MEMS-based method and system for tracking a femoral frame of reference
A system for tracking a femoral frame of reference in computer-assisted surgery comprises a sensor unit. The sensor unit is adapted to be secured to the femur. The sensor unit comprises accelerometer and gyroscope sensors that produce orientation data. A processing unit receives gyroscope and accelerometer data. The processing unit comprises a gyroscope-data calculator to provide calculated acceleration data resulting from movements of the femur, an accelerometer-data calculator to calculate measured acceleration data resulting from movements of the femur, and an acceleration comparator to relate an orientation of the sensor unit to the femur to define a femoral frame of reference. The femoral frame of reference is defined from the comparison between the calculated and the measured acceleration data. A sensor orientation interface provides orientation data for the femur from a tracking of the femoral frame of reference. A method for tracking a femoral frame of reference is also provided.
1. A method for tracking a femoral frame of reference with a sensor unit comprising an accelerometer sensor and a gyroscope sensor configured to move concurrently with a femur, comprising:
obtaining, with at least one processor of a processing unit, gyroscope readings and accelerometer readings resulting from movements of the femur;
calculating, with the at least one processor of the processing unit, acceleration data at least from the gyroscope readings, the acceleration data representative of the movements of the femur;
measuring with the at least one processor of the processing unit, additional acceleration data from the accelerometer readings, the additional acceleration data representative of the movements of the femur;
relating with the at least one processor of the processing unit, an orientation of the sensor unit to the femur using a comparison between the calculated and the measured acceleration data to define a femoral frame of reference; and
tracking with the at least one processor of the processing unit, the femoral frame of reference from the gyroscope readings and from the accelerometer readings.
2. The method according to claim 1 , wherein calculating acceleration data includes calculating a tangential acceleration using estimate values for a trajectory of the femur and a radius of the femur.
3. The method according to claim 1 , wherein calculating acceleration data and using the comparison includes calculating a tangential acceleration using an estimate value for a radius of the femur for deducting a centripetal acceleration from the gyroscope readings, and obtaining an orientation of the gyroscope to deduct an effect of gravity from the accelerometer readings.
4. The method according to claim 1 , wherein relating an orientation comprises performing a numerical optimization using the comparison to obtain an orientation of the sensor unit with respect to the femur.
5. The method according to claim 1 , further comprising calculating at least one of a radius of the femur and a trajectory of the femur from the frame of reference.
6. The method according to claim 1 , wherein defining the femoral frame of reference comprises defining a three-axis coordinate system.
7. The method according to claim 1 , wherein the method is performed on a bone model or cadaver.
8. The method according to claim 1 , wherein obtaining gyroscope readings and accelerometer readings resulting from movements of the femur includes obtaining gyroscope readings and accelerometer readings from the sensor unit at a knee mechanical entry point.
9. The method according to claim 8 , wherein relating an orientation of the sensor unit to the femur using the comparison includes making a center of the femoral head a center of a sphere with the knee mechanical entry point being a radius of the sphere.
10. The method according to claim 1 , further including displaying the femoral frame of reference as axes representative of the femur relative to a coordinate system.
11. A system for tracking a femoral frame of reference in computer-assisted surgery, comprising a sensor unit adapted to be secured to the femur, the sensor unit comprising an accelerometer sensor and a gyroscope sensor both producing orientation data; and a processing unit receiving gyroscope data and accelerometer data and configured for executing the method according to claim 1 .
12. A method for tracking a femoral frame of reference with a sensor unit comprising an accelerometer sensor and a gyroscope sensor configured to move concurrently with a femur, comprising:
obtaining, with at least one processor of a processing unit, gyroscope readings and accelerometer readings resulting from movements of the femur;
calculating, with the at least one processor of the processing unit, acceleration data at least from the gyroscope readings, the acceleration data representative of the movements of the femur;
measuring with the at least one processor of the processing unit, additional acceleration data from the accelerometer readings, the additional acceleration data representative of the movements of the femur;
relating with the at least one processor of the processing unit, an orientation of the sensor unit to the femur using a comparison between the calculated and the measured acceleration data to define a femoral frame of reference; and
tracking with the at least one processor of the processing unit, the femoral frame of reference from the gyroscope readings and from the accelerometer readings;
wherein calculating the acceleration data includes calculating a tangential acceleration using estimate values for a trajectory of the femur and a radius of the femur.
13. A method for tracking a femoral frame of reference with a sensor unit comprising an accelerometer sensor and a gyroscope sensor configured to move concurrently with a femur, comprising:
obtaining, with at least one processor of a processing unit, gyroscope readings and accelerometer readings resulting from movements of the femur;
calculating, with the at least one processor of the processing unit, acceleration data at least from the gyroscope readings, the acceleration data representative of the movements of the femur;
measuring with the at least one processor of the processing unit, additional acceleration data from the accelerometer readings, the additional acceleration data representative of the movements of the femur;
relating with the at least one processor of the processing unit, an orientation of the sensor unit to the femur using a comparison between the calculated and the measured acceleration data to define a femoral frame of reference; and
tracking with the at least one processor of the processing unit, the femoral frame of reference from the gyroscope readings and from the accelerometer readings;
wherein calculating acceleration data and using the comparison includes calculating a tangential acceleration using an estimate value for a radius of the femur for deducting a centripetal acceleration from the gyroscope readings, and obtaining an orientation of the gyroscope to deduct an effect of gravity from the accelerometer readings.