Second Metacarpal Index as a Predictor of Secondary Displacement in Conservatively Treated Distal Radius Fractures

<i>Background and Objectives:</i> Distal radius fractures (DRFs) represent the most common fracture in the elderly population and are typically the first fractures to occur in the sequence of fragility fractures. Although fracture instability is an important criteria for guiding treatment, there is no universal consensus on how to define an unstable DRF. Given the demonstrated influence of bone quality on fracture stability, it has been hypothesized that second metacarpal index (2MCI) may also serve as a predictor of instability in DRFs. This study aimed to evaluate the predictive value of 2MCI for fracture instability and to determine its threshold value beyond which surgical management should be considered. <i>Materials and Methods:</i> A retrospective study was conducted between January 2023 and May 2025 investigating conservatively treated DRFs. Radiographic parameters including 2MCI, volar inclination, radial inclination, and ulnar variance were obtained at three time points: pre-reduction, post-reduction, and at 6 weeks of follow-up time. Univariate and multivariate linear regression analysis and receiver operating characteristic (ROC) analysis were conducted to determine the optimal 2MCI threshold for predicting clinically significant displacement. <i>Results</i>: A strong correlation was found between 2MCI and the changes (∆) in volar inclination (<i>p</i> < 0.001), radial inclination (<i>p</i> < 0.001), and ulnar variance (<i>p</i> < 0.001) in univariate regression analysis. In multivariate regression analysis, 2MCI was an independent predictor of ∆ volar inclination (<i>p</i> < 0.001) and ∆ radial inclination (<i>p</i> = 0.004). For ∆ ulnar variance, both 2MCI (<i>p</i> = 0.003) and initial displacement (<i>p</i> = 0.049) were statistically significant predictors. A 2MCI cut-off value of 0.412 predicted a ∆ volar inclination greater than 10° (sensitivity 80.9% and specificity 74.1%, <i>p</i> < 0.001). <i>Conclusions</i>: The results of this study reveal the potential of the 2MCI as a quantitative marker of both fracture instability and bone fragility. Further prospective validation may demonstrate its role as a standard radiographic parameter in DRFs, as well as in guiding treatment selection.