A biomechanical comparison of crossed and parallel rod configurations in atlantoaxial internal fixation.

BACKGROUND: Posterior atlantoaxial fixation with screw rod forms an approximate "II" shape or "H" increasing transverse link for better stability. In order to improve stability and in consideration of difficult placement of transverse connecting rod, possibility of inadequate bone graft, some scholars have preliminarily researched biomechanics of a novel crossed rod as an approximate "X" configuration of screw rod. PURPOSE: The aim of this study was to evaluate and compare the biomechanics of the crossed and parallel rod configurations in the screw rod system for posterior atlantoaxial fixation on a cadaveric model. METHODS: Six fresh cervical specimens were used to complete the range of motion (ROM) testing by applying pure moments of ± 2.0 nm. Following intact state and under destabilization testing, screws were implanted. The specimens were then tested in the following sequence: Group BLS + PR (C2 bilateral laminar screws + parallel rod), Group BLS + CR (C2 bilateral laminar screws + crossed rod), LPRLS + PR (C2 left pedicle screw and right laminar screw + parallel rod), LPRLS + CR (C2 left pedicle screw and right laminar screw + crossed rod), BPS + PR (C2 bilateral pedicle screws + parallel rod) and BPS + CR (C2 bilateral pedicle screws + crossed rod). The ROM of the C1-2 segments was measured in flexion-extension, lateral bending and axial rotation. Six surgical constructs were compared between the groups and with intact condition, respectively. RESULTS: The six fixed modes significantly increased stability compared with both the intact and destabilization group in flexion-extension, lateral bending and axial rotation (p < .05). In extension, BPS + CR and BLS + CR showed greater stability than BLS + PR (p < .05). During flexion, the six fixation methods showed no statistical significance (p > .05). In left lateral bending, stability of the other five screw rod fixation techniques significantly increased when compared with BLS + PR (p < .05). In the right lateral bending direction, the stability of BLS + PR was worse than that of BPS + CR and BPS + PR (p < .05). In the left axial rotation, stability of BLS + CR, LPRLS + CR and BPS + CR was greater than that of BLS + PR, LPRLS + PR and BPS + PR (p < .05). In the right axial rotation, the stability of BPS + CR and BLS + CR was greater than that of BLS + PR (p < .05). CONCLUSION: The six investigated fixation methods provide sufficient biomechanical stability. The crossed rod configuration can further enhance the axial rotation stability of the screw rod system, which consists of C1 bilateral pedicle and C2 pedicle, or C2 lamina screws. The crossed rod can also improve the stability of the screw rod system made up of C1 bilateral pedicle and C2 lamina screws in lateral bending and extension. The crossed rod configuration is reliable and provides superior stability for clinical application.

Relationship between the laminectomy extension and spinal cord injury caused by acute spinal shortening: goat in vivo experiment.

OBJECTIVE: To investigate the relationship between different types of laminectomy extension and spinal cord injury subsequent to acute spinal shorting after 3-column osteotomy in living goat model. METHODS: A total of 18 healthy goats were selected, and a procedure of bivertebral column resections and total laminectomy of T13 and L1 was completed followed by different laminectomy extensions under the somatosensory evoked potential (SSEP) monitoring. The samples were divided into three groups according to types of subsequent laminectomy extension. In the first group (enlarged resection of upper lamina group), laminectomy extension was performed on 10 mm caudal to T12; in the second group (equidistant enlarged resection of upper and lower lamina group), laminectomy extension was performed on 5 mm caudal to T12 and 5 mm cranial to L2 simultaneously; and in the third group (enlarged resection of lower lamina group), laminectomy extension was performed on 10 mm cranial to L2. The SSEP measured after vertebral resection was set as the baseline, and the SSEP decreased by 50% from the baseline amplitude and/or delayed by 10% relative to the baseline peak latency was set as positive results, which indicated spinal cord injury. Spinal column was gradually shortened until the SSEP monitoring just did not show a positive result. The shortened distance (ΔH) and the changed angle of the spinal cord buckling (Δα) were measured in each group. Neurologic function was recorded by the Tarlov scores at 2 days after the surgery. RESULTS: The safe shortening distances of three groups were 38.6 ± 1.2 mm, 41.5 ± 0.7 mm, 43.7 ± 0.8 mm, respectively; the corresponding changed angles of the spinal cord buckling were 62.8 ± 6.9°, 82.8 ± 7.5°, and 98.5 ± 7.0°. Significant differences of ΔH and Δα were found among the three groups by LSD multiple comparison test (P < 0.05). Strong correlation between ΔH and Δα was shown in each group by Pearson's correlation test. CONCLUSIONS: Different laminectomy extensions after 3-column osteotomy have different effects on the prevention of SCI caused by acute spinal shortening. The enlarged resection of lower lamina is superior to equidistant enlarged resection of upper and lower laminas which is superior to enlarged resection of upper lamina in preventing SCI. These slides can be retrieved under Electronic Supplementary Material.