The Rorschach butterfly, understanding bone biomechanics prior to using nomenclature in bone trauma interpretations

•Nomenclature and fracture characteristics are used to interpret broken bones.•Nomenclature assumes an external loading condition making it inappropriate.•Butterfly fracture describes an injury, limiting interpretative abilities.•Modes of failure interpret bending direction of bone and total bone trauma patterns. Blunt traumas are the most common injuries observed and reported in medical examiner settings. Two common methods to describe bending bone fractures in the anthropological literature include the application of morphology nomenclature and describing characteristic fracture morphology. A nomenclature descriptor of blunt trauma, the butterfly fracture, is commonly used to describe broken long bones. In this paper, a case study of a fractured long bone in a pedestrian vehicle accident is used to highlight the complex interplay of factors involved in bone fracture formation. The application of a butterfly fracture pattern in trauma analysis is useful in establishing the bending direction of a bone, in identifying failure modes, and is valuable in teaching. Yet, butterfly fracture characteristics need to be examined in 3-dimensions for diagnosis of modes of failure, tension, shear and compression, and even then, the bending direction of a broken bone may not provide a reliable indicator of the point of impact (POI); this is especially true when a priori knowledge of the injury is unknown. Common fracture nomenclature, such as oblique, transverse and/or comminuted, as well as eponyms, are medical descriptions of an injury which are impractical to use for interpreting a broken bone from fleshed or skeletonized remains, in establishing a POI and in evaluating total bone trauma (TBT). The examination ofcharacteristic features on the surface of a broken bone associated with the modes of failure is the best approach for establishing the bending direction of a long bone.