The Modified Spinal Instability Spondylodiscitis Score (mSISS): Adaptation and Validation of a Novel Classification System for Spinal Instability in Spondylodiscitis.

Study DesignMulticenter reliability and validation study.ObjectivesTo adapt and validate the Spinal Instability Spondylodiscitis Score (SISS) to create a practical and reliable classification system for assessing spinal instability in pyogenic spondylodiscitis.MethodsThe original SISS was modified through structured consensus meetings within the AO Spine Trauma and Infection Knowledge Forum, resulting in the modified SISS (mSISS). The mSISS incorporates four parameters-location, extent of bone lesion, spinal alignment, and mechanical pain-to classify lesions as stable, potentially unstable, or unstable using computed tomography (CT) and clinical data. Fifteen experienced spine surgeons independently evaluated ten representative cases in two rating sessions. Intra- and interrater reliabilities were calculated using intraclass correlation coefficients (ICC) and Fleiss' Kappa. The gold standard was established by consensus of five expert spine surgeons.ResultsThe mSISS demonstrated excellent intrarater reliability for the total score (ICC 0.90, 95% CI 0.84-0.96). Interrater reliability for the total score was 0.87 (95% CI 0.77-0.97) in the first assessment and 0.89 (95% CI 0.80-0.98) in the second. Reliability of individual parameters ranged from moderate to excellent, with spinal alignment showing the highest variability but remaining within acceptable agreement levels. Agreement with the gold standard ratings was high across all parameters.ConclusionThe mSISS is a simplified and clinically applicable scoring system for the assessment of spinal instability in pyogenic spondylodiscitis, demonstrating strong reliability among expert spine surgeons. Broader international validation is ongoing to support its integration into clinical decision-making.