Abstract:
For replacing a human hip or knee joint and the adjoining bone sections, an implantable prosthesis is provided, comprising a joint replacement part ( 10 ), a shaft replacement part ( 12 ) and a rod-shaped shaft anchoring part ( 16 ) being insertable into an axial cavity ( 14 ) in the shaft replacement part ( 12 ).

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an implantable prosthesis for replacing a human hip or knee joint and the adjoining bone sections, wherein the prosthesis comprises a joint replacement part, a shaft replacement part being connectable to the joint replacement part or integral therewith and having a central cavity, as well as a rod-shaped shaft anchoring part being insertable into the cavity. An implantable prosthesis of this kind is known from EP 1 371 346 B1. 
     After resection of a bone tumor, the removed bone and the joint concerned can be replaced by a tumor prosthesis, and there is initially no difference in leg length. The prosthesis is anchored in the remaining bone, as described, for example, in EP 1 371 346 B1. When a difference in leg length develops due to growth, the bone can be osteotomized and the prosthesis can be provided with a distraction medullary nail instead of the shaft anchoring part for performing the callus distraction method. Distraction medullary nails of this kind are described, for example, in EP 0 432 253 B1. 
     If a second lengthening step is required, this procedure must be performed again. In any case, however, after completion of length growth, the distraction medullary nail, which cannot remain in place for permanent shaft anchoring, must be replaced by a stable shaft anchoring part. Exchange of a distraction medullary nail or replacement by a stable shaft anchoring part is often only possible if the joint components are decoupled, so that extensive operative exposure is required. Thus, in addition to resection of the tumor, at least two, in most cases even three extensive surgical operations are required near the joint in the region of the prothesis, which implies a considerable risk of infection. 
     Therefore the object underlying the invention is to considerably reduce the extent of a repeated surgical operation, and thus the risk of infection, in the case of a set tumor prosthesis while maintaining the advantages of bone lengthening according to the callus distraction method, wherein the artificial joint itself is to be touched as little as possible. 
     SUMMARY OF THE INVENTION 
     This object is achieved on the basis of the implantable prosthesis of above-mentioned type by the fact that the shaft anchoring part, at its side facing away from the joint replacement part, comprises locating means for forming a channel through the bone section at the side facing away from the joint replacement part from the outside, the channel being axially aligned with the shaft anchoring part and reaching up to the shaft anchoring part, and attachment means for engagement with a tool located outside the bone section through the channel for exerting at least a traction force on the shaft anchoring part, enabling to remove it from the bone section through the channel. 
     The locating means provided at the shaft anchoring part thus enables directed drilling or milling of a channel being axially aligned with the shaft anchoring part inserted in the bone section and having a diameter allowing passage of the shaft anchoring part. Thereafter the attachment means at the end side of the shaft anchoring part facing away from the joint replacement part enables engagement of a tool by means of which the shaft anchoring part can be withdrawn from the cavity of the shaft replacement part through the milled channel in the bone. Subsequently a distraction medullary mail can be introduced instead of the shaft anchoring part through the milled bone channel into the cavity of the shaft replacement part and be fixed in a known manner. After the desired length growth of the bone at a previously performed osteotomy has been completed, the distraction medullary mail can be removed again through the channel in the bone and be replaced by a solid shaft anchoring part of corresponding length, which thereafter remains part of the prosthesis. All of these measures can be performed via the side of the bone facing away from the joint replacement part without requiring repeated large operations in the region of the prosthesis, so that the risk of infection is minimized. 
     The locating means can be a radiation source being non-hazardous to patients, which is associated with the shaft anchoring part at its side facing away from the joint replacement part, wherein the channel can be formed in the required design through measuring for the radiation source from outside the bone section. 
     Advantageously, also a wire-shaped guide element protruding centrally from the attachment means can be used as the locating means, wherein the wire-shaped guide element passes through the bone section and protrudes therefrom to the outside. The connection between the guide element and the attachment means is advantageously designed to be releasable. This can be achieved, for example, by the fact that the attachment means has an internally threaded bore and the guide element has a corresponding external thread. 
     In order to facilitate withdrawal of the shaft anchoring part through the milled channel under tractive action of the tool, the shaft anchoring part is appropriately formed such that also a torque can be transmitted by the tool to the shaft anchoring part. 
     For this, the shaft anchoring part additionally includes a polygonal socket in front of the internally threaded bore towards the tool side. The tool used therein is designed to have two portions; Diametrically inside it has an element for traction force transmission, for example a tie rod having an external thread, and diametrically outside it has an element for transmission of the torque to the shaft anchoring part, for example a sleeve-shaped connection element having a polygonal plug. 
     In case a guide element is used as the locating means, the guide element is screwed out of the internally threaded bore forming the attachment means. Thereafter the tie rod can be led through the sleeve-shaped connection element and screwed into the internally threaded bore of the attachment means at the shaft anchoring part, in order to exert axial traction on the shaft anchoring part, while a torque can be transmitted from outside through polygonal engagement between the connection element and the attachment means, whereby the shaft anchoring means can be extracted by means of traction and torque. 
     Advantageously, a drill cutter is provided on the shaft anchoring part at the tool attachment side, which during rotation of the shaft anchoring part by means of the tool located outside the bone takes away any bone parts possibly protruding in the milled channel and thus facilitates withdrawal of the shaft anchoring part. 
     The surface of the shaft anchoring part is smooth, preferably polished and appropriately has a surface roughness depth of 0.1 μm or less. This largely excludes engagement of the living bone with the shaft anchoring part, so that the shaft anchoring part can be withdrawn from the medullary space without large resistance. 
     In order to enable withdrawal of the shaft anchoring part from the cavity of the shaft replacement part without any problems, the inner wall of the cavity is appropriately designed to be slide friction reducing, for example by applying a thin slide coating or by inserting a plastic sleeve. 
     Preferably, in the joint replacement part or the shaft replacement part there is provided a lateral bore opening into the cavity, wherein a cable can be led through said lateral bore, which cable can be connected at its one end to a subcutaneous antenna for supplying energy and at its other end to a drive of a distraction medullary nail when the latter is introduced into the shaft replacement part after the shaft anchoring part has been removed in order to perform bone distraction according to the callus distraction method. 
     During the first surgical operation, connection means can be mounted at the shaft replacement part in the region of its end facing away from the joint replacement part for fixing it to an adjoining bone section; however, said connection means do only gain in importance when a distraction is performed. 
     The shaft anchoring part over its length has an invariable diameter, but it may also taper towards the joint replacement part or have tapered sections, as may also be the case with a distraction medullary nail. 
     The shaft anchoring part can be a massive stabilizer or a medullary nail having an integrated drive for distraction. 
     In the configuration according to the invention, the implantable prosthesis with its joint forming part can be designed in a conventional manner, which has significant advantages, because the prosthesis lock to the tibia component can be designed in an essentially simpler and more stable manner, and the prosthesis does not have to be decoupled any more and can remain permanently in place. Also the connection to the shaft replacement part, the length of which can be designed to be variable and as short as possible, so that only the defect produced by the resection is bridged, can be designed conventionally, for example as a cone and screw connection. 
     In cases where a knee joint and the femur shaft portion near the knee joint are to be replaced, first the tibial joint component is implanted after a resection en bloc of the affected bone portion of the femur. In the case of children preferably a tibia plateau component with a PTFE inlay having a polished shaft anchoring part is used, wherein rotational stability is achieved via pins below the tibia plateau component. In the case of said arrangement the polished shaft anchoring part is able to slide under pressure of the growth cartilage obtained and the remaining growth potential of the proximal tibia growth cartilage can be utilized. 
     Subsequently, a guide wire is introduced into the greater trochanter in the frontal plane in extension of the medullary space and in the lateral plane at the height of the front one-third point of the proximal femur and is pushed forward through the medullary space from proximally up to the defect. The medullary space is milled out gradually via said guide wire as far as necessary for introducing the shaft anchoring part. Then the joint replacement part and the shaft replacement part are chosen depending on the size of the defect. Thereafter the straight shaft anchoring part adapted to the diameter of the medullary space and polished on the outside is introduced into the cavity of the shaft replacement part at the open side thereof and connected in a positive-locking manner by means of bolts or screws. Subsequently, the prosthesis can be placed in situ and be connected, on the one hand, to the guide wire in the central opening of the shaft anchoring part, which guide wire has previously been introduced from proximally, and, on the other hand, via the prosthesis lock to the tibia plateau component. The guide wire is cut off about one centimeter above the trochanter tip. Additionally an L-plate is mounted at the joint replacement part and the shaft replacement part, respectively, at their lateral side, wherein the later sliding segment is already at this time fixed via the L-plate by means of one or two small fragment screws. 
     For lengthening the remaining thigh bone, first the medullary space is milled out gradually corresponding to the diameter of the envisaged distraction medullary nail up to the height of the planned osteotomy by means of a straight cannulated milling tool adapted to the set guide wire. Subsequently, the bone is cut through in a minimally invasive manner, preferably by means of a medullary space saw. The milling operation is continued until the shaft anchoring part has been reached. Therein a possibly existing curvature of the bone can be overcome, because after osteotomy the two bone segments are normally very short and align towards one another. As soon as the shaft anchoring part has been reached, a cannulated extraction tool is introduced via the guide wire, wherein a polygonal plug at the end of the cannulated extraction tool engages in a corresponding polygonal socket in the shaft anchoring part, so that rotational forces for releasing the shaft anchoring part after removal of the positive-locking connection between the shaft anchoring part and the joint replacement part and the shaft replacement part, respectively, can be transmitted. Then the guide wire is screwed out of the attachment means and a tie rod is screwed through the cannulated tool into the thread of the attachment means. Therewith also an axial force can be exerted on the shaft anchoring part and the shaft anchoring part can be extracted towards the proximal direction. Herein a drill cutter at the side of the shaft anchoring part facing away from the joint replacement part can take off smaller intramedullary obstacles. Thereafter the distraction medullary nail is introduced into the cavity of the shaft replacement part through the bone channel formed in this way. The distraction medullary nail is connected in a positive-locking manner by means of bolts or screws, on the one hand, through the provided openings to the joint replacement part and the shaft replacement part, respectively, and on the other hand to the proximal femur. If an electromotive drive with a subcutaneous receiving antenna is used, it may be required depending on the embodiment that the drive is introduced in advance into the shaft replacement part. For this, the cable is introduced in advance through the channel in the proximal femur vai the osteotomy into the opening of the joint replacement part and the shaft replacement part with the aid of a strand or a wire and led out laterally. 
     A similar procedure is performed when replacing a hip joint and the femur shaft portion near to the hip joint. Unless hemiarthroplasty is envisaged, at first the hip socket is implanted after a resection en bloc of the affected bone portion. Subsequently a guide wire is intercondylarly introduced in the frontal plane centrally and in the lateral plane in extension of the distal femur shaft axis and advanced through the medullary space up into the defect. The medullary space is milled out gradually via said guide wire as far as necessary for introducing the shaft anchoring part. The joint replacement part and the shaft replacement part are chosen depending on the size of the defect. Then the straight shaft anchoring part adapted to the diameter of the medullary space and polished on the outside is introduced into the opening of the cavity of the shaft replacement part facing away from the joint replacement part and connected in a positive-locking manner by means of bolts or screws. Thereafter the prosthesis is placed in situ, wherein the guide wire is introduced into the central opening of the shaft anchoring part. The guide wire is cut off in the notch. Additionally an L-plate is mounted laterally at the joint replacement part and the shaft replacement part, respectively, wherein the later sliding segment is already at this time fixed via the L-plate by means of one or two small fragment screws. 
     After length grown has been completed, first the medullary space is milled out gradually corresponding to the diameter of the envisaged distraction medullary nail up to the height of the planned osteotomy by means of a straight cannulated milling tool adapted to the set guide wire. Subsequently, the bone is cut through in a minimally invasive manner, preferably by means of a medullary space saw. The milling operation is continued until the shaft anchoring part of the prosthesis has been reached. Therein a possibly existing curvature of the bone can be overcome, because after osteotomy the two bone segments are normally very short and align towards one another. As soon as the shaft anchoring part has been reached, a cannulated extraction tool is introduced via the guide wire, wherein a polygonal plug at the end of the cannulated extraction tool engages in a corresponding polygonal socket in the shaft anchoring part, so that rotational forces for releasing the shaft anchoring part after removal of the positive-locking connection between the shaft anchoring part and the joint replacement part and the shaft replacement part, respectively, can be transmitted. Then the guide wire is screwed out of the attachment means and a tie rod is screwed through the cannulated tool into the thread of the attachment means. Therewith also an axial force can be exerted on the shaft anchoring part and the shaft anchoring part can be extracted towards the distal direction. Herein a drill cutter at the side of the shaft anchoring part facing away from the joint replacement part can take off smaller intramedullary obstacles. Thereafter the distraction medullary nail is introduced into the cavity of the shaft replacement part through the bone channel formed in this way. The distraction medullary nail is connected in a positive-locking manner by means of bolts or screws, on the one hand, through the provided openings to the joint replacement part and the shaft replacement part, respectively, and on the other hand to the distal femur. If an electromotive drive with a subcutaneous receiving antenna is used, it may be required depending on the embodiment that the drive is introduced in advance into the shaft replacement part. For this, the cable is introduced in advance through the channel in the distal femur via the osteotomy into the opening of the joint replacement part and the shaft replacement part with the aid of a strand or a wire and led out laterally. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Hereinafter an exemplary embodiment of the invention will be explained in greater detail by way of drawings, wherein 
         FIG. 1  shows a schematic view in longitudinal cross-section of an implantable prosthesis with adjoining bone section; 
         FIG. 2  shows a partial view in longitudinal cross-section of the bone section with an alternative embodiment of the portion of the shaft anchoring part of the prosthesis facing away from the joint replacement part; 
         FIG. 3  shows a partial view in longitudinal cross-section of an extraction tool in engagement with the shaft anchoring part; 
         FIG. 4  shows a schematic view in longitudinal cross-section of a replacement of the distal femur by a first embodiment of a distraction medullary nail prior to distraction, the distraction medullary nail being inserted instead of the shaft anchoring part; 
         FIG. 5  in a view like in  FIG. 3  shows the state after the distraction; 
         FIG. 6  in a view like in  FIG. 3  shows a replacement of the distal femur by a second embodiment of a distraction medullary nail prior to distraction, the distraction medullary nail being inserted instead of the shaft anchoring part; 
         FIG. 7  in a view like in  FIG. 5  shows the state after the distraction; 
         FIG. 8  in a view like in  FIG. 3  schematically shows a replacement of the proximal femur by a first embodiment of the distraction medullary nail prior to distraction; 
         FIG. 9  in a view like in  FIG. 7  shows the state after the distraction; 
         FIG. 10  in a view like in  FIG. 7  schematically shows a replacement of the proximal femur by a second embodiment of the distraction medullary nail prior to distraction; 
         FIG. 11  in a view like in  FIG. 9  shows the state after the distraction. 
     
    
    
     DETAILED DESCRIPTION 
     The implantable prosthesis shown in  FIG. 1  comprises a joint replacement part  10 , which is releasably connected to a shaft replacement part  12  for exchange purposes. The shaft replacement part  12  is connected to a bone section  18  through a plate  19  and screws. 
     A cavity  14  in the form of a central blind hole is formed in the shaft replacement part  12 , which cavity  14  has a bottom  15  at the side of the joint replacement part  10  (in practice, the shaft replacement part  12  will surely be bored through, because appropriately the joint replacement part and the shaft replacement part are connected by a central screw) and is open at its side facing away from the joint replacement part  10 . The inner wall of the cavity  14  is provided with a slide friction reducing coating. A shaft anchoring part  16  having a polished wall is inserted in the cavity  14 . At the bottom  15  there is provided a lateral bore  37  in the shaft replacement part  12  for passing through a connection element, which can be connected with its one end to a drive of a distraction medullary nail  30  ( FIGS. 4 to 11 ) inserted instead of the shaft anchoring part  16  and with its end located at the outside of the prosthesis to a subcutaneous receiver for supplying energy to the drive. The shaft anchoring part  16  is connected to the shaft replacement part  12  by means of a screw bolt  20  extending through a transverse bore  22  in the shaft replacement part  12  and in the shaft anchoring part  16 . 
     At its end facing away from the joint replacement part  10 , the shaft anchoring part  16  has attachment means  26  formed by an opening, which is releasably engaged, for example through a threaded joint, by locating means  28  in the form of a guide wire. The guide wire  28  extends from the shaft anchoring part  16 , passes through the bone section  18  and exits it. 
     In the embodiment shown in  FIG. 1  the guide wire  28  forms the guide for a milling tool located outside the bone section  18  for milling a channel  42  ( FIG. 3 ) up to the free end of the shaft anchoring part  16 , which then, after the screw bolt  20  has been released and pulled out, is removed through the milled channel  42  and can be replaced by a medullary nail  30  ( FIGS. 4 to 11 ), which is correspondingly fixed by means of cross bolts  31  and  33 , respectively ( FIGS. 4 to 11 ), after an osteotomy of the bone has been performed along a partition plane  40  shown as a dash-dotted line by means of an inner saw (not shown). 
     In the embodiment shown in  FIG. 2  a cutting drill  38  protrudes from the shaft anchoring part  16 , which during rotation under traction of the shaft anchoring part  16  takes away potentially existing protrusions at the wall of the milled channel  42  ( FIG. 3 ) and thus facilitates withdrawal of the shaft anchoring part  16  from the bone section  18  and subsequent insertion of a distraction medullary nail  30  ( FIGS. 4 to 11 ). 
     For withdrawing the shaft anchoring part  16  from the bone section  18  at its side facing away from the joint replacement part  10 , the shaft anchoring part  16 , as shown in  FIG. 3 , has a polygonal socket  46  at its end side and an adjoining internally threaded bore  48  for engagement with a sleeve-shaped connection element  45 , which fits through the channel  42 , of a tool  44  located outside the bone section  18 . With a polygonal plug located at its end side, the connection element  45  engages in a positive-locking manner in the polygonal socket  46  of the shaft anchoring part  16 . A tie rod  49  passing through the connection element  45  in a rotationally fixed manner is screwed with its end-sided external thread into the internally threaded bore  48  of the shaft replacement part  16 . In this way the tool  44 , when rotated, is able to transmit a traction force and a torque for easily extracting the shaft anchoring part  16 . 
       FIGS. 4 to 11  describe variants of bone lengthening using a known distraction medullary nail  30  inserted instead of the extracted shaft anchoring part  16  ( FIGS. 1 to 3 ). 
       FIGS. 4 and 5  show a replacement of the distal femur bone, for example after resection of a bone tumor. A joint replacement part  10  and a shaft replacement part  12  are shown. A distraction medullary nail  30  having an elongated hole  32  is introduced from proximally in a positive-locking, but axially slidable manner into the shaft replacement part  12 . The distraction medullary nail  30  is fixed, for example, by means of one or, more advantageously, three screws  31 , as shown, in the proximal femur bone  1  (in  FIGS. 1 to 3  bone section  18 ), whereby also very proximal osteotomies  2  (in  FIGS. 1 to 3  partition plane  40 ) become possible. A fixation distally of the osteotomy  2  is done in the proximal end of the elongated hole  32  also by means of a screw  33 . An axial distraction force can be exerted on the bone via said screw  33  by means of a drive in the medullary nail  30 , for example by means of a gear motor, and thus the bone can be lengthened according to the callus distraction method in the osteotomy gap  2  by formation of callus  5 . The physiological antecurvation of the femur does not constitute a hindrance for inserting a straight medullary nail  30 , because the femur is straightened at the osteotomy  2 . An electric cable for connecting an inside drive to an antenna  39  located in the subcutaneous fatty tissue is led out through the lateral bore  37 , wherein energy can be coupled in from the outside via the antenna  39 . 
       FIGS. 6 and 7  essentially correspond to  FIGS. 4 and 5 . Here lengthening is done by means of a distraction medullary nail  30  having a telescopic mechanism  35 , wherein the distraction medullary nail  30  is, on the one hand, anchored by means of screws  31  in the proximal femur bone  1  (in  FIGS. 1 to 3  bone section  18 ) and, on the other hand, fixed in a positive-locking manner by means of a further screw  31  in the shaft replacement part  12 . In order to increase strength in the middle of the shaft, i.e. the point of maximal bending stress, the telescopic medullary nail  30  can be sunk into the shaft replacement part  12  far enough for it to remain, even at its extended state, still sufficiently far with the large diameter in the shaft replacement part  12 . In this configuration the cable for the antenna  39  is led out through the milled-out channel  42 . 
       FIGS. 8 and 9  show a replacement of the proximal femur bone, for example after resection of a bone tumor. A distraction medullary nail  30  having an elongated hole  32  is introduced from distally from the knee joint through the distal femur bone in an axially slidable manner into the shaft replacement part  12 , wherein the distraction medullary nail  30  is anchored, on the one hand, by means of several screws  31  in the distal femur  3  (functionally corresponding to the bone section  18  of  FIGS. 1 to 3 ) and, on the other hand, by means of a further screw  31  in the distal elongated hole and the remaining femur shaft  4 , which is connected to the shaft replacement part  12  through connection means  19  in the form of an L-shaped plate. An axial distraction force can be exerted on the bone via the elongated hole  32  in the medullary nail by means of a drive, for example by means of a gear motor, within the medullary nail  30  and thus the bone can be lengthened according to the callus distraction method (callus  5 ) in the osteotomy gap  2  (partition plane  40  of  FIGS. 1 to 3 ). 
       FIGS. 10 and 11  essentially correspond to  FIGS. 8 and 9 . Here, however, lengthening (callus  5 ) is done by means of a distraction medullary nail  30  having a telescopic mechanism  35  analogously to  FIGS. 6 and 7 , wherein the distraction medullary nail  30  is fixed by means of screws  31  in the distal femur  3  and by means of a further screw  31  in the shaft replacement part  12 . In order to increase strength in the middle of the shaft, i.e. the point of maximal bending stress, the telescopic medullary nail  30  can here also be sunk into the shaft replacement part far enough for it to remain, even at its extended state, still sufficiently far with the large diameter in the shaft replacement part  12 . In this configuration the cable for the antenna  39  is led out through the milled-out channel  42 .