OBJECTIVETo evaluate the effect of crosslinks on the stability of the spine and pedicle screws.

METHODSCompression fracture of the L1 vertebra was produced in 30 fresh thoracic and lumbar vertebrae samples obtained from adult sheep, which were divided into 3 groups (n=10)with lot-drawing method. Four screws were fixed onto the superior and inferior pedicles of vertebral arch close to the fractured vertebrae, with different number of crosslinks (0 in Group A,1 in Group B, and 2 in Group C) on the rods. After fixation, the samples were subject to 10 000 times of fatigue test with 1.5 Hz load on the HY-3080 computer-control electronic universal test machine and HY-1000NM computer-control torsion test machine. The axial compressive stiffness, maximum pullout strength,and range of motion (ROM) of 6 directions, i.e., flexion, extension, left and right lateral bending, and left and right axial rotation of the 3 groups were measured and compared.

RESULTSThere were no statistically significant differences in axial compressive stiffness as well as the ROM of flexion, extension, and left and right lateral bending (all P>0.05). The maximum pullout strength was significantly smaller in Group A and Group B than in Group C [(129.56±29.63)N vs.(294.67±23.25) N,P=0.000;(254.02±36.29)vs.(294.67±23.25)N, P=0.006]. The ROM of left axial rotation was the highest in Group A(13.35°±1.06°), followed by Group B(12.23°±1.06°)and Group C (11.04°±0.74°)(F=13.44, P=0.000; Group B vs. Group A, P=0.000; Group B vs. Group C, P=0.001; Group C vs. Group A,P=0.000). The ROM of right axial rotation was also the highest in Group A(13.56°±1.15°), lower in Group B (12.39°±1.01°) and the lowest in Group C (10.81°±0.51°) (F=21.91, P=0.000; Group B vs. Group A,P=0.002; Group B vs. Group C, P=0.001; Group C vs. Group A, P=0.000).

CONCLUSIONCrosslinks may reinforce the pullout strength of the screws and improve the axial stability of the spine.