BIOMECHANICAL EVALUATION OF A CENTRAL ROD SYSTEM IN THE TREATMENT OF SCOLIOSIS

Objective. To evaluate the efficiency of a central rod system in the t reatment of scoliosis. Design. Biomechanical testing and finite elemen t analysis were performed to compare the stiffness and stress distribu tion of the central rod system with the Cotrel-Dubousset instrumentati on. Background. During the fixation and derotation processes in the tr eatment of scoliosis, the Cotrel-Dubousset instrumentation concentrate s unilateral stress on the vertebral elements. It may be possible to c ause severe injury to the bony tissues especially in the ostaeoporotic patients. A central rod system was developed to reduce the damage to the bony tissue of the vertebrae in the treatment of scoliosis. Method s. Mechanical non-destructive cyclical tests comparing the compression , flexion, extension and torsional stiffness were carried out on the m odels of the central rod system and Cotrel-Dubousset instrumentation u sing a 858 Bionix test system. The stress distribution on the central rod, the Cotrel-Dubousset rod and a vertebral model consisting in four vertebrae each implanted with two screws was evaluated using a genera l purpose finite element software ANSYS R 5.0a. The SOLID73 element wa s employed for three-dimensional modelling. Results. Although the cent ral rod system was found to have lower values in the compression, flex ion and extension stiffness, its torsional stiffness is comparable to the Cotrel-Dubousset instrumentation. Moreover, the stability of the n ew design is satisfactory. Finite element analysis indicates that the maximum equivalent stress on the vertebral model with the central rod system is relatively constant. Conclusions. The central rod system con sists of a central rod which applies fixation and derotation through a connector between the screws on both sides of the vertebral column. T his new design can not only prevent the effect of unilateral stress co ncentration, but is also simpler and easier to implant. However, it is necessary to strengthen the material of the rod or the connection bet ween the rod and the screws.