A Finite Element Analysis on the Effectiveness and Ideal Positions of the Tansfixators in Pedicle Screw Instrumentation

Kim, Myung Duk; Shin, Jung Woog; Lee, Sung Jae; Suk, Se Il; Kim, Won Joong

Abstract

Recent studies suggest that addition of transfixators to pedicle screw instrumentation enhances rotational stability of the constructs. However, the effectiveness of the transfixators along with their ideal numbers and positions still remains unclear. In this study, finite element analysis was performed to suggest the most effective positions of the transfixators based on their numbers. For this purpose, a finite element model of a spinal segment(L3-5) with total vertebrectomy at L4 and pedicle screw instrumentation was developed. The finite element model was made with 3-D 8 node solid elements so that they are more realistic and closer to the actual human vertebrae than the preciously published models. On this model, the transfixator(s) were placed along the pre-determined position(s) along the rod while its number was varied between 1 and 2. Appropriate boundary conditions were designated and rotational moment of 6.4 Nm was applied both in axial rotation and lateral bending. Improvement in rotational stability due to addition of the transfixator(s) was calculated as the percent decrease in motion relative to the case without. Results suggested that with one transfixator the rotational stability increased by the average of 19.0% and 6.1% against axial rotation and lateral bending, respectively. With two, the corresponding improvements were 32.3% and 10.7%. Against axial rotation, it was most effective when transfixator was placed at the middle of the rod(22.9%) for one transfixator and at the 1/3 and 2/3 sites along the rod(35.8%) for two. Against lateral bending, the ideal position(s) were at the proximal end(10.7%) for one transfixator and at the proximal and distal ends of the construct(17.7%) for two. It was also found that adding of a transfixator was more sensitive against lateral bending than against axial rotation(78% versus 64%). In conclusion, base on our biomechanical results, it can be suggested that the ideal positions for the ators are located at the equidistance from the both ends of the rod against axial rotation and at the both ends against lateral bending.