CA1192960A - Bone growth stimulator connector - Google Patents

Abstract

BONE GROWTH STIMULATOR CONNECTOR

Abstract Of The Disclosure There is disclosed a lead for a bone growth stimulator which includes a connector in the vicinity of that part of the lead which is implanted in bone to func-tion as a cathode electrode. The use of a connector, instead of a one-piece lead/electrode which is permanently secured to the stimulator, allows different materials to be used for the bulk of the lead and the electrode, each material satisfying different requirements. During explant of the stimulator, there is no danger of a jagged electrode lead end remaining in the body, and the lead is always "broken" at a predetermined site. Remote disconnection of the lead from the bone site is easier, and the dis-connection force is reproducible from unit to unit. Should stimulation be required for longer than the life of the stimulator, a new device can be connected to the implanted electrode without requiring a totally new electrode implant.
Most important is the fact that electrodes of different materials and shapes may now be designed for use with the same basic stimulator, a family of bone growth stimulators thus being made available without requiring the stocking of numerous costly electronic devices. The particular connector employed has a male/split-female construction which has been found to provide a reproducible disconnection force, while at the same time insuring a connection of maximum integrity.

Description

BONE GROWTH STIr~IUL~TOR CONNECTOR

This invention relates to bone growth stimulators, and more particularly to connectors for use in the lead or leads thereof.
In the c~pending application of Wickham et al, Serial No. 346,235 filed Febrlary 22, 1980, and entitled "Bone Growth Stimulator" t there is disclosed a bullet-shaped bone gxowth stimulator for promoting bone-bone fusion, i.e., bone growth, by electrical stimulation.
The Wickham et al application, describes the general backqround of bone growth stimulators.
SUch a device is simoly a constant current source. One or more cathode leads from the device are implanted in bone, in the area of a fracture. The case itself, typically made of titanium but preferably plated in at least a limited area with platinum, may serve as the anode, although a separate anode lead may be provided for insertion in soft tissue~
In the copending application of Hirshorn et al, Serial Mo. 405,324-9 filed on June 16, 1982 and entitled "~onitorable Bone Growth S-timula~or", there is disclosed an improved bone growth stimulator which is not only hermetically sealed, but which also allows the therapeutic current to be monitored. The stimulator generates con-tinuous pulses at a rate proportional to the magnitude of the current being delivered to the fracture site, and an external frequency counter may be used to thus determine the current magnitude.

6~3 The cathocle lead of ~ typical prior art bone growth stimulator is made of -titanium wire, -the free end of which is ~ormed into a helix by wrapping it tightly in a spiral around a smooth mandril, e.g., the chuck end oE a drill bit. A block of bone, typically one cm wide and 2-3 cm long~ is removed from across the ~racture site, and the non-insulated coiled end of the cathode lead is implanted into the resulting bone slot. During an explant procedure, wha-t is o~ten done is to pull on the cathode lead;
the lead usually breaks jus-t where it exits the bone.
The electrode itself cannot be removed from the bone into which it was previously implanted since the bone grows around it.
This type of construction, in which the lead/
electrode is an integral part of the overall stimulator J
gives rise to several problems. Despite the fact that the lead usually breaks just where it exits the bone, it some-times happens that it breaks elsewhere -- in which case there remains a loose lead in the patient's body. Further-more, there is always a possibility that the lead will not break cleanly and that at the break there may remain a sharp jagged end. It would be highly advantageous to provide a lead which would insure that a break always occurs at a predeter~ined point during an explant procedure, and that the break not have an unpredictable form.
It would also be desirable if bone growth stimulator leads could be made such that the same force is always required to break the lead during an explant procedure.
A considerable force is required to break a prior art lead, and -there is a natural reluctallce on the part oE a surgeon to pull on an implanted lead with such a great force. Also, repeatability in any surgical procedure is d~sirable, and to~ard this end it ~ould be hi~hly advantageous to provide a lead whose breaking force is always approxima-tely the same. Prior art ~itanium-wire leads have exhibited difEerent breaking strengths;especially when a lead is made of twisted titanium strands, due to the different compositions of the strands and the differen-t annealing steps from ba-tch to batch, it has been found that differen-t forces are required to break the leads.
Prior art leads are usually made of titanium because -titanium is probably the best tolerated metal in the human body. However, -ti-tanium is not necessarily the best metal for a lead when it comes to other considerations.
Especially in the case of a lead which is subject to continuous flexing, e.g., where the electrode is in a finger and the stimulator is in the forearm, stainless steel would offer not only greater strength, but also greater Eatigue life. Yet prior art leads have not been made of stainless steel because the electrode tip must remain in the bone and body after the explant procedure, and titanium is better by far in this regard. In the prior art, there has been no way to achieve both the strength (and lower cost) advantage of stainless steel and the tolera-tion advantage of titanium.
Although a bone grow-th stimulator is supposed to do its job within a few months, success is not always achieved. In such a case it may be necessary -to continue stimulation. But the batteries which power the device have a life of only a few months since they deliver a relatively large constant direct current of 20 micro-amperes. Continued stimulation in the prior art requires not only a new implant, but also the placement of a new electrode in the bone. This results from the fact that the lead/electrode comprises an integral part of the overall device.
It has also become apparent that pre-formed electrodes have much to offer. Depending on the particular fracture involved, the size of the electrode helix may vary.
Also, electrodes o completely different shapes have been proposed. It is even possible that electrodes of different materials may be desired by the surgeon, for example, there are advantages to the use of silver electrodes as described in the Hishorn et al a~lication,S.~.405,324-9. T~hat all of this means is that if a hospital is to stock a complete line of bone growth stimulators with integral lead/electrodes, a line which will satisfy every contingency, literally dozens of different stimulator types may have to be kept on hand.
This can give rise to a considerable investment, especially if numerous stimulators of each type must be available.
It is the general object of my invention to provide a bone growth stimulator and a lead/electrode therefor which overcome all of the aforesaid disadvantages of the prior art.
All of the aforesaid disadvantages of the prior axt are overcome by providing a two-part lead/electrode.

The main lengtll o~ the leacl is an integral part of the stimulator, as in ~he prior art. ~u-t instead o~ the end o~ this l~ad being used as the elec~rode, th~ end oE this lead -terminates in one part or a connector. The other mating part of the connector is attached to the electrode itself.
A hospital now need stock only one type of bone growth stimulator -- the bullet-shaped device with a connec-tor-terminated lead. To accommodate differen-t electrode materials and shapes, the hospital need stock only the much less costly connector-terminated electrodes themselves. Prior to implant, the surgeon selects an electrode and "plugs" it into the s-timulator. Thereafter, the implant procedure is the same as that in the prior art.
When it comes time to explant the device, the surgeon pulls on the lead. The "break" is always at the connector since the wires which comprise the lead and the electrode have a pull strength (typically, 15 pounds) which is greater than the disconnect force. The same force ~typically, 3-6 pounds) is always required to disconnect the stimulator from the electrode which remains in the bone.
There is no possibility of a jagged wire end, and the "break"
always leaves an electrode lead of known length remaining in the body; this lead is very short, with the connector remaining close to the bone.
It is now a simple matter to replace a stimulator if that is necessary. Instead of going through another total implant procedure, all that is required is to replace the stimulator itself, and to connect the new :Lead to tile connector which remains at the end of the electrode whlch is already in -the bone. Similarly, the use of a connector allows the stimulator lead to be made of stainless steel, while -the electrode may be made of the conventional titanium or any oth~er desired material.
There are many differen-t types of connectors used in -the electronic ar-ts, and it might be thought that any standard type of connector might satisfy the objectives set forth above. Yet it was found that this is not -thç
case. Experiments have shown that many connector types are not satisfactory. For example, the "simplest"
connector is a male-female, interference-fit type device.
In such a connector, a male cylindrical pin is pushed into a female cylindrical hole. It was found that with pure interference-fit connectors of this type, in order to achieve reproducible pull-out forces the tolerances were so fine that the manufacturing costs would be prohihitive.
Other connector types were also considered but they met with equally little success. It must be borne in mind that what is desired is a pull-out force which is relatively cons-tant and independent of the speed of disconnection (i.e., the same pull resistance is provided whether the lead is pulled slowly or quickly out of the body), and that at the time of disconnection the connector will invariably be saturated with water in view of the moist body environment; pure interference-fit connectors have thus ~ar proved to be unsuitable.

~1~2~

Although -there may be other -types of connectors which may work, only one type has been found -thus ~ar to offer satisfactory perEormance. The male part of the connector consists of a cylindrical pin. Rut the female part of the connector, referred to herein as "split female", consists of a sleeve with a diametrical slit along its length. The connection is established not simply by the interference fit of a pin in a socket. Rather, the two halves of ~he female socket (there being two halves due to the slit) function as bending beams and squeeze the male pin between them. ~ithout excessively close tolerances being required, a disconnection force which varies by no more than a few pounds can be achieved quite easily, even in the presence of different degrees of water saturation.
The disconnect force is reproducible even if male and female connectors are pushed into and pulled out of each other several dozens of times. ~here is absolutely no need for matched pairs. ~s long as certain tolerances are main-tained (to be described below), the male connector of any electrode may function with the female connector attached to the stimulator. (The male and female connectors may - be reversed for any family of devices, if desired.) Further objects, features and advantages of my invention will become apparent upon consideration of the following detailed description in conjunction with the drawing, in which:
FIG. 1 depicts an overall view of a stimulator/
connector/electrode combination as used in accordance with the principles of the invention;

_. . . ., .... . .,, .. , , . . . _. . _ 6~

FIG. 2 depicts an alternative electrode and ls designed to show how, due to the use of a connector, the same stimulator may be used with many different electrodes in the same family;

FIG. 3 is a sectional view through the connector itself, connector part 14 being attached to the stimulator lead and connector part 16 being attached to the electrode;

FIG. 4 is an enlarged side view of the female part of the connector; and FIG. 5 is an enlarged side view of the male part of the connector.

Bone growth stimulator 10 of FIG. 1 is provided with an integral lead assembly 12 and female connector 14.
The stimulator itself is preferably of the type disclosed in the Hirshorn et al a?plication S.N.405,32~-9. Pre-formed electrode 18 is connected by a wire 36 (made of the same material~
to male connector 16. Prior to an implant procedure a particular electrode is selected and connected to the stimulator. FIG. 2 depicts an alt:ernative electrode 18', connected by wire 36' to male connector 16'. This particular electrode may be desirable for use in knee joint applications. (The invention, of course, is not limited to pre-formed electrodes; the surgeon may still form his own electrode from a wire which has a pre-attached connector.) The female connector part itself is shown in FIG.
4; the critical dimensions will be described below, but at this stage the overall shape of the part should be appreciatedO At the left end of female connector 26,

2~

there is an internal bore 26a ~or insertion therein of a stimulator lead (shown by the numeral 20 in FIG. 3).
At the right end of the part, there is a cylindrical bore, with a diametrical slit 26c extending along the length of the resulting sleeve. The slit results in two semi-circular beams 26d, 26e for mating with the pin of the male connèctor.
The male connector part 30 is shown in FIG. 5, and has two main parts. The pin 30a at the left fits into the socket in the female connector, and the bore 30b at the right side of the connector has inserted in it the electrode wire (shown by the numeral 36 in FIG. 3).
Reference should now be had to FIG. 3 which depicts the overall construction of the connector, after which the critical dimensions of FIGS. 4 and 5 will be described.
In the preferred embodiment of the invention, lead 20 consists of multi-stranded twisted stainless steel suture wires (e.g., available from American Cyanimid), preferably of .4 mm diameter. The free end of the lead is inserted in bore 26a of the female connector, and the connector is then swaged at 90-degree intervals around its edge, as shown by the numerals 28, to secure the lead to the connector. The area of each swage is approximately 1 mm x 1 mmO
The swage strength should exceed the pull strength of the connector so that during the explant procedure, the two parts of the connector disconnect rather than the lead and the connector. After the lead is thus attached to the female connector part, Silastic*tubing 22 (Dow Corning medical *Trade Mark _g_ grade) is placed over the entire lead an~ connector part, with the right end of the tubing extending to tlle right end of the connector part. Silastic "A" medical grade adhesive ,~ (Dow Corning) 24 is then injected in-to the left end of the tubing to fill the void between the lead and the tubing.
The left end of the lead 20 is attached to the stimulator as described in thè Hirshorn et al application S.N.405,324-9, with the Silastic tubing 22 adhering to a Silastic top cover by means of Silastic "A" adhesive. It is to be understood that the full length of the lead cannot be shown in the drawing. The lead is actually about 160 mm long.
Silastic materials are used because they are biocompatible, and also because they bond both to other Silastic materials and to metal wires.
Electrode wire 36 typically comprises three twisted strands of annealed titanium wire, with a total diameter of .55 mm. The end of the wire is inserted in bore 30b of the male connector, and the connector is swaged at four points as indicated by the numerals 32, the swage strength once again exceeding the pull-apart strength of the connector.
A short Silastic sleeve 38 is then placed over wire 36 into the larger part of bore 30b. The sleeve i5 flexible and reduces the acuteness of the angle at which wire 36 might otherwise bend where it exits the connector. Silastic sleeve 42 is then placed over the female connector part 30 and tubing 38, as shown. In the last manufacturing step, Silastic adhesive 40 is injected between wire 36 and sleeve 38, with a smooth fillet being formed at the end of the connector and bonding sleeve 42 to sleeve 38.

2~

It shoul(l be notecl that the swagincJ oE eaci~
part of the connector is not near the exit polnt oE the connected wire. The pOSitiOIl of the swages is not arbi-trary.
The most vulnerable point in each wire is in the swage region. Just in case cracks in the wire are formed as a result of the swaging, it is desirable to prevent bending of the wires in the vicinity of the cracks.
Because there can be no bending of either wire in the vicinity of its respec-tive swage, the possibility of failure is minimized. The swaging force i-tself should be kept to a minimum so as to reduce the incidence of wire cracking. The swaging should result in a holding force which exceeds the disconnect force, but which exceeds i-t by at most only several pounds. In the preferred embodiment of -the invention, the disconnect force is 3-6 pounds. In general, however, I contemplate a disconnect force anywhere in the 3-20 pound range. This is not to say that in any particular family of connectors the disconnect force will range over such a wide range. On the contrary, the disconnect force should range over about only six pounds or so in any given family. The point is that for any particular family, the disconnect force should be in a range such as 3-9 pounds, 10-16 pounds, etc. As a practical matter, no advantage is gained by requiring a disconnect force which exceeds six pounds.
Where the two connectors butt up against each other in the vicinity of their respective Silastic sleeves 22, 42, body fluid can flow into the connector. There is no practical way to avoid this. The Silastic insulation

3~
-is provided to minimi~e -the e~posed area of the connector so that leakacJe currents are kept to a minimum, but water does ge-t into the connec-tor. The important thing, however, is that the presence of wa-ter does not materially affect the disconnect force.
It should be no-ted that at the connector site the only exposed sur~faces are those o-f the Silastic sleeves and the titanium electrode wire. Because both Silastic and titanium are biocomatible materials, they prevent excessive fibrotic encapsulation, which encapsulation might otherwise prevent separation of -the two parts of the connector.
The male and female parts of -the connector are , y shown enlarged in EIGS. 4 and 5. Each part is made of unannealed commercially pure grade titanium. In the preferred embodiment of the invention, the parts are made of IMI Grade 130 3-mm or 4-mm cold-drawn titanium rods, available from Imperial Metal Industries Ltd., of Birmingham, England. There are only two critical dimensions on the male part and three critical dimensions on the female part.
The two critical dimensions on the male part are shown in FIG. 5. The holding force is determined by the bending of the two levers 26d, 26e on the right side of -the female connector. The degree of bending is determined by the inner diameter of -the female bore, and the outer diameter of male pin 3Oa. The ou-ter diameter of the male pin is thus one of the critical dimensions. Also, the length of the pin is critical, as shown. The two parts of the -12- ~ r~

connector are pushed into each other all the way, and it is the left erld of the pin before the diameter s~arts to taper that actually bends apart the two levers 26d, 26e of the female connector. Thus the degree of bending is related to both the length of the male pin and its outer diameter, the two critical dimensions shown in FIG~ S.
The three critical dimensions of the female part are the inner diameter of the bore, the leng-th of the bore, and the width of -the slit. The bending force of -the two levers or beams 26d, 26e varies with the inner diame-ter of the bore relative to the outer diameter of the pin, the length of the bore (which determines the mechanical advantage of the levers), and the width of the slit 26e.
The critical width for the slit is shown about 2.5 mm from the right end of the female connector. This dimension in itself is not critical. The slit is invariably of constant width throughout its length, and for test purposes the slit width is measured about 2.5 mm from the end of the connector.
In the illustrative embodiment of the invention, the pull-apart force of the connector is determined by the force applied to the end of the male pin by the deflection of the two semi-cylindrical beams of -the female connector.
The tolerances are relatively easy to achieve, and there are only five critical tolerances in the first place. A
family of connectors made this way provides a pull-out force of 3-6 pounds, no matter which male and female con-nectors are paired, even if they are connected and disconnected dozens of times, and even if the connectors are wet.

It might be thougllt tha-t a split-~emale connector with four seqments rather thc~n only -two would be preEerred because it might be less susceptible to sideway forces.
The symmetry oE the device (with two diametrical slits) would appear to mc~e the connector less dependent on bending moments. However, it has been found that this is not the case to any significant degree, and the increased machining (and deburring) time, and hence unit cost, adds little to the efficacy of the design. For this reason, a single slit in the female connector is preferred.
~ lthough the invention has been described with reference to a particular embodiment, it is to be understood that this embodiment is merely illustrative of the applica-tion of the principles of the invention. Numerous modifi-cations may be made therein and other arrangements may be devised without departing from the spirit and scope of the invention.

Claims (27)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A bone growth stimulator comprising im-plantable current source means and an integrally con-nected lead terminating in a first connector part, and an electrode terminating in a second connector part, said electrode being a wire configured for non-tensioned placement in bone, said first and second connector parts being connected to each other but being subject to disconnection during an explant procedure in response to a pull-apart force in the range 3-20 pounds being ap-plied thereto, said pull-apart force being less than the pull strength of said lead and said electrode.

2. A bone growth stimulator in accordance with claim 1 wherein said pull-apart force is less than six pounds.

3. A bone growth stimulator in accordance with claim2 wherein one of said connector parts includes a male pin, and the other of said connector parts includes a female sleeve with a diametrical slit therealong.

4. A bone growth stimulator in accordance with claim 3 wherein said connector parts are encased in biocompatible plastic except at the end of said sleeve and around said pin.

5. A bone growth stimulator in accordance with claim 4 wherein said lead and said electrode are made of different metals.

6. A bone growth stimulator in accordance with claim 1 wherein one of said connector parts includes a male pin, and the other of said connector parts includes a female sleeve with a diametrical slit therealong.

7. A bone growth stimulator in accordance with claim 6 wherein said connector parts are encased in biocompatible plastic except at the end of said sleeve and around said pin.

8. A bone growth stimulator in accordance with claim 7 wherein said lead and said electrode are made of different metals.

9. A bone growth stimulator in accordance with claim 1 wherein said connector parts are encased in biocompatible plastic except at the ends thereof.

10 . A bone growth stimulator in accordance with claim 9 wherein said lead and said electrode are made of different metals.

11. A bone growth stimulator in accordance with claim 1 wherein said lead and said electrode are made of different metals.

12. A bone growth stimulator comprising implant-able current source means and an integrally connected lead terminating in a first connector part, said first connector part being connectable to a second connector part which is at the end of an electrode, said electrode being a wire configured for non-tensioned placement in bone, said first and second connector parts being subject to disconnec-tion during an explant procedure in response to a pull-apart force in the range 3-20 pounds being applied thereto, said pull-apart force being less than the pull strength of said lead and said electrode.

13. A bone growth stimulator in accordance with claim 12 wherein said pull-apart force is less than six pounds.

14. A bone growth stimulator in accordance with claim 13 wherein said first connector part is encased in biocompatible plastic except at the end thereof.

15. A bone growth stimulator in accordance with claim 12 wherein said first connector part is encased in biocompatible plastic except at the end thereof.

16. A bone growth stimulator/electrode family comprising at least one implantable current source means having an integrally connected lead terminating in a connector part of a first type, and a group of elec-trodes each terminating in a connector part of a second type, each of said electrodes being a wire configured for non-tensioned placement in bone, said first and second type connector parts being connectable to each other but being subject to disconnection during all ex-plant procedure in response to a pull-apart force in the range 3-20 pounds being applied thereto, said pull-apart force being less than the pull strength of said lead and said electrode.

17. A family in accordance with claim 16 wherein said pull-apart force is less than six pounds.

18. A family in accordance with claim 17 wherein one of said connector part types includes a male pin, and the other of said connector part types includes a female sleeve with a diametrical slit therealong.

19. A family in accordance with claim 17 wherein said connector parts are encased in biocompatible plas-tic except at the ends thereof.

20. A family in accordance with claim 17 wherein the leads and the electrodes are made of different metals.

21. A family in accordance with claim 16 wherein one of said connector part types includes a male pin, and the other of said connector part types includes a female sleeve with a diametrical slit therealong.

22. A family in accordance with claim 16 wherein said connector parts are encased in biocompatible plas-tic except at the ends thereof.

23. A family in accordance with claim 16 wherein the leads and the electrodes are made of different metals.

24. A bone growth stimulator electrode cornprising an electrode wire terminating in a first connector part, said electrode wire being configured for non-tensioned placement in bone, said first connector part being con-nectable to a second connector part which is at the end of a lead which is integrally connected to an implant-able current source means, said first and second connec-tor parts being connectable to each other but being subject to disconnection during an explant procedure in response to a pull-apart force in the range 3-20 pounds being applied thereto, said pull-apart force being less than the pull strength of said lead and said electrode wire.

25. A bone growth stimulator electrode in accor-dance with claim 24 wherein said pull-apart force is less than six pounds.

26. A bone growth stimulator electrode in accor-dance with claim 24 wherein said first connector part is encased in biocompatible plastic except at the end thereof.

27. A bone growth stimulator electrode in accor-dance with claim 24 wherein said first connector part includes a case with an internal bore, said electrode wire being secured within said bore, and a flexible sleeve surrounding said electrode wire within said bore to reduce the acuteness of the angle at which the wire exits the bore.