Model validation
ROM data of the intact lumbar spine were compared to the results of the previous studies, which were under the act of the same load as listed in Figure 5. The ROM tendency of each segment was closely correlated with the results of Chen, et al[20]and Zhong, et al[21]. In flexion, the maximum ROM took place at L4-5 and the maximum in extension and bending were respectively seen at L3-4 and L4-L5. The mean values in torsion were under 3°. The ROM of the L2-L5 segments were 11.2°, 10.9°, 12.0°, and 7.1°in flexion, extension, bending, and torsion, respectively. Overall, the ROM discrepancy was within the acceptable range of error. The results of our study confirm the rationality of the model and can be further analyzed.
Range of motion
In Fig 6, there was a significant reduction in ROM at L3/4 for model a, b and c when compared with the intact model for all loading conditions. Model d slightly decreased ROM in axial rotation and lateral bending. The supplemented fixation device provided the additional fixed effect on the fusion segment. Differences in the ROM between model a and b were not significant at less than 1 degree for all loading conditions. The ROM of each instrumented model was shown in more detail in Fig 7.
Flexion-extension
In Fig 7, there were no ROM differences in flexion among four models (90.1% to 98.8% restriction). In extension, model a and model b provided similar stability (97.7% and 98.4% restriction, respectively) when compared with the intact model. Model c reduced 77.5% ROM of the intact model, which was 9.8 times greater than that of model a. Model d reduced the lowest ROM (65.3% restriction), which was less restrictive than that of the model a (15.1 times).
Lateral bending
Model a and model b provided the largest reduction of ROM, by 95.7% and 94.5% restriction in lateral bending, compared with the intact model. Model c presented less than 30% of intact ROM (76.3% restriction). Like flexion-extension, model d reduced the lowest ROM (55.9% restriction), which was 10.3 times greater than that of model a.
Axial rotation
Compared with the intact model, the largest reduction of axial rotation ROM was found in model a. But there was no significant difference in the ROM observed within model a, b and c (73.8%, 68.3%, 64.9% restriction, respectively). Significant differences were found in model d, which merely provided 18.3% ROM restriction when compared with the intact model. In addition, axial rotation ROM was the least restricted mode of kinematic behavior.
The magnitudes of the maximum Von Mises stress in interbody cage
The maximum Von Mises stress in the interbody cage was displayed in Figure 8. In all loading conditions, the stress of the cage was found to be largest in model b. In flexion, the maximum stress of the cage reached 172.6 MPa in the model b, which was significantly increased maximum stress compared with other models. The cage stress in the model b was 13.2, 6.1, and 6.7 times greater than that of model a, c and d in flexion, respectively. Similarly, the peak stress in the model b was 4.8 and 2.3 times greater than that of model a and c in the lateral bending and 2.0 and 1.5 times greater than that of model a and c in the axial rotation. Difference was not significant between model b and d in lateral bending and axial rotation.
The magnitudes of the maximum Von Mises stress on interbody cage-L4 superior endplate interface
In all loading conditions, model d generated the largest endplate stress among implanted models (Fig 9). However, in flexion, the maximum stress caused by the model b exceeded model a, c and d by 3.9, 2.3 and 1.6 times, respectively. The stress in model a and c was 40.9% and 68.9% of that compared with model d. In lateral bending, the maximum endplate stresses caused by the model d exceeded the model a, b, and c by 2.6, 3.0, 5.4 times in left bending and 2.7, 2.5, 1.7 times in right bending. In axial rotation, the largest stress on the pedicle screw was found in the model b, which exceed model a, c and d by 1.7, 1.8 and 1.3 times, respectively.
The maximum displacement(mm) in interbody cage
For interbody cages without supplementary fixation, the maximum displacement of the cage was found to be high in the model d in all loading conditions (Fig 10). In flexion, the displacement caused by the model d exceeded the model a, b and c by 121.3%, 116.8%, and 116.8%, respectively. Greater differences could be seen in lateral bending, the displacement caused by the model d exceeded the model a, b and c by 173.8%, 225.8%, and 166.3%. In extension and axial rotation, model d was slightly higher than that of other models, but the difference was not significant.
The magnitudes of the maximum Von Mises stress on screw-bone interface
The stress peak of the screw-bone interface is investigated to show the load distribution between the vertebrae and the spinal implants. It is important to evaluate the risk of screw loosening and migration[24]. Fig. 11 summarizes maximum Von Mises stress of the screw-bone interface for implanted models. At L3/4 segment, the stress was greater in lateral instrumentation than that of posterior instrumentation in all loading conditions. In flexion-extension, the stress in the model c was 5.7 and 5.1 times greater than that of model a and model b. The largest stress of screw-bone interface was found to be 617.5 MPa in the model c in axial rotation, which exceeded the model a and model b by 4.1 and 3.4 times. Greater differences could be seen in lateral bending, the stress caused by the model c was 7.0 and 6.1 times greater than that of model a and model b. Besides, the stress caused by model b was slightly higher than that of model a in all loading conditions. Particularly in axial rotation, the difference was more than 30 MPa.
The magnitudes of the maximum Von Mises stress in NP-IDP of adjacent intervertebral disc
Fig. 12 included the maximum Von Mises stress in NP-IDP of the superior adjacent level(L2/3) for each instrumented construct. In all loading conditions, the L2/3 NP-IDP caused by four models was slightly higher than that of the intact model, but the differences were not significant.