Spine - 2026-06-15 - Journal Article; Observational Study
Instability in Spinal Infection: Evaluating the Predictive Value of the Spinal Instability Spondylodiscitis Score (SISS).
Neuhoff J, Thavarajasingam SG, Sutherland R, McCaughan H, Abdelmaksoud A, Shiban E, Davies B, Kandziora F, Ringel F, Demetriades AK, Kramer A
Topics
Key Takeaway
The SISS 'unstable' category predicted radiologic deformity progression with 94.5% specificity and 77.8% PPV/NPV (AUC=0.68), but SISS classification was not an independent predictor of functional outcome (mRS 4–6) in multivariate analysis.
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Summary
This two-center retrospective cohort evaluated whether the SISS predicts radiologic progression and functional outcomes in pyogenic spondylodiscitis. In 54 conservatively managed patients, SISS 'unstable' classification showed moderate predictive value for deformity progression (PPV/NPV 77.8%, specificity 94.5%, AUC=0.68), with vertebral destruction >50% as the strongest individual predictor. In multivariate analysis across both treatment groups, age, neurological deficits at admission, and SIRS—not SISS classification—were the independent predictors of poor functional outcome (mRS 4–6).
Key Limitation
The conservatively treated subgroup (n=54) is underpowered for radiologic progression analysis, and the modified Rankin Scale—designed for stroke—is a coarse, non-disease-specific functional measure that may obscure spine-relevant outcome differences.
Original Abstract
STUDY DESIGN
Retrospective observational cohort study.
OBJECTIVE
Evaluating the predictive value of the Spinal Instability Spondylodiscitis Score (SISS) for radiologic progression in conservatively treated patients and its association with functional outcomes in both treatment groups.
SUMMARY OF BACKGROUND DATA
The SISS was developed to assess spinal instability in spondylodiscitis and assist in treatment decisions. However, its ability to predict radiologic and functional outcomes has not been validated.
METHODS
A total of 176 patients diagnosed with primary pyogenic spondylodiscitis at two centers in Germany and the UK were included. Clinical data, imaging studies, and outcomes were retrospectively collected. SISS scores were calculated pretreatment based on MRI/CT imaging. Functional outcomes were measured using the modified Rankin Scale (mRS). Radiologic progression was assessed in conservatively treated patients. Predictive value analyses, correlation, and multivariate regression were conducted to assess the relationship between SISS and both radiologic and clinical outcomes.
RESULTS
On the basis of SISS, patients were classified as stable (14.77%), potentially unstable (44.89%), and unstable (40.34%). In conservatively treated patients (n=54), the "SISS_unstable" category showed moderate predictive value for deformity progression (PPV and NPV both 77.8%, sensitivity 41.2%, specificity 94.5%, AUC=0.68). Vertebral destruction >50% was the strongest predictors of radiologic progression. At a mean follow-up of 121 days correlation between initial SISS scores and unfavorable clinical outcomes (mRS 4-6) was weak (r=0.201, P = 0.002). In multivariate analysis, age, neurological deficits at admission, and systemic inflammatory response syndrome (SIRS) were stronger predictors of poor outcomes, while SISS classification was not significant.
DISCUSSION
Patients triaged as "unstable" were very likely to show radiologic progression, indicating a fair predictive value of the score for spinal deformity. However, instability itself did not correlate with functional outcomes. These findings suggest that instability alone may not capture the multifactorial nature of outcome in spinal infections, but must be interpreted with caution due to small subgroup sizes and the use of a global, less granular outcome measure. Future research should focus on integrating broader patient factors to support clinical decision-making in spondylodiscitis.