polyester it can be formed by techniques that include for example one or more of molding and machining the assembly of the invention includes a plane guide 44 which is formed from a sheet of stainless steel the plane guide is of a sufficient size that it extends from the guide portion 20 at least as far as the greater trochanter 8 as shown in fig1 the thickness of the plane guide is slightly less than the thickness of each of the horizontal and vertical grooves 30 32 38 so that the plane guide is a snug fit in the grooves in particular the snug fit of the plane guide in a selected groove is such that the portion of the plane guide that extends beyond the groove is substantially fixed relative to the portion of the plane guide that is located in the groove against movement out of the plane defined by the groove fig5 shows a proximal femur 2 with a trial implant that comprises a stem trial 10 located within the intramedullary cavity with a trial neck 50 and a trial head 52 fitted to it the trial neck has a mating surface with a spigot and a socket formed on it which cooperate with the spigot and the socket on the neck surface 18 of the stem trial so that the stem trial and the trial neck are located positively relative to one another against translation and rotation when each of the spigots is received in its corresponding socket and the mating surface on the neck portion abuts the neck surface 18 on the stem trial the femur is prepared for receiving the trial instrument of the invention using techniques that are commonly used in hip replacement surgical procedures these can include 1 pre.