In FIG. 1 there is a single, rod-shaped permanent magnet designated by 1. Its direction of magnetization is perpendicular to its axis. Preferred type magnets are rare-earth permanent magnets, coated to be biocompatible. Other possible materials are platinum-cobalt alloys, or, less expensively, ferrites or Alnico. The shaft 2, which can be a Chrome-Nickel steel, is attached by adhesive to the permanent magnet 1 and has at its ends two screws 3 that engage the internal threads on the two tubular structures 4. The preferred material for these are titanium alloys or other non-ferromagnetic alloys that allow the external magnetic field from the external magnetic apparatus to penetrate to the permanent magnet of the prosthesis. The thread between 3 and 4 can be coated with Teflon or lubricated with a biocompatible lubricant such as graphite powder. 5 is a ratchet mechanism, shown enlarged in FIG. 3. The cap 6 serves to facilitate the assembly. The borie, for instance a femur, is shown in outline as 7.
FIG. 2 shows the same prosthesis as FIG. 1, but now in its fully expanded position. 1 to 7 are the same components as in FIG. 1. Where the new growth of the bone has occurred when the prosthesis has been expanded, is shown by 8. It is understood that the length of the prosthesis and therefore the length of the new growth can be anything between the extremes shown in FIGS. 1 and 2, respectively.
FIG. 3 is an enlarged section, taken perpendicular to the axis of the rod, of the ratchet mechanism that serves to prevent the rod from getting shorter when subjected to variable loads from motions by the patient. 2 is a continuation of the shaft and is connected by adhesive or similar means to the permanent magnet. 4 is part of the tubular structure that is also shown in FIGS. 1 and 2. The component 9 is an elastomer with prongs that engage 2. As shown, the combination of 2 and 9 permit rotation of 2 in a counter-clockwise direction that produces a lengthening of the prosthesis, but prevents rotation in the clockwise direction that would cause a shortening of the prosthesis. This type of ratchet is particularly well suited for prostheses with a small diameter, typical of rod-type prostheses. It is understood, however, that a conventional, metallic racket with spring loaded prongs could also be used.
FIG. 4 is a section and view of the external magnetic apparatus, version 1, taken crosswise to the limb of the patient. This apparatus incorporates one or several electromagnets, powered with direct current, with soft iron cores 10 and electric windings 11. The soft iron pole pieces 12 produce the external magnetic field that interacts with the permanent magnet in the prosthesis. The directions of the current in the windings 11 are such as to produce a magnetic North in one of the pole pieces and South in the other. The tube 13 contains the limb of the patient. The assembly, other than 13, is supported by rollers 14 and can be rotated either by hand or by an electric motor around the limb. Slip rings 15 combined with conventional electric brushes that are stationary transmit the current from a stationary direct current power supply to the electromagnets. The rollers 14 run on a stationary ring 16 that is supported by the structural member 17 that is attached to a table on which rests the patient.
FIG. 5 represents a second version of the external magnetic apparatus. In this version, the electromagnets, having cores 10 and windings 11, are powered by short, but intense current pulses. There are at least four electromagnets, here labeled by S1 to S4. The directions of the currents in the winding change from pulse to pulse in such a manner that the same pole piece 12 can be a magnetic North or South. The effect of switching the current directions in the electromagnets is such as to produce an external magnetic field that rotates in space by 90 degrees, inducing a corresponding rotation of the permanent magnet in the prosthesis. The components 13 and 17 are the same as in FIG. 4. There are at four Hall-current sensors 18 that sense the magnetic field of the permanent magnet in the prosthesis and allow the physician to determine the rotational position of the permanent magnet in the prosthesis without need for x-ray examination. To avoid interference from the more intensive external magnetic field, these sensors are activated only between the current pulses.
Although somewhat more complicated than version 1, there are two advantages to version 2. First, the peak currents in a compact device can be much larger than the direct currents in version 1, resulting in a larger torque exerted on the permanent magnet in the prosthesis, which in turn produces a larger force available for the expansion. Because of the short duration of the pulses, with longer pauses between pulses, high peak currents can be tolerated without overheating the windings of the electromagnets. Second, the external magnetic apparatus is simplified since it does not require the mechanical rotation about the patient's limb.
FIG. 6 is an electrical circuit diagram that shows the charging, via a resistor R, and discharging of a capacitor C. The double switch S is either open, or closed to direct the current into the electromagnet L in one direction, or closed to direct the current in the opposite direction. Such circuits can be used separately for each of the electromagnets shown in FIG. 5.
FIG. 7 is a timing diagram for the current pulses, I, that activate the electromagnets S1 to S4 that are shown in FIG. 5. The letter t and the associated arrow indicate the time and its direction. The horizontal lines delineate the start and cut-off of the pulses by the switch S shown in FIG. 6. By example, 19 is defined as a positive pulse, 20 as a negative one. The sequence of pulses that is shown rotates the permanent magnet in the prosthesis in four steps of 90 degrees through a complete revolution. Each step of 90 degrees requires two sets of pulses (as will be clear from FIG. 8). Depending on the desired amount of expansion of the prosthesis, the sequence of pulses can be either shorter than shown, or can be repeated wholly or in part.
FIG. 8 is a schematic hysteresis diagram for a permanent magnet in the prosthesis. The abscissa H0 is the magnetic field strength produced by the external magnetic apparatus at the center of the permanent magnet. The ordinate Mpm is the (variable) magnetization of the permanent magnet. A set, at a given time, of the four current pulses to the four electromagnets will move a point in the diagram from A (at the beginning of the set of pulses) to B to C (at the end of the set of pulses). The next set of pulses will move the point from C to D and back to A. These pairs of sets of pulses can be repeated as is required by the desired amount of expansion of the prosthesis.
While particular forms of the invention have been illustrated and described, it will be apparent that various modifications can be made to the present invention without departing from the spirit and scope thereof
Patent Application
20050261779
