Navigating the Indeterminate Zone: Surgeons' Decision-Making Factors in Treating Vertebral Metastases with Spinal Instability Scores of 7-12

Background The Spinal Instability Neoplastic Score (SINS) classification system, is a validated and the most widely accepted instrument for defining instability in vertebral metastasis (VM), in which lesions scoring between 7 – 12 are defined as indeterminate and the treatment is controversial. This study aimed to determine which variables more frequently are considered by spine surgeons for choosing between the conservative and the surgical treatment of VMs among patients with an indeterminate SINS. Material and Methods A single-round online survey was conducted in ten spine surgeons with expertise in the management of VMs from our AO Spine Region. In this survey, each surgeon independently reviewed demographic and cancer-related variables of 36 real-life cases of patients with vertebral metastases scored between 7 – 12 in the SINS. Bivariate and multivariate analyses were performed to identify significant SINS and non-SINS variables influencing the decision-making on surgical treatment. Results The most commonly variables considered important, were the SINS element ‘mechanical pain’, rated important for 44.4% of the cases, ‘lesion type’ for 36.1%, ‘degree of vertebral collapse’ and the non-SINS factor ‘tumor histology’ rated for 13.9% of cases. By far the factor most commonly rated unimportant was ‘posterior element compromise’ (in 72.2% of cases). Conclusions Surgeons relied on mechanical pain and type of metastatic lesion for treatment choices. Vertebral collapse, spinal malalignment, and mobility were less influential. Spinal mobility was a predictor of surgical versus non-surgical treatment. The only variables not identified either by surgeons themselves or as a predictor of surgery selection was the presence/degree of posterolateral/posterior element involvement.