The self adapting washer for lag screw fixation of mandibular fractures: finite element analysis and preclinical evaluation

Besides rigid fixation, lag screws have distinct advantages compared with p lates in appropriate indications in mandibular fractures. However, in curre nt lag screw systems, the relatively small area of the screw head has to tr ansfer the tensile force which can exceed 1000 N in the symphysis, to the t hin cortical bone plate. Countersinking, which is obligatory in most system s, will weaken the cortical plate. Finite element analysis (FEA) revealed t hat load in this situation can exceed the normal tensile strength of metal and bone. Consequently, a new washer was constructed which both increased t he supporting surface and did not require countersinking, The washer is sel f adapting (SAW) to the cortical plate in a defined position, forming a bal l and socket joint with the screw head. Using the FEA model, a ten-fold red uction in load on bone and metal was observed with the new washer, In a min iature pig mandibular symphysis fracture model, the clinical applicability and a favourable histological reaction were demonstrated, compared with con ventional lag screw designs.