Abstract:
A hip joint prosthesis including a stem assembly attached to the femur and having a generally spherical head and a cup assembly having an outer casing received within the hip socket. The cup assembly outer casing includes threadedly attached upper and lower portions to encapsulate the lining and the lining includes an upper portion having a recess receiving the head and a split lower portion seating the head. The cup assembly also includes an interior spring assembly providing shock absorption between the head and the casing. The stem assembly includes an elongate stem and a cooperating shield attached to the stem, the stem being independently movable following insertion of the stem and shield into the femur.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to artificial joints and particularly to an endoprosthesis for the hip joint. 
     Artificial joints 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. 
     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 members which they replace. 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 a degree of resilience or cushioning. 
     U.S. Pat. No. 4,770,661 discloses a replacement joint having a cup assembly which includes a generally hemispherical metal outer shell threaded at the rim and having an upper part received directly into the hip socket; an interfitting plastic core received within the shell and a split plastic locking ring. The core in turn receives the femoral head and the split plastic locking ring is installed by attaching the plastic parts under the head, cementing the parts together and then threadedly locking the plastic parts to the metal outer shell thereby encapsulating the ball. There is no metal casing encapsulating the head but rather a combined plastic and metal casing. Also lacking is any resilient feature between the head and the socket. U.S. Pat. No. 4,159,544 discloses an exterior spring system between the head and the socket. U.S. Pat. No. 3,648,294 discloses an arrangement which provides a rather complicated piston and cylinder construction within the head. U.S. Pat. No. 4,406,023 discloses the use of a lubricant filled bellows system for providing joint resilience. U.S. Pat. No. 4,199,824 discloses a femoral prosthesis having a flange and serrations to provide additional support and load transfer capability between the joint parts. All five of these prior patents are incorporated herein by reference. 
     The present hip joint replacement represents an improvement over prior art devices and overcomes problems and provides advantages in a manner not revealed in the known prior art. 
     SUMMARY OF THE INVENTION 
     This invention provides an artificial hip joint replacement which provides substantially universal joint movement between a stem assembly which is secured to the hollowed out passage of the femur and a cup assembly, which relatively receives the femoral head and is itself rotatively received within the prepared hip socket, said socket being fitted with a firmly attached socket shell. 
     The structural arrangement of parts provides for rotational movement of the cup assembly about the radial center of the socket and pivotal movement of the stem about the center of the femoral head and circular movement of the head about the axis of the femoral neck. 
     The hip joint prosthesis provides a stem assembly attached to the femur including a generally spherical head and a cup assembly having an outer casing received with the socket and an inner lining receiving the head. 
     It is an aspect of this invention to provide that the cup assembly outer casing includes an upper portion and a lower portion threadedly attached to the upper portion to encapsulate the lining, the lining including an upper portion received within the casing upper portion and having a recess receiving the head, said lining having a lower portion received within the casing lower portion and having a recess seating the head, said lining lower portion being split into at least two portions surrounding the head. 
     It is another aspect of this invention to provide that the cup assembly includes resilient means therewithin providing shock absorption between the head and the casing. 
     It is another aspect of this invention to provide that the stem assembly includes an elongated stem and a shield and means between the stem and the shield permitting independent movement of the stem relative to the shield following insertion of the stem and shield together in an elongate passage into the femur. 
     This invention provides a hip replacement which is relatively inexpensive, easy to manufacture and install and efficient in providing natural joint movement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view of the hip prosthesis as installed; 
     FIG. 2 is a sectional view of the stem assembly; 
     FIG. 3 is a plan view thereof; 
     FIG. 4 is a sectional view taken on Line  4 — 4  of FIG. 2; 
     FIG. 5 is an enlarged simplified detail showing the connection between the stem assembly body and shield; 
     FIG. 6 is an exploded view of the cap assembly; 
     FIG. 7 is a plan view of the split liner portions taken on Line  7 — 7  of FIG. 6; 
     FIG. 8 is an enlarged sectional view of the cup assembly parts as installed; 
     FIG. 9 is a similar view to FIG. 8 illustrating the resilient nature of the cup assembly; 
     FIG. 10 is a section view taken on Line  9 — 9  of FIG. 8; and 
     FIG. 11 is a sectional view taken on Line  10 — 10  of FIG.  8 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now by reference numerals to the drawings and first to FIG. 1, the hip joint prosthesis  1  includes a stem assembly  2  secured within a femur  4  and a cup assembly  6  rotatably connected to the associated hip socket  18 . The stem assembly  2  includes an elongated metal stem  8 , a neck  10 , and a generally spherical head  12 . The spherical head  12  is encapsulated within the cup assembly  6 , the cup assembly  6  rotating freely within a generally spherical metal socket shell  16  cemented within the hip socket  18 . 
     Installation of the hip joint prosthesis  1  is briefly as follows: the socket shell  16 , as shown in FIGS. 6-8, is first attached to the hip socket  18 , then the stem assembly  2 , as shown in FIGS. 2-5, is attached to the femur  4 , and finally the cup assembly  6  is installed, as shown in FIGS. 6-8, thereby completing the hip joint prosthesis. Provided below are a more detailed description of the structural arrangement of parts and the installation procedure for each of the socket shell  16 , the stem-assembly  2 , and the cup assembly  6 . 
     FIGS. 2-5 illustrate the attachment of the stem assembly  2  to the femur  4 . As shown, the elongated metal stem  8 , has an upper shoulder portion  28 , unitarily formed with the inwardly directed neck  10  and the head  12  and a tapered body portion  30  which terminates in a lower distal end  32 . The cross-section of the lower distal end  32  is relatively narrow, but the cross-section of the tapered body portion  30  gradually increases from the distal end  32  to the shoulder portion  28 . 
     The stem assembly  2  includes a generally L-shaped protective shield  40  having a generally vertical shield back portion  42  and a generally horizontal top portion  44 . The vertical inside face of the shield back portion  42  is formed into a plurality of tooth-like triangular steps  46  extending substantially the fill length of said shield. The vertical outside face of the back of the stem body  30  includes corresponding steps  48 , which are compatibly shaped to mesh with the shield steps  46 , as shown in FIG.  5 . The femur  4  includes a predrilled hollowed out passage  70  and a plurality of vertically arranged horizontal openings  60 ,  62  and  64  communicating with said passage  70 . The stem  8  and shield  40  are installed as a unit into said passage  70 , the shield only being coated with cement  66 . The stem  8  will be held in place by the steps  48  on the shield and steps  46  on the stem  8 , as shown in FIG.  5 . The shield top portion  44 , which is positioned adjacent an abutment  56  of the stem assembly shoulder portion  28 , can hammered into place. This allows the cement between the stem and the shield to enter the openings so that when it hardens it will secure the position of the shield in place as shown in FIG.  5 . This will allow the stem  8  to move downward when required without the shield moving. Finally, to secure the stem assembly  2  to the femur  4 , upper and lower nichrome hose clamps  80 , constituting adjustable straps, are attached around the outside of the femur  4  to apply pressure to the outside of said femur, thereby applying inwardly directed pressure to keep the femur  4  from shattering if the femur itself has thinning of the bone walls. 
     If deterioration of the femur  4  interior causes slippage of the stem body  30  relative to the shield  40 , for example by one step, or more, the stem body  30  and the shield  40  will remain tight in femur  4  and no surgery will be needed to reseat the stem as must be done with existing hip prostheses. 
     FIG. 6 illustrates the arrangement of parts the socket shell  16 , which receives the cup assembly  6 . As shown, the metal socket shell  16  is hemispherical having an exterior surface  20 , a interior surface  22  concentric with its exterior surface  20 , and an open circular end defining a rim  24 . Surrounding and unitarily formed with the rim  24  are a plurality of outwardly extending tabs  26 . During installation, the exterior surface of the socket shell  16  is coated with cement and hammered into the hip socket  18  with the socket shell&#39;s open circular end  24  at the bottom of the socket. The outwardly extending tabs  26  help to secure the socket shell  16  to the hip socket  18 . 
     FIGS. 6-11 illustrate the structural arrangement of parts of the cup assembly  6  with greater particularly. As shown, the cup assembly  6  includes upper and lower outer casing portions  90  and  92 . The upper casing portion  90  is generally hemispherical and includes a male threaded portion  94  at the rim. The lower casing portion  92 , which is generally pan-shaped, constitutes a locking ring, and includes an overlapping matching female threaded portion  96  at the rim, such that the two casing portions  90  and  92 , when connected, provide a generally spherical surface, which is received in rotatable relation within the socket shell  16 . The lower casing  92  includes an end opening  98 , large enough to receive the head  12 . 
     Within the casing formed by threadedly connected portions upper and lower casing portions  90  and  92 , respectively, are an upper liner portion  100 , having an insert block  102  slidably received within an opening  122  and a lower liner portion  104  which is split into two semi-annular portions  103  and  105 . 
     The upper liner portion  100  and the insert block  102  are spaced from each other at the upper end to form a gap between them occupied by an upper plate  106 , a lower plate  108  and a spring means sandwiched between them which, in the embodiment shown, is provided by a plurality of annular wave washers  110 . The upper liner portion  100  and the lower split liner portion  104  include interengagable conical surfaces  112  and  114  respectively. Also, the insert block  102  and the lower portion  104 , respectively, are formed to receive the spherical surface of the head  12  as indicated by numerals  116  and  118 . Also the insert block has a seating surface  120 . 
     The liner portions  100  and  102  are sized to permit the wave spring washers to move axially within the casing, whereby to be compressed to provide the resilience necessary to absorb shock loads. This arrangement is shown by reference to FIGS. 8 and 9 in which the resilient movement is indicated by distance “d”, the distance between the moveable insert seating surface  120  and the split liner conical surface  114 . 
     The cap assembly  6  is mounted to the head  12  by preassembling the upper liner portion  100 , the plates  106  and  108 , with the wave washer  110  therebetween, and the lower split liner portion  104 . The casing lower portion  92 , which provides a pan-shaped locking ring, can readily be slipped over head  12  and the two split liner portions  103  and  105  may be emplaced within the locking ring on opposite sides of the head  12 . The head  12  can then be fitted within the insert block  102  and the lower casing portion  92  locking zing threadedly connected to the upper casing portion  90  to encapsulate the head  12  within the casing. 
     Finally, to secure the stem-assembly  2  to the femur  4 , upper and lower femur adjustable straps  80 , preferably ⅜ inch wide Nichrome, are attached around the outside of the femur  4  to apply pressure to the outside  70  urging the outside of the femur  4  inwardly and hold the stem assembly  2  in place within the femur  4 . 
     The step arrangement  46 ,  48  between the stem body  30  and the shield back portion  42  has the advantage that when the stem  8  descends over time within the femur passage  70 , the stem body  30  will move downwardly and engage the next step on the stationary shield  40 . During this process, the clamps  80  maintain their pressure on the outside of the femur  4 . 
     With respect to the material from which the hip joint replacement parts are manufactured and formed, the following are preferred. The socket shell  16 , and the upper casing portion  90  and lower casing  92  are preferably formed from chrome steel. The inner lining portions, namely, the upper portion  100 , the intermediate portion  102  and the split lower portion  104  are preferably formed from hard plastic such as polyethylene. The bearing plates  106  and  108  for the wave washer  110  and the wave washers themselves are preferably formed from stainless steel. The stem assembly head  12  and body  30  are formed from titanium while the expansion shield is formed from stainless steel. Finally, the adjustable hose clamps  80  are formed from nichrome. 
     It will be understood that ongoing improvements in more exotic materials may suggest alternatives to those suggested above, for example, graphite may be used for in lieu of plastic for the casing lining. 
     Accordingly, although the hip replacement prosthesis has been described by making detailed reference to a preferred embodiment, the details of description are not to be understood as restrictive, numerous variants being possible within the scope of the claims hereunto appended.