Biomechanical Testing of the Cairo Anterior Spinal Fixation System (CSS)

A newly introduced Egyptian anterior spinal fixator, the Cairo Spinal System (CSS), was tested biomechanically. It consisted of upper and lower plates, medial and lateral rods, two medial non-slotted screws and two lateral slotted screws. The medial rod passed through the plates, the lateral through the slotted screws. Inserting the medial rod through the plates helped decrease the profile of the medial aspect of the device which lay close the major vessels and obviated the need of adding cross connectors to prevent the windshield wiper movement. Inserting the rods at two different levels made their plane oblique to both the antero-posterior and horizontal axes of the vertebral column. This helped resist both flexion-extension and side bending equally. CSS was compared to Kaneda SR device. The testing machine was a uniaxial servohydraulie testing machine, performing only tension and compression. To allow it to perform different moments (torsion and bending), it was modified according to Crawford and colleagues. To avoid false results caused by impact loading of the specimen, displacement controlled mode of testing was used adjusting the machine velocity at 0.2 mm/sec. Testing was performed on fifteen calf spine specimens. In each specimen, corpectomy of L3 was done and a tricortical iliac graft from the same calf inserted. In preliminary right torsion to failure of the CSS spinal construct, the rod screw/plate junction failed before bone and ligaments at a moment of 0.12 kNm. This was considered as weakness in the device that necessitated its modification to increase its torsional stiffness, most importantly by replacing the washer fixing the rod that passed through the slotted screw, by an I-shaped peg having a V-slotted undersurface. Both CSS and Kaneda SR were next tested to failure in compression, right and left torsion, right and left bending, flexion and extension and the results compared with reference to the mode of failure, the failure load or moment and the stiffness of the construct. In flexion and right bending, failure occurred exclusively in the ligaments and disc, the stabilized segment remaining totally unaffected. Apart from the first right torsion to failure of CSS, failure in right and left torsion, left bending and extension occurred entirely in or started at bone, be it the strut graft or the vertebral body. The load required to cause metal failure was non-physiologic, being far greater than that needed to induce bony or ligamentous failure. Non-physiologic failure loading in compression and left torsion, led to failure of CSS at the rod screw/plate junction at a load of 9.6 kN for compression and a moment of 0.33 kNm for left torsion. The same non-physiologic loading led to pulling out of the Kaneda SR inferior tetra spiked plate and screws at 6.3 kN under compression and failure of its screw rod junction at 0.39 kNm under left torsion. Consequently, it was concluded that CSS resisted compression more than Kaneda SR, wereas, Kaneda SR resisted left torsion more than CSS, but both performed well under normal physiologic compression and left torsion. Stiffness of the spinal construct was the third parameter used in the comparison. The results of stiffness measurement showed a substantial difference in compression stiffness only, where CSS exceeded Kaneda SR by 150 kN/m. The difference in angular stiffness was small. The torsional stiffness of Kaneda SR slightly exceeded that of the final version of CSS. Considering that Kaneda SR had the highest torsional stiffness among all anterior spinal fixation devices, the achieved torsional stiffness of CSS was quite satisfactory. CSS was superior to Kaneda SR in resisting bending, in particular, left bending. This superiority could be attributed firstly to locking of the screws into the plates, preventing movement at the screw/plate junction in the coronal plane during bending and secondly to placing of the rods in an oblique plane.