Abstract:
A joint prosthesis has a bending hinge, which is formed by a hinge fork and an axle pin, which comprises two axle stubs, which are arranged in an installation position for insertion, in which installation position the axle stubs are retracted in a coupling piece, and in an expanded position after implantation by movement in the axial direction into aligned hinge holes of the hinge fork, wherein the axle pin has two bearing areas at the ends of the axle pin and a joining area lying therebetween, and the axle pin is separated in the joining area along a plane extending in the axial direction, which plane intersects with the jacket of the axle pin at two points.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national stage application under 35 USC 371 of International Application No. PCT/EP2012/057165, filed Apr. 19, 2012, which claims the priority of European Application No. 11 163 204.8, filed Apr. 20, 2011, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a joint endoprosthesis having a bending hinge, the axial pin of said bending hinge comprising two pin stubs which are arranged in an installation position, in which they are retracted into the coupling piece, for insertion, and are arranged in a spread position after insertion of the central part by being moved in their longitudinal direction into aligned hinge holes in the hinge fork. 
       BACKGROUND OF THE INVENTION 
       [0003]    Endoprostheses of this kind are used, in particular, as prostheses for knee joints. Owing to the high loading on knee joints by virtue of the body weight of the patient and owing to the complex movement sequence of said knee joints, they are comparatively more susceptible to dysfunction due to wear or disease. The replacement of a knee joint by an endoprosthesis is a complicated operation which puts stress on the patient. It is therefore desirable to be able to implant the endoprosthesis in a manner which is as straightforward and insusceptible to faults as possible in order to avoid complications. 
         [0004]    To this end, a joint endoprosthesis of the kind cited in the introductory part which has a spreadable axial pin has been disclosed (EP 1 381 335 B1). Said joint endoprosthesis comprises a tibial component and a femoral component which are connected to one another in an articulated manner by means of a coupling piece. A bending joint and a rotary joint are formed. The bending joint allows flexion and extension of the knee. To this end, the coupling piece has an axial eye into which an axial pin, which comprises pin stubs, is inserted. The pin stubs are provided with mutually complementary coaxial recesses, with the result that they can be moved toward one another or away from one another in a telescopic manner along their common center axis. For insertion purposes, the pin stubs have to be moved completely toward one another and are therefore located in an installation position in which the pin stubs are retracted into the coupling piece. After the coupling piece has been installed in the mating piece, a hinge fork on the femoral component, said pin stubs are spread and thereby expand into the holes in the hinge fork which are aligned with one another. As a result, the coupling piece is connected to the femoral component in a pivotable manner. This axial pin allows simpler installation, however complications may arise in the event of the pin stubs becoming jammed. 
       SUMMARY OF THE INVENTION 
       [0005]    The invention is based on the object of providing, proceeding from the last-mentioned prior art, an improved joint endoprosthesis which avoids this disadvantage. 
         [0006]    A solution according to the invention can be found as broadly described herein. Advantageous developments are the subject matter of the detailed embodiments described below. 
         [0007]    In a joint prosthesis having a bending hinge which is formed by a hinge fork and an axial pin which comprises two pin stubs which are arranged in an installation position, in which they are retracted in a coupling piece, for insertion purposes, and are arranged in an expanded position after implantation by being moved in the axial direction into aligned hinge holes in the hinge fork, provision is made, according to the invention, for the axial pin to have two bearing regions at its ends and a joining region which is situated between said bearing regions, and for the axial pin to be separated in the joining region along a plane which runs in the axial direction and which intersects the casing of the axial pin at two points. 
         [0008]    The invention is based on the idea of dividing the joining region of the two pin stubs into two half-shafts, specifically by means of a plane which runs along the center axis and intersects the casing of the axial pin at two points. This is generally, but not necessarily, the center plane. Said plane can be composed of several component planes, but it is continuous in most cases. Since said plane runs from one side to the other side, it has a maximum width, that is to say is the same size as the diameter of the pin in the joining region. This produces a plane which extends over the entire diameter and along which the two pin stubs are guided with their maximum width. Guidance over a wide region ensures favorable friction conditions and thereby prevents jamming. Since, moreover, the guide plane intersects the casing at at least two points, a degree of freedom is available for compensating for tilting. This constitutes a considerable advantage over the prior art with closed—for example hollow-cylindrical—guide planes which can have a tendency to become blocked on account of tilting. The invention therefore provides considerable advantages both during the initial implantation and also in the case of any subsequent adjustments. 
         [0009]    The pin stubs preferably complement one another in their joining region. As a result, the same outer contour as that exhibited by the undivided axial pins which are conventionally used can be achieved by simply plug-connecting said pin stubs. The axial pin according to the invention can therefore be used in conventional coupling pieces. Particular preference is given to the pin stubs not only being complementary but even having the same shape. In a well-established embodiment, the pin stubs have half-pegs in the joining region. By way of example, the cross section in the joining region has a half-moon shape. 
         [0010]    The coupling piece advantageously has a separating element with an aperture for receiving the joining region of the pin stubs, with the aperture being non-round. As a result, the two pin stubs can be prevented from rotating in the coupling piece. Therefore, a separate rotation-prevention means, for example in the form of a screw which can be easily lost, is no longer required. 
         [0011]    A movement-prevention means can be arranged on the coupling piece. Said movement-prevention means acts on the two pin stubs and secures them in their spread position. Therefore, a spring, as provided in the prior art for spreading purposes, is not required. In the event of an adjustment, the pin stubs can therefore be moved into the non-spread installation position by simply removing the movement-prevention means, without it being necessary to permanently overcome the counteracting force of the spreading spring for this purpose. 
         [0012]    In order to be able to act on the pin stubs in the coupling piece, an access slot is provided on each coupling piece, preferably for each pin stub. Said access slot is oriented such that its elongate extent lies in the axial direction and has an extent which is at least the same size as the area over which the pin stubs are to be spread. A spreading instrument which can be inserted into the access slots is provided for the purpose of moving the pin stubs. Said spreading instrument is expediently in the form of a clip with two free ends for insertion into the access slots, wherein the free ends can preferably be at an angle of 30 degrees to 75 degrees in relation to one another. Said spreading instrument can be formed from a piece of wire, the wire forming a three-point loop, which faces away from the free ends, in the rear region of said piece of wire. A sliding sleeve can be mounted on said three-point loop, said sliding sleeve moving from an inoperative position, in which the free ends are close to one another, the to a movable spreading position in which the free ends are moved away from one another. 
         [0013]    A fastening guide can be arranged on the spreading instrument. Said fastening guide is designed as a sleeve in which a locking screw is arranged. To this end, said sleeve expediently has an internal thread into which the locking screw can be screwed. The fastening guide is oriented such that it targets the pin when the spreading instrument is mounted on the coupling piece. The length of the locking screw is preferably greater than the distance between the coupling-piece-side end of the fastening guide and the joining region of the pin stubs. This ensures that the spreading instrument, together with the fastening guide, can be removed only if the pin stubs are secured in their spread position. 
         [0014]    The sliding sleeve can have a circumferential radial collar which has a recess in its side which faces the fastening guide. This ensures movement also on a curved clip of the spreading instrument and a straight access path for a screwdriver to the locking screw which is arranged in the fastening guide. 
         [0015]    Furthermore, a pair of adjustment pliers can be provided. Said pair of adjustment pliers has handles at its rear end and receiving fingers at its front end, said receiving fingers being designed to be passed through the access slots and having an oval cross section. The receiving fingers diverge at the front at an angle of preferably between 5 and 25 degrees. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The invention will be explained in greater detail below with reference to the appended drawing which illustrates an advantageous exemplary embodiment. In said drawing: 
           [0017]      FIG. 1  shows a rear view of a joint prosthesis; 
           [0018]      FIG. 2  shows a perspective view of the coupling piece; 
           [0019]      FIGS. 3   a - c  show side views of the coupling piece; 
           [0020]      FIGS. 4   a - c  show views of a detail of a pin stub; 
           [0021]      FIG. 5  shows a plan view of a spreading instrument; 
           [0022]      FIG. 6  shows a side view of the coupling piece with the spreading instrument in section; 
           [0023]      FIG. 7  shows a perspective view relating to  FIG. 6 ; and 
           [0024]      FIGS. 8   a  and  b  show a perspective view of a pair of adjustment pliers and a view of a detail of a pair of adjustment pliers. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The endoprosthesis according to an exemplary embodiment of the invention will be explained using a knee joint endoprosthesis. The endoprosthesis of a knee joint substantially comprises two components  1 ,  2 , one being in the form of a tibial component  1  and the other being in the form of a femoral component  2 . In the femoral component  2 , a femoral bearing half  21 , which has two condyle-like runners  22  which project in the manner of a fork in relation to the tibial component  1 , adjoins a stem  20  which is inserted into a femur of a patient. Said runners are supported on a tibial plateau  12  which is arranged on a tibial bearing half  11  which is fastened to a tibia of the patient by means of a stem  10 . 
         [0026]    A coupling piece  3  is arranged between said stems, said coupling piece a T-shaped piece  30  as a main body with a receiving eye  33 , which is arranged in its upper region, for an axial pin  4  and with a bearing pin  31  for being received in a bearing bushing in the tibial bearing half  11 . 
         [0027]    A first bearing (flexion bearing) allows a pivoting movement between the components  1  and  2 , that is to say realizes the bending movement between the thigh and the lower leg. This pivoting movement about the axis of the axial pin  4  therefore forms the first axis for the movement of the knee joint prosthesis. The pin  31  is oriented transverse to said first axis by way of its center axis which forms a second axis for the rotary movement with which the femur part  2  rotates about the second axis relative to the tibia component  1 . For this rotary bearing, the pin  31  of the coupling piece  3  projects into the bearing bushing. A bearing insert  32  is arranged between said pin and bearing bushing. 
         [0028]    The spreadable axial pin  4  is arranged in the receiving eye  33 . Said axial pin has a plurality of regions, in each case a bearing region  41  at the two ends and a joining region  40  between said two ends. In  FIG. 3   b , said axial pin is illustrated in its installation position in which it is retracted into the receiving eye  33 , and in  FIG. 3   c  is illustrated in its spread position in which the bearing regions  41  project out of the receiving eye  33  on both sides. Said axial pin is passed through an aperture in a separating element  30  in the receiving eye  33 , said aperture having the shape of a flattened oval and preventing rotation of the axial pin  4 . 
         [0029]    The coupling piece  3  is provided with a flat receiving area  35  on its front side (on the left-hand side in the illustration in  FIG. 3   a ). A pocket  36  is formed in the receiving eye  33  at the upper end of said receiving area, said pocket merging with the receiving area  34  in a flat manner at its base. A retaining opening  35  which is designed in the form of a blind hole is provided in the central region of the receiving area  34 . An impact-protection plate  37  can be mounted on the receiving area  34  and is inserted into the pocket  36 . As a result, the pocket  36  is secured against lifting off from the receiving area  34  by way of its upper edge, specifically also under the action of force from the front (from the left-hand side in  FIG. 3   a ) when the extended stop position of the flexion bearing is reached. The impact-protection plate  37  is protected against movement, in particular in the downward direction, by means of a projection which is formed on the rear face of said impact protection plate and engages in the retaining opening  35  in an interlocking manner. 
         [0030]    The axial pin  4  has a separating plane  49 , which runs along to form its center axis, in the joining region  40 . Said separate plane intersects the casing in the joining region  40  at two diametrically opposite points (see  FIG. 3   a ). The axial pin  4  is thereby divided into two pin stubs  42 ,  43 . The two said pin stubs have a projecting half-peg  44 ,  45 , which pegs form the cross section of the axial pin  4  in the joining region  40  when combined. 
         [0031]    Furthermore, each of the two pin stubs  42 ,  43  has a cavity  46 ,  47  in the shape of a half moon. Said cavities are shaped to complement the half-pegs  44 ,  45 , with the result that each half-peg enters the cavity  46 ,  47  when the pin stubs are pushed together (for the installation position, as illustrated in  FIG. 3   b ). In the spread position, the half-pegs  44 ,  45  move out of the cavities  46 ,  47 , said half-pegs being securely guided by the wide separating plane  49 . Owing to this wide guidance in a flat plane, the spreading process does not lead to jamming or to tilting; the same applies when said half-pins are pushed together in the case of adjustment of the knee joint prosthesis. The design of the pin stubs  42 ,  43  is illustrated in detail in  FIG. 4 . Said pin stubs further have an actuator opening  48 , which is designed to receive a spreading instrument  5 , on their casing. Two actuator openings  48  are preferably provided for each pin stub, specifically symmetrically in relation to the separating plane  49 . One is used in each case, specifically depending on the installation position. In the illustrated exemplary embodiment, the pin stubs  42 ,  43  are the same shape, that is to say the same element can be used both as pin stub  42  and also as pin stub  43 . 
         [0032]    In order to spread the pin stubs  42 ,  43 , two access slots  38  are formed on the receiving eye  33 . Said access slots are oriented on an aligned line with an orientation of their elongate extent in the direction of the center axis of the axial pin  4 . A fastening opening  39  is provided above and centrally between said access slots. A pointed instrument can be inserted through the access slots  38 , said instrument engaging in one of the two actuator openings  48 . The pin stub  42  is removed from the other pin stub, and vice versa, by moving the instrument outward. 
         [0033]    The spreading instrument used for this purpose is illustrated in greater detail in  FIGS. 5 and 6 . Said spreading instrument comprises a wire clip  50  with two free ends  51  at its front end, said free ends diverging at an angle α. Said wire clip forms a three-point loop in its rear region, a sliding sleeve  54  being mounted on said three-point loop. Said sliding sleeve has a radially projecting collar  56 , which is provided with a recess  55 , on its side which faces away from the ends  51 . The sliding sleeve  54  can, between its spreading position, as illustrated in  FIG. 5 , be moved forward into an inoperative position, as a result of which the free ends  51  are pushed into a position in which they are closer together. A certain adjustment force is required for this purpose, with the result that the sliding sleeve  54  functions as a means for preventing unintentional movement. The recess  55  ensures that the collar  56  does not collide with the wire clip  50  during the movement. Said recess also ensures unimpeded straight access to the fastening sleeve  60 . 
         [0034]    The fastening sleeve  60  is part of a fastening guide  6  which is arranged at the end of the wire clip  50 . The fastening sleeve has an internal thread  62  in its inner opening  61 , a locking screw  63  being screwed into said internal thread. In the inoperative position, the locking screw  63  projects only slightly by way of its tip  64 , specifically to such an extent that it engages in the opening  39  by way of its thread and in this way secures the fastening guide to the coupling piece  3 . 
         [0035]    During installation, the sliding sleeve  5  is moved backwards for the purpose of spreading the axial pin  4 , as a result of which the free ends  51 , which are inserted into the actuator openings  48  in the pin stubs  42 ,  43  through the access slots  38 , are moved away from one another and thereby spread apart the pin stubs  42 ,  43 . Once said pin stubs have reached their spread position (see  FIGS. 3   c  and  7 ), the locking screw  63  is turned further, until it enters the separating plane  49  by way of its tip and secures the pin stubs  42 ,  43  by clamping and, if there is a corresponding receiving opening  49 ′, even in an interlocking manner. If the locking screw  63  has been turned sufficiently far, it is no longer in engagement with the internal thread  62 . The fastening guide  6  is therefore free, and the spreading instrument  5  can then be removed (for the first time). This ensures that the spreading instrument can be removed only when the pin stubs  42 ,  43  are spread. 
         [0036]    In the event of an adjustment, it is necessary to move the pin stubs  42 ,  43  out of the spread position (see  FIG. 3   c ) back into the installation position (see  FIG. 3   b ). To this end, a set of adjustment pliers  7  is provided with a handle  70  at the rear end and with receiving fingers  70  on jaws  71  at the front end (see  FIG. 8 ). The dimensions of the receiving fingers are such that said receiving fingers can be inserted through the access slots  38  and engage with the actuator openings  48 . The receiving fingers  72  are preferably arranged in a converging manner. This means that they are oriented so as to point toward one another, specifically through an angle β of approximately 10 degrees. This angle is selected such that the receiving fingers  72  are approximately parallel in the position of the set of adjustment pliers  7  when the pin stubs  42 ,  43  have reached their installation position (see  FIG. 3   b ), for the purpose of simpler removal.