Abstract:
A femoral hip-joint prosthesis having a tapered stem for implantation by cement. The prosthesis has a collarless shoulder at a proximal end and a stem extending therefrom to a distal end in tapered fashion. A logarithmically extending curvature of anterior and posterior side faces of a first subsidiary section of the stem combine with straight converging lines of a second subsidiary section of the stem to uniformly distribute pressure in the prosthesis within the femoral hip bone.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to a femoral hip-joint prosthesis having a tapered stem, for implantation by cement. 
     2. Description of Related Art 
     Laid-open patent application WO 91/18561 A1 discloses a femoral hip-joint prosthesis which is designed as a wedge-shaped intramedullary stem with a collarless shoulder. In order to secure a stem of this kind in the canal of a femur, a type of bone cement is used which normally comprises a mixture of polymethyl methacrylate (hereinafter PMMA) polymer and methyl methacrylate monomer, and optionally contains a styrene copolymer of PMMA. These and other types of cement used for the purpose of securing the stem in the canal of the femoral bone are subject to a phenomenon known as creeping. Although the bone cement in the hardened state appears to be rigid, it is subject to tiny movements over the course of time, and these lead to disturbance of the microscopic denticulations constituting the cement/implant interface and the cement/bone interface. This effect can cause the stem to loosen with time. The known stem is designed in such a way that there is slight adhesion between the bone cement and the surface of the stem. If creeping of bone cement occurs, then the wedge-shaped stem easily sinks in the bone cement and automatically settles again within the bone cement. In order to prevent loosening of the stem, it is crucially important that as far as possible the creeping of the cement and sinking of the stem do not disturb the microscopic denticulations of the cement/bone interface. 
     A disadvantage of the known stem is the fact that because of the sinking of the stem in the femur, the pressure force exerted on the bone cement increases, particularly at the cement/implant interface, but also at the cement/bone interface, and in this case excess creeping or fissuring of the bone cement occurs in particular at those points where there is a great increase in pressure force. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to develop a femoral hip-joint prosthesis for implantation by cement in such a way that the bone cement is exposed to less stress, and in particular in such a way that there is minimal disturbance of the microscopic denticulations at the cement/bone interface. 
     This object is achieved with a femoral hip-joint prosthesis having a tapered stem for implantation by cement as described herein. Further advantageous embodiments of the invention are also described herein using the tapered stem femoral hip-prosthesis for implantation by cement having a logarithmically extending curvature in a first subsidiary section and a converging straight line in a second subsidiary section. 
     The invention is achieved in particular with a femoral hip-joint prosthesis for implantation by cement, having a collarless shoulder at the proximal end and having a stem which extends in a straight line from the shoulder to the distal end and tapers from the proximal end toward the distal end, the stem having a quadrilateral cross section and an anterior side face and a posterior side face, and the two side faces, in the direction of extension of the stem, having, in a first subsidiary section beginning at the shoulder, a logarithmically extending curvature, and the two side faces thereafter, in a second subsidiary section, extending in a straight line and converging. 
     The femoral hip-joint prosthesis according to the invention, hereinafter also referred to as a femoral stem, has an anterior side face and a posterior side face which, in the longitudinal direction, in a first subsidiary section beginning at the shoulder, each have a logarithmically extending curvature so that the cross section of the stem tapers toward the distal end. The anterior side face and the posterior side face each have, following the first subsidiary section, a second subsidiary section, which second subsidiary sections extend in a straight. line and converge toward the distal end. 
     The femoral stem according to the invention has the advantage that its configuration in the longitudinal direction corresponds approximately to the anatomical course of the medullary canal of the femur which has a trumpet-shaped structure in a first, upper subsidiary section, whereas the medullary canal, in a second, lower subsidiary section, has side walls which extend in straight lines, slightly narrowing in the distal direction. In a preferred embodiment, the femoral stem has a configuration which is adapted to the anatomical course of the medullary canal in such a way that the bone cement coming to lie between the stem and the femur has an approximately constant layer thickness. On account of the logarithmically extending curvature, the femoral stem according to the invention additionally has such a configuration that upon sinking of the femoral stem, the cement, particularly in the first, upper subsidiary section, experiences as uniform as possible a compression, which in the first subsidiary section effects an approximately uniform increase in force oriented perpendicular to the direction of extension of the femoral stem, so that stress peaks or fissures in the bone cement are avoided. 
     The advantage of this logarithmically extending configuration can of course also be exploited, in the first subsidiary section, by means of correspondingly logarithmically curved lateral and medial sides of the femoral hip-joint prosthesis. 
     A further advantage of the design of the femoral hip-joint prosthesis according to the invention lies in the fact that by avoiding stress peaks in the bone cement, the cement/bone interface has a fairly uniform distributed pressure load. It is known that in the case of a cement/bone interface with a non-uniform pressure load, regression of the bone can occur at areas where the pressure loads are high, which could result in additional loosening of the femoral stem. 
     The femoral stem according to the invention has a quadrilateral cross section with edges, the cross section being in particular square, rectangular or trapezoidal. It can prove advantageous to round off these edges in order to reduce the notch effect which these edges exert on the bone cement upon sinking of the femoral stem. In addition to the notch effect, the stress peaks acting on the bone cement are of course also reduced. In an advantageous design, the edges have a radius of between 1 and 3 mm. The hip-joint prosthesis according to the invention has the further advantage that because of the quadrilateral cross section of the stem, torsional forces or shearing forces acting on the femoral stem can advantageously be transmitted to the bone cement. The quadrilateral design of the cross section of the stem and the rounding-off of the edges permit optimum functioning with respect to the two opposing requirements below: 
     A femoral stem with many edges leads to high stress peaks as it sinks, or even to stress fractures in the cement, so that a round cross section would be best. 
     On the other hand, a round cross section would generate a high shear stress in the bone cement in the event of torsional forces acting on the femoral stem, so that a cross section with corners would be more advantageous. 
     In a preferred embodiment, the femoral stem has a highly polished surface, as a result of which the adhesion of the cement at the cement/implant interface is reduced or avoided. The action of the shearing forces which occur along the stem as the femoral stem sinks, or also the shearing forces which occur in the cement upon torsion or bending of the stem, are additionally greatly reduced in this way. 
     An illustrative embodiment of the femoral hip-joint prosthesis according to the invention is explained below with reference to FIGS.  1  through  3 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a femoral hip-joint prosthesis from the medial direction, with the femur indicated; 
     FIGS. 2 a ,  2   b  show cross sections of the stem along the line A—A and the line B—B, respectively, according to FIGS. 1 and 3, respectively, with the medullary canal and compact substance indicated; 
     FIG. 3 shows the femoral hip-joint prosthesis from the anterior direction, with the femur indicated; 
     FIG. 4 shows purely schematically the extended curves y 1 (x) and y 2 (x) describing the surfaces  28  and  28 ′, respectively; 
     FIG. 5 shows purely schematically the extended curves y 3 (x) and y 4 (x) describing the surfaces  36  and  38 , respectively; 
     FIGS. 6 a-d  show a prosthesis with three subsections,  26 ,  32  and  47 , wherein the third subsection has a circular cross section; and 
     FIGS. 7 a ,  7   b  show a prosthesis with parallel faces near the distal end. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a view, from the medial direction, of a femoral hip-joint prosthesis  10  which is embedded in a femur  12  represented diagrammatically in cross section. The femoral hip-joint prosthesis  10  comprises a shoulder  34  which on one side merges seamlessly into a stem  18  and on the other side has a neck  16  which opens into a conical peg  14  on which a joint head (not shown) can be secured. The cortical substance  20 , which merges into the compact substance  21 , and the spongy substance  22  and medullary canal  24  are visible on the cross-sectioned femur  12 . The femoral hip-joint prosthesis  10  is held in the femur  12  by means of a bone cement which fills the empty space represented in the medullary canal  24 , although for the sake of improved clarity the bone cement has not been represented. In its longitudinal direction, the stem  18  has, in a first subsidiary section  26  beginning at the shoulder  34 , an anterior side face and a posterior side face  28 ,  28 ′, respectively, which have. a curvature running logarithmically in the longitudinal direction. To clearly illustrate the curve of the side faces  28 ,  28 ′, a dot-and-dash vertical line  30  has been drawn alongside the stem  18 , beginning at the shoulder  34 . In a second subsidiary section  32  adjoining the first subsidiary section  26 , the anterior and posterior side faces  28 ,  28 ′ run in a straight line, the two side faces  28 ,  28 ′ converging toward the distal end. 
     FIG. 4 shows for clarity and purely schematically, the extended curves y 1 (x) and y 2 (x) describing the surfaces  28  and  28 ′ respectively, in which the curvature is admittedly low. 
     FIG. 3 shows the same femoral hip-joint prosthesis  10  according to FIG. 1 from the anterior direction. The shoulder  34  with neck  16  and peg  14  located at the proximal end of the stem  18  can clearly be seen from this perspective. In the illustrative embodiment shown, the medial side face  36  and the lateral side face  38  have, like the anterior and posterior side faces  28 ,  28 ′, a first subsidiary section  26 , arranged in the proximal area, with side faces  36 ,  38  extending in a logarithmic curve in the longitudinal direction, merging thereafter into a second subsidiary section  32  which has faces extending in a straight line. The logarithmic curvature of the medial and lateral side faces  36  and  38  begins adjoining the shoulder  34 . 
     This configuration, according to the invention, of the side faces  28 ,  28 ′ and, if appropriate, also of the side faces  36 ,  38 , is on the one hand adapted to the natural trumpet-shaped structure in the first subsidiary section  26  and, in the second subsidiary section, to the straight-line configuration of the medullary canal  24  of the femur  12 . The stem  18  is preferably fitted in the medullary canal  24  in such a way that a layer of cement with preferably an approximately constant thickness of 2 mm, for example, is obtained between the stem  18  and the femur. The function and effect of the logarithmic curvature of the side faces  28 ,  28 ′ and, if appropriate, of the side faces  36 ,  38  in the first subsidiary section  26  are fully explained by the mathematical explanations which follow. As has already been mentioned, the stem according to the invention is designed in such a way that after it has been implanted, further sinking into the cement-filled medullary canal is possible. As is shown in FIG. 3, the logarithmic curvature of the side faces  28 ,  28 ′,  36 ,  38  of the stem  18  means that the change in the space which the stem  18  additionally takes up as it sinks into the cement-filled medullary canal  24  remains constant or approximately constant in the direction of extension of the stem, so that the pressure also exerted approximately perpendicular to the cement/bone interface of the femur  12  is uniformly distributed in the longitudinal direction, and, as a result of this, no area with an excessively high pressure value occurs at the cement/bone interface. 
     For clarity, and purely schematically, FIG. 5 shows the extended curves y 3 (x) and y 4 (x) describing the surfaces  36  and  38 , respectively, which are determined, consistent with that shown in FIG. 3, according to: 
     
       
           y ( x )= K ln|x|   Equation 1 
       
     
     where K is a constant, y(x) describes the curvature of the lateral surface, y denotes the direction along the longitudinal axis (or a direction parallel thereto) and x a direction perpendicular thereto. Of course, in the extended curves shown, not the whole logarithmic curve of x→∞ is shown. Rather, only a segment of each curve is shown to describe the corresponding surfaces  28 ,  28 ′,  36  and  38  as appropriate. 
     The effect of a stem  18  sinking easily into the medullary canal  24  is explained below with reference to the example of the side face  36 , although these statements do of course also apply to the side faces  28 ,  28 ′ and  38 , if they are designed with a logarithmically extending curve. The distance taken up by the stem  18  in the original position, on a horizontal I from a center line  42  of the stem  18  up to, for example, the medial side face  36 , is given by x 1 . The distance taken up by the stem  18  in the original position, on a horizontal II, is given by x 2 . If the stem  18  now sinks by a constant displacement path K 1  in the femur  12 , then the distances taken up by the stem  18  on the horizontals I and II change by dx 1  and dx 2 , respectively. A concept, according to the invention, of the stem shown is that on each horizontal i the relationship of the change in the distances dx 1  to the original distance x 1  taken up by the stem  18  does not change, so that the relationship of these two distances to one another is constant, or equal to a constant K 2 :                ε        (     x   1     )       =         dx   1       x   1       =     K   2               Equation                 2                                
     The sinking of the stem  18  thus effects, at each horizontal i, a constant expansion ε which effects, perpendicular to the direction of extension of the stem, a constant expansion of the bone cement and thereby a constant increase in the pressure force in the bone cement. 
     Thus, on each horizontal i, the distance dx 1  additionally taken up after the sinking of the stem  18  is given as: 
     
       
           dx   i   =x   i   ·K   2   Equation 3 
       
     
     The gradient y′ of the equation y=f(x 1 ) describing the curvature of the side face  36  at the point of the horizontal i is defined by the gradient of the tangent at this point, which is calculated as follows from the quotient of the constant displacement path K 1  and the distance dx i  additionally taken up:                  y   ′          (     x   i     )       =       tan        (     dx   i     )       =       K   1       dx   i                 Equation                 4                                
     with equation 2 gives:                  y   ′          (     x   i     )       =         K   1       K   2       ·     1     x   i                 Equation                 5                                
     The following general curve equation is obtained for the course of the side faces  28 ,  28 ′  36 ,  38 , where the y direction runs parallel to the center line  42 : 
     
       
           y ( x )= K ln|x|   Equation 6 
       
     
     A side face  28 ,  28 ′,  36 ,  38  extending as a function of the logarithm thus has the property that the space additionally taken up through the sinking of the stem  18  in the cement-filled medullary canal  24  effects a constant expansion  6  of the bone cement, which results, in the direction of extension of the stem  18 , in an approximately uniform increase in the pressure transmitted from the stem  18  to the bone via the cement, or a uniform pressure distribution. 
     FIG. 2 a  shows a cross section of the stem  18  along the section line A—A, which also forms the transition between the first and second subsidiary sections  26 ,  32 . The stem  18  is embedded in the cement-filled medullary canal  24 , the cement at the cement/bone interface being surrounded by the compact substance  21 . At the section line A—A shown, the stem  18  has a rectangular cross section  40 . Toward the distal end of the stem  18 , the cross section  40  of the stem  18  changes in such a way that, as can be seen from FIG. 2b in a section along the section line B—B, the cross section  40  has a square shape. The stem  18  is again embedded in the cement-filled medullary canal  24  and surrounded by the compact substance  21 . Both illustrated cross sections of the stem  18  have edges with rounded corners  44 . Along its entire length shown in FIGS. 1 and 3, the stem  18  has, for example, rounded edges  44 , of which the radius is preferably between 1 and 3 mm. 
     The cross section of the stem  18  could also be designed in another quadrilateral shape, for example as a trapezoid or a parallelogram. Either this same cross-sectional shape extends over the entire length of the stem  18 , or the different cross-sectional shapes merge one into another. A stem  18  with a cross section of such a rectangular design also preferably has rounded edges  44 , the edges  44  of the stem  18  and the comers  44  of the cross section  40  preferably having a radius of between 1 and 3 mm. At the distal end  46  (FIGS. 1,  3  and  6   a-d ), the stem  18  can also have a circular cross section, which is essentially a third subsidiary section of the stem  18 , since at this point there is usually no transmission of torsional and shearing forces. 
     Starting from the distal end  46 , the side faces  28 ,  28 ′ and  36 ,  38  of the stem  18  extend parallel to one another along a length of up to 5 mm as seen, for example, in FIGS. 7 a  and  7   b . This end section, which is essentially a third subsidiary section of the stem  18 , can be used for receiving a centering sleeve which allows the distal end  46  to be centered in the medullary canal  24 . 
     In the illustrative embodiment shown, the point of transition from the logarithmically curved side faces  28 ,  28 ′ and  36 ,  38  of the first subsidiary section  26  to the rectilinearly converging side faces  28 ,  28 ′ and  36 ,  38  of the second subsidiary section  32  is arranged at the level of the section line A—A. This point of transition can also be located, for example, in relation to the longitudinal direction, in the central area of the stem  18 , but preferably approximately in the lower third of the stem  18 , so that the second subsidiary section  32 , starting from the distal end  46 , extends along half, preferably a third, of the stem  18 . 
     The femoral stem  18  is made, for example, of a biocompatible metal such as titanium, or a titanium alloy or a CoCrMo alloy having highly polished surfaces.