2022, Vol. 26 ›› Issue (18): 2801-2806

Biomechanical stability of crossed-rod fixation and parallel-rod fixation of atlantoaxial posterior screw-rod system

Wang Datian1, Qiu Feng2   

Abstract: BACKGROUND: Preliminary study has shown that the crossed rod fixation of the short segment screw rod fixation system composed of C1 bilateral pedicle screws and C2 unilateral lamina screw can increase its axial rotation stability. Therefore, posterior atlantoaxial crossed rod fixation may be a simple and effective new internal fixation framework. Whether in the screw rod system composed of C2 bilateral lamina screws or bilateral pedicle screws, the connection mode of crossed rod may further enhance the biomechanical stability of its structure compared with the traditional parallel rod.   
OBJECTIVE: To evaluate the biomechanical stability of the fixation technique for the crossed rod and parallel rod by the C1 bilateral pedicle screws combined with C2 bilateral lamina screws or bilateral pedicle screws.
METHODS:   Six fresh adult craniocervical specimens were used to test the biomechanical stability. Following intact state and under destabilization testing, C1 bilateral pedicle screws, C2 bilateral lamina screws and bilateral pedicle screws were implanted in each specimen. The specimens were then tested in the following sequence: C2 bilateral laminar screws + parallel rod, C2 bilateral laminar screws + crossed rod, C2 bilateral pedicle screws + parallel rod, and C2 bilateral pedicle screws + crossed rod. The ranges of motion of the C1-2 segments were measured in flexion-extension, lateral bending, and axial rotation.   
RESULTS AND CONCLUSION: (1) The four fixed modes significantly increased stability compared with both the intact and destabilization group in flexion-extension, lateral bending, and axial rotation (P < 0.05). (2) When C2 was implanted with bilateral pedicle screws, whether crossed rod or parallel rod connection was stronger than that composed of C2 bilateral lamina screws. (3) The crossed rod fixation could enhance the stability of the internal fixation system composed of C2 bilateral lamina screws in extension, lateral flexion and rotation, and the stability of the screw rod system composed of C2 bilateral pedicle screws in the direction of rotation. (4) Therefore, crossed rod fixation can further improve the biomechanical properties of atlantoaxial posterior screw-rod system and can be used as a choice for clinical application.
Key words: atlantoaxial instability, atlantoaxial fixation, crossed rod, parallel rod, biomechanics