BIOMECHANICAL EVALUATION OF THE SCHUHLI NUT

The schuhli nut is a device designed to lock an AO 4.5-mm cortical scr ew to a 4.5-mm dynamic compression plate independent of bony contact w ith the plate, The nut engages the screw below the plate, elevating th e plate, and locking the screw at a 90 degrees angle, thus preventing toggling, Photoelastic modeling and biomechanical testing on sheep tib ias were done to determine the mechanical properties of constructs usi ng schuhli nuts, Use of schuhli nuts was shown to decrease stress in t he bone below the plate, The initial axial stiffness of a construct fi xed with schuhli nuts is less than a construct with standard screws, b ut the rate of loss of stiffness with cyclic loading is similar, When a cortical defect is present at the near cortex and the screw engages the far cortex only, the use of a schuhli nut significantly improves t he stability of the construct compared with a standard screw alone, an d behaves mechanically the same as a standard construct with intact co rtices, This indicates that the schuhli nut acts as a substitute for a deficient cortex, The schuhli nut can be useful in osteoporotic bone because it prevents the screw from stripping the threads in the bone a s the screw is advanced, It also serves to lock the screw to help prev ent the screw from backing out, The schuhli nut may be a useful tool t o improve stability in the treatment of complex fractures, reconstruct ions, or in pathologic bone.