This study aimed to conduct a comparative experimental and numerical evaluation of two designs for a polycentric prosthetic knee joint with a four-bar mechanism: a design made entirely of AL7075-T651 alloy and a hybrid design of PA6-CF/AL7075-T651, to assess the trade-off between mass reduction and structural performance. Tensile and flexural tests were performed to determine the mechanical properties of the materials, and a fatigue test was performed on PA6-CF. Ground reaction force measurements also supported the selection of a 1000 N load for numerical analysis. Both designs were evaluated using the finite element method under three representative loading conditions of the heel strike, midstance, and toe-off phases, employing Von Mises stress, total deformation, and safety factor. The results showed that the AL7075-T651 design achieved higher stiffness and a greater safety margin, with stresses ranging from 109.05 to 127.88 MPa and safety factors ranging from 3.93 to 4.61. The hybrid design recorded von Mises stresses ranging from 197.76 to 299.62 MPa and safety factors ranging from 1.68 to 2.54. Stress separation by material in the hybrid design showed that the highest stress in the PA6-CF components was 25.19 MPa, which is lower than the experimentally measured yield stress of 50 MPa. Conversely, the hybrid design reduced the joint mass from 750 g to 350 g, a decrease of 53.3%. The results show a clear trade-off between mass and structural performance: the aluminum design offers higher stiffness and a greater safety margin, while the hybrid design achieves a significant mass reduction while keeping component stresses within yield limits under the studied loading conditions.