A modular spacer device for a knee joint includes a tibial element adapted to be constrained to an end of the tibial bone and a femoral element adapted to be constrained to an end of the femoral bone and to be articulated on the tibial element. The tibial element has first and second surfaces opposite to each other, and the femoral element has first surface and second surfaces opposite to each other. The first surface of the femoral element is convex and laterally has a curved, ammonite-shaped contour with a curvature radius that increases starting from a rear section with a curvature radius R1, a first central section having a curvature radius R2, a second central section having a curvature radius R3, and a front section having a curvature radius R4, with R1≤；R2≤；R3≤；R4.
