An apparatus registers a patient's pelvis during surgery by establishing a patient-specific, supine pelvic reference plane. The apparatus may include an elongated support arm having three legs, at least two of which may be moveable along the arm. The first leg may contact the patient's right anterior superior iliac spines (ASIS), the second leg may contact the left ASIS, and the third leg may contact an anterior aspect of the ischium of the patient's pelvis below the acetabulum of the hip being operated on. The distances between the first leg and the second and third legs may be determined preoperatively so that the legs dock to the desired points. The apparatus may further include a direction indicator configured to point in a desired orientation for inserting an acetabular cup component in the patient's acetabulum.
