This paper presents the results in two parts. First, the ROM, disc stress, and facet force in the instrumented groups were compared to those in the INT group. Second, the biomechanical behavior of the screw, NS, and PS was compared to that of Nt45.
The intervertebral ROM
At the implanted L3-L4 levels, the ROM in all models decreased compared with the INT, do a degree of 56-62% in extension, which is similar to flexion (57-60%). The ROM further decreased by 4-18% in torsion and 49-57% in lateral bending. In contrast, the ROM of the cephalic adjacent L2-L3 levels all increased; by 17-19% during flexion, 13-15% during extension, 1-5% during torsion and 14-17% during lateral bending (Table 3).
Overall, the differences between these models were within 5% for both flexion and extension. There was a downward trend in ROM changes at all levels as the NS diameter decreased (Figure 2).
Intervertebral disc stress
At the implanted L3-L4 level, the disc stress in all models decreased when compared with INT (Table 4). The disc stress decreased by 31-33% in flexion, where the maximum occurred in the anterior superior edge; 42-43% in extension, where the maximum occurred in the posterior inferior edge; 3-19% in torsion, where the maximum occurred in the left inferior edge; and by 45-55% in lateral bending, where the maximum occurred in the left superior edge.
In contrast, the disc stress of the cephalic L2-L3 adjacent levels all increased. This parameter increased by 24-27% in flexion, where the maximum occurred in the anterior superior edge (Figure 3); by 12-14% in extension, where the maximum occurred in the posterior inferior edge; by 0-7% in torsion, where the maximum occurred in the left inferior edge; and by 21-24% in lateral bending, where the maximum occurred in the left superior edge.
Overall, the differences between these models were within 10%, except for torsion at the implant level. The NS diameter was inversely proportional to the disc stress at the implant level. It did not markedly influence the disc stress at the adjacent level (Figure 4).
Facet joint contact force
There was no facet force during flexion in any of the models. At the implanted L3-L4 levels, the facet force in all models decreased compared with the INT, by 87-95% in extension, 4-37% in torsion and 79-100% in lateral bending. The Nt45 model showed the greatest decrease (Table 5).
In contrast, the facet force of the adjacent cephalic L2-L3 levels all increased, by 23-25% and 40-55% in extension and lateral bending, respectively, where the maximum occurred on Nt35, while it increased by 2-11% in torsion, and the maximum occurred on Nt45.
Screw stress
In flexion, the screw stress on Nt35 increased by 1% when compared with Nt45, whereas the other models decreased by 4-6%. The screw stress increased by 5-7% and 11-15% during extension and lateral bending, respectively, and decreased by 23-53% in torsion. For all motions, the screw stress of all models ranged between 98.9 and 211 MPa, and the maximum occurred on Nt45 in torsion (Table 6).
PS stress
In flexion and lateral bending, the PS stress increased by 4% and 6% on Nt15 compared to Nt45. Except for these two models, the other models decreased by 26-74%. There was no obvious trend in PS stress as the NS diameter changed (Figure 5). In all motions, the PS stress of all the models was between 2.7 and 17.4 MPa, and the maximum occurred on the Nt45 in torsion (Table 6).
NS stress
In flexion, extension, torsion, and lateral bending, the NS stress increased by 24-168%, 16-156%, 48-75% and 14-117% when compared with Nt45 (Table 6). The NS stress increased as the NS diameter decreased, and the maximum stress occurred in the lower-third portion (Figure 6). For all motions, the NS stress of all models was between 39.5 and 383 MPa, and the maximum occurred on Nt25 in torsion (Figure 7).
SHE rod stress
The SHE rod stress comprises both the PS and NS stresses. In flexion, extension, torsion, and lateral bending, the SHE rod stress increased by 18-152%, 9-137%, 40-65% and 12-111% when compared with Nt45, and the greatest increase occurred on Nt15. The SHE rod stress increases as the NS diameter decreases. For all motions, the SHE rod stress of all models was between 44.6 and 389 MPa, and the maximum occurred on Nt25 in torsion.