A transtibial patellar tendon bearing (PTB) prosthesis for below the knee amputees is disclosed. The prosthesis includes a socket, and a foot prosthesis attached to a modular shank component using a foot rotator device. Using the rotator device, the foot component of the prosthesis may be smoothly and axially rotated internally or externally .+-.150.degree. with respect to the shank component about the axis of the shank component. The wide range of axial rotation enables a more even load distribution on the whole lower limb of an amputee and enables an amputee to comfortably assume a wide range of biomechanical positions, such as sitting or kneeling. A recoil spring, having a selectable tension, is positioned within the rotatable component and selectively limits and retains the relative rotation of the upper portion and the lower portions. A locking mechanism selectively prevents rotation when the foot component is engaged.