Abstract:
An orthopaedic implant used in arthroplasty of the fingers comprises a first element for a proximal phalanx and a second element for an adjacent distal phalanx, each element having a respective stem for implantation in the bone and an interphalangeal articulation head, the head of the first element having a biconvex surface of articulation, with a central valley, and the head of the second element having a biconcave surface of articulation designed to cooperate with said convex surface and comprising a central crest. At rest, the surfaces are congruent in the frontal plane of the articulation in the area of the condyles and non-congruent in the sagittal plane.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US national phase of PCT application PCT/FR2010/000010, filed 8 Jan. 2010, published 15 Jul. 2010 as 2010/079288, and claiming the priority of French patent application 0900054 itself filed 8 Jan. 2009, whose entire disclosures are herewith incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an orthopedic implant used in arthroplasty of the fingers, comprising a first element for a proximal phalanx and a second element for an adjacent distal phalanx. 
     A proximal phalanx is to be understood to mean a phalanx located on the body side or member side (hand or foot) in question, and a distal phalanx is to be understood to mean a phalanx located toward the outside of this member, according to the orientation rules conventionally adopted in anatomy that will moreover be used below. 
     The invention also relates to a method for installing such an implant. 
     The invention finds one particularly important, although not exclusive, application in the medical field of proximal interphalangeal joint prostheses for the hand (known in the field by the initials PIP). 
     However, it can also and in particular be used advantageously for interphalangeal finger joint prostheses for the thumb (thumb IP) or for distal interphalangeal prostheses (DIP) for the hand or foot, by simple geometric adaptation to the bone sites in question. 
     There are already known interphalangeal joint prostheses comprising first and second elements, each element having a stem for implantation in the bone and a complementary interphalangeal pivot head. 
     For example, there is known (EP 1 339 362) [U.S. Pat. No. 6,699,292] an implant comprising a first element provided with a head having a convex bicondylar pivot surface having a central valley, and a second element provided with a complementary head having a biconcave pivot surface designed to cooperate with the convex surface so that there is a congruence between these surfaces in the sagittal plane. 
     Congruence is to be understood to mean the fact of coinciding or of fitting perfectly with one another in the sagittal plane. 
     The congruence in the sagittal plane therefore means that the point of contact between the two articular surfaces in this plane always passes through the same line of surface contact during movement. 
     It should be recalled that a sagittal or parasagittal plane is a plane parallel to the median plane when the human body is in the upright position. 
     Such an arrangement, although it allows good guidance of the joint, is sometimes prejudicial to the service life of the prosthesis. This is because it may give cause jamming and thus a failure of the prosthesis in up to 20% of cases, depending on the surgeon carrying out the procedure; in addition, it causes significant wear on the articular surfaces of the prosthesis, requiring a new operation. 
     Prostheses are also known (EP 0 572 339) [U.S. Pat. No. 5,405,399] having barrel-shaped bicondylar surfaces with different radii of curvature. 
     Here again, this type of implant that is used in particular as a shoulder prosthesis, poses problems of wear and is moreover not very suitable for joints of small size such as those of the fingers. 
     In order to attempt to alleviate these drawbacks, prostheses made from bioceramic, for the fingers or toes, have been envisaged (DE 198 207 48). 
     Although such prostheses are more resistant to wear, they nevertheless present a risk of shattering and are also complex to install. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an orthopedic implant and a method that satisfy the practical requirements better than those known previously, in particular in that it wears very little despite numerous joint movements, and in that it is very easy to install while adapting to the constraints associated with the subsequent functions of arthroplastic joints. 
     It is moreover of low cost and allows a high degree of modularity, that is to say a high degree of variance in terms of size. 
     To this end, the present invention proposes an orthopedic implant used in arthroplasty of the fingers, comprising a first element for a proximal phalanx and a second element for an adjacent distal phalanx, each element having a respective stem for implantation in the bone and an interphalangeal pivot head, the head of the first element having a biconvex condylar pivot surface, having a central valley, and the head of the second element having a biconcave pivot surface designed to fig generally complementarily with the biconvex surface and having a central ridge, characterized in that the surfaces at rest are congruent in the frontal plane of the pivot in the area of the condyles and are not congruent in the sagittal plane. 
     At rest is to be understood to mean an absence of external pressure on the surfaces of the pivot. 
     The congruence in the frontal plane in the area of the condyles means that the zone of contact between the two articular surfaces is formed by two horizontal lines that pass vertically over the surface during movement. 
     Surprisingly, this congruence in the frontal plane associated with no congruence in the sagittal plane ensures in the presence of articular fluid an excellent lubrication of the joint during movement and thus considerably limits wear on the articular surfaces and stiffening. 
     In advantageous embodiments, use is moreover made of one and/or the other of the following arrangements:
         the radii of curvature (R 1 , R′ 1 ) of the condyles of the biconvex surface of the first element are identical to the radii of curvature (R 2 , R′ 2 ) of the respective biconcave surface of the second element in the frontal plane, the radius of curvature (R 3 ) of the proximal surface of the first element being smaller than the radius of curvature (R 4 ) of the respective distal surface of the second element in the sagittal plane.       

     Radius of curvature in the frontal plane is to be understood to mean the radius of the curve formed by the intersection of the pivot surface with a transverse plane orthogonal to the surface of the head;
         the radii of curvature are such that R 1 =R′ 1 =R 2 =R′ 2 ;   the material of the heads is designed to allow the heads to cooperate by friction along a line of friction in the frontal plane in the absence of longitudinal pressure and along a crush zone in the frontal plane in the event of longitudinal pressure.       

     Longitudinal pressure is to be understood to mean a pressure exerted in the longitudinal direction of one or of both phalanxes in question. In other words, this arrangement makes it possible in particular to obtain a maximum congruence under load, transforming the two lines of contact mentioned above into a strip of contact that thus distributes the forces and further reduces in a surprising manner wear on the articular surfaces;
         the material of the biconvex surface of the head of the first element is a flexible polymer material having a Young&#39;s modulus E≦30 Gpa.       

     The two articular surfaces are thus of different rigidity, the most flexible material that is most subject to wear being on the condyle side. 
     It will be noted in this regard that, since the condylar surface is in general and by design thicker than the distal surface, this permits a lesser deformation of the flexible material. 
     Furthermore, since the distal surface passes over a large part of the condylar surface, between the straight position and the flexed position, there is thus a better distribution of wear. 
     A compromise is thus obtained between a very high elasticity close to that of anatomical cartilage and a greater rigidity so as to limit the deformation and the long-term wear. 
     Advantageously, it may also be the material known by the name CFR PEEK, these initials referring to Carbon fiber reinforced polyetheretherketone having a Young&#39;s modulus of around 18 Gpa;
         the Young&#39;s modulus of the bicondylar surface is E≦2 Gpa. More specifically and in a nonlimiting manner, the flexible material is a polyethylene known by the name UHMWPE (these initials referring to ultra high molecular weight polyethylene) with E≦1 Gpa;   the respective facing surfaces of the heads of the two elements placed in contact define an intercondylar space of non-zero volume along the entire length of the central valley, allowing an improved lubrication of the joint between the heads.       

     Such an intercondylar space for example of between 15 mm 3  and 30 mm 3 , for example 25 mm 3 , makes it possible to further improve lubrication of the joint, the contact between the articular surfaces at this location therefore being non-congruent;
         in the frontal plane, the valley of the bicondylar head of the first element has a radius of curvature R 5 , the ridge of the biconcave surface of the second element having for its part a radius of curvature R 6 &gt;R 5 ;   the implantation stems have the shape of a polygonal post that is proportioned so as to be received inside the medullary cavity of a phalanx and are provided with an end stop to prevent them from being excessively pushed in.       

     Advantageously, the material and the shape of the stem have an effective modulus of elasticity or Young&#39;s modulus E≦30 GPa. 
     The effective or apparent Young&#39;s modulus is to be understood to mean an overall Young&#39;s modulus of the stem having a behavior identical to that of a material that intrinsically has this Young&#39;s modulus. 
     More specifically, it is possible to provide two apparent stiffnesses of different stems depending on the patients, i.e. a rigid stem for young patients (i.e. around 20 GPa) and a more flexible stem for elderly patients (i.e. around 5 to 6 GPa);
         at least one implantation stem has a shape that is at least partially open in the dorsal-palmar plane;   at least one implantation stem has an H-shaped, U-shaped or V-shaped cross-section and/or has a duckbill-shaped slot or a recessed semicylindrical shape on the palmar side or a half-tube shape.       

     Advantageously, the stems are open and/or are covered with a partial or full osseointegrating coating to improves anchoring;
         since the implantation stems of the elements are removable relative to the respective heads, they comprise means for attachment to the heads;   the attachment means are formed by the conical fitting of a post secured to the head into a recess formed in the respective stem;   the conical fitting has a Morse taper, that is to say having a cone angle of around 5%: however, this cone angle may also be smaller, for example around 3% or 2.5%;   the biconcave surface comprises a base provided with a connection post of predetermined thickness.       

     Providing a base of predetermined thickness thus permits a number of predetermined thicknesses that the surgeon will be able to choose and adapt to the cut made during the operation;
         the attachment means between the parts of an element formed by the stem and the respective head moreover comprise a recess or a groove formed in one of the parts able to receive a plate of complementary shape that is secured to the other part;   the biconvex bicondylar pivot surface and the respective biconcave surface have uniform radii of curvature in the frontal plane and variable radii of curvature in the sagittal plane;   the attachment means are identical between on the one hand a plurality of heads of different size and on the other hand identical or different stems of first elements and/or second elements.       

     Such an arrangement allows a high degree of modularity. 
     The invention also proposes a method of installing an orthopedic implant for arthroplasty of the fingers, of the type described above. 
     The invention will be better understood on reading the following description of embodiments that are given below by way of nonlimiting examples. The description refers to the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic top view of the bones of a hand in which an implant according to the invention has been installed; 
         FIG. 2  shows in perspective the biconvex head of a first element according to one embodiment of the invention; 
         FIG. 3  is a perspective view of a second element having a biconcave head designed to cooperate with the head of the first element of  FIG. 2 ; 
         FIG. 4  is a schematic top or dorsal view showing an implant according to the invention, with the joint in the extended position; 
         FIG. 5  is a sectional side view of the implant of  FIG. 4 , with the palmar side facing upward; 
         FIGS. 6 and 7  are respectively sectional views along VI-VI and along VII-VII in  FIGS. 4 and 5  showing the radii of curvature in the sagittal and frontal planes of the pivot surfaces according to the embodiment of the invention more particularly described here; 
         FIG. 8  shows the head of  FIG. 2  under longitudinal stress, causing a crush zone to appear in the case of using a more flexible head material according to one embodiment of the invention; 
         FIGS. 8A and 8B  are sectional views of the joint respectively in a position with the finger flexed and with the finger straight, showing the zone of crush contact between the surfaces; 
         FIG. 9  shows an implant stem that can be used with the head of  FIG. 8 , according to one embodiment of the invention; 
         FIG. 10  is a sectional side view of an implant element having a stem and a head according to one embodiment of the invention; 
         FIG. 11  schematically shows in side view three embodiments of heads for a second element according to the invention, with bases of different thickness; 
         FIGS. 12 to 16  show in perspective from behind five embodiments of a stem for implantation according to the invention; 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic top view of a skeletal hand  1 , having an orthopedic implant  2  according to the invention; 
     This implant comprises a first element  3  for a proximal phalanx  4  and a second element  5  for a distal phalanx  6 ; 
     Each element comprises a respective stem  7 ,  8  for implantation in the bone and a respective interphalangeal pivot head  9 ,  10 . 
     Hereinbelow, the same reference numbers will be used to denote similar or identical elements. 
       FIG. 2  shows the head  9  of the first element  3 . 
     It has a biconvex condylar pivot surface  11  having a central valley  12 . 
       FIG. 3  for its part shows the second element  5  having an implantation stem  8  and the head  10 . 
     The latter has a biconcave surface  13  designed to cooperate with the biconvex surface  11  of the head of the first element. 
     This biconcave surface  13  comprises a central ridge  14 . 
     The surfaces  11  and  13  are designed so as to be congruent in the area of the two condyles  15  and  16  and of the respective concave surfaces  17  and  18  in the frontal plane of the pivot and so as not to be congruent in the sagittal plane. 
     More specifically,  FIGS. 4 ,  5 ,  6  and  7  show the embodiment more particularly described here of this congruence or of noncongruence. 
     To this end, the surfaces of the condyles and the facing surfaces have radii of curvature such that, with reference to  FIG. 7 , in the frontal plane F, the radii of curvature R 1 , R′ 1  of the condyles having a convex surface are identical or substantially identical to the radii of curvature R 2 , R′ 2  of the respective concave surfaces of the second element. 
     Substantially identical is to be understood to mean a value equal to ±5%. 
     By contrast, the radius of curvature R 3  of the proximal surface of the first element (see  FIG. 6 ) is smaller than the radius of curvature R 4  of the respective distal surface of the second element in the sagittal plane S, for example between 1.1 and 1.5 times smaller, for example 1.2 times smaller. 
     This makes it possible to obtain congruences and non-congruences as mentioned above. 
     For example, R 3  is between 3.4 mm (size S), 3.75 mm (size M) and 4.5 mm (size L) for a radius R 4 =5 mm. 
     Advantageously, the radii R 1 , R′ 1 , R 2 , R′ 2  are all equal for example to a value between 4 mm and 6 mm, for example 5 mm. 
       FIG. 8  shows the contact zone  20  obtained with a first element head made from elastic material when a longitudinal pressure (arrow H in  FIG. 8 ) is exerted. 
     This is because the line of friction in the frontal plane then is applied to a zone  20 , which makes it possible to better distribute the friction forces and thus unexpectedly to minimize the long-term wear on the head. 
     In other words, and by virtue of the combination of the material of this head, which may for example be a polymer of the UHMWPE type, and the respective radii of curvature mentioned above, an excellent damping and minimized wear are thus obtained. 
     Still with reference to  FIGS. 6 and 7 , there is provided an intercondylar space  21  of a volume greater than zero along the entire length of the central valley  12  facing the ridge  14 , which space facilitates the liquid lubrication, for example a theoretical volume that will depend on the pressure on the prosthesis of around 20 mm 3 , for example 24 mm 3 . 
     To this end, the radius of curvature R 5  of the valley and the radius of curvature R 6  of the ridge are such that R 6  is greater than R 5 , for example 4.5 mm for the former and 2.5 mm for the latter. 
     According to the invention more particularly described here, the implant comprises two elements  3  and  5 , each provided with a head  9 ,  10  and with a respective removable stem  7 ,  8  that can be detached from the head for fixation in the bone. 
       FIGS. 8A and 8B  show the change in the zone  20  in the course of the movement, with the finger flexed ( FIG. 8A ) and with the finger straight ( FIG. 8B ). 
     It can be seen that the zone  20  covers half the condyle but remains more or less at the same location on the distal part. The most fragile surface is thus chosen for the condyle side according to the embodiment of the invention more particularly described here. 
       FIG. 9  shows an example of such an implantation stem  22  made from titanium. 
     The stem  22  has a part elongated substantially around an axis  23 , slightly curved, for example with a radius of curvature r of 100 mm for a stem for a proximal element of large size and of 90 mm for a proximal element of medium or small size or between 50 mm (large size), 40 mm (medium size) and 10 mm (small size) for the distal elements. 
     The stem is provided with a part designed to be fitted into the central hole of the marrow of the phalanx, namely a part  24  of substantially polygonal shape, for example of hexagonal shape with a slightly frustoconical end  25  that enables good introduction and lateral blocking in terms of indexing in the modular cavity of the phalanx. 
     The implantation stem is removably mounted by attachment means  26  on the head. 
     These attachment means comprise a conical orifice  27  (see also  FIG. 10 ), into which there is inserted a conically tapered post  28  that is secured to the head  29  and has for example a cone angle of 2 to 3°. 
     The stem also comprises a stop plate  30  that cooperates with a complementary recess in the form of a groove  31  (see  FIG. 8 ) that allows a good securing and a good indexing of the head  29  to the stem  24  when the post  28  is pushed into the recess  28 ′ and the plate  30  is locked into the recess  31 . 
     On the second element side, advantageously the head  10  has a base (see  FIG. 11 )  32 ,  33 , of  34  of variable thickness that makes it possible to adjust the thicknesses at the time of installation of the implant so as to optimize functioning of the joint during the surgical operation. 
     This base may for example have a thickness of 1 mm, 2 mm; 1.7 mm, or 2.2 mm. 
     Advantageously, the convex bicondylar pivot surface and the biconcave surface have uniform radii of curvature in the frontal plane and different radii of curvature in the sagittal plane. 
     The fact of standardizing or of making the dimensions of the posts  28  and of the orifices  27  uniform furthermore makes it possible to adapt indifferently a head of predetermined size to a stem of different size, thereby allowing a high degree of modularity. 
     Other means of mechanically assembling the head to the stem are of course possible. 
     The differences in thickness between the different bases may range from 0.5 mm to 1.5 mm per size. 
       FIGS. 12 to 16  show stems for a distal or proximal element of different shapes and lengths according to embodiments of the invention. 
     The stem  33  of  FIG. 12  has on one side a stop plate  34  of the type described with reference to  FIG. 9 , the plate being formed with a throughgoing hole for receiving the post of the head. 
     The solid part  35  extends away from the plate as a flat beveled portion  36  provided with a slot  37  that is open along its entire length and that is of small thickness, for example 2 mm in the dorsal-palmar plane, which gives it a standard duckbill shape. 
       FIG. 13  shows another embodiment of the stem  38  showing a part  39  capable of being pushed into the bone, the part being substantially pyramidal or frustoconical with an H-shaped cross-section, thereby exposing on either side of the dorsal-palmar plane  40  substantially parallelepipedal recesses from the duckbill-shaped end  41  to the stop plate  42 . 
       FIG. 14  shows another stem  43  having a part  44  of frustoconical shape provided on its lower surface with a groove  45  that gives the stem a recessed shape. 
       FIG. 15  for its part shows in cross-section a half-stem  46  in the shape of a substantially cylindrical tube  47  having an end  48  provided with the plate  49 , formed with a throughgoing hole  50  in order to complementary fit with the post of the head as described above, the opening  50  being extended by a bore  51  along the entire length of the tube  47 . 
       FIG. 16  shows a stem  52  in the shape of a folded sheet, having an end part  53  with a U-shaped cross-section. 
     In the embodiments of the invention more particularly described here, the stems have rough outer surfaces. 
     This roughness is obtained for example by blasting with sand or corundum, or by means of transverse fluting (not shown), making it possible to increase the adhesion in the bone. 
     In order to further improve anchoring, an osseointegrating coating is advantageously provided, for example of the hydroxyapatite (HAP) type or formed from microporous titanium by plasma deposition. 
     The installation of a prosthesis according to the invention during an implantation operation will now be described. 
     After opening the finger and cutting away the damaged bone parts in order to install the prosthesis, in a manner known per se, (first bone resection step) the housings for the stems are prepared (introduction of rasps) and then the fixing stem is fitted into the medullary canal. 
     The space available for the installation of the heads is then verified, and then the proximal condylar head  9  is installed and is fixed onto the stem in a precise and locked manner by way of the Morse cone. 
     A test is then performed with a phantom of the other distal head. 
     Then, after choosing the thickness as a function of the prior resection of the bone, the stem  8  is installed on the other side in the distal phalanx and the joint is finalized by installing the head  10 . 
     The fact that the head  10  can have several different thicknesses for its base therefore permits adjustment during the operation. 
     All of these operations are carried out in a manner known per se using dedicated ancillary equipment. 
     As will be understood, and as is also clear from the above, the present invention is not limited to the embodiments more particularly described. On the contrary, it encompasses all the is variants and in particular those in which the two heads  9  and  10  are made from flexible material, those in which the stems are made from biocompatible polyethylene or from a metal other than titanium, those in which the prosthesis is a thumb or big toe IP prosthesis, or an IPD prosthesis that involves different dimensions and a different design of the articular surfaces so as to allow adequate lateral mobility.