A Novel X-Ray-Based Screening Model for Bilateral L5 Spondylolysis

Abstract

Background: Lumbar spondylolysis is commonly evaluated initially with lumbar radiography, but radiographs may miss pars interarticularis defects, particularly at L5. This study aimed to characterize radiographic and morphological features associated with bilateral L5 spondylolysis and to develop and externally evaluate an X-ray-based screening nomogram for radiographic risk stratification.

Methods: This cross-sectional study included patients who underwent lumbar radiography and computed tomography (CT) between 2018 and 2025. Patients with CT-confirmed bilateral L5 spondylolysis were compared with controls without spondylolysis. Radiographic parameters and CT-based morphological features were measured and analyzed separately, and interobserver reliability was assessed. Propensity score matching (1:1) was used to balance baseline characteristics. Factors associated with L5 spondylolysis were identified using multivariable logistic regression, and an X-ray-only nomogram was constructed using significant radiographic variables. Model performance in the development cohort was assessed using discrimination, calibration, decision curve analysis, and clinical impact curves with internal bootstrap validation. External validation applied the original development-cohort coefficients without refitting and primarily assessed discrimination in an independent dataset.

Results: After propensity score matching, 114 patients were included in the development cohort, with 57 patients in each group. Compared with controls, patients with L5 spondylolysis had larger sacral slope, L5 lamina inclination angle (L5 LIA), and pedicle internal inclination angle, as well as smaller sacral inclination and L5 lumbar index. Multivariable analysis identified smaller sacral inclination, smaller L5 lumbar index, larger sacral slope, larger L5 LIA, and larger pedicle internal inclination angle as factors independently associated with L5 spondylolysis. L5 LIA showed the strongest individual diagnostic performance (area under the curve [AUC] = 0.879; 95% CI, 0.813–0.944). The X-ray screening model incorporating the L5 lumbar index, sacral inclination, sacral slope, and L5 LIA achieved excellent discrimination in the development cohort (AUC = 0.916; bootstrap-corrected concordance index = 0.911) and maintained strong discrimination in the external validation dataset when the development-cohort coefficients were applied without refitting (AUC/C-index = 0.949; 95% CI, 0.922–0.977).

Conclusions: An X-ray-based nomogram incorporating key sagittal alignment and morphological parameters showed strong diagnostic discrimination for bilateral L5 spondylolysis in both the development cohort and the external validation dataset. This model should be interpreted as a radiographic screening and risk-stratification aid rather than a tool for prospective prognosis. CT-based analysis also showed that patients with L5 spondylolysis had a larger pedicle internal inclination angle, a morphological difference with potential relevance to preoperative planning.

Keywords

lumbar vertebrae / nomograms / radiography / spondylolysis / tomography / X-Ray computed

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The Author(s) 2026.

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