Wrist motion induced stress in scaphoid after osteosynthesis

The therapy of fractures and non unions of the scaphoid bone still presents a problem due to long consolidation time and high rate of persisting non unions. In order to improve therapy concepts various problems have to be considered. Re-establishing the local blood circulation and the presence of osteoconductive and osteoinductive tissue, this is made possible mainly by the operation technique, as well as the drilling process, in order to guarantee a relatively rigid fixation of the fragments. On the subject of the scaphoid bone there seem to be wide gaps in knowledge and there is a need for optimisation in the basic biomechanical examinations up to the surgical management of the scaphoid bone. Further findings serve as a foundation for the optimal fixation of fragments. In a primary examination we plan to osteotomise scaphoid bones from cadaver forearms, they will then be subjected to movements of the wrist, load forces and moments, which are measured with the help of a specially constructed sensor. We assume that above all things the rotation strain, which the scaphoid bone is subjected to during movement and burden of the wrist leads to a delay in the healing of the fracture in conservative and operative therapy. The results received from the measurements will be used to examine and possibly complete current biomechanical mathematical models like finite elements technique. The results established in the first part of the study will be used in a further study in which singular headless screws are examined in vitro in isolated scaphoid bones, which have been osteotomised, above all for their rotation, bending and stability. This examination will be substantiated by a further examination of the scaphoid bone fracture model, which will be prepared by a rapid prototyping technique from patients` micro CT data. In the examination series implants will be manufactured which, apart from the advantage of a singular traction screw technique, are mostly distinguished by higher rotation stability. The higher stability will also be tested using in vitro measurements in comparison to conventional screw systems.

The paper "Wrist motion induced stress in scaphoid after osteosynthesis" had the goal to reveal biomechanical causes for delayed healing of bone fractures of the scaphoid bone. Using the determined forces caused by wrist movements between the scaphoid fragments, weaknesses of conservative therapy and of single screw osteosynthesis should be uncovered and benefits of an new developed bone screw set be detected. To prove forces and moments, which move the fractured scaphoid and its fragments through load and movement of the wrist, we have conducted experiments on the wrist of people who left their corpse to the Anatomical Institute of the Medical University. All of the used wrists were anatomically intact. The forces and moments were measured by an implanted sensor in an artificially induced fracture of the scaphoid bone. In the first experiment, the forces and moments were generated only by activating the fingers to perform closure of the fist, without movements of the wrist. In the second experiment, the forces and moments within the scaphoid were caused by movements in the wrist and performed with and without fist closure. In both experiments, an attempt was made to stabilize the scaphoid bone fragments advers the adjacent carpal bones, using kirschner wires. The measurements were carried out and compared with and without additional kirschner wire wrist stabilisation. None of the occuring forces and moments, which reached a considerable extent, could be neutralized by the additional kirschner wire osteosynthesis. These forces in and around the scaphoid were not only mechanically determined by a sensor, but also with a numerical method, the finite element analysis. In particular, the forces that occurred at the scaphoid axis in rotation were determined and confirmed. These forces and moments were assessed to test conventional scaphoid headless screws and our newly developed scaphoid bone screw set in further studies under realistic conditions. On the basis of mechanical tests with artificial bones, functioning and characteristics of conventional headless scaphoid screws were reviewed by us. The results were used for the development of the bone screw set. The two different versions of the set both consist out of two components: a main screw, which, in variation 1, can be interlocked by a pin through a diagonal perforation and in variation 2, wherein the lead screw includes a longitudinal groove in which a bolt is inserted, that extends over the cross section of the first screw. Both variations of the bone screw set were compared for their pullout resistance and rotational stability with conventional headless screws. In particular the variant, in which the bolt is obliquely passed through the first screw, has significant advantages over all mechanical tested screws in rotation and pullout resistance. If these results can be transferred to a clinical application, it is likely to assume an advantage of the bone screw set over conventional screws.