# Novel Design of Expandable Spinal Cage for Efficient Lumbar Spine Fusion Operation
## 5 Results and Discussions  
This study successfully demonstrated the achievement of key design objectives for the expandable spinal cage. First, the cage provides sufficient height and angle expansion, ensuring the required intervertebral space and lordosis angle restoration. Second, the height and angle can be adjusted independently. While several existing cages offer height and angle adjustments in linked or fixed increments, this design allows individual control of both height and lordosis adjustments. Third, the height and angle adjustments can be continuously and finely tuned, enabling gradual adjustments during surgery to achieve precise alignment and fit, as opposed to limited incremental adjustments. The main characteristics of the proposed design are compared with existing cages in Table 2.  
These improvements were made possible through the novel linkage mechanism introduced in this cage design. The internal linkage mechanism translates the surgeon's operation into independent height and angle adjustments, while the slider component facilitates smooth, continuous tuning. Traditional rack-and-pinion or wedge designs typically offered only fixed expansion stages or lacked independent adjustment capabilities. In contrast, the spinal cage proposed in this study can be customized during surgery based on patient-specific anatomical features, allowing for sufficient disk height and spinal curvature restoration. This represents a significant improvement compared to static cages with no adjustment capabilities or conventional expandable cages with coarse incremental adjustments. FEM analysis was utilized to predict the mechanical behavior of the expandable cage and compared with actual mechanical testing results. As shown in Figures 16 and 17, differences between FEM simulations and experimental outcomes were observed. The primary cause of these differences was attributed to the deformation of the testing equipment. During experiments, the test apparatus underwent slight elastic deformation under load, which was not considered in the FEM analysis. To compensate, tests compressing only the equipment without specimens were conducted, revealing approximately 0.11 mm deformation under a 1000 N load. This data allowed for the calculation of equipment deformation corresponding to applied loads, which was then integrated into the FEM analysis for calibration. As shown in Figures 18 and 19, The calibrated FEM results closely matched the experimental data, enhancing the reliability of the FEM model. However, some discrepancies occurred in low-load regions, likely due to slight looseness caused by manufacturing and assembly tolerances in specimen. Specifically, there was a designed tolerance of 0.2 mm which was not present in the FEM model. While the FEM model assumes perfectly fitted components, the actual specimen may have had minor gaps that delayed initial contact between parts. As the load increased, these gaps closed through deformation and interlocking, resulting in load-displacement behavior that aligned with FEM predictions. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Model**} & **Height** & **Angle** & **Independent** & **Continuous** \\  & **Adjustable Range** & **Adjustable Range** & **Adjustment** & **Adjustment** \\ \hline Our study & 8.4 mm & 12.3\({}^{\circ}\) & Possible & Possible \\ \hline Tyche [20] & 2 mm & 4\({}^{\circ}\) & Impossible & Impossible \\ \hline Rise [21] & 7 mm & 0\({}^{\circ}\) & Impossible & Possible \\ \hline Sable [21] & 8 mm & 22\({}^{\circ}\) & Impossible & Possible \\ \hline [PERSON] et al. [27] & 4 mm & 10\({}^{\circ}\) & Impossible & Impossible \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison with existing model. Figure 16: Comparison of compression experiment result with calibrated analysis result. Figure 17: Comparison of compression-shear experiment results with calibrated analysis results. Figure 18: Shifted compression analysis results to compare trends in analysis and experimental results.