ARTIFICIAL JOINT, ESPECIALLY FOR FINGER JOINTS

Abstract: The invention relates to a finger joint prosthesis, comprising a pair of titanium pins (6) designed to be surgically implanted in the bones adjacent to the joint, and an intermediate articulating body joined to the pins, said body containing a helical spring package (8, 9) which biases the finger joint to the extended position.

Description: Artificial joint, especially for finger joints. The present invention relates to a joint prosthesis, comprising a pair of anchoring members surgically im- plantable in the bones adjacent to the joint and an articulating body for joint to the anchoring members. A finger joint prosthesis of this type developed in recent years consists of a pair of titanium screws which can be surgically implanted in the bones adjacent to the finger joint and an intermediate articulating body of silicon rubber, which is injection moulded over the flange por¬ tions of the screws. Each titanium screw is inserted into a bore in the longitudinal direction of the respective finger bone and in time is integrated with the bone tissue. The silicon rubber, which makes up the joint itself, serves as a hinge with a slight amount of re¬ silience. The advantage of making the joint itself of silicon rubber in this manner is that it is possible to obtain a simple and reliable design without friction producing elements such as in a ball-and-socket joint. it has, however, been shown that silicon in the joint itself affects the bone tissue in such a manner as to dissolve the bone adjacent to the joint. This results, inter alia, in a weakening of the anchoring of the titanium screws. Another disadvantage is the very limited resilient force in the silicon joint acting to straighten out a bent finger. The purpose of the present invention is to provide a joint prosthesis of the type described by way of introduction, which, without use of silicon or another elastomer, pro¬ vides a simple and reliable joint in which there can be achieved a well-balanced spring-back effect. This is achieved according to the invention by virtue of the fact that the articulating body comprises at least one helically wound spring wire having one end joined to one of the anchoring members and the other end joined to the other anchoring member. In the joint according to the invention, a helical spring means forms the elastic connection between the bones. The wire material, the wire diameter and the number of coils determine the spring force and the spring path or joint angle, which makes it possible to adjust these parameters in a simple manner so that a joint can be achieved which also increases mobility in persons suffering from rheuma¬ tism. In a preferred embodiment of the joint prosthesis accord¬ ing to the invention, two helically wound spring wires are used which are so intertwined that the windings of one wire lie between the windings of the other wire. Each wire has one end joined to one anchoring member and the other end joined to the other anchoring member in such a manner that each anchoring member is joined to both wires at either end of the spring package. This design avoids the overturning moment which is otherwise exerted by a simple helix if the wire ends at opposite ends of the helix are joined to individual anchoring members. In the embodiment according to the invention with double springs and "dia¬ gonal" anchoring, the overturning moments cancel each other out so that the spring force will be symmetrical relative to the longitudinal centre line of the joint. The invention will be described below with reference to an example shown in the accompanying drawings, in which Figure 1 shows a schematic X-ray picture of a finger with a joint prosthesis according to the invention implanted in a finger. Figure 2 shows a perspective view of an embodiment in schematic form to illustrate the principle of the in¬ vention, and Figures 3 and 4 show a longitudinal section and a cross- -section, respectively, through a preferred embodiment. In Figure l, l and 2 designates two bones in a finger. The facing ends of the bones have been cut off to provide room for a joint prosthesis, generally designated 3, consisting of an articulating body 4, a pair of stainless plates 5 and a pair of stainless pins 6a. The pins 6a are inserted into bores in titanium pins 6b, previously surgically im¬ planted and held securely by bone growth. The principle and functioning of the articulating body 4 itself are illustrated in Figure 2. The articulating body comprises a cylindrical carrier 7, on which two metal wires 8 and 9 of spring steel, e.g. stainless 18/8 steel, are helically wound in such a manner that the wires are entwined in each other. The wires are wound so that the ingoing end (10 or 11) and the outgong end (12 or 13) of each wire are oppositely directed. The ends 10-13 of the wires are joined in pairs to a pair of plates 14, 15 to which pins 6 are fixed. The mounting of the wire ends 10-13 in the manner described results in the individual helical springs formed by the wires 8, 9 being subjected to oppositely directed overturning moments "a" and "b", which cancel each other out during an articulating move¬ ment as indicating by the arrow "c". Figures 3 and 4 show the preferred embodiment of the joint prosthesis of Figure 1 on a very large scale. Components corresponding to those in Figures 1 and 2 have been given the same reference numerals. The practical embodiment according to Figures 3 and 4 differs from the sketch in Figure 2 showing the principle of the invention only in that the flat anchoring elements in Figure 2 have been given the form of a pair of concentric sleeves 14 and 15. Preferably, they are made of stainless steel and have on their facing surfaces 16, 17 a polyethylene coating so that the friction between the sleeves 14, 15 will be minimal upon relative rotation during the articulating movement. in the embodiment in Figure 3, the articulating body 4 is separate from the pins 6b and is only joined thereto by means of a bayonet connection which allows the joint to be easily replaced should this become necessary. For this purpose, each plate 5 is eguipped with a driver pin 18, 19, respectively, with a flange 30. The drivers are each fixed in an individual bore 20, 21 in the cylin- der 14 or 15 and the flanges 30 are fixed in the corre¬ sponding cavities 31 in the plates by means of lock screws 32. The outermost cylinder 14 is provided with a peri¬ pheral through slot 22, through which the driver 19 for the inner sleeve 15 extends. The peripheral extension of said slot determines the maximum angle of articulation. In the embodiment shown in Figure 3, the slot is so long that it permits articulation between 0° and 90°. The ends 10, 11 and 12, 13, respectively, of the wire springs 8 and 9, respectively, are fixed in the outer sleeve 14 and the inner sleeve 15, respectively, in the same pattern as shown in Figure 2. The respective wire ends 10, 11 extend through a peripheral slot 23 in the inner sleeve (Figure 4), the peripheral extent of which corresponds to the extent of the slot 22 and out through a bore 24 in the outer sleeve 14, where it is locked by means of a bent portion. Each wire end 12, 13 is fixed in a bore 25 (Figure 4) in the inner sleeve 15. The spring package formed by the wire springs 8, 9 can either be entirely unloaded in the completely extended position shown in Figure 3, or can have a slight pre- tensioning so that the lateral surface of the driver 19 joined to the inner sleeve 15 is lightly pressed against the upper end edge 26 of the slot 22. In the position shown in Figures 3 and 4, representing the straight ex- tended position of the joint, there is a small play "s" between the wire windings and the outer surface of the cylinder 7 and the inner surface of the inner sleeve 15. This play is required to provide for the expansion of spring 8 and the contraction of spring 9 during articu- lation. A preferred embodiment has been described above, in which the springs 8, 9 are wound about a cylinder 7. This has a single purpose, namely to provide an inner limit for the wire helices to prevent the wire springs from interfering with each other, other dispositions of the wire windings are. however, conceivable so that the cylinder can be eliminated. It is also theoretically possible to achieve a resilient joint with a single helical spring, which, how- ever, does not provide the optimum solution obtained by the above described embodiment.

Claims: CLAIMS l. Joint prosthesis, comprising a pair of anchoring members surgically implantable in the bones adjacent to the joint, and an articulating body (4) joined to the anchoring members, c h a r a c t e r i z e d in that the articulating body comprises at least one helically wound spring wire (8, 9) having one end (10, 11) joined to one (6, 14) of the anchoring members, and the other end (12, 13) joined to the other anchoring member (6, 15). 2. Joint prosthesis according to Claim l, c h a r a c t ¬ e r i z e d in that the articulating body (4) comprises a cylindrical carrier (7), on which the spring wire (8, 9) is wound. 3. Joint prosthesis according to Claim l or 2, c h a r ¬ a c t e r i z e d in that the articulating body (4) com¬ prises two helically wound spring wires (8, 9) which are entwined so that the windings of one wire lie between the windings of the other wire, and that each wire has an end (io, 11) joined to one anchoring member (6, 14) and an¬ other end (12, 13) joined to the other anchoring member (6, 15). 4. Joint prosthesis according to one of Claims 1-3, c h a r a c t e r i z e d in that the articulating body (4) comprises a pair of concentrically mounted, cylind¬ rical sleeve elements (14, 15), which surround the cylind¬ rical carrier (7), and that one end (10, 11) of said wire spring(s) (8, 9) is joined to one sleeve (14) while the other end (12, 13) is joined to the other sleeve (15), so that the sleeves are rotatable relative to each other against the force of the helical spring or springs exerted by the spring wire(s). 5. Joint prosthesis according to Claim 4, c h a r a c t ¬ e r i z e d in that the anchoring members comprise a pin (6, 19, 6, 20) joined to an individual sleeve (14, 15), the surgically implantable (1, 2) portion thereof consiβt- ing of titanium, and that the outer sleeve (14) has a peripheral extending through slot (22), through which the pin (19) in the inner sleeve (15) extends. 6. Joint prosthesis according to Claim 4 or 5, c h a r - a c t e r i z e d in that the sleeves (14, 15) consist of stainless steel and that their frictional surfaces (16, 17) facing each other are coated with a polyethylene layer 7. Joint prosthesis according to one of Claims 1-6, c h a r a c t e r i z e d in that the spring wires (8, 9) consist of stainless steel and have a diameter of about 0.5 mm.

Inventors: PERSSON BENGT, PERSSON, BENGT
Applicants: VOLVO AB, AB VOLVO
Application year: 1990
Main classification: A61F2
IPC codes: A61F2, A61L27