Effect of Transverse Position and Numbers on the Stability of the Spinal Pedicle Screw Fixation during the Pedicle Cortex Perforation.

Objective To evaluate the effect of transverse position and numbers on the stability of the spinal pedicle screw fixation during the pedicle cortex perforation. Methods The vertebral compression fracture was performed in the L1 vertebral body using the Chiba's method from 60 fresh thoracic and lumbar vertebrae samples of sheep(T13-L3),which were randomly divided into 6 groups(A,B,C,D,E,and F)using a lottery method. Bilateral pedicles of vertebral arch of T14 and L2 were inserted pedicle screws,connecting titanium rods to fix T14-L2 segments. Then the samples of groups B,C,D,E,and F were removed a quarter of right side of lateral T14 thoracic pedicle cortical,which were considered the pedicle cortex perforation model. Finally,each group was fixed on different numbers of crosslinks:group A(0 crosslink,Intact),group B(0 crosslink,NCL),group C(1 crosslink,1/2 of the rods,MCL),group D(1 crosslink,1/3 of the rods close to T14,PCL),group E(1 crosslink,2/3 of the rods away from T14,DCL)and group F(2 crosslinks,1/3 and 2/3 of the rods respectively,TCL). After all samples were subject to 10 000 times of fatigue test with biomechanics test machines,the axial compressive stiffness,range of the motion(ROM)of the 6 directions(flexion,extension,lateral bending,and axial rotation),and the maximum pullout of the screws of the T14 pedicle cortex perforation were measured and compared among these 6 groups. Results The axial compressive stiffness in groups A,C,D,E,and F was significantly higher than that in group B(all P<0.05),and group A was significantly higher than group F(P<0.05) . The maximum pullout in groups A,C,D,E,and F were significantly higher than that in group B(all P<0.05),and group A was significantly higher than group F and groups C,D,and E were significantly lower than group F(all P<0.05). ROM of flexion,extension,lateral bending,and axial rotation in groups A,B,C,D,E,and F were significantly lower than that in group B(P=0.000),and ROM of left and right axial rotation in groups C,D,and E were significantly higher than in that group F(P=0.000). Conclusions During the pedicle cortex perforation,adding of one crosslink can improve the stability of the pedicle cortex perforation,and adding of two crosslinks can approximately achieve the same stability as the pedicle screw fixation with no pedicle cortex perforation. The location of the crosslink has no obvious effect on the short segment of spinal fixation.