This study investigates the biomechanical behaviour of Ti-6Al-4V, Co-Cr-Mo alloy, and Stainless Steel in knee implants using Finite Element Method (FEM) analysis across various flexion angles (−8°, 5°, 15°, 30°, 60°, 90°, and 120°) and internal/external rotational conditions (5°, 6°, and 7°). Key parameters assessed include Von Mises stress, deformation, contact area, and contact pressure under physiologically relevant loads. Deep flexion angles (90° and 120°) produce peak stress concentrations in the femoral and tibial components, especially at their interfaces, increasing the risk of wear and fatigue. The polyethylene (PE) insert exhibited localized strain peaks, highlighting its role in load transfer. Maximum deformation occurred at 120°, raising concerns about implant stability. In contrast, moderate flexion (−5° to 30°) showed uniform stress distributions. Even minor internal/external rotations significantly altered stress profiles, stressing the need for precise rotational alignment in implant design. These findings advocate for material-specific optimization, especially for PE inserts, and highlight flexion-related loading as a critical factor in implant fatigue. Future work will include experimental validation and long-term fatigue studies across varied patient anatomies and activity levels.