The knee-disarticulation prosthesis is very similar to the above-knee prosthesis, except for the lower part of the socket and the knee mechanism. Before the introduction of the present day polycentric knee units, sockets for the prosthesis were usually made of leather and metal hinges were used to attach the socket to the shin. This type of prosthesis is still preferred by a few preferred even though it is bulky and control of the leg during the swing phase is difficult. However, most prefer one of the polycentric designs where the knee mechanism can be installed within the shin due to its special design.