Implant Evaluation Using Acoustic Emissions

1 . A method of identifying a loosened implant in a joint, the method comprising the steps of:

positioning a plurality of acoustic sensors at a respective plurality of locations around the joint,

causing the joint to be moved,

receiving signals from the acoustic sensors during the movement of the joint,

identifying signals from two or more of the plurality of acoustic sensors that correspond to a common acoustic event,

identifying a position of the acoustic event within the joint, and

providing a health indication related to the joint.

2 . The method of claim 1 , wherein the step of identifying a position comprises:

calculating a first time delay between a first time of receipt of a first signal from a first acoustic sensor of the plurality of acoustic sensors and a second time of receipt of a second signal from a second acoustic sensor of the plurality of acoustic sensors, and

calculating a first geometric surface of possible locations of the acoustic event from the first time delay.

3 . The method of claim 2 , wherein the step of identifying a position further comprises:

calculating a second time delay between the first time of receipt and a third time of receipt of a third signal from a third acoustic sensor of the plurality of acoustic sensors,

calculating a second geometric surface of possible locations of the acoustic event from the second time delay, and

determining a line of intersection of the first and second geometric surfaces.

4 . The method of claim 2 , wherein the step of identifying a position further comprises determining where the first geometric surface intersects the implant.

5 . The method of claim 2 , wherein the step of calculating a first time delay comprises:

identifying one or more types of tissue between the implant and the first acoustic sensor,

identifying one or more signal paths from the implant to the first acoustic sensor, and

adjusting the first time delay according to predetermined speeds of signal propagation within each of the one or more types of tissue and the signal paths.

6 . The method of claim 1 , wherein the step of identifying a position comprises:

calculating a first magnitude difference between a first signal amplitude of a first signal from a first acoustic sensor of the plurality of acoustic sensors and a second signal magnitude of a second signal from a second acoustic sensor of the plurality of acoustic sensors, and

calculating a first geometric surface of possible locations of the acoustic event from the first amplitude difference.

7 . The method of claim 6 , further comprising:

adjusting the first time delay according to predetermined speeds of signal propagation within each of the one or more types of tissue and the signal paths.

8 . A method of identifying a loosened implant in a joint, the method comprising the steps of:

positioning an acoustic sensor at a location proximate to the joint,

causing the joint to be moved,

receiving a signal from the acoustic sensor during the movement of the joint,

analyzing the signal to identify an attribute that is associated with a state of joint health, and

providing a health indication related to the joint.

9 . The method of claim 8 , wherein the step of analyzing the signal comprises:

calculating a rise time and a magnitude from the signal,

comparing the rise time to a first threshold and the magnitude to a second threshold, and

determining that the signal is indicative of a loose implant when the rise time exceeds the first threshold and the magnitude exceeds the second threshold.

10 . The method of claim 8 , wherein the step of analyzing the signal is performed only when the signal comprises a primary frequency within a predetermined band.

11 . The method of claim 10 , wherein the predetermined band is associated with the implant.

12 . The method of claim 8 , wherein the step of analyzing the signal comprises:

calculating a power spectral density (PSD) of the signal,

calculating a first partial power of the PSD within a predetermined first frequency band,

comparing the first partial power to a first threshold, and

determining that the signal is indicative of a loose implant when the first partial power exceeds the first threshold.

13 . The method of claim 12 , wherein the predetermined band includes a resonant frequency associated with the implant.

14 . The method of claim 8 , wherein the step of analyzing the signal comprises:

calculating a power spectral density (PSD) of the signal,

calculating a first partial power of the PSD within a predetermined first frequency band,

calculating a second partial power of the PSD within a predetermined second frequency band, and

comparing the first partial power to the second partial power.

15 . The method of claim 14 , wherein the step of comparing comprises:

calculating a ratio of the first partial power to the second partial power, and

determining that the signal is indicative of a loose implant when the ratio exceeds a threshold.

16 . The method of claim 14 , wherein the step of comparing comprises:

calculating a difference between the first partial power and the second partial power, and

determining that the signal is indicative of a loose implant is loose when the difference exceeds a threshold.

17 . The method of claim 8 , wherein the step of analyzing the signal comprises:

calculating a power spectral density (PSD) of the signal,

calculating a first maximum value of the PSD within a predetermined first frequency band,

calculating a second maximum value of the PSD within a predetermined second frequency band, and

comparing the first maximum value to the second maximum value.

18 . The method of claim 8 , wherein the step of providing a health indication related to the joint comprises:

evaluating the total number of acoustic events indicative of a loose implant to determine a diagnostic indication of a loose implant.

19 . An apparatus for identifying a loosened implant in a joint, the apparatus comprising the steps of:

a plurality of acoustic sensors configured to be placed in contact with a patient's skin at a respective plurality of locations around the joint,

a processor configured to receive signals from the acoustic sensors during the movement of the joint, wherein the processor is configured to:

identify signals from two or more of the plurality of acoustic sensors that correspond to a common acoustic event,

compare an attribute of the signals,

identify a position of the acoustic event within the joint, and

provide a health indication related to the joint.