Abstract:
Wrist prosthesis with a carpus element ( 2 ) to be implanted in the metacarpal bones, a radial element ( 1 ) to be fixed on the bone of the forearm, and, arranged between these, a transverse axis articulation ( 3 ) whose proximal part ( 9 ) is connected to the radial element ( 1 ). According to the invention, the proximal part ( 9 ) of the transverse axis articulation ( 3 ) is available in two embodiments, the first of which is rigid per se and the second of which comprises a further articulation ( 25, 28 ), and that [sic] both embodiments of the proximal part ( 9 ) of the transverse axis articulation ( 3 ) are connected or can be connected releasably to the radial element ( 1 ) and to the distal part ( 18 ) of the transverse axis articulation ( 3 ).

Description:
BACKGROUND OF THE INVENTION 
     The human wrist can execute pivoting movements in all spatial directions, the most important being the pivoting in the dorsal-palmar plane. In a known wrist prosthesis (DE-U 295 00 476), two articulations are therefore provided one behind the other or one within the other, one of them being designed as a transverse axis articulation which is able exclusively to execute a hinge movement for the dorsal-palmar pivoting, the other of them being designed as an elongate socket which is arranged to move essentially transverse to the former. In the case of an unstable ligament apparatus or certain muscle imbalances, prostheses are preferred in which the transverse pivoting is suppressed, with only the transverse axis articulation being provided (DE-U 70 12 087). Hitherto, it has been necessary to decide in each case between different makes, which is difficult when the decision can only be made during surgery. 
     SUMMARY OF THE INVENTION 
     The invention is therefore based on the object of making available a prosthesis which allows a decision to be made during surgery. 
     Accordingly, the proximal part of the transverse axis articulation is connected releasably to the radial element to be anchored in the ulna and is either designed to be rigid per se or is equipped with the further articulation permitting the transverse movement. A prosthesis system is thus obtained which makes it possible to make a decision during surgery as to whether a transverse mobility should be allowed or not and, accordingly, one or other of the alternative embodiments of the proximal part of the transverse axis articulation is used, while the other prosthesis parts remain the same. In particular, this system makes it possible for those elements of the prosthesis to be anchored in the bone to be implanted before a decision has to be taken regarding the transverse mobility of the prosthesis. A subsequent exchange is also made possible by this means. 
     The connecting arrangements provided between the proximal part of the transverse axis articulation and the radial element are expediently equipped for transmitting force with positive locking. According to a further feature of the invention, these can be pushed into one another essentially in the ulnoradial direction. In this way, a reliable connection is achieved which is independent of any screwing or clamping mechanisms. On the other hand, the said prosthesis part can be inserted without any great distraction between the already implanted carpal and radial elements. 
     The transverse axis articulation comprises, on one side, a T-type journal and, on the other side, a bearing part which comprises two aligned bores for receiving the T-type journal or its bearing. The T-type journal is pushed into these bores from one end, the outer of these two bores being slotted in the longitudinal direction to allow passage of the central pin of the T-type journal. The insert opening at the end of the bore is then closed. According to the invention, a threaded closure can be provided for this purpose. 
     The boundary between the mutually releasable distal and proximal parts of the transverse axis articulation can coincide with these bearing surfaces. However, since the bearing functions on the one hand, and the functions relating to the releasability and mutual fixing of the articulation parts, on the other hand, do not necessarily go together, it may be more expedient for all the bearing elements of the transverse axis articulation to be arranged on the distal part. 
     The carpus element has a plurality of distally extending pegs which are to be inserted into the metacarpal bones and which are connected by a transverse member extending essentially perpendicular to the pegs. It is known to arrange the articulation axis of the transverse axis articulation at an acute angle thereto. According to the invention, this angle is chosen in a range between 0° and 15°, preferably in a narrower range of 5° to 10°. 
     In the known prosthesis, the socket of the additional universal joint is arranged steeply inclined in relation to the transverse direction. According to the invention, this inclination is reversed in favor of an almost symmetrical design in which the edge tangent, i.e. the connecting line of the edges of the articulation surface of the socket in the palmar plane, encloses an angle of at least 5° and at most 25°, preferably from 10° to 15°, with the radius transverse direction or the complementary angle with the longitudinal direction of a stem to be fixed in the ulna. This angle can be greater in the sagittal plane. The difference between this angle and the angle mentioned in the preceding paragraph gives the directional difference between the carpus element and the radius element, which preferably lies between 0° and 10°. Correspondingly, the angle which the bearing axis encloses with the radius transverse direction, in the rigid version of the proximal part of the transverse axis bearing, is advantageously of the same order of magnitude. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in greater detail below with reference to the drawing which shows an advantageous illustrative embodiment of the invention, and in which: 
     FIGS. 1 and 2 show the dorsal view of a prosthesis according to the invention for the right hand, in both alternative embodiments, at about twice the natural scale, 
     FIGS. 3 and 4 show the corresponding side views in the direction of arrows III and IV, respectively, in FIGS. 1 and 2, 
     FIG. 5 shows a view, corresponding to FIG. 1, of the carpal element with exposed distal part of the transverse axis articulation, 
     FIGS. 6 and 7 show a side view and a plan view of the olive of the further articulation, 
     FIG. 8 shows a sectional view of the transverse axis articulation on a larger scale, 
     FIGS.  9 - 11  show corresponding views of a similar illustrative embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The prosthesis according to FIG. 1 comprises a radial element  1 , a carpus element  2  and a transverse axis articulation  3 . The radial element comprises a stem  4  which is to be implanted in the bone cavity at the distal end of the ulna. It ends in a connecting arrangement  35  with ribs and grooves which extend approximately in the ulnoradial direction and which are dovetail-shaped. By means of rib engagement  36  in corresponding grooves of the mating piece, the flanks of the dovetail groove are prevented from escaping from the engaged position in the event of high forces having to be transmitted. After implantation of the stem  4 , the proximal part  9  of the transverse axis articulation  3  is capable of being pushed with its connecting arrangement onto that of the stem from the ulnar direction. The positively locking parts of the connecting arrangements provide essentially all of the force transmission. To secure the engagement and the correct relative position, a securing device can be provided, for example a screw  10 . 
     The proximal part  9  of the transverse axis articulation  3  comprises two mutually aligned bore sections  11 ,  12 , the axis  13  of which extends approximately in the ulnoradial direction, i.e. approximately perpendicular to the center axis  14  of the stem  4  in the palmar plane. Situated between the bore sections  11  and  12  there is a cutout  15  which permits up to 70° flexion and extension. The end of the bore section  11  is closed. The outer end of the bore section  12  is initially open. The bore section  12  is slotted at  17  on either side, preferably on the distal side. 
     The distal part  9  of the transverse axis articulation  3  is formed by a T-type journal  18 ,  19  whose length and diameter match the bore sections  11 ,  12  and whose neck piece  19  is pivotable inside the cutout  15  in the assembled state. Details of the embodiments are discussed later with reference to FIG.  8 . On assembly, the journal  18  is pushed from the direction of the open side into the bore  11 ,  12 , the neck piece  19  sliding through the slot  17  of the bore section  12 . Since the axis  13  and the direction of the ribs and grooves  5  to  8  are approximately parallel, it is possible, after implantation of the carpus element  2  and of the radial element  1  and upon insertion of the proximal part  9  of the transverse axis articulation  3 , for the latter to engage simultaneously with the connecting arrangement  35  of the radial element  1  and with the journal  18 . 
     The neck piece  19  of the T-type journal is connected rigidly to a crosspiece  21  of the carpus element  2 , from which crosspiece  21  the pegs  22 ,  23 ,  24  of the carpus element  2  issue in the distal direction. They lie in a palmar plane and are to be anchored in the metacarpal bones. The crosspiece  21  of the carpus element  2  can be made plate-shaped. Transverse to the palmar plane, its dimensions should be reduced so that it does not obstruct the route of the tendons and nerves. 
     The axis  20  of the T-type journal  18  extends at an acute angle α to the crosspiece  21  of the carpus element  2 . In the example shown, this angle is approximately 10°. The crosspiece  21  extends at 90° to the pegs  22  to  24 . The bores  11 ,  12  in the proximal part of the transverse axis articulation  3  extend, in relation to the transverse direction  37  of the radial element  1 , likewise at an angle γ of approximately 10°, so that the longitudinal directions of the pegs  22  to  24  on the one hand and of the stern  4  [sic] on the other hand are aligned with one another or parallel to one another. 
     It will be readily appreciated that the prosthesis according to FIG. 1 permits a pivoting movement exclusively about the axis  13  of the transverse axis articulation. This prosthesis is therefore suitable particularly in those cases where the natural control of the remaining degrees of freedom of the natural hand by the ligament apparatus and the associated muscles is no longer possible. 
     The releasable, rigid connection between the stem  4  and the proximal part  9  of the transverse axis articulation  3  allows this part  9  to be replaced by the part shown at a corresponding position in FIGS. 2 and 4. The embodiment according to FIG. 2 corresponds fully to that according to FIG. 1, except for the presence of a modified proximal part  9  of the transverse axis articulation . The latter is formed by a socket  25  which forms a concave articulation surface  26  pointing in the distal direction, and a complementary convex articulation surface  27 , pointing in the proximal direction, on the olive  28  in which the bore sections  11 ,  12  for the transverse axis bearing are contained. The socket part can be designed so that its height can be varied, e.g. in order to additionally compensate for carpal height reductions as a result of bone defects. The articulation surfaces  26 ,  27  are preferably not spherical, but elongate in the ulnoradial direction. In the palmar plane (FIG.  2 ), the radius of curvature and the extension of the articulation surfaces are greater than in the sagittal lane (FIG.  11 ). The angle β which the edge tangent  29  encloses with the transverse direction of the radial element is about 10° to 15°. 
     The articulation surfaces  26 ,  27  permit a pivoting movement (abduction and adduction) in the palmar plane. To ensure that this is possible, their cross-sectional shapes essentially match, viewed in radial planes relative to the common pivot axis. Since the radii of curvature transverse to the palmar plane are essentially smaller than the radius of curvature in the palmar plane, then, in the event of a palmar-dorsal movement (for which the transverse axis articulation is intended), they cannot slide on one another and maintain their full-surface contact and so counteract such a movement. However, they do not act rigidly under this load and therefore permit a certain degree of pronation or supination. Nor are they completely rigid in respect of torsional stress. This satisfies the aim that the prosthesis components should be connected not completely rigidly to one another, in order to relieve the peak forces acting on their anchoring in the bone. 
     As will be seen from FIGS. 5 and 8, the journal  18  of the distal transverse axis articulation comprises a bearing bore  39  which receives a pin  40  which is rigidly connected to two end pieces  41 ,  42  which are located on each side of the journal  18  and secure the pin  40  therein. The external diameters of the end pieces  41 ,  42  are a little greater than the external diameter of the journal  18  and are slightly less than the diameter of the bore  11 ,  12  in the proximal part  9  of the transverse axis bearing. In this embodiment of the prosthesis, the part  18  does not therefore lie on the inner surface of the bore  39 , but the end pieces  41 ,  42 . This makes it possible to exempt the bore  11 ,  12  of the bearing function. The latter is instead taken over by the pin  40  in the bore  39 , with the end pieces  41 ,  42  taking over the axial forces. 
     To secure the journal  18  and the associated arts in the bore  11 ,  12 , and to transmit the axial forces to the proximal part of the transverse axis articulation, a locking screw  43  is arranged transversely in the end piece  42  (FIG.  8 ). Its overall length is smaller than the diameter of this end piece  42 , so that it does not impede assembly when it is screwed with its collar  44  as far as a corresponding limit stop  45  in the bore contained in the end piece  42 . After assembly, it is unscrewed until its head attachment  46  passes into the aligned bore  47  of the olive  28  or of the proximal part  9  of the transverse axis articulation . For this purpose, the head  46  is provided with key surfaces  48  which are accessible via the bore  47  when the prosthesis is used for the right hand. In the case of the left hand being operated on, the opposite side of the prosthesis faces the operating surgeon. Thus, a corresponding bore  49  is provided on the opposite side, this bore  49  giving access to key surfaces  50  provided on the screw base. 
     The further illustrative embodiment according to FIGS. 9 to  11  is identical to the above-described embodiment, unless expressly stated otherwise. 
     The open end of the bore  11 ,  12  is provided with a thread which permits closure by means of a threaded stopper  16 . Instead of this, a snap ring or the like can also be provided. 
     In the embodiment according to FIG. 9, the axis  13  of the transverse axis articulation is arranged approximately perpendicular to the axis  14  of the radial element. The result of this is that the carpus element with its pegs  22  to  24 , on the one hand, and the radial element  4 , on the other hand, are set at a slight angle to one another. 
     Correspondingly, the socket  25 ,  26  in the embodiment according to FIG. 10 is symmetrical to the axis  14  of the radial element. 
     For the connecting arrangements between the proximal part  9  of the transverse axis articulation and the radial element  14 , an arrangement, differing from the above-described embodiment, of ribs  5 ,  8  and grooves  6 ,  7  is provided on the radial element, these cooperating with corresponding ribs and grooves of the proximal transverse axis part. 
     Finally, a difference between this illustrative embodiment and the one described previously is that the journal  18  of the transverse axis articulation , on the one hand, and the bore  11 ,  12 , on the other hand, directly form the articulation surfaces sliding on one another.