*2.4. Model Validation*

The FE method requires strict validation of the model because an inaccurate model may lead to incorrect and unreliable results [9–11]. Comprehensive experimental validation of the FE model of the reconstructed pelvis is not carried out before the surgery because a definite forecast of the bone stress state is required before performing surgery.

However, the evaluation of the general adequacy of the model was carried out in accordance with several parameters. First, the FE mesh was assessed based on several criteria (aspect ratio, skewness, warping), and mesh quality was evaluated based on element size, type, and shape. Second, the physical and mechanical characteristics of materials ensuring model accuracy were obtained from reliable sources [9,12,14,16,18,19].

Kinematic boundary conditions for the model and contact regions were applied and approved by the medical studies described in other articles [11,26,27]. The loading scheme, which was used in current research and based on the loading of the structure with the hip joint force acting as a reaction force, was the same as used previously [28]. It allowed us to describe the stress-strain state in a more accurate way.

The preliminary frequency analysis proved that the reconstructed pelvis assembly was joint and adjusted correctly, and all the connections were set up properly.

Another validation based on the displacement distribution (Figure 6) showed that the values obtained in the current study were within range close to the ranges described in papers [1,10,29]. The results of the analysis of the stress and strain fields were also close to previous results [1,2,10], proving the principal qualitative and quantitative conformance. *Materials* **2021**, *14*, x FOR PEER REVIEW 10 of 24

**Figure 6.** Total displacement in the "bone–endoprosthesis" system for the walking condition and a screw load of 1000 N: (**a**) typical displacement distribution (mm); (**b**) displacement values for the full gait cycle at the spherical center of the left femoral head. **Figure 6.** Total displacement in the "bone–endoprosthesis" system for the walking condition and a screw load of 1000 N: (**a**) typical displacement distribution (mm); (**b**) displacement values for the full gait cycle at the spherical center of the left femoral head.
