Virtual trial reduction system for hip arthroplasty and coordinate systems therefor

1. 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.

2. The method of claim 1 in which the axial rotation of the pelvis is calculated as a function of the transaxial displacement and the distance between the x-ray source and the image plane.

3. The method according to claim 1 wherein said first landmark comprises the image point of the public symphysis.

4. The method according to claim 1 wherein the second landmark comprises the midpoint of a line between corresponding points on said image of the left and right sacroiliac joints.

5. The method according to claim 1 wherein said displacements are normalized with respect to the separation between a further pair of landmarks on the pelvis.

6. The method according to claim 5 wherein said further pair of landmarks comprises the left and right teardrops.