Patent Publication Number: US-2006020345-A1

Title: Prosthesis device for the ankle articulation

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is a continuation-in-part of U.S. application Ser. No. 10/019,994, filed Nov. 9, 2001, which application is the U.S. national phase of International Application No. PCT/IB00/00638 filed May 12, 2000, which claims priority from and the benefit of Italian application BO99A 000253 filed May 13, 1999. The disclosure of application Ser. No. 10/019,994 filed Nov. 9, 2001 is also hereby expressly incorporated by reference into this application. 
    
    
     TECHNICAL FIELD  
      The present invention relates to a prosthetic joint for replacement of the human ankle and to methods for the design of shapes and sizes of the relevant components, particularly for a two-component prosthesis with components fixed to the talus and to the tibia respectively, and designed to allow motion compatible with the functions of the retained ligaments.  
     BACKGROUND ART  
      In total ankle replacement, the degenerated articular surfaces of the natural bones are removed and replaced with an artificial joint called a prosthesis. The goals for the replaced joint are a) to relieve the pain, b) to restore the original mobility, and c) to restore the original stability.  
      Therapy resistant ankle pain and the disadvantages of ankle arthrodesis (i.e. fusion of the talus to the tibia with consequent loss of mobility) led to the development of numerous ankle joint prostheses. At least 18 designs were introduced throughout the Seventies of the previous century but, after early encouraging results, ankle arthroplasty soon acquired a bad reputation based on many long-term follow-up clinical-radiographic studies. The frequent failures of the previous implants have been related by the present inventors mainly to the inadequate restoration of the original mobility and stability of the ankle complex, caused by poor knowledge of the guiding/stabilizing role played by the ligaments involved. The relative contributions of the ligamentous structures and articular surfaces of the joint to its passive and active stability had not yet been fully understood.  
      The designs devised by the pioneers (1970-1979) mostly featured two-component prostheses with one component attached to the talus and the other to the tibia. These designs have been classified as constrained, semi-constrained and non-constrained. The two-component designs have been also equivalently categorized as congruent (spherical, spheroidal, conical, cylindrical, sliding-cylindrical) or incongruent (trochlear, bispherical, concave-convex, convex-convex), according to extent of the conformity of the shapes of the two prosthetic articular surfaces. The non-constrained incongruous type can be enable restoration of the normal multi-axial three-planar motion exhibited by the healthy joint compatible with its ligaments but leads to poor resistance to wear and deformation of the prosthetic surfaces due to high local stresses resulting from small contact areas; this type also exhibits poor inherent stability of its non-conforming articular surfaces. The constrained congruent designs can be expected to provide better stability and larger contact areas between the components and better performance in terms of resistance to wear and deformation of the prosthetic surfaces due to a better pressure distribution; but they also provide inadequate restoration of the characteristic multi-axial three-planar motion of the healthy joint, which also involves rolling as well as sliding at the articulating surfaces and may be incompatible with the retained ligaments. Cylindrical or conical designs can also provide high stability since the surfaces are forced into total conformity under load, restricting motion to a single plane. The highest failure rates were shown by the constrained designs. Non-constrained designs with incongruent articular surfaces showed only slightly better results. Failure of non-constrained incongruous designs may also be attributed to the poor quality of the ultra-high molecular weight polyethylene used for the tibial component.  
      Ankle joint replacement design therefore has to address the following traditional dilemma also encountered in knee prosthesis design. An unconstrained or semi-constrained type of two-component prosthesis that allows for the necessary axial and transverse as well as flexion/extension mobility requires incongruent contact, which leads to inadequate load-bearing capacity at the interface between its articular surfaces. Conversely, a congruent type of prosthesis produces undesirable constraining forces that overload the anchoring system needed to secure fixation of the prosthetic components to the bones. Meniscal bearing prostheses can resolve the dilemma by providing complete congruence over the entire range of positions of the joint with minimally constrained fixed components to enable the soft tissues still to restore the physiological multi-axial motion of the joint. However, potential problems pertaining to the risk of subluxation and/or dislocation can be envisioned. These risks depend respectively on the degree of distracting stability of the joint and the degree of entrapment of the bearing element in between the bone-anchored components.  
      Although most of the more recent ankle prosthesis designs feature three components, a few of these still have two components. They comprise a curved metal component fixed to the talus and a metal component fixed to the tibia, with articulating surfaces similar in shape respectively to that of the natural talar and tibial articular surfaces when seen in the sagittal plane. The latter component has a plastic bearing for articulating with the former, and a metal on the back for fixation to the bone. These designs claim original natural shapes to allow restoration of the original mobility, and potentially stability at the replaced joint (see documents U.S. Pat. No. 5,326,365, U.S. Pat. No. 3,889,300, U.S. Pat. No. 3,896,503, U.S. Pat. No. 3,975,778, U.S. Pat. No. 3,839,742, U.S. Pat. No. 3,886,599, U.S. Pat. No. 3,896,502, U.S. Pat. No. 3,872,519, U.S. Pat. No. 4,021,864, U.S. Pat. No. 4,069,518, U.S. Pat. No. 4,156,944). These designs also attempt to allow the multi-axial nature of the rotation at the ankle and minimize the loads transmitted to the bones by the means of fixation elements. They allow plantar- and dorsi-flexion (motion in the sagittal anatomical plane) and very partially the characteristic internal/external axial rotation (in the transverse plane) at the ankle joint. With these designs, some abduction and adduction movement (in the frontal plane) of the talus relative to the tibia is also allowed, though resulting in partial loss of contact between the two components. However, no attempt at all was made to relate the shapes of the surfaces of the implanted components to the geometry of the retained ligaments or to understand which joint structures should enable and control the mobility of the prosthetic joint as well as of the natural joint. The present inventors have demonstrated mathematically that the use of non-physiological shapes can still be compatible with the retained ligaments and result in physiological motion at the ankle joint as well (Leardini A, Catani F, Giannini S, O&#39;Connor J J. Computer-assisted design of the sagittal shapes of a ligament-compatible total ankle replacement. Med Biol Eng Comput. 2001; 39(2):168-175). Recent improvements in the manufacture of ultra-high molecular weight polyethylene may tempt designers to return to two-component configurations which are compatible with the ligaments in the belief that the problems of wear and deformation exhibited by early two component designs will be avoided because of improvements in material properties.  
      Despite the increasing number of total ankle prosthesis used in current clinical practice, to this day there are no designs with clinical results comparable to those achieved with ankle arthrodesis or to clinical results obtained with total hip and total knee replacements. Aseptic loosening of the tibial and/or of the talar components is the most frequent cause of failure but complications also include impingement and wear together with subluxation of the mobile meniscal bearing, deep infection, dehiscence of the surgical wound, lateral talofibular joint impingement, subsidence of the talar component. The relationship between cause of failure and the etiology of the degenerative disease has been studied by many authors with a large variation of the results reported.  
      When seen in the sagittal plane, the natural articular surface of the tibia at the ankle joint appears to be concave, with a radius of about 2.4-2.6 cm. The natural articular surface of the talus when seen in the sagittal plane appears to be convex, with a radius of about 2.0-2.2 cm so that the natural ankle joint appears to be a slightly incongruous articulation. The present invention uses shapes for articular surfaces of the fixed components which each differ significantly from the shapes of the natural articular surfaces while nonetheless reproducing natural strain patterns within the retained ligaments and minimizing subsequent wear of the components and loosening of the implant.  
      The design of an ankle joint prosthesis can aim to replicate exactly the complete original anatomical geometry of both natural articular surfaces, a path followed by many of the designers of constrained two-part prostheses in the 1970&#39;s. Studies by the inventors showed that when the shape of the articular surface of one of the fixed components is chosen to be non-physiological, the shape of the articular surface of the other fixed component should also be non-physiological but can nonetheless be designed so as to restore natural strain patterns in the ligaments. These patterns can be restored only when the shapes of the prosthetic articular surfaces and the geometry of the retained ligamentous structures are compatible or approximately compatible, i.e. during passive motion of the ankle, the articular surfaces are designed to move in mutual contact while maintaining some ligament fibers at or close to a constant length over the range of dorsi-plantar flexion.  
      In summary, one of the main innovative elements of the present invention is that it uses non-physiological shapes for the surfaces of both the components attached to the bones but nonetheless allows restoration of the functions of the ligaments in controlling and limiting the movement of the ankle joint complex while minimizing the risk of wear, dislocation or loosening of the components.  
     DISCLOSURE OF INVENTION  
      The presently proposed use of surface shapes for the two components fixed to the bones both differing significantly from the natural was not anticipated in U.S. Pat. No. 4,087,466 or in any later patent specifications.  
      In order to restore the natural articular load bearing pattern, an ideal human joint prosthesis should reproduce the type of motion and the original pattern of ligament slackening/tightening exhibited by the healthy natural joint. Previous studies by the present authors on intact cadaver ankle joints (Leardini, A. and O&#39;Connor, J. J. and Catani, F. and Giannini, S. Kinematics of the human ankle complex in passive flexion—a single degree of freedom system, J Biomechanics 1999, 32-2:111-118. Leardini, A. and O&#39;Connor, J. J. and Catani, F. and Giannini, S. A geometric model for the human ankle joint, J Biomechanics 1999, 32-6:585-591) seem to be qualitatively consistent with what has been already observed in the knee joint. The type of motion is complex and multi-axial, including rolling as well as sliding between the articular surfaces. In addition to a relative sliding, the talus rolls forward on the tibial mortise during dorsi-flexion and backwards during plantar-flexion. These studies revealed that the articular surfaces of the natural joint are slightly non-congruous to allow the rolling component of their relative motion. It has been shown that, under passive minimally-loaded conditions, the articular surfaces and the ligaments of the natural joint prescribe a unique envelope for the position of the axis of rotation. The changing positions of the axis of rotation suggests that the hinge-like fixed-axis concept for the ankle joint commonly described in the literature is an oversimplification and does not reflect the actual kinematic pattern of motion. The aforementioned studies by the inventors and several other studies have described a nearly-isometric pattern of rotation of the calcaneo-fibular ligament on the lateral side of the joint and tibio-calcaneal ligament on the medial side in comparison with all the other ankle ligaments (Colville, M. R. and Marder, R. A. and Boyle, J. J. and Zarins, B., Strain measurement in lateral ankle ligaments, Am J Sports Med 1990, 18(2), 196-200; Bruns, J. and Rehder, U., Ligament kinematics of the ankle joint, Zeitschrift fur Orthopadie und lhre Grenzgebiete 1993, 131(4), 363-369). A few recent studies also claim an anterior shift of the contact area at the tibial mortise during dorsiflexion (Kitaoka, H. B. and Kura, H. and Luo, Z. P. and An, K. N., Contact features of the ankle joint, Proceedings of 42nd Annual Meeting of Orthopaedic Research Society, Atlanta (Ga.), 19-22 February 1996, 396) as described in the aforesaid publications by the inventors. These observations have been confirmed very recently by the inventors in specifically designed experiments (Stagni R, Leardini A, Ensini A. Ligament fibre recruitment at the human ankle joint complex in passive flexion. J Biomech. 2004 December; 37-12:1823-9; F. Corazza, R. Stagni, V. Parenti Castelli, A. Leardini Articular contact at the tibiotalar joint in passive flexion, Journal of Biomechanics, 2005 June; 38(6):1205-12).  
      In conclusion, it has been shown that a) the most anterior fiber of the calcaneo-fibular and of the tibio-calcaneal ligaments of the healthy human ankle joint remain exactly or nearly isometric during passive motion and that these fibers control and guide passive ankle motion in its predefined and preferential passive path, whereas the other ligament fibers slacken and lengthen so as to limit but not to guide motion; b) the axis of rotation in the sagittal plane passes through or close to the intersection as seen in the sagittal plane of the most isometric fibers of the calcaneo-fibular and tibio-calcaneal ligaments and therefore moves anteriorly and proximally during dorsiflexion when these fibers change direction as they rotate about their origins and insertions on the bones; c) the contact area between the natural articular surfaces moves anteriorly on the tibial mortise during passive dorsi-flexion and posteriorly during passive plantar-flexion. As with the knee, the slackening and the tightening of the ankle ligaments may be explained in terms of their instantaneous positions with respect to the moving axis of rotation.  
      These observations imply that the natural articular surfaces of the bone segments must fulfill the requirement that they can be moved passively in mutual contact while fibers within the calcaneo-fibular and tibio-calcaneal ligaments remain at or close to constant length. This condition is achieved in the natural joint because the common normal to the articular surfaces at their point of contact passes through or close to the axis of rotation at the intersection as seen in the sagittal plane of those fibers in each position of the joint (this theorem is called the common normal theorem). For ankle joint replacement, it is suggested that the shapes of the articular surfaces of the prosthesis components must also satisfy the common normal theorem in order to be compatible with the geometry of the retained ligamentous structures.  
      When the geometry of the two isometric ligament fibers is known in terms of the locations of their origins and insertions on the bones and when the shape in the sagittal plane of one articular surface is chosen by the designer, the shape in the sagittal plane of the complementary surface of the other articular segment for a two component prosthesis can be deduced from the common normal theorem for it to be compatible with ligament isometry: to avoid interpenetration or separation of the two bones during passive motion of the joint, the normal at the contact point on the complementary second surface should ideally pass through the intersection as seen in the sagittal plane of the isometric fibers of the calcaneo-fibular and tibio-calcaneal ligaments.  
      When the articular surface of the talar component as seen in the sagittal plane is chosen to be a concave circle with a radius close to that of the natural surface, the talar surface determined from the common normal is found to be a convex surface with curvature very similar to that of the natural talus. However, when the designer chooses a non-physiological surface shape in the sagittal plane for one of the components of a two component prosthesis and applies the common normal theorem, the result is a design with non-congruent surfaces in the sagittal plane and a non-physiological surface shape for the second component. When the surface of the tibial component in the sagittal plane is chosen to be convex, flat or concave, the calculated shape of the compatible talar component is found in each case to be convex but with a radius of curvature up to 70% longer than that of the natural talus.  
      When the surface of the tibial component in the sagittal plane is chosen to be either flat or a convex or a concave circle, exact application of the common normal theorem yields a shape for the compatible surface of the talar component in the sagittal plane that is convex but not precisely circular. Motion approximately compatible with the ligaments can be achieved by replacing the calculated non-circular shape of the compatible talar surface with a circular surface which provides a closely fitting approximation to the ideal shape. The common normal to the articular surfaces at their point of contact then passes close to but not exactly through the intersection of the most isometric fibers of the calcaneo-fibular and tibio-calcaneal ligaments, as seen in the sagittal plane. However, because all these solutions yield surfaces for a two-component prosthesis which are non-congruous, a higher wear rate is possible because of the higher contact stress due to small contact areas developed between non-conforming surfaces but recent developments in the manufacture of ultra high molecular weight polyethylene may make the use of such a two-component non-congruous prosthesis feasible. Moreover, because of the need to achieve overall surface/ligament compatibility in the replaced joint, the accuracy required for implanting two components can be critical for the overall success of the implant and the risk of erroneous positioning can be high.  
      Compatibility between ligaments and articular surfaces can be retained even when introducing a third mobile component to the prosthesis. Again choosing a flat, convex or concave circular shape for the tibial component as seen in the sagittal plane, the ligament-compatible shape of the talar component in the sagittal plane can be approximated by a circular surface. However, the use of a flat or a concave tibial component with a convex talar component having a radius in the sagittal plane longer than that of the natural joint and that of prior art three component prostheses increases the danger of dislocation of the meniscal bearing because of reduced entrapment as a consequence of a smaller difference between the thickest and thinnest regions of the bearing. The present invention is based mainly on issues related to ankle joint function in the sagittal plane but it has been developed in three dimensions to provide a two component prosthetic ankle joint device, comprising a first component for attachment to the distal tibia having an articular bearing surface that is generally curved in a concave manner, and a second component for attachment to the proximal talus having an articular bearing surface that is generally curved in a convex manner in the sagittal plane and curved partly in a concave manner in the frontal plane (a so-called anticlastic surface having principal curvatures of opposite signs). The two components are to be secured respectively to the tibia and to the talus with the articular surfaces of these components in mutually opposite disposition. The shapes of the articular surfaces of these components can not be chosen to achieve full congruency at interface over the range of movement. Rather, the shapes of the articular surfaces at the interface can be chosen to ensure that the axis of flexion of the ankle during passive movements passes through or close to the intersection of the most isometric fibers of the calcaneo-fibular and tibio-calcaneal ligaments, as seen in the sagittal plane, thereby providing an articulation that is substantially ligament-compatible. The thickness of the plastic bearing fixed to the tibial component can be selected at surgery to be the most appropriate to restore the original tensioning pattern of the ligaments: thicker or thinner bearings would involve joint rigidity or laxity respectively. A thicker bearing would of course last longer as for the process of the possible wear. As in the intact joint, the articular surfaces do not constrain the relative motion of the bone segments but merely allow the replaced joint to perform the motions directed by the ligamentous mechanism. A modular thickness of the plastic bearing has also the advantage that it can accommodate partially for surgical technique errors entailing the erroneous level of the bone cuts. Lastly, it has been observed that when total ankle replacement is performed, the subtalar joint complex is frequently affected too. Total ankle replacement certainly does have to cope with an affected ankle (talocrural) joint but also, very often, with an affected subtalar (talocalcaneal) joint complex, and therefore it should be aimed at restoring the function of both joints. A full conforming component articulation in the frontal plane allows abduction/adduction as well as axial rotation and flexion/extension movements (in the frontal, transverse and sagittal planes respectively) to occur at the tibio-talar interface, complementing the abduction/adduction movements which, in the healthy joint, occur mainly at the sub-talar joint complex. However smaller contact areas would be exhibited at the articulating surfaces in the former two cases. In this two-component solution there is a reduced risk of subluxation and dislocation for the bearing. 
    
    
     DESCRIPTION OF THE DRAWINGS  
      The technical features of the invention, according to the aforesaid aims, can be clearly noted from the content of the claims set out below and its advantages shall become more readily apparent in the detailed description that follows, made with reference to the accompanying drawings, which show an embodiment provided purely by way of non limiting indication, in which:  
       FIG. 1  is an overall view in the sagittal plane of a prosthesis device according to the invention;  
       FIGS. 2   a ,  2   b  and  2   c  schematically show the mechanism of relative motion expected between the prosthesis components as guided by the most isometric fibers of the calcaneo-fibular and tibio-calcaneal ligaments in an replaced joint in three characteristic positions: in maximal plantar-flexion, neutral and maximal dorsi-flexion respectively;  
       FIG. 3  is an overall view in the frontal plane of the prosthesis device shown in the preceding figures. With reference to  FIGS. 1-3 , the reference number  1  globally indicates an ankle prosthesis device comprising two components: a first tibial component  2 , and a second talar component  3  opposite to the first tibial component  2 . The first component  2  is able to be attached to a tibial bone segment  2   a  and has a first bearing surface  5  presenting a shape different to the shape of the corresponding bearing surface of the natural joint. The second component  3  is able to be attached to the talar bone  3   a  and has a second bearing surface  6  presenting a shape different to the shape of the corresponding bearing surface of the natural joint. The first and the second bearing surface  5 ,  6  are arranged to face each other. The first and the second bearing surface  5 ,  6  are in mutual contact to ensure that the original joint movement be restored. 
    
    
      Each of the articulating bearing surfaces  5 ,  6  presents a respective shape designed on the basis of the shape of the opposite articulating bearing surface  5 ,  6  according to the common normal to their point of mutual contact at every position over the range of passive plantar and dorsi-flexion. In particular, the shape of each articulating bearing surface  5 ,  6  is determined by the common normal theorem so that the first and second articulating bearing surfaces  5 ,  6  be compatible with isometric ligament rotation. For instance, once the shape of the first articulating bearing surface  5  or the tibial arc has been imposed, the shape of second articulating bearing surface  6  or tarsal arc is deduced by using the common normal theorem. Thus, to be compatible with the isometric rotation of the anatomical calcaneo-fibular and tibio-calcaneal ligament fibers  9 ,  10  ( FIG. 1, 2   a ,  2   b  e  2   c ) the tibial and talar surfaces  5 ,  6  can be of any shape, but when the shape of one articular surface  5  of the first component  2  is chosen, the shape of the complementary surface  6  of the other component  3  is deduced accordingly from the common normal theorem so that the articulation be ligament-compatible.  
      Advantageously, the common normal  26  to the point of mutual contact of the articulating bearing surfaces  5 ,  6  at every position over the range of passive plantar and dorsi-flexion passes close to, and preferably through, the intersection, in the sagittal plane, of the most nearly isometric fibres in the tibio-calcaneal ligament on the medial side of the joint and calcaneo-fibular ligament on the lateral side of the joint.  
      According to the common normal theorem one of the two articulating bearing surfaces  5 ,  6  can be substantially planar, in the sagittal plane, while the other articulating bearing surface  5 ,  6  can be convex. Alternatively, one of the two articulating bearing surfaces  5 ,  6  can be concave and the other convex, like those shown within the above listed figures, or both articulating bearing surfaces  5 ,  6  can be convex, in the sagittal plane.  
      To minimise the resection of the talus bone  3   a , the talar component  3  should be convex in the sagittal plane ( FIGS. 1, 2   a ,  2   b , and  2   c ). The inventors have found that curved, convex, multicentric and multiradial shapes of the articular surface  6  of the talar component  3  can be ligament-compatible with planar, concave, or convex shapes of the articular surface  5  of the tibial component  2 . According to a preferred embodiment of the present invention, both the first articulating bearing surface  5  and the second articulating bearing surface  6  present respectively radius of curvature  5   a ,  6   a  included within a predetermined range of radius preferably different, and advantageously close to, the radius of the corresponding surfaces of the natural joint. In particular, the first articulating bearing surface  5  presents, in the sagittal plane, a radius of curvature  5   a  which can be imposed between a value of 24 mm (for a concave configuration) and an infinity value (planar configuration) or between an infinity value (planar configuration) and a value of 20 mm (for a convex configuration) while the second articulating bearing surface  6  presents, in the sagittal plane, a radius of curvature  6   a  comprised between 22 mm and 34 mm and in any case to form with the articulating bearing surface  5  a couple of shapes compatible with the isometric rotation of the ligaments  9 , 10 .  
      Advantageously, the radius of curvature  6   a  of the second articulating bearing surface  6  can be up to 75% longer than the natural talar articulating bearing surface of the natural joint. Naturally, also in such a case the radius of curvature  5   a  of the first articulating bearing surface  5  is determined by the common normal theorem on the basis of the shape and the radius of curvature  6   a  of the second articulating bearing surface  6 .  
      Always referring to the above figures, the first component  2  is preferably made of two portions  2   b ,  2   c . The first portion  2   b , preferably made of metal, like the second component  3 , ensures that the first component  2  be tightly attached to the tibial bone segment  2   a , whereas the second portion  2   c , preferably made of plastic, advantageously UHMWPE, is fastened to the first portion  2   b  by properly connecting means (not shown) and defines the above mentioned first articulating bearing surface  5 . The connecting means allows that the second portion  2   c  can be detached from the first portion  2   b  to be substituted by another second portion  2   c  having different sizes better suited for the installation on the patient&#39;s body and performed bone cut at operation. The connecting means also ensures that the engagement among the first and the second portion  2   b ,  2   c  be fixed and immovable after implantation of the prosthesis ankle device.  
      The selection of a concave cylindrical or spherical shape with radius  5   a  for the articulating bearing surface  5  of the first tibial component  2  and a convex cylindrical or spherical shape with radius  6   a  of the second component  3  can be made for the better degree of antero/posterior and medial-lateral entrapment of ankle device with the associated smaller risk of subluxation and dislocation.  
      So, given a convex circular arc in the sagittal plane of the articular surface  5  of the tibial component  2 , together with the geometry of the two ligament fibers  9 ,  10  in at a variety of positions of dorsi-/plantar-flexion, a series of contact points for the optimal dome of the articular surface  6  of a talar component  3  in the sagittal plane are deduced from the common normal theorem. A circular arc that best approximates these points is then adopted as to guarantee large contact areas in all these joint positions. The radius  6   a  of the resulting dome-shaped articular surface  6  of the talar component  3  in the sagittal plane can be even 1.5 times longer than those of prior three and two-component designs.  
      The inventors have found that only with a much more ample arc radius  6   a  for the talar component  3  it is possible to reproduce the characteristic pattern of sliding and rolling during ankle flexion ( FIGS. 2   a ,  2   b  and  2   c ).  
      Therefore, the criteria pertaining to the restoration of the original kinematics pattern and to the minimization of the risk of dislocation for the ankle joint are both met.  FIGS. 2   a ,  2   b  and  2   c  schematically show, with different dimensional scales, in the sagittal plane, the kinematics of the ankle once it has been replaced with a prosthesis embodied by the device  1  according to the invention.  
      During ankle flexion ( FIGS. 2   a ,  2   b  and  2   c ), the second talar component  3  slides and rotates in the sagittal plane on the first tibial component  2 . The line of centers  26  joining the center of curvature  28  of the tibial component  2  to the center of curvature  29  of the talar component  3  passes through or close to the intersection  27  of the most isometric fibers of the calcaneo-fibular  9  and tibio-calcaneal  10  ligaments, thus satisfying the common normal theorem. With ankle joint motion constrained by this type of mechanism, the estimated elongation of the two ligament fibers  9 ,  10  is less than 0.2% of resting length. Hence, the design allows both for the restoration of the original pattern of ligament slackening/tightening.  
      The tibial component  2  has a highly polished articular surface  5  and  5 , two cylindrical bars  12  ( FIGS. 1, 2   a ,  2   b ,  2   c  and  3 ) covered with a porous coating, and positioned on the upper surface  2   d  of the first portion  2   b , which is porous coated as well, and are provided for anchoring the tibial component  2  to the distal tibia  2   a.    
      Both articulating bearing surfaces  5 ,  6 , are partly anticlastic, the central portion of each surface  5 ,  6  having two mutually transverse curvatures, in opposite directions, as the surface of a saddle. The articular surface  5 , the lower surface of the tibial component  2 , is a concave arc in the sagittal plane ( FIGS. 1, 2   a ,  2   b  and  2   c ), with a center of curvature  28 , and a generally convex arc in the frontal plane ( FIG. 3 ). The articular surface  6 , the upper surface of the talar component  3 , is a convex arc in the sagittal plane ( FIGS. 1, 2   a ,  2   b  and  2   c ), with a center of curvature  29  in the talar body and a radius that can be about 1.5 times the length of the radius of the natural talar articular surface, and a generally concave arc in the frontal plane ( FIG. 3 ).  
      The upper surface  6  is a surface of revolution, generated by rotating a generatrix curve about a medial-lateral fixed axis, orthogonal to the sagittal plane of  FIGS. 1 and 2   a - 2   c , i.e. belonging to the frontal plane of  FIG. 3 . The generatrix curve is concave in the frontal plane ( FIG. 3 ), presenting a central concave circular arc, the sulcus  14 , between two lateral convex circular arcs  15 . The articulating surface immediately proximal—coinciding with the articular surface  5  of the tibial component  2 —shows a circular concave arc in the sagittal plane ( FIGS. 1, 2   a ,  2   b  and  2   c ), and has a convex central arc and two concave lateral arcs when seen in the frontal plane ( FIG. 3 ). These are to be fully conforming to the equivalent arcs  14 ,  15  of the talar component  3 , i.e. they have the same radii of curvature when seen in the frontal plane.  
      When the prosthesis is in neutral position ( FIG. 2   b ), the arc  6  in the sagittal plane is slightly longer posterior than anterior to the mid longitudinal axis  13  of the tibia  2   a  ( FIG. 2   b ) because of the larger range of ankle motion in plantarflexion ( FIG. 2   a ) than in dorsiflexion ( FIG. 2   c ), both in the natural and in the replaced joint. In this position, the line of centers  26  is located, in the sagittal plane, slightly forward relative to the mid longitudinal axis  13  of the tibia  2   a  ( FIG. 2   b ).  
      The consequences of the above arrangements are that:  
      1) The interface between the tibial  2  and the talar  3  components, is capable of independent relative movement by virtue of the complementary nature of the corresponding coupled surfaces  5 ,  6 . More specifically, the tibial  2  and the talar  3  components are capable of mutual rotation about an axis passing at the intersection of the isometric ligaments  9 ,  10  as seen in the sagittal plane ( FIGS. 2   a ,  2   b  and  2   c ). The deriving ability to perform relative motions between the tibial  2  and talar  3  components is accordingly extensive and can include rolling, gliding, twisting, and combinations thereof, such as they take place in the natural ankle joint complex. The backwards and forwards motion of the line of centers  26  has significant effect on the mechanics of the articulation since, in the absence of friction, it is also the line of action of the compressive force transmitted across the joint. Its backwards position in plantar-flexion maximizes the lever-arm available to the dorsi-flexor muscles running in front of the joint. Its forwards position in dorsi-flexion maximizes the lever-arm available to the plantar-flexor muscles running in back of the joint.  
      2) The shapes of the bearing surfaces  5 ,  6  of the tibial  2  and talar  3  components can reproduce the natural pattern of relative motion of the corresponding bony segments even though these surfaces both differ significantly from the shapes of the natural tibial and talar articular surfaces. Therefore, the mechanical interactions between the shapes of the surfaces  5 ,  6  and the forces in the surrounding muscles and ligaments and particularly in the related ligament fibers  9 ,  10  which control the stability of the joint, will be physiological as well.  
      3) All relative positions of the components once implanted, under passive conditions, are obtained as positions of minimum stored energy shared among the ligamentous structures of the joint. The shape of the bearing surfaces  5 ,  6  of the two components  2 ,  3  were designed to allow relative motion without resistance through mutual sliding without separation or inter-penetration, while the fibers  9 ,  10  of the isometric ligament rotate about their origins and insertion points without stretching or slackening. Other ligament fibres remain slack during passive motion, except at the limits of that motion. No input of energy is expected to be necessary to displace the replaced joint along this neutral passive path because no tissue deformation is necessary.  
      4) In particular, to obtain this series of minimum energy positions between the tibial  2  and talar  3  components, the talar component  3  is guided by the ligaments to slide forward while rolling backward with respect to the tibial component  2  during plantarflexion and to slide backward while rolling forward during dorsiflexion.  
      5) The interface between tibial component  2  and the talar component  3  allows ligament-controlled motion in the sagittal plane. Rotations in the transverse and frontal planes are possible, but imply further reduction of the contact area and separation of the components a ball-and-socket joint, providing at least one degree of rotational movement to the bones. A further degree of freedom is allowed at the interface between the tibial component  2  and the talar component  3 , when the former component can slide congruently on the latter along the sulcus  14 ,  15  which extends along the dome of the components  2 ,  3  mostly antero-posteriorly. Dorsi/plantarflexion in the sagittal plane is allowed at the tibial component  2 —talar component  3  interface. Pure translations in the proximo-distal, antero-posterior and medio-lateral directions are resisted respectively by tension of the ligaments. In the later case there is also an inherent resistance of the articulating surfaces.  
      6) The implantation of the two bone-anchored components  2 ,  3  has to be carried out most carefully. The center  28  of the tibial lower spherical arc  5  and the center  29  of the talar upper arc  6  must lie on the same line  26  in the sagittal and frontal plane ( FIGS. 1-3 ), and this line  26  must pass through or close to the intersection  27  of the most isometric fibers of the calcaneo-fibular  9  and tibio-calcaneal  10  ligaments, thus satisfying the common normal theorem.  
      7) It is here thought that the tibial  2 , talar  3  components will all have to be made in a number of dimensions to accommodate patients of different size. The number and dimension of different sizes does not constitute a limitation of the present invention. Because of its strategic importance in restoring original joint function, the first portion—of the tibial component  2  should also be made in various thicknesses. The interval between different thicknesses can even be very small but in any case it should be large enough to allow surgeons to detect differences in articular mobility and stability during the operation.  
      Development of the invention since its initial conception has shown that, while a variety of potentially advantageous forms are possible within the more general scope of the invention the above consequences can result from a partly anticlastic-tibial  5  and partly anticlastic-talar  6  prosthetic surfaces. However, they could also result from a relatively simple form of the invention in which the talar surface  6  is part-spherically or cylindrically shaped, and the engaged bearing surfaces  5  of the tibial component  2  is planar. In this way the two-component prosthesis is able to assist the joint in reproducing the original pattern of ligament slackening/tightening, still with articulating surfaces designed according to the present invention.  
      The relevance of this general application of the invention is based on a particular view of the form and function of the passive structures of the joint, these elements being the articular surfaces  5 ,  6  and the adjacent ligaments  9 ,  10 . This view holds that, during the passive motion of the joint, the articular surfaces  5 ,  6  serve to maintain the fibers  9 ,  10  of the ligaments at a constant length and that the ligaments themselves act in such a way as to maintain these articular surfaces in contact. The articular surfaces  5 ,  6  serve predominantly to transmit compressive forces, and the ligaments and the muscle tendons to control and limit the surface movements while themselves serving to resist and transmit tensile forces. Thus, there is interdependence between all the elements of a joint, and this interdependence is vital to the overall performance of a natural joint having incongruent surfaces which can provide little inherent stability.  
      The advantages and novelty of the illustrated device  1 , with respect to prior designs can be listed as follows: 
          the multiaxial pattern of movement of the natural ankle joint can be closely simulated without significant distortion of the natural controlling and stabilising mechanism. The necessary compatibility of the articular surfaces  5 ,  6  with the isometric rotations of the ligaments  9 ,  10 , is obtained with arcs of curvature of the tibial and talar components  2 ,  3  that significantly different from those of the natural anatomical shapes, and thus from that of all designs of the prior art;     the device  1  features conforming surfaces in both the tibial  2 —talar  3  component articulations in the frontal plane in all the positions of the joint;     medial-lateral entrapment of the ankle joint device  1  is guaranteed by the sulcus  14  running on the dome of the tibial and talar components  2 ,  3 , avoiding the sharpened limiting interfaces as used in the prior art to prevent dislocation and separation which certainly entail a high risk of wearing at the sharpened interfaces;     unlike all previous two-component cylindrical and ball-and-socket designs, the axis of joint rotation represented by the point  27  in the sagittal plane is not fixed as imposed by the congruity of the articular surfaces, but rather is able to move relative to both tibial  2   a  and talar  3   a  bones to assist the joint in performing the original pattern guided by the isometric rotation of certain ligament fibers  9 ,  10  ( FIGS. 2   a ,  2   b  and  2   c ).        

      In order to restore the original compatibility between articular surfaces and ligaments at the human ankle joint, not only should the prosthesis components  2 ,  3  be designed according to the criteria set out above, but should be implanted in their definitive relevant position with great care and precision.  
      The invention thus conceived is clearly suitable for industrial application; moreover, it can be subject to numerous modifications and variations, without thereby departing from the scope of the inventive concept. Furthermore, all components can be replaced by technically equivalent elements.