Virtual trial reduction system for hip arthroplasty and coordinate systems therefor

A method for providing, without using a patient tracker, a patient-specific pelvic coordinate system from a single near AP intra-operative image of the patient, said method comprising:

• forming a single intra-operative fluoroscopic image of the patient's pelvis in the near AP direction using an x-ray source;
• defining first and second landmarks on anatomically separated regions of said pelvis on said image, said landmarks being separated from each other in at least an anterior-posterior direction;
• determining the transaxial displacement of said landmarks on said image;
• determining the axial displacement of said landmarks on said image;
• calculating by a processor the axial rotation of the pelvis using the transaxial displacement as a measure of the axial rotation of said pelvis with respect to the plane of said image, and
• calculating by a processor the transaxial rotation of the pelvis using said axial displacement as a measure of the transaxial rotation of said pelvis with the respect to the plane of said image.