A METHOD TO DETERMINE THE 3-D STIFFNESS OF FRACTURE FIXATION DEVICES AND ITS APPLICATION TO PREDICT INTER-FRAGMENTARY MOVEMENT

Inter-fragmentary movement considerably influences the fracture healin g process. Large shear movement delays while moderate axial movement s timulates the healing process. To be able to control the mechanical si tuation at a fracture site and to achieve optimal bony healing it is e ssential to understand the relationship between inter-fragmentary move ment, bony loading and fixation stiffness. A 6 x 6 stiffness matrix is introduced which completely describes the linear relationship between the 6 inter-fragmentary movements and the resulting bony loading (3 f orces and 3 moments). Further, it is illustrated that even in relative ly stiff external fixateur constructs simple axial loading of the bony fragments leads to complex inter-fragmentary movement. When the 3-D s tiffness description is multiplied by the load state in sheep tibiae: movements similar to those measured in vivo are calculated. The relati onship between axial compression and medio-lateral or dorso-ventral sh ear varies depending on the mounting plane of the external fixateur. T he authors conclude that a single value is not sufficient to describe the mechanical relationship between inter-fragmentary movement and bon y loading. Only a complete description of fixation stiffness allows pr ediction of inter-fragmentary movement and differentiation between var ious configurations of fixation devices and their potential for mechan ically promoting bony healing. (C) 1998 Elsevier Science Ltd. All righ ts reserved.