## 4 Discussion  
Graft failure with subsidence and breakage leading to non-union are major concerns in ACDF surgery and have been shown to be associated with the use an autologous bone substitute, stand-alone cages, allospacers, reinforcement with anterior plates and screws, and posterior fixation, as well as age and other factors [13; 15; 32; 33; 34; 35]. However, no study has analyzed associations of biomechanical stress with cervical spacer design, length, and the ratio of cortical and cancellous portions of the spacers. It is expected that the cortical and cancellous portions play a different role once they are fused after insertion [10]. The cortical portion usually supports the endplates until the cancellous portion can form a firm union of bone. It could be postulated that a larger cortical component could result in breakage of allograft spacers or more subsidence into the vertebral body through the endplates if they fail to be fused properly postoperatively because of the different properties of E and \(v\) of the allograft-cortical bone and the cancellous portions of the recipient vertebral body [36]. Cortical breakage or subsidence could then result in a decrease of disc height and lead to foraminal restenosis [37]. It is commonly accepted that greater stress on the endplate and spacers and less contact with the surface of the cancellous portion could lead to an increased risk of subsidence and delayed fusion [10]. In one clinical study, CO-type allospacers, which have a smaller cancellous fusion bed, exhibited more breakage and displacement with disc height loss causing fixation instability, compared with other spacers [13]. In our study, the smaller cortical portion conversely led to an increased subsidence risk and a relatively higher risk of allograft spacer failure despite the wider fusion bed of cancellous bone, especially upon flexion and extension. We analyzed the results of predicted FE study using von Mises stress. The reasons are as follows: Generally, stress has direction, but von Mises stress is scalar and not a vector. Therefore, it is easy to analyze stresses in various directions, such as principal stress, at complex loading on the human body. Additionally, the von Mises is a theoretical measure of stress used to estimate yield failure criteria and is also popular in fatigue strength calculations. While significant differences in von Mises stress on different types of spacers and endplates were noted (Figure 4), the results did not fall in line with our initial hypothesis that longer spacers could supply a wider surface area of stress distribution and result in less von Mises stress on allograft spacers and endplates. The longer spacers showed a higher level of concentrated stresses on the posterior wall of the allograft spacers and in the contacted endplate area in flexion and extension. This could be explained by the nutracker mechanism with anterior cortical-bite positioning of allograft spacers (Figure 1), wherein the longer spacer exerts greater compression force at the posterior margin of the allograft spacer and contacting endplate, as well as greater posterior shear force from the superior anterior corner to the inferior posterior corner of the allograft spacer. This could lead to allograft spacer failure or graft subsidence, even though it is known that posterior endplates are stronger than the anterior component [38]  
The ROM in all motion modes was easily understandable because of the decreased ROM in the fused segment of C5-6 and increased ROM in the adjacent C4-5 and C6-7 segments. The longer spacers produced greater stress on the endplate and posterior complex of the vertebral body in flexion and extension. Along with higher preloading on the distal cervical segment, this increased stress could play a role in increased subsidence and result in a decreased ROM relative to the intact segment of C6-7. This needs to be clarified in further research. This study had a few limitations.