JOINT PROSTHESIS

Abstract: The present invention relates to a joint prosthesis, preferably for small joints such as finger-joints. The prosthesis comprises at least one distal prosthesis element (1) and at least one proximal prosthesis element (2), a joint (3) by means of which the said prosthesis elements (2, 3) are connected in an articulated manner, and securing means (4) for securing the said distal part (1) on the distal bone shaft (6) of the joint and the said proximal part (2) on the proximal bone shaft (5) of the joint. A feature of the invention is that at least one of the said prosthesis elements (1, 2) has at least one attachment part (12, 22) which can be applied on the peripheral part of the bone shaft (5, 6) on which it is intended to be attached. Such a prosthesis permits a minimal surgical defect, since preferably only the joint head needs to be cut away.

Description: Title: Joint prosthesis Technical field: The present invention relates to a joint prosthesis, which permits permanent anchoring in human joints, according to the preamble of patent claim 1 below. The joint is primarily intended to be used on small joints such as finger- oints. However, it is not limited to these, but can also be used on large joints such as elbow-joints or knee-joints. Prior art and problems: On the market today there is essentially one type of finger-joint prosthesis. This is made of silicone plastic and is applied in the medullary cavity of the bone proximally and distally of the joint in question. The middle section of the prosthesis has a waist, which is designed to increase the flexibility in the middle of the joint. The ends of the bones facing the joint are cut away in the operation, and the prosthesis is introduced into the medullary cavity. The ends of the prosthesis thus move relatively freely in each medullary cavity in the axial direction during movements in the joint. The prosthesis is thus not anchored against the bones. In many cases this movement produces, after a certain time, on account of inadequate physiological loading, attrition of the cortex which surrounds the medullary cavity. This results in an increased risk of fracture formation in the bone shafts. An advantage of the present method is the relatively uncomplicated surgical technique. One problem, besides the risk of local attrition of bone, is that the stability in the joint remains poor after the operation since the construction of the prosthesis precludes lateral stability on account of the use of silicone plastic which is soft and yielding. In addition, the use of silicone can, due to wearing off of silicone fragments, give rise to cyst formations. The patients who nowadays mainly receive finger-joint prostheses are those with degenerative finger-joint diseases such as rheumatoid arthritis, and patients with traumatic finger-joint injuries. The indications for surgery are mainly pain and inadequate ability to use the fingers as a result of poor stability in the finger- joints. Since the risks of the bone suffering local attrition and of the stability after the operation not being optimal are nowadays relatively great, finger-joint prostheses are mainly reserved for very serious cases of finger-joint disease where the requirements regarding usability have not been set particularly high. This means that it is primarily the elderly and severely handicapped who predominantly undergo surgery, since these patients are expected to have a lower activity level and a shorter remaining lifetime. The large group of patients under 60 years of age with serious symptoms and at the same time with relatively high requirements for usability may not undergo surgery for finger-joint prostheses due to the doubts regarding the reliability of the joint replace¬ ments on offer today with respect to their long-term results and the usability of the operated finger-joint. e. There is at present a Swedish method for skeletal anchor¬ ing of dental bridges with dental implants, where the anchoring stability is initially good and can be main¬ tained for over 20 years following the skeletal implanta- tion. This unique anchoring stability for the skeletal implant is probably dependent on establishing a contact, free of connective tissue, between the bone tissue and the implant, so-called osseointegration. The precon¬ ditions for an anchoring situation of this type have been stated as being the use of biocompatible materials with high corrosion resistance, such as commercially pure titanium, combined with a controlled non-traumatic surgical method and the avoidance of direct loading by means of the implant being introduced in a two-stage procedure, where only an anchoring element is implanted in the first stage, and it is only in a second operation that a loaded function element is added. In addition, there is now wide experience of the use of ceramic materials for implantation in body tissues. The ceramics are characterized by rigidity combined with great corrosion resistance and no output of substances injurious to the body. Solution and advantages: The aim of the present invention is to provide a joint prosthesis which builds on the experience gained from the abovementioned Swedish method for jaw implantation and which eliminates the disadvantages which are associated with the use of the abovementioned known prostheses. The said aim is achieved by means of an arrangement according to the present invention, whose characteristics emerge from patent claim 1 below. A further aim of the present invention is to produce a joint prosthesis by means of which the surgical defect upon prosthetic surgery is reduced, that is to say the cutting away of intact bone tissue can be reduced iSi connection with the operation by means of the fact that only the joint head, the proximal joint end, need be cut away, and which in its preferred embodiment does not require any further cutting of spongy osseous tissue. The present invention aims to create the preconditions for a satisfactory anchoring stability, which means that it is possible to extend the range of indications for joint surgery, in particular finger-joint surgery, to more active persons of younger age groups and to afford greater joint stability than is now the case. Description of figures: The invention will be illustrated in greater detail below on the basis of exemplary embodiments and with reference to the attached drawings, in which : Fig.l shows a side view of the prosthesis implanted in a finger-joint. Fig. 2 shows an exploded view of a preferred embodiment of the invention. Fig. 3 shows an exploded view of a second embodiment of the invention. Fig. 4 shows an exploded view of a further embodiment. Fig. 5 shows the preferred embodiment of the anchoring element and the plate and screw cooperating with it, and Figs. 6 and 7 shows two embodiments in which the prosthesis elements also comprise parts projecting into the medullary cavity. Preferred embodiments: Fig. 1 thus shows a prosthesis in accordance with the invention, which has been implanted in a finger-joint. The joint prosthesis shown consists of a distal prosthesis element 1 which is secured on the distal bone shaft 6 of the joint by securing means 4, a proximal prosthesis element 2 which is secured on the proximal bone shaft 5 of the joint by securing means 4, and & joint 3 by means of which the said prosthesis elements 1, 2 are connected in an articulated manner. The broken line in Fig. 1 shows how the distal part 6 of the joint can pivot about the joint 3 of the prosthesis. Fig. 2 shows the construction of a preferred form of a prosthesis according to the invention and how it is " intended to be secured. The distal prosthesis part 1 here consists of a band-shaped material. A 0.5 mm titanium plate has been found to be very suitable for this pur- pose, but other biocompatible materials with corres¬ ponding strength characteristics are also conceivable. such as reinforced polymer material. This distal part is provided with an upper and a lower attachment part 12 provided with holes 14. This distal part 1 is moreover intended to be provided with means 11 for fixing the outer part of a bearing member 32. This is preferably achieved by means of the fact that the band-shaped material is allowed essentially to surround the outer circular surface of the bearing member 32. For anchoring of the distal prosthesis element 1, there are anchoring members 41 which preferably consist of two through titanium screws which are provided with internal threads. Coaxial to these internal threads there is a raised part 45 whose thickness at least approximately corresponds to the thickness of the band- shaped material of the prosthesis element, and the holes 14 which are arranged in the attachment parts 12, 22 of the prosthesis element have a diameter which corresponds to the diameter of the said raised part 45. In this way tensile loads in the prosthesis can be transferred directly to the said anchoring members 41 via the said raised parts 45, so that the dimension of the securing members 42 can be kept down. When the prosthesis element is mounted on the anchoring members 41, this is achieved suitably by means of screws 42 which are screwed into the internal thread of the anchoring member 41. As emerges from Fig. 2, both anchor¬ ing members 41 are of the through type, but only one of them has to be designed with internal threads at both end surfaces, since one end of the attachment parts of the distal prosthesis element 1 is secured with a downward- directed screw 42 in the said anchoring member 41, while the other end of the prosthesis element is secured by " means of an upward-directed screw 44 in the same anchor¬ ing member 41. In order to make space for the distal prosthesis element 1, it is in most cases not necessary to carry out any cutting of the distal bone shaft 6. However, a slight scraping may be necessary in order to better fit the prosthesis element. As emerges from Fig. 2 , the joint head has been cut away from the proximal bone shaft 5, which is necessary in order to make space for the prosthesis. The proximal prosthesis element is attached in principle in the same way as the distal part 1, but the proximal prosthesis element has only one attachment part 22. This attachment part 22 is provided with two holes 24 which, in a manner corresponding to the distal part, cooperate with securing members 41. The proximal prosthesis element is moreover provided with two coaxial hollows 21 which are intended to cooperate with an axle element 31 in the joint 3. Fig. 2 shows two alternative embodiments of bearing members 32 which can be used in the joint prosthesis. The lower of these alternatives has a ball or roller bearing 32, advantageously made of a ceramic, which is intended to be mounted on an axle element 31. In the second alter- native there is a slide bearing, again advantageously made of ceramic material. In this case it is intended that the axle element 31 should preferably be integrated with the bearing body 32 (but not necessarily) . As can be seen, this slide bearing has a slightly spherical outer ' surface, by which means it is possible for a prosthesis joint to be given a certain desired and controlled lateral movement. Even if a joint essentially free from play is desirable, it is an additional advantage if such a joint affords a certain lateral movement, as in a physiological joint. In the preferred form of the invention, the anchoring elements 41 and the function elements are introduced in separate operation stages, with an intermediate load-free period of at least 3 months. Upon implantation of the function elements, the distal part 1 is attached in such a way that its intermediate part 11 essentially surrounds and fixes the bearing member 32 which is mounted on the axle element 31 which in turn is positionally fixed on the proximal prosthesis element 2. Fig. 3 shows a second embodiment of the invention. Here, alternative means for attaching the distal part are shown, consisting of a band-shaped attachment part 12 which is integral and surrounds a large part of the « periphery of the bone shaft 6. Besides the securing screws 42 which are fixed in the same way as described above, this prosthesis part is also secured with trans- verse screws 43 which are directly anchored in the cortex. The part 11 surrounding the bearing can in this case either be connected to the attachment part 12 of the prosthesis element at both its ends, or else consist of a one-side secured element 11 which must then have great rigidity. The proximal prosthesis element 2 also has a similar asttachment principle, and use is therefore also made here of transverse directly-anchored screws 43 which are arranged approximately at right angles to the securing screws 42 which are intended to be screwed into the anchoring members 41. Fig. 4 shows a further exemplary embodiment of an arrangement according to the invention. One change compared to what is described above is that component elements of each prosthesis part are here welded together. Thus, the figure shows a proximal prosthesis element 2 consisting of an attachment part 22 in the form of a plate which is connected to two flanges 7 by means of welds 71. The flanges 7 are provided with holes 21 which are intended to receive the axle ends of an axle element which is designed in accordance with the upper alternative shown in both Figs. 2 and 3. The distal part 1 is shown in the state in which it has not yet been welded together. The plate 12 is arranged with two longitudinal bend lines 17, so that the edge areas of the plate 12 have an angle corresponding to the angle of the bevelled surfaces 111 of the axle-fixing means 11. The through hole 110 in the fixing means 11 is intended for an axle element 31, 32. Fig. 4 also shows that those end surfaces 12A, 22A of the plates 12 and 22 respectively facing awa from the joint are bevelled in such a way that they form a downward-directed pointed edge. This design is to reduce the friction against soft tissue. Fig. 5 illustrates a possible embodiment of a plate 12 for achieving lower friction against soft tissue. The last-mentioned aim is achieved by using countersunk screws 42 (hexagonal socket cap) and by means of the fact that the holes 14 in the plate are provided with conical recesses 15 which correspond to the conicity of the screw head 42. In this way it is possible to obtain a fixing of the plate 12 which does not have any components project¬ ing above the upper surface of the attachment plate 12. Fig. 5 also shows that the anchoring member 41 is designed with a raised part 45, which on both sides has parallel opposite surfaces 46. The holes 14 in the plate 12 are also designed in a corresponding manner, that is to say wit recesses 16 which correspond to the shape of the raised part 45. A more secure connection between the anchoring member 41 and the plate 12 can be obtained in this way. In order for it to be possible for the screw head 42 to *' be screwed down to a level flush with the upper surface 12 of the plate, the raised part 45 is limited in terms of its height in relation to the thick¬ ness of the plate 12, and the raised part 45 is designed with a conical depression. The recesses 46 on the raised part 45 further serve as bearings, for example for an open end wrench when the anchoring members 41 are to be screwed into ' the bone. The figure also shows an expedient modification of the securing element 41. The element is~ in fact designed with two different diameters in the threaded paits, in which respect the threaded part 47 which lies nearest the raised part 45 has a greater diameter ' than the lower part 48. Between the two threads 47, 48 there is a section provided with a hole 49, this section having a smooth outer surface. The hole 49, which is parallel to the bevels 46, serves as a channel for blood and medullary fluid. In order to be self-tapping, the threads are designed with slots 47A. The threads will preferably "take" simultaneously, which requires that securing elements 41 with varying relative sizes should be prefabricated. Fig. 6 shows a further modification of a joint prosthesis in accordance with the invention. The prosthesis parts shown are largely designed in the same way as those shown in Fig. 4 and described above. In addition, each prosthesis part 1, 2 is designed with tongue-like members 112, 212 which are intended as positive locks at the ends of each bone shaft, by virtue of the fact that at least the proximal bone shaft is designed with a slit-shaped recess in towards the medullary cavity. The tongues 112, 212 are advantageously welded securely on the respective part/parts 11, 7. The embodiment in Fig. 7 moreover shows how the tongue- like members 112, 212 can be extended further into the medullary cavity and are in this case provided with a smooth-bored hole 113, 213 in line with the nearest fixture hole 14, 24 intended to cooperate with its respective fixture screw 41. The diameter of these holes 113 and 213 corresponds to the diameter of the smoot-h section of the fixture screw 41 situated between the threads 47, 48. These prosthesis parts are preferably implanted in a one-stage procedure, that is to say at the same time as the anchoring members 41. The hole placed nearest the joint, and the hole 113 or 213 which is coaxial with it, is then penetrated during the operation by a direct-anchoring fixture screw 41. The anchoring of the screw 41 in the cortical parts is achieved with the aid of holes arranged in the bone tissue in a correspond- ing manner (different diameters). The plate 112 on the proximal part 1 can, if so desired, be introduced without any additional measures into the medullary cavity, since the joint head here has already been removed. At the distal part 2, on the other hand, it is necessary, in accordance with the above, to create space for the plate 212 by making a slit in the medullary cavity, a slit which is advantageously milled out with the aid of a fixture. The invention is not limited to what is described above, but can be varied within the limits as have been set out in the following patent claims. It is thus possible to design the parts of the prosthesis in a number of bio¬ compatible materials, such as pure titanium, titanium alloys, HD-polyethylene, a number of ceramics, etc. In addition, it is not necessary for the entire component to be made of this biocompatible material, and instead in many cases it is only necessary for its surface to be made of such a material, that is to say a "coated" component. As has already been mentioned, the joint is primarily designed to be used in connection with small joints of the finger-joint type, but it is also possible to use the prosthesis on joints of larger dimensions, such * -as knee- or elbow-joints. In such a case it is conceivable for only one prosthesis element, preferably the proximal one, to be anchored inside the medullary cavity of the bone shaft 5. In addition, and in particu- lar in such, a case, it is not necessary for all the anchoring members 41 to be of the through type. The raised part 45 on the anchoring members 41 can advant¬ ageously have other than a circular or partial-circle form, for example a rectangular form which possibly also gives even better guiding. Moreover, the invention is not limited to the use of only one bearing member, but instead it is also conceivable to " use two bearing members 32, and in this case the securing hollows 21 for the axle element 31 would be arranged in a centrally positioned bracket, on the sides of which the two bearings would be arranged.

Claims: Patent Claims 1. Joint prosthesis, preferably for small joints such as finger-joints, comprising at least one distal prosthesis element (1) and at least one proximal prosthesis element (2), a joint (3) by means of which the said prosthesis elements (2, 3) are connected in an articulated manner, and securing means (4) for securing the said distal part (1) on the distal bone shaft (6) of the joint and the said proximal part (2) on the proximal bone shaft (5) of the joint, characterized in that at least one of the said prosthesis elements (1, 2) has at least one attachment part (12, 22) which can be applied on the periosteal part of the bone shaft (5, 6) on which it is intended to be attached. 2. Joint prosthesis according to patent claim 1, characterized in that the said prosthesis is intended to replace a physiological joint, where preferably only the joint head has been cut away. 3. Joint prosthesis according to patent claim 1, characterized in that the said securing means (4) com¬ prise securing elements (41, 42, 43) of which the majority are intended to be applied essentially trans¬ versely with respect to the principal extent of the said prosthesis element (1, 2). 4. Joint prosthesis according to patent claim 1, characterized in that the said securing elements (4) comprise anchoring members (41) of which at least one is designed to be of the through type. 5. Joint prosthesis according to patent claim 4, characterized in that each anchoring member (41) is provided with at least one internal thread for cooperation with a fixing screw (42). 6. Joint prosthesis according to patent claim 4, characterized in that at least one of the said through anchoring members (41) is surrounded by a section of the attachment part (12, 22) which can be inside the medul¬ lary cavity or on the underside of the bone, in the latter case for cooperation with two counter-directed fixing screws (42, 44). 7. Joint prosthesis according to patent claim " 5, characterized in that the end surface (surfaces) of the anchoring member (41) in the area for the said internal thread has a raised part (45) which is intended to cooperate with the peripheral part of the holes (14, 24) designed for this in each attachment part (12, 22). 8. Joint prosthesis according to patent claim 4, characterized in that at least the surface of the said anchoring members is made of biocompatible material. 9. Joint prosthesis according to patent claim 6, characterized in that the said anchoring members (41) are applied in a first stage, and in that the other parts of the prosthesis are applied in a second stage after incorporation of the anchoring members (41) in the bone tissue. ._ 10. Joint prosthesis according to patent claim 4, characterized in that at least one of the said attachment parts (12, 22) has an inner curved surface (13, 23) which is concave in shape, whose curvature essentially corres¬ ponds with the curvature of the periosteal part of the bone shaft (5, 6) on which it is intended to be applied. " 11. Joint prosthesis according to patent claim 1, characterized in that the said joint (3) comprises an axle element (31) which is positionally fixed with respect to a first of the said prosthesis elements (1, 2) 11. Ledprotes enligt patentkravet 1, k a n n e t e c k - n a d a v att namnda led (3) omfattar ett axelelement (31) som ar lagesfixerat med avseende pa ett fόrsta av namnda proteselement (1,2) samt atminstone ett lagerorgan (32) som ar onterat vid namnda axelelement (31) och lagesfixerat med avseende pa ett andra proteselement (1,2) . 12. Ledprotes enligt patentkravet 11, k a n n e t e c k- n a d a v att namnda axelelement (31) halles lagesfixerat medelst tva koaxiella urtagningar (21) hos den proximala protesdelen (2) . 13. Ledprotes enligt patentkravet 11, k a n n e t e c - n a d a v att en namnda lagerorgan (32) vasentligen om- slutande del (11) av den distala protesdelen (1) haller detta organ (31) fixerat. 14. Ledprotes enligt patentkravet 11, k a n n e t e c - n a d a v att namnda lagerorgan (32) ar ett kul- eller rull-lager. 15. Ledprotes enligt patentkravet 11, k a n n e t e c k- n a d a v att namnda lagerorgan (32) ar ett glidlager. 16. Ledprotes enligt patentkravet 1, k a n n e t e c k - n a d a v att samtliga i protesen ingaende detaljer ar gjorda av ett biokompatibelt material.

Inventors: ALBREKTSSON BJOERN, WENNBERG STIG, ALBREKTSSON, BJOERN, WENNBERG, STIG
Applicants: ALBREKTSSON BJOERN, WENNBERG STIG, ALBREKTSSON, BJOERN, WENNBERG, STIG
Application year: 1990
Main classification: A61F2
IPC codes: A61F2