FORCES REQUIRED TO INITIATE SLIDING IN 2ND-GENERATION INTRAMEDULLARY NAILS

Second-generation intramedullary nails, which allow the fixation screw that is placed in the femoral head to slide distally and thus allow c ompression of the fracture of the femoral neck, have become a popular option for the treatment of ipsilateral fractures of the femoral neck and shaft. However, the sliding characteristics of the screw within th e barrel of the nail or the side-plate have not been assessed biomecha nically, to our knowledge, The goal of the current study was to invest igate the forces required to initiate sliding of the proximal screw in intramedullary devices and to compare these forces with those require d to initiate sliding of hip screws, The loading configuration simulat ed the typical angle of 135 degrees between the intramedullary nail an d the proximal screw. The forces required to initiate sliding of the p roximal screw with the screw extended fifty-one, seventy-six, eighty-s ix, and 102 millimeters beyond the proximal end of the barrel, were me asured for three different types of second-generation intramedullary n ails (Recon, ZMS, and Gamma), a sliding compression hip screw and an i ntramedullary hip screw and these forces were then compared. With each amount of extension of the screw, the hip screws required lower force s to initiate sliding than did the second-generation intramedullary de vices. Of the second-generation devices, the Gamma nail required the h ighest forces to initiate sliding; the Recon and ZMS nails required 20 to 40 percent lower forces compared with the Gamma nail. None of the devices jammed in any of the loading configurations that were tested. When the extension of the screw was increased, higher forces were requ ired to initiate sliding, CLINICAL RELEVANCE: Since sliding allows con tinued compression of the fracture, surgeons should be aware that, com pared with hip screws, the second-generation nails that me tested requ ired higher loads to initiate sliding and to generate subsequent compr ession of the fracture.