WO2001024697A1

WO2001024697A1 - Device and method for measuring skeletal distraction

Info

Publication number: WO2001024697A1
Application number: PCT/US2000/041096
Authority: WO
Inventors: Takis C. Kasparis
Original assignee: Orthodyne, Inc.
Priority date: 1999-10-06
Filing date: 2000-10-06
Publication date: 2001-04-12
Also published as: AU1963101A

Abstract

A method and apparatus for extracutaneously measuring and monitoring movement of an implant within a patient. The device (750) includes a housing (752) having a sensor portion, a control computer (762), and a memory (764). The sensor portion includes first and second sensors (776, 778). The first sensor (776) measures and provides a signal or reading corresponding to the strength of the magnetic field of the earth, and the second sensor (778) measures and provides a signal or reading corresponding to the strength of the magnetic field of a magnet associated with the movement of the implant. The control computer (762) receives the signals from the first and second sensors (776,778), and is operable to correct the second signal for the effects of the earth's magnetic field or power source fluctuation. The corrected second signal is used by the control computer (762) to determine the amount of movement of the implant. This information, along with other information associated with the determination, is stored in the memory portion (764) for later retrieval and/or processing. The device may also include various other components operable by the control computer, such as a clock, energy manager, LCD display, LEDs, and a battery back-up system.

Description

DEVICE AND METHOD FOR MEASURING SKELETAL DISTRACTION

BACKGROUND OF THE INVENTION The present invention relates generally to surgical implements for use in the lengthening of bones, and more specifically to an apparatus and method for measuring and/or monitoring intramedullary skeletal distraction.

The problem of limb-length discrepancies resulting from congenital, postinfectious, and post-traumatic disorders is one that has received the attention of many researchers. Various devices have been known in the art that can be attached to the ends of a sectioned bone and made to lengthen progressively, the lengthening causing growth of bone tissue at the site of sectioning and thus a commensurate lengthening of the bone. The devices are then removed when the desired length is achieved or they may be left in place as a splint. Skeletal distraction devices fall generally into either external or internal devices. External distractors, usually comprising pins passing through soft tissue and bone, can carry non-negligible potential risks of infection, pain, and muscle contractures. One benefit, however, is the accessibility of this type of device to manipulation extracutaneously. Internal devices are designed for placement entirely within the body and often within the medulla of a long bone.

An attempt to obviate the need for directly contacting the elongation members to create the distraction has been made by Grammont et al. (U.S. Pat. No. 5,074,882; Trans. 37^th Ann. Mtg. Orthopaedic Research Soc, Vol. 16, p. 657, 1991). As in previous prostheses, two telescoping tubes are used to stimulate progressive elongation of a limb. A related embodiment of the device and method to be discussed herein, the disclosure of which is hereby incorporated by reference, may be found in "Intermedullary Skeletal Distractor and Method." U.S. Pat. No. 5,505,733, issued to D. Justin and J.D. Cole.

Another problem associated with the use of external and internal distractors is determining and measuring the amount of lengthening or movement of the distractor. Measurements of elongation of the distractor are required and useful for many purposes, including, for example, evaluation of the progress of bone growth and determining the proper treatment for the patient. This measuring and monitoring process requires the patient to make multiple trips to the doctor's office in order for measurements to be made. Typically, such measurements are made by x-ray evaluation. Additional doctor visits increase the expense associated with using skeletal distractors, and cause the patient to devote significant time for treatment and evaluation.

As disclosed in U.S. Patent No. 5,074,882, a magnet may be mounted on the distraction device. The magnet rotates with a moveable portion of the distraction device thereby providing an indication of the number or extent of rotations. A sensor was provided to detect magnetic field strength and polarity. With this device, the patient may be relied upon to take the necessary measurements. Experience has indicated that patients do not remember to take measurements at timely intervals, or forget to take measurements altogether, such mistakes may cause errors in the accuracy of measurements. Also, there is the risk that the patient will lose the data or take the measurements improperly. Further, the sensor did not provide reliable compensation for background magnetic fields or changes in the voltage of the power source, thereby causing inaccurate readings. All of these problems, among others, keep the doctor from making an appropriate evaluation of the progress of the patient, thus increasing the costs and risks associated with the use of skeletal distractors.

Therefore a need remains for a device which, among other things, monitors and measures skeletal distraction in an efficient, simple, and reliable manner, yet affords flexibility in use and function. The present invention is directed to satisfying these needs, among others. SUMMARY OF THE INVENTION

The present invention provides a device for extracutaneously monitoring and measuring movement of a movable implant by reading the strength of the magnetic field of a magnet associated with movement of the implant.

The present invention further includes a method for extracutaneously measuring and monitoring movement of an internal fixation apparatus within a patient. The fixation apparatus is provided with a magnet indicative of movement of the apparatus. The method includes providing a measuring and monitoring device having a control computer, a memory portion and a sensor portion including a first sensor and a second sensor. At least one of the sensors reads the magnetic field of the magnet within the apparatus and provides a signal corresponding thereto to the control computer. The control computer uses the signal to determine the movement of the apparatus. In one aspect of the invention, a device for extracutaneously measuring the movement of an implant in the human body is provided. The device has a housing with a control computer. The control computer has a first memory portion for storing algorithms that allow the control computer to control operation of the device. There is also a memory portion connected to the control computer for storing data related to each measurement. A first sensor senses the strength of the earth's magnetic field, and provides a first reading corresponding thereto to the control computer. A second sensor senses the strength of the magnetic field of the magnet disposed within the implant, and provides a second reading corresponding thereto to the control computer. The control computer determines the position of the magnet from the second reading after correcting the second reading for bias caused by the magnetic field of the earth as determined by the first reading. The movement of the implant is calculated by comparing the determined position of the magnet to the last position of the magnet retrieved from storage in memory, thus providing a measurement of the movement of the implant. Preferably, the device further includes a clock connected to the control computer. The control computer associates a date and time with the measurement of movement, and this data is provided to the memory portion for storage. In another preferred aspect, the device includes an alarm connected to the control computer, and the control computer control is operable with the alarm to provide an audible signal to a user of the device that a measurement is needed. In yet another preferred aspect, the device includes an energy management system designed to minimize power consumption and extend operating time for the device. Preferably, the device includes a primary power source providing power to the sensors, computing device, memory, and clock. Since the sensors utilize a large amount of power, the energy management system utilizes various parameters to minimize the amount of time the sensors are operational. More preferably, a second power source is provided that may be utilized to maintain clock functions and/or memory when the primary power source is too weak or removed from the device.

In another preferred aspect, the device includes an alphanumeric display connected to the control computer. In another aspect, the device includes an analog-to-digital converter connected between the first and second sensors and the control computer. The converter receives first and second signals from the sensors in analog form and converts them to digital form for further processing by control computer. In another aspect of the invention, a method for determining movement of an internal fixation apparatus is provided. The fixation apparatus has a magnet indicative of the position of the fixation apparatus. The method includes providing a device for measuring and storing data relating to movement of the fixation apparatus. The device has a first and second sensor, a control computer connected to the sensors to receive signals therefrom, and a memory portion connected to the control computer. The device is initialized when powered on, and the control computer retrieves a previous magnet position from the memory portion of the control computer. The first and second sensors also take a reading of the magnetic field of the earth. The device is initialized when the first and second sensors are recording the same reading for the earth's magnetic field. After initializing the device, the sensor portion of the device is positioned proximate a measuring point on the patient. A reading of the position of the magnet is taken by the second sensor. The reading is used by the control computer to calculate the movement of the fixation device by relating the position of the magnet to the previous position of the magnet. In one preferred aspect of the method, the control computer corrects the reading of the second sensor based upon the reading of the first sensor to minimize the effect of the earth's magnetic field.

In still a further preferred aspect, the first and second sensor sense a substantially identical background magnetic field. The outputs of each sensor are utilized to determine a compensation factor between the sensor outputs. This provides a method for calculating a value to compensate for differences in sensor output as a result of internal sensor differences and variations in supply voltages.

In another preferred aspect, the memory portion has a capacity to store data associated with at least 2000 measurements. However, it is contemplated that more or less memory may be available to store any desired number of measurements. Moreover, it is contemplated that in a preferred aspect of the invention, stored data may be transferred to an external device, including but without limitation, a computer, monitor, or printer.

In yet another preferred aspect, the method includes allowing the user of the measuring and monitoring device to view stored data prior to taking a reading.

Further objects and advantages of the present invention will be apparent from the description of the preferred embodiments that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the skeletal distractor (a) assembled and in its most shortened position and (b) in exploded view.

FIG. 2 is (a) a longitudinal view and (b) a longitudinal sectional view of the first cylindrical member. The views in (a) and (b) are rotated 90 degrees from each other along the cylindrical axis.

FIG. 3 is (a) a longitudinal view and (b) a longitudinal section view of the second cylindrical member. The views in (a) and (b) are rotated 90 degrees from each other along the cylindrical axis.

FIG. 4 illustrates the elongated rod. The views in (a) and (b) are rotated 90 degrees from each other along the cylindrical axis.

FIG. 5(a) and (b) are cross-sectional views of two embodiments of the indicator mechanism with the piston (a) within the cutout and (b) entirely within the bore in the elongated rod.

FIG. 6 depicts an exploded view of the overrunning roller clutch used in the distractor.

FIG. 7 illustrates the skeletal distractor positioned within the medullary cavity of a bone.

FIG. 8 illustrates an alternate embodiment of the device having a bent first cylindrical member.

FIG. 9 is a perspective view of the keyring of the present invention. FIG. 10 is an exploded view of the device having a magnetic position - indicating device.

FIG. 11 is a perspective view of the magnetically driven embodiment of the skeletal distractor, using (a) a static magnet or (b) an electromagnet; (c) an exploded view of the device, and (d) a cross-sectional view of the second cylindrical member. FIG. 12 is a perspective view of the magnetic position-indicating device in place within a bone.

FIG. 13 is a perspective view of an alternate one-piece assembly of a roller clutch. FIG. 14 is an exploded view of an alternate embodiment of the overrunning roller clutch.

FIG. 15 is a perspective view of one preferred embodiment measuring device for use with magnetically driven skeletal distractors.

FIG. 16(a) is a block diagram of the measuring device of FIG. 15. FIG. 16(b) is a top view of a board layout for the measuring device of FIG.

15.

FIG. 17 is a detailed schematic drawing of the measuring device of FIG. 15.

FIG. 18 is composed of FIGs. 18(a) and 18(b), and is a flowchart of one preferred embodiment of an algorithm executable by a controller of the measuring device of FIG. 15.

FIG. 19 is a flowchart of one preferred embodiment of an algorithm executable by a computer connected to the measuring device of FIG. 15.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.

The present invention relates generally to skeletal distractors, and to methods and devices for measuring and/or monitoring elongation of the sectioned bone. In a most preferred form, the measuring and monitoring device is used extracutaneously with the intramedullary skeletal distractors described herein. However, it should be understood the measuring and monitoring device may likewise be used with external distractors any other intramedullary distractors, so long as the measuring and monitoring device can assist with the determination of the elongation of the sectioned bone in accordance with the principles discussed herein. Moreover, the sensing device and method may be used with any implantable device, instrument, or structure to determine location and orientation. The intramedullary skeletal distractor, shown assembled and in exploded view in FIGS, la and lb, respectively, will be referred to generally by the reference numeral 10. Device 10 comprises a first 20 and a second 30 cylindrical member, shown in cross section in FIGS. 2 and 3, respectively, an elongated rod 40, shown in FIG. 4, and an indicator mechanism 50, shown in FIG. 5. In this embodiment the clutch means comprise a first clutch 60 and a second clutch 70 (FIG. 1), both overrunning roller clutches that permit rotation in one direction and lock movement in the other direction. It can be appreciated by one skilled in the art that other types of clutches may be substituted, such as spring, spiral-band, friction, magnetic, or sprag clutches. In detail, first cylindrical member 20 (FIG. 2) has a first end 202, a second end 204, an outer diameter 206, an inner wall 214, an outer wall 216, and a partial longitudinal bore 208. Bore 208 communicates with second end 204 but does not proceed through the first end 202. Adjacent first end 202 is angled bore 212, through which a screw 604 is inserted to anchor first cylinder member 20 to a proximal section 602 of bone 60 (see FIG. 7). Angled bore 212 does not communicate with longitudinal bore 208. In the preferred embodiment, angle 218, measured from the cylinder axis of first end 202 to the axis of bore 212 is in the range of 45-60 degrees for the case of, for instance, a femur or a humerus. This angle permits the anchoring means to engage the thickest portion of bone and thus provide the greatest strength.

For the case of a tibia, a slightly different embodiment of first cylindrical member 80 is provided. As shown in FIG. 8, first end 802 of first cylindrical member 80 is slightly bent, typically at an angle 810 of approximately 10 degrees from the cylinder axis. The location of the bend 804 occurs between the first end 802 and end of bore 808 closest the first end 802. In this embodiment, instead of angled bore 212, two diametric bores 806 and 808 are provided adjacent first end 802 through which a pair of screws may be inserted to anchor first cylindrical member 80 to the proximal section of the tibia. Returning to FIG. 2, within first end 202 and communicating with angled bore 212, but not communicating with longitudinal bore 208, is threaded longitudinal bore 210. Threaded bore 210 is utilized during insertion and retrieval of the device by threading an extension member (not shown) into bore 210. Bore 210 is also used as a passageway for the insertion of a drill guide to assist in the correct placement of screw 604. In an alternate embodiment, a nail may be used in place of screw 604. With either a screw or a nail, bore 210 may also be used for a locking set screw 606 to secure the screw or the nail in place.

Bore 208 has three stages, which proceeding from second end 204, are first section 220, having the largest diameter 222 and a length 221 ; second section 224, having intermediate diameter 226; and third section 228, having the smallest diameter 230 and having an octagonal internal wall profile in axial cross section. Outer wall 216 has a tapered section 232 at second end 204, through which is a pair of opposed slots 234 and 236, having a width 238 for engaging keyring 95, to be discussed in the following.

Second cylindrical member 30, shown in detail in FIG. 3, has first end 302, second end 304, outer diameter 306, longitudinal bore 308, outer wall 310, and inner wall 312. Outer diameter 306 is dimensioned to slidably engage first section 220 of bore 208 in first cylindrical member 20. Outer diameter 306 is further dimensioned to be larger than the diameter 226 of second section 224; therefore, the depth to which second cylindrical member 30 can be inserted into first cylindrical member 20 is determined by the length 221 of first section 220. When assembled (see FIG. 1), first end 302 is inserted (after elongated rod 40, vide infra) into bore 208 from second end 204 of first cylindrical member 20. In the preferred embodiment, second end 304 is tapered 315 and has a rounded edge 313 to facilitate insertion. A pair of opposed slots, 314 and 316, not communicating with bore 308, run from adjacent second end 304 to a section 318 of first end 302. Slots 314 and 316 have a width 315 perpendicular to the axis of a second cylindrical member 30. Near second end 304 is a pair of diametric bores 320 and 322, proceeding through and having a larger diameter than the width 315 of slots 314 and 316. Bores 320 and 322 are utilized for screws 607 and 608, which secure member 30 to a distal section 610 of bone 60 (see FIG. 7).

Beginning from the second end 304, bore 308 has three stages, first section 334, having diameter 326; second section 336, which is threaded, having diameter 338; and third section 340, having diameter 342.

Distal plug 90 (FIG. lb) is dimensioned to fit into bore 308 of the second cylindrical member 30 at second end 304 and extends from second end 304 past the location of bore 320. Distal plug 90 serves to strengthen the distal end and to block intramedullary tissue from entering bore 308 during insertion and elongation. Distal plug 90 also has a pair of circumferential bores 902 and 904 extending therethrough and communicating with bores 322 and 320, respectively. Keyring 95 (FIG. 9) is a cylindrical member dimensioned to be press fit onto the second end 204 of first cylindrical member 20. Keyring 95 has an inner wall 952, from which a pair of opposed protrusions 954 and 956, each having a width 958, extend into bore 960. When assembled, protrusions 954 and956 engage opposed slots 234 and 236 in first cylindrical member 20 and also opposed slots 314 and 316 in second cylindrical member 30. The purpose of keyring 95 is to prevent excessive rotation of first 20 and second 30 cylindrical members and yet permit sufficient relative rotation to activate the clutches. Thus, the width 958 of protrusions 954 and 956 must be dimensioned smaller than slot widths 238 and 315 to permit a relative rotation sufficient to activate the clutch mechanism and is also within the limits of natural anatomical limb rotations. In the preferred embodiment, a rotation of 3 degrees is permitted.

Elongated rod 40, shown in FIG. 4, has a first end 402, and second end 404, length 408, and diameter 406. Extending from second end 404 is threaded portion 410 of rod 40, dimensioned to mate with threaded section 336 of second cylindrical member 30. Extending from first end 402 is nonthreaded portion 412. Extending into nonthreaded portion 412 are partial 414 and full 416 diametric bores. When assembled (see FIG. 1), first end 402 resides within the full extent of bore 208 of first cylindrical member 20 and is threaded into threaded section 336 of second cylindrical member 30, with which it is rotatably engaged and longitudinally extendable thereby, as will become apparent in the following. Affixed to nonthreaded portion 412 of elongated rod 40 at first end 402 is indicator mechanism 50 (see FIG. 1) shown in detail in FIGS. 5a and 5b. Indicator mechanism 50 comprises indicator housing 52, piston 54, and spring 53.

Indicator housing 52 is a hollow octagonal element having an outer periphery 520 dimensioned to closely fit within third section 228 of bore 208 of first cylindrical member 20. Indicator housing 52 also has a bore 522 that has four substantially identical cutouts 526 that are equally spaced radially (at 90 degrees from each other) about bore 522, cutouts 526 having a radial extent 528. Bore 522 further has a minimum diameter 524 over the bore 522 regions away from cutouts 526. Minimum diameter 524 is dimensioned to encompass and closely engage nonthreaded portion 412 of elongated rod 40. Piston 54 and cutouts 526 are shaped so as to closely engage each other. The radial profile 542 of piston 54 has a sloping edge 544 and a substantially straight edge 546 approximately collinear with the radius of elongated rod 40. The radial extent 548 of pistons 54 is greater than the radial extend 528 of cutouts 526. Cutouts 526 have a radial profile 541 having a sloping edge 543, which curves oppositely from curve 544, and a substantially straight edge 545. The sloping and straight edges of the piston and a cutout substantially align when the piston resides within the cutout.

When assembled (see FIG. 1), spring 53 is inserted into bore 414 in rod 40 and piston 54 is inserted into bore 414 atop spring 43. Indicator housing 52 is the fit over first end 402 of rod 40 so that piston 54 resides in one cutout 526 and extends partially into bore 414. It can be seen that relative rotation between indicator housing 52 and rod 40 is opposed in a first direction 548 toward straight edge 546, but that it is possible in a second direction 549 toward sloping edge 544. If rotation in second direction 549 is to occur, however, a sufficient torque must be exerted to overcome the coefficient of friction of the mechanism and the spring constant of spring 53, so that spring 53 is compressed by piston 54 being pushed into bore 414. When sufficient rotation in the second direction occurs so that pistons 54 reside completely within bore 414, as shown in FIG. 5(b), less torque is required to continue rotation. When a rotation of 90 degrees is achieved, piston 54 reaches the next cutout 526, and an audible sound is emitted as piston 54 snaps into cutouts 526 as spring 53 is released.

In a subembodiment, an audible sound may be a signal that sufficient forcible torque has been applied to overcome the resistance of the indicator mechanism and thus alert the patient that progressive elongation may occur.

In another subembodiment, a magnetic field may be applied to move the piston into the bore 714, releasing the indicator mechanism. It can be seen that further subembodiments may comprise different numbers of cutouts, also equally radially spaced. For instance, three cutouts would be spaced 120 degrees apart. Having the flexibility afforded by such a variety of indicator mechanisms permits one to custom design elongation parameters without altering any of the other elements.

Also affixed to nonthreaded portion 412 of elongated rod 40 is indicator bearing 55, a cylindrical member having a longitudinal bore 552 dimensioned to closely engage nonthreaded portion 412 (see FIG. 1). Diametric bore 554 communicates with bore 416, and locking pin 58 inserted through bores 554 and 416, retaining bearing 55 upon rod 40. When assembled, bearing 55 resides within third section 228 of bore 208 in first cylindrical member 20, further toward second end 204 than and adjacent to indicator mechanism 50.

First clutch 60, the structure of which is shown in FIG. 6, has an outer periphery 620 dimensioned to fit sufficiently tightly within the second section 224 of bore 208 of first cylindrical member 20 that rotational motion can be communicated therebetween. In an alternate embodiment, as shown in FIG. 10, first clutch 60 screws into a threaded portion in the second section 224 of bore 208, and is held in place by retaining bushing 64. First clutch 60 further has an inner diameter 604 dimensioned to closely engage nonthreaded portion 412 of elongated rod 40 and also communicate rotational motion therebetween. When assembled, first clutch 60 is mounted on nonthreaded portion 412 between indicator bearing 55 and threaded portion 410. First clutch 60 permits slippage between rod 40 and first cylindrical member 20 when rotation occurs in a first direction and communicates rotation therebetween in the second direction. Second clutch 70, identical in structure to first clutch 60 shown in FIG. 6, has an outer periphery 702 dimensioned to fit sufficiently tightly within widest section 340 of second cylindrical member 30 that rotational motion can be communicated therebetween. Second clutch 70 further has an inner diameter 740 dimensioned to engage threaded portion 410 of rod 40 and move longitudinally therealong. The directionality of second clutch 70 permits locking between rod 40 and second cylindrical member 30 when rotation occurs in the first direction; rotation in the second direction causes rotational slip, thus allowing linear movement between rod 40 and member 30. When the device 10 is fully assembled, rotation in the second direction causes second clutch 70 to move along threaded portion 410 of rod 40 toward second end 404, and consequently threaded portion 410 moves in a longitudinal direction out of threaded section 336 of bore 308 in second cylindrical member 30. Such relative longitudinal movement serves to push second cylindrical member 30 out of the bore 208 of first cylindrical member 20, elongating device 10.

In detail, clutches 60 and 70 comprise cylindrical clutch housing 702, clutch cage 704, end cap 706, four clutch springs 708, and twelve clutch rollers 710.

Clutch housing 702 has a bore 711 having inner surface 712, a first end 701, and a second end 703. Inner surface 712 has sixteen asymmetrically shaped radial cutouts 713 having a gradual slope 714 in a first direction and a sharper slope 716 in a second direction.

Clutch cage 704 comprises a first ring-shaped end 718 and a second ring- shaped end 720 spaced apart by four equally spaced support members 722 affixed to the facing surfaces 719 and 721 of the ends 718 and 720, respectively. Each of these facing surfaces 719 and 721 has a circumferential channel 726 and 727, respectively, therein. Second end 720 is dimensioned to slide within first end 701 of clutch housing 702; first end 718 is larger than clutch housing 702 and thus will not slide past first end 701 of clutch housing 702. Each clutch roller 710 comprises a substantially cylindrical member having a first 734 and a second 736 narrowed end. One set of three clutch rollers 710 resides between adjacent support members 722, the first ends 734 positioned within channel 727 and the second ends 736 positioned within channel 726. When assembled, with cage 704 within housing 702, clutch rollers 710 are biased against sharper slope 716 by springs 708, which are mounted on support members 722. Rollers 710 cannot rotate in the second direction past the sharper slope 716 of the housing, but they can rotate in the first direction past the gradual slope 714. This selective rotational ability provides the clutch directionality.

To complete the assembly, end cap 706, having a depending lip 738 dimensioned to fit without housing bore 711 , is placed over housing 702 at second end 703.

Since both clutches 60 and 70 are overrunning roller clutches, very small rotations can effect elongation. In practice, torsions as small as 1 degree, well within the range of normal physiological movement, will cause elongation of device 10. Therefore, no external manipulation is necessary, and, rather than the several larger elongations per day required of previously disclosed devices, small progressive longitudinal increments can occur throughout the day, a more desirable situation for stimulation of bone growth. The only external manipulation required is that necessary to release the indicator mechanism 50. In practice, for human subjects, the device is designed to permit 0.20-0.25 mm of elongation per 90 degree rotation of the elongated rod 40, and manipulation to release this mechanism is recommended four to six times per day, for a total elongation of 1 mm per day.

In an alternate embodiment of the clutch construction, shown in FIG. 14, the clutch 61 has a cylindrical clutch housing 612, a clutch cage 614, an end cap (not shown), eight clutch springs 618, and eight clutch rollers 610. The principle of operation is identical to that described for clutches 60 and 70, except that there is one clutch spring 618 for every clutch roller 610.

Another alternate embodiment of the roller clutch is shown in FIG. 13 which illustrates a one-piece roller/spring/cage assembly 62 that is dimensioned to be inserted into housing 702. In this assembly 62, which can be made, for example, by laser cutting or metal injection molding, the generally cylindrical "cage" portion 622 of the structure is fenestrated, having openings 624 in which the spring portions 626 and roller portions 628 are supported by interconnections between each other and, for the spring portions 626, with the cage portion 622. In this clutch 62 design a first end 625 of each leaf spring 626 joins the roller 628 near the center thereof. A second end 627 of each leaf spring 626 joins a longitudinal section 621 of the cage 622 also near the center thereof. Each roller portion 628 is supported only the leaf spring's first end 625, thereby permitting some movement relative thereto. Under tension, the clutch operates in similar fashion to those discussed above, in that the roller portions 628 are restrained from rotating in the second direction by the shape of the housing 702 and are permitted to rotate in the first direction. In this case, however, the rollers 628 do not rotate fully; rather they turn slightly, exerting a force on the leaf springs 626. The spring portions 626 comprise a loop of metal having sufficient flexibility to permit clutch rotation under physiological conditions but sufficient stiffness to restrain free motion.

In the embodiment shown in FIG. 13, there are 13 roller/spring units. While this number is meant to be exemplary and nonlimiting, it should be noted that the construction of the clutch must strike a balance. The fewer the rollers, the larger they are in diameter, which limits the diameter of the elongated rod and the wall thickness of the cylindrical members; the greater the number of rollers; the smaller they are in diameter, which permits either the elongated rod or the wall thickness of the cylindrical members to be greater and thereby, stronger. However, if the number of rollers becomes too large, the clutch would be unmachinable and too weak; therefore, a workable range is likely from 8 to 16 rollers for this embodiment, although this is not intended as limiting, as new materials might indeed make it possible to achieve operable clutches having a larger number of rollers.

The method of utilizing the above-described device comprises the following steps (see FIG. 7): An incision at the head of the sectioned bone 60 to be treated is made, through which the distractor 10 is inserted into the medulla of the bone. The first 20 and the second 30 cylindrical members are affixed by screws 604, 607, and 608 to the proximal 602 and distal 610 sections of the bone 60, respectively. When any torsional movement in the first direction occurs, the second clutch moves along the threaded portion of the elongated rod, pushing the second cylindrical member away from the first, elongating the device. Successive rotations continue to telescope the device until 0.25 mm elongation is reached, at which point the indicator mechanism locks against further elongation. At the end of a quarter-day, either a second party or the patient him/herself forcibly rotates the limb until the piston is pushed within the bore in the elongated rod and clutch movement can occur again. When the limb has been stimulated to grow at bone section point 601 to the length desired, the distractor is removed from the bone 60 or left in place as an internal splint.

In a second embodiment 12, shown in FIG. 10, means are provided that are in communication with the elongated rod 42 for determining from an extracutaneous location an amount of elongating telescopic movement that has occurred between the first 20 and the second 30 cylindrical members from a predetermined starting position. This movement-determining means comprises a signal-producing means, wherein the signal produced is indicative of a rotational position of the elongated rod 42.

In a specific embodiment, the signal-producing means comprises a magnet 43, such as a rare-earth magnet, having a pole (N-S) axis 432 oriented generally diametrically relative to the elongated rod 42. The magnet 43 is rotationally constrained relative to the elongated rod 42, so that a movement of the elongated rod 42 is communicated to the magnet 43 and the magnetic field is thereby indicative of the rotational position of the elongated rod 42. As shown in FIG. 10, the elongated rod 42 has a partial longitudinal bore

422 from the second end 424, within which the magnet 43 is dimensioned to reside. Close engagement is provided by a plug 44 that tightly encompasses the magnet's second end 434 and fits within the bore 422 sufficiently tightly that rotational movement is communicated between the magnet 43 and the elongated rod 42.

Within the normal range of physiological movements, it is expected that the magnet 43 will rotate fully approximately once per day. As previously disclosed, a measurement may be taken periodically with, for example, an electronic Hall- effect sensor 49, which detects the direction and magnitude of the magnetic field (FIG. 12). A microprocessor 48 in communication with the sensor 49 counts each time the field changes from north to south and relates that to one-half of the thread pitch, which gives a measure of the lengthening that has occurred. Alternatively, a sensor could be worn by the patient continually and checked as often as desired.

In still a further embodiment of an intramedullary skeletal distractor 14 (FIGS.l la-d), the first 20 and second 32 cylindrical members and the elongated rod 42 are essentially as described above. However, here the rod 42 is coupled to the second cylindrical member 32 so that a relative rotational movement of the rod 42 is translated into a relative axial movement between the rod 42 and the second cylindrical member 32.

Specifically, the rod 42 has a threaded portion 426 that is positioned to engage a complementarily threaded portion 362 of the second cylindrical member's bore 334. Thus a rotation of the rod 43 effects a longitudinal movement between the rod 42 and the second cylindrical member 32.

This embodiment 14 further comprises a rotation-effecting means affixed to the elongated rod 42 that is responsive to an extracutaneous rotation-producing signal. In a specific embodiment, the rotation-effecting means comprises a material responsive to a magnetic signal, such as a magnet 45, so that an extracutaneous circumferentially directed magnetic signal causes a rotation of the responsive material and a corresponding rotation of the elongated rod 42. In order to retain the elongated rod 42 within the bore of the first cylindrical member 20, a bushing 64 is provided that screws into a position adjacent the first cylindrical member's first end 202. The bushing 64 is dimensioned to fit over the elongated rod's first end 421 but not in movement- producing contact. A retaining screw 63 is then inserted into a threaded bore 423 in the rod's first end 421, serving to retain the busing 64 in surrounding relation to the rod 42.

The method of using this embodiment of the invention, therefore, comprises the step of delivering a magnetic signal from a second magnet 47 to the magnet 45. The magnetic signal should have a circumferential component that is sufficient to drive the magnet 45 and rod 42 rotationally. Operationally, the second magnet 47 is positioned extracutaneously next to the limb containing the distractor 14 and is rotated in a direction to achieve lengthening (shown as a movement of the magnet 47 counterclockwise in FIG. 11).

Referring now to FIG. 15, there is illustrated a perspective diagrammatic view of an improved measuring and monitoring device 750 usable with the skeletal distractors described above. In its most preferred form, the measuring device 750 is a highly sensitive digital magnetometer operable to detect and measure the strength of magnetic fields. The measuring device 750 senses the magnitude and direction of a magnetic field of a magnet, such as the magnet 43 of FIG. 10, which is embedded in the distractor 12 and rotates with rotation of the distractor. However, other uses for device 750 with other embodiments of skeletal distractors are not excluded. Moreover, device 750 may have application for other implantable devices in which orientation of at least a portion of the device is important to its function. Such devices may include, but without limitation, catheters, guidewires, percutaneaus instrumentation, and diagnostic instruments. Preferably, the device 750 is configured and operable such that the "user" of the device is the patient with an implanted skeletal distractor. Of course, the measuring device 750 may also be used by persons other than the patient for the same purposes, such as a doctor or caregiver of the patient. Thus, reference below to "user" below generically describes the person using the device 750. Reference is now made additionally to FIGS. 16(a) and 16(b), a block diagram and circuit board layout of measuring device 750. The measuring device 750 includes a control computer 762 interfacing with various components, displays, and sensors of the measuring device 750 in order to control its operation. The control computer 762 is preferably microprocessor based and is central to the operation and function of measuring device 750. It is contemplated that the term control computer refers generally to computational devices. For example, but without limitation, a control device may comprise one or more programmable logic arrays or microcontrollers without deviating from the invention. Control computer 762 includes a memory, an analog-to-digital (A/D) input, a communication port, and a number of digital I/Os. Control computer 762 is operable to manage and control many operations and functions in accordance with software algorithms and operational data typically stored within the memory (not shown) of control computer 762. The control computer 762 may use any type of memory, such as ROM, RAM, EPROM, EEPROM, FLASH MEMORY, and/or any other type of memory known in the art. Control computer 762 is electrically connected to A/D converter 775 at

I/Ol via signal path 796. A/D converter 775 receives analog signals from sensors 776, 778 along signal paths 792, 794, respectively. The analog signals correspond to readings of the strength of the magnetic field of the earth and of the magnet field of the magnet within the distractor, as described above. A/D converter 775 converts the analog signals from sensors 776, 778 to digital form for further processing by control computer 762 in accordance with software algorithm(s) stored in its memory. While conversion from analog to digital is utilized in a preferred embodiment, it will be understood that the analog sensor signal may be utilized for comparison without conversion to a digital output. Preferably, sensors 776, 778 are Hall-effect sensors operable to determine the position of the magnet within the distractor by measuring the magnitude and direction of the magnetic field of the earth and of the magnet. However, the present invention also contemplates other types of sensors capable of measuring and transmitting a signal corresponding to the relative elongation of the distractor or the position of the magnet disposed therein, including digital sensors. In a preferred embodiment, the magnet disposed in the distractor is a relatively weak two pole magnetic source. Further, it is contemplated that the indication magnet disposed in the implantable device may have more than two poles. Use of a multiple pole magnetic source may increase accuracy in determining orientation of the magnetic source. Thus, it is preferred that dual sensors 776, 778 be provided in order to compensate for the bias created by the magnetic field of the earth when sensing the magnetic field of the magnet.

In a preferred embodiment, shown in Fig 16 (b), sensors 776 and 778 are separated by a distance Dl . As is well known, the field strength of magnetic fields at a rate of _D ², where D is the distance from the magnetic source. Dl represents a distance greater than the anticipated distance the magnetic field of the magnetic source may be sensed, thus limiting or preventing the first and second sensors from both sensing the magnetic field associated with the implanted device. The sensors are spaced such that the anticipated magnetic field strength of the magnetic source disposed within the implant is sensed by the first magnet and not by the second magnet.

In a further aspect of the preferred embodiment, the positive pole 777 of sensor 776 is oriented the same as positive pole 779 of sensor 778 with respect to magnetic field M. Magnetic field M represents the background magnetic field generated by the earth and/or, for example electrical equipment. It will be understood that with the first and second sensors having substantially identical alignment or orientation in magnetic field M, the magnetic field present at each sensor should be substantially identical. Thus, the difference between the sensor values generated by the first and second sensor, respectively, provides the compensation factor for any measurement variation between the sensors. The first and second sensor combination provides improved sensing capabilities in comparison to a single sensor. With only a single sensor, the orientation of the sensor in the earth's magnetic field and any additional background magnetic fields (often produced by electronic equipment) would have to be maintained once the sensor had been calibrated away from the implanted magnetic source. Changes to the single sensor orientation in the background magnetic field present before measuring could adversely affect the measurements of the indication magnet located in the implanted device.

Utilizing more than one magnetic sensor preferably provides advantages over using only a single magnetic sensor. Using at least two sensors permits compensation for the difference between the magnetic sensors when they are in the same magnetic field. This field can be the earth's magnetic field, magnetic fields generated by adjacent electrical devices or a combination of these and any other magnetic field sources. Further, in a preferred embodiment the sensing components are Hall effect devices which vary an output voltage in response to a change in magnetic field strength. With a single sensor, weakening of the power source may also create a change in voltage from the sensor thereby generating a false indication of a change in magnetic field strength. With more than one sensor, taking the difference between sensor output voltages and utilizing this value as a compensating factor, reduces or eliminates the false readings even as the power source output voltage is weakened. Referring to Fig 16 (a), also connected to control computer 762 is memory portion 764. Preferably, the memory portion 764 is EEPROM type memory and is connected to control computer 762 via signal path 780 at the port I/O3. Memory portion 764 is operable to store data gathered by the sensors 776, 778 as processed by control computer 762, such as the elongation of the distractor, position and polarity of the magnet, and date and time of the measurement, for example. It is preferred that the data stored in memory portion 764 cannot be erased accidentally by the user or during loss of power to the device. In one embodiment, the memory portion 764 can store data associated with about 2000 measurements. More or less memory may be utilized to increase or decrease the number of measurements that may be stored. Other types of memory, such as RAM, EPROM, FLASH MEMORY, and/or any other type of memory known in the art are also contemplated, but preferably the memory should have high security for data retention even with a loss of primary power. Memory portion 764 may also be supplemented by external memory connected thereto (not shown). In an alternate embodiment, the memory of the control computer 762 is configured and/or partitioned to perform the function of memory portion 764.

The measuring device 750 also includes a housing 752 supporting a liquid crystal display (LCD) portion 756, and push buttons 757 operable by the user to initiate and control various functions and operation of the device 750, as described more fully below. The LCD 756 is electrically connected to control computer 762 via signal path 782 at port OUT3, and is operable to display alphanumeric messages to the user. It is comtemplated that any display device may be substituted with other suitable display devices, including but without limitation, flourescent displays, LED's, or analog displays. An energy manager unit 766 is electrically connected to control computer 762 at OUT4 via signal path 781.

Energy manager is controlled by a managing software to limit power consumption. A significant consumer of power are the two sensors. In a preferred embodiment, each of the sensors utilize approximately 12mA each. Thus, the energy manager is utilized to limit the overall amount of the time the sensors are energized. The second large user of power is the processor for the system. Thus, the processor unit is placed in a sleep mode after measurements to further conserve power. Further, energy manager 766 shuts down peripherals and components of device 750 when not in use or required. The internal clock and calendar system 774 of the measuring device 750 is coupled to control computer 762 at I/O2 via signal path 790. A back-up battery 785 is coupled to the clock 774 via electrical connection 789 and ensures continuous functioning of the clock 774 when the primary power source becomes too weak and during changing of the primary power source. The back-up battery 785 is preferably a lithium battery but other power sources such as, but without limitation, NiCad, NiMH or Supercaps may also be utilized.

The LED's 768 are electrically connected to control computer 762 at OUT2 via signal path 784. Preferably, the LED's 768 include at least a red light and a green light to provide various indications and prompts to the user as measuring device 750 performs various functions or requires input from the user. In a most preferred form, a first colored LED 768, such as red, corresponds to a north polarity, and a second colored LED 768, such as green, corresponds to a south polarity. Of course, it is understood that the colors of LED's 768 may be any color, and any one color may be assigned to correspond to the north polarity and a second color to correspond to the south polarity of the magnet.

The alarm 770 is electrically connected to control computer 762 at OUT1 via signal path 786. Alarm 770 is used, for example, to signal the user that it is time for a measurement to be taken, that the device is taking a measurement, or to alert the user that attention to some aspect of the device 750 or the distractor is required.

Control computer 762 may be connected to an external computer 772 via a communications bus 798 connected to communications input (COM) of control computer 762. Computer 772 may then be used to store the measurement data from memory portion 764 and to perform further processing and analysis of the data. These further functions are described in more detail below. This also allows data stored in memory portion 764 to be erased, thus providing storage space for additional measurements. In one embodiment, it is contemplated that the measuring device is connected remotely to a computer 772 via a telecommunications line and modem device (not shown). This would add further convenience for the patient and doctor in that data stored in memory portion 764 of the device 750 may be downloaded and observed by the doctor without requiring a visit from the patient.

Push buttons 757 are electrically coupled to control computer 772 at INI via signal path 788. In one preferred embodiment, push buttons 757 include three push buttons that are designated in the algorithm stored in control computer 762 to perform specific functions when pressed alone or in combination with another push button. In one embodiment push button 758 is designated as the "STORE" button; push button 759 is designated as the "RESET" button; and push button 760 is designated as the "RESTART" button. It is should be understood that other types and variations on the designations and number of push buttons as would occur to one skilled in the art are also contemplated herein. The push buttons 757 are operable so the user can signal the device 750 in accordance with the algorithm in the memory of control computer 762 to perform various operations, such as those discussed below.

Referring now to FIG. 17, a schematic drawing illustrates an example of the electrical interconnection of the various components of measuring device 750 discussed above with control computer 762. Various modifications to the schematic in FIG. 17 are contemplated herein as would occur to one of ordinary skill in the art.

The measuring device 750 provides extracutaneous measurement and monitoring of the lengthening rate of the distractor by the user. As previously mentioned, the sensor and magnet combination may have many uses in providing extracutaneous indication of the orientation of implants, instruments, and diagnostic equipment inserted into the body. In a preferred application, measurements can be taken by the patient, thus minimizing the need for doctor visits and X-rays. The measuring device 750 has a control computer and clock/alarm system that increases the probability of the patient taking measurements in a timely manner. The memory portion 764 stores the data associated with each measurement for evaluation and observation by the user and/or by the doctor. Since the sensors detect rotation of the magnet, it may be important to know the interval between measurements, particularly if one or more measurements have been skipped by the patient. This increases the effectiveness of using intermedullary skeletal distractors and enhances the ability of the doctor to evaluate and treat the patient. The measuring device 750 also eliminates problems associated inaccurate recordation and retention of measurement data. The measuring device 750 also provides for accurate measurement of the magnetic field of the magnet disposed within the implant by compensating for the bias created by the magnetic field of the earth.

Referring now to FIGs. 18(a)- 18(b), one preferred embodiment software algorithm for determining and storing measurements and data relating to movement of the distractor is shown in flowchart form. The algorithm 850 of FIG. 18(a) is preferably executable by the control computer 762 to receive signals or readings from sensors 776, 778 and process the signals to determine movement of the distractor and to store the measurement along with other data associated with the measurement in memory portion 764.

Algorithm 850 starts at step 811, and execution continues at step 812 when measuring device 750 is powered on by the user and the control computer 762 initializes the device 750. Preferably, the user turns the device on by pressing the "RESTART" button 760, and the measuring device 750 powers off automatically after completion of the measurement or other function. In one alternate embodiment, an ON/OFF switch is provided. Once measuring device 750 is powered on, LCD 756 is reset and various messages and/or information may be displayed in LCD 756. For example, LCD 756 can display the current date and time, preferably in a 24-hour mode. The LCD 756 also displays other information upon powering and initialization of the device 750. For example, the total number of measurements taken by the device may be displayed (indicating the amount of data storage available in memory portion 764), and/or the total elongation of the distractor since the initial measurement. These initial messages may be displayed for a preset time or until removed by the user. Once the initial messages are cleared, control computer 762 prompts LCD 756 to display the polarity of the last reading of the magnet. Preferably, LED 758 provides an indication of the polarity by illuminating a particular color associated therewith. For example, north polarity may be indicated by illuminating a red color in the LED 758, and south polarity may be indicated by illuminating a green color in the LED 758.

From step 812, algorithm execution continues at step 822, where the user has the option of observing stored measurement data in memory portion 764 or taking a measurement. If it is desired to observe stored measurement data, execution of algorithm 850 proceeds at step 824, where the operation of the device 750 is controlled by the algorithm 855 of FIG. 18b. It should be understood that data corresponding to each measurement of distractor elongation is recorded in memory portion 764 for later observation and evaluation. In addition to downloading the measurement data from memory portion 764 to a computer 772, the stored measurement data can be observed on LCD 756 in accordance with algorithm 855. The data stored for each measurement and displayed by LCD 756 may include various information. For example, the data for each measurement can include the date and time of the measurement, an index or count of the measurement, the elongation of the distractor, the polarity of the magnet at the measurement, and/or the strength of the magnetic field. Measurements may be sequenced on the display at a preprogrammed rate.

Referring now to FIG. 18(b), algorithm 855 begins execution at step 852 and proceeds at step 854 where the user initiates observation of the data stored in memory portion 764. In order to initiate observation of the data, the user presses and holds RESTART button 760. The user then presses and holds pressed STORE button 758. With both buttons pressed, the user then releases RESTART button 760. LCD 756 then displays a message, indicating the device 750 is ready to access the stored data in the observe data mode. The user may then release the

STORE button 758. Execution of algorithm 855 then proceeds at step 857 to send the unit's serial number and at step 856, where the stored data is retrieved from memory portion 764 by control computer 762 and sent to the control computer 762 for display on LCD 756. The data is displayed on LCD 756 in a sequencing manner, scrolling the data from the initial measurement to the last measurement. During scrolling of the data, execution of algorithm 855 proceeds at step

858. The user has the option to stop the scrolling of data to observe a particular measurement. If it is desired to observe a particular measurement, execution of algorithm 855 continues at step 860. The user stops the scrolling by pressing the RESET button 759 to freeze data on the LCD 756. When it is desired to continue scrolling data, execution of algorithm 855 continues once again at step 858. When all the measurements are displayed, execution of algorithm 855 continues to step 862 where execution ends and control computer 762 automatically powers off the device 750.

Returning now to FIG. 18(a), if it is not desired to observe data stored in memory portion 764, execution continues at step 813 where the measuring device 750 enters an active cycle. In one embodiment, execution of the algorithm 850 at the active cycle of step 813 has control computer 762 proceeding to analyze measurements recorded in memory portion 764 during the previous days. If a particular day includes an unacceptably slow or fast elongation of the distractor, measuring device 750 will signal the user via alarm 770. LCD 756 will indicate the date of the particular day, and the amount of lengthening that occurred during that day. The measuring device 750 will also prompt the patient to call the doctor's office via an alphanumeric message in LCD 756.

From step 813, execution of algorithm 850 proceeds to step 814 where control computer 762 determines whether the battery powering device 750 has a voltage above a first threshold voltage. Preferably, the measuring device 750 checks the condition of the main power source battery to ensure accurate measurements when it is powered on. If the voltage of the power source is above the first voltage, the program continues at step 818. If the battery is below the first threshold voltage, then execution algorithm 850 continues at step 815 where it is determined whether the battery is too weak for reliable operation. If the battery is below a second threshold voltage necessary for reliable operation, the sensitivity of the sensors may be degraded and unable (because of the potential for inaccurate voltage differences from the sensors) to provide reliable accurate readings of the magnetic field. Further, low voltage from the power source reduces the low-end sensitivity of the device for sensing weak magnetic fields. If the battery is too weak, algorithm execution continues at step 816 where MESSAGE 1 is displayed in LCD 756, indicating to the user the battery needs to be replaced. As discussed above, data storage is preferable maintained without power and minimal operations, such as clock and calendar functions, may continue with power supplied from backup battery 785. Execution of algorithm 850 then proceeds to step 834 and the measuring device 750 powers down in sleep mode. If the main battery is above the second voltage threshold, then program execution continues from step 815 to step 817, where control computer 762 prompts LCD 756 to display MESSAGE2 to indicate to the user that the battery is low, and that the main battery will soon need to be replaced. However, the measuring device 750 is still functional, and program execution continues to step 818.

In a preferred embodiment of the device, a 9 volt power supply is provided. The first threshold voltage is set at approximately 7.0 volts. The second threshold voltage is set at approximately 6.5 volts. At step 818, algorithm 850 directs control computer 762 to enter a measuring mode, where the device 750 is readied to take a measurement of the magnitude and direction of the magnetic field of the magnet positioned within the distractor. When the measuring device 750 is not positioned adjacent the measurement location on the patient, the sensors 776 and 778 both provide a voltage signal corresponding to the background magnetic field M. The difference between the signal supplied by sensor 776 and the signal supplied by sensor 778 is determined. The value of the difference between the signals is designated as a compensation value. This compensation value is used to compensate for the variation in the first and second sensors when they are placed in the sensing mode adjacent the indication magnet disposed within the patient. In effect, the system is calibrating the sensors in the substantially uniform background magnetic field M before a measurement is taken. This calibration also takes into account the variations of sensor performance at various power supply voltages experienced as the power supply weakens. After calibration, device displays "0" indicating it is ready for measurement. From step 818, execution of algorithm 850 proceeds at step 819 where the user ensures the device is ready to take a measurement. If the LCD 756 does not display "0" when the device 750 is positioned away from the measuring point, the device 750 is not ready to take a measurement. Execution of algorithm 850 proceeds to step 820, by the user pressing the "RESET" push button 759 to return execution of algorithm 850 to step 818. Execution of algorithm 850 is repeated as described above from step 818 until "0" is displayed in LCD 756, indicating the device 750 is ready to take a measurement. Once the device displays "0", alarm 770 preferably provides a hunting sound indicting that the device is ready for a measurement. The hunting sound ceases once a difference in magnetic field strength is detected indicating a measurement is being taken.

During sensing, sensor 776 is positioned adjacent the indicator magnet attached to the implant. The change in magnetic field adjacent sensor 776 creates a change in the sensor signal voltage. Execution of algorithm 850 proceeds from step 819 to step 826. The sensor 776 produces a signal corresponding to the intensity of the magnetic field of the magnet. The signal is transmitted to control computer 762 for further processing to compute movement of the distractor based on magnetic field strength and polarity. As previously described above, second sensor 778 is positioned on the circuit board a distance Dl from the first sensor 776. The circuit board is positioned in sensor device 750 such that sensor 778 is spaced more distant (at least Dl) from the magnetic source. In a preferred aspect, distance Dl is approximately 3 inches and is intended to inhibit sensor 778 from sensing any significant magnetic field generated by the magnetic source. Thus, sensor 778 provides a signal corresponding the background magnetic field. This signal from sensor 778, the signal from sensor 776, and the compensation factor are utilized by the computer to determine the indicator magnets field strength and polarity. Preferably, the first signal is reduced by the second signal and the compensation factor to compensate for differences in the sensors. The resulting signal represents the signal corresponding to the magnetic field strength of the indicator magnet. Additionally, the control computer 762 preferably records the date and time of the measurement, the count of the measurement, and the polarity of the magnet in memory portion 764. An indication that a reading has occurred may be communicated by control computer 762 to the user through LCD 756 and/or illuminating LED's 768. Additionally, a characteristic beeping sound from alarm 770 could be produced. Different frequency beeps may be utilized to distinguish between north and south polarity. Preferably, the intensity of the color display from LED 768 duration of the beeping sound of alarm 770 will be proportional to the intensity of the magnetic field.

Once a measurement is taken, execution of algorithm 850 proceeds at step 828. If the measurement indicates zero, the current elongation is computed and displayed. Then algorithm execution returns to step 818 where the algorithm 850 returns control computer 762 to the measuring mode at step 818, and execution of algorithm 850 continues as described above.

If the measurement indicated that elongation has occurred, execution of algorithm 850 proceeds to step 830 where the amount of elongation is computed. It should be understood that step 830 may also be performed with step 826 if a measurement that is not "0" is taken. In order to compute the elongation, the algorithm relates the rotational position of the magnet within the distractor to the pitch of the threads upon which the implant rotates and to the position of the magnet at the previous measurement.

After computation of the elongation, execution of algorithm 850 continues at step 832 where the measurement and data associated therewith is stored in memory portion 764. In order to enter the calculated elongation into memory portion 774, the user depresses the "STORE" button 758. LCD 756 then displays to the user the magnet position and the computed elongation of the distractor. Execution of algorithm 850 then continues at step 834 where execution of algorithm 850 ends and control computer 762 powers down the device 750 into a sleep mode. In a most preferred embodiment of the algorithm 850, as shown in steps 835 and 837, control computer 762 is programmed to sound alarm 770 as a reminder to the user to take a measurement within pre-programmed hours. Preferably, measurements are taken every three hours between the hours of 9:00 a.m. and 9:00 p.m. The exact timing of the alarm will vary somewhat depending on when the last measurement was taken. In a preferred embodiment, alarm 770 beeps twice every minute until a measurement is taken. Alternatively, the alarm may be programmed to generate a tone for a desired period (e.g. 10 seconds) followed by a long silence (e.g. 5 minutes). The alarm continues until a user restarts the program at step 811. Further, the unit may sound the alarm for a set period of time (i.e. 30 minutes) and then record a no measurement message in memory to alert the health care provider that a measurement was skipped. This feature may save battery life. If desired or necessary, measurements may also be taken at any time during non-alarm hours by powering on the measuring device 750 manually and proceeding as described above.

The measuring device 750 may also be used to track the position of the magnet without taking a measurement. In order to do this, the device 750 is powered on, and control computer 762 executes algorithm 850 from steps 811 to 818 as described above. To track the magnet, the device 750 is moved slowly the perimeter of the leg with sensor portion 754 adjacent the magnet. As it is moved, the sensor 778 detects the magnitude and direction of the magnetic field. The LED 768 will alternately display red or green, depending on the polarity of that portion of the magnet adjacent the sensor. The point of maximum intensity of the LED display indicates the position of the magnet within the distractor. However, the point of maximum intensity of the magnetic field is not necessarily aligned with the poles of the magnet because the intensity of the magnetic field depends on the orientation of the magnet with respect to its polarity and the distance from the magnet.

The user may also want to determine whether the magnet in the distractor, and thus the ditstractor, is moving without taking a measurement. A first method for making this determination is accomplished by tracking the poles of the magnet. As device 750 is moved around the leg with sensor portion 754 adjacent the magnet, the LED's 768 will identify red and green regions. These regions are separated by "zero" or "dead" regions on either side of the leg where LED's 768 display no color. At these locations, the sensor 778 is recording a 0 reading. The orientation of a pole is approximately halfway between the zero regions. To pinpoint the orientation of the pole corresponding to the red LED 768, the measuring device 750 is moved around the leg with sensor portion 754 adjacent the magnet to determine where the red LED area begins and ends. The pole position is in the middle of that region. The same is repeated to determine the location of the pole corresponding to the green LED. To determine if the magnet is rotating, the location of a particular pole is located as described above. The location method described above is then repeated at a later time to determine if the location of the pole of the magnet has changed. Alternatively, movement of the magnet within the distractor may be determined by tracking the "zero" or "dead" regions. In this alternative method, the particular "zero" or "dead" region being tracked is identified by noting which color comes first and which comes second as device 750 is moved around the leg in the same direction.

It should be understood that recording measurements or reading at points other than specified measurement location on the patient will result in incorrect data being stored in the memory portion 764. Thus, such readings of the magnet position should not be recorded, and magnet tracking is not a substitute for measurement, which must be taken at regular intervals at the same location to enable proper monitoring and evaluation of elongation of the distractor.

Although the algorithms 850 and 855 instruct control computer 762 to automatically power off the device 750, it may be desirable to power off the device 750 prior to recording any measurements. One alternate method for directing control computer 762 to power off the device 750 requires the user to press RESTART button 710 and hold it down, and then press the RESET button 709. While holding both buttons down, the RESTART button 710 is released, and control computer 762 powers off device 750. The control computer 762 is also programmable to display various error messages in LCD 756 to provide information to the user. For example, clock 774 may be programmed to display an error message instead of the correct time in LCD 756. Th error message indicates a clock battery failure. The device 750 is still functional to sense the position of the magnet and measure the elongation of the distractor. The error message, however, will indicate to the user that, until the device 750 is serviced, measurement information will be needed to be recorded manually, and that the alarm 720 will not function to prompt the user to take measurements. In another example, LCD 756 indicates to the user that memory portion 764 is at capacity, and future measurements cannot be stored therein. Again, the device 750 is still operable to take measurements, but the data will have to be recorded manually by the user.

Referring now to FIG. 19, a method for downloading data from memory portion 764 is illustrated in flowchart form. Flowchart 925 begins at step 926, and continues at step 928 where measuring device 750 is connected to a computer 772, as described above. Preferably, computer 772 is PC based, and includes a terminal program, such as ProComm or TERMINAL. The downloading of data from memory portion 764 of device 750 is preferably performed by the physician or other qualified person, who will then store the data permanently for analysis and/or processing. Alternatively, the stored data may be transferred to a monitor for viewing or to a printer for generating a hard copy. Further, it is desirable that each unit have a unique identification number either electronically imprinted in the circuitry or imprinted on the housing exterior, or both. This identification number will permit the physician or other qualified person to track the source of the information to an individual patient.

Once connected, the method of flowchart 925 continues at step 930 where a display prompt is provided to the user on the monitor of computer 772. The prompt is produced by pressing the RESTART button 760 after connecting device 750 to computer 772 and running the terminal program. The method of flowchart 925 then proceeds to step 932 where the user enters an indication of the desired operation to be performed at the display prompt. There are many functions which the user may perform at step 934 with the device 750 connected to the computer 772. These functions are initiated by the user of computer 772 providing various commands at the prompt described as follows. For example, in one function of computer 772, the user may reset the clock and calendar. The computer 772 requests various entries for the date, time, etc. When the entries are completed, the device 750 will automatically restart to verify and update the clock 774 accordingly. Another function that may be performed by computer 772 is to reprogram and enable/disable the alarm settings and the alarms. In another function, computer 772 initializes the memory portion 764 to erase the data stored therein. When the memory portion 764 is initialized, the memory of control computer 762 retains the polarity and position of the magnet corresponding to the last measurement taken by device 750. New measurements by device 750 continue using the data from the last measurement. Initialization of device 750 should only be performed after the data in memory portion 764 has been downloaded to computer 772, as described below. Preferably, memory portion 764 is configured so that if initialization occurs prior to transferring the data, the data may still be recovered if no new measurements are taken.

In yet another function that may be performed by computer 772 with device 750, the memory portion 764 of device 750 may be erased. When this function is performed, measurement data in memory portion 774 is erased completely. Again, transfer of data from memory portion 764 should occur prior to performing this function. Complete erasing of memory portion 764 is completed prior to giving the device 750 to a new patient. In another function, measurement data from the memory portion 764 of the device 750 can be transferred and saved on the computer 772 for further processing. After initiating this function at the display prompt of computer 772, the user presses the RESTART button 760 of device 750 and holds pressed. The user then presses the STORE button 758 and holds pressed. With both buttons now pressed, the RESTART button 760 is released. A message on LCD 756 is displayed, and the user may then release the STORE button 758. The device 750 will start sequencing through the past measurement data and transfer the measurement data to computer 772.

After the desired function at step 934 is selected and completed, the method of flowchart 925 continues to step 936, and control computer 762 powers off the device 750.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.

Claims

What is claimed is:

1. A device for extracutaneously measuring movement of an implantable device in the human body, comprising: a control computer having a memory associated therewith; a first sensor for sensing a background magnetic field, said first sensor providing a first signal to said control computer; and a second sensor for sensing a magnetic field of a magnet disposed within the implantable device, said second sensor providing a second signal to said control computer; whereby said control computer determines a corrected value of the magnetic field of the magnet disposed within the implantable device from said second reading by using said first reading to correct for bias in said second reading caused by the background magnetic field.

2. The device of claim 1, further comprising a clock connected to said control computer, whereby said control computer is operable to define measurement data by associating a date and time with said corrected value, whereby said control computer may compare corrected values to determine movement of the magnet disposed within the implantable device.

3. The device of claim 2, wherein said device further comprises an alarm connected to said control computer, said control computer operable with said alarm to provide an audible signal to a user of the device.

4. The device of claim 3, wherein said audible signal reminds the user to take the extracutaneous measurements at predetermined clock intervals.

5. The device of claim 1, further comprising an energy manager connected to said control computer to determine sensor activity and reduce power consumption.

6. The device of claim 2, further comprising a first power source and a second power source, said first power source connected to said control computer and said second power source connected to a clock and calendar segment of said control computer, said second power source supplies said clock and calendar segment with power upon loss of power from said first power source.

7. The device of claim 2, further comprising an alphanumeric display connected to said control computer.

8. The device of claim 1 , wherein said memory portion has a capacity to store measurement data associated with at least 2000 readings.

9. The device of claim 1, wherein said control computer including means to compensate for variations in sensitivity between said first and second sensors.

10. The apparatus of claim 1, wherein said memory portion is an EEPROM.

11. The device of claim 1 , further comprising an analog-to-digital converter connected between said first and second sensors and said control computer, said converter receiving first and second readings from said sensors in analog form and converting said first and second analog signals to digital form before sending said signals to said control computer.

12. The device of claim 1, wherein said correction for bias created by the background magnetic field allows the control computer to determine the change in rotation of the magnet.

13. A method of determining movement of an internal fixation apparatus within a body, the internal fixation apparatus having a magnet indicative of the relative position of the fixation apparatus, comprising: providing a device for measuring and storing measurement data relating to the movement of the fixation apparatus, the device having a sensor portion having a first and second sensor, a control computer connected to the sensor portion to receive at least one reading from said sensor portion, and a memory portion connected to said control computer for storing the measurement data; initializing the device by retrieving from the stored measurement data the last position of the magnet, and by further having the first sensor take a reading of the earth's magnetic field; positioning the first sensor of the device proximate the magnet, the second sensor maintained relatively distant from the magnet; taking a reading of the magnet magnetic field with the first sensor a reading of the earth's magnetic field with the second sensor; calculating the magnetic field strength of the magnet by compensating ro the earth's magnetic field; and determining the movement of the fixation device by relating the position of the magnet within the fixation apparatus to the last position of the magnet retrieved from the memory portion.

14. The method of claim 13, further comprising storing the determination of movement in the memory portion.

15. The method of claim 14, wherein providing a device includes providing a device having a clock connected to the control computer, and further including associating a date and time with the determination of movement and storing therewith in the memory portion.

16. The method of claim 15, further comprising associating a magnet polarity and a magnet position with the determination of movement and storing therewith in the memory portion.

17. The method of claim 16, further comprising providing an option to view stored data prior to taking a reading.

18. The method of claim 16, further including downloading the data from the memory portion to a computer.