Abstract:
A femoral component of a replacement hip joint is provided which has a stem for fixing in cement in a medullary cavity by cement and having a separate proximal component provided with a neck for a ball head. An attachment system is provided for securing the proximal component to the stem which allows selection of an infinite number of angularly displaced positions about a proximal/distal axis, and adapted to allow fixation of the two parts after insertion of the stem into the prepared medullary cavity and without causing torsional loads on the stem. The stem extends proximally to the level of the bone cut which allows the proximal component to rotate after the stem is implanted. If an infinite number of angularly displaced positions is not required, then a Vernier coupling system with a large member of positions can be used.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     This invention related to a femoral component of a replacement hip joint of the “Exeter” type which has a collarless stem including a shoulder for fixing in a medullary canal by cement. 
     2. Discussion of Prior Art 
     The “Exeter” type femoral component of the kind shown in British Patent No. 1 409 054 is well known and comprises a neck which carries a ball head for cooperation with an acetabular socket. The neck is connected to a tapered collarless stem. Thus, there is no collar for resting either on the bone or the cement in the area where the stem joins the neck of the implant. This type of stem has evolved so that the stem can be given a highly polished finish to help it slide down inside the bone cement and the present invention relates to this type of femoral component. 
     In certain circumstances, it can be difficult to locate the femoral component in the medullary socket with the neck and ball at the precise height and angle required. This can be caused by the bone which are structurally imperfect or by difficulties in reaming a suitable cavity. It is particularly pronounced when cavities have to be lined with bone fragments prior to cementing or when there are irregularities and the present invention is intended to overcome some of the difficulties experienced. 
     It is known from U.S. Pat. No. 5,931,871 to provide a femoral component with a separate proximal component but in this application, the angle of the proximal component with regard to the stem about a proximal/distal axis is fixed and there is no provision for adjustment. 
     U.S. Pat. No. 4,051,559 also shows the use of a separate proximal component and this is provided to allow it to be placed in position on a cylindrical stem which is intended to be screwed directly into the bone. The proximal component is provided with a collar which is intended to rest against the cut and prepared bone and there is no provision for a stem to slide down inside bone cement as is required by the Exeter type hip stem. Moreover, the angular adjustment about a proximal/distal axis may be too coarse to allow final accurate adjustment with an Exeter type component. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a femoral component of a replacement hip joint has a stem for fixing in cement in a medullary cavity by cement and having a separate proximal component provided with a neck for a ball head, and including an attachment system for securing the proximal component to the stem which allows selection of an infinite number of angularly displaced positions about a proximal/distal axis, and adapted to allow fixation of the two parts after insertion of the stem into the prepared medullary cavity and causing minimal torsional loads on the stem. The stem extends proximally to the level of the bone cut which allows the proximal component to rotate after the stem is implanted. The attachment system includes a vernier coupling. 
     Thus, with the component according to the present invention, it is possible to insert the stem into a prepared medullary cavity and subsequently add the proximal component at the precise angle about the proximal/distal axis and then complete fixation of the two parts without causing torsional loads on the stem which might cause the stem to move in the cement and create cavities or cause other damage. This also prevents the stem from being twisted about the proximal/distal axis which might alter the precisely prepared angle of the proximal component. 
     If an infinite number of positions is unnecessary, the use of a Vernier coupling in the attachment system enables a very large number of positions to be used and provides a simple and inexpensive method of manufacture. In one preferred embodiment, the boss and cavity are tapered and in another they are cylindrical. 
     Preferably, the attachment system include a male element on the stem adapted to cooperate with the female structure on the proximal component. Thus, the male element may comprise a boss on the proximal portion of the stem and the female structure can be provided by a cavity to receive and cooperate with the boss, and a release or locking element can be provided for preventing axial movement after initial fixing. 
     In one embodiment, the boss can be tapered to engage a tapered cavity on the proximal component and the release element may comprise an axially extending locking screw extending from the proximal component into the tapered boss. 
     Alternatively, the release or locking element may comprise a cotter pin adapted to locate on a boss extension, the pin extending tangentially to the extension. 
     In another construction, the release can be provided by a circumferential groove in the boss which can be engaged by a radially extending lock pin or screw in the proximal component. 
     According to another preferred construction, the boss can be cylindrical and with this arrangement, the release element can again be provided by a tangentially extending cotter pin located in the proximal component. If desired, the cotter pin can be located in a groove in the boss. 
     With these arrangements, the release element may comprise a radially extending pin or screw in the proximal component which can engage a peripheral groove in the boss. 
     In another alternative construction, the boss can be cylindrical or tapered and be provided with a peripheral groove and the release element can comprise a locking plate with means to move it radially to engage said groove. The corresponding cavity is also cylindrical or tapered to match the boss. 
     In yet another embodiment, the proximal member and stem can each be provided with a predetermined number of openings arranged at the same radius about an axis substantially parallel to the long axis of the prosthesis, one ring of openings having one more opening than the other, and a locking member adapted to enter one of each ring of holes to provide a Vernier coupling. Thus, one ring of holes can be provided in the male element and the other in the proximal component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be performed in many ways and some embodiments will now be described by way of example and with reference to the accompanying drawings in which: 
     FIG. 1 is a part cross-sectional diagrammatic side elevation of a femoral component using a collarless stem for fixing in cement in a medullary cavity according to the present invention; 
     FIG. 2 is a part cross-sectional part view of an alternative embodiment; 
     FIG. 3 is a side elevation of a cotter pin for use in the embodiment shown in FIG. 2; 
     FIG. 4 is a part cross-sectional side elevation of another alternative embodiment; 
     FIG. 5 is a part cross-sectional side elevation of another alternative embodiment using a cylindrical boss; 
     FIG. 6 is a side elevation of a cotter pin for use with the embodiment shown in FIG. 5; 
     FIG. 7 is a part cross-sectional side elevation of yet another alternative embodiment; 
     FIG. 8 is a side elevation of a cotter pin for use in the embodiment shown in FIG. 7; 
     FIG. 9 is another part cross-sectional side elevation of another alternative embodiment; 
     FIG. 10 is a part cross-sectional side elevation of yet a further alternative embodiment; 
     FIG. 11 is a plan view of a locking plate for use in the construction shown in FIG. 10; 
     FIG. 12 is a diagrammatic part cross-sectional side elevation of a femoral component according to the invention; 
     FIG. 13 is a partial plan view of the construction shown in FIG. 12; 
     FIG. 14 is a plan view on the line  14 — 14  of FIG. 12; 
     FIG. 15 is a view similar to FIG. 12 of an alternative embodiment; and 
     FIG. 16 is a view similar to FIG. 15 of yet another alternative embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in the drawing, the femoral component comprises two parts, a shoulderless stem  1  and a proximal component  2 . The proximal component has an engagement portion  3  and a neck  4  with a tapered spigot  5  to receive a ball head  6 . 
     The portion  3  is provided with a tapered cavity  7  which is adapted to cooperate with a tapered boss  8  provided on the proximal end of the stem  1 . The taper can be a morse taper so that when pressed together the two parts tend to lock in position. 
     In order to prevent the taper inadvertently coming apart during use an attachment system is provided which, in FIG. 1, comprise a screw  10  which extends through an opening  11  in the proximal component and passes into a threaded bore  12  in the tapered boss  8 . The screw  10  carries a lock nut  13  and has a hexagonal, or other appropriately shaped head  14 . In order to tighten and lock screw  10 , two especially shaped spanners  15  and  16  are provided. 
     In order to insert the femoral component according to the invention, the medullary cavity of the bone to which it is to be fitted is suitably prepared and lined with cement. The stem  1  is now inserted, perhaps using an appropriate inserter, but due to the construction of the present invention, its angular position about a proximal/distal axis is not vital and it can therefore be inserted so that it can take up the best and appropriate position in the cavity. Thus, it can be positioned so that it has the maximum amount of cement about it so that it is riot close to any of the cavity walls. 
     With the stem located in the cavity, the proximal component can be carefully fitted so that it projects angularly about the proximal/distal axis as is required. In order to hold the tapers in position it may be necessary to deliver a light blow in the proximal/distal axis direction to ensure that the tapers seat and grip. 
     In order to prevent inadvertent release of the tapers, the screw  10  is now inserted to a marked position where it is known that there is sufficient thread within the boss  8 . The locking screw  13  is now initially gently tightened down by hand and the spanner wrenches  15  and  16  are now employed to ensure that the screw  10  between the tapered boss  8  and the top of the portion  3  is pulled up tightly to again ensure that the tapers hold. In order to do this, the spanner  15  is used to hold the screw head  14  and prevent rotation and the spanner is employed to tighten the nut  13 . Due to the juxtaposition of the spanners this can be achieved without applying any torsion to the stem  1  and thus prevents inadvertent movement of the possibility of loosening the stem in the rapidly drying cement. 
     The spanners are, of course, removed after the femoral component and the stem have been locked together. The screw may be removed, as required, without altering the engagement of the tapers. 
     FIGS. 2 and 3 show an alternative construction again using a tapered boss and the same reference numerals are used to indicate similar parts as in FIG.  1 . In the construction, the tapered boss is indicated by reference numeral  20  and has a cylindrical extension  21 . The proximal component  2  is again provided with a tapered socket  22  which in this construction extends upwardly as a cylindrical opening  23 . 
     In this construction, the releasable means includes a cotter pin  24  as shown in FIG.  3 . This cotter pin has a flat side  25  and is shaped to pass through a hole  26  provided in the portion  3  and shaped so that the flat side of the tapered pin  24  engages the cylindrical extension  21 , the flat side being tangential to the extension. 
     The stem is inserted as described with regard to FIG.  1  and the proximal component again placed in position. In order to locate the proximal component in place, the cotter pin is inserted through the opening  26  and tightened by a nut (not shown) on a threaded portion  27 . This pulls the cotter pin through on its taper firmly engaging it with the extension  21 . Once again, it will be seen that no torsional twisting movement is applied to the stem  1  as the releasable locking means are placed in position. 
     In FIG. 4, the same reference numerals are again used to indicate similar part to those shown in FIG. 1 but in this construction an extended tapered boss  30  is used and the socket  32  extends through the portion  3 . The tapered boss  30  has a groove  33  which is lined with a bonded synthetic plastics material indicated by reference numeral  34 . The portion  3  has a threaded opening  35  to receive a set screw  36 . 
     With this embodiment, the parts are again assembled together in the manner described with regard to the previous figures and the releasable locking system is provided by tightening the screw  36  which extends substantially radially to the proximal/distal axis. 
     FIGS. 5 and 6 show another embodiment according to the invention in which the same reference numerals are again used as those employed in FIG. 1 to indicate similar parts but in this construction the tapered boss of FIGS. 1 and 4 is replaced by a cylindrical boss  40  which extends through an open socket  41  in the proximal component. A cotter pin  42  is again employed, this having a flat side  43  and a threaded end  44 . The portion  3  is provided with a suitably shaped opening  40 ′ to receive the pin which is inserted and locking in position in a similar manner to that described with regard to FIG.  2 . 
     FIGS. 7 and 8 also show a construction in which a cotter pin is used but in this arrangement the cylindrical boss  50  is provided with a peripheral groove  51  into which a tapered cotter pin  52  can extend. The cotter pin has a cylindrical tapered shank  53  and is provided with a threaded end  54 . The pin is passed through an opening  55  in the portion  3 , one side of the pin engaging the opening  55  and the other engaging in the groove  51 . Once again, it is locked in position by means of a screw on the thread  54 . 
     FIG. 9 shows another construction using a boss with a groove  51  similar to that shown in FIG. 7 but in this construction the cotter pin is replaced by a locking screw  57  which engages the groove by passing through a threaded opening  58  in the portion  3 . 
     FIGS. 10 and 11 show another construction in which a boss  50  is provided with a groove  51  but the screw  57  of FIG. 9 is replaced by a locking plate  60 . This plate  60  has a central opening  61  and is located in a slot  62  in the portion  3 . The plate is provided with a projecting screw  63  on which is located a tightening nut  64 . It will be seen from FIG. 1 that the inner edges of the opening  61  are bevelled to accurately locate in the groove  51 . 
     In order to operate this arrangement, the plate is located in position in the slot  62  before the proximal component is placed on the boss  50 , the angular position is carefully located and the boss is held in position by tightening up the nut  64  which pulls the plate into a position where it can lock against the groove  51 . 
     It will be appreciated that in all constructions described above, it is possible to lock the two parts together without creating a torsional twisting movement to the stem  1 . 
     As shown in FIGS. 12-14, the femoral component according to the invention comprises a Vernier coupling provided by a ring of openings  80  in a proximal component  72 . The openings  80  are arranged on a radius r about a proximal/distal axis A. 
     A second ring of openings  81  are provided on a stem  71  and are arranged on the same radius r about the same axis A as the openings  80  in the proximal component. These openings  81  extend into the wall of a boss  78  whilst the openings  80  are arranged to open through a proximal surface  82  of the proximal component surrounding the open end of a tapered cavity  77 . As will be seen from FIG. 12, the distal end of the openings  80  extend into the tapered cavity  72  at a position in line with the proximal ends of the openings  81  in the boss and the distal end  83  of each of the openings  81  is towards the distal end of the boss. 
     A locating pin  84  having a head  85  is provided which is a push or tight fit in the openings  81 . In an alternative embodiment (not shown) the openings  81  are threaded and the pin  84  is replaced by a screw which can engage the thread. 
     The Vernier effect is created in the construction shown in FIG. 12 by providing six openings  80  and seven openings  81 . This allows small increments of angular adjustment between the proximal component and the stem  71 . 
     If desired, more accuracy of adjustment can be arranged by providing more openings  80  and  81  but only six and seven respectively have been shown in the drawings so that they are clear. 
     If a pin  84  is used, then any pressure placed on the femoral component is in direction of the proximal/distal axis. If a screw is used, then any torsional twisting movement of the stem when the proximal component is fitted need only be very light. 
     In order to fit the femoral component, a medullary cavity is prepared in the usual way and the stem  1  is first inserted, being allowed to take up any angular position about a proximal/distal axis as is desired by the surgeon, thus enabling the maximum cement mantle to be achieved. The proximal component is now fitted and the correct angle assessed by the surgeon, the very small increments of angular movement allowing good accuracy. The pin  84  or screw can be lightly inserted and then the femoral component tapped or have pressure applied to ensure that the taper between the boss and the cavity locks. The pin or screw  84  is now driven fully home thus ensuring that the proximal component and the stem are firmly locked together. 
     An alternative embodiment is shown in FIG.  15  and the same reference numerals are used to indicate similar parts. In this arrangement however a cylindrical boss  90  is employed on the stem  71  and is a close sliding fit in a cylindrical cavity  91  in the proximal component  72 . The end of the cavity  91  is closed and the end wall is provided with a ring of openings  92  which are again around a fixed radius based on a proximal/distal axis A. A further ring of openings  93  are provided in the proximal end of the boss  90  which are again on the same radius and about the same axis A. Once again there is one more hole  93  than holes  92  and a Vernier coupling is provided by the insertion of a pin or screw  94  into an appropriate pair of mating holes at the desired angular position. 
     FIG. 16 shows another alternative construction again employing the same reference numerals to indicate similar parts. In this construction a cylindrical boss  90  is again shown but this carries a ring of blind openings  95  which are aligned in a plane B with a second ring of openings  96  provided in the proximal component  72 . There are six openings  95  and seven openings  96  and a pin or screw  97  is provided to achieve the Vernier coupling. This arrangement could also be employed with a tapered boss and cavity of the kind shown in FIG.  12 .