An alignment apparatus, for correct orientation of an acetabular cup of a prosthetic hip for insertion into a pelvis, includes a medical instrument mounted on a bone screw 21 attached to a patients ASIS and comprises four members 22,30,40,60 movable and lockable relative to each other, and including a ball and socket, a three dimensional spirit level 64, a laser direction indicator 65 and a pointer 63 in conjunction with a scale 46 to indicate an angle of orientation. An alignment device (10, Figure 3a) comprising a rod (11) with a three dimensional spirit level (12) and a pair of slidably mounted, extendable end plates (15), each for engagement with an ASIS of a patient, is initially used to establish the correct position of a patient lying on an operating table. A laser beam (75, Figure 6)) of an introducer (70) may be aligned with a laser beam (66) from the alignment device to position the cup into the hip joint socket.
