Motion tracking system with inertial-based sensing units

A joint monitoring apparatus comprising: a first inertial monitoring unit (48A) configured to engage and detect movement of a first portion of a patient's joint using a first inertial monitoring sensor (36) comprising an accelerometer (56) , a gyroscope (58) and a magnetometer (60), and to collect and transmit data indicative of the detected movement; a first ultrasound module (52) located in a fixed position with respect to the first inertial monitoring unit (48A), wherein the first ultrasound module is configured to transmit a first ultrasound pulse to the first portion of the patient and receive an echo of the first ultrasound pulse; a second inertial monitoring unit (48B) configured to connect to and detect motion of a second portion of the patient's joint using a second inertial monitoring sensor (36) comprising an accelerometer (56), a gyroscope (58), and a magnetometer (60), and for collecting and transmitting data indicative of detected motion, wherein the first patient portion is coupled to the second patient portion via the articulation; a second ultrasound module (52) located in a fixed position with respect to the second inertial monitoring unit (48B), wherein the second ultrasound module is configured to transmit a second ultrasound pulse to the second portion of the patient and to receiving an echo of the second ultrasound pulse; a processor (448); and a memory that includes program code that, when executed by the processor (448), causes the processor (448) to: determine a first distance between a bone in the first portion of the patient and the first ultrasound module (52 ) based on the echo of the first ultrasound pulse; and determining a second distance (52) between a bone in the second portion of the patient and the second ultrasound module (52) based on the echo of the second ultrasound pulse; process accelerometer data from the first inertial monitoring sensor (36) including a calibration calculation that compensates for scale and bias to calculate tilt angles of the first inertial monitoring unit (48A) in quaternion; process data from the magnetometer (60) of the first inertial monitoring sensor (36) including calibrating the scale and bias by setting bias and scale factors resulting in a calculated pitch of the first inertial monitoring unit (48A) in quaternion having a linear response; process data from the accelerometer (56) of the second inertial monitoring sensor (36) including a calibration calculation that compensates for scale and bias to calculate tilt angles of the second inertial monitoring unit (48B) in quaternion; process data from the magnetometer (60) of the second inertial monitoring sensor (36) including calibrating the scale and bias by setting the bias and scale factors that result in a calculated pitch of the second inertial monitoring unit (48B) at quaternion that has a linear response; calculate the azimuth of the first and second inertial monitoring units (48A, 48B) using the calculated quaternion pitch and quaternion pitch angles; prepare a global reference model using quaternion pitch angles and quaternion pitches; process the angular velocity data of the gyroscope (58) of the first inertial monitoring unit (48A) including a calibration calculation that compensates for scale and bias to calculate the relative change in orientation of the first inertial monitoring unit (48A ) in quaternion; process angular velocity data from the gyroscope (58) of the second inertial monitoring unit (48B) including a calibration calculation that compensates for scale and bias to calculate the relative change in orientation of the second inertial monitoring unit (48B ) in quaternion; prepare a local reference model using the quaternion orientations; and use the global reference model and the local reference model to determine an orientation of the patient's joint.