An experimental technique is developed to study the transfer of load through spinal Instruments when subjected to cyclic loading. İn this study two series of tests are conducted on in vitro calf spines comprising of three vertebrae. First series consists of testing intact vertebrae without instrument cyclically (cross head speed 50 mm/minute). The change of stiffness is studied both in short and long period of loading. Second series of testing are performed on fractured and instrumented vertebrae. İn order to asses the power of cor-rection of the spinal instrument the change in the participation factor of the vertebra is studied. For this pur-pose a model is developed to simulate the burst fracture where the transected vertebral body is replaced by a plastic spacer with a certain elasticity. Furthermore, strain gages were placed to the Central parts of the posterior rods. The load carried by this setup is designated as Ps. This model, in the presence of spinal Instruments are then tested under the aforementioned loading and the load carried is called as Pappi- At the end of tests axial stiffens versus load curves drawn at the end of quasi-static tests supply the necessary preliminary information for the adequate prediction of the behavior of vertebrae (intact and/or instrumented) under cyclic loading. Alıcı and TSRH spinal 'Instruments are compared with respect to the additional stiffness supplied by the ‘Instruments and with respect to the participation factor (Participation factor of the instrument = 1 - Ps/Pappi) of the instrument. Results of the experiments indicate that Alıcı spinal instrument, under axial cyclic loading can compete favorably well with the other two 'Instruments.

Keywords: Spine, Biomechanics, Cyclic Loading