Biomechanical Effects of Lateral Inclination C1 and C2 Pedicle Screws on Atlantoaxial Fixation

OBJECTIVE To assess the biomechanical effect of lateral inclination C1 and C2 pedicle screws on the atlantoaxial fixation through vitro human cadaveric study. METHODS From January 2016 to December 2017, fresh-frozen cadaveric cervical spines with intact ligaments from eight donated cadavers at an average age of 71.5 ± 10.6 years, comprising of six males and two females, were collected. There were no fracture and congenital malformation in all specimens according to the imaging examination. The range of motion (ROM) of the specimens were tested in their intact condition and destabilized condition. Next, the specimens were randomly divided into two groups to ensure no differences in sex and age: Group 1 was medial inclination C1 pedicle screw and C2 pedicle screws (C1 MPS-C2 PS) and Group 2 was lateral inclination C1 pedicle screw and C2 pedicle screws (C1 LPS-C2 PS). The ROM of the fixation scenarios were recorded. Thereafter, all the specimens with fixation constructs were tested for 1,000 cycles of axial rotation and tensile loading to failure was carried out collinearly to the longitudinal axis of all the screws, the data were documented as screw pullout strength (SPS) in newtons. All the recorded data subjected to quantitative analysis. RESULTS The ROM of specimens was increased significantly in destabilized condition and significantly reduced in fixation condition compared with intact condition. In C1 LPS-C2 PS groups, the C1 -C2 cervical segment showed 3.96° ±1.21° and 3.75° ± 1.33° in flexion and extension direction, 2.85° ± 0.91° and 2.96° ± 0.71° in right and left lateral bending, 2.20° ± 0.43° and 2.15° ± 0.40° in right and left axial rotation. In C1 MPS-C2 PS groups, it showed 4.24° ±1.31° and 3.98° ± 1.21° in flexion and extension direction, 2.76° ± 1.10° and 3.23° ± 0.62° in right and left lateral bending, 2.20° ± 0.46° and 2.21° ± 0.42° in right and left axial rotation. There was no statistically significant difference on ROM and screw pullout strengths (764.29 ± 129.00 N vs 714.55 ± 164.63 N) between the two groups. However, there was one specimen in the C1 MPS-C2 PS group showing rupture the inferior wall of the left screw trajectory owing to the relatively thin posterior arch of the atlas, the screw pullout strength was significantly reduced (left pullout strength value: 430.5 N, right pullout strength value: 748.4 N). Therefore, in the case of the thin posterior arch of the atlas, the C1 LPS-C2 PS group had strong long-term biomechanics. CONCLUSION The lateral inclination C1 pedicle screw can achieve the same biomechanical strength as the traditional atlas pedicle screw. However, for the case where the posterior arch of the atlas is relatively thin, a lateral inclination C1 pedicle screw is more suitable.