Application of transoral atlantoaxial reduction plate III in treatment of complicated atlantoaxial dislocation

Objective To evaluate the biomechanical characteristics and the clinical advantage of transoral atlantoaxial reduction plate(TARP)Ⅲ.Methods Design of TARP-Ⅲ was based on TARP-Ⅱ.The screw hole in the axis was moved 1-2 mm upwards and inwards in a plate which turned a vertebral screw into a pedicle screw or an articular process screw.A polyaxial self-lock ring and polyaxial guiding drill were added to the crew hole of the plate.Finally,the withdrawal resist ence force of the three axis screws was tested and TARP-Ⅲ was used in 44 patients with complicated irreducible atlantoaxial dislocation.The axis was fixed with the pedicle screw or the articular process screw.Results The maximum withdrawal resist ence force of the anterior pedicle screw,the articular process screw and the vertebral screw in the axis was(593.1 ± 97.8)N,(469.9 ± 73.3)N and(395.2 ± 75.1)N respectively,with statistical difference between groups among three fixation methods(P ＜ 0.05).All 44 patients were followed up for 5-38 months(average 18 months),which showed complete anatomic reduction in 36 patients and appropriate anatomic reduction in eight,with basic correction of the angles between the brain stem and the spinal cord and sufficient decompression of the spinal cord.The decompression rate of the cervical spinal cord was average 88.2% according to the Yin evaluating method of cervical cord decompression.The improvement rate of spinal cord function was average 76.6% according to Japanese Orthopaedic Association(JOA)score.Conclusion With the design of polyaxial self-lock mechanism,TARP-Ⅲ with the pedicle screw or the articular process screw surpasses TARP-Ⅱ with vertebral screw in aspect of biomechanics. Key words: Atlas; Axis; Dislocations; Biomechanics