%)	<0.001	
 L3–5 (2 levels)	64 (38.6%)	52 (76.5%)		
Etiology	
 Isthmic spondylolisthesis	26 (15.7%)	2 (2.9%)	0.001	
 Degenerative spondylolisthesis	56 (33.7%)	12 (17.6%)		
 Disc herniation	22 (13.3%)	10 (14.7%)		
 Spinal stenosis	16 (9.6%)	8 (11.8%)		
 Multiple	46 (27.7%)	36 (52.9%)		
Pfirrmann grade (cranial)	
 1	—	—	0.768	
 2	20 (12.0%)	6 (8.8%)		
 3	88 (53.0%)	38 (55.9%)		
 4	58 (34.9%)	24 (35.3%)		
 5	—	—		
Facet grade (cranial)	
 0	24(14.5%)	16(23.5%)	0.189	
 1	80(48.2%)	24(35.3%)		
 2	40(24.1%)	16(23.5%)		
 3	22(13.3%)	12(17.6%)		
BMI, kg/m²	26.3±3.5	26.9±4.5	0.330	
Follow-up, mo	68.9±12.8	70.7±15.6	0.402	
Medical comorbidities	
 Coronary artery disease	24 (14.5%)	10 (14.7%)	0.961	
 Diabetes mellitus	42 (25.3%)	20 (29.4%)	0.518	
 Hypertension	108 (65.1%)	46 (67.6%)	0.705	
 Cerebral infarction	2 (1.2%)	2 (2.9%)	0.375	
 Osteoporosis	6 (3.6%)	7 (10.3%)	0.043	
Values are presented as number (%) or mean ± SD unless otherwise indicated.

ASD indicates adjacent segment disease; BMI, body mass index.

Evaluation of Spinopelvic Alignment and Risk Factors of ASD

PI demonstrated significant correlations with PL (Pearson’s r=0.447), SL (Spearman’s rho= 0.395), LDI (Spearman’s rho=−0.300), and PI-LL (Spearman’s rho=0.418) (all with P values <0.01). However, the correlation with DL was not significant (Spearman’s rho=0.098, P=0.133). As depicted in Figure 2, linear regression analysis uncovered significant a linear and positive correlation between PI and PL (P<0.001).

Figure 2 Linear regression analysis uncovered a linear and positive correlation between PI and PL (PL=0.4049×PI−3.337, R2=0.199, P<0.001).

The ASD group showed significant variations in preoperative SVA, LL, and PL compared with the non-ASD group (all P values of less than 0.05). Patients with ASD had lower postoperative DL, LL, SS, and PI values compared with those without ASD, again with all P values of less than 0.05 (Table 2). Analyzing the position of the lumbar apex, it was found that 64.7% (44 of 68) of patients in the ASD group had a mismatched lumbar apex, in contrast to 41% (68 of 166) of patients of non-ASD group (P=0.001).

TABLE 2 Spinopelvic Parameters of the Non-ASD and ASD Groups

	Non-ASD	ASD	P	
Preoperative	
 PT,°	18.3±7.5	17.9±8.6	0.731	
 PI,°	51.2±9.3	49.2±9.9	0.128	
 SS,°	32.9±6.9	31.3±8.9	0.173	
 LL,°	45.1±10.6	39.2±15.0	0.004	
 DL,°	28.6±8.6	26.1±11.0	0.090	
 PL,°	16.5±9.3	13.2±9.4	0.014	
 SL,°	19.6±7.8	18.9±11.1	0.663	
 LDI, %	65.5±21.2	67.5±23.0	0.519	
 SVA, mm	11.6±36.8	28.5±47.7	0.010	
PI-LL	
 ≤10°	110 (66.3%)	46 (67.6%)	0.839	
 >10°	56 (33.7%)	22 (32.4%)		
Postoperative	
 PT,°	17.1±6.0	16.4±7.6	0.501	
 PI,°	51.7±9.2	48.9±9.7	0.042	
 SS,°	34.6±6.7	32.6±8.2	0.047	
 LL,°	48.9±9.6	43.9±11.1	0.001	
 DL,°	31.3±8.2	27.4±8.5	0.001	
 PL,°	17.6±8.9	16.5±7.7	0.377	
 SL,°	20.8±7.3	21.0±6.7	0.803	
 LDI, %	64.9±15.3	63.2±16.6	0.442	
 SVA, mm	7.8±28.7	7.8±28.1	0.994	
PI-LL	
 ≤10°	132 (79.5%)	48 (70.6%)	0.141	
 >10°	34 (20.5%)	20 (29.4%)		
Lumbar apex	
 Matched	98 (59.0%)	24 (35.3%)	0.001	
 Mismatched	68 (41.0%)	44 (64.7%)		
Values are presented as number (%) or mean ± SD unless otherwise indicated.

ASD indicates adjacent segment disease; PT, pelvic tilt; PI, pelvic incidence; SS, sacral slope; LL, lumbar lordosis; DL, distal lordosis; PL, proximal lordosis; SL, segmental lordosis; LDI, lordosis distribution index; SVA, sagittal vertical axis.

Multivariate analysis finally found four independent risk factors of ASD (Table 3). Age was determined to be a risk factor, with an odds ratio (OR) of 1.057 [95% confidence interval (CI): 1.014–1.103, P=0.009]. L3–5 fusion significantly increased risk (OR = 3.195, 95% CI: 1.494–6.832, P<0.001), as did a mismatched lumbar apex (OR = 3.165, 95% CI: 1.625–6.163, P=0.003). Conversely, greater DL appeared to be protective, with an OR of 0.950 (95% CI: 0.912–0.990, P=0.014).

TABLE 3 Univariate and Multivariate Analyses

	Univariate analysis	Multivariate analysis	
Variables	OR (95% CI)	P	OR (95% CI)	P	
Age	1.065 (1.030–1.101)	<0.001	1.057 (1.014–1.103)	0.009	
Sex	
 Male	Reference				
 Female	0.587 (0.332–1.036)	0.066	—	—	
Etiology	
 Isthmic spondylolisthesis	Reference				
 Degenerative spondylolisthesis	2.786 (0.581–13.357)	0.200	—	—	
 Disc herniation	5.909 (1.168–29.885)	0.032	—	—	
 Spinal stenosis	6.500 (1.224–34.528)	0.028	—	—	
 Multiple	10.174 (2.264–45.729)	0.002	—	—	
Fusion level	
 L4–5 (1 level)	Reference		Reference		
 L3–5 (2 levels)	5.180 (2.727–9.839)	<0.001	3.195 (1.494–6.832)	<0.001	
Osteoporosis	2.581 (0.802–8.036)	0.112	—	—	
Postoperative PI	0.969 (0.939–0.998)	0.043	—	—	
Postoperative SS	0.961 (0.923–0.999)	0.049	—	—	
Postoperative LL	0.951 (0.923–0.980)	0.001	—	—	
Postoperative DL	0.944 (0.911–0.978)	0.002	0.950 (0.912–0.990)	0.014	
Lumbar apex	
 Matched	Reference		Reference		
 Mismatched	2.642 (1.471–4.746)	0.001	3.165 (1.625–6.163)	0.003	
CI indicates confidence interval; DL, distal lordosis; LL, lumbar lordosis; OR, odd ratio; PI, pelvic incidence; SS, sacral slope.

Subanalysis by the Location of Lumbar Apex

When patients were stratified by the location of lumbar apex, those who were identified as caudal or cranial group showed a higher likelihood of developing ASD compared with those with an ideal lumbar apex position (P=0.0042) (Fig. 3). Patients in the ideal group, in comparison to the other two groups, had an older age and a more severe degree of disc degeneration (all P values of less than 0.05). Spondylolisthesis was found to be more commonly diagnosed in the cranial group, whereas the majority of patients in the caudal group were diagnosed with lumbar stenosis or disc herniation (P=0.001). Significant differences were also detected among groups regarding sex, body mass index, and cerebral infarction (all P values of less than 0.05) (Table 4).

Figure 3 Comparison of incidence of ASD among ideal match, cranial from ideal (proximal migration), and caudal from ideal (distal migration) groups.

TABLE 4 Baseline Characteristics of Ideal Match, Cranial From Ideal, and Caudal From Ideal Groups

	Ideal match (n=122)	Caudal from ideal (n=32)	Cranial from ideal (n=80)	P	
Age at surgery, y	61.7±8.9	58.8±14.5	58.1±9.2	0.028	
Sex	
 Male	58 (47.5%)	24 (75.0%)	20 (25.0%)	<0.001	
 Female	64 (52.5%)	8 (25.0%)	60 (75.0%)		
Fusion level	
 L4–5 (1 level)	62 (50.8%)	14 (43.8%)	42 (52.5%)	0.699	
 L3–5 (2 levels)	60 (49.2%)	18 (56.3%)	38 (47.5%)		
Etiology	
 Isthmic spondylolisthesis	14 (11.5%)	6 (18.8%)	8 (10.0%)	0.001	
 Degenerative spondylolisthesis	36 (29.5%)	0 (0%)	32 (40.0%)		
 Disc herniation	16 (13.1%)	6 (18.8%)	10 (12.5%)		
 Spinal stenosis	14 (11.5%)	4 (12.5%)	6 (7.5%)		
 Multiple	42 (34.4%)	16 (50.0%)*	24 (30.0%)†		
Pfirrmann grade (cranial)	
 1	—	—	—	0.029	
 2	10 (8.2%)	4 (12.5%)	12 (15.0%)		
 3	58 (47.5%)	18 (56.3%)	50 (62.5%)		
 4	54 (44.3%)	10 (31.3%)	18 (22.5%)		
 5	—	—	—		
Facet grade (cranial)	
 0	16 (13.1%)	10 (31.3%)	14 (17.5%)	0.091	
 1	50 (41.0%)	14 (43.8%)	40 (50.0%)		
 2	36 (29.5%)	6 (18.8%)	14 (17.5%)		
 3	20 (16.4%)	2 (6.3%)	12 (15.0%)		
BMI, kg/m²	26.2±3.5	25.0±3.8	27.5±4.0	0.004	
Follow-up, mo	70.0±13.9	65.9±10.4	70.0±14.4	0.306	
Medical comorbidities	
 Coronary artery disease	18 (114.8%)	4 (12.5%)	12 (15.0%)	0.939	
 Diabetes mellitus	32 (26.2%)	8 (25.0%)	22 (27.5%)	0.960	
 Hypertension	74 (60.7%)	24 (75.0%)	56 (70.0%)	0.195	
 Cerebral infarction	0 (0%)	0 (0%)	4 (5.0%)	0.013	
 Osteoporosis	6 (4.9%)	1 (3.1%)	6 (7.5%)	0.593	
Values are presented as number (%) or mean ± SD unless otherwise indicated.

P values were calculated with one-way analysis of variance analysis, chi-square test, or Fisher exact test.

* Four patients had a diagnosis of degenerative spondylolisthesis.

† Sixteen patients had a diagnosis of degenerative spondylolisthesis.

ASD indicates adjacent segment disease; BMI, body mass index.

Although the PI value of caudal group was highest, values of pre- and postoperative PT, SS, and LL were similar among three groups (all P values of less than 0.05). Preoperatively, significant differences were detected in the DL, PL, S,L and LDI among groups (all P values of less than 0.05). After surgery, spinopelvic parameters were improved in all three groups, and there were significant differences in the postoperative DL, PL and LDI among groups (all P values of less than 0.05) (Table 5).

TABLE 5 Spinopelvic Parameters of Ideal Match, Cranial From Ideal, and Caudal From Ideal Groups

				P	
	Ideal match (n=122)	Caudal from ideal (n=32)	Cranial from ideal (n=80)	Among groups	Ideal vs. caudal	Ideal vs. cranial	Cranial vs. caudal	
Preoperative	
 PT,°	17.3±7.2	20.0±7.9	18.8±8.5	0.160	0.087	0.190	0.467	
 PI,°	49.3±9.4	53.3±8.4	51.7±9.7	0.047	0.030	0.077	0.395	
 SS,°	32.0±7.8	33.4±7.8	32.8±7.2	0.554	0.349	0.426	0.732	
 LL,°	44.7±11.5	44.2±13.9	41.1±12.7	0.124	0.827	0.045	0.240	
 DL,°	30.1±8.0	32.1±9.6	22.8±9.4	<0.001	0.246	<0.001	<0.001	
 PL,°	14.6±8.3	12.1±11.7	18.3±9.5	0.002	0.167	0.005	0.001	
 SL,°	21.1±8.0	20.1±9.2	16.3±9.4	0.002	0.577	<0.001	0.062	
 LDI, %	69.5±19.3	77.2±25.9	56.3±19.8	<0.001	0.061	<0.001	<0.001	
 SVA, mm	10.4±37.6	24.2±45.0	22.7±43.1	0.059	0.089	0.037	0.858	
Postoperative	
 PT,°	15.9±5.4	18.1±7.8	17.8±7.2	0.065	0.094	0.041	0.858	
 PI,°	49.7±9.5	53.9±7.9	51.6±9.6	0.057	0.024	0.150	0.248	
 SS,°	33.7±7.5	35.8±7.8	33.8±6.5	0.330	0.153	0.984	0.179	
 LL,°	48.0±10.7	49.7±12.2	45.8±8.6	0.142	0.386	0.146	0.069	
 DL,°	31.8±7.3	37.2±7.4	25.0±7.4	<0.001	<0.001	<0.001	<0.001	
 PL,°	16.2±8.3	12.6±8.5	20.8±7.6	<0.001	0.025	<0.001	<0.001	
 SL,°	21.3±6.7	22.1±6.7	19.6±7.9	0.169	0.599	0.109	0.119	
 LDI, %	67.4±13.7	76.9±15.1	54.8±13.3	<0.001	0.001	<0.001	<0.001	
 SVA, mm	7.4±28.9	5.8±26.7	9.2±28.9	0.832	0.782	0.662	0.574	
Values are presented as number (%) or mean ± SD unless otherwise indicated.

Among groups p values were calculated with one-way analysis of variance (ANOVA) analysis. P values of comparisons between two groups were calculated with least significance difference (LSD) post hoc test following one-way ANOVA analysis.

DL indicates, distal lordosis; LDI, lordosis distribution index; LL, lumbar lordosis; PI, pelvic incidence; PL, proximal lordosis; PT, pelvic tilt; SL, segmental lordosis; SS, sacral slope; SVA, sagittal vertical axis.

DISCUSSION

LL represents the angle formed by the superior endplates of L1 and S1, and individuals with comparable LL measurements may display varying lumbar configurations.6,8,10 Thus, the shape of the lumbar spine could be considered as a more significant sagittal parameter. Our previous research confirmed that Roussouly classification can be used for predicting the occurrence of ASD.4 However, there are several limitations to this approach. First, Roussouly classification requires the consideration of numerous parameters when assessing whether patients have matched spinal shape, making the evaluation process complicated. This reduces the reliability and reproducibility of the results, limiting its direct clinical application. Second, according to previous studies, the majority of spinal shapes tend to progress towards a lower type when degenerative diseases occur, but there are rare instances where a low type may evolve into a higher type.16 Due to the limited number of cases, we were unable to conduct an in-depth analysis of these infrequent situations. Finally, when assessing the current type using Roussouly classification, conflicts can arise between different parameters. For example, a patient with degenerative diseases may exhibit a reduction in LL, which suggests a progression towards a lower type, but at the same time, the lumbar apex may show proximal migration, which is a characteristic of a higher type.6 Therefore, Roussouly classification has flaws and should be simplified when it comes to evaluating patients with degenerative diseases.

Recent studies have reported that the lumbar apex is one of the most important parameters which could affect lumbar spine shape.9 Pan et al. 6 then found that there was a correlation between PI and lumbar apex position in asymptomatic adults, meaning that as PI increased, the location of the apex was increasingly proximal. Subsequent researches indicated that repositioning the lumbar apex to its ideal location may decrease mechanical complications following surgery for adult spinal deformity.11,12,20 Thus, the current study incorporated the position of apex into the analysis and identified four features that could contribute to the ASD development, including age, fusion level, DL, and lumbar apex. If the lumbar apex was not aligned with the corresponding ideal location, the patients may be at a higher risk of developing ASD.

Concurrently, DL and LDI have been used as a tool to evaluate the potential risk of developing ASD after lumbar fusion procedures.5 Zheng et al. 21 observed that abnormal postoperative LDI significantly increased the risk of ASD compared with normal LDI levels. Nevertheless, considering the linear correlation between LDI and PI, it is unreasonable to establish a fixed threshold of 50-80% as the ideal range for LDI.22 In addition, previous studies have demonstrated that L4–S1 lordosis tends to remain relatively constant (around 35°) and shows independence from PI values.10 With increase of PI values, proximal levels are recruited to increase total lumbar lordosis, leading to decreased LDI.4,5,10,22 Our study also confirmed a significant correlation between PI and PL, as well as between PI and LDI. Thus, normal LDI range shifts with PI variations, but the ideal value of DL may be similar in the different cohorts.4,5

To figure out the relationship between spinopelvic alignment and lumbar apex, and their contribution to the occurrence of ASD, a subanalysis by the location of apex was conducted. We noticed that when the lumbar apex was not identical to the theoretical position, patients were more susceptible to severe sagittal imbalance. The diagnosis of spondylolisthesis was common in the cranial group. Because the spondylolisthesis could change the orientation of vertebral body and make it become more parallel to sacrum, a significant loss of DL and subsequently anterior displacement of the axis of gravity were frequently detected.4 The loss of DL could be partially compensated for by pelvic retroversion to maintain sagittal balance, but this capacity is limited by the PI value and aging and osteoarthritis of the hip.1,23 To prevent the trunk from leaning forward, the upper lumbar spine is positioned posteriorly by extending cranial adjacent segments.24 Due to increased PL and decreased DL, these patients had a significant reduction of LDI and the location of apex became more proximal than before. If the position of apex and DL are not adequately restored postsurgery, the reversal of pelvic retroversion is impeded and proximal lumbar levels are compelled to maintain an increased extension to preserve sagittal balance, as depicted in Figure 4. However, such a compensatory mechanism leads to abnormally high pressure on the posterior spinal structures, and consequently exposes adjacent segments to an elevated risk of retrolisthesis and degenerative changes.24

Figure 4 A case with proximal migration of apex. A and B, A 51-year-old woman with lumbar degenerative spondylolisthesis and spinal stenosis at L4–5, the ideal location of lumbar apex was L4–5 based on the PI of 42°. C, She underwent pedicle screw fixation from L4 to L5, and TLIF at L4–5. The upright lateral radiograph showed that PT was 11°, SS was 31°, LL was 42°, DL was 15°, LDI was 36%, and lumbar apex was L3–4, indicating the proximal migration of apex. D and E, At 5 years of follow-up, she complained of recurrent low back pain and leg numbness. The upright lateral radiograph showed hypertension and retrolisthesis at the adjacent segment, and MRI detected occurrence of L3–4 disc herniation.

In contrast, the majority of patients in the caudal group were diagnosed as lumbar stenosis or disc herniation which did not significantly change the shape of DL. Because of high PI values, the location of apex of these patients was more proximal than patients with low PI. Meanwhile, patients with high PI values had a good capacity to compensate for sagittal imbalance through pelvis retroversion.25 Thus, when degenerative diseases involved the lower lumbar spine, the apex of these patients tended to migrate and locate within the DL. Sagittal balance improved after the symptoms and stenosis were alleviated through surgery, but a low lumbar apex could lead to the posterior displacement of the axis of gravity. Our results showed that caudal group had lowest PL, suggesting that proximal levels were recruited to decrease lordosis to compensate for backward trunk. Distal migration of apex and reduction of PL made DL represent the major part of LL, resulting in an abnormally high LDI (Fig. 5). A well-balanced LL could convert to a longer but flat LL or a nonharmonious back with thoracolumbar kyphosis and short hyperlordosis.16 At the transition zone from thoracolumbar kyphosis to lumbar lordosis, the discs are significantly angled, leading to a higher likelihood of degeneration and retrolisthesis.24,25 Furthermore, when the back is flat, the pressure on the disc is at its peak, increasing the likelihood of early disc degeneration or herniation.24,25

Figure 5 A case with distal migration of apex. A and B, A 71-year-old man with lumbar degenerative spondylolisthesis and disc herniation at L3–4 and L4–5, the ideal location of lumbar apex was L3–4 based on the PI of 61°. C, He underwent pedicle screw fixation from L3 to L5, and TLIF at L3–4 and L4–5. The upright lateral radiograph showed that PT was 18°, SS was 43°, LL was 62°, DL was 45°, LDI was 73%, and lumbar apex was L4–5, indicating the distal migration of apex. D and E, At 3 years of follow-up, he complained of recurrent low back pain, and leg pain and numbness. The upright lateral radiograph showed disc collapse at the adjacent segment, and MRI detected occurrence of L2–3 spinal stenosis.

Based on the above-mentioned results, it can be observed that TLIF may not have an advantage in reconstructing the segmental sagittal alignment and restoring the position of apex. In future studies, it would be necessary to compare the effects of different surgical approaches such as TLIF, posterior lumbar decompression and fusion, posterior lumbar interbody fusion, anterior lumbar interbody fusion and oblique lateral interbody fusion on the segmental sagittal alignment and lumbar apex position. Among the different fusion techniques, the selection of fusion cage size and the placement position of fusion cage may be crucial factors determining the postoperative sagittal morphology and lumbar apex position. Therefore, we need to pay attention to the variations in these parameters.

Limitations

This study had several limitations. First, patients with lumbosacral fusion were not enrolled. Significant differences were observed in the clinical features between patients who underwent lumbosacral fusion and those who underwent float fusion at our centers. Previous studies have also reported differences in the compensatory mechanism and the rate of ASD between lumbosacral fusion and float fusion. To eliminate influences of lumbosacral fusion and reduce the bias, these patients were not included. Second, the robustness of the findings was constrained by the relatively small case series. The limited number of cases in the caudal group reduced the generalizability of the conclusions. To reach more powerful conclusions, an expansion of the dataset is necessary. Finally, the development of ASD following lumbar fusion surgery may be influenced by the passage of time. It is conceivable that some cases currently classified as non-ASD could progress to ASD as more time elapses. Therefore, to mitigate this potential bias, the implementation of a long-term follow-up study is recommended.

CONCLUSION

In summary, the position of the lumbar apex significantly influenced the risk of ASD following short-level TLIF for lumbar degenerative conditions. Proximal and distal migration of apex had different compensatory mechanisms to prevent sagittal imbalance. Minimizing ASD incidence requires careful adjustment of DL to realign the lumbar apex to its optimal position.

Key Points

If the lumbar apex was not aligned with the corresponding ideal location, the patients may be at a higher risk of developing ASD.

With increase of PI values, proximal levels are recruited to increase total lumbar lordosis, leading to decreased LDI.

Proximal and distal migration of apex had different compensatory mechanisms to prevent sagittal imbalance.

Minimizing ASD incidence requires careful adjustment of DL to realign the lumbar apex to its optimal position.

Institutional Review Board approval was obtained before enrolling patients in the database. Informed consent was obtained from each patient before enrollment.

The manuscript submitted does not contain information about medical device(s)/drug(s). No relevant financial activities outside the submitted work.

This work was supported by the National Natural Science Foundation of China (grant no. 82160555) and Natural Science Foundation of Xinjiang Uygur Autonomous Region (grant no. 2022D01A317).

The authors report no conflicts of interest.
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