Leg alignment for surgical parameter measurement in hip replacement surgery
A CAS system and method comprises a first reference in fixed relation with the pelvis and a registration tool. A sensor apparatus tracks the first reference and tool. A controller unit receives tracking data for the first reference and tool. A calculator tracks the pelvic frame of reference, and the tool to produce a femoral frame of reference at two sequential operative steps. A reference orientation adjustor receives tracking data for the pelvic frame of reference to orient the femoral frame of reference in a reference orientation with respect to the pelvic frame of reference, and to produce a reference adjustment value as a function of the reference orientation. A surgical parameter calculator receives tracking data from the tool to calculate surgical parameters as a function of the reference adjustment value.
1. A method of measuring surgical parameters in computer-assisted surgery so as to guide an operator in inserting a hip joint implant in a femur, comprising:
i) digitizing a frame of reference of a pelvis, the frame of reference of the pelvis being trackable in space;
ii) digitizing a pre-reduction frame of reference of the femur as a function of the frame of reference of the pelvis, the pre-reduction frame of reference comprising a femoral axis;
iii) obtaining a reference orientation for the femoral axis of the pre-reduction frame of reference of the femur with respect to the frame of reference of the pelvis;
iv) digitizing a reduction frame of reference of the femur by virtually orienting the femoral axis in said reference orientation as a function of the frame of reference of the pelvis, after initiation of implant reduction;
v) providing the surgical parameters associating the femur to the pelvis and measured as a difference between the first and second frames of reference of the femur.
2. The method according to claim 1 , wherein the femoral axis is a mechanical axis, and wherein obtaining a the reference orientation comprises the reference orientation having the mechanical axis of the pre-reduction frame of reference of the femur at a predetermined angle of a vertical axis of a patient within a frontal plane of the frame of reference of the pelvis.
3. The method according to claim 2 , wherein obtaining the reference orientation comprises the reference orientation having the mechanical axis where the predetermined angle is 3°.
4. The method according to claim 2 , wherein digitizing the frame of reference of the pelvis comprises digitizing two points on an iliac crest and a point on one of pubic tubercles to obtain the frontal plane of the frame of reference of the pelvis.
5. The method according to claim 2 , wherein obtaining the reference orientation comprises having the mechanical axis of the pre-reduction frame of reference of the femur pass through a center of rotation of the femur, and the reference orientation has the center of rotation of the femur and of an acetabulum coincident.
6. The method according to claim 1 , wherein the step iv) includes updating the frame of reference of the pelvis by digitizing a center of rotation of the acetabulum subsequently to the implant reduction.
7. The method according to claim 1 , wherein the surgical parameters are at least one of a limb length discrepancy and a medio-lateral offset.
8. The method according to claim 1 , wherein the steps i) and ii) are performed with respect to a single trackable reference secured to the pelvis.
9. The method according to claim 1 , wherein the method is performed on a bone model or a cadaver.