Abstract:
A joint prosthesis assembly includes a stem for disposition in a canal of a first bone constituting a first portion of the joint, a neck fixed to the stem, a collar fixed on the neck and having a peripheral groove, and an articulating head fixed on the neck. The assembly further includes a cup for disposition in a second bone constituting a second portion of the joint, an insert disposed in the cup to provide an interior lining for the cup, the insert being configured to receive the articulating head for movement therein, and the insert having a peripheral groove therein. A sleeve-shaped membrane is provided with a first O-ring fixed thereto at a first end thereof for disposition in the insert groove, and a second O-ring fixed thereto at a second end thereof for disposition in the collar groove. The membrane is thereby disposed to capture particulate debris generated by the articulating head in the insert. There is further contemplated a method for installing the assembly.

Description:
REFERENCE TO PENDING PRIOR PATENT APPLICATION  
       [0001]    This application claims the benefit of pending prior U.S. Provisional Patent Application Serial No. 60/127,197, filed Mar. 31, 1999 by Gregory W. Stocks for ENCAPSULATED HIP REPLACEMENT PROSTHESIS. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to the field of orthopedic surgery in general, and more specifically to a joint prosthesis assembly and method for installing same.  
         BACKGROUND OF THE INVENTION  
         [0003]    A natural hip joint is a ball and socket joint in which a hemispherical head moves freely within a concave socket.  
           [0004]    Artificial hip joints are an approximation of natural hip joints. More particularly, artificial hip joints consist of a metal (or metal alloy) head which articulates inside a metal (or metal alloy) cup. Often the metal socket is lined with very dense plastic, typically Ultra High Molecular Weight Polyethylene (UHMWPE), so as to reduce friction in the artificial bearing.  
           [0005]    In 1994, 165,000 total hip replacements (THR&#39;s) were performed in the United States. Due to the success of the THR operation, the improved quality of life it provides, and the increasing elderly population, this number is expected to increase in the future.  
           [0006]    The expected life of an artificial hip is less than 10 years in active adults and about 10-20+ years in less active adults. When the artificial hip wears out, it must be replaced with a new artificial hip. This procedure is typically referred to as a “revision”. In 1994, THR revision surgery accounted for approximately 20% of all THR surgery, and is expected to increase inasmuch as many patients are receiving THR&#39;s at an earlier age. According to The National Center For Health Statistics, the annual number of total hip revisions will increase to 157,000 by the year 2010 and 219,000 by the year 2030.  
           [0007]    THR revision surgery is typically more complex, time-consuming and complication-prone than standard THR surgery.  
           [0008]    Accordingly, it is desirable to find a way to extend the life of the artificial hip.  
           [0009]    According to the National Institutes of Health, “the principle cause of prosthesis failure appears to be the generation of particles, which, in turn, cause inflammation and bone resorption around the prosthesis (osteolysis)” (NIH Consensus Statement: Total Hip Replacement. 1994; 12:1-31). The problem of osteolysis in THR involves three aspects: (a) the creation of debris particles within the artificial joints; (b) the migration of the debris particles to the bone around the prosthesis; and (c) a cellular reaction to the debris particles.  
           [0010]    Extensive research has been published regarding the aforementioned first and third aspects of the problem.  
           [0011]    More particularly, some current work is aimed at reducing the production of wear particles at the bearing surface of the artificial joint by using new metal-on-metal bearing designs, or modifying existing metal-polyethylene hips; however, it is unlikely that these modifications will eliminate wear particles completely, some of which have been found in the liver, spleen, and lymph nodes, and have been implicated as a cause of cancer.  
           [0012]    Research aimed at reducing osteolysis by better understanding and controlling the biological cellular response, the third aspect described above, is widespread. Currently, a clinically useful approach has not been elucidated, although anti-inflammatory medications and anti-osteoclast medications show promise. Many investigators, however, have reservations about treating a localized problem with a systemic agent.  
           [0013]    The present invention addresses the problem of osteolysis by focusing on the second aspect identified above, i.e., the migration of the debris particles to the bone around the prosthesis. More particularly, the present invention provides an improved “Sealed-Bearing” total hip replacement (SB-THR) which incorporates the use of a limiting membrane that encapsulates the bearing and prevents debris particles from migrating from the prosthesis to the surrounding bone. The SB-THR could potentially eliminate osteolysis in the primary THR, thereby sparing thousands of patients from the ordeal of revision surgery and effecting significant savings in health care costs to society.  
           [0014]    In addition, the SB-THR has the additional advantage that it can prevent so-called “third bodies” from entering the space between the femoral head and the pelvic socket, where they can accelerate wear on the head and the socket. Such “third bodies” typically comprise small pieces of bone which are created during the milling process when the femoral canal is prepared or the socket seat is prepared; and/or small pieces of the prosthesis which may flake off the outer surface of the prosthesis, e.g., hydroxyapetite or porous metal for encouraging bone ingrowth.  
           [0015]    There has been some prior work in the area of the sealed-bearing concept. See, for example, U.S. Pat. Nos. 4,731,088 (Collier), 5,514,182 (Shea), and 5,755,807 (Anstaett et al.).  
           [0016]    Accepting that the sealed bearing membrane is advantageous in eliminating or discouraging migration of debris particles, it becomes critical that the sealing membrane itself not present a problem, as by disconnection from a member of the joint.  
           [0017]    A further complication lies in the fact that a sealing membrane in the hip joint flexes, when a leg is moved, not only in a bending direction, but also in a twisting or rotating direction, in which the sleeve-like membrane twists around its central axis.  
           [0018]    There is, therefore, a need for a membrane adapted to encapsulate a bodily joint, such as a hip joint, so as to prevent migration of debris particles. There is further a need for securely attaching the membrane to the members of the joint. A related further need exists for effecting long term attachment of the membrane to the members of the joint, such that upon gradual elimination of holding power of the initial attachment means, other attachment means grow stronger and increase the holding security of the membrane. A still further need exists for compensating for the bend-and-twist movement of the membrane in hip assemblies, such that undue strain from repetitive movement does not weaken the structure of the membrane. And a still further need is to provide an improved total joint replacement assembly which may be used in joints other than the hip, e.g., the knee, the shoulder, the elbow, etc.  
         OBJECTS OF THE INVENTION  
         [0019]    It is, therefore, an object of the invention to provide a sealed-bearing type of total hip replacement assembly and/or other total joint replacement assembly.  
           [0020]    A further object of the invention is to provide such an assembly which includes an encapsulating membrane which captures and prevents migration of debris particles.  
           [0021]    A further object of the invention is to provide such a membrane having thereon means for accurately and securely attaching the membrane to supporting structures at either end thereof.  
           [0022]    A still further object of the invention is to provide such a membrane as permits localized ingrowth of tissue to further secure the membrane to the supporting structures.  
           [0023]    A still further object of the invention is to provide such a membrane configured to reduce stress fatigue of the membrane resulting from long term use of the membrane.  
           [0024]    A still further object of the invention is to provide a method for installing such an assembly.  
         SUMMARY OF THE INVENTION  
         [0025]    With the above and other objects in view, as will hereinafter appear, a feature of the present invention is the provision of a joint prosthesis assembly, the assembly comprising a stem for disposition in a canal of a first bone constituting a first portion of the joint, a neck fixed to the stem, a collar fixed on the neck, the collar having a peripheral annular groove therein, and an articulating head fixed on the neck, the assembly further comprises a cup for disposition in a second bone constituting a second portion of the joint, and an insert disposed in the cup to provide an interior lining for the cup, the insert being adapted to receive the articulating head for movement therein, the insert having a peripheral annular groove therein. A sleeve-shaped membrane is provided with a first O-ring fixed thereto at a first end thereof for disposition in the insert groove, and a second O-ring fixed thereto at a second end thereof for disposition in the collar groove. The membrane is thereby disposed to capture particulate debris generated by the articulating head in the insert.  
           [0026]    In accordance with a further feature of the invention, there is provided a joint prosthesis assembly comprising a stem for disposition in a canal of a first bone constituting a first portion of the joint, a neck fixed to the stem, a collar fixed on the neck, and an articulating head fixed on the neck. The assembly further comprises a cup for disposition in a second bone constituting a second portion of the joint, and an insert disposed in the cup to provide an interior lining for the cup, the insert being adapted to receive the articulating head for movement therein. A sleeve-shaped membrane is provided with a first connector structure fixed thereto at a first end thereof for connection to the insert, and a second connector structure fixed thereto at a second end thereof for connection to the collar. The membrane is thereby disposed to capture particulate debris generated by the articulating head in the insert. A first end portion of the membrane proximate the first end of the membrane and a second end portion of the membrane proximate the second end of the membrane have apertures therein permitting ingrowth of tissue to further secure the membrane in an operative position, and a middle portion of the membrane is devoid of apertures therein permitting ingrowth of tissue.  
           [0027]    In accordance with a still further feature of the invention, there is provided a joint prosthesis assembly comprising a stem for disposition in a canal of a first bone constituting a first portion of the joint, a neck fixed to the stem, a collar fixed on the neck, and an articulating head fixed on the neck. The assembly further comprises a cup for disposition in a second bone constituting a second portion of the joint, and an insert disposed in the cup to provide an interior lining for the cup, the insert being adapted to receive the articulating head for movement therein. The assembly further includes a sleeve-shaped membrane having a first connector structure fixed thereto at a first end thereof for connection to the insert, and a second connector structure fixed thereto at a second end thereof for connection to the collar, the membrane being thereby disposed to capture particulate debris generated by the articulating head in the insert. The first and second ends are circularly displaced from each other around a central axis therethrough by an angle of about 35°-55°.  
           [0028]    In accordance with still another feature of the invention, there is provided a joint prosthesis assembly comprising a stem for disposition in a canal of a first bone constituting a first portion of the joint, a neck fixed to the stem, a collar fixed on the neck, and an articulating head fixed on the neck. The assembly further includes a cup for disposition in a second bone constituting a second portion of the joint, and an insert disposed in the cup to provide an interior lining for the cup, the insert being adapted to receive the articulating head for movement therein. The assembly further comprises a sleeve-shaped membrane having a first connector structure fixed thereto at a first end thereof for connection to the insert, and a second connector structure fixed thereto at a second end thereof for connection to the collar. The membrane is thereby disposed to capture particulate debris generated by the articulating head in the insert. A middle portion of the membrane is formed so as to be pre-stressed in a circular direction around a central axis therethrough, by an angle of about 35°-55°.  
           [0029]    In accordance with a still further feature of the invention, there is provided a method for installing a joint prosthesis assembly. The method comprises the steps of providing a joint prosthesis assembly comprising a stem for disposition in a canal of a first bone constituting a first portion of the joint, a neck fixed to the stem, a collar fixed on the neck, the collar having a peripheral annular groove therein, an articulating head fixed on the neck, a cup for disposition in a second bone constituting a second portion of the joint, an insert disposed in the cup to provide an interior lining for the cup, the insert being adapted to receive the articulating head for movement therein, the insert having a peripheral annular groove therein, and a sleeve-shaped membrane having a first O-ring fixed thereto at a first end thereof for disposition in the insert groove, and a second O-ring fixed thereto at a second end thereof for disposition in the collar groove. The method includes the further steps of mounting a first sub-assembly of the stem, neck, collar and articulating head on the first bone, mounting a second sub-assembly of the cup and insert on the second bone, mounting the membrane around the articulating head, fitting the articulating head in the insert, placing the first O-ring in the insert groove to fix the membrane first end to the insert, and placing the second O-ring in the collar groove to fix the membrane second end to the collar. The membrane is thereby disposed to capture particulate debris generated by the articulating head in the insert.  
           [0030]    The above and other features of the invention, including various novel details of construction and combinations of parts, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular assembly and method embodying the invention are shown by way of illustration only and not as limitations of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    The above and other objects and features of the present invention will be more fully disclosed in the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts, and further wherein:  
         [0032]    [0032]FIG. 1 is a diagrammatic illustration of a natural hip joint;  
         [0033]    [0033]FIG. 2 is a diagrammatic exploded view, partly in section, of a prior art total hip replacement assembly;  
         [0034]    [0034]FIG. 3 is a diagrammatic, partly sectional, view of the prior art assembly of FIG. 2 installed;  
         [0035]    [0035]FIG. 4 is an elevational exploded view of one form of total hip replacement assembly, less an encapsulating membrane portion, illustrative of an embodiment of the invention;  
         [0036]    [0036]FIG. 5 is a diagrammatic elevational illustration of an encapsulating membrane portion for use with the assembly of FIG. 4;  
         [0037]    [0037]FIG. 6 is a side elevational view of an insert portion of the assembly of FIG. 4;  
         [0038]    [0038]FIG. 7 is an enlarged illustration of a boxed portion of the insert portion of FIG. 6;  
         [0039]    [0039]FIG. 8 is a diagrammatic, partly sectional, view of the assembly of FIGS. 4 and 5 combined and in operative position;  
         [0040]    [0040]FIG. 9 is a diagrammatic perspective view of the encapsulating membrane portion wherein, for clarity of illustration, selected structures have been omitted from view;  
         [0041]    [0041]FIGS. 10 and 11 are diagrammatic views of an encapsulating membrane, illustrating an operational feature of prior art membranes; and  
         [0042]    [0042]FIGS. 12 and 13 are similar to FIGS. 10 and 11, but illustrative of a feature of the inventive encapsulating membrane. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0043]    Referring to FIG. 1, it will be seen that a natural hip joint  20  comprises a femoral head portion  22 , which is generally hemispherical in shape, and which articulates freely in an acetabulum  24 , forming a natural bearing. A natural semi-permeable sack or bursa (not shown), generally referred to as the capsule of the hip joint, surrounds the bearing, enclosing synovial fluid, which serves to lubricate the bearing. The femoral head portion  22  extends to a femoral neck portion  26 , which is sufficiently narrow to allow full range of motion.  
         [0044]    A prior art artificial hip (FIG. 2) approximates the major components of the natural joint  20 . The artificial hip is typically made of metal or metal alloy. Traditionally, a hip prosthesis consists of a femoral sub-assembly  30  and an acetabular sub-assembly  32 . The artificial femoral subassembly  30  includes a stem  34 , which is inserted into a canal  36  in the patient&#39;s femur  38  for support and load distribution. During hip replacement surgery, an incision is made and the patient&#39;s hip is dislocated, revealing the patient&#39;s femoral head  22  (FIG. 1) and acetabulum  24  (FIG. 1). The patient&#39;s femoral head  22  (FIG. 1) is removed and the canal  36  (FIG. 2) is drilled into the patient&#39;s femur  38 . The artificial stem  34  is fitted into the canal  36 , often using bone cement for anchorage. When bone cement is not used, part of the stem  34  may be textured or covered with Titanium mesh (or similar network) to promote bone in-growth. Often, there is a lip  40  for the distribution of load across the femur  38 . An end of the femoral sub-assembly  30  is an articulating head  42  which may be permanently attached or press fit onto a femoral neck  44 , in which case the femoral neck  44  is generally in the shape of a Morris taper to ensure a tight, slip-resistant fit.  
         [0045]    The acetabular sub-assembly  32  comprises a cup  46  in which the articulating head  42  articulates. The acetabular cup  46  may be screwed and/or glued into the patient&#39;s acetabulum  24  (FIG. 3). Often, the outer surface of the cup  46  is covered in Titanium mesh, or similar network, to promote bone in-growth. An insert  48 , typically of plastic material, is usually placed between the articulating head  42  and the acetabular cup  46  to reduce load and friction in the bearing. The insert  48  may be made of high density plastic such as Ultra High Molecular Weight Polyethylene (UHMWPE). Once installed, the patient&#39;s hip is relocated and the incision is closed.  
         [0046]    The assembly of the present invention is adapted to encapsulate the bearing of the prosthesis, to prevent migration of wear particles that are generated by the articulation of the femoral head portion  42  and the acetabular cup  46 , or insert  48 . In addition, the assembly of the present invention can prevent so-called “third bodies” from migrating into the bearing area of the prosthesis. Improvements which are made to the current artificial hip prosthesis, and described herein, include an increased offset dual-groove UHMWPE insert  50  (FIG. 4), a femoral collar  52 , and a sleeve-shaped encapsulating membrane  54  (FIG. 5).  
         [0047]    The encapsulating membrane  54  is made of biocompatible material, such as Cytoplast SB™, and can easily be extruded into any desired thickness. The sleeve shape of the membrane  54  preferably is generally frusto-conical, having a larger circular acetabular end  56 , a smaller circular femoral end  58 , and a middle portion  60  extending therebetween. Both ends  56 ,  58  of the membrane  54  have a connector structure, preferably an attached O-ring  62 ,  64 , respectively, made of the same or similar material as the membrane  54  for fastening the membrane  54  to the insert  50  and femoral collar  52 . Each O-ring  62 ,  64  is provided with tabs  66 , preferably set equidistant from each other along its circumference, to provide a surgeon with a place to hold for pulling the membrane  54  into place and adjusting its location. The tabs  66  may be made of the same material as the membrane  54  and may be textured to provide additional “growth points” for the natural bursa to attach when it regenerates after surgery. The end  56  of the membrane  54  with the larger diameter is, preferably, provided with a flexible flap  68  that extends along at least a portion of the membrane&#39;s circumference. A suture  70  is contained in the flap  68  to aid in the installation of the membrane  54 , as will be discussed below.  
         [0048]    Referring to FIGS. 6 and 7, it will be seen that the insert  50  is provided with two grooves  72 ,  74 . The groove  72  is located close to the open end of the insert  50  and is used to seat and secure the O-ring  62  on the larger end  56  of the membrane  54 . The groove  74  is disposed approximately 1.5 mm behind groove  72  and is used to seat the suture  70 , which acts as a guide to direct the placement of the O-ring.  
         [0049]    During installation, visibility in the area of the membrane end  56  can be limited. To attach the end  56  of the membrane to the insert  50 , the surgeon grasps both ends of the suture  70  and fits the suture  70  in the groove  74 , substantially by “feel” in some cases. The suture  70  is secured, as by tying, in the groove  74 . The tabs  66  are then pulled until the O-ring  62  pops into place in the groove  72  (FIG. 7). In effect, suture  70  acts as something of a pursestring to secure (sometime substantially by feel alone) flexible flap  68  to groove  74  in insert  50 ; thereafter, tabs  66  can be used to manipulate O-ring  62  into groove  72  in insert  50 .  
         [0050]    The femoral sub-assembly  30  is provided with the elliptical femoral collar  52  (FIGS. 4 and 8) around the femoral neck portion  44 , with a groove  76  formed along its perimeter. The O-ring  64  from the smaller end  58  of the membrane  54  is seated in the groove  76 .  
         [0051]    As shown in FIGS. 5, 8 and  9 , the encapsulating membrane  54  includes the first (acetabular) end portion  56 , the middle portion  60 , and the second (femoral) end portion  58 . The end portions  56 ,  58  are the portions which are anchored to the joint members, while the middle portion  60  is not directly anchored to joint members or tissue. The ends  56 ,  58  are fabricated so as to be porous, or otherwise are provided with apertures or recesses or texture or surface treatment or the like, all of which constructions are intended to be encompassed by the general term “apertures”, so as to encourage tissue ingrowth. The middle portion  60  is fabricated so as to discourage tissue ingrowth, as by being made substantially non-porous, or otherwise without apertures or recesses or texture or surface treatment or the like of sufficient size to permit ingrowths, all of which constructions are intended to be encompassed by the general term “devoid of apertures”.  
         [0052]    Thus, if after a period of years the O-rings  62 ,  64  suffer from fatigue and their holding power is diminished, the ingrowth of tissue through the membrane end portions  56 ,  58  serves to maintain the membrane securely affixed at either end.  
         [0053]    By way of example but not limitation, encapsulating membrane  54  may be formed out of PTFE, with end portions  56 ,  58  being formed out of expanded PTFE so as to encourage tissue ingrowth, and with middle portion  60  being formed out of non-expanded PTFE so as to prevent tissue ingrowth.  
         [0054]    If desired, membrane  54  may include an antibiotic to minimize the risk of infection associated with hip replacement surgery. The antibiotic may be applied to the surface of the membrane as a coating, or it may be impregnated into the material of the membrane.  
         [0055]    In FIGS. 10 and 11, there is illustrated the known fact that in hip joints, when a proximate leg is flexed, the membrane  54  covering the joint twists, as for example, in the direction indicated by arrow  80  in FIG. 11. It is known that the twist around the central axis c-c is about 45°±10°. Thus, the membrane  54  is continually moved from a substantially relaxed position, as indicated in FIG. 10 wherein hypothetical straight lines  82  illustrate the relaxed, unstressed state of the membrane middle portion  60 , to a stressed condition, indicated in FIG. 11 wherein hypothetical curved lines  82 ′ illustrate the stressed state of the membrane middle portion  60 . It will be apparent that continually stressing and relaxing the membrane  54  in this manner eventually introduces fatigue into the membrane structure, which may lead to the rupture or tearing of the membrane and the escape of debris.  
         [0056]    It has been found to be beneficial to provide the membrane with a pre-set twist of about 45° in the direction opposite to arrow  80 , that is, in the direction of arrow  84  in FIG. 12. The pre-setting of the twist may be built into the membrane middle portion  60 , as by alignment of fibers in the material of the middle portion, or may be introduced by the surgeon at the time of the THR operation. In the latter case, the surgeon manually shifts one end of the membrane through angle a, preferably an estimated 45°, before securing that end in place. Thereafter, upon flexing of the proximate leg, the “stressed” state, indicated by hypothetical lines  86  in FIG. 12, is relieved, indicated by hypothetical lines  86 ′ in FIG. 13, by movement of the middle portion  60  of the membrane  54  in the direction of arrow  88  around the central axis c-c.  
         [0057]    Installation of the above-described assembly requires the steps of mounting the femoral sub-assembly  30 , including the stem  34 , neck  44 , collar  52  and articulating head  42  on the first bone  38 , mounting the acetabular sub-assembly  32 , including the cup  46  and insert  50  on the second bone  24 , mounting the membrane  54  around the articulating head  42 , fitting the articulating head  42  in the insert  50 , placing the first O-ring  62  in the insert groove  72  to fix the membrane first end  56  to insert  50 , and placing the second O-ring  64  in the collar groove  76  to fix the membrane second end  58  to the collar  52 , the membrane being thereby disposed to capture particulate debris generated by the articulating head  42  in the insert  50 . In addition, membrane  54  can also prevent so-called “third bodies” from migrating into the bearing area of the prosthesis.  
         [0058]    Alternatively, if desired, the membrane&#39;s second O-ring  64  can be seated in collar groove  76  before its first O-ring  62  is seated in insert groove  72 .  
         [0059]    In cases in which the aforesaid twist is desired, and is not formed or pre-set in the membrane  54 , the above-described method is modified to include the further step of twisting the membrane  54  circularly about its central axis c-c, about 35°-55°, before securing the last of the first O-ring  62  to insert  50  and the second O-ring  64  to the collar  52  with the membrane  54  in a twisted state.  
         [0060]    The method described herein, and particularly the step of placing the first O-ring  62  in the insert groove  72  preferably further includes the steps of placing the strand of suture  70  in the insert suture groove  74 , tightening and tying the strand of suture  70  in the insert suture groove  74 , and manipulating the tabs  66  to pull the first O-ring  62  toward the insert peripheral annular groove  72  until the first O-ring  62  snaps into the insert peripheral annular groove  72 , to fix the membrane first end  56  to the insert  50 .  
         [0061]    There is thus provided an improved sealed-bearing total hip prosthesis including an encapsulating membrane having (1) improved means for securely mounting at the site, (2) means for further securing at the site over time, and (3) means for relieving stress fatigue therein. In addition, there is provided an improved total joint replacement assembly which may be used in joints other than the hip, e.g., the knee, the shoulder, the elbow, etc.  
         [0062]    There is further provided a method for installing the above-described prosthesis assembly.  
         [0063]    It is to be understood that the present invention is by no means limited to the particular construction and method steps herein disclosed and/or shown in the drawings, but also comprises any modifications or equivalents within the scope of the claims.