**1. Introduction**

The human pelvis is a geometrically complex, biomechanical structure that carries the weight of the human body and stabilizes and protects inner organs. The pelvis can be damaged due to problems with the primary implant, infections, accidents, or bone tumours, which usually involve a large area of tissue removal and affect the patients' lives. Due to the complex anatomical structure, the reconstruction of pelvic biomechanics after the loss of bone structure is still a challenge [1]. Various implants are used for different types of pelvis injuries, such as modular pelvis prostheses, saddle prostheses, pedestal cups, and custom-made pelvis prostheses. Among them, custom-made endoprostheses are matched with the patient's bones, which in turn can reduce the risk of infection, dislocation, or failure of the implant [2]. Therefore, a custom prosthesis design is in demand when it is required to treat a complex bone fracture or replace a primary serial implant.

Previous research [3] presented pelvis reconstruction by applying a fibula and a variation of the methods of internal fixation of the implant. In this study, a vertical load of 500 N was applied to the L3 lumbar vertebrae, and the pelvis was considered to be in a bipedal standing position. As a result, the stress concentration in the fibula implant was extremely high, but this effect was minimized by internal fixation, which partially transferred the stresses from the fibula to the screw system. Additionally, a high stress concentration was detected in the implant. Among the four methods of fixation, the best

**Citation:** Maslov, L.; Borovkov, A.; Maslova, I.; Soloviev, D.; Zhmaylo, M.; Tarasenko, F. Finite Element Analysis of Customized Acetabular Implant and Bone after Pelvic Tumour Resection throughout the Gait Cycle. *Materials* **2021**, *14*, 7066. https://doi.org/10.3390/ma14227066

Academic Editors: Oskar Sachenkov and Antoniac Iulian

Received: 13 October 2021 Accepted: 18 November 2021 Published: 21 November 2021

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method was a double rod system with an L5-S1 pedicle and iliac screws, which provided the lowest stress concentration and the lowest displacement of the pelvis.

A previous study [4] describes a modular endoprosthesis for the damaged half of the pelvis. In the course of this research, a comparative analysis of the stress distribution between the healthy and reconstructed pelvis was carried out in three static positions: sitting, standing, and standing on the foot of the injured side. The loads and boundary conditions were similar to those described in the study above [3]. In the healthy pelvis, the stress distribution was concentrated in the upper region of the acetabulum, arcuate line, sacroiliac joint, sacral midline, and, in particular, the upper region of the greater sciatic notch. In the reconstructed pelvis, the stress distribution was concentrated on the proximal area of the pubic plate, the top of the acetabulum, the connection between the CS fixator and acetabular cup, and the fixation between the prosthesis and sacroiliac joint. The stress distribution in the reconstructed pelvis was similar to the stress distribution in the healthy pelvis in the three different static positions.

Generally, the clinical efficacy and biomechanical features of the implants used for pelvic injuries should be evaluated through biomechanical experiments in vitro. However, irregular geometry and material heterogeneity of the pelvis often make mechanical experiments challenging [5].

In modern orthopaedic biomechanics, a computational approach was developed for analysing the stress and strain distributions of a hip joint endoprosthesis [6]. The study under consideration is based on the finite element (FE) method to investigate stresses on the bones and implant.

The FE method has proven to be a powerful tool in reducing the cost and time in many biomechanical studies and has become an important tool for understanding overall biomechanical behaviour. Nevertheless, many factors, such as material properties, anatomical geometry, the integrity of the human structure, and boundary conditions, could influence the accuracy of FE results [7].

Thus, the FE method is becoming increasingly popular in pelvis biomechanics research and plays a critical role in failure analysis and revision prosthesis design [2]. Although some FE analyses of custom-made prostheses have been carried out, studies of the influence of the prestress of the screws on the biomechanical performance of a reconstructed pelvis for walking patterns are rarely reported [2].

The aim of this study was to investigate the stress distribution of the pelvis reconstructed by individual endoprostheses with different screw forces and then identify which force value is optimal for tightening the implant and the bone. After that, the stress distribution in the "bone–endoprosthesis" system was obtained for typical walking loads and chosen screw forces.
