Imaging analysis of adjacent segment disease combined with retrolisthesis after posterior lumbar interbody fusion: a preliminary cross-sectional comparative study

Introduction

Posterior lumbar interbody fusion (PLIF) is a commonly performed surgical procedure for treating lumbar degenerative diseases. Long-term studies have demonstrated its clinical efficacy in resolving lumbar disc herniation, lumbar spondylolisthesis, and lumbar stenosis (1). However, biomechanical changes in the spine following PLIF result in increased stress on the adjacent segments above and below the fusion, potentially leading to adjacent segment disease (ASD). Based on widely cited definitions in the spinal literature, adjacent segment degeneration (ASDeg) is defined as asymptomatic radiographic changes at levels adjacent to a spinal fusion, without clinical symptoms or requirement for intervention; meanwhile, ASD (or ASDis) is defined as radiographic degeneration accompanied by clinical symptoms such as pain or neurogenic claudication that generally warrant further treatment or revision surgery (2). ASD can be also categorized into symptomatic ASD and imaging ASD. The diagnostic criteria for symptomatic ASD included the recurrence of radiating lower limb pain after the initial surgery, and radiological evidence of degeneration at the adjacent fused segment indicating revision surgery (3,4). Lau et al. reported that the incidence of ASD following lumbar spinal fusion is approximately 13.4%, which contributes to increased patient pain, additional financial costs, and a substantial societal burden (5,6).

ASD can be classified based on whether there is slippage of the adjacent vertebrae and the direction of this slippage into three types: retrolisthesis, no-slip, and anterolisthesis. In our clinical practice, we have observed that many patients with ASD following lumbar fusion frequently present with retrolisthesis of the upper vertebra. However, to our knowledge, few studies have reported on ASD combined with retrolisthesis, and its imaging characteristics remain unclear. Qu et al. were the first to compare the three sagittal alignment types of ASD—retrolisthesis, anterolisthesis, and no-slip—and highlighted that the radiological and biomechanical characteristics of these degeneration types may differ (7). They reported differences in the Roussouly type and spinopelvic parameters between patients with ASD of different sagittal degeneration patterns. However, studies on the radiological parameters of the corresponding segments, such as intervertebral disc height and degree of degeneration, remain lacking.

Thus, this study aimed to investigate the epidemiology of ASD combined with vertebral retrolisthesis and to conduct a preliminary analysis of the radiological characteristics of patients with ASD, categorized into those with vertebral retrolisthesis, no-slip, and anterolisthesis. Furthermore, we sought to analyze the factors contributing to vertebral slippage in different directions and preliminarily examine the associated risk factors. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1066/rc).

Methods

Patients

This retrospective, cross-sectional comparative study was approved by the Ethics Committee of the Third Hospital of Hebei Medical University (approval No. 2024077-1) and was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. Informed consent was waived for this retrospective cross-sectional study, as it involves secondary analysis of de-identified data from routine clinical practice without direct participant intervention, posing minimal risk. Obtaining consent was impracticable due to difficulties in tracing all subjects.

We retrospectively analyzed all patients who underwent surgical treatment for symptomatic ASD at our institution between November 2020 and July 2024. The surgical indications for ASD included low back pain, radicular symptoms, or intermittent claudication that significantly impaired the quality of life and did not improve after 3 months of conservative treatment.

The inclusion criteria were as follows: (I) symptomatic ASD requiring surgical intervention; (II) age >18 years; and (III) first-time surgical treatment for lumbar disc herniation, lumbar spinal stenosis, or lumbar spondylolisthesis, with PLIF as the operative procedure (1). Meanwhile, the exclusion criteria were as follows: (I) presence of lumbar conditions such as combined lumbar fractures, tumors, tuberculosis, or ankylosing spondylitis; (II) incomplete or unclear preoperative lateral lumbar spine radiographs, lumbar spine computed tomography (CT), or lumbar spine magnetic resonance imaging (MRI); and (III) CT findings indicating severely disrupted the facet joints of the fused segments due to initial PLIF surgery

Patients were categorized into three groups based on the presence and direction of vertebral slippage in the diseased segments with ASD: the retrolisthesis group (group A), the no-slip group (group B), and the anterolisthesis group (group C). A detailed overview of the patient selection process, including inclusion and exclusion criteria, is presented in the patient enrollment flow diagram in Figure 1.

Figure 1 Flowchart of patient enrollment. ASD, adjacent segment disease; CT, computed tomography; MRI, magnetic resonance imaging; PLIF, posterior lumbar interbody fusion; TB, tuberculosis.

Retrolisthesis was defined as a posterior displacement of the superior vertebral body ≥3 mm on standard standing lateral radiographs (8), no-slip as the absence of anterior or posterior slippage of the superior vertebra in the affected segments, and anterolisthesis as an anterior slip of the superior vertebral body by ≥3 mm (9).

In addition, we collected preoperative imaging data from the initial surgery for patients in each group and performed a comparative analysis.

Imaging measurements

The patients’ radiographic data were obtained from the picture archiving and communication system (PACS), and two surgeons independently performed the measurements. To assess intra- and interobserver reliability, 30 patients were randomly selected, and the two surgeons repeated the measurements after a 2-week interval. The following radiographic and imaging parameters were assessed in this study:

• Lumbar disc height: on the lateral lumbar spine X-ray, plumb lines were drawn from the anterior edge, midpoint, and posterior edge of the inferior endplate of the superior vertebral body to the extension line of the superior endplate of the inferior vertebral body at the affected segment. The average of these measurements was used to determine the intervertebral disc height of the segment (10,11) (Figure 2).
• Lumbar lordosis (LL), upper lumbar lordosis (uLL), and lower lumbar lordosis (lLL): the LL angle was defined as the angle between the upper endplate of the L1 vertebra and the endplate of the S1 vertebra. The uLL angle was defined as the angle between the upper endplate of L1 vertebra and the upper endplate of L4 vertebra, and the lLL angle was defined the angle between the upper endplate of the L4 vertebra and the endplate of the S1 vertebra (Figure 3).
• Lumbar disc dehydration scores (Pfirrmann classification) (12): for lumbar disc dehydration, grade A was assigned a score of 0, grade B a score of 1, and so on, with higher scores indicating more severe disc degeneration (13) (Figure 4).
• Facet joint degeneration score: according to the Weishaup classification (14), grade 0 was assigned a score of 0, grade 1 a score of 1, and so on, with higher scores indicating more severe facet joint degeneration. The bilateral sum was used to represent the patient’s overall facet joint degeneration score (Figure 5).
  Facet joint angles (FJAs) and facet joint tropism: under the CT axial bone window, the level of the inferior vertebral body at the endplate was selected. The FJA was measured by drawing line b along the median sagittal plane through the vertebral body and by drawing line a to connect the anterior medial and posterior lateral points of the facet joints (Figure 4). The FJA for the segment was calculated by summing the left and right angles. The absolute value of the difference between the bilateral FJA measurements was recorded as the facet tropism (FT) (7,15). This study compared the FJAs of the L3–4 segment, which exhibited the highest proportion within the ASD group.
• Relative muscle cross-sectional area (RCSA): given that most of the fused segments were located at the L4–S1 vertebrae, substantial surgical disruption of the MF and erector spinae (ES) muscles prevented reliable quantitative analysis at those levels. To minimize the influence of surgical trauma, axial T2-weighted lumbar MR images at the level of the upper endplate of the L2 vertebra were selected to measure the cross-sectional areas of the MF and ES muscles (Figure 6A).
  Meanwhile, the cross-sectional area of the psoas major (PS) muscle was assessed at the level of the L4 upper endplate (Figure 6B). To account for variability due to body size and fat infiltration, vertebral body cross-sectional areas at the L2 and L4 vertebrae were also measured, and the RCSA was calculated by normalizing the muscle area to the corresponding vertebral area.

Figure 2 Representative image of the measurements for the LL, uLL, and lLL angles and lumbar disc height. lLL, lower lumbar lordosis; LL, lumbar lordosis; uLL, upper lumbar lordosis.

Figure 3 Facet joint degeneration score. The degree of degeneration from left to right is graded from mild to severe, with scores of 0–3, respectively.

Figure 4 Disc dehydration score. The degree of dehydration from left to right is graded from mild to severe, with scores of 0–4, respectively.

Figure 5 The measurement of FJAs. The FJA was measured by drawing line b along the median sagittal plane through the vertebral body and by drawing line a to connect the anterior medial and posterior lateral points of the facet joints (Figure 4). The FJA for the segment was calculated by summing the left and right angles. The absolute value of the difference between the bilateral FJA measurements was recorded as the FT. FJAs, facet joint angles; FT, facet tropism.

Figure 6 Representative images of paraspinal muscle cross-sectional area measurement. (A) The measurement of ES and MF at the level of the upper L2 endplate; (B) the measurement of PS at level of the upper L4 endplate. ES, erector spinae; MF, multifidus; PS, psoas major; VB, vertebral body.

All segmentations were performed manually using ImageJ software (National Institutes of Health, Bethesda, MD, USA) by two trained independent observers based on clearly identifiable muscle fascial boundaries. The average of the two measurements was used for analysis.

Statistical analysis

Statistical analyses were conducted with SPSS version 29.0 (IBM Corp., Armonk, NY, USA). A Chi-squared test was applied to compare categorical variables (e.g., gender) between the three groups. One-way analysis of variance (ANOVA) was used to evaluate differences in continuous variables. Intraclass correlation coefficients (ICCs) were calculated to assess both intra- and interobserver reliability of imaging measurements. Binary multiple logistic regression was performed on variables that were significantly different between the retrolisthesis and no-slip groups to identify independent risk factors associated with retrolisthesis in patients with ASD.

Results

A total of 165 patients who met the inclusion and exclusion criteria were included in this study. Of these, 79 were male and 85 were female, with a mean age of 62.21 years (range, 27–80 years). Among the patients, 145 (87.88%) had ASD segments at the cranial end, while 20 (12.12%) had ASD segments at the caudal end. Group A (retrolisthesis group) included 102 cases (61.8%), group B (no-slip group) included 43 cases (26%), and group C (anterolisthesis group) included 20 cases (12.1%). ICC analysis indicated high intraobserver and interobserver reliability for all measured parameters (range, 0.875–0.959). In addition, we collected preoperative imaging data from the initial surgery for a subset of patients and conducted a comparative analysis. This included 21 patients in the retrolisthesis group and 5 patients in the no-slip group.

Baseline data

In this study, there was no statistically significant difference in age or the number of fused segments between the three groups. Anterolisthesis was observed in 16 women (80%) and 4 men (20%), representing a statistically significant difference (P=0.028). The average time from the initial fusion surgery for all patients was 6.86±4.55 years, with no statistically significant difference between the three groups (P=0.458). Specifically, the mean intervals were 6.56±4.41 years for the retrolisthesis group, 7.60±5.01 years for the no-slip group, and 6.70±4.02 years for the anterolisthesis group. The average body mass index (BMI) was 26.77±0.28 kg/m², and similarly, there was no statistically significant difference between the three groups (P=0.946) (Table 1).

Radiographic measurement

There was no statistically significant difference between the three groups in terms of LL and uLL; however, the lLL angle was significantly greater in the retrolisthesis group than in the anterolisthesis group (24.22±8.61 vs. 18.05±9.84; P=0.005). The retrolisthesis group had significantly lower disc heights in the affected segments as compared to the no-slip group (0.76±0.25 vs. 0.96±0.23, P<0.001) and significantly more severe disc degeneration (3.58±0.72 vs. 2.98±0.64; P<0.001).

Regarding the facet joint degeneration score, the retrolisthesis group showed more severe facet joint degeneration than did the no-slip group (3.58±0.72 vs. 2.98±0.64; P<0.001). Similarly, the anterolisthesis group exhibited greater facet joint degeneration than did the no-slip group (3.45±0.76 vs. 2.98±0.64; P=0.014).

For the RCSA of the paraspinal muscles, the MF RCSA was significantly smaller in the retrolisthesis group as compared to both the no-slip group (0.17±0.70 vs. 0.21±0.67; P=0.004) and anterolisthesis group (0.17±0.7 vs. 0.23±0.75; P<0.001). The ES RCSA was also smaller in the retrolisthesis group than in the no-slip group but smaller in the no-slip group than in the anterolisthesis group (2.11±0.49 vs. 2.34±0.54; P=0.016). Finally, the RCSA in the anterolisthesis group was smaller than that in the retrolisthesis group (0.73±0.27 vs. 0.93±0.35; P=0.016) (Table 2).

In this study, a total of 83 cases of ASD at the L3–4 vertebrae were identified, with 61 cases in group A (retrolisthesis group), 13 cases in group B (no-slip group), and 9 cases in group C (anterolisthesis group). The FJA in the anterolisthesis group was significantly smaller than that in both the retrolisthesis and no-slip groups (P=0.004 and P=0.043), while there was no statistically significant difference between the retrolisthesis and no-slip groups in this regard. Additionally, no statistically significant difference was found in FT between the three groups (Table 3).

Logistic regression analysis

In logistic regression analyses, MF and ES RCSA below the mean were classified as atrophy, while values above the mean were considered normal. With reference to the no-slip group, we identified the risk factors for ASD with retrolisthesis to be reduced disc height [odds ratio (OR) =11.185; 95% confidence interval (CI): 1.276–98.045; P=0.029], severe disc degeneration (OR =0.400; 95% CI: 0.194–0.823; P=0.013), and MF atrophy (OR =4.087; 95% CI: 1.378–12.122; P=0.011) (Table 4).

Comparison of preoperative imaging before the first surgery

We compared baseline date and imaging findings prior to the initial surgery between the two groups. Regarding baseline date, no statistically significant differences were observed in terms of sex, age, or BMI. In terms of imaging findings, the retrolisthesis group had significantly lower preoperative disc height as compared to the no-slip group (P=0.025), whereas no significant difference was found for disc dehydration scores. Additionally, the MF RCSA of the ES was significantly smaller in the retrolisthesis group prior to the initial surgery (P=0.006), and there was no statistically significant difference in ES RCSA or PS RCSA (Table 5).

Discussion

Although numerous studies have addressed ASD following lumbar fusion, there is little research on the clinically common phenomenon of ASD combined with retrolisthesis. In this study, we reported for the first time that patients with ASD combined retrolisthesis accounted for 61.8% of those with symptomatic ASD, representing the largest proportion among the three patterns of ASD: superior vertebrae retrolisthesis, anterolisthesis, and no slip (7). Our findings further revealed that patients with ASD and retrolisthesis of the adjacent vertebrae exhibited severe lumbar disc and facet joint degeneration, as well as pronounced atrophy of the MF and ES muscles. This study preliminarily establishes a relationship between lumbar disc degeneration, paravertebral muscle atrophy, and ASD with retrolisthesis, identifying MF atrophy and lumbar disc degeneration as significant risk factors. Furthermore, a comparative analysis of preoperative imaging data from the initial surgery in a subset of patients was conducted, which further corroborated our conclusions.

Previous studies have suggested that the LL angle of patients with retrolisthesis is less than that of patients with no slip (8,16,17). In contrast, our study found that the LL angle in patients with ASD and retrolisthesis was greater than that in those without slippage, although the difference in total LL angle was not statistically significant. Rothenfluh et al. reported that patients with a mismatch between pelvic incidence and LL had a 10-fold higher risk of requiring revision surgery as compared to controls (18). This may be because the baseline LL angle in those with retrolisthesis group was relatively lower, and surgical correction might have overcompensated, making the upper lumbar vertebrae more prone to retrolisthesis.

The lumbar disc plays a crucial role in bearing longitudinal loads and maintaining spinal stability (7,19). Previous studies have shown that the lumbar disc bears nearly half of the antitorque moment during segmental movement of the lumbar spine (4). Yoganandan et al. confirmed that when the human body is upright, the lumbar vertebrae and intervertebral discs bear about 80% of the gravitational pressure (20). In our view, the applied pressure can be decomposed into two components: one transmitted to the spine and another as a shear force parallel to the endplate. With the operated segment frequently being located in the lower lumbar spine and the segment with ASD typically positioned above the lordosis apex, a shear force directed posteriorly along the endplate occurs when the body is in an upright position. This shear force is referred to as the physiological posterior slip force. Our study found that lumbar disc degeneration in the retrolisthesis group was more pronounced as compared to that in the no-slip group, indicating that disc degeneration significantly contributes to the development and progression of retrolisthesis in patients with ASD. Degeneration impairs the segment’s ability to maintain spinal stability, making it more challenging to counteract the physiological posterior slip force and thus increasing the likelihood of retrolisthesis in the superior vertebra. Additionally, facet joints are crucial for maintaining spinal alignment and preventing vertebral slippage.

In this study, facet joint degeneration in the retrolisthesis group was more severe than that in the no-slip group. Yang et al. reported through finite element modeling that the facet joints of the lumbar spine bear approximately 3% to 25% of the spinal load (21). We hypothesize that retrolisthesis significantly increases pressure on the facet joints posterior to the vertebrae. van Schaik demonstrated that elevated pressure on the lumbar facet joints correlates with a substantial increase in the load on the joint cartilage (22). This pressure escalation damages the cartilage and adversely affects the metabolic processes of the synovial fluid and cartilage tissue, impairing their protective and nutritional functions and leading to progressive degeneration. Consequently, the redistribution of longitudinal spinal pressure resulting from retrolisthesis markedly increases the pressure on the facet joints, which explains the severe degeneration observed in the retrolisthesis group. Furthermore, it has been widely established that the FJAs in patients with anterolisthesis are significantly more sagittal (23,24). However, few studies have investigated the FJA in patients with retrolisthesis. In this study, the FJA in the anterolisthesis group was found to be smaller and more sagittal than that in both the retrolisthesis and no-slip groups, which aligns with previous research (23). Conversely, the FJA in the retrolisthesis group was larger and more coronal than that in the anterolisthesis group. This study is the first to examine the FJA in patients with retrolisthesis on a segmental basis. Future research should involve larger sample sizes and include patients who hospital primarily due to anterolisthesis or retrolisthesis.

Degeneration of the paravertebral muscles can result in decreased muscle tension, which impairs the ability to counteract spinal loads, leading to lumbar instability and, ultimately, lumbar spondylolisthesis. Duan et al. demonstrated that the incidence of paravertebral muscle degeneration following L4–5 TLIF is closely related to the occurrence of ASD at the L3–4 vertebrae (25). In our study, the MF RCSA and ES RCSA in the retrolisthesis group were significantly smaller than those in the other two groups, confirming that the degeneration of the paravertebral muscles is related to retrolisthesis. However, there was no statistical significance in the PS in the retrolisthesis and no-slip groups. The possible explanation is that when retrolisthesis occurs, the PSs located at the front of the vertebral body increase their contractile force to restore the spine’s upright posture, with compensatory hypertrophy enhancing overall spinal stability. Qu et al. demonstrated that patients with the Roussouly I spinal type are more prone to developing ASD with retrolisthesis following lumbar fusion, although their findings were limited to the relationship between spinopelvic types and ASD (7). We further found that the degeneration of the paravertebral muscles was closely associated with the development of ASD and the retrolisthesis of the adjacent vertebrae. We speculate that a combination of spinopelvic type and paravertebral muscle atrophy contributes to ASD with retrolisthesis.

Another novel finding was that 61.8% of patients with symptomatic ASD also exhibited retrolisthesis of adjacent vertebrae. We hypothesize that patients with preoperative retrolisthesis observed on radiographs may have a higher likelihood of requiring secondary surgery for ASD. Furthermore, MF muscle atrophy was identified as a risk factor for ASD with retrolisthesis. Therefore, we hypothesize that during postoperative follow-up after lumbar fusion surgery, if retrolisthesis of adjacent vertebrae is observed on radiographs, early initiation of lumbar muscle-strengthening exercises may be beneficial in reducing the risk of developing ASD. However, this hypothesis requires validation through prospective, controlled studies.

This study involved several limitations which should be noted. (I) The design was preliminary, retrospective, and cross-sectional in nature and was conducted at single center, which limits our ability to determine the sequence of retrolisthesis and ASD development. Further longitudinal studies are required to establish a causal relationship. (II) A portion of the patients had their initial surgeries at other institutions, and their preoperative imaging data were unavailable. Consequently, the spinopelvic parameters could not be obtained, and the presence of adjacent segment degeneration at the time of the first surgery could not be fully excluded. This missing information might have introduced selection bias. (III) This study inevitably involved a degree of sampling bias, as it only included patients who underwent revision surgery for symptomatic ASD. As a result, the cohort may represent a more severe spectrum of ASD cases, possibly underrepresenting patients with mild or asymptomatic ASD who did not require surgical intervention. Future prospective studies including both surgical and nonsurgical patients with ASD may provide a more comprehensive understanding of the epidemiology and risk factors of ASD. (IV) Subgroup analysis indicated certain patterns in imaging across vertebral slip patterns, but limited sample sizes (notably the no-slip group) entail reduced statistical power, higher frequency of type II errors, and a compromised reliability in intergroup comparison. These findings should be interpreted and generalized with caution, and further validation with larger, more balanced studies is needed. (V) Another limitation is that no formal correction for multiple comparisons was applied, which might have increased the risk of type I error. Given the exploratory nature of the study, we prioritized identifying potential associations, but the results should be interpreted with caution. Future studies with larger sample sizes and adjusted statistical models are needed to confirm these findings.

Conclusions

Among the three patterns of ASD examined in this study, ASD combined with retrolisthesis was the most common, which was associated with severe degeneration of discs and facet joints, along with significant atrophy of the MF. Reduced disc height, severe disc degeneration, and atrophy of the MF were identified as risk factors for ASD combined with retrolisthesis. In summary, we hypothesize that early lumbar muscle-strengthening exercises may reduce ASD risk in patients with adjacent vertebral retrolisthesis detected on postoperative follow-up radiographs after lumbar fusion; validation via prospective, controlled studies is warranted.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1066/rc

Data Sharing Statement: Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1066/dss

Funding: This work was supported by the Natural Science Foundation of Hebei Province (No. H2020206656 to L.W.).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1066/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was approved by the Ethics Committee of the Third Hospital of Hebei Medical University (No. 2024077-1). This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. Informed consent was waived for this retrospective cross-sectional study, as it involves secondary analysis of de-identified data from routine clinical practice without direct participant intervention, posing minimal risk. Obtaining consent was impracticable due to difficulties in tracing all subjects.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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