Position sensing system for orthopedic applications

0001The present invention relates generally to intrabody tracking systems, and specifically to wireless devices for tracking the position and orientation of an object in the body.

0002Various methods and systems are known in the art for tracking the position of a medical probe or implant inside the body of a subject.

0003For example, <patcit id="pcit0001" dnum="US5391199A"><text>U.S. Patents 5,391,199</text></patcit> and <patcit id="pcit0002" dnum="US5443489A"><text>5,443,489</text></patcit> to Ben-Haim, describe systems wherein the coordinates of an intrabody probe are determined using one or more field sensors, such as a Hall effect device, coils, or other antennae carried on the probe. Such systems are used for generating three-dimensional location information regarding a medical probe or catheter. Preferably, a sensor coil is placed in the catheter and generates signals in response to externally-applied magnetic fields. The magnetic fields are generated by three radiator coils, fixed to an external reference frame in known, mutually-spaced locations. The amplitudes of the signals generated in response to each of the radiator coil fields are detected and used to compute the location of the sensor coil. Each radiator coil is preferably driven by driver circuitry to generate a field at a known frequency, distinct from that of other radiator coils, so that the signals generated by the sensor coil may be separated by frequency into components corresponding to the different radiator coils.

0004<patcit id="pcit0003" dnum="WO9605768A"><text>PCT Patent Publication WO 96/05768</text></patcit>, <patcit id="pcit0004" dnum="US6690963B"><text>U.S. Patent 6,690,963</text></patcit> and the corresponding <patcit id="pcit0005" dnum="US414875A" dnum-type="L"><text>U.S. Patent Application 09/414,875</text></patcit>, to Ben-Haim et al. (published as U.S. Patent Application Publication <patcit id="pcit0006" dnum="US20020065455A1"><text>US 2002/0065455 A1</text></patcit>), describe a system that generates six-dimensional position and orientation information regarding the tip of a catheter. This system uses a plurality of sensor coils adjacent to a locatable site in the catheter, for example near its distal end, and a plurality of radiator coils fixed in an external reference frame. These coils generate signals in response to magnetic fields generated by the radiator coils, which signals allow for the computation of six location and orientation coordinates.

0005<patcit id="pcit0007" dnum="US6239724B"><text>U.S. Patent 6,239,724</text></patcit> to Doron et al. describes a telemetry system for providing spatial positioning information from within a patient's body. The system includes an implantable telemetry unit having (a) a first transducer, for converting a power signal received from outside the body into electrical power for powering the telemetry unit; (b) a second transducer, for receiving a positioning field signal that is received from outside the body; and (c) a third transducer, for transmitting a locating signal to a site outside the body, in response to the positioning field signal.

0006The above-mentioned <patcit id="pcit0008" dnum="US029473A" dnum-type="L"><text>U.S. Patent Application 10/029,473</text></patcit> to Govari, published as <patcit id="pcit0009" dnum="US20030120150A" dnum-type="L"><text>U.S. Patent Application Publication 2003/0120150</text></patcit>, describes apparatus for tracking an object. The apparatus includes a plurality of field generators, which generate electromagnetic fields at different, respective frequencies in a vicinity of the object, and a radio frequency (RF) driver, which radiates a RF driving field toward the object. A wireless transponder is fixed to the object. The transponder includes at least one sensor coil, in which a signal current flows responsive to the electromagnetic fields, and a power coil, which receives the RF driving field and conveys electrical energy from the driving field to power the transponder. The power coil also transmits an output signal responsive to the signal current to a signal receiver, which processes the signal to determine coordinates of the object.

0007<patcit id="pcit0010" dnum="WO9729683A"><text>WO 97/29683</text></patcit> describes a tracking system including three reference field transducers mounted on a rigid frame which can be used to wirelessly track the position of a probe in the body. A controller receives and processes signals to determine the position of the probe. The rigid frame can be attached to an operating table so that the frame can be tilted relative to the operating table and slid along the operating table.

0008<patcit id="pcit0011" dnum="EP1321097A"><text>EP-1321097</text></patcit> describes a tracking system including a wireless position sensor which can be implanted in bone, a controller and a pad with three generator coils which can be attached to an underside of an operating table to generate a magnetic field.

0009<patcit id="pcit0012" dnum="EP1382308A"><text>EP-1382308</text></patcit> describes a system including a wireless sensor disposed in an orthopaedic appliance and a hand-held locator pad having four magnetic field generator coils to generate a magnetic field so that the position of the sensor can be detected.

0010The present invention provides a surgical tracking system, comprising: a wireless position sensor, which is adapted to be implanted in a bone of a subject, and responsively to externally-applied magnetic fields within a working volume of the tracking system, to generate and transmit sensor signals indicative of coordinates of the position sensor within the bone; a plurality of field generator coils, which are adapted to generate the magnetic fields so as to define the working volume, each field generator coil having a coil axis; a reference structure, to which the field generator coils are fixed in predetermined locations, and which is movable relative to the subject in order to position the working volume so as to intercept the bone; a system controller, which is coupled to receive and process the sensor signals so as to determine the coordinates of the position sensor within the bone; and an operating table having an opening, characterised in that the reference structure is configured to be inserted into the opening in the operating table to slide the reference structure in and out of the table in order to shift the position of the working volume along the table, and each of the plurality of field generator coils is tilted relative to the reference structure so that each coil axis is inclined relative to the reference structure and the coil axes are directed diagonally inward of the reference structure to lower the centroid of the working volume toward the operating table.

0011The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> is a schematic, pictorial illustration of a magnetic tracking system used in surgery;</li><li><figref idref="f0002">Figs. 2A and 2B</figref> are schematic, partly sectional illustrations, showing insertion of implantable position sensors into the bone of a patient;</li><li><figref idref="f0003">Figs. 3A</figref> and <figref idref="f0004">3B</figref> are schematic, pictorial illustrations showing details of wireless position sensors;</li><li><figref idref="f0005">Fig. 4</figref> is a schematic, pictorial illustration showing details of a two-part position sensor;</li><li><figref idref="f0006">Fig. 5</figref> is a schematic, pictorial illustration showing a surgical tool and a position sensor used to track coordinates of the tool;</li><li><figref idref="f0007">Fig. 6A</figref> is a schematic, pictorial illustration showing an operating table and a location pad that is inserted into the table, in accordance with an embodiment of the present invention;</li><li><figref idref="f0007">Fig. 6B</figref> is a schematic, pictorial illustration showing the location pad of <figref idref="f0007">Fig. 6A</figref> after insertion into the operating table, and showing the working volume of the location pad, in accordance with an embodiment of the present invention;</li><li><figref idref="f0008">Fig. 7</figref> is a schematic, pictorial illustration showing adjustment of the magnetic tracking system of <figref idref="f0001">Fig. 1</figref> for use in a knee operation;</li><li><figref idref="f0009">Fig. 8</figref> is a schematic, pictorial illustration of a magnetic tracking system for use in surgery;</li><li><figref idref="f0010">Fig. 9</figref> is a schematic, pictorial illustration of a magnetic tracking system for use in surgery;</li><li><figref idref="f0011">Figs. 10A</figref> and <figref idref="f0012">10B</figref> are schematic, pictorial illustrations of a magnetic tracking system for use in surgery; and</li><li><figref idref="f0013">Fig. 11</figref> is a schematic, pictorial illustration of a magnetic tracking system for use in surgery.</li></ul>

Detailed Description:

0012<figref idref="f0001">Fig. 1</figref> is a schematic, pictorial illustration of a magnetic tracking system 20 for use in surgery.

0013In the pictured example, a surgeon 22 is preparing to perform a procedure on a leg 24 of a patient 26. The surgeon uses a tool 28 to implant or place an implant, generally designated 30, (in this example a bone screw 30) in the bone of leg 24. Both the tool and the implant, i.e. the screw, contain miniature, wireless position sensors, which are described in detail hereinbelow. Each sensor generates and transmits signals that are indicative of its location and orientation coordinates, in response to an external magnetic field produced by a set of field generator coils 32 (also referred to as radiator coils). Typically, multiple screws 30 with position sensors are implanted by surgeon 22 at key locations in the patient's bone. Additionally or alternatively, similar position sensors may be fixed to other implants 30, such as a prosthetic joint or intramedullary insert or other implants 30 such as a nail, rod, pin, staple, bone or tissue anchor, or other orthopedic device, in order to permit the position of the implant 30 to be monitored, as well. For example, the use of such position sensors in a hip implant is shown in the above-mentioned <patcit id="pcit0013" dnum="US029473A" dnum-type="L"><text>U.S. Patent Application 10/029,473</text></patcit>.

0014Field generator coils 32 are driven by driver circuits 34 to generate electromagnetic fields at different, respective sets of frequencies {ω₁}, {ω₂} and {ω₃}. Typically, the sets comprise frequencies in the approximate range of 100 Hz - 30 kHz, although higher and lower frequencies may also be used. The sets of frequencies at which the coils radiate are set by a computer 36, which serves as the system controller for system 20. The respective sets of frequencies may all include the same frequencies, or they may include different frequencies. In any case, computer 36 controls circuits 34 according to a known multiplexing pattern, which provides that at any point in time, no more than one field generator coil is radiating at any given frequency. Typically, each driver circuit is controlled to scan cyclically over time through the frequencies in its respective set. Alternatively, each driver circuit may drive the respective coil 32 to radiate at multiple frequencies simultaneously.

0015For the purposes of system 20, coils 32 may be arranged in any convenient position and orientation, so long as they are fixed in respect to some reference frame, and so long as they are non-overlapping, that is, there are no two field generator coils with the exact, identical location and orientation. Typically, for surgical applications such as that shown in the figures, coils 32 comprise wound annular coils about 15-20 cm in outer diameter (O.D.) and about 1-2 cm thick, in a triangular arrangement, wherein the centers of the coils are about 80-100 cm apart. The coil axes may be parallel, as shown in this figure, or they may alternatively be inclined, as shown, for example, in <figref idref="f0007">Figs. 6A and 6B</figref>. Bar-shaped transmitters or even triangular or square-shaped coils could also be useful for such applications.

0016In orthopedic and other surgical applications, it is desirable that coils 32 be positioned away from the surgical field, so as not to interfere with the surgeon's freedom of movement. On the other hand, the coils should be positioned so that the working volume of the tracking system includes the entire area in which the surgeon is operating. At the same time, the locations and orientations of coils 32 should be known relative to a given reference frame in order to permit the coordinates of tool 28 and screw 30 to be determined in that reference frame.

0017In order to meet these potentially-conflicting requirements, coils 32 are mounted on a reference structure 40. In the example of <figref idref="f0001">Fig. 1</figref>, structure 40 comprises multiple arms 42, which are fixed to an articulated base 44. Alternative reference structures and configurations are shown in the figures that follow. Arms 42 hold coils 32 in known relative positions. Base 44, however, is capable of tilting, turning and changing the elevations of arms 42, so as to enable surgeon 22 to position coils 32 in convenient locations. The movement of base 44 may be controlled by computer 36, so that the computer is also aware of the actual locations of coils 32.

0018Alternatively or additionally, an image registration procedure may be used to calibrate the positions of coils 32 relative to patient 26. An exemplary registration procedure, based on X-ray imaging, is described in <patcit id="pcit0014" dnum="US6314310B"><text>U.S. Patent 6,314,310</text></patcit>, whose disclosure is incorporated herein by reference. Further alternatively or additionally, a reference sensor, fixed to patient 26 or to the operating table in a known location, may be used for calibration. The use of reference sensors for this purpose is described, for example, in the above-mentioned <patcit id="pcit0015" dnum="US5391199A"><text>U.S. Patent 5,391,199</text></patcit>.

0019The position sensors in screw 30 and tool 28 typically comprise sensor coils, in which electrical currents are induced to flow in response to the magnetic fields produced by field generator coils 32. An exemplary arrangement of the sensor coils is shown in <figref idref="f0003">Fig. 3A</figref> below. The sensor coils may be wound on either air cores or cores of magnetic material. Typically, each position sensor comprises three sensor coils, having mutually orthogonal axes, one of which is conveniently aligned with the longitudinal axis of tool 28 or of screw 30. The three coils may be concentrically wound on a single core, or alternatively, the coils may be non-concentrically wound on separate cores, and spaced along the longitudinal axis of the tool or screw. The use of non-concentric coils is described, for example, in the above-mentioned <patcit id="pcit0016" dnum="WO9605768A"><text>PCT Patent Publication WO 96/05768</text></patcit> and in the corresponding <patcit id="pcit0017" dnum="US414875A" dnum-type="L"><text>U.S. Patent Application 09/414,875</text></patcit>. Alternatively, the position sensors may each comprise only a single sensor coil or two sensor coils. Further alternatively, screw 30 and tool 28 may comprise magnetic position sensors based on sensing elements of other types known in the art, such as Hall effect sensors.

0020At any instant in time, the currents induced in the sensor coils comprise components at the specific frequencies in sets {ω₁}, {ω₂} and {ω₃} generated by field generator coils 32. The respective amplitudes of these currents (or alternatively, of time-varying voltages that may be measured across the sensor coils) are dependent on the location and orientation of the position sensor relative to the locations and orientations of the field generator coils. In response to the induced currents or voltages, signal processing and transmitter circuits in each position sensor generate and transmit signals that are indicative of the location and orientation of the sensor. These signals are received by a receiving antenna (shown, for example, in <figref idref="f0007">Fig. 6A</figref>), which is coupled to computer 36. The computer processes the received signals, together with a representation of the signals used to drive field generator coils 32, in order to calculate location and orientation coordinates of screw 30 and tool 28. The coordinates are used by the computer in driving a display 46, which shows the relative locations and orientations of the tool, screw and other elements (such as prosthetic implants) to which position sensors have been fixed.

0021Although in <figref idref="f0001">Fig. 1</figref>, system 20 is shown as comprising three field generator coils 32, in other examples, different numbers, types and configurations of field generators and sensors may used. A fixed frame of reference may be established, for example, using only two non-overlapping field generator coils to generate distinguishable magnetic fields. Two non-parallel sensor coils may be used to measure the magnetic field flux due to the field generator coils, in order to determine six location and orientation coordinates (X, Y, Z directions and pitch, yaw and roll orientations) of the sensor. Using three field generator coils and three sensor coils, however, tends to improve the accuracy and reliability of the position measurement.

0022Alternatively, if only a single sensor coil is used, computer 36 can still determine five position and orientation coordinates (X, Y, Z directions and pitch and yaw orientations). Specific features and functions of a single coil system (also referred to as a single axis system) are described in <patcit id="pcit0018" dnum="US6484118B"><text>U.S. Patent 6,484,118</text></patcit>.

0023When a metal or other magnetically-responsive article is brought into the vicinity of an object being tracked, such as screw 30 or tool 28, the magnetic fields in this vicinity are distorted. In the surgical environment shown in <figref idref="f0001">Fig. 1</figref>, for example, there can be a substantial amount of conductive and permeable material, including basic and ancillary equipment (operating tables, carts, movable lamps, etc.), as well as invasive surgery apparatus (scalpels, scissors, etc., including tool 28 itself). The magnetic fields produced by field generator coils 32 may generate eddy currents in such articles, and the eddy currents then cause a parasitic magnetic field to be radiated. Such parasitic fields and other types of distortion can lead to errors in determining the position of the object being tracked.

0024In order to alleviate this problem, the elements of tracking system 20 and other articles used in the vicinity of the tracking system are typically made of non-metallic materials when possible, or of metallic materials with low permeability and conductivity. For example, reference structure 40 may be constructed using plastic or non-magnetic composite materials, as may other articles in this vicinity, such as the operating table. In addition, computer 36 may be programmed to detect and compensate for the effects of metal objects in the vicinity of the surgical site. Exemplary methods for such detection and compensation are described in <patcit id="pcit0019" dnum="US6147480A"><text>U.S. Patents 6,147,480</text></patcit> and <patcit id="pcit0020" dnum="US6373240B"><text>6,373,240</text></patcit>, as well as in <patcit id="pcit0021" dnum="US44828903A" dnum-type="L"><text>U.S. Patent Applications 10/448,289 filed May 29, 2003</text></patcit> and <patcit id="pcit0022" dnum="US10632217B" dnum-type="L"><text>10/632,217 filed July 31, 2003</text></patcit>.

0025<figref idref="f0002">Fig. 2A</figref> is a schematic, sectional illustration showing implantation of screw 30 into a bone 50, such as the femur of patient 26.

0026To insert the screw, surgeon 22 makes an incision through overlying soft tissue 52, and then rotates the screw into bone 50 using tool 28, for example. Alternatively, the screw may be inserted percutaneously, without prior incision. Note that in the example of <figref idref="f0002">Fig. 2A</figref>, screw 30 has no wired connection to elements outside the body. Typically, screw 30 is between about 5 and 15 mm long, and is about 2-4 mm in diameter. To avoid interfering with reception and transmission of signals by the sensor that it contains, screw 30 typically comprises a non-magnetic material, which may comprise metals, alloys, ceramics, plastics or a combination of such materials. The configuration and operation of the circuits in screw 30 are described hereinbelow with reference to <figref idref="f0003">Figs. 3A</figref> and <figref idref="f0004">3B</figref>.

0027<figref idref="f0002">Fig. 2B</figref> is a schematic, sectional illustration showing another position sensor device 54.

0028Device 54 comprises an implantable screw 56, which is coupled by wires 58 to an external unit 60. Screw 56 is inserted into bone 50 in substantially the same manner as is screw 30 (leaving wires 58 to pass out of the patient's body through soft tissue 52). In this case, however, because some elements of device 54 are contained in external unit 60, screw 56 may generally be made smaller than screw 30. For example, screw 56 may be between about 5 and 10 mm long, and between about 2 and 4 mm in diameter. The reduced screw size is helpful in reducing trauma and possible damage to bone 50. Further details of device 54 are shown in <figref idref="f0005">Fig. 4</figref>.

0029<figref idref="f0003">Fig. 3A</figref> is a schematic, pictorial illustration of a wireless position sensor 70 that is contained in screw 30

0030Sensor 70 in this example comprises three sets of coils: sensor coils 72, power coils 74, and a communication coil 76. Alternatively, the functions of the power and communication coils may be combined, as described in the above-mentioned <patcit id="pcit0023" dnum="US029473A" dnum-type="L"><text>U.S. Patent Application 10/029,473</text></patcit>. Further alternatively, although communication coil 76 is shown in <figref idref="f0003">Fig. 3A</figref> to be wound in a plane that is perpendicular to the longitudinal axis of screw 30, the communication coil or antenna may alternatively be arranged along the length of sensor 70, roughly parallel to the longitudinal axis of the screw. Coils 72, 74 and 76 are coupled to electronic processing circuitry 78, which is mounted on a suitable substrate 80, such as a flexible printed circuit board (PCB). Details of the construction and operation of circuitry 78 are described in <patcit id="pcit0024" dnum="US029473A" dnum-type="L"><text>U.S. Patent Application 10/029,473</text></patcit> and in the above-mentioned <patcit id="pcit0025" dnum="US706298A" dnum-type="L"><text>U.S. Patent Application 10/706,298</text></patcit>.

0031Although for simplicity, <figref idref="f0003">Fig. 3A</figref> shows only a single sensor coil 72 and a single power coil 74, in practice sensor 70 typically comprises multiple coils of each type, such as three sensor coils and three power coils. The sensor coils are wound together, in mutually-orthogonal directions, on a sensor core 82, while the power coils are wound together, in mutually-orthogonal directions, on a power core 84. Typically, each of the three power coils comprises about 30-40 turns of wire having a diameter of at least about 40 µm, while power core 84 is a ferrite cube of about 1.5-2 mm on a side. Each of the three sensor coils typically comprises between about 700 and 3000 turns of 11 µm diameter wire, while sensor core 82 is a ferrite cube of about 1.8-2.4 on a side. (It will be understood that these dimensions are given by way of example, and the dimensions may in practice vary over a considerable range.) Alternatively, the sensor and power coils may be overlapped on the same core, as described, for example in <patcit id="pcit0026" dnum="US75475104A" dnum-type="L"><text>U.S. Patent Application 10/754,751, filed January 9, 2004</text></patcit>. It is generally desirable to separate the coils one from another by means of a dielectric layer (or by interleaving the power and sensor coils when a common core is used for both) in order to reduce parasitic capacitance between the coils.

0032In operation, power coils 74 serve as a power source for sensor 70. The power coils receive energy by inductive coupling from an external driving antenna (shown, for example, in <figref idref="f0007">Fig. 6A</figref>). Typically, the driving antenna radiates an intense electromagnetic field at a relatively high radio frequency (RF), such as in the range of 13.5 MHz. The driving field causes currents to flow in coils 74, which are rectified in order to power circuitry 78. Meanwhile, field generator coils 32 (<figref idref="f0001">Fig. 1</figref>) induce time-varying signal voltages to develop across sensor coils 72, as described above. Circuitry 78 senses the signal voltages, and generates output signals in response thereto. The output signals may be either analog or digital in form. Circuitry 78 drives communication coil 76 to transmit the output signals to a receiving antenna (also shown in <figref idref="f0007">Fig. 6A</figref>) outside the patient's body. Typically, the output signals are transmitted at still higher radio frequencies, such as frequencies in the rage of 43 MHz or 915 MHz, using a frequency-modulation scheme, for example. Additionally or alternatively, coil 76 may be used to receive control signals, such as a clock signal, from a transmitting antenna (not shown) outside the patient's body. Although certain frequency ranges are cited above by way of example, those skilled in the art will appreciate that other frequency ranges may be used for the same purposes.

0033In another example, not shown in the figures, sensor coils 72 are non-concentric. In this embodiment, each of the sensor coils typically has an inner diameter of about 0.5-1.3 mm and comprises about 2000-3000 turns of 11 µm diameter wire, giving an overall coil diameter of about 1-1.9 mm. (As above, these dimensions are given only by way of example, and the actual dimensions may vary.) The wire size of the sensor coils can range from 10-31 µm, and the number of turns between 300 and more than 3000, depending on the maximum allowable size and the wire diameter. The effective capture area of the sensor coils is typically made as large as feasible, consistent with the overall size requirements. The sensor coils are typically cylindrical, but other shapes can also be used. For example, barrel-shaped or square coils may be useful, depending on the geometry of screw 30.

0034<figref idref="f0004">Fig. 3B</figref> is a schematic, pictorial illustration of a wireless position sensor 90,

0035Sensor 90 differs from sensor 70, in that sensor 90 comprises a battery 92 as its power source, instead of power coils 74. Battery 92 may be of any suitable type, either single-use or rechargeable. In other respects, the operation of sensor 90 is substantially similar to that of sensor 70, as described above. Use of battery 92 has the advantages of supplying higher operating power to electronic processing circuitry 78, while avoiding the need to irradiate patient 26 with an intense electromagnetic field in order to provide inductive RF power to the sensor. On the other hand, incorporating battery 92 in sensor 90 typically increases the length of the sensor, by comparison to sensor 70, and therefore may require the use of a longer screw 30 to contain the sensor. In addition, the operating lifetime of sensor 70 is effectively unlimited, while that of sensor 90 is limited by the lifetime of battery 92.

0036<figref idref="f0005">Fig. 4</figref> is a schematic, pictorial illustration showing details of device 54.

0037The external features of device 54 and its implantation in bone 50 were described above with reference to <figref idref="f0002">Fig. 2B</figref>. Device 54 comprises an internal sensing unit 94, which is contained in screw 56. Typically, sensing unit 94 contains only sensor coils 72, and possibly or optionally elements of circuitry 78. This arrangement allows the size of screw 56 to be minimized. External unit 60 typically contains a battery 96 and circuit elements 98, which comprise some or all of circuitry 78 (depending on how much of circuitry 78 is located within sensing unit 94), as well as communication coil 76. The battery may thus be replaced when necessary, without removing screw 56 from the bone. On the other hand, whereas sensors 70 and 90 are contained completely within screw 30, and thus leave no elements protruding outside the patient's body, device 54 can operate only when external unit 60 is connected outside the body to wires 58 that are operatively connected to sensing unit 94 and communicate with sensing unit 94.

0038<figref idref="f0006">Fig. 5</figref> is a schematic, pictorial illustration showing details of tool 28.

0039Tool 28 comprises a handle 100 and a shaft 102. A tool sensor 104 fits snugly into a suitable receptacle inside handle 100. Sensor 104 comprises sensing and communication circuits 106, which are powered by a battery 108. Typically, circuits 106 comprise three sensing coils, a communication coil and processing circuitry, as in sensor 90 (<figref idref="f0004">Fig. 3B</figref>). The sensing coils are similar to coils 72, and sense the location and orientation of sensor 104 relative to the magnetic fields generated by field generator coils 32 (<figref idref="f0001">Fig. 1</figref>). The communication coil conveys position signals to computer 36. The operation of circuits 106 is thus similar to that of the circuits in sensors 70 and 90, although elements of circuits 106 may be made larger and consume greater power than the corresponding elements in sensors 70 and 90.

0040Tool sensor 104 may be permanently housed inside tool 28, or the sensor may alternatively be removable (to replace battery 108, for example). Because the geometry of tool 28 is known, the location and orientation of handle 100, as indicated by sensor 104, indicates precisely the location and orientation of the distal tip of shaft 102. Alternatively, the tool sensor 104 may be miniaturized and may thus be contained inside shaft 102. Optionally, the tool sensor 104 may be calibrated before use in order to enhance the precision with which the shaft position is measured.

0041<figref idref="f0007">Figs. 6A and 6B</figref> are schematic, pictorial illustrations showing insertion of a location pad 110 into an opening in an operating table 112, in accordance with an embodiment of the present invention. Pad 110 may be used as the reference structure in system 20 (<figref idref="f0001">Fig. 1</figref>), in place of structure 40. Pad 110 comprises an integral unit, which holds three field generator coils 32 in fixed positions. The unit is typically made from non-magnetic material, such as carbon fiber, fiberglass, plastic or ceramic. The field generator coils in this case are angled diagonally inward. In <figref idref="f0007">Fig. 6A</figref> pad 110 is shown prior to insertion into the table, while in <figref idref="f0007">Fig. 6B</figref> the pad has been slid into place.

0042Location pad 110 is also seen in <figref idref="f0007">Fig. 6A</figref> to comprise an optional power coil 114 and a communication coil 116. Power coil 114 is coupled by wires (not shown) to driver circuits 34, and generates an electromagnetic field to provide power inductively to power coils 74 in sensor 70 (<figref idref="f0003">Fig. 3A</figref>), as described above. (When a battery-powered sensor is used, the power coil is not required.) Communication coil 116 receives signals transmitted by communication coil 76 in sensors that are implanted in the patient's body, as well as from tool sensor 104. Communication coil 116 may also be used to transmit control signals, such as a clock signal, to the implanted sensors and tool sensor. Communication coil 116 is coupled by wires (not shown) to computer 36. The computer processes the signals received from communication coil 116 in order to determine the locations and orientations of the sensors. Coils 114 and 116 may be printed on the surface of pad 110, as shown in <figref idref="f0007">Fig. 6A</figref>, or they may alternatively comprise printed circuit traces or wire-wound coils contained inside pad 110.

0043<figref idref="f0007">Fig. 6B</figref> schematically shows a working volume 118 created by field generator coils 32 when driven by driver circuits 34. The surface of the working volume represents the outer limit of the region in which tracking system 20 is able to determine sensor coordinates to within a certain accuracy i.e. the location coordinates or position and orientation coordinates of the sensor. The required accuracy is determined by functional considerations, such as the degree of positioning precision required by surgeon 22 in performing the surgical procedure at hand. Typically, the outer surface of working volume 118 represents the limit in space at which tracking accuracy drops to the range of 1-2 mm. Tilting the field generator coils, as shown in <figref idref="f0007">Figs. 6A and 6B</figref>, typically lowers the centroid of the working volume. Because pad 110 is rigid, it cannot be raised and lowered or tilted, as can structure 40 in <figref idref="f0001">Fig. 1</figref>. Pad 110 may, however, be slid in and out of table 112 in order to shift the position of working volume 118 along the table, so that the working volume intercepts the bone 50 or portion of the bone 50 on which the surgeon in to operate.

0044<figref idref="f0008">Fig. 7</figref> is a schematic, pictorial illustration showing how reference structure 40 may be adjusted for use in surgery on a knee 120 of patient 26.

0045The patient lies on an operating table 122, which folds as shown in the picture to give the surgeon convenient access to the patient's knee joint. Base 44 of structure 40 tilts accordingly, so that the working volume of field generator coils 32 encompasses the area of knee 120, while still permitting the surgeon unimpeded access to the area.

0046<figref idref="f0009">Fig. 8</figref> is a schematic, pictorial illustration showing a reference structure 130 for supporting field generator coils 32.

0047Structure 130 comprises arms 132, which hold coils 32. The arms are fixed to an articulated boom 134, which permits the height and angle of the field generator coils 32 to be adjusted relative to the position of the patient on an operating table 136: Boom 134 may be carried by a wheeled cart 138, so that structure 130 can be positioned at either side of table 136 or at the foot or head of the table. Cart 138 may also contain computer 36 and/or driver circuits 34. To reduce clutter over operating table 136, structure 130 may be integrated with an overhead lamp 140, as shown in the figure. In this configuration, lamp 140 illuminates the area of the working volume of coils 32. An additional lamp 142 is shown for completeness.

0048<figref idref="f0010">Fig. 9</figref> is a schematic, pictorial illustration showing a reference structure 150 supporting field generator coils 32.

0049Structure 150 comprises an articulated boom 154, which holds arms 152 to which coils 32 are attached. In this example, structure 150 is tilted and positioned over the area of the patient's knees, to provide functionality similar to that shown in <figref idref="f0008">Fig. 7</figref>.

0050<figref idref="f0011">Figs. 10A</figref> and <figref idref="f0012">10B</figref> are schematic, pictorial illustrations showing another reference structure 160.

0051Structure 160 comprises a semicircular holder 162 for field generator coils 32, which is mounted on a base 164. Whereas the reference structures an the examples shown above are configured to position coils 32 in a plane that is roughly parallel to the long axis of the bone to be operated upon (such as the femur or the fibula), the plane of structure 160 is roughly perpendicular to this axis. Typically, for proper positioning of the working volume, structure 160 is placed so that the bone axis passes through the circle defined by the positions of coils 32, i.e., so that holder 162 partly surrounds the bone axis.

0052Structure 160 may be mounted on a cart 166 with wheels, enabling it to be positioned either at the foot (<figref idref="f0011">Fig. 10A</figref>) or head (<figref idref="f0012">Fig. 10B</figref>) of table 122. An adjustment slot 167 or other mechanism in base 164 permits holder 162 to rotate about the patient. A hinge permits base 164 to tilt, while telescopic legs 170 permit the entire structure to be raised or lowered. Structure 160 may thus be positioned flexibly, at the convenience of the surgeon, depending on the type of procedure that is to be carried out. The configuration of <figref idref="f0011">Fig. 10A</figref>, for example, may be convenient for hip surgery, while that of <figref idref="f0012">Fig. 10B</figref> is convenient for knee surgery.

0053<figref idref="f0013">Fig. 11</figref> is a schematic, pictorial illustration showing a magnetic tracking system 180 for use in surgery.

0054In this example, the tracking system is integrated into an operating table 182. The operating table may be custom-made for this purpose, and may thus comprise little or no magnetic material. A reference structure 184 is fixed to the underside of table 182 by an articulated mount that permits structure 184 to be rotated, tilted, raised and lowered, so as to position field generator coils 32 as required for the surgical procedure in question. A telescopic base 186 of table 182 contains driver circuits 34 and computer 36. Positions and orientations of position sensors and tools are shown on display 46, which is likewise integrated with table 182. System 180 thus permits the surgeon to operate with only minimal added encumbrance due to the use of magnetic position tracking.

0055It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention is defined by the appended claims.