MEMS-based method and system for tracking a femoral frame of reference

1. A system for tracking a femoral frame of reference in computer-assisted surgery, comprising:

a sensor unit adapted to be secured to a distal end of a femur, the sensor unit comprising an accelerometer sensor and a gyroscope sensor both configured for producing orientation data;

a processing unit configured for receiving gyroscope data and accelerometer data and comprising:

a gyroscope-data calculator for providing calculated acceleration data using at least gyroscope data resulting from movements of the femur about a proximal end thereof;

an accelerometer-data calculator for calculating measured acceleration data using accelerometer data resulting from the movements of the femur; and

an acceleration comparator for relating an orientation of the sensor unit to the femur to define the femoral frame of reference, the acceleration comparator performing a numerical optimization to determine the orientation of the sensor unit, the numerical optimization using at least a plurality of estimated radius values between the proximal end and the distal end of the femur with the calculated acceleration data and the measured acceleration data to determine a radius value corresponding to the acceleration data and the measured acceleration data at the distal end of the femur;

a sensor orientation interface for providing orientation data for the femur from a tracking of the femoral frame of reference.

2. The system according to claim 1 , wherein the gyroscope-data calculator provides the calculated acceleration data using an estimated trajectory value and the estimated radius values for the femur.

3. The system according to claim 1 , wherein the gyroscope-data calculator provides the calculated acceleration data using the estimated radius values for the femur for deducting centripetal accelerations from the gyroscope data, and using an orientation of the gyroscope sensor for deducting gravity from the accelerometer data.

4. The system according to claim 1 , wherein the femoral frame of reference defined by the acceleration comparator is a three-axis femoral coordinate system.

5. The system according to claim 1 , wherein the acceleration comparator calculates at least one of a femur length and a femoral trajectory.

6. The system according to claim 1 , further comprising filters in any one of the sensor unit and the processing unit to reduce noise from any one of the gyroscope data and the accelerometer data.

7. A method for tracking with a tracking system a femoral frame of reference with a sensor unit comprising an accelerometer sensor and a gyroscope sensor secured to a femur, comprising:

obtaining with the tracking system gyroscope readings and accelerometer readings from the sensor unit and resulting from movements of the femur;

calculating acceleration data with the tracking system using at least the gyroscope readings;

measuring acceleration data with the tracking system using the accelerometer readings;

comparing with the tracking system the calculated and the measured acceleration data using at least a plurality of estimated radius values in a numerical optimization to determine a radius value corresponding to the calculated acceleration data and the measured acceleration data at a distal end of the femur, whereby an orientation of the sensor unit is related to the femur;

relating with the tracking system the orientation of the sensor unit to the femur using the comparison to define the femoral frame of reference; and

tracking with the tracking system the femoral frame of reference from the gyroscope readings and from the accelerometer readings.

8. The method according to claim 7 , wherein calculating acceleration data comprises using an estimated value for a trajectory of the femur and the estimated radius values of the femur.

9. The method according to claim 7 , wherein calculating acceleration data comprises using the estimated radius values for a radius of the femur for deducting a centripetal acceleration from the gyroscope readings, and obtaining an orientation of the gyroscope to deduct the gravity from the accelerometer readings.

10. The method according to claim 7 , further comprising calculating at least one of a radius of the femur and a trajectory of the femur from the frame of reference.

11. The method according to claim 7 , wherein defining the femoral frame of reference comprises defining a three-axis coordinate system.

12. The method according to claim 7 , wherein the method is performed on a bone model or cadaver.

13. The method according to claim 7 , wherein defining the femoral frame of reference comprises setting an axis of the femoral frame of reference on a mechanical axis of the femur.

14. The method according to claim 7 , further comprising displaying information pertaining to an orientation of a tool relative to the tracked femoral frame of reference.

15. The method according to claim 14 , wherein displaying information pertaining to an orientation of a tool relative to the tracked femoral frame of reference comprises displaying information indicative of a varus-valgus value of the tool.

16. The system according to claim 1 , wherein the numerical optimization is performed by using one of a gradient descent algorithm and a simulated annealing algorithm.