## 4 Discussion
Unlike the normal model, the sagittal angle of C6-C7 in the fusion model was modified to more lordosis. Hence, postoperative sagittal alignment in the fusion model trends towards imbalance. The alteration of sagittal alignment could be the result of the alteration of the A-P SFs, further leading to an imbalanced structure. [PERSON] et al. [25] found that the decrease in the A-P SFs during the flexion movement could be correlated with the sagittal imbalance in the thoracic-lumbar region. A systematic review of ASD etiology stated that abnormal stresses resulting from imbalanced alignment after spinal fusion appear to lead to ASD [5]. However, there is still no definitive evidence that ASD is distinctly related to a decrease in SF. The relationship between ASD and SF should be investigated in further research. Thirdly, significant increases in FJF s for C4-C5 and C6-C7 in the fusion model were observed in comparison to the NS (Figure 5). Moreover, according to the estimated load-sharing ratios, the proportions of FJF s after fusion were obviously increased in the middle range of the head extension (Figure 7). The average increase in FJF s at C3-C4 or C6-C7 was approximately 20%; for C4-C5, the increase was 47.4%. According to a cadaveric experiment by [PERSON] et al. [12], the FJF after spinal fusion was increased by roughly 25% at adjacent segments. Although the predicted average increase ratio at the superior adjacent segment was greater than the experimental result, the FJF of the FS was significantly increased, overall. In the actual cervical spine system, the role of the facet joint is to share the external loads, as a complementary role to the disc [26]. After fusion surgery, the facet joint at adjacent segments would be expected to share more spinal loading. The increasing load-sharing ratios of the facet joint might biomechanically explain adjacent segment facet degeneration after spinal fusion. Finally, there are no significant differences in translations of ICRs at adjacent segments between the FS and the NS. These results indicated that cervical spinal fusion may not affect the joint internal motion during the head flexion-extension movement. Moreover, a similar phenomenon was found in the in vivo cervical spine kinematics observation [27]. Thus, cervical spinal fusion might merely affect the motion redistribution among adjacent segments, not the translations of ICRs. For this numerical biomechanical study, several limitations should be pointed out. In terms of kinematics inputs, the maximal ranges of motion of the head and cervical spine were used, rather than the actual daily activities. Limited ranges of motion and the motion pattern redistribution after spinal fusion were remarkable features for the maximal ranges of motion. However, in daily activities, the global ranges of motion are similar for the FS and the NS. Hence, \"loss of motion\" will be compensated by adjacent segments, thus achieving the desired motion [7]. Due to different motion patterns, cervical spinal loading in daily activities might be slightly different. Furthermore, the adopted sagittal alignment in the fusion model could be another limitation. In the present study, the fusion model only considered a kind of postoperative sagittal alignment (imbalance lordosis). Other kinds of postoperative sagittal alignments were not investigated. As mentioned above, the alteration of A-P SFs could result from abnormal sagittal alignment. Hence, the effects on spinal loading among different sagittal alignments need to be further researched. Additionally, only single-level cervical spinal fusion was studied. In terms of multi-level fusion, the fusion length could have a significant influence on the adjacent segmental biomechanics. Hence, further investigation on the biomechanics of multi-level fusion can be performed using the developed cervical spinal fusion model.