Lumbar disc herniation (LDH) is a common condition among middle-aged adults, characterized by displacement of disc material that may compress the spinal canal or nerve roots. Diagnosis typically relies on clinical assessment supported by imaging. While conservative management is effective for most patients, surgical intervention is indicated for those with severe symptoms such as motor weakness or cauda equina syndrome. The degree of spinal canal compromise is an important factor influencing treatment outcomes and the likelihood of requiring surgery.

Previous studies have shown that radiographic findings, particularly the extent of canal compromise, are strong predictors of conservative treatment failure (1–4). Reported degrees of compromise vary widely, ranging from approximately 20% (3, 4) to 50% in surgical cohorts (5). Factors such as reduced intervertebral disc space, smaller spinal canal dimensions, and the biomechanical loading environment of the lumbar spine may contribute to the severity of disc herniation. Adolescent patients with narrower spinal canals are more likely to undergo surgical treatment (6). In addition, spinopelvic alignment parameters—including pelvic incidence (PI) and sagittal vertical axis (SVA)—have been reported to differ between surgical and nonsurgical groups in LDH and other degenerative spine conditions (7–9).

Although magnetic resonance imaging (MRI) is the gold standard for evaluating disc displacement and canal compromise, it is costly and often unavailable in many primary and secondary care settings. In contrast, standing radiographs are universally accessible, even in resource-limited environments, and remain the first-line imaging modality for patients presenting with lumbar symptoms. Identifying radiolographic alignment parameters that correlate with MRI-determined LDH severity could help clinicians prioritize referrals, improve triage accuracy, and optimize MRI utilization where resources are constrained.

Given the limited evidence directly linking standing radiographic parameters with MRI-defined severity in LDH, we propose that altered spinopelvic alignment, decreased disc height, and reduced sagittal canal dimensions may be associated with the degree of herniation. To our knowledge, no prior study has evaluated whether standing spine radiographs can predict the severity of LDH. This study therefore aims to assess radiological parameters obtained from standing full-spine lateral radiographs as potential predictors of spinal canal compromise in healthcare settings with limited MRI access.

In this study, we examined whether radiographic alignment parameters measurable on standing full-spine lateral radiographs are associated with the severity of LDH. Among all evaluated parameters, only SVA demonstrated a statistically significant association with severe canal compromise (≥50%). In contrast, DHI, pelvic parameters, and PW:SBW showed no significant association with the degree of canal compromise.

We selected a ≥50% threshold for canal compromise based on the representative mean value reported in surgical cohorts by Sutheerayongprasert et al. (5). Demographic characteristics—including sex, age, BMI, smoking status, and neurological deficits—did not differ significantly between patients with mild (<50%) and severe (≥50%) canal compromise. The higher prevalence of diabetes mellitus observed in the mild group (7.3% vs. 0%, p = 0.04) is likely attributable to the small sample size. This finding is consistent with Kaiser et al. (14), who reported no association between demographic factors and severe high canal compromise presentations such as cauda equina syndrome. Although smoking, diabetes, and obesity have been associated with recurrent LDH (15), our results suggest these factors may not substantially influence the initial severity of canal compromise.

Herniation characteristics differed between groups. Central and extrusion-type herniations were more common in the severe group, whereas paracentral and protrusion-type herniations predominated in the mild group. The mean canal compromise was significantly greater in the severe group (59.7%) than in the mild group (38.33%), consistent with previous reports (5). DHI did not differ significantly between groups, suggesting that although reduced disc height has been linked to recurrence risk (16), it may not correlate with the initial severity of canal compression.

Regarding spinopelvic parameters, PI and LL did not differ significantly between groups; however, SVA was markedly higher in patients with severe canal compromise (28.25 mm vs. 6.86 mm, p = 0.002). This finding is consistent with prior studies by Endo et al. (7) and Fei et al. (17), which reported increased SVA in LDH cohorts. LL tended to be higher in the mild group, in line with earlier observations (17). The finding that PI values were within normal ranges aligns with reports by Barrey et al. (18) and normative data from the Thai population reported by Pluemvitayaporn et al. (19). Collectively, these findings suggest that while PI and LL may not be directly associated with canal compromise severity, increased SVA may reflect compensatory sagittal alignment changes in patients with more substantial canal compression.

Importantly, SVA should not be interpreted as a risk factor for canal compromise. Rather, it is a global alignment parameter that appears to be associated with MRI-defined severity. This association must be interpreted cautiously, as SVA can be influenced by pain-related postural adaptation, compensatory mechanisms, and individual differences in pain tolerance. These factors may limit the generalizability of SVA as a predictive measure. Moreover, the discriminatory performance of SVA was modest (AUC = 0.60). Although multivariable analysis showed that SVA ≥50 mm was associated with severe canal compromise (OR = 3.376, 95% CI: 1.658–6.877), its sensitivity was low (41%) despite relatively high specificity (83%) at a cutoff of 52 mm.

From clinical perspective, these findings do not support the use of SVA as a stand-alone screening or diagnostic tool. Instead, SVA may serve as an adjunctive parameter in settings where access to MRI is limited. In patients with clinically suspected LDH, an elevated SVA (≥52 mm) may prompt consideration of earlier MRI to clarify the extent of canal compromise. Early identification of patients with severe canal compromise is relevant because such patients are more likely to require surgical intervention (3, 4, 11) and delays in surgery have been associated with slower recovery and increased healthcare costs (12, 20, 21). Nevertheless, these implications must be interpreted caution given the limited sensitivity of SVA, its susceptibility to postural variation, and the absence of clinical severity measures in this study (3, 4, 11, 12, 20, 21).

From a health system perspective, SVA assessment may assist triage decision in primary and secondary care environments with limited MRI availability, where referral timing is often uncertain. While MRI is clearly indicated for patients with progressive neurological deficits or cauda equina syndrome, guidance for other cases is less clearly defined and often recommends MRI only after four weeks of unsuccessful conservative treatment (22, 23). In this context, SVA may help identify patients who could benefit from earlier evaluation, but it should not replace clinical judgment or MRI when available.

This study has several limitations. The retrospective design introduces potential selection bias and treatment details were not systematically collected, limiting the ability to correlate canal compromise with clinical outcomes. Although intra- and inter-observer reliability were not recalculated, the use of established measurement methods supports reproducibility. Chen et al. (24) demonstrated high reliability for the modified Farfan's method used to measure DHI, and Yamada et al. (25) reported high reliability for spinopelvic parameters, including PI, SS, and PT. Prospective studies incorporating clinical severity measures, symptom-based stratification, and longitudinal outcomes are needed to confirm these findings and define the clinical role of SVA more precisely.

This study demonstrates that increased sagittal vertical axis is associated with more severe spinal canal compromise in patients with lumbar disc herniation. Although SVA alone cannot reliably determine the severity of canal compromise and should not be used as a screening or exclusion tool, it may provide adjunctive information in clinical assessment, particularly in settings where access to MRI is limited. When interpreted alongside clinical findings, SVA help identify patients who could benefit from earlier MRI evaluation. Further prospective studies are required to validate these findings and to define the appropriate role of SVA within clinical decision-making pathways.