Abstract:
A femoral prosthetic component of a replacement hip joint comprises a stem for fixing in a medullary cavity. A separate proximal element is provided with a neck for receiving a modular ball head or having an integral ball head. A dove tail-shaped system is provided for securing the proximal component to the stem which includes a sliding tongue and groove joint. The tongue being provided on the stem by an undercut rail which extends in a medial-lateral direction and the groove being provided in the separate proximal element. The tongue and groove being tapered along their lengths.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to a femoral prosthetic component for a replacement hip joint which comprises a stem for fixing in a medullary canal, separate proximal element provided with a neck to receive a ball head or having a ball head and connections, such as tapered connectors, for securing the proximal component to the stem which will enable a modular construction so that stems and heads of different sizes and/or shapes can be mated together.  
         [0002]     French Patents Nos. 2,626,168 and 2,721,200 both show femoral prosthetic components which have separate stems and proximal elements which can be fastened together but both constructions show the use of a tapered plug provided on the separate proximal element and which is located in a socket in the stem. Such constructions are expensive to produce and it can be difficult to remove the plug from the stem once it has been placed in position.  
         [0003]     U.S. Pat. No. 5,336,268 shows an adjustable hip joint endoprosthesis which has an adjustable prosthesis head which includes a link element which can slide in an undercut groove on the stem. The position of the link element can be adjusted in a medial/lateral direction and is locked in position by a threaded tension pin.  
         [0004]     U.S. Pat. No. 5,800,560 also shows an adjustable hip joint prosthesis which is multi-dimensionally adjusted on the neck portion and which includes a dovetail shaped groove in a plate which can be secured to the stem and on which is a second plate which is movable in relation to the first and which also carries a second dovetail shaped groove which is normal to the direction of the first and which carries a tapered cone to receive a ball head. Thus the construction allows adjustment in two planes and can be firmly fastened in any position by means, for example, of screws.  
         [0005]     Neither of the above US Patent specifications shows a construction in which the undercut grooves are tapered so that the stem and separate proximal element are accurately located in a set position by the use of tapers.  
         [0006]     The present invention is intended to provide an improved construction which is easier for the surgeon to operate.  
       SUMMARY OF THE INVENTION  
       [0007]     According to the present invention a femoral prosthetic component of a replacement hip joint comprises a stem for fixing in a medullary cavity, a separate proximal element provided with a neck to receive a ball head or having a ball head and means for securing the proximal component to the stem which include a sliding tongue and groove joint, the tongue being provided on the stem by an undercut rail which extends in a medial-lateral direction the groove being provided in the separate proximal element, and the tongue and groove being tapered along their lengths.  
         [0008]     Thus, with this construction it is easy for the surgeon to slide the proximal element into place, and their tapered lengths allow the parts to wedge together, and it is also relatively easy to remove the proximal element if an alternative element is required by releasing the wedging action of the tapers.  
         [0009]     Preferably the widths and/or depth of the tongue and groove are tapered along their lengths.  
         [0010]     In one preferred construction the opposed side walls of the tongue can be inclined to each other provide a dovetail shape.  
         [0011]     In another convenient construction the tongue and groove have co-operating cross-sections which are substantially T-shaped.  
         [0012]     The cross arms of the T-shapes can have downwardly projecting extensions to engage co-operating troughs in the groove.  
         [0013]     Elements for locking the stem and proximal element together are provided, for example, by means of a set screw.  
         [0014]     The tongue can be provided as first and second lengths with a gap between them and the groove can also have first and second lengths with a gap between them to allow the grooves to be located on the tongue at an intermediate position in the length thereof and then moved lengthwise to the final located position.  
         [0015]     The stem and/or proximal element can be made from metal, a synthetic material or a ceramic material.  
         [0016]     The invention also includes a kit of parts to provide a modular construction of the femoral prosthetic component as set forth above and which includes two or more alternative stems and/or two or more alternative proximal elements which are of different sizes and shapes and which have appropriate sliding tongue and groove joints which can be assembled together to form said prosthetic component.  
         [0017]     As used herein when referring to bones or other parts of the body, the term “proximal” means close to the heart and the term “distal” means more distant from the heart. The term “inferior” means toward the feet and the term “superior” means toward the head. The term “anterior” means toward the front part or the face and the term “posterior” means toward the back of the body. The term “medial” means toward the midline of the body and the term “lateral” means away from the midline of the body. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The invention can be performed in various ways and some embodiments will not be described by way of example and with reference to the accompanying drawings in which:  
         [0019]      FIG. 1  is a side elevation of a femoral prosthetic component according to the present invention;  
         [0020]      FIG. 2  is an isometric view of the separate proximal element and part of the stem of the kind shown in  FIG. 1  and ready for assembly together;  
         [0021]      FIG. 3  is a plan view of the upper end of the stem shown in  FIGS. 1 and 2 ;  
         [0022]      FIG. 4  is an isometric view showing the use of means for locking the parts together;  
         [0023]      FIG. 5  is a part cross-sectional elevation on lines V-V on  FIG. 4 ;  
         [0024]      FIG. 6  is an exploded view showing an alternative construction;  
         [0025]      FIG. 7  is an isometric view of a stem utilizing a T-shaped tongue rail; and  
         [0026]      FIG. 8  is a part cross-sectional elevation of the stem as shown in  FIG. 6  secured to a separate proximal element. 
     
    
     DETAILED DESCRIPTION  
       [0027]     As shown in FIGS.  1  to  3  of the drawings a femoral prosthetic component of a replacement hip joint according to the present invention generally denoted as  100  comprises a distal stem  1  for fixing in a medullary cavity. The stem can be of any suitable size of shape and can be made from metal, for example a stainless steel or a composite synthetic material. In the preferred embodiment, a proximal end  1 ′ of the stem has a fin  2  on its lateral side to assist location.  
         [0028]     The proximal end of the stem  1 ′ is shaped to provide a flat platform  3  on which is located a wedge-shaped undercut rail  4 . The rail  4  tapers in width and depth from its medial to its lateral end and is undercut to that the cross-sectional width of his upper end surface  5  is greater than the cross-sectional width of its base  6  (along surface  3 ). The opposed side walls  7  of the rail are inclined to each other to provide a dovetail shape and rail  4  provides a tongue for a tongue and groove joint.  
         [0029]     The upper lateral corner of the proximal stem  1 ′ is chamfered as indicated by reference numeral  8 .  
         [0030]     A separate proximal element  10  is provided for use with the stem  100  comprises a neck portion  11  and a tapered trunion or spigot  12  to receive a ball head (not shown) in known manner. Ball heads of this type are provided with a tapered socket to mate with spigot  12 .  
         [0031]     In an alternative construction (not shown) the ball head could be integral with proximal element  10 . If a separate ball head is provided it could be made of metal, a synthetic material or ceramic material.  
         [0032]     In the preferred embodiment the proximal element  10  has a flat planer base  13  in which is provided a tapered groove  14 . The walls of the groove and the dimensions of the taper are arranged so that the groove is a sliding fit over the tongue provided by the rail  4  and the tapering sides of the walls  15  of the groove mate with side walls  7  of the rail to provide a dovetail tongue and groove joint. It will be seen that the lengthwise taper of the tongue is in a medial-lateral direction and the taper can be a Morse taper so that when the two parts are pushed into position they tend to wedge together, the inclined opposed side walls of the joint preventing any inclination to come part and again providing a taper locking connection. Typically the taper angle is between 1° and 2°.  
         [0033]     If it is desired to take the parts apart it is merely necessary to apply a sharp blow to one of the parts in the appropriate direction to release them from the tapers.  
         [0034]     In the construction described above the rail is tapered in both width and depth (walls  7 ) but, alternatively, it could only be tapered in one or the other.  
         [0035]      FIGS. 4 and 5  show a construction in which the same reference numerals are used to indicate similar parts to those shown in FIGS.  1  to  3 . In this arrangement a locking element is included for releasably locking the separate proximal element together and to enhance the locking effect of the tapers on the tongue and groove joint. This may be achieved by use of a set screw  16  which is screwed into a threaded bore  17  in the stem. The lower part of separate proximal element  10  is cut away to provide a recess  18  in which the head of the set screw is located. The set screw  16  therefore acts as a stop to prevent proximal element  10  sliding backwards (laterally) and detaching from the stem  100 . As seen in  FIG. 2  the medial end of undercut rail  4  is provided with a chamfer  19 . This chamfer is omitted in the construction shown in  FIGS. 4 and 5  and is replaced by threaded bore  17 .  
         [0036]      FIG. 6  shows the stem  1  and separate proximal element  10  and the same reference numbers are used to indicate similar parts as in FIGS.  1  to  3 . In this arrangement however undercut rail  4 , which acts as the tongue, is provided as a first medial-lateral length  20  and a second medial-lateral length  21  with a gap  23  between them. Similarly tapered groove  14  in separate proximal element  10  is also formed with a first medial-lateral length  24  and a second medial-lateral length  25  with a gap  26  between them. The dimensions of the gap  26  are arranged to be slightly larger than the length of the first length  20  of the tongue and gap  23  in the tongue is arranged to be slightly larger than the walls of the second length  25  of the groove so that proximal element  10  can be placed downwardly over stem  100  until flat planar base  13  of the proximal element engages the flat platform  3  of the stem with the proximal element located approximately halfway along rail  4 . The proximal element can now be moved in a medial-lateral direction to fully engage the rail in the groove and provide the tongue and groove connection.  
         [0037]     This construction enables the parts to be put together after the stem has been inserted in the bone and requires a short medial-lateral distance for engagement. Thus it will be appreciated that the medial-lateral dimension required is only half that of the construction shown in  FIGS. 1, 2 , and  3 .  
         [0038]      FIGS. 7 and 8  show another alternative construction and in which the same reference numerals are used to indicate similar parts to those shown in  FIGS. 1, 2  and  3 . In this construction however tapered dovetail shaped rail  4  of the construction shown in  FIGS. 1, 2  and  3  is replaced by a rail  30  which is of T-shaped cross-section. The cross arms  31  of the T have downwardly projecting extensions  32  with rounded extremities. The T-shaped rail is again tapered in depth and width along its medial-lateral length.  
         [0039]     Proximal element  10  is provided with a T-shaped co-operating groove  35  which is shaped and dimensioned to engage the tapered T-shaped rail  30  and lock in position in a similar manner to that described with regard to dovetail shaped rail  4  in the other constructions. The groove  35  is relieved at  36  to provide a small space between the top of the T of the rail and the bottom of the groove  35  and the lower corners are chamfered as will be seen from  FIG. 8 . With this arrangement flat planar base  13  of proximal element  10  is arranged to engage flat platform  3  on stem  100 . The rounded extremities of the downwardly projecting extensions  32  of cross arms  31  engage suitably shaped rounded troughs  33  in groove  35  so that the wedging effect acting on proximal element  10  is between troughs  33  and flat planar base  13 .  
         [0040]     As mentioned above groove  35  and rail  30  are tapered in a similar manner to the construction shown in the other  FIGS. 1-4  and the taper can again be a Morse taper so that when the two parts are pushed into position they tend to wedge together.  
         [0041]     If required the positive stop provided by the set screw  16  can also be provided in the construction shown in  FIGS. 7 and 8  and similarly T-shaped rail  31  and groove  35  can be provided with gaps in a similar manner to the construction shown in  FIG. 6  to reduce the dimensions required to achieve engagement.  
         [0042]     The invention provides a kit of parts which can include a number of stem elements and a number of proximal elements which could have different shapes and sizes and all of which have the sliding tongue and groove joint so that any stem can be connected to any proximal element to provide the shape required by the surgeon.  
         [0043]     The separate proximal element can be made from any suitable material, for example metal or a synthetic plastics material.  
         [0044]     Due to the modular arrangement proximal elements can be used which can be angled or shaped to fit the requirements of the patient and can be fitted to a suitable length of thickness of stem. This provides a very large variety of shapes and sizes.  
         [0045]     The made up component can be used for revision surgery or the initial fitting of a prosthesis.  
         [0046]     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.