A Fracture Healing Odyssey: Kinematic Comparison of Unions and Nonunions in Human Lower-Extremity Long Bones Treated with Intramedullary Nailing: A Retrospective Cohort Study.

Abstract

Fracture nonunion remains a major clinical challenge. Although radiographic scores such as RUST (Radiographic Union Score for Tibial fractures) and mRUST (the modified version) are widely used to assess healing, there has been limited investigation into their longitudinal use. This study evaluated whether a log-logistic statistical model of radiographic healing scores over time could quantify differences between fractures that healed and those that went on to nonunion. This retrospective cohort analysis added serial radiographs of 49 femoral shaft nonunions to radiographs from previous cohorts. Tibiae and femora were scored with RUST and mRUST, respectively. Scores were plotted over time and fitted to a log-logistic curve to generate a healing ("position") equation. Model-derived parameters (Yinf [ultimate score], k [a curve shape parameter], and thalf [time to half of the score change]) were compared between fractures that did and did not unite. Secondarily, first and second derivatives were used to describe the fracture healing "velocity" and "acceleration" for each group. A total of 613 fractures were analyzed: 234 tibial (196 healed, 38 nonunion) and 379 femoral (330 healed, 49 nonunion). In femoral fractures, Yinf, k, and thalf all differed significantly (p < 0.008) between healed and nonunion cases. In tibial fractures, Yinf and k differed (both p < 0.008), whereas thalf did not (p = 0.30). The first and second derivatives at the population level suggest early differences in healing kinematics between fractures that heal and those that go on to nonunion. This proof-of-concept study demonstrates that log-logistic modeling of radiographic scores can quantify differences in radiographic healing between femoral and tibial shaft fractures treated with intramedullary nails that will go on to union versus nonunion. Differences in healing velocity and acceleration may emerge early, supporting the potential of the longitudinal framework for earlier identification of fractures at risk for nonunion. Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.