Abstract:
A modular hip implant that can be custom fit to an individual patient, including a shock absorption system that absorbs compressive stresses that are imparted to the implant. The size of the femoral ball member, size of the femoral stem, femoral neck length, and tension in the shock absorption system are all individually adjustable parameters, depending on the particular patient. A unique coupling member houses a modular spring mechanism that serves as the shock absorber. The coupling member is received into the ball member to an adjustable depth, the adjustment of which varies the length of the femoral neck. The length of the femoral neck can be adjusted during surgery without requiring additional parts.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates in general to an orthopaedic hip implant, and in particular to ‘Modular Hip Prosthesis’ that can be custom fit a patient. The present invention also relates to an implant having a versatile spring mechanism, a coupling member, and a ball member, which are attachable and detachable by the surgeon before the surgical procedure, during the surgical procedure, or during a postoperative revision surgery should it become necessary.  
           [0002]    Prosthetic implants for the replacement of a portion of a patient&#39;s hip joint are well known, and are typically available as a two to three component systems. The femoral stem component includes a shank at its distal end which extends into the canal of the femur bone and is fixed therein using bone cement or other means known in the art. At the proximal end of the femoral stem is a neck which typically terminates in a spherical ball that is adapted to cooperate with the patient&#39;s acetabulum or a prosthetic acetabular cup fixed into the patient&#39;s acetabulum. The ball, neck and femoral stem are typically formed in one piece from suitable material. The acetabular cup is typically formed as metal cup or bowl and optionally, can be provided with a plastic insert that is fixed therein to enhance the sliding engagement between the ball and the acetabular cup.  
           [0003]    Conventional one-piece femoral component designs are available in different sizes, but they do not allow enough flexibility for variations in individual parameters of the patient&#39;s anatomy. Parameters such as femoral neck length, femoral shank length and diameter, and femoral head size can vary independently of one another. For example, the length of the femoral neck can vary independently of the size of the femoral head or length of the medullary canal. It can thus be difficult to find a suitably fitting implant when selecting the implant from the different sizes available as one-piece femoral components.  
           [0004]    To address this problem, modular implant systems are known. For example: U.S. Pat. No. 4,938,773 to Strand discloses a femoral stem which can be fitted with interchangeable, different size femoral neck portions. Such a system is undesirably limited by the availability of different size components. Further, a large quantity of different size components must be produced and stocked to ensure that all patients can be fitted.  
           [0005]    Similarly, U.S. Pat. No. 5,507,830 to Demane et al discloses a modular hip prosthesis, which includes a plurality of different size tubular sleeves that can be attached to cylindrically shaped stem of the femoral component, thereby allowing the surgeon to extend the femoral stem length as necessary. Also disclosed are interchageable frustroconically shaped extension sleeves for the extension of the neck length of the prosthesis. Removable pads are provided for attachment to the mid-section of the prosthesis for cross-sectional configuration thereof. Again such a system is limited by the availability of the implant kit and various tools. Demane et al, did not disclose a spring or cushion mechanism, Demane et al, did not disclose a coupling member and the threaded method of neck length adjustment using the ball member and the coupling member.  
           [0006]    Lawes et al, U.S. Pat. No. 5,258,033 discloses a hip implant consisting of a femoral component that has an outer surface shaped to conform to an acetabular socket and has an internal bore adapted to receive a femoral head. The head of the femoral bone has been shaped and dimentioned to provide a spigot to enter the bore and be locked in place by a cement mantle between the bone and the walls of the bore. A spacer is included which creates a void to allow the components to move further over the spigot and relock should the cement creep or because of any movement in the bone after assembly. The spacer is included for preventing cement from reaching the bottom of the bore. Lawes et al, U.S. Pat. No. 5,258,033 do not allow enough flexibility for variations in the femoral neck lengths for variations in individual patients hip anatomy. Lawes et al also do not disclose a spring or cushion mechanism to prevent shock or stress at the hip joint or to the prosthesis to prevent related complications.  
           [0007]    Amino et al U.S. Pat. No. 5,362,311 discloses a method of custom-fitting modular hip prosthesis in which a tapered hole of a ceramic stem head to be fixed, a truncated conical sleeve having a thickness of 50 μm to 5.0 mm is compressedly held between a wall surface of the tapered hole and an outer circumferential surface of the tapered cone such that they are taperedly engage with each other, whereby the length of the stem neck can be optionally changed. Again such a system is undesirably limited by having different truncated conical sleeves of different thicknesses. Also Amino et al do not disclose a spring or cushion mechanism to absorb shock or a versatile neck length adjustment method.  
           [0008]    U.S. Pat. No. 5,389,107 to Nassar et al discloses a hip prosthetic implant, having an elongate element that extends coaxially from the ball section of the femur component. The elongate element slidably extends into a chamber formed by a tubular insert that is secured in the femur. Contained at the bottom of the chamber is a spring against which the elongate element abuts, thereby providing shock absorption. A pin member extends from the bottom of the chamber and slidably fits into a bore formed in the elongate element. A second spring is disposed between the pin and the bottom of the bore to provide further shock absorption. U.S. Pat. No. 5,389,107, the invention is limited in scope for the surgeon and the patient, did not disclose adjustable neck length of the prosthesis, did not disclose versatile attachable or detachable head or a spring mechanism, attachable or detachable coupling member.  
           [0009]    SU 171883-A1, discloses a modular hip implant that includes a spiral spring that has its ends rigidly mounted to the bottom of the recess of the head and is rigidly attached to the smooth end face of the neck. The threaded end of the neck is screwed into the threaded canal of the base and fixed. The base has a threaded canal under the neck, a lock nut in the form of a threaded washer with a hexahedral opening. The possibility of correcting the neck length is achieved by using special keys and lock nut. The invention discloses a plastic stopper, a special syringe, a biologically inert plastic paste or bone cement. The invention is limited in scope, particularly the prosthetic head can not be separated from from the neck, since the spring member is rigidly afixed and bonded between these two components. The invention is not versatile in case of need, for spare parts removable and replacement during primary or revision surgery for head or spring or neck members. Bone and tough scar tissue may grow into the plastic stoper, into the lock nut, into the base channel and into the hexagonal opening of the neck member, and may cause considerable difficulty during revision surgery for the removal of head and neck members using the special keys. Spare parts replacement can not be done and the whole modular prosthesis needs to be removed in such a situation and the surgery can become difficult, this may lead to increased morbidity to the patients. Also special keys with handle and lock nut are required for neck length adjustment, using the threaded aperture in the base, which maneuver can be fiddling and trouble-some in the blood stained, deep , wet operative field.  
           [0010]    Another problem with artificial hip prosthesis is that countless compressive stresses are transmitted thereto from daily activities such as walking, running, exercising, sitting and standing. These compressive stresses can often result in the loosening of the prosthesis, acetabular erosion, protrusion of the prosthetic head into the acetabulum or pelvic bone, dislocation of the prosthetic head from the socket or cup, chronic pain and suffering of the patient.  
           [0011]    What is needed is an improved modular hip prosthesis that provide neck length adjustments, spring mechanism for cushion effect and shock absorption to prevent complications. Also what is needed is modular prosthesis whose parts can be easily adjusted or replaced with surgeons hands or with simple tools. Also cut down the costs to medicare or to the patients with less inventory of instruments, less operating time, and a versatile prosthesis modular in nature with adjustable, removable and also replaceble parts by the surgeon hands.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention provides a modular hip implant that can be custom fit to an individual patient and that includes a shock absorption system that absorbs compressive stresses that are imparted to the implant.  
           [0013]    In one form thereof, the present invention provides a modular hip prosthesis. The hip prosthesis comprises a ball member having an outer surface adapted to cooperate with an acetabular socket and a femoral stem having a shank adapted to be inserted and secured into a medullary cavity of a femur. The femoral stem has a neck at a proximal end thereof which is connected to the ball member. A spring mechanism is disposed intermediate the ball member and the neck, and provides cushioning movement between the femoral stem and the ball member. The spring mechanism is detachably connected to the neck and detachably connected to the ball member.  
           [0014]    In a preferred form, the modular hip prosthesis further comprises a bore disposed in the ball member. A coupling member houses the spring mechanism, and the connection of the ball member to the spring mechanism is through the coupling member. The coupling member is received in the bore to an adjustable depth, adjustment of which causes corresponding adjustment of the distance the neck extends from the ball member. More preferably, the bore and the coupling member comprise corresponding threads, the coupling member being threadingly received in the bore. Still more preferably, the spring mechanism includes a first connector at a first end thereof connecting the neck to the spring mechanism. The first connector and the neck include complementary threads, such that the first connector is threadingly connected to the neck. The spring mechanism includes a second connector at a second end thereof connecting the spring mechanism to the coupling member.  
           [0015]    In another form thereof, the present invention provides a method of custom fitting a hip prosthesis to an individual patient. In this method, a ball member is selected from a plurality of different size ball members, depending upon the size of the acetabular socket into which the ball member is to be inserted. A femoral component is selected from a plurality of different size femoral components, and the neck of the selected femoral component is attached to a coupling member. A depth that the coupling member is to be inserted into the selected ball member is determined. Such depth corresponds to an individual patient. The coupling member is installed into the ball member to the determined depth.  
           [0016]    In a preferred form of the inventive method, a spring mechanism is installed in the prosthesis to allow cushioning movement of the neck of the selected femoral component relative to the ball member. More preferably, the spring mechanism is selected from a plurality of spring mechanisms having spring elements of different spring constants or stiffnesses. The spring stiffness can be calibrated to the weight of the patient. Further, the length of the neck can be adjusted intraoperatively to compensate for errors in neck length obtained from preoperative imaging techniques.  
           [0017]    One advantage of the present invention is that the spring mechanism absorbs much of the compressive stresses imparted to the implant during daily activities such as walking, running and exercising. Because the spring mechanism contracts and expands to absorb load bearing, shock and compressive stresses imparted to the hip joint during weight bearing and mobilization, the implant is less likely to loosen, and the useful life of the implant is therefore lengthened. The spring mechanism also reduces other complications, such as dislocation of the femoral stem from the acetabulum, acetabular damage and erosion, and protrusion of the femoral ball member into the acetabulum and pelvis during a sudden jarring event, such as a fall.  
           [0018]    Another advantage of the present invention is that the spring mechanism is a modular component such that a spring element having a specific stiffness can be selected.  
           [0019]    Another advantage of the present invention is that the length of the femoral neck can be changed without adding or interchanging parts, unlike the above-described prior art implants which require a plurality of interchangeable parts. Instead, the present invention employs a single coupling member that can be installed in the ball member to a depth which corresponds to the desired length of the femoral neck.  
           [0020]    Yet another advantage of the present invention is that the length of the femoral neck can be adjusted intraoperatively. While pre-operative imaging techniques can be used to determine the appropriate length of the femoral neck, such techniques are often only an approximation of actual surgical conditions. With the present invention. adjustments to the length of the femoral neck can be made during surgery by adjusting the depth to which the coupling member is inserted into the ball member so that an exact preoperative neck length need not be entirely relied upon. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0021]    The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:  
         [0022]    [0022]FIG. 1 is an exploded side sectional view of a modular hip implant incorporating the present invention;  
         [0023]    [0023]FIG. 2 is side sectional view of the modular hip implant of FIG. 1;  
         [0024]    [0024]FIG. 3A is a fragmentary sectional view illustrating the femoral neck of a femoral stem component;  
         [0025]    [0025]FIG. 3B is a sectional view illustrating a spring mechanism incorporating the present invention;  
         [0026]    [0026]FIG. 3C is a sectional view illustrating a coupling member incorporating the present invention;  
         [0027]    [0027]FIG. 3D is a sectional view illustrating a ball member incorporating the present invention;  
         [0028]    [0028]FIG. 4A is a sectional view illustrating the femoral neck extending from the ball member a first distance;  
         [0029]    [0029]FIG. 4B is a sectional view illustrating the femoral neck of FIG. 4A extending from the ball member a second distance less than the first distance; and  
         [0030]    FIGS.  5 A- 5 E are sectional views illustrating alternate embodiments of the spring mechanism in accordance with the present invention. 
     
    
       [0031]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set forth herein illustrates preferred embodiments of the invention, in several forms, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION  
       [0032]    [0032]FIGS. 1 and 2 show a modular hip implant  20  including a femoral ball member  22  having an outer surface  24  adapted to cooperate with an acetabular socket (not shown) or a prosthetic acetabular cup (not shown) as is known in the art. Coupling member  26  forms a tubular threaded insert which is threadingly received in threaded bore  28  of ball member  22 . A spring mechanism  30  includes a spring element  32 , shown in FIGS. 1 and 2 as a coiled spring having connectors  34  and  36  at opposite ends thereof. Connector  34  is received in a threaded opening  38  formed in femoral neck  40  of femoral stem  42 . Connector  36  abuts against cap  44  of coupling member  26  and is secured thereto by means of a screw  46  received through threaded aperture  48  in cap  44  and threaded aperture  50  formed in connector  36 .  
         [0033]    Femoral stem  42  includes a shank  52  adapted to be inserted into a medullary cavity  54  (FIG. 2) of a patient by means of bone cement or other fixation means known in the art. Femoral stem  42  includes a circular hole  56  adapted for a medical instrument to be hooked thereto to remove the prosthesis should such become necessary during a surgical procedure. Triangular shaped fenestration  58  is provided to allow bone and scar tissue to grow therein and thereby prevent loosening or rotation of femoral stem  42 . More than one fenestration can be provided, as is known in the art. Square shaped notch  60  is provided to accommodate an impactor or other suitable surgical instrument for implanting shank  52  into cavity  54 . Femoral stem  42  includes a flange  62  that abuts femur bone portion  64  (FIG. 2) and prevents shank  52  from migrating downward into cavity  54  of femur  55 .  
         [0034]    With reference to FIGS.  1 - 4 , spring mechanism  30  includes a threaded connector  34 , shown in FIGS. 4A and 4B as a threaded plug fixed to an end of spring element  32 . The end of spring element  32  can be welded to or embedded within plug connector  34 . The other end of spring element  32  can also be welded to or embedded within disk-shaped connector  36 . Connector  34  is threadingly received into threaded opening  38  and, at the other end of spring mechanism  30 , disk-shaped connector  36  is secured to cap  44  by means of screw  46 .  
         [0035]    It can be appreciated that the connection of neck  40  to coupling member  26  and thus to ball member  22  is through spring mechanism  30 , which is disposed intermediate ball member  22  and neck  40 . Similarly, the connection of spring mechanism  30  to ball member  22  is through coupling member  26 . It can be also be appreciated that spring mechanism  30  is removable from coupling member  26  and thus from modular hip implant  20 . Thus, hip implant  20  provides the flexibility of accepting a spring mechanism having different spring constants, or stiffnesses, if desired. Such may be desirable depending on the age, weight and activity level of the patient.  
         [0036]    The spring mechanism is designed to absorb shock and vibrations produced by daily activities such as walking, running, exercising, and even simple load-bearing activities such as sitting and standing. Because the spring mechanism absorbs some of the shock and vibrations imparted to the implant, it is less likely that such shock and compressive stresses will cause the implant to loosen or fracture over a period of time. Further, because the stiffness of the spring can be pre-selected, its cushioning effect can be adjusted for an individual patient. For example, a spring element  32  that is too stiff will frustrate the load-sharing purpose of the implant. On the other hand, if the spring element is not stiff enough, the implant will experience too much movement. With the present invention, the stiffness of the spring can be selected to provide the appropriate cushioning effect.  
         [0037]    As shown in FIG. 1, coupling member  26  is formed as a substantially hollow, tubular insert having an open distal end to receive neck  40 . Neck  40  has an outer surface  66  that corresponds to a substantially smooth inner surface  68  of coupling member  26  such that neck  40  slidably engages coupling member  22 . Various low-friction, bio-compatible coatings can be applied to the two surfaces  66 ,  68 . Preferably, the two mating surfaces are cylindrical, although other complementary shapes are contemplated. At least half and preferably two-thirds of neck  40  should be housed within coupling member  26  to adequately secure neck  40  to coupling member  26 . The spring mechanism and the sliding engagement between surfaces  66  and  68  combine to provide a cushioning movement between neck  40  and ball member  22 .  
         [0038]    With reference to FIGS. 4A and 4B, one of the features of modular hip implant  20  is that the extent to which neck  40  extends from ball member  22  is an adjustable parameter, depending on characteristics of the individual patient. That is, coupling member  22 , to which neck  40  of stem  42  is removably attached, can be screwed into bore  28  to a depth that corresponds to the desired extension distance of neck  40  from ball member  22 . Thus, the present invention avoids the necessity of interchangeable sleeves of different sizes to produce different length necks. Instead, a continuous range of neck lengths are made possible with a single coupling member  26 . For example, the configuration shown in FIG. 4A can accommodate a patient needing a larger femoral neck length whereas the configuration in FIG. 4B may accommodate a person needing a shorter femoral neck length. Further, it is possible with the present invention to adjust the femoral neck length at the time of surgery, which might be desirable, for example, when preoperative data used to establish femoral neck length are inaccurate.  
         [0039]    Alternate embodiments of the spring mechanisms are possible. For example, as shown in FIG. 5A, spring mechanism  130  can include spring element  132  having threaded connectors  134  and  136  at opposite ends thereof. Coupling member  126  has a portion of its interior formed with a thread  144  which threadingly engages threads  146  formed on connector  136 . This arrangement provides an additional means to adjust the length that neck  140  extends from the ball member because the relative position of connector  136  can be varied by the extent to which it is screwed into coupling member  126 .  
         [0040]    As shown in FIG. 5B, neck  240  can be formed with a threaded fastener  238  which screws into a threaded aperture  248  formed in connector  236 . Spring element  232  is fixed to connector  236  at one end and is fixed to cap  244  of coupling member  226  at its other end.  
         [0041]    As shown in FIG. 5C, neck  340  can be formed with a threaded fastener  338  which screws into a threaded aperture  348  disposed in connector  336 . Spring element  332  can be formed of a compressible and elastic material such as silicone, closed gel foam, rubber or the like. Sufficient elastic material is placed in the cavity  380  such that connector  336  is biased against stop  382  which can be formed as an annular ridge or as a crimped portion on the inside of coupling member  326  as shown.  
         [0042]    [0042]FIG. 5D illustrates an embodiment similar to the embodiment described with reference to FIGS.  1 - 4 , except that cap  444  of coupling member is formed with an internally threaded bore  448  that receives threaded fastener  446  extending from connector  436 . On the other end of spring element  432  is attached a threaded connector  434  that screws into bore  438  formed in neck  440 .  
         [0043]    [0043]FIG. 5E illustrates an embodiment wherein spring mechanism  530  is fixed within coupling member  526 . An end of spring element  532  is fixed to cap  544  of coupling member  526 . As with other embodiments described above, threaded connector  534  is received in bore  538  formed in neck  540 .  
         [0044]    [0044]FIG. 5F illustrates an embodiment wherein spring mechanism  630  includes a piston-cylinder spring element  632  connected at one end to connector  634  and connected at its other end to connector  636 . Spring mechanism  630  is detachably connected to coupling member  626  by means of screw  646  that passes through aperture  648  and is threadingly received in threaded opening  650 . Connector  634  is received in threaded bore  638  formed in neck  640 .  
         [0045]    The advantages of the modular features of the present invention can be better understood with reference to a description of custom fitting a modular hip implant to an individual patient. Ball member  22  can be selecting from a plurality of different size ball members, depending upon the size of the acetabular socket into which the ball member is to be inserted. The acetabular socket can be the patient&#39;s acetabulum or a prosthetic acetabular cup that is fixed into the patient&#39;s pelvic bone. The femoral stem component  42  is selected from a plurality of different size femoral stems. The femoral stems may vary, among other parameters, by length and/or diameter of the femoral shank, angle of femoral neck with respect to the femoral shank, and length and/or diameter of femoral neck.  
         [0046]    A spring mechanism  30  is selected for the individual patient and installed into the coupling member  26 . The stiffness of spring element  32  can be chosen based upon various patient factors, such as weight and activity level. With reference to FIG. 1, connector  36  is placed against cap  44  such that apertures  48  and  50  are aligned. Screw  46  is then threadingly advanced through the apertures, thereby securing spring mechanism  30  to coupling member  26 . Femoral stem  42  is connected to spring mechanism  30  and thus coupling member  26  by aligning connector  34  with bore  38  and turning coupling member  26  such that connector  34  is screwed into bore  38 .  
         [0047]    After coupling member  26  is secured to stem  42  as just described, coupling member  26  is inserted into bore  28  of the selected ball member to a specific depth. Based upon patient data such as computer assisted tomography images (CAT scans). magnetic resonance imaging (MRI) and the like, the appropriate length of the patient&#39;s femoral neck can be determined preoperatively and then correlated to determine the corresponding depth to which coupling member  26  should be inserted into ball member  22 . Advantageously, if the length of the neck as determined preoperatively does not exactly match the actual length needed as determined during surgery, the length of the neck can be adjusted during surgery by turning the ball member  22  relative to coupling member  26 .  
         [0048]    While this invention has been described as having an exemplary design, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.  
         [0049]    This invention of modular hip prosthesis is used in the following manner:  
         [0050]    The hip joint area is opened up using the standard surgical approach, the femoral head and neck of the patient is dislocated from the acetabular cavity. The femoral head and neck bones are cut away and removed from the operative field.  
         [0051]    The medullary canal ( 54 ) Of the femur bone ( 55 ) is cleaned and reamed and a channel is made in the femur medullary canal for the insertion of the prosthetic stem ( 42 ).  
         [0052]    The modular hip prosthesis ( 20 ) that is custom made as in FIG. 2 , containing the ball member ( 22 ), the coupling member ( 26 ) the spring mechanism ( 30 ), the neck ( 40 ), and the stem member( 42 ) is selected for the surgical procedure.  
         [0053]    The prosthetic stem ( 42 ) is fixed into the medullary canal ( 54 ) of the femur bone ( 55 ) in such a way the flange ( 62 ) rests on the proximal end of ( 64 ) femur bone ( 55 ).  
         [0054]    The desired neck ( 40 ) length adjustment can be made during the surgical procedure by the surgeon, wherein the ball member ( 22 ) is threaded to an adjustable depth by screwing method using the surgeon&#39;s hands, wherein the coupling member ( 26 ) threadingly received into the threaded bore ( 28 ) of the ball member ( 22 ). To adjust the neck length,no special keys or tools or instruments are necessary.  
         [0055]    After the prosthetic neck length adjustment is made, the ball member ( 22 ) is reduced into the acetabular cavity.  
         [0056]    The surgical wound is closed.