Abstract:
An artificial joint is configured for surgical insertion between two bones. The joint includes first and second parts supported for relative movement, and structure that can be selectively used to facilitate relative fixation of the first and second parts in a manner preventing the relative movement thereof. A method involves surgically inserting such a joint between two bones, and completing the surgical procedure with the first and second parts movable relative to each other. A different method relates to a situation where such a joint was previously surgically installed, and involves modifying the joint in situ to fix the first and second parts against relative movement.

Description:
BACKGROUND  
       [0001]     Spinal columns have a plurality of vertebrae that are separated by discs. A disc may be displaced or damaged due to trauma or disease, resulting in disruption of the annulus fibrosis, and the eventual protrusion of the nucleus pulposus into the spinal canal. This condition is commonly referred to as a herniated or ruptured disc. The extruded nucleus pulposus may press on the spinal nerve, thereby causing nerve damage, pain, numbness, muscle weakness and/or paralysis. Alternatively, the normal aging process may cause a disc to deteriorate. For example, as a disc ages, it dehydrates and hardens, and this in turn reduces the effective thickness of the disc. As a result, there can be pain, decreased mobility, and/or instability of the spine.  
         [0002]     It has become fairly common to surgically remove a damaged or problematic disc, in and to replace it with an artificial disc. One type of artificial disc is designed to secure the adjacent vertebrae against movement with respect to each other, and this is commonly known as fusion of the two vertebrae. When two vertebrae are fused in this manner, the rest of the spinal column provides sufficient movement to accommodate the needs of the patient.  
         [0003]     A different type of artificial disc is designed to preserve motion between two vertebrae. This type of disc is designed to operate reliably for many years after it has been surgically implanted in a patient, typically for the natural lifetime of the patient. Nevertheless, in rare situations, problems may eventually develop. For example, even where the artificial disc is still functioning properly, the patient may be subjected to trauma or disease that leads to a physiological condition causing pain, numbness, muscle weakness or the like during the movement permitted by the artificial disc. Alternatively, trauma or long-term wear may cause the artificial disc itself to experience a problem that causes pain or discomfort during the movement permitted by the artificial disc. When one of these types of problems develops, the current solution is to subject the patient to another major surgical procedure, in which the motion preservation disc is surgically removed, and replaced with a new artificial disc. The new disc may be either a motion preservation disc or a fusion disc, depending on the particular circumstances of the patient.  
       SUMMARY  
       [0004]     One form of the invention involves an artificial joint for surgical insertion between two bones, the joint including: first and second parts supported for relative movement; and structure that can be selectively used to facilitate relative fixation of the first and second parts in a manner preventing the relative movement.  
         [0005]     A different form of the invention involves a method of carrying out a surgical procedure that includes: inserting between two bones an artificial joint having first and second parts that are movable relative to each other and that each cooperate with a respective bone, the joint having structure that can be selectively used to facilitate fixation of the first and second parts against relative movement; and completing the surgical procedure with the first and second parts movable relative to each other.  
         [0006]     Still another form of the invention relates to a method that involves an artificial joint disposed between two bones and having first and second parts movable relative to each other; wherein the method includes modifying the joint in situ to fix the first and second parts against relative movement.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a diagrammatic perspective view of an artificial joint that is an intervertebral disc, and that embodies aspects of the present invention.  
         [0008]      FIG. 2  is a diagrammatic perspective view, partly in section, showing the disc of  FIG. 1  implanted between two vertebrae.  
         [0009]      FIG. 3  is a diagrammatic perspective view, partly in section, showing an intervertebral disc that is an alternative embodiment of the intervertebral disc of  FIGS. 1 and 2 .  
         [0010]      FIG. 4  is a central sectional side view of the disc of  FIG. 3 .  
         [0011]      FIG. 5  is a diagrammatic perspective view, partly in section, of an intervertebral disc that is an alternative embodiment of the intervertebral disc of  FIGS. 3 and 4 .  
         [0012]      FIG. 6  is a diagrammatic view similar to  FIG. 5 , but showing a different operational position. 
     
    
     DETAILED DESCRIPTION  
       [0013]      FIG. 1  is a diagrammatic perspective view of an apparatus that is an artificial joint, in particular an intervertebral disc  10 .  FIG. 2  is a diagrammatic perspective view, partly in section, showing the disc  10  after surgical insertion between two vertebrae  12  and  13 . With reference to  FIGS. 1 and 2 , the disc  10  includes two parts  16  and  17  that are vertically spaced, a central body  19  disposed between the parts  16  and  17 , and an annular sheath  21 . The sheath  21  encircles the central body  19 , and extends vertically between the parts  16  and  17 .  
         [0014]     The parts  16  and  17  each include a respective shell  26  or  27 . The shells  26  and  27  each have a concave inner surface, and a convex outer surface. Further, the shells  26  and  27  each have a central post  28  or  29  that projects vertically toward the other thereof. An opening  31  or  32  extends vertically through each shell  26  or  27 , and through the post  28  or  29  thereof. The outer end of each opening  31  and  32  is threaded. The shells  26  and  27  each have a respective annular groove  33  or  34  extending circumferentially around the periphery thereof. The shells  26  and  27  each have an upwardly-extending flange  36  or  37  on a rear side thereof, and a respective opening  38  or  39  extends horizontally through each of the flanges  36  and  37 . The parts  16  and  17  also include respective plugs  42  and  43 . The plugs  42  and  43  each threadedly engage the threaded outer end of a respective one of the openings  31  and  32 . The shells  26  and  27  and the plugs  42  and  43  can each be made from a wide variety of biocompatible materials. In the embodiment of  FIGS. 1 and 2  they are made from titanium, but they could alternatively be made from stainless steel, a titanium alloy, a polymeric material such as polyethylene, or any other suitable material.  
         [0015]     Each of the parts  16  and  17  has on the convex outer surface thereof a respective coating  46  or  47  that promotes ingrowth of bone material, in order to help fixedly couple the parts  16  and  17  to the bones  12  and  13 . In the embodiment of  FIGS. 1 and 2 , the coatings  46  and  47  are defined by a plurality of sintered beads made of a biocompatible material. In the embodiment of  FIGS. 1 and 2  they are made from titanium, but could alternatively be made from stainless steel, a titanium alloy, a polymeric material such as polyethylene, or any other suitable material.  
         [0016]     The central body  19  is annular, with a vertical axial opening therethrough. The opposite ends of this opening receive the respective posts  28  and  29 , with sufficient clearance to allow relative transverse movement. The central body  19  has convex top and bottom surfaces that each slidably engage the concave inner surface on a respective one of the shells  26  and  27 . The central body  19  is resiliently deformable, and has surface regions that are harder then the interior region. This allows the central body  19  to be sufficiently deformable and resilient so that the disc  10  functions to provide resistance to compression and also to provide damping, while still providing adequate surface durability and wear resistance. In addition, the material of the central body is selected so that the surfaces are very lubricious, in order to decrease friction between the central body and each of the rigid shells  26  and  27 .  
         [0017]     The material used to make the central body  19  is a biocompatible polymeric material that is slightly elastomeric, and that may be coated or impregnated to increase surface hardness, lubricity or both. Coating may be carried out by any suitable technique, such as dip coating, and the coating solution may include one or more polymers. The coating polymer may be the same as or different from the polymer used to form the interior of the central body, and may have a different Durometer hardness than that of the interior material. The coating thickness can be greater than about 1 mil, for example from about 2 mil to about 5 mil. Examples of suitable commercially-available materials include polyurethanes such as polycarbonates and polyethers, including CHRONOTHANE P 75A or P 55D (P-eth-PU aromatic, CT Biomaterials), CHRONOFLEX C 55D, C 65D, C 80A, or C 93A (PC-PU aromatic, CT Biomaterials), ELAST-EON II 80A (Si-PU aromatic, Elastomedic), BIONATE 55D/S or 80A-80A/S (PC-PU aromatic with S-SME, PTG), CARBOSIL-10 90A (PC-Si-PU aromatic, PTG), TECOTHANE TT-1055D or TT-1065D (P-eth-PU aromatic, Thermedics), TECOFLEX EG-93A (P-eth-PU aliphatic, Thermedics), or CARBOTHANE PC 3585A or PC 3555D (PC-PU aliphatic, Thermedics).  
         [0018]     The disc  10  includes two retaining rings  61  and  62  that each sealingly hold a respective axial end of the sheath  21  within a respective one of the grooves  33  or  34 . An annular chamber  66  is defined within the disc  10 , between the sheath  21 , the periphery of the central body  19 , and the peripheral edges of the shells  26  and  27 . In the embodiment of  FIGS. 1 and 2 , the rings  61  and  62  are made of titanium, but they could alternatively be made of any other suitable biocompatible material, including stainless steel, a titanium alloy, or a synthetic material. The sheath  21  is made from a biocompatible material that is durable and flexible, and that can be slightly elastic. For example, the sheath  21  can be made from a segmented polyurethane having a thickness ranging from about 5 to about 30 mils, and more particularly from about 10 to 11 mils. Examples of suitable commercially-available materials include BIOSPAN-S (aromatic polyetherurethaneurea with surface modified end groups, Polymer Technology Group), CHRONOFLEX AR/LT (aromatic polycarbonate polyurethane with low-tack properties, CardioTech International), CHRONOTHANE B (aromatic polyether polyurethane, CardioTech International), and CARBOTHANE PC (aliphatic polycarbonate polyurethane, Thermedics).  
         [0019]     A fitting  71  is mounted on the sheath  21 , in angular alignment with the flanges  36  and  37 . The fitting  71  extends through the sheath  21 , and has a passageway  72  that can provide communication between the annular chamber  66  and the exterior of the disc  10 . In the embodiment of  FIGS. 1 and 2 , the fitting  71  is manufactured with an integral portion that completely obstructs the passageway  72 , so as to prevent fluid flow in either direction through the passageway  72 . As discussed in more detail later, the obstruction can be selectively punctured at a later point in time, in order to allow fluid flow. As an alternative to the obstruction, the fitting  71  could have a valve to control fluid flow through the passageway  72 , such as a simple spring-biased ball valve of a known type. The fitting  71  can be made from a wide variety of materials that are biocompatible. In the embodiment of  FIGS. 1 and 2  the fitting is made from a polymeric material such as polyethylene, so that the integral obstruction in the passageway  72  can be punctured without difficulty. However, the fitting  71  could be made from any other suitable material. If it included a valve rather than the integral obstruction, then it could be made from materials such as titanium, stainless steel, or a titanium alloy.  
         [0020]     Following manufacture of the disc  10 , the disc  10  is surgically inserted in a known manner between two vertebrae, such as the vertebrae shown at  12  and  13  in  FIG. 2 . Not-illustrated screws can optionally be inserted through the openings  38  and  39  in the flanges  36  and  37 , in order to engage the bones  12  and  13  and thus securely hold the disc  10  in place. Over time, and as mentioned above, bone growth will occur into the sintered coatings  46  and  47 , thereby further securing the disc  10  to the bones  12  and  13 .  
         [0021]     After surgical insertion of the disc  10 , and after recovery of the patient, the disc will facilitate a degree of relative movement between the bones  12  and  13 . In particular, the shells  26  and  27  can each carry out limited lateral sliding movement relative to the central body  19 . Since the cooperating surfaces on the central body  19  arid the shells  26  and  27  are curved, the relative movement will effectively be limited pivotal movement about any of various horizontal axes. In addition, the inherent resilience of the central body  19  will allow a limited degree of vertical compression that permits movement of the shells toward each other, and also a limited degree of relative rocking movement of the shells that is effectively limited pivotal movement about horizontal axes.  
         [0022]     In rare cases, it is possible that a problem may develop over time. For example, even where the disc  10  is still functioning properly, the patient may experience trauma or disease that leads to a physiological condition causing pain, numbness, muscle weakness or the like during the relative vertebral movement permitted by the disc  10 . As another example, trauma or long-term wear may cause the disc  10  itself to experience a problem that causes pain or discomfort to the patient during the movement permitted by the disc. In either case, the standard solution with pre-existing artificial discs is to subject the patient to a further major surgery in order to replace the artificial disc with a different artificial disc. In contrast, the disc  10  allows a different approach.  
         [0023]     More specifically, in a relatively minor surgery, a small incision is made in the skin and muscle of the patient, order to allow access to the fitting  71 . The obstruction within the passageway  72  is punctured with a sharp and sterile object, in order to permit fluid flow through the passageway  72 . One end of a tube  91  is then coupled to the fitting  71  in any suitable manner, so that the passageway  72  is in fluid communication with the passageway that extends through the tube  91 . A syringe  92  or other suitable device is then used to inject a fluid material through the tube  91  and fitting  71 , in order to fill the chamber  66  with the material. The material then cures or hardens, preferably in a relatively short period of time. Since this material engages the entire peripheral edge of each of the shells  26  and  27 , the shells  26  and  27  will become fixed against relative movement when the material hardens. Consequently, the disc  10  will be converted from one operational mode in which the shells  26  and  27  are capable of relative movement to a different operational mode in which the shells  26  and  27  are fixed against any relative movement. The material injected into the chamber  66  is a biocompatible material. In the embodiment of  FIGS. 1 and 2 , the material is a known epoxy, where two components are mixed together in a fluid state and then injected into the chamber  66 , where the mixture chemically hardens. The material could alternatively be any other suitable material, including any of a number of known cements that are initially fluid but then harden.  
         [0024]     After the material has been injected, the tube  91  is detached from the fitting  71 , and the opening  72  is closed. For example, a small plug may be force-fit into the opening  72 . Alternatively, the opening  72  could be closed in any other suitable manner. The small incision made through the skin and muscle of the patient is then sutured or stapled. If necessary, the patient is kept immobilized until the material in the chamber  66  has had time to harden. However, in the embodiment of  FIGS. 1 and 2 , the material hardens in a relatively short period of time, so that it is fully hardened by the time the surgeon finishes closing the incision and the patient is released to the recovery room. This is a minimally invasive procedure that can be performed on an outpatient basis, and permits the patient to be up and around in a day or two, as opposed to the long recovery time needed for a major surgery in which an artificial disc is removed and replaced with another.  
         [0025]      FIG. 3  is a diagrammatic perspective view, partly in section, showing a disc  110  that is an alternative embodiment of the disc  10  of  FIGS. 1 and 2 . The disc  110  includes two parts  116  and  117 , and a sheath  121  that envelopes the parts  116  and  117 . Approximately half of the sheath  121  has been removed in  FIG. 3 , so that the parts  116  and  117  can be seen.  FIG. 4  is a central sectional side view of the disc  110  of  FIG. 3 .  
         [0026]     The parts  116  and  117  each include a respective plate-like center portion  126  or  127 . The center portion  126  has in the underside thereof an approximately hemispherical recess with a concave surface  131 . The center portion  127  has on an upper side thereof an approximately hemispherical projection with a convex surface  132 . The surfaces  131  and  132  slidably engage each other, to facilitate approximately pivotal movement of the parts  116  and  117  with respect to each other.  
         [0027]     The part  116  has on the upper side of its center portion  126  an upwardly-extending projection or keel  136 . Similarly, the part  117  has on the lower side of its center portion  127  a downwardly-extending projection or keel  137 . The projections  136  and  137  each have a pair of transverse openings extending therethrough. Before the disc  110  is inserted between two vertebrae, the surgeon creates a recess in each vertebra. Then, when the disc  110  is surgically implanted, the projections  136  and  137  are each received in one of those recesses. This helps to anchor the disc  110  in the proper position. Further, as bone growth occurs over time, there will be bone growth into the transverse openings through the projections  136  and  137 , thereby helping to anchor the disc  110  in place. The parts  116  and  117  can be made from a wide variety of biocompatible materials. In the embodiment of  FIGS. 3 and 4 , the parts  116  and  117  are made from a cobalt-chrome-molybdenum metallic alloy (such as ASTM F799 or F-75). The parts  116  and  117  could alternatively be made from stainless steel, titanium, a titanium alloy, a polymeric material such as polyethylene, or any other suitable material.  
         [0028]     The sheath  121  is made of a biocompatible material that is durable and flexible, and that may be slightly elastic. For example, the sheath  121  can be made from materials of the type discussed above in association with the sheath  21  of  FIGS. 1-2 . The sheath  121  may optionally be made from a material that promotes bone growth. Also, to facilitate bone growth, the top and bottom portions of the sheath  121  can be roughened. Alternatively, the top and bottom portions of the sheath  121  may optionally be coated with a known type of material that promotes bone growth. A variety of bone-growth promoting substances are known in the art. One example is a hydroxyapatite coating formed of calcium phosphate.  
         [0029]     As best seen in  FIG. 4 , an annular chamber  166  is present within the sheath  121 , and extends around the hemispherical projection having surface  132 , between the peripheral edges of the center portions  126  and  127  of the parts  116  and  117 . As shown in  FIG. 4 , a fitting  171  is mounted in an opening through the sheath  121 , on a rear side of the disc  110 . The fitting  171  is similar to the fitting  71  that was discussed above in association with  FIGS. 1 and 2 , and has a passageway  172  extending therethrough. The fitting  171  initially includes an obstruction or valve within the passageway  172 , in the same manner as the fitting  71 .  
         [0030]     A tube  174  is provided within the chamber  166 , and has one end fixedly secured to the inner side of the fitting  171 . The opening through the tube  174  communicates with the passageway  172 , and effectively serves as an extension of the passageway  172 . The other end of the tube  174  is positioned on a side of the chamber  166  that is remote from the fitting  171 . Although  FIG. 4  shows only a single tube  174 , it would alternatively be possible to have a plurality of tubes that are all coupled to the fitting  171 , and that each extend from the fitting  171  to a respective different location within the chamber  166 .  
         [0031]     When the disc  110  is surgically implanted in a patient, the parts  116  and  117  are initially capable of relative movement, due to the sliding engagement of the surfaces  131  and  132 . If necessary, at a later time, a material can be injected into the chamber  166  in a fluid state, through the fitting  171  and the tube  174 . The material then hardens within the chamber  166 . The engagement of this hardened material with the peripheral surfaces of the parts  116  and  117  serves to fix the parts  116  and  117  against relative movement. The injection of this material is carried out in a minor surgical procedure that is similar to the procedure already described above in association with the embodiment of  FIGS. 1-2 . Accordingly, to avoid redundancy, the surgical procedure is not described again here.  
         [0032]     In a not-illustrated variation of the embodiment of  FIGS. 1 and 2 , a lubricant is provided within the disc  10  at the time it is initially manufactured. In particular, after the disc  10  has been substantially fully assembled, and after it has been sterilized, one of the plugs  41  and  42  is installed in one of the openings  31  and  32 , and then a lubricant is introduced through the other of the openings  31  and  32 . The lubricant may be any suitable material,. such as saline, hyaluronic acid, mineral oil, or the like. The other of the plugs  41  and  42  is then installed in the other opening.  
         [0033]     Later, when it becomes necessary to introduce a material such as cement into the chamber  66 , there will be a need to remove most or all of the lubricant that is in the chamber  66 . In that event, the fitting  71  and the tube  90  may each have two passageways, one of which carries the material that is being injecting into the chamber, and the other of which allows the lubricant to escape from the chamber. With respect to the passageway for the material being injected, the disc  10  would include a tube similar to that shown at  174  in  FIG. 4 , so that the injected material is introduced on a side of the chamber  66  remote from the fitting  71 . As the injected material progressively fills the chamber  66 , it forces the lubricant to progressively flow to the fitting  71 , and then out through the extra passageway in the fitting  71  and tube  90 .  
         [0034]      FIG. 5  is a diagrammatic perspective view, partly in section, of an intervertebral disc  210  that is an alternative embodiment of the intervertebral disc  110  of  FIGS. 3 and 4 .  FIG. 6  is a diagrammatic view similar to  FIG. 5 , but showing a different operational position of the disc  210 . The disc  210  includes two parts  216  and  217  that are generally similar to the parts  116  and  117  described above in association with  FIGS. 3 and 4 , except for the differences discussed below.  
         [0035]     The part  216  has an approximately rectangular recess  223  in the center thereof. A cylindrical hole extends horizontally through the part  216 , and has two portions  224  and  225  of different diameter. The portion  224  is of smaller diameter than the portion  225 , and communicates at its inner end with the recess  223 . The outer end of the portion  225  opens through an exterior surface of the part  216 . The part  217  has an upwardly projecting post  251 , and an opening  252  extends horizontally through the upper end of the post  251 .  
         [0036]     The disc  210  includes a pin  253  that is axially slidably disposed within the opening  224  and  225  in the part  216 . The pin  253  has an annular groove near its inner end. A coil spring  256  encircles the pin  253 , and resiliently urges the pin  253  to move axially outwardly. The recess  223  in the part  216  is filled with a material  258 . As shown in  FIG. 5 , the material  258  engages the groove  254  in the pin  253 , and prevents the pin  253  from being moved axially outwardly by the spring  256 .  
         [0037]     In the embodiment of  FIGS. 5 and 6 , the material  258  is a material that is commercially available under the tradename TERFENOL-D from Etrema Products, Inc. of Ames Iowa. Normally, the material  258  is relatively rigid. However, when subjected to an appropriate field of electromagnetic energy, the material  258  undergoes a shape change. This permits the spring  256  to move the pin  253  outwardly to the position shown in  FIG. 6 , where the outer end of the pin  253  engages the opening  252  in the post  251  on the part  217 . This mechanically locks the parts  216  and  217  against any relative movement, even after the electromagnetic field is removed and the material  258  returns to its original shape. The electromagnetic field can be applied to the material  258  without any need to make any incision in the patient.  
         [0038]     Instead of the TERFENOL-D product discussed above, the material  258  could alternatively be any other suitable material that. can transition between two states, such as hard and soft states. For example, the material  258  could be a polyethylene material having an electrically conductive part embedded in it. When subjected to a rapidly varying magnetic field, an electric current is induced in the electrically conductive part and causes it to heat up, which in turn heats the polyethylene in order to soften it sufficiently so that the pin  253  is released.  
         [0039]     Although selected exemplary embodiments have been disclosed above in detail, many modifications and variations are possible. For example, it would alternatively be possible to provide a disc having a cam or other mechanical element that can be selectively manually moved between two positions in which it respectively permits and obstructs relative movement of two parts. As another alternative, a mechanical element that is not initially present in the disc could be selectively manually inserted in order to obstruct relative movement of two parts. Persons skilled in the art will readily appreciate that many other modifications and variations are possible without departing from the spirit and scope of the invention, as defined by the claims that follow.  
         [0040]     The foregoing description uses spatial references such as “horizontal,” “vertical,” “top,” “upper,” “lower,” “bottom,” “left,” and “right”, in relation to orientations that are shown in the drawings. These spatial references are used for purposes of convenience, and are not intended to limit the scope of protection provided by the claims that follow. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures.