Effect of lordotic rod curvature and preloading on postoperative stability of thoracolumbar fractures / 中华创伤杂志

ABSTRACT

Objective To study the effect of different rod curvature on the postoperative stability and stress of thoracolumbar junction fracture using the finite element simulation.Methods (1) Thoracolumbar finite element model of T11 to L1 from three-dimensional CT data of a 30-year-old healthy male volunteer was established,including the assignment of cortical bone,cancellous bone,disc,ligaments and facet joints.On this basis,the T12fracture model was also established.T11 to L1 bilateral pedicle screw fixation was loaded,and the rod connection was divided into straight rod group and pre-bended rod group (lordotic 15°-25°) according to angle of the rod.A 400 N stress was loaded on the top of the upper endplate of T11 to simulate the upper part body gravity,while applying a 10 N · m torque to generate flexion and extension,lateral flexion and rotation.Stress distribution of different methods of pre-bending for thoracolumbar fractures after reduction was compared.(2) A retrospective cohort analysis was made on 56 cases of thoracolumbar fractures surgically treated from July 2012 to July 2016,including 31 cases in straight rod group and 25 cases in pre-bended rod group.Two groups were compared in angle between adjacent level before operation,after operation and at final follow-up.Results (1) In flexion,extension,lateral bending and rotation,both rod bending methods effectively controlled the thoracolumbar junction displacement.The peak stress of connecting rod (151,315,369,377 MPa respectively) in pre-bended rod group was lower than that in straight rod group (110,239,281,189 MPa respectively) (P ＜ 0.05),and straight rod group appeared relatively obvious stress concentration.(2) Mean follow-up time was 21.4 months (range,4-33 months).Preoperative kyphosis angle was (21.7 ± 7.4)°in straight rod group and (20.3 ± 6.8)° in pre-bended rod group (P ＞ 0.05).Postoperative lordosis angle in straight rod group was (3.3 ± 1.2) °versus (8.3 ± 2.8) ° in pre-bended rod group (P ＜ 0.05).At the final follow-up,the lordosis angle in straight rod group was reduced by (8.7 ± 2.3) ° versus (3.9 ± 1.7)°in pre-bended rod group (P ＜0.05).Implant failure was seen in 3 cases in straight rod group,but there was no implant failure in pre-bended rod group.Conclusion Pre-bended (lordotic 15°-25°) and pre-loaded rods used in internal fixation of thoracolumbar fractures may reduce the stress of rods,decrease the incidence of implant failure and facilitate the recovery of spine stability.