The Truly Intermediate Subgroup Within the SINS "Potentially Unstable" Category: The Matryoshka Doll Phenomenon.

Objective: A significant dilemma exists for the surgical plan of spinal metastases with a spinal instability neoplastic score (SINS) of 7-12. Our aim is to trim down this range of "potential instability" and find a virtual cut-off value for instrumentation.

Methods: We performed a retrospective study on 60 patients who underwent surgical intervention for vertebral metastasis. They were segregated into 'instrumented' and "noninstrumented" groups. The primary endpoint of the study was to see whether surgical stabilization was done.

Results: Although univariate analysis showed the overall SINS score, involvement of posterior elements, and mechanical pain to be the factors that significantly affected our decision making in favor of stabilization, only the SINS score was found to be statistically significant on multivariate analysis. On plotting the proportion of patients undergoing stabilization at each SINS score we found the curves to crossover between SINS 8 and 9, ran almost parallel to each other at values 9 and 10 and then seemed to diverge from each other significantly at values above 10. Taking SINS 9 as the cut-off value above which instrumentation is advised, the receiver operating characteristic curve had a sensitivity of 67.57% (95% confidence interval 50.21% to 81.99%) and specificity of 73.91% (95% confidence interval 51.59% to 89.77%). The area under the curve was 0.79 (0.67-0.91).

Conclusions: SINS 9-10 seems to be the actual ambiguous subset in the wider subgroup of the 'potentially unstable' category SINS 7-12, like a 'Matryoshka doll'. A SINS value of 9 can be seen as a cut-off value for instrumentation.