The choice between an external and an internal fixator in orthopaedics is often not so simple because it depends on many factors, including the nature of the injury and the condition of the patient along with various limitations such as infections or shape of the bones and fractures. The purpose of this paper is to determine the guidelines for choosing the design of an external or internal fixator for different fracture conditions along with their safe application where the discussion shows how the correct choice of fixator reduces the rate of operative and postoperative complications. To be able to define such guidelines, it is necessary to cover various areas of biomechanics and orthopaedic surgery. That goal was achieved through the research of bone fractures, their types and causes, and their healing mechanisms. Furthermore, indications and contraindications for each group of fixators and their design solutions are defined. Finally, an ergonomic analysis of the application of the fixator was performed. Here, the importance of the connection between the bone and the pin, the arrangement of the fixator elements and the plane of application of external fixators and the influence of the correct plane of compression, i.e., the stabilization of fragments using internal fixators, is indicated. Accordingly, and with other findings, guidelines are defined for an easier choice of the correct design of the fixator, from which, using a sequence of questions, the appropriate fixator is selected for the associated condition of the bone and surrounding tissue, the shape of the fracture or the location of the bone.