Abstract:
An endoprosthesis utilizes a biomechanical structure based on the new inventive lever model of first degree. Contrary to the widespread practice, the inventive structure has a stem placed inside the femur bone, which is not rigidly attached to this bone. Accordingly, the bone, following its known tendency to slightly pivot away from the medial plane of the body in response to the loads, does not directly contact the stem. The neck portion experiences even greater loads then the stem. The supporting anchor has a region extending laterally from the stem and pressing against a supporting surface of the housing, which also extends laterally from the body of the housing. Since the stem, supporting anchor and neck are typically constitute a one-piece component, the lateral surfaces of the supporting anchor and housing are in continuous frictional contact during displacement of the stem and housing relative to one another.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     Replacement devices such as artificial joints in general and especially those for the hip have been known for many years. Such replacement devices include substitute members for the two parts of the natural joint, namely the femoral head, which is joined to the femur, and the hip socket, which receives and cooperates with the head to provide a natural universal joint.  
         [0002]     Replacement of the natural hip joint parts is necessary when deterioration has occurred to one or both of the natural femoral head and socket. Ideally, the replacement members should reproduce the structure and function of the original members. For example, it is important that the femoral head be securely attached to the femur, that the head be received within the socket and that the resulting joint be produced with the desirable degree of resilience or cushioning.  
         [0003]     All known prior art, for example U.S. Pat. No. 4,770,661 and U.S. Pat. No. 4,159,544 disclose a system associated with resorptions of bone tissue, protrusion of the bottom of acetabulum, and loosening of the stem due to a relatively high pressure upon the bone. Furthermore, localized pressure generated by the prosthesis parts causes greater than naturally occurring displacement of the stem from its initial position. Particularly, in the upper part, the stem tends to move medially, while in the lower part, it moves outwardly laterally.  
         [0004]     Currently used endoprosthesis models typically take into consideration the biomechanics of the femur, which is rather similar to a console. This view is applied both to normal hip joints and total joint replacements and is usually realized by a system configured to rigidly fix the stem of endoprosthesis to the surrounding bone. Typically, to further this goal, the stem may have various geometrical forms for compressive fixation; alternatively or in addition to the specifically designed forms, it is not uncommon to cement the stem to the bone.  
         [0005]     The clinical practice and numerous data show that this approach may not fully take into consideration the following: 
        even after milling of the channel through the femur diaphysis, the femur bone still contains a live tissue requiring circulation of tissue fluid, which may be detrimentally affected by cementing, and     during walking, the femur bone rotates around its long axis thus gradually weakening a bond between this bone and the prosthesis. (See  Guide to prosthetics,  N. Kondrashin, Moscow, “Medicine” 1988, pages 87,88).        
 
       SUMMARY OF THE INVENTION  
       [0008]     Thus, a primary object of the present invention is to improve the stability of hip joint endoprosthesis.  
         [0009]     A further object of the present invention is to substantially reduce and eventually to completely eliminate complications associated with at least some of the currently used endoprosthesis models.  
         [0010]     Still a further object is to provide the recipients of the inventive hip joint endoprosthesis with a fully functional life.  
         [0011]     To achieve these objects, an endoprosthesis configured in accordance with the invention utilizes a biomechanical structure based on the lever model of first degree. In other words, contrary to the widespread practice of cementing the entire endoprosthesis to the femur bone, the inventive structure has at least one component, a stem placed inside the femur bone, which is not rigidly attached to this bone. Accordingly, the bone, following its known tendency to slightly pivot away from the medial plane of the body in response to the loads, does not directly contact the stem.  
         [0012]     As a consequence, while in the known prior art structures, such a direct contact would rather rapidly wear out the bond between the housing and stem, the inventive structure avoids such a drawback exhibiting a long functional life.  
         [0013]     Yet the ability of the inventive structure to withstand numerous loads is not compromised due an elongated outer housing extending beyond the distal (lower) end of the stem and reliably coupled to the bone. Such a coupling is ensured by a plurality of holes formed in the outer housing and allowing bone tissue to penetrate through the holes and bond with the outer housing. Furthermore, the holes provide circulation of the tissue fluid indispensable for maintaining the ingrown tissue.  
         [0014]     In accordance with a further aspect of the invention, the endoprosthesis includes a sleeve made from porous material and located between the stem and outer housing. Accordingly, the sleeve reduces the friction during rotation of the stem and outer housing relative to one another, which in turn minimizes formation of granules typically originated during such a friction and known to weaken the bone. Material of the sleeve also plays an important role in maintaining the circulation of the tissue fluids within the inventive prosthesis.  
         [0015]     Still another aspect of the invention is directed to further minimizing frictional forces between the neck portion of the stem, which extends angualrly from the stem towards the bone&#39;s head, and the housing. Formed with a supporting anchor and a neck inserted into the head of femur, the neck portion experiences even greater loads then the stem. In particular, the supporting anchor has a region extending laterally from the stem and pressing against a supporting surface of the housing, which also extends laterally from the body of the housing. Since the stem, supporting anchor and neck are typically constitute a one-piece component, the lateral surfaces of the supporting anchor and housing are in continuous frictional contact during displacement of the stem and housing relative to one another. To minimize deterioration of these surfaces, the inventive structure includes at least one plate inserted into one of the a supporting surface of the housing, which also extends laterally from the body of the housing. Since the stem, supporting anchor and neck are typically constitute a one-piece component, the lateral surfaces of the supporting anchor and housing are in continuous frictional contact during displacement of the stem and housing relative to one another. To minimize deterioration of these surfaces, the inventive structure includes at least one plate inserted into one of the contacting surfaces of these components and made of material with a low friction coefficient.  
         [0016]     The above and other objects, features and advantages of the present invention will become more clearly understood from the following description referring to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a schematic representation of biomechanics of the femur with total joint replacement.  
         [0018]      FIG. 2  is a longitudinal partially sectional view of the inventive endoprosthesis.  
         [0019]      FIG. 2   a  is a longitudinal partially sectional view of an embodiment of the  FIG. 2 ;  
         [0020]      FIG. 2   b  is a partial view from arrow B of the  FIG. 2 ;  
         [0021]      FIG. 2   c  is a partial sectional view of the region C of an embodiment of the  FIG. 2 ;  
         [0022]      FIG. 3  is a schematic view of a supporting anchor seen from arrows III-III of the  FIG. 2 ; and  
         [0023]      FIG. 4  is a schematic view of a supporting rim of the metal casing seen from arrows IV-IV of the  FIG. 2 .  
         [0024]      FIG. 5  is a cross-sectional view V-V of the  FIG. 2 ;  
         [0025]      FIG. 6  is a longitudinal sectional view of the inventive cup (sectional view VI-VI of the  FIG. 2 ).  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     Referring to  FIGS. 1-2 , the anatomical structure of the hip joint includes the femur diaphysis, further referred to as a bone  35 , neck and head  11 . If a situation requires the hip replacement, it is performed by removing the neck, forming a channel  36  within the bone  35  and introducing a stem  13  ( FIG. 2 ) of a prosthesis  100  into the channel  36 . Formed integrally with the stem  13  is a neck portion  12  of the prosthesis  100 , which extends angularly from the stem  13  into a cup  7  that is attached to the head of the bone.  
         [0027]     As follows from the clinical practice, loading of the prosthesis cup leads to the greatest bone resorption of the femur diaphysis in its upper medial part  34 , as this is where the most of the body weight load is applied. The prosthesis stem  13  and particularly its distal end  37 , are pushed laterally away form the median plan  39  ( FIG. 4 ), as shown by an arrow “38” in  FIG. 1  due to the inherent motion of the bone  35  occurring as a result of applied loads.  
         [0028]     If the prosthesis were constructed in accordance with the convenient practice as a console model, displacement of the bone  35  would gradually destroy cement typically bonding the stem and bone  35 . In contrast, the inventive prosthesis  100  is configured as a lever of the first kind with an axis  2  of the lever characterized by a substantial degree of freedom between the stem  13  and the bone  35 . As a result, the bone  35  is free to follow its natural pattern of rotating, and the bond between a housing  14  of the prosthesis  100  and the bone  35  is not affected.  
         [0029]     The pressure by the prosthesis stem  13  on the lateral side of the femur diaphysis channel is increased by the physiological abduction of the femur bone (incline to the medial), equal 8° relative to the vertical body axis.  
         [0030]     Placement of the endoprothesis  100  into the bone  35  is realized by severing its neck along a plane extending substantially perpendicular to the medial plane MP. Of course, small deviations from the perfectly perpendicular plane ranging within a few degrees would not drastically reduce the effectiveness of the prosthesis. To completely remove the damaged hip joint, the cup of the bone is removed from the bone&#39;s head. As a result, upon milling the channel  36  for the stem  13  ( FIGS. 1, 2 ), the housing  14  and stem  13  are introduced into the channel  36 , while the neck portion  12  of the prosthesis  100  terminates in the bone&#39;s head  11  in accordance with the normal course of the standard operation disclosed in detail below.  
         [0031]     Rigidly coupling the cup  7  to the bone  35 &#39;s head while coupling the housing  14  to the bone  35  completes the replacement procedure. Coupling the housing  14  to the bone  35  is realized by the inventive configuration of the housing  14  provided with a plurality of holes  15 ,  151  ( FIGS. 2, 2   a,    2   c ) allowing the bone  35  tissue to ingrow and absorb within the housing  14 . The number of these holes and their shape is arbitrary, but, based on experimental data and individual anatomy of the recipient, may be limited to 16-18 holes, preferable elliptical ( 151 ) each having dimensions of about 2-3 to 4-6 mm or round (15) each having a diameter of about 34 mm, patterned spirally or in staggered arrangement around the circumference of the housing  14 . Preferably, the housing  14  has an annular periphery narrowing towards its distal end. The material the housing  14  is made preferably from Titanium or Titanium alloys. Advantageously, the housing  14  is manufactured from biologically friendly material including, but not limited to, various metals as stainless steel, Tantalum.  
         [0032]     A flange  21  of the housing  14  ( FIGS. 2 and 2   a ) extending corolla-like on the whole length of the substantially horizontal section  41  of the bone  35  (the bone is displayed as a dotted line on  FIG. 2 ) from one side of the greater trochanter to the other. A rim  25  with blades  25   a  formed on and extending transversely to the flange  21  along the lateral side of the bone  35  and receives screws  40  ( FIGS. 2, 2A ) penetrating the adjoining portion of the bone  35 . The number of screws  40  is not limited and may be increased by inserting multiple screws at the appropriate locations selected by the operating surgeon. In time, the housing  14  and the bone  35  are reliably coupled to synchronously pivot about an axis  38  (this axis extends substantially along the central portion of the stem  13 ) extending parallel to the median plane  39  ( FIG. 4 ) Ability of the housing  14  to withstand linear loads generated by the recipient of the prosthesis  100  is conditioned by the reliable bond  14  is made preferably from Titanium or Titanium alloys. Advantageously, the housing  14  is manufactured from biologically friendly material including, but not limited to, various metals as stainless steel, Tantalum, and preferably has an optimal wall thickness of up to about 0.3-0.5 mm.  
         [0033]     The diameter of the stem  13  is dimensioned somewhat smaller than the inner surface of the housing  14  to prevent direct contact between these components of the prosthesis  100 . When the cup  7  is rigidly coupled to the bone  35 &#39;s head, displacement of the stem  13  does not directly affect displacement of the housing  14  pivotal along with the bone  35 . Accordingly, these components pivot independently from one another in a rather natural manner.  
         [0034]     To reinforce the stem/housing combination, the prosthesis  100  further includes a sleeve  16  ( FIGS. 2, 2   a ) coextending with the housing  14  and wrapping the outer periphery of the stem  13 . Mutual displacement of the stem  13  and the housing  14  may cause undesirable displacement of the distal end  37  ( FIG. 2 ) of the stem  13 . Dimensioning the housing  14  and the sleeve  16  so that both of these elements project beyond the distal end of the stem  13  at about 15-20 mm allows these components to minimize the stem&#39;s pressure on the bone  35  thereby protecting this bone  35  and precluding the formation of granulations  6   a  ( FIG. 1 ).  
         [0035]     The proximal end of the sleeve  16  is provided with a collar  17  abutting a lower portion of the stem  13  near a heel  22  to ensure proper positioning of the sleeve within the housing  14 .  
         [0036]     The collar  17  made from Teflon™ (polytetrafluoroethylene) and the sleeve  16  is made of Teflon fabric, for example from medical felt from ftoroplast  4 ″ (produced with this name in Russia) and having porous structure. The sleeve  16  is not affected by the ingrown tissue and immune against a tissue fluid that usually is excretion in the body. Most importantly, the sleeve  16 , having a thickness of about 1-1.5 mm, has a low friction coefficient and does not inhibit displacement of the housing  14 .  
         [0037]     The neck portion  12  of the prosthesis  100  ( FIG. 2 ) terminates in the cup  7 , whereas its lower portion  20  supports the stem  13  and has heel  22  extending substantially along the entire length of the housing&#39;s flange  21 . The sizes and the area of the housing&#39;s flange  21  exceed the sizes of the heel  22  ( FIGS. 2B, 3  and  4 ), and a configuration of support flange  21  for the prosthesis  100  for the left and right leg do not coincide. It is due to that the bone  35  and the housing  14  connected to it tend a turn in external concerning the plane  39 . Therefore the border of the support flange  21  from internal side to the plane  39  can be less then the diameter of the housing  14  ( FIGS. 2B and 4 ).  
         [0038]     As a consequence, unless dealt with, the contact region between the flange  21  and the heel  22  of the lower portion  20  have the opposing surfaces of the flange  21  and heel  22  extended substantially perpendicular to the longitudinal body of the housing  14  may inhibit the independent rotation of the stem  13  and the housing  14  relative to one another. To minimize this unavoidable friction, at least one of the juxtaposed surfaces of these components has spaced recesses  23  ( FIG. 2 ) each receiving a respective insert  24  The recesses  23  each are dimensioned to have a diameter substantially equal to the diameter of the heel  22  of the lower portion  20  and a depth of about  5 - 8  mm and a length of about  5 - 7  mm corresponding to the dimension of the insert  24 . The latter is made from low friction material such as Teflon or Kynar™ (Fluorine containing Synthetic Resin) and fixed to the heel  22  by any of conventional methods, for example by adhesive or by screws (not shown in the drawings) and has a height up to 1 mm more then depth of the recesses  23 . While the opposing surface, for example the surface of the flange  21 , as shown in  FIGS. 2, 2   b,    2   c  can also be recessed to receive similar or other implants, it is preferred that this surface would be simply polished or covered by material with a low friction coefficient such as polycrystalline diamond as describe in U.S. Pat. No. 6,425,922 which is incorporated herein by reference. Overall, the place between the heel  22  with the insert  24  and the surface of the flange  21 , said surface are polished or covered by material with a low friction coefficient, is the place of support of the axis  2  of the levers as the lever of the first kind with maximum loading on the given area of the bone  35  this support area reduced the possibility of granulations  6  and due to a good sliding action between the housing  14  and the sleeve  16 , between the heel  22  with the insert  24  and the polished or covered by material with a low friction coefficient surface of the flange  21 , the stem  13  of the endoprosthesis  100  can rotate slightly during walking without causing rotational stress to the femur diaphysis, thereby protecting this bone  35  and precluding the formation of granulations  6 . It enables to apply the endoprosthesis according the present invention at the situation requires the second hip replacement when the bone  35  is weakened or damaged by use of conventional endoprosthesis during the first hip replacement.  
         [0039]     The endoprosthesis cup  7  ( FIGS. 2 and 6 ) has a supporting crown  8  lying on the whole cortical arc of the acetabulum and on the pubic bone  35  with its lower part. The supporting crown  8  has a plurality of holes  9 , for example 6-8 holes each dimensioned to have a diameter of about 3-4 mm for tissue fluid filtration, and 9-12 blades  10  with holes dimensioned to be traversed by medical screws. Thus, prosthesis cup  7  is precluded from protrusion through the acetabulum and from displacing during body movements.  
         [0040]     The stem  13  and the neck portion  12  define a one-piece component made preferably from Titanium or Titanium alloys. Advantageously, the housing  14  is manufactured from biologically friendly material including, but not limited to, various metals as stainless steel, Tantalum. The endoprosthesis stem  13  is subjected to the lesser load from the medial then from the lateral side. This created an opportunity of making a trough  18  ( FIGS. 2 and 5 ) with the depth of no more then ⅓ of the stem&#39;s diameter, and with the outside circumference length of about ⅓ of the stem (120°). The trough starts at about 10 mm below the joining of the stem  13  with the neck  12  of the endoprosthesis, and ends at about 10 mm above the stem&#39;s distal end  37 . The bottom  18  of the trough  13  is preferably rectangular in its transverse section and filled with bone  35 -replacing mass  19  of bioactive material including, but not limited to calcium phosphates—calcium phosphate ceramics (hydroxyapatite). This and similar materials are suffused with one of numerous modern antibiotics to create an antibacterial environment throughout the diaphysis channel receiving the stem  13  and the housing  14 . Bone  35  ceramics is placed in the area of the smallest load on the stem, wrapped by the porous sleeve  16  and is not subjected to mechanical stress.  
         [0041]     The surgery directed to the installlation of the endoprosthesis  100  is performed regularly. After accessing the hip joint, transverse osteotomy of the neck  12  and the head  11 , of the femur is performed, moving to the base of the greater trochanter, but not including the great trochanter itself. The acetabulum and the femoral channel are milled, the prosthesis cup is inserted into the acetabulum, pressing the crown  8  of the cup tightly and fixing it with surgical screws through the blades  10  to fix the cup  7  tightly and support it by the outside edges of the acetabulum. The metal housing  14  is inserted into the femoral channel. The stem  13 , wrapped into the teflon sleeve  16  is inserted into the casing  14  in such a way that the anchor support  20  of the neck  12  rests on the polished surface  25  of the casing rim  14 . Manipulating the patient&#39;s extremity, the head of the prosthesis is inserted into the cup  7 . This ends the technical part of the hip replacement procedure.  
         [0042]     Hemostasis is performed, the drainage is created, and muscles are reattached (depending on the technique), wound closure is performed. This ends total hip replacement procedure.  
         [0043]     Having described the preferred embodiments of the invention by referring to the accompanying drawings, it should be understood that present invention is not limited to this precise embodiment but various changes and modifications thereof could be made by one skilled in the art without departing from the spirit or scope of the invention as defined in the appended claims.