# US 7,611,520 B2 · Method and apparatus for finding the position of a mechanical axis of a limb

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- Number: US 7,611,520 B2
- Document ID: US7611520B2
- Status: Granted
- Kind: B2
- Application number: 11/205,761
- Filed: 2005-08-16
- Published: 2006-04-27
- Granted: 2009-11-03
- Earliest claimed: 2000-08-31
- Assignees: Smith & Nephew Orthopaedics AG
- Inventors: Holger Broers; Werner Berner; Bernhard Hauri; Thomas Hauri
- Web page: https://patentgrep.com/patent/US7611520B2

## CPC

- A61B 5/4504: {Bones (A61B5/4547 takes precedence)}
- A61B 5/103: Measuring devices for testing the shape, pattern, {colour,} size or movement of the body or parts thereof, for diagnostic purposes (measuring instruments specially adapted for dentistry A61C19/04)
- A61B 5/1079: {using optical or photographic means}
- A61B 17/155: {Cutting femur}
- A61B 34/20: Surgical navigation systems; Devices for tracking or guiding surgical instruments, e.g. for frameless stereotaxis
- A61B 5/6878: {Bone}
- A61B 2034/105: {Modelling of the patient, e.g. for ligaments or bones}
- A61B 2034/108: {Computer aided selection or customisation of medical implants or cutting guides}
- A61B 2034/2055: {Optical tracking systems}
- A61B 2034/2068: {using pointers, e.g. pointers having reference marks for determining coordinates of body points}
- A61B 2034/2072: {Reference field transducer attached to an instrument or patient}
- A61B 2090/3983: {Reference marker arrangements for use with image guided surgery}
- A61B: DIAGNOSIS; SURGERY; IDENTIFICATION (usefulness limited to only animals A61D)

## Abstract

A method of finding the position of a mechanical axis running in the longitudinal direction through a limb that is rotatably supported by its first end in a center of rotation that is not fixed and is not accessible for the mechanical determination of coordinates, by means of an optical coordinate-measurement device with an indicator to detect measurement-point coordinates in one rotational position of the limb such that, for each rotational position, a multi-point indicator rigidly attached near the second end of the limb signals several measurement-point coordinates, and, from the sets of measurement-point coordinates detected in a plurality of rotational positions, at least one group is selected that can be assigned to the same site of the center of rotation, and the measurement-point-coordinate sets assigned to this site of the center of rotation are used to calculate the mechanical axis.

## Claims

### Claim 1

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 includes:
determining a first reference point at the first end of the limb; and
calculating the position of the center of rotation by rotating the limb about the second end to one of a variety of rotational positions and measuring the orientation of the limb at the rotational position, the mechanical axis extending through the first reference point and 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.

### Claim 2 (depends on 1)

The method of claim 1, wherein the position of the center of rotation is calculated iteratively via a least-squares method to minimize error in said calculation.

### Claim 3 (depends on 1)

The method according to claim 1, wherein the evaluation is accomplished by means of an iterative calculation by a least-squares method regardless of the immobility of the body.

### Claim 4

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.

### Claim 5

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.