Determining femoral cuts in knee surgery

1. A method for determining a distal cut thickness and posterior cut thickness for a femur in a knee replacement operation, the method comprising:

performing a tibial cut on a tibia;

performing soft tissue balancing based on a desired limb alignment;

creating a reference coordinate system in a computer aided surgery system for an uncut femur and said tibia;

registering said femur and said tibia to said reference coordinate system using tracking devices;

measuring an extension gap between said femur and said tibial cut while in extension using said femur and tibia as registered in said reference coordinate system;

measuring a flexion gap between said femur and said tibial cut while in flexion using said femur and tibia as registered in said reference coordinate system;

calculating a distal cut thickness and a posterior cut thickness for said femur using said measured extension gap and said measured flexion gap and taking into account a distal thickness and posterior thickness of a femoral implant; and

performing said femoral posterior cut according to said posterior cut thickness subsequently to the calculating of the posterior cut thickness; and

performing said femoral distal cut according to said distal cut thickness and based on the flexion gap resulting from the performed femoral posterior cut registered in said reference coordinate system.

2. A method as claimed in claim 1 , wherein, said distal cut thickness and said posterior cut thickness are calculated such that a post-cut gap from said tibia to said femur is equal in extension and in flexion.

3. A method as claimed in claim 2 , wherein said performing a tibial cut comprises obtaining a tibial cut that is substantially perpendicular to a mechanical axis of said limb.

4. A method as claimed in claim 3 , wherein said performing a tibial cut comprises using a positioning device to align said substantially perpendicular cut.

5. A method as claimed in claim 1 , wherein said performing a femoral cut comprises obtaining a femoral cut that is substantially parallel to said tibial cut such that said extension gap and said flexion gap are substantially rectangular after said femoral cut has been done.

6. A method as claimed in claim 1 , wherein said taking into account a distal thickness and posterior thickness comprises selecting a femoral implant having a distal thickness and a posterior thickness such that a desired distal cut thickness and posterior cut thickness may be obtained.

7. A method as claimed in claim 1 , wherein said performing a tibial cut comprises registering said tibial cut to said reference coordinate system.

8. A method as claimed in claim 7 , wherein said performing soft tissue balancing comprises spacing said femur and said tibia while in extension.

9. A method as claimed in claim 8 , wherein said spacing comprises placing a tensor device between said femur and said tibia and tensing ligaments connecting said femur and said tibia.

10. A method as claimed in claim 7 , wherein said desired limb alignment is based on a computed limb alignment.

11. A method as claimed in claim 7 , wherein said measuring a flexion gap and measuring an extension gap comprises using said tracking devices to obtain said measurements.

12. A method as claimed in claim 11 , wherein said measuring a flexion gap comprises determining a most posterior point of a reference of said femur on said reference coordinate system and measuring a distance between said tibial cut and said most posterior point of said femur.

13. A method as claimed in claim 12 , wherein said registering said cut comprises creating a plane along said tibial cut on said reference coordinate system and said determining a most posterior point of a reference comprises raising said plane on said tibial cut until said most posterior point of said femur is reached.