Measuring skeletal distraction

1. A system, comprising:

a bone-lengthening device;

a first transponder and a second transponder, the transponders being coupled to the bone-lengthening device and spaced apart from each other along a length of the bone-lengthening device, the transponders each being operable to transmit a radio-frequency signal in response to an interrogation signal; and

a movable handheld reader comprising

a radio-frequency transceiver operable to transmit a wireless interrogation signal and detect responsive signals from the first transponder and the second transponder,

one or more processing devices, and

one or more storage devices storing instructions that are operable, when executed by the one or more processing devices, to cause the movable handheld reader to perform operations including:

transmitting the wireless interrogation signal from the movable handheld reader at each of a plurality of positions with respect to the bone-lengthening device,

detecting response signals from the first transponder and the second transponder produced in response to the wireless interrogation signal transmitted from the movable handheld reader, comprising detecting, for each transmission of the wireless interrogation signal from the plurality of positions, (i) a corresponding first response signal from the first transponder, and (ii) a corresponding second response signal from the second transponder,

determining delays between the transmission of the wireless interrogation signal from the movable handheld reader and detection of the response signals from the first transponder and the second transponder by the movable handheld reader, and

determining a distance spanned by the transponders based on the delays comprising:

determining, for each transmission of the wireless interrogation signal, a first delay between the transmission of the wireless interrogation signal and detection of the corresponding first response signal from the first transponder;

identifying a shortest delay of the first delays;

determining a related delay between the transmission of the wireless interrogation signal that resulted in the shortest delay and detection of the corresponding second response signal; and

determining the distance spanned by the transponders based on the shortest delay and the related delay.

2. The system of claim 1 , wherein the operations further comprise:

accessing a stored distance,

determining a difference between the determined distance and the stored distance, and

indicating the difference on a user interface provided on a display of the movable handheld reader.

3. The system of claim 1 , wherein the bone-lengthening device is a telescoping intramedullary nail.

4. The system of claim 1 , wherein the first transponder is a passive RFID tag and the second transponder is a passive RFID tag.

5. The system of claim 1 , wherein the bone-lengthening device has a longitudinal axis, and the first transponder and the second transponder are attached to the bone-lengthening device at different locations spaced apart along the longitudinal axis of the bone-lengthening device.

6. The system of claim 1 , wherein the movable handheld reader is a cellular phone.

7. The system of claim 1 , wherein the operations further comprise storing the distance in association with a time or date that measurement of the distance occurred.

8. The system of claim 1 , wherein the operations further comprise:

determining that the distance does not satisfy a threshold; and

in response to determining that the distance does not satisfy the threshold, providing an alarm.

9. The system of claim 1 , wherein the operations further comprise determining, using the distance spanned by the transponders, (i) a change in length of a bone based on the distance spanned by the transponders or (ii) an amount of distraction of bone fragments that has occurred during a particular period of time.

10. The system of claim 1 , wherein the bone-lengthening device has a first portion that is configured to move relative to a second portion of the bone-lengthening device, wherein the first transponder is fixed to the first portion of the bone lengthening device and the second transponder is fixed to the second portion of the bone lengthening device.

11. The system of claim 1 , wherein the bone-lengthening device is an implant configured for subcutaneous implantation and wherein the first transponder and the second transponder are configured for subcutaneous implantation, and

wherein the movable handheld reader is an extracutaneous device configured to communicate the first transponder and the second transponder while the first transponder and the second transponder are subcutaneously implanted.

12. The system of claim 1 , wherein the bone-lengthening device is an intramedullary nail, and the first transponder and the second transponder are arranged on the intramedullary nail for placement in an intramedullary cavity of a bone.

13. The system of claim 1 ,

wherein transmitting the wireless interrogation signal from the movable handheld reader comprises transmitting the wireless interrogation signal from the movable handheld reader at a plurality of locations along a path while the movable handheld reader is moving along the path relative to the bone lengthening device;

wherein detecting the response signals from the first transponder and the second transponder comprises detecting, by the movable handheld reader, response signals from the first transponder and the second transponder corresponding to each transmission of the wireless interrogation signal at the plurality of locations along the path; and

wherein determining a distance spanned by the transponders based on the delays comprises determining the distance spanned by the transponders using response signals corresponding to transmissions of the wireless interrogation signal at the plurality of locations along the path.

14. The system of claim 13 , wherein the path is a substantially linear path.

15. The system of claim 1 , wherein the movable handheld reader is configured to determine the distance spanned by the transponders based on the interactions of a single transceiver with the first transponder and the second transponder.

16. The system of claim 1 , wherein the movable handheld reader comprises a pressure sensor configured to sense pressure against a side of the movable handheld reader,

wherein the movable handheld reader is configured to measure pressures with the pressure sensor during placement and movement of the movable handheld reader while the movable handheld reader transmits the wireless interrogation signal, and

wherein the movable handheld reader is configured to assess variations in the pressures measured while the movable handheld reader moves and transmits the wireless interrogation signal.

17. The system of claim 16 , wherein the operations further comprise:

determining that a difference between the measured pressures exceeds a threshold; and

in response to determining that the difference between the measured pressures exceeds the threshold, outputting, by the movable handheld reader, an alarm or an indication that new data should be acquired using the radio-frequency transceiver.

18. The system of claim 1 , wherein the operations further comprise displaying a user interface on a display of the movable handheld reader that instructs a user to move the movable handheld reader along a path.

19. A system, comprising:

a bone-lengthening device;

a first transponder and a second transponder, the transponders being coupled to the bone-lengthening device and spaced apart from each other along a length of the bone-lengthening device, the transponders each being operable to transmit a radio-frequency signal in response to an interrogation signal; and

a reader comprising

a radio-frequency transceiver operable to transmit a wireless interrogation signal and detect responsive signals from the first transponder and the second transponder,

one or more processing devices, and

one or more storage devices storing instructions that are operable, when executed by the one or more processing devices, to cause the reader to perform operations including:

transmitting multiple transmissions of the wireless interrogation signal from the reader, each of the multiple transmissions of the wireless interrogation signal being transmitted with the reader located at a different location relative to the bone lengthening device,

detecting, for each of the multiple transmissions of the wireless interrogation signal, a corresponding first response signal from the first transponder and a corresponding second response signal from the second transponder,

determining, for each of the multiple transmissions of the wireless interrogation signal, a first delay between the transmission of the wireless interrogation signal and detection of the corresponding first response signal from the first transponder,

identifying a shortest delay of the first delays,

determining a related delay that is a delay between the transmission of the wireless interrogation signal that resulted in the shortest delay and detection of the corresponding second response signal, and

determining a distance spanned by the transponders based on the shortest delay and the related delay.