Abstract:
The present invention relates to the electrical stimulation of bone growth utilizing implantable bone fixation devices and implants to which are attached a screw of nonconductive material powered by a battery for the purpose of creating an electrical-magnetic field to promote bone healing and bone formation. The electric magnetic field is directed to the bone around the device through a battery of a rechargeable type and can include a radio frequency identification device. A constant current is generated in a range of 5-20 micro amperes to stimulate bone healing and bone formation.

Description:
RELATED APPLICATIONS 
     This is an application claiming priority from U.S. Provisional Application No. 60/907,622 filed Apr. 11, 2007. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     REFERENCE TO SEQUENCE LISTING, A TABLE OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX 
     None. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention is generally directed toward a battery powered implantable bone growth stimulator and more specifically to a threaded screw made of nonconductive material upon which a hermetically sealed battery casing is mounted to provide electrical stimulation for bone growth. 
     2. Background of the Invention 
     The present invention is directed toward the electronic stimulation of bone (osteogenesis) through or around an orthopedic bone fixation device with an attached implantable bone growth stimulator. It has long been known that the application of electric currents (electric stimulation) can speed bone growth and healing. The electronic stimulation of bone growth has been used in the treatment of fractures, nonunion of bone and to hasten rates of bone fusion as early as the 1800&#39;s. Yasuda, in the 1950&#39;s in Japan studied the effect of electricity in the treatment of fractures. E. Fukudain “On the piezoelectric effect of bone”, J Physiol. Soc. Jpn. 12:1158-62, 1957, and Yasuda, J. Kyoto Med. Assoc. 4: 395-406, 1953 and showed that electric signals could enhance fracture healing. Both direct current capacitively coupled electric fields and alternately pulsed electro magnetic fields affect bone cell activity in living bone tissue. 
     Bone has bioelectrical properties with naturally occurring generated stress potentials. When the bone is stressed, it will carry an electropositive charge on the convex side and an electronegative charge on the concave side. Wolff&#39;s Law demonstrates that bone will form new bone in areas of compression and bone will be resorbed in areas of tension. This biological response to stress in bone creates mechanically generated electrical fields or “strain related potentials. Areas of active growth in bones carry an electronegative charge. When a bone fractures, the bone becomes electronegative at the fracture site. On a cellular basis it has been discovered that osteoblasts are activated by electronegative charges. Research on the effects of electrical forces on bone cells in bone formation and healing has demonstrated that bone healing can be hastened and enhanced by electricity. Studies have shown that by implanting an electrical stimulation device and applying an electrical current around the bone, that bone formation is increased around the cathode (negative electrode) and decreased around the anode (positive electrode). Further research of the use of bone growth stimulators has discovered that the optimal current for bone growth with electrical stimulation is believed to be between 5 and 20 micro amperes. 
     K. S. McLeod and C. T. Rubin in “The effect of low frequency electrical fields on osteogenesis”, J. Bone Joint Surg. 74a:920-929, 1992, used sinusoidal varying fields to stimulate bone remodeling. They found that extremely low frequency sinusoidal electric fields (smaller than 150 Hz) were effective in preventing bone loss and inducing bone formation. They also found strong frequency selectivity in the range of 15-30 Hz. Fitzsimmons et al. in “Frequency dependence of increased cell proliferation”, J Cell Physiol. 139(3):586-91, 1985, also found a frequency specific increase in osteogenic cell proliferation at 14-16 Hz. 
     U.S. Pat. No. 5,292,252 issued Mar. 8, 1994. discloses a stimulator healing cap powered by an internal small battery. The cap can be reversibly attached to a dental implant, and stimulates bone growth and tissue healing by application of a direct current path or electromagnetic field in the vicinity of bone tissue surrounding the implant, after the implant is surgically inserted. 
     Another dental device described in U.S. Pat. No. 4,027,392 issued Jun. 7, 1972 discloses an embodiment of a bionic tooth powered by a battery including an AC circuit. The microcircuitry indicated by its  FIG. 3  is not shown as being incorporated within the cap. 
     Another related device is disclosed by in U.S. Pat. No. 5,738,521 issued Apr. 14, 1998 which describes a method for accelerating osteointegration of metal bone implants using AC electrical stimulation, with a preferably symmetrical 20 mu·A rms, 60 KHz alternating current signal powered by a small 1.5 V battery. However, this system is not a compact, self-powered stimulator cap, but is externally wired and powered. 
     Osteogenetic devices are as described in U.S. Pat. No. 6,605,089 issued Aug. 12, 2003 which discloses a self contained implant having a surgically implantable, renewable power supply and related control circuitry for delivering electrical current directly to an implant which is surgically implanted within the intervertebral space between two adjacent vertebrae. Electrical current is delivered directly to the implant and thus directly to the area in which the promotion of bone growth is desired. 
     U.S. Pat. No. 6,034,295 issued Mar. 7, 2000 discloses an implantable device with a biocompatible body having at least one interior cavity that communicates through at least one opening with the surrounding body so that tissue surrounding the implantable device can grow through the opening. Two or more electrodes are contained within the device having terminals for supplying a low-frequency electrical alternating voltage and at least one of which is located inside the cavity. U.S. Pat. No. 5,030,236 issued Jul. 9, 1991 also discloses the use of electrical energy that relies upon radio frequency energy coupled inductively into an implanted coil to provide therapeutic energy. However, none of these devices perform satisfactory osteogenesis promotion, while leaving the implant member or stem essentially unchanged in appearance and mechanical properties. 
     The art that relates specifically to bone growth stimulation by small, self powered electrical means is very limited and most of the bone graft stimulation has been undertaken using power sources located outside the patient&#39;s body. Another problem that occurs when the implant is self powered is that the power short circuits against the metal screw or device. 
     There is thus a widely recognized need for a practical, self-powered osteogenesis implant that can generate electrical stimulation signals. It would also be extremely advantageous that such implants, when used for example in hip or knee implants, should require minimal changes to both appearance and mechanical integrity and function of the implants. The primary goal of such devices would be to increase bone density and implant bone contact ratio around any new implant as a routine common clinical practice. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided an osteogenesis device including an implant member in the nature of a nonconductive screw having a battery cap mounted thereto to provide electrical signals from the cap to the tip of the screw to function as an electrical bone growth stimulation device. In another embodiment, a universal cap mount with an internal electrical source is mounted on a standard pedicle screw to provide electrical bone growth. 
     It is still another object of the invention to provide a self container power source and generating circuit in the implant. 
     It is yet another object of the invention to provide a powered electrical screw implant which does not short out when used for electrical stimulation. 
     It is another object of the present invention to provide an electrical bone growth promotion implant in which an active cathode is fully contained within the bone fusion mass. 
     It is a further object of the invention to provide a method of fixation of fractures that not only stabilizes the bone but also enhances bone healing with the use of electricity that can be applied through or around the implant. 
     It is yet another object of the invention to provide an implant to which a bone growth stimulator can be attached to enhance bone formation at spinal fusion sites. 
     It is still another object of the invention to provide a self powered implant with a tissue-contacting body having an external surface in contact with biological tissue and having a hollow enclosure, a conductive element in electrical communication with the hollow enclosure and electrically isolated from the external surface, and an electrical stimulation mechanism located within the hollow enclosure for providing electrical stimulation to the biological tissue through the conductive element. 
     It is yet another object of the present invention to provide an electrical bone growth promotion implant in which the power source can be wholly or partially supplied or recharged by externally applied sources; 
     It is another object of the invention to provide an implantable bone growth stimulator implant that can be attached to an intramedullary nail or rod to enhance bone formation and healing at fracture or fusion sites. 
     It is still another object of the invention to provide an implantable bone growth stimulator that can provide a D. C., constant current source. 
     It is yet a further object of the present invention to provide an implantable bone growth stimulator that can be attached to an orthopedic implant in combination with an internal or external implantable cathode and anode that are sized to enhance bone growth stimulation. 
     It is another object of the present invention to provide an implantable fixation implant for cooperation with an internal power supply where the fixation implant serves to treat avascular necrosis; 
     It is still another object of the present invention to provide an implantable bone growth stimulator and orthopedic implant to which a radio frequency identification device can be embedded or attached. 
     These and other objects, advantages, and novel features of the present invention will become apparent when considered with the teachings contained in the detailed disclosure along with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the inventive electrical bone screw assembly with component cap parts shown in phantom; 
         FIG. 2  is an exploded view of the bone screw assembly of  FIG. 1 ; 
         FIG. 3  is a perspective view of the electrical bone screw assembly shown in  FIG. 1  showing the cap casing stem threadably mounted in the screw head and the cathode mounted in the lumen of the screw shaft; 
         FIG. 4  is perspective view of the battery cap housing; 
         FIG. 5  is a perspective view of the cathode mounted in the cap housing stem; 
         FIG. 6  is the assembled electrical screw assembly; 
         FIG. 7  is a schematic of the battery casing, current circuit and lead wire; 
         FIG. 8  is an electrical diagram of the circuit constant current source; 
         FIG. 9  is a perspective view the driver used to insert the electrical bone screw; 
         FIG. 10  is an enlarged view of the driver tip of the driver shown in  FIG. 9  inserted into the screw head to apply torque to the screw; 
         FIG. 11  is a perspective view of the bone growth stimulator assembly attached to an internally threaded screw mounted in a bone plate; 
         FIG. 12  is a perspective view of an external lead which can be mounted on the electrical screw assembly forming a cathode; 
         FIG. 13  is a perspective view of the external cathode lead shown in  FIG. 12  attached to the electrical screw assembly shown in  FIG. 11 ; 
         FIG. 14  is a perspective view of an inventive pedicle screw electrical stimulation device using the cathode lead shown in  FIG. 12  mounted to spinal vertebrae; 
         FIG. 15  is an enlarged view of the pedicle screw electrical stimulation device of  FIG. 14 ; 
         FIG. 16  is a perspective view of the pedicle screw electrical stimulator device of  FIG. 15  showing the universal mount and battery casing in phantom; 
         FIG. 17  is a cross sectional view of the pedicle screw electrical stimulation device of  FIG. 15 ; 
         FIG. 18  is a view of the battery cap of  FIG. 15  with elements shown partially in phantom; and 
         FIG. 19  is a schematic showing the inventive surgical screw used in the hip to treat vascular necrosis. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The best mode and preferred embodiment of the present invention is shown in  FIGS. 1-8 . The cannulated threaded screw  20  is preferably manufactured out of a non-electrically conductive material such as the non-bioabsorbable polymer PEEK (Polyther-ether-ketone) or other type of hardened material such as ceramic, PSU (polysulphone) or PEKK (Polyether-ketone-ketone) or compositions of the same or any of a wide variety of suitable poly (ether-co-ketone) materials which are commercially available. Because the screw is insulated (nonconductive material or conductive material with nonconductive material to the tip, the current flows around the screw from the insert to the tip of the insert and does not actually flow through the screw which prevents shortage of current which is different from other electrical stimulation devices. 
     Alternatively, the cannulated threaded screw  20  can be manufactured out of conductive material such as stainless steel, titanium, titanium alloys or other conductive metal or allograft cortical bone with an inner insulated sleeve which is inserted through the screw lumen. 
     The electrical threaded screw  20  is preferably constructed of non conductive material as previously described with a head  22  defining torque receiving means in the nature of a cutouts  25  which may be four or more in number with a threaded shank  26  extending therefrom. The shank defines a through going lumen  28  which is centrally axially located within the shank and has external threads  30  formed along at least a portion of the shank. The head  22  also defines a chamber  32  at the proximal end of the lumen  28  which is threaded to receive a threaded stem  42  of battery casing  40  as is shown in  FIG. 4 . If desired the chamber  32  can be formed to fit a snap casing stem  43  such as that shown in  FIG. 2 . 
     The casing  40  is preferably disc shaped and hermetically sealed. The casing  40  is formed with a housing  41  and a cap  49  which is press mounted over the housing  41 . Mounted in the housing  41  is an integrated circuit board  45  and a battery  44  which is electrically connected to a chip  46  which has a circuit  48  as shown in  FIG. 8 . The battery  44  is held in place by battery clip  54  and a sealing ring  56  and sealing top member  58  are held in place by leaf spring  59  when the cap  49  is mounted over housing  41 . The circuit board  45  provides a constant current source via connector member  57  to a cathode lead wire  50  which is encased in a silicon insulating tube  51 . The lead wire and insulating tube  51  are positioned through the lumen  28  of screw shank  26  so that the tip  52  of the cathode extends outside of the shank body. The current which is produced ranges between 5 and 50 micro amperes with the preferred range being between 5 and 20 micro amperes and the most preferred range is 20 micro amperes. Rechargeable lithium batteries are an alternative way to power the bio-implantable microsystem. Power is delivered remotely to charge the implanted battery which eliminates the necessity for battery replacement. Thus the tip  52  acts as a cathode and the casing  40  acts as an anode. 
     The circuit diagram shown in  FIG. 8  shows a representative current of 20 micro amperes which can be modified as desired by changing the resistor  55  in the circuit and the case housing forms the anode for the circuit. An RFID chip can be mounted in the casing  40  allowing easy identification of the implant outside of the patient&#39;s body with the additional benefit that it can be used to power the implant. The electrical screw assembly when implanted in the bone and set to generate a current of 20 micro amperes is particularly effective in the treatment of avascular necrosis. 
     Alternatively the present invention can use a signal conditioning circuit for a remotely rechargeable system. A rechargeable lithium ion battery powers this circuit. The desired output, then goes directly to the electrodes. A second rechargeable lithium ion battery may be included to serve as a back up and in this embodiment a lithium ion charging chip is included which is connected to the designed integrated circuit through a logic interface. The two batteries would work in tandem thus when one battery powers the integrated circuit, the other battery gets recharged and vice versa providing an uninterruptible output. The integrated circuit optionally can use a series of charge pumps or transistors to get the required boost in voltage. This alternate integrated circuit uses voltage detector circuits to detect battery voltages, has a voltage regulator, pulse generator circuits, logic circuits and requisite switches. 
     The top surface  41  of cap  49  is flat and is provided with an angular cutout  41 ( a ) which allows torque to be generated by an outside tool driving the threaded stem  42  into the screw head chamber  32  so that it is securely mounted to the head of the screw. 
     As can be seen in  FIGS. 9 and 10  a driver  60  is formed with an end  62  having projections  64  which fit in the cutouts  25  of the screw head so that torque can be applied to the screw head driving the screw into the bone of the patient. Once the screw has been implanted into the patient, the battery casing  40  and associated cathode  50  are mounted to the screw  20  by applying torque with a tool mounted in cutout  41 ( a ) and screwing the stem  42  into threaded chamber  32  or pushing the stem  43  as shown in  FIG. 2  into a snap on chamber formed in screw head  22 . The device then provides an electrical current through the portion of the patients bone which is fractured or has a defect to promote bone growth. 
     The electrical screw assembly  20  can be used in connection with a bone plate  70  as shown in  FIGS. 11 and 13 . In the usage shown in  FIG. 13  an external lead  80  is formed with an electrically conductive washer  82  secured to one and having a spiral section  84  at the distal end. The washer  82  is mounted between the screw  20  and casing  40  as shown in  FIG. 13 . The lead wire  84  can have one or more sections insulated to provide variances in the electrical field. The washer  82  is mounted around stem  42  and is positioned between the casing  40  and the top surface of the screw head  22  so that the external spiral lead wire  84  extends past the bone plate  70  allowing a primary electrical field to be formed between the cathode spiral lead wire and the anode of the casing. 
     The electrical screw assembly  20  can also be used in connection with a pedicle screw electrical stimulation device  90  as seen in  FIGS. 14-18 . As seen in  FIGS. 14-18 , the device  90  has a flexible support mount  92  which fits over and can be universally attached to any make of pedicle screw  200  seen in  FIG. 14  as being screwed into adjacent vertebrae  300 . 
     The support mount  92  is in the form of a base mount member  93  with a central aperture  94  defined in the top surface which receives the snap lock stem  43  of cap member  40 . The base mount member  93  has an inwardly projecting flexible rim assembly  96  which is cammed outward by the action of the stem  43  which is forced into it and snaps back against the lesser diameter of the stem  43 ( a ) to hold the stem  43  in fixed position within the chamber  99  formed by an insert member  120 . Surrounding the central aperture  94  are a plurality of locking recesses  100  as shown in  FIG. 16 , which additionally act as spacers and can selectively receive and hold the lock button  49  of the battery casing  40  as best shown in  FIG. 18  so that the battery casing  40  cannot be rotated on the top of the pedicle screw  200 . The side wall  95  of the base mount member  93  extends down over the head of the pedicle screw  200  and is formed with a curved cut away channel  102  and a viewing aperture  104  which allows the support mount to be flexibly mounted over the top of the pedicle screw. The cut away channel  102  is best seen in  FIGS. 15 and 16 . The base mount member  93  additionally defines curved cutouts  97  which fit over a support rod  208  as shown in  FIG. 14  holding the support rod  208  in place in the pedicle screw transverse bore  202 . A threaded interior insert  120  as seen in  FIG. 17  is threaded in the pedestal screw  200  and is used to lock the stem  42 / 43  of battery casing  40  to the pedicle screw  200 . As shown in  FIG. 17 , the threaded insert  120  defines a chamber  122  which receives a snap on stem  43  to hold the battery casing  40  in a fixed mounted position. An electrical field is generated between the anode and cathode to accelerate bone growth of the fractured vertebrae. The support mount  92  can also be mounted onto an intramedullary nail, pedicle screw rod, surgical plate, surgical washer or plate rod. 
     The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. However, the invention should not be construed as limited to the particular embodiments which have been described above. Instead, the embodiments described here should be regarded as illustrative rather than restrictive. Variations and changes may be made by others without departing from the scope of the present invention as defined by the following claims: