Abstract:
A prosthetic ankle assembly including an astragalar component with an upper articular surface that forms part of an ankle joint prosthesis and a lower surface with a shape adapted to engage an upper face of an astragalus. An astragalocalcanean rod is positioned in an astragalocalcanean channel extending from the lower face of the calcaneus to the upper surface of the astragalus. A first end of the astragalocalcanean rod is engage with the lower surface of the astragalar component and a second end of the astragalocalcanean rod is engaged with a lower region of the calcaneus. A distance between the second end of the astragalocalcanean rod is adjustable relative to the lower surface of the astragalar component to adjustably compress the calcaneus against the astragalus.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to prior French Application No. 06 09001, filed Oct. 13, 2006, entitled ANKLE PROSTHESIS FOR THE ARTHRODESIS OF THE CALCANEUM. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a prosthetic ankle assembly and to a surgical method for implanting such an assembly, and in particular, to the treatment of an ankle in which the articulation between the astragalus and the calcaneus are damaged. 
       BACKGROUND OF THE INVENTION 
       [0003]    One possible treatment involves fusing the three bones represented by the calcaneus, astragalus and tibia, as is proposed in US-A-2005/0107791. A long compression screw internally connects the tibia and the calcaneus by passing through the astragalus, in such a way as to block these three bones relative to one another. This results in a considerable loss of mobility for the patient, since the ankle is totally blocked. 
         [0004]    Another possible treatment involves fusing the calcaneus and astragalus, with the aid of a dedicated arthrodesis rod and re-establishing the articular capacity between the astragalus and tibia, by implanting an ankle joint prosthesis. However, the two articulations then have to be treated in two stages: the arthrodesis of the calcaneus and astragalus requires a consolidation period of at least six months, which delays the implantation of the articular prosthesis between the astragalus and the tibia. 
         [0005]    US-A-2005/0288792 discloses an astragalar component of an ankle joint prosthesis with which it is possible to re-establish the articular capacity between the astragalus and the tibia, while at the same time using an astragalocalcanean rod that extends from a lower side of the astragalar component to the upper part of the calcaneus, in so doing passing right through the astragalus. In one embodiment, the upper end of this rod is screwed directly into internal threads of the astragalar component, which threads open out on the lower side of this component. Before being implanted, the component and the rod are firmly immobilized relative to each other by screwing. When this screwed assembly is implanted, the rod guarantees good anchoring of the astragalar component, since the latter is thus fixed both to the astragalus and also to the upper part of the calcaneus, which explains why the prosthetic assembly is more specifically intended for revision of an arthroplasty of the ankle. However, the assembly does not effectively block the articulation between the calcaneus and the astragalus, since relative movements around and/or along the rod remain possible and, in practice, appear quickly. 
         [0006]    FR-A-2 220 235 proposes a prosthetic ankle assembly comprising an astragalar component of an ankle joint prosthesis in the form of a support designed to be fixed to the astragalus of a patient. This support is intended to cooperate in an articulated manner with a tibial rod, with interposition of an intermediate articulated component. In its lower part, the astragalar support is joined to three anchoring feet, namely two anterior feet and one posterior foot, which are intended respectively to be inserted into corresponding cavities formed in the astragalus and in the calcaneus. As regards the posterior foot, one embodiment entails this foot being formed by at least one sagittal screw that passes through the support from top to bottom, until bearing with its head on the support, while its stem is inserted, without being screwed, into the cavity of the calcaneus, plugged with cement. The benefit of using such a screw is that, if necessary, it can be unscrewed in order to facilitate the removal of the support. Thus, the posterior foot of this prosthetic assembly provides a stable means of bearing on the calcaneus, but, even in the form of a screw inserted into cement, this foot does not totally block the articulation between the astragalus and the calcaneus in an effective manner. 
         [0007]    WO-A-01/30264 and US-A-2005/124995 for their part disclose a prosthetic ankle assembly comprising an astragalocalcanean rod that is introduced into a cavity formed beforehand through the astragalus and in the upper part of the calcaneus. This rod is used to anchor an astragalar plate, which is fitted in place and fixed to the astragalocalcanean rod after the rod has been placed in the aforementioned bone cavity. Hence, this rod does not provide any effective blocking of the articulation between the calcaneus and the astragalus so that, in order to limit the relative movements between these two bones, additional screws are fitted between the astragalar plate and the astragalocalcanean rod, in order to stabilize the position of the latter. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    The present invention is directed to a method and apparatus that blocks the articulation between the calcaneus and the astragalus and to re-establish an articular capacity between the astragalus and the tibia of a patient, all in a single intervention. 
         [0009]    To this end, the subject matter of the invention is a prosthetic ankle assembly, comprising an astragalar component which forms part of an ankle joint prosthesis and which is designed to be fixed to the astragalus of a patient. An astragalocalcanean rod of which a first longitudinal end is designed to be fixed to the astragalar component, by its shape complementing that of a dedicated part of the astragalar component. The dedicated part is provided on a lower side of this component intended to be directed towards an upper face of the astragalus. The astragalocalcanean rod is equipped, at its second longitudinal end, with an engagement feature for osseous engagement of the calcaneus of the patient, so as to approximate and thereby compress the calcaneus against the astragalus when the first end of the rod is fixed to the dedicated part of the astragalar component. 
         [0010]    When the prosthetic assembly according to the invention is implanted, its astragalar component permits articular movements between the astragalus and the tibia of a patient, especially by being connected to a prosthetic tibial component fixed to the tibia, and to a prosthetic bearing interposed between the astragalar and tibial components. On the other hand, the astragalocalcanean rod blocks the articulation between the calcaneus and the astragalus, by these two bones being compressed against each other. In practice, this bone compression is obtained by fixing the first end of the rod in the astragalar component, which engages the second end of the rod directly with the calcaneus. For this purpose, a fixation means and the engagement means provided respectively at the ends of the rod are structurally configured to obtain a progressive compression effect between the calcaneus and the astragalus as the second end of the rod advances into the calcaneus, whilst the first end is coupled to the dedicated part of the astragalar component resting on the astragalus. 
         [0011]    In practice, the rod is introduced from a lower face of the calcaneus into an astragalocalcanean channel that has been drilled beforehand through the calcaneus and the astragalus and that opens out on the dedicated part of the astragalar component when the latter is placed on an upper face of the astragalus. Thus, advantageously in a single surgical intervention, the astragalar component and the astragalocalcanean rod are implanted by being coupled to each other during the intervention, while the rod passes right through the calcaneus and the astragalus, in order to compress them and thereby immobilize them against each other. In other words, the operation of arthroplasty between the astragalus and the tibia and the operation of arthroplasty between the astragalus and the calcaneus are performed during the same surgical intervention, which limits the surgical risks faced by the patient and which allows the surgeon to use the same surgical routes for the two operations. 
         [0012]    In addition, the astragalocalcanean rod ensures immobilization of the astragalar component relative to the astragalus and to the calcaneus, which improves the fit of the astragalar component. Moreover, in the event of revision of the prosthetic assembly according to the invention, the bearing action on the calcaneus is safe and reliable in the sense that this bone is firmly fused to the astragalus. 
         [0013]    According to other advantageous characteristics of this prosthetic ankle assembly, taken either singly or in all the technically possible combinations:
       the astragalocalcanean rod is designed to extend in length from the upper face of the astragalus, on which the astragalar component is fixed, as far as a lower face of the calcaneus directed away from the astragalus;   the engagement means comprises an external thread designed to engage in the bone substance of the calcaneus when the first end of the astragalocalcanean rod is fixed in the dedicated part of the astragalar component;   the engagement means comprises a ring, fixedly or moveably surrounding the second end of the astragalocalcanean rod and adapted to be pressed against a lower face of the calcaneus, opposite to the astragalus, when the first end of the rod is fixed to the dedicated part of the astragalar component;   the engagement means for engaging the calcaneus has a maximum transverse dimension strictly greater than the maximum transverse dimension of a fixation means for fixing the first end of the astragalocalcanean rod to the astragalar component;   either the first end of the astragalocalcanean rod or the dedicated part of the astragalar component is equipped with an external thread, while the other of them forms a substantially complementary internal thread;   the astragalar component is equipped, on its lower side, with an anchoring means for osseous anchoring in the astragalus, inside of which anchoring means the dedicated part is arranged;   the anchoring means includes a hollow stub in which the external thread or internal thread of the astragalar component is delimited, preferably in a manner substantially coaxial to the rest of the stub;   the astragalocalcanean rod is equipped, at its second longitudinal end, with a driving profile, preferably driven in rotation on itself, designed to cooperate with a tool for coupling the rod to the astragalar component;   the profile or another profile for driving the rod is designed to cooperate with a tool for disconnection between the rod and the astragalar component;   the assembly also comprises a tibial component of an ankle joint prosthesis, designed to be fixed to the tibia of the patient, and a prosthetic bearing designed to be interposed between the astragalar component and tibial component.       
 
         [0024]    The invention also relates to a surgical method for implanting a prosthetic ankle assembly, the assembly comprising an astragalar component of an ankle joint prosthesis, and an astragalocalcanean rod of which a first of its two longitudinal ends is designed to be fixed to the astragalar component, by its shape complementing that of a dedicated part of the component, and of which the second longitudinal end is equipped with an engagement means of osseous engagement. The method of implantation comprising steps of: 
         [0025]    preparing an upper face of an astragalus to receive an astragalar component; 
         [0026]    forming an astragalocalcanean channel between an the upper face of the astragalus and a lower face of a calcaneus; 
         [0027]    locating the astragalar component on the upper face of the astragalus; 
         [0028]    inserting an astragalocalcanean rod into the astragalocalcanean channel from the lower face of the calcaneus; 
         [0029]    engaging a first end of the astragalo-calanean rod with the astragalar component and a second end of the astragalo-calanean rod with a lower region of the calcaneus; and 
         [0030]    adjusting a distance between the second end of the astragalocalcanean rod relative to a lower surface of the astragalar component to adjustably compresses the calcaneus against the astragalus. 
         [0031]    This method makes it possible, in a single surgical intervention, to implant a prosthetic ankle assembly of the kind defined above, in order to permit arthrodesis of the calcaneus and astragalus and, at the same time, arthroplasty between the astragalus and the tibia of a patient. Arthrodesis refers to a surgical fixation of a joint, ultimately resulting in bone fusion. Basically, the procedure is artificially induced ankylosis performed to relieve pain or provide support in a diseased or injured joint. 
         [0032]    According to an advantageous embodiment of this method, during step i), a substantially cylindrical recess is hollowed out in the upper face of the astragalus in order to receive a hollow bone-anchoring stub which is provided on the astragalar component and in which the dedicated part is arranged, and, during step ii), the astragalocalcanean channel is drilled from the upper face of the astragalus, in a continuation of the recess. 
         [0033]    In this case, to make matters easier, a drill bushing is used to drill the astragalocalcanean channel, by resting this bushing in the recess formed during step i). 
         [0034]    Advantageously, in order to promote the fusion of the calcaneus and the astragalus, the method comprises, before step ii), an additional surgical step during which bone substance is inserted between the astragalus and the calcaneus by arthroscopy. 
         [0035]    Moreover, after consolidation of the arthrodesis of the calcaneus and astragalus, the method advantageously comprises a postoperative step in which the astragalocalcanean rod is withdrawn by disconnecting its first end from the dedicated part of the astragalar component. 
     
    
     
       BRIEF DESCRIPTIONS OF THE SEVERAL VIEWS OF THE DRAWING 
         [0036]    The invention can be better understood on reading the following description given purely by way of example and made with reference to the drawings, in which: 
           [0037]      FIG. 1  is a front view, in partial cross section, of a prosthetic ankle assembly implanted in the left ankle of a patient in accordance with an embodiment of the present invention; 
           [0038]      FIG. 2  is an exploded side view, according to the arrow II indicated in  FIG. 1 , of part of the prosthetic assembly, depicted without the bones of the ankle; 
           [0039]      FIGS. 3A and 3B  are schematic cross sections, along the line III-IIi in  FIG. 1 , illustrating two successive steps in the implantation of the prosthetic assembly; and 
           [0040]      FIG. 4  is a view similar to  FIG. 3B , illustrating a variant of the prosthetic ankle assembly in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]      FIG. 1  shows schematically the lower ends of a tibia T and of a peroneal bone P or fibula of a human being, and also the corresponding astragalus A and calcaneus C, thereby illustrating the constituent bones of the left ankle of a patient.  FIG. 1  also shows a prosthetic ankle assembly  1  comprising four separate components implanted in the ankle of the patient, namely a tibial component  10 , an astragalar component  20 , a prosthetic bearing  30 , interposed between the tibial and astragalar components, and an astragalocalcanean rod  40 . In  FIG. 1 , only the bearing  30  is shown in frontal section, whereas the broken lines are used to symbolize the contours of the components concealed by bone substance. 
         [0042]    For convenience, the description that follows relates to the bones of the ankle in their anatomical position, that is to say the terms “posterior” or “rear”, “anterior” or “front”, “upper”, “lower”, etc., are to be understood in relation to the ankle of the patient when standing upright on a substantially horizontal surface. 
         [0043]    The tibial component  10  comprises a plate  11  which is attached in a fixed manner to the lower end of the tibia T. For this purpose, the plate  11 , on its upper face  11 A, is joined, via a sagittal fin  12 , to a hollow bone-anchoring stub  13 . Additional and/or alternative means for anchoring the plate  11  are conceivable, provided that they effectively immobilize the tibial component at the lower end of the tibia T. 
         [0044]    On its lower side, the plate  11  delimits a plane surface  11 B intended to form a sliding support for the plane upper surface  30 A of the bearing  30 . 
         [0045]    The astragalar component  20  comprises a main block  21  attached in a fixed manner to the upper end of the astragalus A. For this purpose, the block  21  is made integral with a hollow bone-anchoring stub  22  that extends downwards from the lower side  21 B of the block  21 , as is represented by broken lines in  FIG. 1  and as can be clearly seen in  FIG. 2 . 
         [0046]    On its upper side, the block  21  delimits an articular surface  21 A intended to cooperate with a matching articular surface  30 B delimited by the lower side of the bearing  30 . In sagittal section, the surface  21 A has an arched profile, with its concavity directed downwards, as can be seen in  FIG. 2 . The articular surfaces  21 A and  30 B are thus designed to slide against each other along this curved profile, in what is an essentially antero-posterior direction. Advantageously, these surfaces  21 A and  30 B between them permit a medio-lateral clearance, which combines with the curved movement of sliding in the antero-posterior direction. 
         [0047]    As is shown in  FIG. 2 , the rod  40  extends lengthwise about a completely vertical central axis X-X. This rod comprises, in succession along its length, a first threaded end  41 , an externally smooth and regular part  42 , and a second threaded end  43 . The end  41 , the regular part  42  and the end  43  are successively of increasing size, such that, on the one hand, the maximum diameter d 41  of the external thread  41 A of the end  41 , that is to say the maximum transverse dimension of this end  41  in the area of the crest of the thread, is less than or equal to the external diameter of the part  42 , and, on the other hand, the maximum diameter d 43  of the external thread  43 A of the end  43 , that is to say the maximum transverse dimension of this end  43  at the crest of the thread  43 A, is strictly greater than the diameter of the regular part  42 . 
         [0048]    The external thread  41 A of the end  41  is designed to be received by screwing into a complementary internal thread  22 A which, as is indicated by broken lines in  FIG. 2 , is delimited by the inner face of the hollow stub  22  and is preferably centred on the central longitudinal axis of this stub. 
         [0049]    At its end  43 , the rod  40  delimits a recess  44  which is substantially centred on the axis X-X and opens out axially downwards to the outside, as is indicated by broken lines in  FIG. 2 . This recess  44  has a transverse profile  44 A, for example of square shape, intended to cooperate with a complementary tool for driving the rod  40  in rotation on itself about the axis X-X. 
         [0050]    A surgical method for implanting the prosthetic ankle assembly  1  will now be described. 
         [0051]    In a first step, the bones of the patient&#39;s ankle have to be prepared. For this purpose, the surgeon uses cutting means (not shown) in such a way as to, on the one hand, resect the lower end of the tibia T and form a substantially plane surface T 1 , and, on the other hand, resect the upper end of the astragalus A and form a multi-facet surface A 1  comprising two main surfaces which are inclined relative to each other, as can be seen in  FIG. 3A . In the face A 1 , a cylindrical recess A 2 , indicated by broken lines in  FIG. 3A , is hollowed out in an overall vertical direction, for example by means of a milling cutter. This recess A 2  thus defines a central longitudinal axis A 3 . 
         [0052]    At this stage of the surgical intervention, the upper face A 1  of the astragalus A is in a configuration suitable for receiving the astragalar component  20 , it being understood that the recess A 2  is made substantially complementary to the stub  22  in order to anchor this component  20  on the astragalus. However, before fitting the astragalar component  20  in place, the surgeon drills through the astragalus A and calcaneus C, from the upper face A 1  of the astragalus to a lower face C 1  of the calcaneus, in such a way as to form an astragalocalcanean channel AC indicated by broken lines in  FIG. 3A . This channel AC is drilled in a rectilinear continuation of the recess A 2 , by being substantially centred on the axis A 3 . To do this, the surgeon advantageously uses a drill bushing  50 , indicated partially and schematically in  FIG. 3A . This bushing  50  has a tubular shape designed to be received in a substantially complementary manner in the recess A 2 , substantially coaxial to the recess. In this way, the bushing  50  bears in the recess A 2  in such a way as to guide the introduction and advance of a drill bit successively through the astragalus and the calcaneus, as is indicated by the arrow  51  in  FIG. 3A . The surgeon thus drills the astragalocalcanean channel AC rapidly and with precision, using ancillary equipment traditional in the field of orthopaedic surgery. 
         [0053]    After the astragalocalcanean channel AC has been drilled, the surgeon places the astragalar component  20  on the upper face A 1  of the astragalus A and places the astragalocalcanean rod  40  in the channel AC. More precisely, the rod  40  is introduced into the channel AC from the lower face C 1  of the calcaneus, with its end  41  directed upwards. The internal diameter of the channel AC is made substantially equal to the external diameter of the regular part  42 , such that the rod is easily advanced through the channel in an upward translational movement and parallel to the axis A 3 , until the end  41  reaches the point where the channel AC opens into the recess A 2 . As this recess is occupied by the stub  22 , advancing the rod  40  requires the latter to be driven in rotation on itself about the axis X-X, in such a way as to screw the external thread  41 A into the internal thread  22 A, as is indicated by the arrow  60 . In practice, the rod is driven in rotation by means of a tool whose head, indicated only by broken lines and reference number  61  in  FIG. 3B , is engaged with the profile  44 A in the recess  44 . 
         [0054]    It will be understood that, by giving the rod  40  a suitable lengthwise dimension, the screwing of the end  41  into the stub  22  is accompanied by the screwing of the end  43  into the calcaneus, the external thread  43 A engaging directly in the bone substance of the calcaneus at the point where the channel AC opens out on the lower face C 1  of the calcaneus. In other words, the rod  40  has a length substantially equal to that of the astragalocalcanean channel AC, such that the rod can extend through the astragalus and the calcaneus with its end  41  in the area of the upper face A 1  of the astragalus and with its end  43  in the area of the lower face C 1  of the calcaneus. 
         [0055]    The twin screwing of the external threads  41 A and  43 A causes the astragalus A and the calcaneus C to be gradually approximated to each other, thereby compressing these two bones against each other in the area of their respective contacting surfaces. It will be appreciated that the structural features of these external threads, in particular their respective pitch, are chosen so as to achieve compression values that are sufficient to efficiently block the astragalus and the calcaneus against each other, without generating excessive stress in the bones. 
         [0056]    The other components of the prosthetic assembly  1  are implanted in parallel with this. The tibial component  10  is attached and immobilized on the lower end of the tibia T, with its plate  11  resting flat against the face T 1 , then the prosthetic bearing  30  is attached between the components  10  and  20 . The assembly  1  is then in the implantation configuration illustrated in  FIG. 1  and the surgical intervention is concluded. 
         [0057]    After consolidation of the arthrodesis between the astragalus A and the calcaneus C, which generally takes between six and twelve months, the astragalocalcanean rod  40  can be withdrawn by unscrewing its ends  41  and  43 . The surgeon then advantageously uses the profile  44 A by introducing a suitable disconnecting tool into the latter. To promote and accelerate this arthrodesis, bone substance can advantageously be inserted between the astragalus and the calcaneus before implantation of the prosthetic assembly  1 . Before jointly drilling right through the astragalus A and the calcaneus C, an arthroscope is inserted into the junction area between the astragalus and the calcaneus in order to place bone substance there. 
         [0058]      FIG. 4  shows a variant of the astragalocalcanean rod  40 , which differs from the embodiment considered in  FIGS. 1 to 3B  only at its lower end  43 . The external thread  43 A is cancelled, so that the external surface of the end  43  is smooth and extends in the straight continuation of the external surface of the regular part  42 , except at its end area that forms a bulge  43 B radially protruding externally from the rest of this surface. Unlike the embodiment of  FIGS. 1 to 3B , the end  43  of the rod  40  of  FIG. 4  is associated with a ring  45  externally surrounding this end. This ring has an internal diameter that is both substantially equal or greater than the external diameter of end  43 , except at its end bulge  43 B, and lower than the maximum external diameter of this bulge. In practice, the ring  45  is pulled around the rod  40  by being introduced from upper end  41  while the rod is still not introduced into the astragalocalcanean channel AC. 
         [0059]    When the external thread  41 A of the end  41  of the rod  40  is screwed into internal thread  22 A of the astragalar component  20 , by driving in rotation the rod by means of the tool whose head is engaged within the recess  44 , the ring  45  is thus interposed between the lower face C 1  of the calcaneus C and the end bulge  43 B of the rod until this bulge drives the ring  45  in a pressed contact against the face C 1  of the calcaneus. Thus, the ring  45  transmits and distributes the clamping stress on the face C 1  of the calcaneus, in the same way as a washer. The calcaneus is thus compressed against the astragalus A, until to efficiently block the astragalus and the calcaneus against each other. 
         [0060]    A radial clearance between the internal face of the ring  45  and the external face of the end  43 , above its end bulge  43 B, allows adjustment of the position of the ring with respect to the rod during its pressing against the calcaneus C. Thus, this relative mobility between the ring and the end  43  compensates for a potential misalignment between the central axis of the ring and the axis X-X of the rod. 
         [0061]    According to a not shown variant, the ring  45  is fixedly secured at the lower end  43  of the rod  40 , for example being directly integral with the rod. In this case, in order to increase the pressing surface of this fixed ring against the lower face C 1  of the calcaneus C during the screwing of the external thread  41 A into the astragalar component  20 , this calcanean face C 1  is, if necessary, beforehand wrought, so that a substantially plane surface, inscribed in a plane substantially perpendicular to the axis A 3  of the astragalocalcanean channel AC, is delimitated around the point where this channel opens into the recess A 2 . 
         [0062]    Configurations other than the external thread  43 A and the ring  45  can be provided at the lower end  43  of the astragalocalcanean rod  40  in order to engage the calcaneus C and compress it against the astragalus A when screwing the upper end  41  into the astragalar component  20 . For example, the lower end  43  can have an outer surface in the shape of a truncated cone or flare widening towards the bottom. 
         [0063]    Other variations on the prosthetic ankle assembly  1  and on the implantation method that have been described above are also conceivable. By way of example: 
         [0064]    The central axis of the internal thread  22 A can be offset from the central axis of the stub  22 , the drill bushing  50  then being provided with a similar offset in order to drill the astragalocalcanean channel AC in the possibly inclined continuation of the recess A 2 ; 
         [0065]    The structure of internal thread  22 A and external thread  41 A can be reversed, such that the end  41  of the rod  40  forms an internal thread that can be screwed around an external thread formed in the stub  22 , for example delimited by a central pin internal to this stub; more generally, other types of means permitting coupling through complementary shapes between the upper end of the rod  40  and the lower side  21 B of the astragalar component  20  are conceivable, provided that these means lead to engagement of the lower end of the rod with the calcaneus C in order to compress the latter against the astragalus A; thus, a coupling by means of a bayonet structure is possible, and also coupling structures with stressing obtained exclusively through rectilinear translation; 
         [0066]    By virtue of the fact that they bear slidingly on each other at their surfaces  11 A and  30 A, the tibial component  10  and the bearing  30  are movable relative to each other, which generally leads the ankle prosthesis comprising the elements  10 ,  20  and  30  to be designated as a prosthesis with a “movable bearing”; however, the astragalar component  20  can alternatively be joined to a “fixed” bearing, that is to say a bearing placed fixedly against a tibial component; 
         [0067]    The astragalar component  20  and the astragalocalcanean rod  40  are not necessarily to be used jointly with a tibial component and a prosthetic bearing such as the elements  10  and  30 ; the astragalar component can in fact be made to cooperate in an articulated manner directly with the lower anatomical end of the tibia T if the latter has sufficient articular capacity; the prosthetic ankle assembly can then be designated as a partial prosthetic assembly, whereas the prosthetic assembly  1  in  FIG. 1  is to be considered as a total prosthetic assembly; and/or 
         [0068]    If necessary, the fixation of the astragalar component  20  to the astragalus A can be strengthened by additional means. 
         [0069]    Patents and patent applications disclosed herein, including those cited in the Background of the Invention, are hereby incorporated by reference. Other embodiments of the invention are possible. It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.