## 4 Discussion  
### Radiological Data and FEA Simulation  
Starting from patient-specific data, the segmentation workflow of medical images led to a reliable three-dimensional virtual model of the knee joint, with a focus on the main bones and cartilage tissue that compose it. This will support various clinical applications and allows for a better understanding of the knee biomechanics through a realistic 3D-printed model. Subject 1 (D), characterized by a greater volume, indicates potential fluid infiltration into the pathologic cartilage, leading to swelling and, thus, more intense strain [37]. In addition, the cartilage surface of Subject 2 appears to exhibit greater homogeneity compared to that of Subject 1 (D), accounting for the uneven distribution of holes and deformities in the latter.  
Upon analyzing the Finite Element Analysis (FEA) results in Figure 5, it is evident that the stress-strain curves display a linear trend with remarkably similar slopes. The mechanical performance of biological cartilage tissue is intricately influenced by various factors, such as age, osteoarthritis (OA), and physical activity, making it challenging to establish a standardized mechanical profile [38]. Despite the distinct biochemical composition, the synthetic cartilage demonstrates promising overall elastic properties, thanks to the manual tunability achieved by adjusting the proportions of elastic or gel-like materials. This adaptability allows for the calibration of synthetic cartilage to match the desired reference values. The results position the printed materials as the endpoints on a spectrum, with \(Soft\) representing the lower limit and \(Medium\) representing the upper limit of the mechanical properties akin to knee joint cartilage (Figure 9). As stresses and deformations escalate, the impact of geometric variations becomes more pronounced, underscoring the role of osteoarthritis in modifying the mechanical response of cartilage [37]. These insightful comparisons underscore the potential of 3D printing and synthetic materials in effectively mimicking the elastic properties of natural cartilage tissue.  
Figure 8: Stress-strain behavior for compressive and tensile tests of fiber-reinforced samples mimicking the three layers of native cartilage tissue. The tensile chart included a curve for the plain material (without fibers) to estimate the properties of the matrix itself.  
The FEM analyses carried out on the models with supports were used to verify that the created supports were actually able to simulate the correct distribution of loads in the knee. Despite surface modifications caused by the supports, the results of the stresses and strain distributions on the cartilages remained almost unchanged (Figure 6). Moreover, the materials' behavior in the stress-strain graphs remained consistent, affirming the minimal influence from the introduced supports on force transmission. These results validated the implementation of the mechanical setup with 3D-printed patient-specific phantoms shown in Figure 2.  
Conventional testing techniques usually do not take into account tissue morphology and geometrical structure, in favor of concentrating only on the mechanical properties [44]. One significant simplification relates to the native cartilage's biomechanical behavior, which is bi-phasic poro-viscoelastic [45; 46], but in this instance, it is assumed to be linear isotropic elastic. Nonetheless, several studies have found that this is suitable for instantaneous tissue deformation [26; 28]. Moreover, other anatomical components of the knee joint, such as the tibial cartilages, menisci, and ligaments were not taken into account for the scope of this study. Even though these structures are essential to the health and function of the joint, their absence made it possible to gain a greater awareness of the behavior of pathological femoral cartilage and how it affects the health of the knee joint. This understanding permitted us to identify potential failure mechanisms associated with damage to the femoral cartilage.