What injury makes an isolated small coronoid fracture? Morphological analysis using three-dimensional computed tomography reconstruction

Small, isolated coronoid fractures involving less than one-third of the coronoid height are often underestimated on plain radiographs due to minimal displacement and preserved joint congruity. However, these fractures can be associated with significant soft tissue injuries, leading to occult elbow instability. This study aimed to investigate the mechanisms and characteristics of isolated small coronoid fractures and to analyze their three-dimensional (3D) morphology using computed tomography (CT)-based modeling. We retrospectively reviewed 27 cases of isolated, noncomminuted coronoid fractures involving less than one-third of the coronoid height. Fractures were classified using the O'Driscoll and Adams systems. Injury mechanisms, associated soft tissue injuries, and joint instability were evaluated using radiographs, CT, magnetic resonance imaging, and operative findings. 3D CT modeling was used to analyze fracture morphology, including fragment size, orientation, and anatomical involvement. Posteromedial rotatory instability (PMRI) was the most common diagnosis (66.7%). According to O'Driscoll's classification, anteromedial (AM) subtype 1 fractures predominated (44.4%). 3D CT revealed significant differences in fragment volume and orientation between PMRI, tip, and anterolateral subtypes. Soft tissue injuries such as lateral ulnar collateral ligament and medial collateral ligament tears were frequently observed across all subtypes, even in the absence of frank dislocation. Small, isolated coronoid fractures often reflect underlying complex elbow instability, particularly PMRI, despite their benign radiographic appearance. Detailed evaluation using 3D CT and assessment of associated ligamentous injuries are essential for appropriate diagnosis and treatment planning. These findings highlight the limitations of current classification systems and suggest a need for a revised system incorporating injury mechanism and associated soft tissue damage.