INTRODUCTION
Oblique lumbar interbody fusion (OLIF) utilizes a completely different entry route compared to traditional posterior approaches such as posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF), providing indirect decompression and mechanical stability [4,6,10,14]. However, because OLIF preserves the posterior structures, it does not allow posterior column shortening, which is essential for achieving segmental lordosis in deformity correction. Whether sufficient lordosis can be achieved without this shortening remains controversial [18,21,22].
Previous studies have emphasized factors such as cage insertion angle and anterior positioning as key determinants of postoperative alignment [3,5]. In this study, we analyzed not only these known intraoperative variables, but also preoperative radiographic factors such as disc height (DH) and disc angle (DA). We further evaluated potential contributors that were underexplored in prior literature, including facet joint degeneration and the presence of anterior or posterior bridging osteophytes.
To minimize anatomical heterogeneity and inter-level variability in alignment, we limited our analysis to single-level OLIF procedures at L4/5. This segment was specifically selected because it is the most commonly affected level in lumbar degenerative disease and is anatomically complex. Due to variations in pelvic morphology and anterior vascular anatomy, L4/5 often presents difficulty in achieving ideal orthogonal cage insertion. As a result, this level tends to show greater variability in annulotomy location and cage trajectory, making it a suitable target for investigating how preoperative and intraoperative factors influence DA change in OLIF [5].
The aim of this study was to identify radiographic and cage-related predictors of postoperative DA loss following L4/5 OLIF, and to develop a risk stratification model to support personalized surgical planning and improve alignment outcomes.
MATERIALS AND METHODS
Ethical approval
This retrospective study was approved by the Institutional Review Board (IRB) of Pusan National University Yangsan Hospital (IRB No. 55-2025-050), with informed consent waived.
Patient selection
Between January 2021 and January 2023, patients who underwent OLIF surgery at our institution were retrospectively reviewed. Inclusion criteria were : 1) diagnosis of degenerative spondylosis with segmental instability unresponsive to medical therapy for more than 3 months, 2) availability of postoperative magnetic resonance imaging (MRI) within 1 week, 3) OLIF performed exclusively at the L4/5 level, 4) use of posterior percutaneous pedicle screw fixation, and 5) a minimum follow-up (FU) duration of one year with radiographic evaluation. Exclusion criteria included : 1) absence of postoperative MRI, 2) OLIF performed at levels other than L4/5, 3) FU period shorter than 1 year, 4) incomplete imaging data, and 5) performance of additional posterior deformity correction procedures, such as Smith-Petersen osteotomy.
Surgical technique
Procedures followed standard OLIF techniques [1,6,10,11,15,23]. Annulotomy was typically performed between the anterior and middle thirds of the disc space. In certain cases, anatomical constraints—such as the sympathetic chain or the left iliac vein—necessitated a slightly more posterior annulotomy. Cage insertion was performed with attention to orthogonality, although pelvic morphology at L4/5 often limited a perfectly orthogonal trajectory. Fluoroscopic guidance was used to ensure accurate cage positioning and to avoid over-insertion.
Radiographic and clinical assessment
Radiographic measurements included DA and DH, assessed in neutral, flexion (DAFLEX), and extension (DAEXT) views, obtained at three time points : preoperatively (PRE), postoperatively (POST), and at 1-year FU. The DA was defined as the Cobb angle between the superior endplate of L4 and the inferior endplate of L5. The DH was measured as the perpendicular distance from the midpoint of the lower endplate to the opposing upper endplate on lateral radiographs. ΔDH and ΔDA represent temporal changes over the course of treatment. Specifically, ΔDHPOST-PRE refers to the difference between the postoperative and preoperative DH. Subsidence was assessed in terms of both DH and DA, and was defined as a decrease in either parameter at FU compared to the immediate postoperative period (i.e., DHFU < DHPOST or DAFU < DAPOST).
Spinopelvic parameters—sacral slope (SS), pelvic incidence (PI), pelvic tilt (PT), lumbar lordosis (LL), and L4-S1 angle—were measured using standing lateral radiographs [13]. Lordosis distribution index (LDI) was calculated as L4-S1/LL [19] (Fig. 1).
Computed tomography was used to assess endplate sclerosis, facet degeneration (graded by the Kushchayev system) [7], and anterior or posterior bridging osteophytes across the disc space. Region of interest (ROI) values were measured at the mid-axial level of the L4 vertebral body, excluding cortical bone, to assess localized bone density [17].
MRI findings included preoperative diagnostic classification and spinal stenosis severity based on the Schizas grading system. Postoperative MR images were used to assess cage location, insertion angle, and the presence of endplate injury [12,16,20]. Cage position was quantified using X (left-right; Cage_X) and Y (anteroposterior; Cage_Y) coordinates relative to the center of the intervertebral disc. The insertion angle was defined as the Cobb angle between the cage trajectory and a reference line connecting the bilateral facet joints. Endplate injury was defined as a >2 mm protrusion of the cage beyond the superior or inferior endplate margin on sagittal MR images [3] (Fig. 2).
Clinical variables included age, sex, body mass index (BMI), bone mineral density (BMD), prior L4/5 laminectomy, estimated blood loss (EBL) and use of bone-enhancing agents such as teriparatide or romosozumab.
Statistical analysis
Statistical analyses were performed using SPSS (v26.0; IBM, SPSS Inc., Chicago, IL, USA). Continuous data were expressed as means±standard deviation, and categorical data as frequencies or percentages. Group comparisons used t-tests or Mann-Whitney U tests for continuous variables and chi-square or Fisher’s exact tests for categorical variables. Univariate logistic regression was performed to identify factors associated with postoperative angle loss. Variables with p<0.10 were entered into a multivariate logistic regression (forward stepwise). Odds ratios (ORs) with 95% confidence intervals (CIs) were reported. Receiver operating characteristic (ROC) curves were generated for continuous predictors; cutoffs were defined using the Youden index. Multivariate model performance was compared to individual predictors via area under curve (AUC).
A risk scoring system was developed based on significant predictors in multivariate analysis, assigning 1 point per factor. Angle loss incidence was then analyzed across risk score groups using chi-square trend testing. Significance was defined as p<0.05.
RESULTS
Patient demographics and surgical characteristics
From January 2021 to January 2023, 436 patients underwent OLIF surgery. Of these, 297 were excluded due to non-L4/5 or multilevel procedures. An additional nine were excluded for insufficient imaging, and 50 for FU loss before 1 year. A total of 669 patients were screened, of whom 80 met all inclusion criteria and were included in the final analysis (Supplementary Fig. 1). The mean age was 66.38±7.14 years, and 42.5% were male. The most common diagnosis was spondylolisthesis (53.8%), followed by spinal stenosis (37.5%) and revision surgery with instability (8.7%). The mean BMI was 25.45±3.00, and the average BMD T-score of lumbar spine was -0.46±1.65. Prior to surgery, 12.5% of patients were treated with teriparatide, and 6.3% received romosozumab for at least 3 months. The majority of cages used were 45 mm in length (88.7%), 12 mm in height (51.3%). All cages had a 6° angle and 18 mm width (Table 1).
Preoperative and postoperative radiological findings
Radiologically, the mean DHPRE was 6.52±2.85 mm, and the DAPRE was 4.34°±3.50°. The mean DAFLEX and DAEXT was 0.56°±3.53° and 5.82°±3.93°, respectively. The average SS and PI were 30.22°±8.62° and 48.83°±10.87°, respectively. Postoperatively, the mean increase in DH and DA was 5.22±2.11 mm and 1.08°±3.76°, respectively. At final FU, 58.8% of patients showed a decrease in DA (SubsidenceANGLE), while 56.3% exhibited radiographic SubsidenceHEIGHT (Table 2).
To better understand the dynamic behavior of the L4/5 segment over time, segmental DA and height were plotted at three time points (PRE, POST, and FU), stratified by whether angle gain or angle loss was observed. As shown in Fig. 3, patients with angle gain experienced a marked increase in DA immediately postoperatively, with partial preservation at FU. In contrast, the angle loss group started with a higher preoperative angle but experienced a sharp decrease following surgery, which persisted at FU. In terms of DH, both groups showed a notable increase postoperatively, with the angle loss group demonstrating a slightly greater increase. These trends highlight that angle gain is not solely dictated by DH restoration, but also influenced by segmental flexibility and cage positioning, justifying the focus on angular parameters in our study.
Correlation analysis summary
To explore potential inter-variable relationships, correlation analysis was performed among key radiologic and surgical parameters. Notably, DAPRE showed a strong positive correlation with DAEXT and a significant negative correlation with ΔDAPOST-PRE. Additionally, ΔDAPOST-PRE was moderately associated with Cage_Y, suggesting a biomechanical link between cage placement and angular correction or loss (Supplementary Fig. 2). These findings supported the selection of variables for subsequent multivariate analyses.
Risk factors for postoperative angle loss
Univariate analysis revealed that patients with angle loss had significantly greater DAPRE, reduced SS, and negative value of Cage_Y (Table 3). In multivariate logistic regression, three factors were identified as independently associated with postoperative angle loss. Patients with a greater DAPRE were more likely to experience angle loss after surgery. Specifically, for each 1° increase in DAPRE the odds of angle loss increased by approximately 38%. A lower SS was also significantly associated with angle loss, suggesting that a flatter lumbosacral alignment may reduce the capacity to maintain angular correction following interbody fusion. Furthermore, negative value of Cage_Y were more prone to postoperative angular loss, indicating the importance of optimal anterior cage positioning. EBL was marginally associated with angle loss, with lower EBL trending toward increased risk, although this finding did not reach conventional levels of statistical significance (Fig. 4).
In addition to the significant predictors, several demographic and radiographic factors—including BMI, BMD status, facet joint degeneration grade, bridging osteophytes, endplate sclerosis, ROI values, and spinal stenosis severity—were evaluated but did not show significant associations with postoperative angle loss in either univariate or multivariate analyses. These findings suggest that segmental morphology and cage trajectory may play a more dominant role in influencing angular outcomes than baseline degenerative changes or general bone quality (Table 4).
Predictive value and stratification by risk factor burden
To determine clinically meaningful thresholds for predicting postoperative angle loss, ROC curve analyses were conducted. A DAPRE greater than 6.15° predicted angle loss with an AUC of 0.736 (p<0.001), demonstrating good discriminatory power (sensitivity, 0.600; specificity, 0.840). To determine clinically meaningful thresholds for predicting postoperative angle loss, ROC curve analyses were conducted. A DAPRE greater than 6.15° predicted angle loss with an AUC of 0.736 (p<0.001), demonstrating good discriminatory power (sensitivity, 0.600; specificity, 0.840). An SS less than 32.30° also predicted angle loss with an AUC of 0.659 (p=0.018), though with more modest discriminative ability (sensitivity, 0.500; specificity, 0.767). Posterior cage positioning (Cage_Y <1.905 mm) was likewise associated with angle loss (AUC, 0.641; p=0.036; sensitivity, 0.320; specificity, 0.933). EBL less than 75 mL showed moderate predictive value (AUC, 0.725; p=0.001), but was excluded from the risk model due to its borderline significance in multivariate analysis (p=0.052) and limited preoperative applicability.
Based on the three significant variables—DAPRE >6.0°, SS <32.0°, and Cage_Y <1.9 mm—a simple risk stratification model was constructed. One point was assigned for each risk factor, yielding a total risk score from 0 to 3. The incidence of angle loss increased progressively with the number of risk factors : 0% in patients with no risk factors, 13.6% in those with one, 40.0% in those with two, and 81.3% in those with all three. This trend highlights the cumulative effect of multiple risk factors on postoperative DA outcomes (Fig. 5 and Table 5). To demonstrate the potential clinical relevance of our risk prediction model, we summarized the clinical course of representative cases with and without all identified risk factors in a Supplementary Fig. 3.
Predictors of angular subsidence
Given that angle loss can evolve over time, we conducted a separate analysis focusing on angular subsidence, defined as the reduction in DA from the immediate postoperative period to the final FU. A detailed summary of angular subsidence findings is provided in Supplementary Table 1. According to the multivariate logistic regression results for significant variables, more posterior cage positioning (i.e., lower Cage_Y values) and greater initial angular gain (i.e., high ΔDAPOST-PRE values) were independently associated with increased risk of angular subsidence. Notably, more posterior cage positioning significantly increased the odds of angular subsidence (OR, 1.583; p<0.001), while larger ΔDAPOST-PRE was inversely associated with this outcome (OR, 0.720; p=0.001) (Table 6). These findings emphasize the biomechanical relevance of cage position and magnitude of angular correction in predicting postoperative DA maintenance (Table 6).
DISCUSSION
This study analyzed how preoperative radiographic parameters and cage-related factors affect segmental lordosis changes after single-level L4/5 OLIF surgery. The results showed that a greater DAPRE was significantly associated with a higher risk of immediate postoperative angle loss. In addition, low SS and posterior cage positioning (low value of Cage_Y) were also linked to a greater risk of angle loss. Furthermore, a larger ΔDAPOST-PRE was associated with a higher incidence of angular subsidence at FU. These findings provide important insights into predicting segmental alignment outcomes following OLIF.
According to previous studies, OLIF has been reported to offer better angle correction compared to TLIF or PLIF due to less muscle dissection, reduced blood loss, and the use of larger cages [6,8,14,15]. However, this advantage is only partially supported by our findings. In our cohort, a greater number of patients than expected experienced postoperative angle loss. These results suggest that while OLIF can effectively increase DH, this does not always translate into an increase in DA (Fig. 3). Notably, insufficient segmental lordosis at L4/5 can negatively impact global alignment, reduce the LDI, and potentially lead to flatback deformity or an increased risk of adjacent segment disease [9]. Jang et al. [5] discussed the influence of spondylolisthesis reduction on adjacent segment degeneration, suggesting that proper DA restoration at the spondylolisthesis level can decrease the demand for hyperlordosis at adjacent segments. This complements our findings by highlighting the broader implications of segmental alignment on overall spinal balance.
Although prior studies have emphasized that anterior cage placement is beneficial for achieving greater segmental lordosis [10,22], our study quantified this relationship using the Cage_Y coordinate. We confirmed that more posterior cage placement was significantly associated with both angle loss and subsidence, supporting the existing body of evidence. Chen et al. [2] reported that anterior cage placement (higher cage center point ratio) leads to greater segmental lordosis restoration and a lower incidence of cage subsidence. Our findings corroborate this, emphasizing that more anterior cage positioning significantly enhances lordotic gain and reduces the risk of subsidence. The likely reason anterior cage positioning improves angle gain lies in the interaction between the cage and the concave curvature of the vertebral endplate, which increases the angle between the cage and endplate and thereby enhances the potential for lordotic correction (Fig. 6).
A key contribution of this study is the quantitative identification of preoperative DA as a powerful independent predictor of angle maintenance, with the highest OR (Exp(B)=7.9). Even alone, DA showed strong predictive value and should be considered in preoperative planning. Wu et al. [22] demonstrated a significant negative correlation between preoperative DA and postoperative DA change, indicating that larger preoperative angles are associated with reduced postoperative correction. This supports our observation that higher DAPRE may act as a negative prognostic factor for achieving intended angular correction. SS >32.30° also showed predictive value (Exp(B)=3.6), although relatively lower, and combining it with DA allowed for a more nuanced evaluation of sagittal alignment. For instance, patients with high SS and low DA typically presented with balanced sagittal alignment, favoring lordosis correction via OLIF. In contrast, those with low SS and high DA were more likely to have structural imbalance and experience angle loss after correction. This study is among the first to categorize patients based on DA-SS combinations and demonstrate their relevance for predicting outcomes.
We developed a simple additive risk score model using DA, SS, and Cage_Y. The frequency of angle loss and subsidence increased proportionally with the risk score. Although a weighted model based on Exp(B) values was also evaluated, the AUC difference was negligible. Therefore, we adopted the simple scoring model for its clinical practicality. The performance of both models is shown in the Supplementary Fig. 4.
Clinically, this risk model is applicable to preoperative planning. For high-risk patients (e.g., DA >6°, SS <31°), surgeons may consider anterior cage positioning, additional posterior shortening. In contrast, low-risk patients may achieve satisfactory correction with standard OLIF alone.
We intentionally limited our analysis to the L4/5 segment to eliminate confounding variables and better assess the effect of cage-related factors. The L2/3 level is often affected by rib interference, increasing the risk of endplate injury. L3/4 tends to have consistent annulotomy and trajectory, limiting variability in cage positioning [2]. Multilevel surgery introduces rod contouring effects, making single-level analysis more suitable for evaluating cage-specific impact.
Another limitation is that only 6° cages were included. Given OLIF’s emphasis on indirect decompression without posterior shortening, using 12° cages could pose biomechanical disadvantages such as increased endplate stress or suboptimal cagebone contact. Furthermore, 12° cages were used infrequently in our series. Their exclusion helped avoid selection bias, but it limits the generalizability of our findings regarding high-angle cages.
This study is also limited by its retrospective single-center design, and it did not compare OLIF with posterior shortening or high-angle cage cases. Nonetheless, the use of standardized surgical techniques, consistent imaging metrics, and multivariate regression supports the robustness of our findings. This study provides a valuable foundation for future research and contributes to the development of more precise correction strategies in OLIF surgery.
CONCLUSION
In single-level L4/5 OLIF surgery, postoperative DA loss was independently associated with greater preoperative DA (DAPRE), lower SS, and more posterior cage positioning (Cage_Y). Based on these three parameters, we developed a simple additive risk score that stratifies patients by their risk of angle loss and subsidence. This model is clinically applicable and may support alignment-focused surgical planning and individualized decision-making in lumbar degenerative disease.
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