WO2009125188A1 - Capteur - Google Patents

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Publication number
WO2009125188A1
WO2009125188A1 PCT/GB2009/000931 GB2009000931W WO2009125188A1 WO 2009125188 A1 WO2009125188 A1 WO 2009125188A1 GB 2009000931 W GB2009000931 W GB 2009000931W WO 2009125188 A1 WO2009125188 A1 WO 2009125188A1
Authority
WO
WIPO (PCT)
Prior art keywords
sensor
movement
equilibrium
wheatstone bridge
components
Prior art date
Application number
PCT/GB2009/000931
Other languages
English (en)
Inventor
John Tudor Taylor
Shiying Hao
Anthony William Miles
Original Assignee
University Of Bath
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by University Of Bath filed Critical University Of Bath
Priority to GB1017922A priority Critical patent/GB2471799A/en
Publication of WO2009125188A1 publication Critical patent/WO2009125188A1/fr

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Classifications

    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/07—Endoradiosondes
    • A61B5/076—Permanent implantations
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/0002—Remote monitoring of patients using telemetry, e.g. transmission of vital signals via a communication network
    • A61B5/0031—Implanted circuitry
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/07—Endoradiosondes
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/103—Detecting, measuring or recording devices for testing the shape, pattern, colour, size or movement of the body or parts thereof, for diagnostic purposes
    • A61B5/11—Measuring movement of the entire body or parts thereof, e.g. head or hand tremor, mobility of a limb
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/45—For evaluating or diagnosing the musculoskeletal system or teeth
    • A61B5/4504—Bones
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/45—For evaluating or diagnosing the musculoskeletal system or teeth
    • A61B5/4528—Joints
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
    • A61B5/6846—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be brought in contact with an internal body part, i.e. invasive
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D18/00—Testing or calibrating apparatus or arrangements provided for in groups G01D1/00 - G01D15/00
    • G01D18/008—Testing or calibrating apparatus or arrangements provided for in groups G01D1/00 - G01D15/00 with calibration coefficients stored in memory
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/12—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means
    • G01D5/14—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage
    • G01D5/20—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying inductance, e.g. by a movable armature
    • G01D5/22—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable using electric or magnetic means influencing the magnitude of a current or voltage by varying inductance, e.g. by a movable armature differentially influencing two coils
    • G01D5/2291—Linear or rotary variable differential transformers (LVDTs/RVDTs) having a single primary coil and two secondary coils
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B2560/00—Constructional details of operational features of apparatus; Accessories for medical measuring apparatus
    • A61B2560/02—Operational features
    • A61B2560/0204—Operational features of power management
    • A61B2560/0214—Operational features of power management of power generation or supply
    • A61B2560/0219—Operational features of power management of power generation or supply of externally powered implanted units
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B5/00—Measuring for diagnostic purposes; Identification of persons
    • A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
    • A61B5/6846—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be brought in contact with an internal body part, i.e. invasive
    • A61B5/6867—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be brought in contact with an internal body part, i.e. invasive specially adapted to be attached or implanted in a specific body part
    • A61B5/6878—Bone

Definitions

  • the invention relates to a sensor for example for in vivo motion detection of orthopaedic implants.
  • Micromotion is the recoverable relative movement between an implant and bone associated with the implant.
  • the amplitude of micromotion is associated with the elasticity of the construct. It is known that micromotion amplitude of less than 30 ⁇ m is required for good bone ingrowth (osseointegration) and fixation. Consequently in, for example, uncemented hip and knee prostheses where micromotions in excess of 150 ⁇ m may result in fibrous tissue formation associated with loosening of the implant, it is advantageous to monitor this motion.
  • Migration is the non-recoverable movement of the implant as it slowly embeds itself into the host bone tissue as a consequence of loading and has a larger amplitude than micromotion, typically of the order of mm.
  • Total hip and knee arthroplasty is the second most common elective surgical procedure in the UK today with nearly 50,000 hips and a similar number of knees replaced through the NHS every year. Although these are highly successful procedures, implants do fail and their replacement (revision) is an expensive and burdensome procedure. Since it is well known that migration and micromotion are significant indicators of the likely longevity of total joint replacements, a system to enable their measurement in vivo will be a very powerful tool to aid in the early diagnosis of failing joint replacements and in guiding the design of future, optimized implants.
  • US 7,256,695 discloses a microsensor using an inductively powered Linear Variable Differential Transducer (LVDT).
  • LVDT Linear Variable Differential Transducer
  • imaging techniques such as Roentgen stereophotogrammetric analysis (RSA) are available for monitoring migration.
  • RSA Roentgen stereophotogrammetric analysis
  • a movement sensor comprising first and second movement components movable relative to each other about an equilibrium position and a sensor component arranged to sense the relative positions of the first and second movement components, the sensor further comprising a processor arranged to identify an equilibrium state of said movement components and set the corresponding position of the movement components as the equilibrium position.
  • Figure Ia shows a schematic block diagram of an arrangement according to the present invention
  • Figure Ib shows a schematic diagram of a Wheatstone bridge with signal processing unit
  • Figure Ic shows a schematic diagram of a LVDT suitable for use with a prosthesis
  • Figure 2 shows a hip prosthesis showing the position of the sensor and non-contact linkage with the pin
  • Figure 3 shows a knee prosthesis showing the position of the sensor and non-contact linkage with the pin
  • Figure 4 shows a strain gauge sensor for monitoring load transfer and fracture healing
  • Figure 5 shows a block diagram of a sensor and calibration circuitry.
  • the arrangement provides an apparatus suitable for, and a method, measuring and monitoring micromotion and migration of an object, such as an implant.
  • the arrangement generally comprises a movement sensor designated generally 100 including movement components 102, 104 forming, for example, an LVDT in which component 102 comprises a rod or core moveable relative to a coil 104.
  • the respective components are mounted to the implant 106 and the bone 109, such that movement of the implant relative to the bone is sensed by the sensor 100.
  • the sensed movement is sensed by circuitry 108 and a processor and controller 110 processes the sensed values and control parameters of the sensor circuitry 108 as described in more detail below. Telemetry of the results, additional control information and inductive powering is provided by additional circuitry 112 allowing communication with an external device such as a PC 114 connected to an external telemetry/powering device 116.
  • the processor 110 is arranged to identify an equilibrium state, for example when there has been no relative movement or only relative movement below a threshold value for a predetermined period of time.
  • the corresponding relative position of the sensor components is defined as the equilibrium position and the sensor component circuitry 108 is recalibrated accordingly.
  • the sensor remains within its optimum operating range, for example a linear region of operation.
  • the shift in the equilibrium position can be logged and provided to a user as an indication of migration of the system over time.
  • components 104, 108, 110, 112 can be provided in a single integrated assembly as appropriate.
  • the measurement of micromotion can be viewed as a small-signal displacement either side of a static equilibrium point. Furthermore, since the effective position of the equilibrium point is set in the calibration process, described below, and its parameters stored in internal memory, any changes in the position of the operating point resulting from gross movement i.e. migration between readings will be readily disceraable. Hence, in the case of a sensor device used with a prosthesis, both key clinical parameters can be measured separately.
  • an LVDT (essentially a Wheatstone bridge) consisting of a pair of coaxial cylindrical coils, Ll and L2, connected together (or a single cylindrical coil with a centre-tap) is used as the sensor component 104.
  • the coils are connected to two pairs of electrically- variable linear resistors, Rl and R2, R3 and R4, to form a modified type of Wheatstone bridge which is driven by a voltage controlled i.e. variable frequency sinusoidal oscillator (VCO) Vi n 12.
  • VCO variable frequency sinusoidal oscillator
  • the output of the circuit is connected to a control unit 10 that consists of amplifiers, signal processing sub-units, calibration and control. Four wired connections are needed to the electronics — two from the outer ends and one common from the centre. A fourth connection (not shown) is common ground.
  • the coils Ll and L2 104 are mounted in the bone cavity and engage with but are not in contact with a short ferrite rod forming the complementary sensor component 102 which can be mounted on the end of a pin (not shown) in the implant 106.
  • the nominal position of the ferrite rod is at the mid-point of the coils, but this will vary according to the accuracy of the surgical assembly procedure and also component tolerances.
  • Figure Ic shows a schematic diagram of the LVDT arrangement for use in a prosthesis, such as a hip prosthesis.
  • the two coils Ll and L2 are approximately 14mm long and have a diameter of approximately 7mm.
  • Ll and L2 are co-axial and are separated by a gap of approximately 4mm.
  • the coils, Ll and L2 are inserted in the bone cavity together with other devices associated with the sensor.
  • the ferrite rod is approximately 8mm long and is located at the mid point between the two coils as equilibrium position as shown. In practice there is a slight off-set of the ferrite rod.
  • the ferrite core comprises rod 202 connected to the implant (not shown) by a rod 201. It will be understood that the dimensions will vary according to the intended application of the sensor.
  • An outer protective sheath (not shown) can be included - adding around lmm to the diameter and length. This should be sufficient to also include any inductive loop (not shown) that may be required for the application to provide power.
  • a small static offset (due e.g. to component tolerance and/or imprecision in the surgical assembly) in the initial position of the rod if uncompensated will upset this equilibrium situation and both the sensitivity and the linear relationship between displacement and output voltage are rapidly lost.
  • a static offset of less than about 1 mm will render the device useless as a micromotion sensor for use with a hip or knee prostheses.
  • the useful range of static offset of the sensor will depend on the final application and the scale of the sensor in use.
  • a self-calibrating arrangement and method have been devised in which independent adjustment of the oscillator frequency and the values of the four variable resistors restore the equilibrium situation, even in the presence of a significant static offset.
  • the self-calibrating arrangement allows the equilibrium point to effectively move away from the geometric centre of the coils by means of an electrical calibration procedure.
  • Figure 5 shows a block diagram of the sensor 100, 108 and control unit 10 that includes calibration circuitry.
  • the detail of the controller 110 for controlling the sensor and calibration circuitry is not shown but it can comprise standard control circuitry as will be well known to the skilled person.
  • the interface between the calibration circuitry 10 and the controller 110 can take any appropriate form - for example, counters followed by a digital-to-analogue converter can be used to control all input signals and provide suitable circuitry so that any signal requiring measurements are of sufficient signal to noise level to be useful.
  • Sensor block 108 shows coils Ll and L2 which make up the sensor 104. As described according to figure Ib these are connected to resistors Rl to R4. Since R2 and R4 are fixed resistors only 3 connections are needed to the control data connections of processor 10 although 5 are show in the diagram. IRl 201, IR2 202, IR3 203, IR4 204 and IVOC 205 are used to control the resistors Rl to R4 and the VOC 12 respectively. The two outputs from the Wheatstone bridge are connected to amplifiers 206 and 207. The output of amplifier 206 is connected to the peak detector 208 and the phase detector 209. Prior to the peak detector 208 the signal is processed by a rectifier/smoothing circuitry (not shown).
  • Peak detector 208 provides output 210 which can be used in the post- calibration process to determine migration and micromotion.
  • the output of amplifier 206 is also used together with the output of amplifier 207 in the phase detector 207 which provides OPhase output 211. Again, this is used in the post- calibration process to determine the migration and micromotion of the implant 106.
  • the calibration process consists of a 3 -step algorithm requiring no knowledge of the component values at any stage.
  • the oscillator frequency is adjusted to maximise the output voltage which will be non-zero at this point to accumulated imbalances.
  • Rl and R2 are adjusted to set the phase angle between the input (oscillator) and the output to be +/-90°.
  • R3 and R4 are adjusted to force the residual output to be zero.
  • the values of the resistor pairs Rl, R2 and R3, R4 are linked, therefore it is only be necessary to adjust one voltage to control one pair of resistances.
  • the second and third steps of this process may be repeated until the values are within a desired tolerance. This iteration process may be automated using suitable control devices/routines.
  • the calibration process consists of a three step algorithm requiring no knowledge of the actual component values at any point in the process.
  • the resulting parameters are used for example with a look up table to determine the displacement (migration) and to calibrate the slope for displacement (i.e., to interpret micromotion).
  • the starting point for the algorithm is to set all components to their initial values.
  • R2 and R4 are set to values fairly close to their final values (400 ⁇ and 100 ⁇ respectively) while the other pair of resistors, Rl and R3, are set to as low a value as possible (about 10 ⁇ ).
  • the calibration process consists of a quasi-differential search which adjusts Rl and R3 only. The advantage of this approach is that only two components need to be adjusted during the calibration process and since Rl and R3 are always reset to their minimal values at the start of a cycle, the search is monotonic and uni-directional.
  • R2 is set to about 400 ⁇ and R4 to 100 ⁇ .
  • Rl and R3 are set to as small a value as practicable (10 ⁇ ). These values are suitable for the case where Ll ⁇ L2 ⁇ 800 ⁇ H, i.e., when the rod is near the geometric centre of the coils.
  • the oscillator frequency is swept until the amplitude of the observed output is maximum, beginning at the minimum frequency (50 kHz). This is the resonant frequency of the system. This is achieved by control of VCO 12 via IVCO 205 and measuring value of OPeak 210. The starting point is always the lowest frequency (the previous resonant frequency will have been stored for migration measurement). The loop counter is set to zero.
  • Rl is adjusted until the phase angle between VCO output and the Wheatstone bridge is +/-90° by value from OPhase 211.
  • Rl is adjusted up from the smallest possible value, for example 10 ⁇ .
  • the residual output is minimised by adjusting R3 from its minimum value.
  • R3 is controlled via IR3 203 and the condition detected using OPeak 210.
  • a bridge consisting of a pair of coils of dimension suitable for implantation in a typical hip prosthesis generates an output linearly related to displacement of about 10 ⁇ V/ ⁇ m when driven with an input sinusoid of amplitude 100 mV and calibrated in the manner described hereinabove. Further it has been shown that the gradient near the equilibrium point is independent of the calibration process, for gross displacements of up to about ⁇ lcm.
  • the resistance values required are typically quite small (order of 10-1000 ⁇ ) with quite small changes ( ⁇ 10%) and so the resistors can be realised as MOSFETs biased in the linear region, allowing convenient voltage control.
  • analogue components and/or subsystems required to implement the algorithm, and therefore allow the sensor to be self-calibrating are: peak detectors, phase-locked loops, and a voltage controlled oscillator - with a frequency range of about 50 kHz - 300 kHz and an output voltage of about 100 mV (pk).
  • peak detectors phase-locked loops
  • These components and/or subsystems of the sensor device can be implemented using a standard CMOS integrated circuit process.
  • Overall control may be managed by a microcontroller which may be integrated along with the analogue and/or digital parts of the system.
  • a small amount of memory will be required for the processor and controller 110 to store the calibration data in the implant between cycles of use, the micromotion data need not be stored but can be transmitted to an external unit by means of telemetry.
  • the telemetry system can be based on a bi-directional transcutaneous link consisting of an external coil (not shown) which in the case of a hip or knee implant is strapped to the patient's leg close to the site of the implant. This interacts inductively with the LVDT coils which will double as the implanted part of the link.
  • an external coil not shown
  • LVDT coils which will double as the implanted part of the link.
  • the external coil which is fitted to the patient when micromotion measurements are to be made, interfaces with a PC or other suitable processor, which is used to control the data logging process.
  • the link transmits commands to the implant and also supplies the necessary power.
  • the power consumption of the system is likely to be on the order of 1 mW during data download and zero at other times. As a result, it is quite feasible to supply all power requirements via the link, no implanted batteries being required.
  • the invention has applications in any situation where an LVDT or Wheatsone Bridge circuit is used to monitor small displacements, in particular where baseline changes (e.g. migration) and fluctuations (e.g. micromotion) occur during the monitoring period. It is particularly useful where access to the sensor is limited or impossible once it is in position.
  • the primary objective is the design of a device which can be assembled quickly and easily as part of a surgical procedure that can measure the parameters discussed hereinabove and output them to an external computer by means of telemetry. The device therefore must be small enough to fit inside the bone cavity adjacent to the implant and require no post-operative adjustment or re-alignment.
  • the invention can be applied, for example, to a hip motion microsensor device that can be used in conjunction with the uncemented hip replacement prostheses currently being employed by surgeons performing these operations.
  • the procedure would involve implanting the microsensor device at the tip of the femoral stem during surgery and the placement of the device in the case of a hip prosthesis is shown in Figure 2.
  • a pin 30 is inserted into the bone 20 without cement.
  • the LVDT 22 sensor is located below the pin 30 in the bone cavity 28.
  • the LVDT 22 is mounted independently of the pin in order that it may detect motion of a rod 26 mounted on the pin.
  • the LVDT 22 is located next to other electronics 24 associated with the sensor, allowing detection and monitoring of both micromotion and migration.
  • FIG. 3 shows a suitable arrangement in the case of a knee prosthesis.
  • the prosthesis 32 extends into the bone 34, connecting to the upper bone 40.
  • the rod 38, LVDT and associated electronics 36 are located below the prosthesis, but are not in direct contact with the prosthesis.
  • the particular values required of the device will depend on the application in use. For use with a knee or hip prosthesis suitable design requirements have been described hereinabove. Further, the sensor may be usefully designed to detect displacement about the equilibrium point to an accuracy of 5 ⁇ m, which is adequate to record micromotion. As described above, in order to eliminate the effects of static offsets, the self-calibration algorithm may be carried out.
  • any appropriate sensor may be used, for example a strain sensor.
  • FIG. 4 A device used in this way is shown in figure 4.
  • the intramedullary nail 44 is inserted in the bone 42 and secured with screws 46.
  • a strain sensor 48 is attached to the intramedullary nail 44.
  • the sensor 48 may, for example consist of 3 sensors 50 placed at 45° intervals so that the direction of strain may be determined. Indeed it will be seen that by providing sensors detecting strain in two or more directions additional information can be extrapolated such as axial, flexural and torsional loads.
  • the senor may be used in other applications where micromotion and migration occur, for example aerospace, industrial, civil infra-structure and other medical applications.

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  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • General Health & Medical Sciences (AREA)
  • Molecular Biology (AREA)
  • Veterinary Medicine (AREA)
  • Public Health (AREA)
  • Animal Behavior & Ethology (AREA)
  • Surgery (AREA)
  • Biophysics (AREA)
  • Pathology (AREA)
  • Biomedical Technology (AREA)
  • Heart & Thoracic Surgery (AREA)
  • Medical Informatics (AREA)
  • General Physics & Mathematics (AREA)
  • Dentistry (AREA)
  • Oral & Maxillofacial Surgery (AREA)
  • Power Engineering (AREA)
  • Physiology (AREA)
  • Rheumatology (AREA)
  • Orthopedic Medicine & Surgery (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Prostheses (AREA)
  • Length Measuring Devices With Unspecified Measuring Means (AREA)

Abstract

La présente invention concerne un capteur de mouvements qui comprend des premier et deuxième composants de mouvement amovibles l’un par rapport à l’autre autour d’une position d’équilibre et un composant de capteur conçu pour détecter les positions relatives des premier et deuxième composants de mouvement. Le capteur comprend en outre un processeur conçu pour identifier un état d’équilibre desdits composants de mouvement et définir la position correspondante du composant de mouvement comme la position d’équilibre.
PCT/GB2009/000931 2008-04-11 2009-04-09 Capteur WO2009125188A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
GB1017922A GB2471799A (en) 2008-04-11 2009-04-09 A sensor

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB0806632A GB0806632D0 (en) 2008-04-11 2008-04-11 A sensor
GB0806632.6 2008-04-11

Publications (1)

Publication Number Publication Date
WO2009125188A1 true WO2009125188A1 (fr) 2009-10-15

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Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2293502A (en) * 1939-04-13 1942-08-18 Gen Electric Electric measuring apparatus
FR2243413A1 (en) * 1973-09-12 1975-04-04 Commissariat Energie Atomique Electrical indicator of long linear movements - prism shaped magnetic core inside twin A.C. coils in wheatstone bridge
US5801645A (en) * 1996-07-18 1998-09-01 Allen-Bradley Company, Inc. Automatic paired LVDT probe balancing
US20020024450A1 (en) * 1999-12-06 2002-02-28 Townsend Christopher P. Data collection and storage device
US20020147416A1 (en) * 2000-05-01 2002-10-10 Southwest Research Institute Passive and wireless displacement measuring device
US20040113790A1 (en) * 2002-09-23 2004-06-17 Hamel Michael John Remotely powered and remotely interrogated wireless digital sensor telemetry system
US20060247773A1 (en) * 2005-04-29 2006-11-02 Sdgi Holdings, Inc. Instrumented implant for diagnostics
US20070179739A1 (en) * 2006-02-01 2007-08-02 Sdgi Holdings, Inc. Implantable pedometer

Patent Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2293502A (en) * 1939-04-13 1942-08-18 Gen Electric Electric measuring apparatus
FR2243413A1 (en) * 1973-09-12 1975-04-04 Commissariat Energie Atomique Electrical indicator of long linear movements - prism shaped magnetic core inside twin A.C. coils in wheatstone bridge
US5801645A (en) * 1996-07-18 1998-09-01 Allen-Bradley Company, Inc. Automatic paired LVDT probe balancing
US20020024450A1 (en) * 1999-12-06 2002-02-28 Townsend Christopher P. Data collection and storage device
US20020147416A1 (en) * 2000-05-01 2002-10-10 Southwest Research Institute Passive and wireless displacement measuring device
US20040113790A1 (en) * 2002-09-23 2004-06-17 Hamel Michael John Remotely powered and remotely interrogated wireless digital sensor telemetry system
US20060247773A1 (en) * 2005-04-29 2006-11-02 Sdgi Holdings, Inc. Instrumented implant for diagnostics
US20070179739A1 (en) * 2006-02-01 2007-08-02 Sdgi Holdings, Inc. Implantable pedometer

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
GILBERT J L ET AL: "A computer-based biomechanical analysis of the three-dimensional motion of cementless hip prostheses", JOURNAL OF BIOMECHANICS, PERGAMON PRESS, NEW YORK, NY, US, vol. 25, no. 4, 1 April 1992 (1992-04-01), pages 329 - 333,335, XP022873592, ISSN: 0021-9290, [retrieved on 19920401] *
SHIYING HAO ET AL: "An implantable system for the in vivo measurement of Hip and Knee migration and micromotion", SIGNALS AND ELECTRONIC SYSTEMS, 2008. ICSES '08. INTERNATIONAL CONFERENCE ON, IEEE, PISCATAWAY, NJ, USA, 14 September 2008 (2008-09-14), pages 445 - 448, XP031361804, ISBN: 978-83-88309-47-2 *

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GB0806632D0 (en) 2008-05-14
GB201017922D0 (en) 2010-12-01
GB2471799A (en) 2011-01-12

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