A computer assisted surgery system for determining a change in a property of a hip joint of a patient caused by an arthroplasty procedure carried out on the hip joint, the system comprising at least one data processing device in communication with at least one storage device and a tracking system which allows the positions of the patient's bones to be tracked within a reference frame of the tracking system, using a first trackable marker and a second trackable marker, and wherein the hip joint comprises a pelvis and a femur and wherein the first trackable marker is attachable to the pelvis and the second trackable marker is attachable to the femur, the storage device storing instructions executable by the data processing device to cause the computer assisted surgery system to carry out the data processing operations of: determining the relative position in the reference frame of the tracking system of the pelvis and the femur with the hip joint in a first position from determined and stored relative positions of a first trackable marker attached to the pelvis and a second trackable marker attached to the femur; determining the position in the reference frame of the tracking system of a pre-operative centre of rotation of the hip joint relative to the pelvis; determining the position in the reference frame of the tracking system of the femur relative to the pre-operative centre of rotation using the relative position of the pelvis and femur; determining the position in the reference frame of the tracking system of a post-operative centre of rotation (COR po) of the hip joint resulting from a prosthetic component to be used in the hip joint by determining the position in the reference frame of the tracking system of the post-operative centre of rotation (COR po) of the pelvis relative to the first trackable marker attached to the pelvis; determining the position in the reference frame of the tracking system of the femur relative to the post-operative centre of rotation by determining the position in the reference frame the tracking system of the post-operative centre of rotation (COR po) of the femur relative to the second trackable marker attached to the femur; and determining the difference between the position in the reference frame of the tracking system of the femur relative to the pre-operative centre of rotation and the position in the reference frame of the tracking system of the femur relative to the post-operative centre of rotation by mapping the position in the reference frame of the tracking system of the post-operative centre of rotation of the femur onto the post-operative centre of rotation (COR po) of the pelvis to determine any change (232, 234) in the property of the joint. determining the relative position in the reference frame of the tracking system of the pelvis and the femur with the hip joint in a first position from determined and stored relative positions of a first trackable marker attached to the pelvis and a second trackable marker attached to the femur; determining the position in the reference frame of the tracking system of a pre-operative centre of rotation of the hip joint relative to the pelvis; determining the position in the reference frame of the tracking system of the femur relative to the pre-operative centre of rotation using the relative position of the pelvis and femur; determining the position in the reference frame of the tracking system of a post-operative centre of rotation (COR po) of the hip joint resulting from a prosthetic component to be used in the hip joint by determining the position in the reference frame of the tracking system of the post-operative centre of rotation (COR po) of the pelvis relative to the first trackable marker attached to the pelvis; determining the position in the reference frame of the tracking system of the femur relative to the post-operative centre of rotation by determining the position in the reference frame the tracking system of the post-operative centre of rotation (COR po) of the femur relative to the second trackable marker attached to the femur; and determining the difference between the position in the reference frame of the tracking system of the femur relative to the pre-operative centre of rotation and the position in the reference frame of the tracking system of the femur relative to the post-operative centre of rotation by mapping the position in the reference frame of the tracking system of the post-operative centre of rotation of the femur onto the post-operative centre of rotation (COR po) of the pelvis to determine any change (232, 234) in the property of the joint.