'Fast Ossifier' phenotype in DISH Diffuse idiopathic skeletal hyperostosis (DISH) is a common but under-recognised condition, characterised by abnormal ossification of spinal and extraspinal entheses. Long regarded as a slowly progressive disorder, DISH in fact displays substantial clinical heterogeneity. Recent work from the Camargo Cohort has described the Fast Ossifier (FO) phenotype, a subset of patients who experience accelerated ossification together with early trabecular bone decline. This discovery challenges the traditional view of DISH as a uniform, indolent condition of the elderly, and highlights the importance of having tools that can identify individuals at higher risk of an aggressive disease course before extensive skeletal damage -and potentially metabolic and vascular dysregulation- occurs. Fast Ossifier Stratification Index (FOSSI) While the FO construct provided a strong biological framework, it lacked a practical means of clinical recognition. Imaging criteria remain insensitive at early stages, and no biomarker had proven able to anticipate the FO trajectory. To address this gap, we elaborated the Fast Ossifier Stratification Index (FOSSI), a pair of sex-specific indices (FOSSI-F for women, FOSSI-M for men) derived from the propensity score framework. FOSSI bridges epidemiological modelling and clinical practice, transforming latent statistical information into interpretable risk categories. It captures distinct biological pathways (insulin resistance in women, inflammatory–endocrine drivers in men) and provides validated thresholds to stratify risk of accelerated ossification. Thus, FOSSI addresses a long-standing gap: enabling early identification and preventive strategies in a condition where timely intervention could alter disease trajectories. The creation of PS-based FOSSI represents a technological innovation that aligns with recent trends in the propensity score literature, where authors have recommended leveraging the extensive covariate data embedded in PS for predictive modelling and risk stratification. The rationale, development, and internal validation of FOSSI have been submitted for publishing consideration. FOSSI Online Calculator In the same line, and with the aim of facilitating the use of FOSSI, promoting its external validation, and sharing the tool, we released the free and publicly accessible 'FOSSI Online Calculator'. By entering simple patient data, the calculator computes FOSSI-F or FOSSI-M and classifies the result into clear risk categories. In short, it fulfils a long-standing need: providing clinicians and researchers with a practical way to recognise early those at risk of rapid progression, opening the door to closer monitoring and preventive strategies. FOSSI Plot Script To enhance the interpretability of the Fast Ossifier Stratification Index (FOSSI), we provide an accompanying plotting script that generates probability curves and individualized visualizations. The script displays the logistic curve relating FOSSI values to the probability of belonging to the Fast Ossifier (FO) group, with reference cut-off points derived from ROC analysis. Coloured risk zones delineate the probability spectrum, thereby translating statistical results into an intuitive clinical framework. The visualization allows users to position a given patient on the curve and immediately identify both the predicted probability of FO status and the corresponding risk category (low, intermediate, or high). In men, the curve shows a steep and nearly vertical slope at the discriminative threshold, indicating sharp separation between FO and non-FO individuals; in women, the slope is more gradual, reflecting a broader metabolic spectrum. The incorporation of expected FO prevalence further grounds the graph in clinical reality, contextualizing the risk estimates. Overall, the plot serves as a practical extension of the FOSSI equations: while the index provides a numerical score, the graph transforms that score into a visually accessible probability of accelerated ossification, reinforcing its role as a stratification tool for research and clinical decision-making.

Contents: Version 1: FOSSI calculator Version 2: FOSSI calculator + R-script for plotting + 2 plots (example) Version 3: FOSSI calculator + R-script for plotting + improved plots + storage The R-script has been tested in RStudio. Patient's FOSSI value must be located at line 86 (FOSSI-F) or 126 (FOSSI-M). To store the plots, select a folder, copy the path and include the following line at the top of the script: setwd("C:/xxx/xxx/xxx/Folder/")

No funding was received for conducting this work, and there are no financial or non-financial interests to disclose.