SE531934C2 - Prosthesis device for joints - Google Patents

Description

20 25 30 531 E34 fasteners which are adapted to be connected to connecting leg parts. This prosthetic device eliminates many of the disadvantages associated with other known artificial prosthetic devices for joints that are available today. However, unwanted tissue ingrowth in this system can occur between the spring turns of the spring, and this can cause a deviating tissue reaction which can lead to inhalation and stiffness of the joint. Furthermore, the springs of the system are always susceptible to abrasion and friction and the endurance of the springs is essential for the function of the prosthetic device.

SUMMARY OF THE INVENTION The overall object of the present invention is to provide a prosthetic device for joints which obviates the disadvantages associated with articulated prosthetic devices for joints available today. In particular, the aim of the present invention is to improve the artificial link shown in International Patent Application WO 97/26846.

An important factor is the durability of the springs of the prosthetic device. The spring will be exposed to friction and fatigue caused by movements of the joint, and will eventually be released. Durability and a long functional life of the spring are of great importance.

Another important factor is the unfavorable tissue reactions that can be caused by unwanted growth of tissue between the spring turns of the spring, which can lead to inflammation and stiffness in the joint.

In order to meet the objectives, a prosthetic device for joints useful for implantation in humans and animals is provided, the device comprising a hinge body comprising two or more substantially helical springs arranged between two fastening elements which are adapted to be connected to connecting leg parts, each fastening element comprises a base plate connected to the hinge body, and a fastening part protruding from the base plate. The springs are connected to the base plate in conical holes, where the wider end of a conical heel faces the spring. The fasteners can protrude from each other at an angle in the range between 20 and 30 degrees when the springs are in their rest position. In alternative embodiments, the fasteners may protrude from each other at an angle in the range of 23 to 27 degrees or in the range of 24 to 26 degrees.

Each of the fastening parts can protrude from the respective base plate at an angle of 10 to 15 degrees in relation to the longitudinal direction of the hinge body. In alternative embodiments, they protrude at an angle of 11.5 to 13.5 degrees in relation to the longitudinal direction of the joint body or at an angle of 12 to 13 degrees in relation to the longitudinal direction of the joint body.

The prosthetic device as above can be designed as a prosthetic device where each spring has closely adjacent turns, which are closed in their rest position and where each spring is a cylindrical helical spring.

The prosthetic device is preferably intended for reconstruction of the metacarpophalangeal joints (MCP), the proximal interphalangeal joints (PlP) or the distal interphalangeal joints (D1P) in humans.

Brief Description of the Drawings The present invention will be described in more detail below, by means of a number of preferred embodiments and with reference to the accompanying drawings, of which: Fig. 1 illustrates a finger having one of its joints exchanged for a prosthetic device according to the present invention.

Fig. 2 illustrates, in a side view, a prosthetic device according to the present invention with a bending angle of 25 degrees.

Fig. 3 is a photograph illustrating the encapsulation of a tightly wound spring according to the present invention. Fig. 4 illustrates the endurance of a bending fatigue test for springs with different wire diameters.

Fig. 5 illustrates the endurance of a bending fatigue test for springs having different spring coil diameters.

Fig. 6 illustrates the endurance in bending fatigue test for springs having different lengths of the spring coil.

Fig. 7 illustrates the endurance in bending fatigue test for springs with different wire materials and wire diameters.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The term "substantially helical" springs means a helical spring having at least one winding turn and a structure in which minor deviations from the helical shape can occur, without adversely affecting the properties of the spring.

By the term "longitudinal direction of the hinge body", which is used throughout below, is meant the axial direction in which the hinge and its terminating leg portions extend in an extended position.

Fig. 1 is a side view of a preferred embodiment of a prosthesis device for an intermediate joint having a hinge body 1 of two parallel cylindrical helical springs 1 arranged in a plane perpendicular to the bending plane of the hinge body 1 and the finger. These two springs 1 are, at each end, via a base plate 2 and a fastening part 3 attached to anchoring means 4 in connecting leg parts 5.

During their research in the field of implantable prosthesis joints, the inventors have found that the joint angle of the implant in accordance with the present invention should be within a particular range in order to be optimal. The fastening parts 3 3 15 20 25 30 531 934 of the present invention are arranged to protrude away from each other at an angle in the range 20 to 30 degrees, forming a bending angle. Fig. 2 illustrates the prosthetic device according to the present invention where the fastening parts 3 according to the present invention protrude away from each other at an angle of about 25 degrees. Following surgery, the natural resting position of the hand in the metacarpophalangeal joint (MCP) and the proximal interphalangeal joint (PlP) is about 20 to 30 degrees relative to the longitudinal direction of the joint. Thus, the springs of the prosthetic device of the present invention when implanted in the MCP or PlP joints will be closed and in an unloaded rest position when the body joint is in its natural rest position.

This will lead to the positive effect of a faster formation of a capsule around the springs, since a closed spring enables / promotes the encapsulation process.

Fig. 3 illustrates the encapsulation of a spring of an articulated joint according to an embodiment of the present invention. In accordance with this invention, a tightly wound tension spring is preferably used. Such a dense cylindrical structure does not inhibit, after implantation in biological tissue, the rapid formation of a thin outer tissue membrane around the helical spring, i.e. a kind of physiological encapsulation. This encapsulation reduces the risk of adverse tissue reactions.

When implanted in the MCP or PlP joints, the bending motion to which the springs will be subjected extends from a full extension of the angles, i.e. a bend of 0 degrees, to a full downward bend, i.e. 90 degrees.

With the 20 to 30 degree bending angle of the prosthesis device of the present invention, a full bend (90 degrees) of the joint will only bend the springs 60-70 degrees, and a full stretch of the joint will bend the springs 20-30 degrees in the opposite direction. Thus, the maximum bending angle of the springs is reduced, thereby the load and wear on the springs, which results in increased durability and a reduced risk of spring breakage. In accordance with the present invention, two or two springs are used to support the joint body 1 in the prosthetic device. Studies have shown that optimal stability is usually achieved when several springs are used.

The joint prosthesis device of the present invention, i.e. the springs, base plates 2 and fastening parts 3, can be manufactured from any biocompatible material that is suitable and approved for implantation, e.g. titanium or stainless steel.

The choice of material for the capercaillie threads is very important. For many implantable devices, titanium is a preferred material. However, in this application, the fatigue resistance of titanium wire has not been judged to be acceptable. Therefore, in a preferred embodiment of the present invention, a suitable material has been found to be MP 35N (Fort Wayne Metals, IN), a material which i.a. used for pacemaker electrodes. The manufacture of this wire material involves a drawing process in which the properties of the wire can be adjusted within certain limits. In one embodiment of the invention, the MP 35N wire material has a strength of 2170 MPa.

The purpose of the design of the hinge body 1 according to the invention is primarily to distribute, over a longer distance, the deformation of the hinge body induced by the bending movement even if the movement itself must be included within the short distance corresponding to the normal extent of the hinge movement. the. As a result, the risk of damage to the material is reduced. A large movement in the artificial joint is "downshifted" to a large number of smaller movements over the entire length of the coil spring. The fact that the springs themselves constitute the supporting, bendable and stabilizing basic structure of the hinge body 1 eliminates the need for a further deformable support hinge body, e.g. 10 15 20 25 30 E31 5311 Each spring can be attached to the base plate 2 in any conventional manner which gives the hinge body 1 sufficient stability and durability.The springs can be attached to the plate on the inside of an opening in the base plate 2. This opening A hole can be made conical so as not to wear on the springs.The springs can be attached to the base plate 2 by means of a weld seam, by means of gluing or by "screwing" the ends of lj the spring turns of the spring over the base plate 2 and is locked in position, e.g. by means of a latch or a recess' in the plate. Laser welding has been proven to be a good method for attaching the springs to the base plate. These fastening methods and other similar methods can be used separately or in combination.

The base plates 2 are adapted to be connected, via the fastening parts 3, to fixed anchoring means 4 which are inserted into connecting leg parts 5 on each side of the prosthesis device for joints. Several conventional, suitable methods of attachment are available. An established method means, as mentioned above, that the fastening parts 3 in the form of shafts, which protrude from the base plates 2, are inserted into longitudinal channels in the longitudinal direction of the anchoring means 4. The anchoring means 4 can be made of ceramic material, titanium or any other material having suitable biological and mechanical properties.

The helical springs to be used as hinge body 1 are preferably tension springs. A tension spring is tightly wound and offers good lateral stability in the system and resistance when exposed to tensile stress. Such a dense cylindrical structure induces after implantation in biological tissue the formation of a thin outer tissue membrane around the helical spring, i.e. a kind of physiological encapsulation. From Figure 3 it can be seen that the spring wire 310 forms a completely closed structure which is encapsulated in a thin membrane 320. This membrane has a natural and healthy appearance. If it had been amateurish, it would have been opaque, swollen, and provided with a different kind of vascularization. In the picture, the blood vessels are healthy and of natural size and quality. This encapsulation reduces the risk of adverse tissue reactions. Tension springs do not allow any further compression in the system, but provide satisfactory stability and elasticity in lateral rocking.

The prosthetic device for joints according to the present invention can be provided in different embodiments depending on the position in the body where they are implanted and how they are intended to function. Thus, a number of similar or different, substantially parallel springs may be used in combination, one or more of the springs may differ from the others, and / or vary within themselves with respect to the winding cross section, pitch, pitch angle and / or wire cross section.

One or fl your springs can also be arranged substantially concentrically inside one or större your larger springs, or put into each other according to a principle similar to that of e.g. a DNA molecule. By the terms “substantially parallel” and “substantially concentric” is meant that a minor deviation in the longitudinal direction between different springs is included in the scope of protection of the winding.

The springs in their longitudinal direction are preferably cylindrical, but can also be convex or concave. Below is a detailed description of some different embodiments.

Joint body with a varying number of fi edges Two or fl parallel springs arranged in a plane which is perpendicular to the bending plane of the joint body 1 provides good lateral stability and maintains satisfactory mobility in a plane (see the embodiment in fi g. 2). When reconstructing a joint, both bending and stretching are made possible in this way, as well as a limited amount of lateral movement. Such a joint body 1 is particularly suitable for the reconstruction of the bone joints (MPC joints) and the intermediate joints of the joints (PIP joints).

Joint body with a varying number of springs and a varying position thereof By varying the position of the capers on each base plate 2, the mechanical properties of the prosthesis device for joints can be modified and controlled. For example, one or more springs may be arranged inside one or more larger springs or at suitable positions. More than two springs in the joint body increase reliability if one of the springs breaks.

Joint body where the fi feather wire has a varying cross-section The mechanical and biological properties of the fi feathers can be varied by varying the cross-section of the trees. Its cross section can e.g. be round, oval, transverse or oblique in different planes.

Joint body with fi springs with varying length The length of the springs can be varied as needed. A longer spring is less stressed than a short one, but is less stable than the shorter one. In small conductors, e.g. in the intermediate and outer joints, it is usually advantageous to have very “short” veins. The length of the spring is also affected by the desire to remove as few bones as possible. In one embodiment of the present invention, the total length of the joint body, i.e. the distance between the adjacent legs, of the prosthetic device for PIP joints 6 to 8 mm.

Joint sprocket with springs that have a varying coil diameter The coil diameter of the springs can, as with conical screw springs, be varied in different ways along the length of the springs. The spring can be, e.g. convex (“egg-shaped”) with its diameter in the center or concave (“hourglass-shaped”) with its smallest diameter in the middle. In this way, the winding turns of the springs when they are bent and stretched can be moved relative to each other in a way that is not possible with a normal cylindrical spring, which may be desirable in some applications.

As mentioned above, the base plate 2 of the joint prosthesis device of the present invention can be made of any suitable biocompatible material. The design of the base plate 2 is not critical to the present invention, but may be of any suitable plate-like shape. However, the surface of the base plate 2 which is directed towards the fastening part 3 may be angled relative to the longitudinal direction of the hinge body 1. Furthermore, the surface which faces the hinge body 1 of a base plate 2 for two springs may be angled, ie so that the springs, which are arranged in a plane perpendicular to the plane for bending the hinge body 1, in the resting position in the longitudinal direction of the hinge body 1 form arcs which are bent away from each other. Such a design provides an even and soft lateral stability in the hinge body 1. If desired, the base plate 2 can also be designed so that än more than two springs form arcs in the rest position.

After implantation of a prosthetic device for joints according to the present invention, a thin capsule consisting automatically of a connective tissue membrane, as mentioned above, is automatically formed around each spring. This phenomenon is most pronounced when a tightly wound tension spring is used. If necessary, each spring can be provided with an artificial outer capsule with the aim of minimizing unwanted tissue ingrowth in the system. Such a capsule may consist of a thin membrane of a loose or homogeneously deformable material with suitable biological and mechanical properties. The membrane may be resorbable or not.

The joint body 1 in its entirety can be encapsulated with a surrounding housing which has a very tight winding and a configuration (for example oval or rectangular) which allows containment of the joint body 1. When designing such a "capsule", priority is given to the "membrane effect". When using a tightly wound thin wire, a very insignificant widening between the winding turns of the spring thus takes place, even in the case of large movements in the system. In this way, two different types of springs can be allowed to complement each other in advantageously, ie a system with inner springs which is responsible for the mechanical properties of the structure, supplemented by an outer enclosing spring system which is mainly intended to have a barrier effect.These two spring systems should is made of a material that has suitable biological and mechanical properties.

The formation of an outer biological membrane around one or more springs in the joint body is facilitated if the spring or springs, before implantation, are surrounded by a tube of biocompatible, selectably resorbable material, without said tube forming a supporting body for the prosthetic device for joints.

In this way, a biological capsule is automatically formed around the spring after implantation.

The present invention is particularly useful for the reconstruction of the bone joints (MCP joints) and the intermediate and outer joints of the fingers (P1P and D1P joints).

The thigh can also be used at the wrist, at the tombase osteoarthritis and as a bone replacement for the os trapezium or as an articular joint between the first metacarpal bone and the os trapezium and / or between the os trapezium and the boat bone. The invention can also be used as a bone replacement for intervertebral discs or individual vertebrae in the spine. The present invention is of course also applicable to other similar joints and bone systems in the body, even where replacement structures are currently rare, but which may be of interest in the future, e.g. for the joints of the foot. 10 15 20 25 531 934 12 By selecting a suitable size and configuration of the springs included in the hinge body, the contour of the normal hinge head can be imitated in a cosmetically advantageous manner.

The dimensions of the components of the joint prosthesis device are not limited, but of course vary depending on the dimensions of the joint or bone part to be replaced in the human or animal in question.

Load reduction by special choice of wire diameter Bending of the joint causes twisting of the wire in the spring coil 1. A thicker diameter of the wire will lead to greater load on the wire which has been verified in tests and simulations see Fig. 4. In the figure representing a diameter of 0.5 mm compared to other diameters with respect to the developed load for a certain bending angle. To extend the life of the joint, different diameters were tested and a diameter of 0.45 mm was found to have a longer life. This is seen as a minor strain in the diagrams in Figures 4 and 7.

However, it has also been found that it is not appropriate to reduce the diameter too much, since the stability and force required to bend the joint at a certain angle will decrease. These findings from tests and simulations suggest that the diameter of the wire should not be less than 0.40 mm.

Test reduction due to a certain choice of coil diameter The diameter of the spring coil 1 also affects the force needed to bend the joint a certain angle. The spring coil diameter 1 also affects the stability and length of the wire. This is reflected in Figure 5 where a diameter of the spring coil 1 of 5 mm is compared with other spring coil diameters 53 "! 934 13 diameters with respect to induced stress for a certain bending angle. It can be seen that a smaller diameter of the spring coil makes the joint stiff.The length of the wire becomes shorter, thereby increasing the stress on the wire.However, the upper diameter of the spring coil is limited by the available space where the joint is to be implanted.Therefore, to reduce stress and still have a diameter that fits in available space, a diameter of 6 mm is preferred.

Stress reduction by adjusting the length of the spring strut The length of the artificial joint is limited upwards by the anatomy of the patient. The longer the artificial joint, the more patient bones must be removed to make room for the eden. However, there is always a desire to remove as few bones as possible. It was discovered that a longer wire has less stress than a shorter one, which can be seen in Fig. 6. Here, a spring coil length of 7 mm is compared with a free spring coil length of 6, 9.5 and 12 mm with respect to induced stress depending on bending angle.

It can be seen that the material stress in a short fi coil that has a length of 6 mm crosses the stress threshold already at approximately a 45 degree bending angle. The stresses in the material of a .5 coil of 9.5 mm in length pass the stress threshold only at approximately 87 degrees of bending angle. Increased stress resistance by selecting suitable material in the spring coil wire The wire material is of great importance. For many implantable devices, titanium is preferred. However, tests for this application have shown that the fatigue of a titanium wire having dimensions suitable for the application is not acceptable. The inventors have searched for another material and found that the MP35N wire material is suitable for the present application. Materials with similar characteristics regarding biocompatibility and strength can of course also be suitable. As mentioned above, the manufacture of this material involves a drawing process in which the properties of the wire can be changed within certain limits. A wire material that is subjected to a drawing process that gives a strength of 2170 MPa was selected as the most preferred material for the present application. Fig. 7 shows the endurance in bending fatigue tests for springs with different wire materials and wire diameters. A wire diameter of 0.45 mm and an MP 35 material with a strength of 2170 MPa can be bent at large angles repeatedly without reaching the loading threshold.

Conical hole Another feature of the mechanical design of the artificial joint is that the hole in which the spring coil is welded to the plate has been made conical to avoid abrasion and friction between the spring coil material and the plate.

Example 1 A prosthetic device adapted for the MCP and PIP leads is shown in fi g. 2.

Preferably it has a length of about 6 to 8 mm and a width of about 4 to 6 mm, each of two coil springs preferably has a length of 4 to 6 mm, a winding diameter of 4 to 6 mm and a wire diameter of 0.4 to 0.55 mm, even more preferably 0.45 mm. The base plate 2 preferably has a thickness of 1 to 2 mm. The fastening part 3 preferably has a diameter of 2 to 3 mm and protrudes 4 to 6 mm from the base plate.

Test method - The finite element method, FJEM calculation: 10 15 20 53.1 534 15 Fig. 4 shows the endurance of the springs during repeated bending during testing using FEM calculations. Real tests point in the same direction.

From the figure it can be seen that a load threshold for the material, in this case the previously mentioned alloy MP 35, is reached for a spring with a wire diameter of 0.7 mm already at a bending angle of 25 degrees.

A corresponding spring with a wire diameter of 0.5 mm reaches the load threshold when the bends are approximately 57 degrees.

An additional corresponding spring with a wire diameter of 0.45 mm will not reach the load threshold until the bends are over 70 degrees.

Therefore, a spring with a wire diameter of 0.45 mm will be very well suited for use in an artificial joint in accordance with the present invention with a bending angle of 25 degrees, since the bending angle of the spring will never rise above 70 degrees, and thus will to withstand the repeated elevation to which it is subjected when used as a replacement for a finger joint.

Claims (15)

10 15 20 25 30 35 531 934 lb PATENT REQUIREMENTS A joint prosthesis device to be implanted in humans and animals, comprising a joint body (1) comprising two or more substantially helical springs arranged between two fastening elements, which have been adapted to be attached to adjacent leg parts (5), each fixed element comprising a base plate connected to the joint body and a fixed part (3) projecting from the base plate (2), characterized in that the springs are connected to the base plate in conical holes, the wider end of a conical hole facing the spring. The prosthetic device according to claim 1, characterized in that the fastening parts (3) protrude from each other at an angle in the range of 20 to 30 degrees, when the springs (1) are in their rest position. The prosthetic device according to claim 1, characterized in that the fastening parts (3) protrude from each other at an angle in the range of 23 to 27 degrees, when the springs (1) are in their rest position. The prosthetic device according to claim 1, characterized in that the fastening parts (3) protrude from each other at an angle in the range 24 to 26 degrees, when the springs (1) are in their rest position. The prosthetic device according to claim 1, characterized in that each of the fastening parts (3) protrudes from the respective base plate (2) at an angle of 10 to 15 degrees in relation to the longitudinal direction of the hinge body (1). The prosthetic device according to claim 1, characterized in that each of the fastening parts (3) protrudes from the respective base plate (2) at an angle of 11.5 to 13.5 degrees in relation to the longitudinal direction of the hinge body (1). The prosthetic device according to claim 1, characterized in that each of the fastening parts (3) protrudes from the respective base plate (2) at an angle of 12 to 13 degrees in relation to the longitudinal direction of the hinge body (1). The prosthetic device according to claim 1, characterized in that each helical spring has closely adjacent spring turns, which abut close to each other in the rest position. The prosthesis device for joints according to any one of the preceding claims, characterized in that each spring is a helical spring of the tension type. 10 15 20 531 934 l? The prosthetic device for joints according to any one of the preceding claims, characterized in that each spring is cylindrical in its longitudinal direction. The prosthetic device for joints according to any one of the preceding claims, characterized in! in that the prosthetic device is intended for reconstruction of the metacarpophalangeal joints (MCP), the proximal interphalangeal joints (PIP) or the distal inguinal phalangeal joints (DIP). The prosthetic device for joints according to claim 1, wherein said springs have a wire diameter of 0.40 mm to 0.55 mm. The prosthetic device for joints according to claim 1, wherein said springs have a spring coil diameter of between 4.0 mm and 6.0 mm. The prosthetic device for joints according to claim 1, wherein said springs have a spring length of between 6.0 mm and 8.0 mm. The device according to any one of claims 12 to 14, wherein the spring wire is made of the material MP35.