Method and apparatus for finding the position of a mechanical axis of a limb

A method for implanting a prosthesis on a limb, comprising:

• calculating the position of a mechanical axis extending longitudinally through a limb without piercing a skin on the limb beyond a first exposed end of the limb, the limb being freely movable and rotatably supported in a body via a movable center of rotation at a second end of the limb, wherein the calculating is done iteratively via a least-squares method, and wherein calculating the position of the mechanical axis comprises:
• rotating the limb about the second end to a plurality of rotational positions of the limb;
• detecting measurement-point coordinates for each of the plurality of rotational positions, the measurement-point coordinates for each rotational position of the limb defining a plurality of measurement-point coordinate sets;
• recording the measurement-point coordinate sets;
• and
• selecting at least one of the measurement-point coordinate sets and assigning said selected group to the center of rotation;
• aligning a cutting jig on the limb with respect to the calculated mechanical axis;
• attaching the cutting jig to the first end of the limb, the cutting jig guiding a cutting device thereon to resect the first exposed end of the limb;
• and
• implanting a prosthesis on the limb at the resected first end.

A method for implanting a knee prosthesis, comprising:

• calculating the position of a mechanical axis extending longitudinally through a femur without piercing a skin on the femur above a first exposed end of the femur, the femur being freely movable and rotatably supported in a hip via a movable center of rotation at a second end of the femur, wherein the calculating is done iteratively via a least-squares method, and wherein calculating the position of the center of rotation includes:
• determining a first reference point at the first end of the femur and calculating the position of the center of rotation, the mechanical axis extending through the first reference point and the center of rotation;
• and
• rotating the femur about the second end to a variety of rotational positions and measuring the orientation of the femur at the rotational positions;
• aligning a cutting jig on the first end of the femur with respect to the calculated mechanical axis, the cutting jig guiding a cutting device thereon;
• operating the cutting device to resect the first exposed end of the femur;
• and
• implanting a prosthesis on the resected first end of the femur.