## 2 Materials and Methods  
### Model Development  
The advancement of implant design and surgical treatment methods remains a critical focus in modern orthopedics. A key objective is to create implants that closely replicate the anatomical shape and biomechanical properties of human bones. This approach ensures better functional integration and improved clinical outcomes. Among the materials considered for implant development, titanium has emerged as the optimal choice due to its superior mechanical properties, biocompatibility, and corrosion resistance. [PERSON] et al. [39] demonstrated that titanium plates exhibit excellent strength under maximum load conditions compared to alternative materials, further supporting its selection for orthopedic applications [15]. Consequently, titanium was chosen for the design and development of the novel derotation plate used in this study. The engineering team at D. Serikbayev East Kazakhstan Technical University modeled the derotation plate using the CT scans of patients diagnosed with Crowe type IV dysplastic oxarthrosis. The CT scan data were processed in DICOM format and imported into Materialise Mimics 25.0 software to generate detailed 3D anatomical models. Subsequently, a fragment of the femur was fabricated using a Designer XPRO 3D printer, employing fused deposition modeling (FDM) technology for layer-by-layer material deposition. The femoral derotation plate was initially conceptualized using Computer-Aided Design (CAD) and Computer-Aided Manufacturing (CAM) tools within the SolidWorks platform. A prototype was then fabricated using Stereolithography (SLA) technology with a Formlabs Form 3 3D printer (Formlabs, Somerville, MA, USA). This preliminary model underwent a meticulous visual inspection to assess its design accuracy and anatomical compatibility with patient-specific femoral structures. Adjustments were incorporated into the design to optimize its fit, functionality, and biomechanical performance. The derotation plate was specifically engineered to enhance biomechanical stability during total hip arthroplasty (THA) with subtrochanter shortening osteotomy in patients with Crowe type IV dysplastic oxarthrosis. Its design focuses on several key principles. Firstly, the plate ensures rigid fixation of the osteotomy site, stabilizing the femoral fragments postoperatively and reducing the risk of nonunion. Secondly, the plate effectively prevents rotational and axial instability by incorporating angled side wings in both the proximal and distal regions, significantly limiting femoral fragment displacement. Additionally, unlike conventional plates that often interfere with the femoral prosthesis, this derotation plate features screw holes oriented in the sagittal plane, preventing intersections with the implanted femoral stem. The carefully selected screw angles (20\({}^{\circ}\), 16\({}^{\circ}\), 8\({}^{\circ}\), and 4\({}^{\circ}\)) allow for optimal placement while maintaining strong and reliable fixation. A critical aspect of the design is its role in minimizing complications by optimizing the load distribution and reducing the stress-shielding effect. This approach enhances postoperative healing and mitigates risks, such as implant instability and delayed bone consolidation. The plate dimensions (length: 92 mm, width: 18 mm, thickness: 3.5 mm) and the use of titanium alloy (BT-6) were carefully selected to balance strength and flexibility, thereby supporting effective osseointegration. For final manufacturing, a titanium alloy (BT-6) was chosen due to its high strength-to-weight ratio, corrosion resistance, and durability, making it ideal for orthopedic applications. The alloy was precisely cut using an air plasma cutter to achieve the required dimensions. The machining process was meticulously programmed using MasterCAM 2018 (v20.0), ensuring sub-millimeter precision.