Abstract:
An implant retention device is provided to assist in restraining movement of a nuclear implant and to assist in preventing expulsion of the nuclear implant through an incision portal or defect in the annular wall. In one form, the implant retention device comprises an expulsion prevention member associated with the nuclear implant and is configured to transition between an unexpanded position and an expanded position. In another form, a method for restraining a nuclear implant includes cutting an opening in the annulus, shifting an implant retention device into an unexpanded position, inserting the implant retention device through the opening, and shifting the retention device to an expanded position.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/871,641, filed Dec. 22, 2006, and U.S. Provisional Application No. 60/948,273, filed Jul. 6, 2007, both of which are hereby incorporated by reference as if reproduced herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to artificial intervertebral implants and devices for securing and retaining the implant in an intervertebral space. 
     BACKGROUND OF THE INVENTION 
     The most common orthopedic condition for which professional medical treatment is sought is lower back pain. Although many factors may be responsible for causing lower back pain, a principal factor is damage or degeneration of an intervertebral spinal disc resulting in impingement on the nerve system, specifically the spinal cord, located within the spine. Such impingement may result in, for instance, loss of mobility, urinary and fecal incontinence, and sciatica or pain experienced in the extremities. 
     Damage to or degeneration of a spinal disc can result from a number of factors such as abuse or age. The disc itself is composed primarily of an annulus and a nucleus contained therein. The annulus is a fibrous annular piece that attaches to the adjacent vertebrae and contains the nucleus, which is in turn a gel-like viscous material capable of shock absorption and flowable to permit poly-axial rotation and resilient compression of the vertebrae and spine. Most frequently, disc degeneration results from damage occurring to the annulus such that the flowable nucleus material may leak or seep out of the annulus. Disc degeneration also can occur in other ways, such as by being deprived of nutrient flow leading to a dried and susceptible to damage disc. Because the nuclear material is flowable, extensive damage to the annulus is not necessary for leakage to occur. 
     A recent, though not new, development for spinal surgery is a procedure known as disc arthroplasty for restoring or reconstructing the disc using a prosthesis to replace a portion or entirety of the damaged disc. The primary objective of disc arthroplasty is to restore or maintain the normal disc anatomy and functions, while addressing and treating the causes of the pain. 
     Two types of prostheses for disc arthroplasty are currently believed to merit further development by medical science and research. One type is a total disc prosthesis, or TDP, where the entire spinal disc is replaced after radial discectomy. A typical TDP includes structures that together mimic the properties of a natural disc. 
     The other type is a disc nucleus prosthesis, or DNP, that is used to replace only the nucleus of a spinal disc after a nucleotomy while retaining the annulus of the disc and, possibly, the end plates intact. As discussed above, failure of the natural disc does not require extensive damage to the annulus, and the annulus would often be capable of retaining a non-flowing prosthetic nucleus. Implantation of a DNP involves clearing of the natural nucleus from the annulus through the procedure known as nucleotomy, and inserting the DNP within the annulus. Accordingly, disc nuclear prostheses (DNPs) are typically smaller and require less extensive surgery than TDPs do. 
     An issue related to DNPs is implant extrusion, defined as the tendencies for an implant not to remain seated, and for the implant to back out of its intended seat in the nuclear space. To prevent this, many designs for disc implants attempt to secure to the end plates of the vertebrae by providing securement features on the implant. The nuclear implants may have one or more restraining features, such as, for example, keels or other implant protrusions that seat into the bone, apertures integrated into the implant for bone in-growth such as a porous surface or coatings, or screws to screw the implant to the bone. These and other similar features restrain the implant in a predetermined orientation to the surrounding boney bodies to thereby properly support the skeletal structure and prevent damage of any soft tissues. These features, however, may violate the integrity of the end plates to a degree where revision surgery is limited. Violation of the vertebrae by the securement may cause bleeding, or calcification of the end plate, either of which can result in pain, loss of mobility, necrosis, or deterioration of any implant device. 
     Some arthroplasty devices are designed to float or sit unrestrained within a ligamentous joint capsule. These devices may rely purely on the soft tissue holding the replacement device in the predetermined position. An unrestrained intervertebral artificial nucleus device would benefit from an intact annulus to secure the implant in the predetermined position and prevent its expulsion into the sensitive nerve structure located just outside the annulus. The health of the annulus, however, is often compromised through the degenerative disc disease process and may not be intact. The annulus may have tears or may be poorly nourished and weak such that it cannot adequately serve by itself to restrain the nucleus replacement device within the confines of the annulus. Additionally, the annulus is typically at least partially incised during surgery to make an opening for removal of the diseased nucleus material and to serve as a window for placing the nucleus replacement device in its predetermined position. It is possible for this window to serve as an undesired expulsion portal for the nucleus implant. 
     For these and other reasons, the implant retention devices described herein may be utilized to assist in the retention of a nuclear implant, particularly those that do not have other restraining features, in a predetermined skeletal relationship. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the present invention, an implant retention device is provided to assist in restraining movement of a nuclear implant and to assist in limiting expulsion of the nuclear implant through an incision portal or defect in the annular wall. Generally, an implant is provided with an implant retention device including a movable or expandable blocking member disposed on the implant. The implant retention device has an unexpanded or compressed orientation, wherein the blocking member is not expanded, and the implant has a compact configuration or size capable of fitting through an annular wall opening. The implant retention device also has an expanded configuration, wherein the blocking member is expanded, and the implant has a second, larger contour or size, such that the implant is kept from being expelled from the nuclear space through the annular wall opening. The unexpanded orientation is intended to allow the implant to be inserted through the annular opening and into the nuclear space. Once the implant is implanted in the nuclear space, the implant retention device may be shifted to the expanded orientation, wherein the expandable member is actuated or allowed to expand. The implant contour or size when the implant retention device is in the unexpanded orientation may be slightly larger than the annular wall opening, as the opening may stretch somewhat during insertion of the implant. However, when the implant or expandable member is referred to herein as being smaller or having a span less than that of the annular opening, it is implied that the implant&#39;s or expandable member&#39;s span is less than the stretched opening. In addition, the size or contour of the implant with the retention device in the expanded orientation is configured to keep the implant from backing through the annular wall opening, despite any stretching of the opening. In effect, the surfaces of the movable or expandable member will interact with the annulus or other spinal tissues in the intervertebral space, such as a vertebral endplate, thereby interfering with movement of the implant through the annular opening. The movable or expandable member may be integrated with the implant, or may be attached separately as described below. In addition, the movable or expandable member may be made from a resilient material, such that it expands due to its own internal forces when released from a compressed position, or alternatively, the expandable member may be moved due to external forces acted thereon, such as those initiated by a surgeon or by a compressed spring. 
     In one embodiment, an implant device may comprise a pair of resilient prongs disposed on opposing sides of the nuclear implant. The prongs are configured to be shifted from a compressed position with a span less than the stretched opening in the annular wall to an expanded position with a span greater than the opening in the annular wall with the implant inserted through the annular wall opening to keep the implant in the nuclear space. The implant is preferably formed with an upper and lower portion, such as top and bottom shells, with the prongs optionally being disposed on either of the top or bottom shell of the implant. The prongs preferably extend beyond a trailing end of the implant toward the annular opening. By one approach, the pair of resilient prongs may be integrally formed in the implant. Alternatively, a mounting attachment may be provided that is configured to engage with the nuclear implant, wherein the resilient prongs are attached to the mounting attachment. The mounting attachment may comprise, for example, a band that engages with a corresponding groove or feature in the implant. The band may be rectilinear to engage with a rectilinear feature on the implant or may be curvilinear to surround at least a portion of the perimeter of the implant. By another approach, a pin may be inserted through a hole in the implant, with one of the pair of prongs being mounted on each end of the pin. The implant may be configured and arranged to accommodate the pair of prongs on each end of the implant. 
     In accordance with another aspect, a method is provided wherein an annular opening is formed in the annular wall by cutting the annular wall for insertion of a nuclear implant into a nuclear space. The implant retention device is shifted to an unexpanded position relative to the implant such that the implant and implant retention device together have a size that is insertable through the annular opening. The implant retention device is then inserted through the annular opening in the unexpanded position. The implant retention device is then shifted to an expanded position such that the implant retention device has an expanded size greater than the size of the annular opening to keep the nuclear implant from being expelled from the nuclear space through the annular opening. The implant retention device may be disposed on the nuclear implant, such as a pair of opposing resilient prongs disposed on a trailing end of the implant, wherein an unexpanded or compressed position is formed by compressing together end portions of the prongs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a first embodiment of an implant retention device secured to a nuclear implant and inserted into a nuclear space of a spinal section; 
         FIG. 2  is a partial side view of the implant retention device, nuclear implant, and spinal section of  FIG. 1  taken along line  2 - 2  thereof; 
         FIG. 3  is a cross-sectional top view of the spinal section of  FIG. 2  taken along line  3 - 3  thereof and showing the nuclear implant and the implant retention device; 
         FIG. 4  is an exploded perspective view of the implant retention device and nuclear implant of  FIG. 1 ; 
         FIG. 5  is a partial cross-sectional top view of the implant retention device and nuclear implant of  FIG. 1 ; 
         FIG. 6  is a perspective view of a second embodiment of an implant retention device secured to a nuclear implant and inserted into a nuclear space of a spinal section; 
         FIG. 7  is a partial front view of the implant retention device, nuclear implant, and spinal section of  FIG. 6 ; 
         FIG. 8  is a partial side view of the implant retention device, nuclear implant, and spinal section of  FIG. 7  along line  8 - 8  thereof; 
         FIG. 9  is a cross-sectional top view of spinal section of  FIG. 8  along line  9 - 9  thereof and showing the implant retention device and nuclear implant; 
         FIG. 10  is a bottom view of a top shell of the nuclear implant of  FIG. 6  showing the implant retention device secured thereto; 
         FIG. 11  is a perspective view of a third embodiment of an implant retention device secured to a nuclear implant and inserted into a nuclear space of a spinal section; 
         FIG. 12  is a partial front view of the implant retention device, nuclear implant, and spinal section of  FIG. 11 ; 
         FIG. 13  is a partial side view of the implant retention device, nuclear implant, and spinal section of  FIG. 12  along line  13 - 13  thereof; 
         FIG. 14  is a cross-sectional bottom view of the spinal section and a top shell of the nuclear implant of  FIG. 12  along line  14 - 14  thereof and showing the implant retention device secured thereto; 
         FIG. 15  is a perspective view of the implant retention device of  FIG. 11 ; 
         FIG. 16  is a side view of the top shell of the nuclear implant of  FIG. 12 ; 
         FIG. 17  is a bottom view of the top shell of the nuclear implant of  FIG. 16  taken along line  17 - 17  thereof; 
         FIG. 18  is a perspective view of a fourth embodiment of an implant retention device secured to a nuclear implant and inserted into a nuclear space of a spinal section; 
         FIG. 19  is a perspective view of the implant retention device of  FIG. 18 ; 
         FIG. 20  is a partial side view of the implant retention device, nuclear implant, and spinal section of  FIG. 18 ; 
         FIG. 21  is cross-sectional top view of spinal section of  FIG. 20  along line  21 - 21  thereof and showing the nuclear implant with the implant retention device attached thereto; 
         FIG. 22  is a perspective view of a fifth embodiment of an implant retention device secured to a nuclear implant and inserted into the nuclear space of a spinal section; 
         FIG. 23  is a partial left side view of the implant retention device, nuclear implant, and spinal section of  FIG. 22 ; 
         FIG. 24  is a cross-sectional top view of the spinal section of  FIG. 23  along line  24 - 24  thereof and showing the nuclear implant and implant retention device attached thereto; 
     
    
    
     DETAILED DESCRIPTION 
     Generally speaking, pursuant to these various embodiments, implant retention devices are disclosed herein, with each device directed to maintaining a nuclear implant in position within a nuclear space and limiting the implant from backing out through an opening in the annulus. It shall be understood that retention refers to retaining, restraining, controlling, or maintaining the implant within the nuclear space to limit the expulsion of the implant out of the nuclear space through the annular opening. Referring now to the drawings, and in particular to  FIGS. 1-3 , a first embodiment of an implant retention device  1000  is shown. A nuclear implant  50  is inserted through an annular opening  30  in the wall of the annulus  22  and positioned within a nuclear space  24  of a disc  26  located between adjacent upper vertebra  42  and lower vertebra  44 . The annular opening  30  is typically created during surgery prior to implantation of the nuclear implant  50  to serve as a portal for removing the nucleus and as a surgical window for inserting the implant  50  into the nuclear space  24 . However, the annular opening  30  can also be formed from a tear or weakening of the annulus  22 . The annular opening  30  generally extends from the exterior of the annulus  22  through to the nuclear space  24 . The implant retention device, for this and other implant retention device embodiments disclosed herein, is generally shown being used in conjunction with a nuclear implant  50  comprising a top shell  52  and a bottom shell  54 . Each shell  52 ,  54  has a peripheral shape of an oval or racetrack shape, such that the implant has a length along a longitudinal axis and a width shorter than the length transverse to the longitudinal axis. A concave recess is formed in the top shell  52  and a corresponding dome surface is formed in the bottom shell  54 , with the dome surface being received in the concave recess to allow for relative translational motion and movement between the top shell  52  and the bottom shell  54 . The nuclear implant  50  shown and described herein is used as an illustrative example, with other configurations of nuclear implants or nuclear replacement devices being contemplated for use in conjunction with the implant retention devices discussed herein. 
     The implant retention device may be in the form of a feature or features such as a blocking member formed on or attached to the implant that prevent the expulsion of the implant from the joint capsule. For example, a nuclear implant or replacement device may be modified to include one or more resilient prongs as part of the upper or lower components of the nuclear implant. In the following embodiments utilizing the prong feature, the prongs are illustratively shown as being mounted or secured to the top shell  52  of the implant  50 . It should be noted that the prong feature of the implant retention device may alternatively be incorporated into the bottom shell. By having the prongs secured to either of the top or the bottom shell, the basic function of the nuclear implant is not interfered with or impeded. The prongs are preferably mounted directly to the nuclear implant. Therefore, once the implant is inserted, movement restraint is provided without the need for an additional procedure for installation of the implant retention device. 
     Referring now to  FIGS. 1-5 , an embodiment of an implant retention device  1000  is shown having a pair of resilient arcuate prongs  1008  secured to the top shell  52  of a nuclear implant  50 . The prongs  1008  may be formed of a resilient material, such as a polymer or a resilient metal, but may be made of any other known metals, ceramic, plastic, composite material, or elastomer. The prongs  1008  may be fastened or otherwise attached to one or more sides of the nuclear implant  50 . The prongs  1008 , as shown, are attached to each lateral side  60 ,  62  of the top shell  52  of the implant  50  at a point generally between the center and trailing end  56  of the implant  50 . The top shell  52  of the implant  50  has a generally oval-shaped perimeter; however, indented portions  1004  may be formed in the opposing lateral sides  60 ,  62  of the top shell  52  perimeter to accommodate the prongs  1008 , as shown in  FIGS. 4 and 5 . The indented portions  1004  allow an end portion  1010  of each prong  1008  to mate with the top shell  52  and sit flush thereagainst. A through hole  1012  is formed in the top shell  52 , extending between the opposing indented portions  1004 . A pin  1014  is inserted in the through hole  1012 , with the pin  1014  sized such that mount end portions  1030  of the pin  1014  extending beyond the sides  60 ,  62  of the implant  50  such that each prong  1008  can be mounted onto the mount end portions  1030  of the pin  1014 . Each prong  1008  has a hole  1022  therethrough at the end  1010  to receive the pin  1014 . As shown, when each prong  1008  is disposed on the pin  1014 , an end portion  1032  of the pin  1014  extends beyond the outer edge of each prong  1008 . A snap ring  1028  or other securing mechanism (not shown) may be affixed to the exposed end of the pin  1014  to secure the  1008  prong to the top shell  52 . The snap ring  1028  thus substantially prevents the prong  1008  from slipping off of the pin  1014  and maintains the prong  1008  in position against the top shell  52 . The prongs  1008  are sized in length to extend beyond the trailing end  56  of the implant  50  once mounted to the implant  50 . The arcuate prongs  1008  curve away from the implant  50  to give the implant  50  a greater width at the trailing end  56  thereof. 
     The prongs  1008  are secured adjacent the trailing end  56  of the implant  50  and positioned adjacent the annular opening  30  upon insertion of the implant  50  into the nuclear space  24 . Prior to insertion of the implant  50 , the resilient prongs  1008  are compressed in at the ends  1016  such that the prongs  1008  move toward each other. By compressing the prongs  1008 , the ends  1016  of the prongs  1008  move inward to reduce the span of the prongs  1008  such that the prongs  1008  extend generally linearly from the implant  50  or curve in toward the implant  50 . With the prongs  1008  in the compressed position, the implant  50  is then able to fit through the annular opening  30  such that the implant  50  can be inserted into the nuclear space  24 . The prongs  1008  may be compressed by an insertion instrument or tool, such that the prongs  1008  are shifted from an expanded configuration to a compact configuration. Once the implant  50  is inserted, the instrument is operable to shift the prongs  1008  from a compact configuration to an expanded configuration. Alternatively, the retention device, in the form of a blocking member, may be shifted from a compact configuration to an expanded configuration by numerous other methods, such as using a temporary adhesive to hold the member in the compact configuration until after the retention device is inserted into the nuclear space  24 . In another form, the blocking member may be held by a string or wire disposed thereabout, such that the retention device may be shifted to an expanded configuration simply by pulling or removing the string or wire. In yet another form, the blocking member may be formed out of a heat-activated shape memory material, such as Nitinol, such that the blocking member is in the compact configuration at room temperature, but is shifted to an expanded configuration when subjected to higher temperatures, such as body temperature. 
     After the implant has been inserted, the prongs  1008  are released to expand to their original position curved away from the implant  50 , thus giving the implant  50  a greater width at the trailing end  56  adjacent the annular opening  30 . The prongs  1008  will expand outward to a diameter or span greater than size of the annular opening  30 . The implant  50  is incapable of recompressing the prongs  1008 . In addition, any forces exerted on the prongs  1008  from the inner wall of the annulus  22  are incapable of recompressing the prongs  1008 . If the implant  50  has forces within the nuclear space  24  to force it out towards the annular opening, the flexible prongs  1008  will stop the implant  50  from being expelled through the annular opening  30  due to the increased span of the implant  50  provided by the prongs  1008  and interference of the prongs  1008  with the inner annular wall. 
     By another approach, the prongs may be part of a component which snaps to the perimeter of the top shell of the implant. Referring now to  FIGS. 6-10 , an additional embodiment of the implant retention device  1100  is shown. In this embodiment, a pair of arcuate resilient prongs  1108  are integral with a band  1110  configured to engage with the perimeter of the top shell  52  of the implant  50 . The prongs  1108  and band  1110  may be formed of a resilient material, such as a polymer or a resilient metal, but may be made of any other known metals, ceramic, plastic, composite material, or elastomer. The band  1110  includes a linear portion  1112  that extends between the pair of prongs  1008 , with the linear portion  1112  fitting into a groove  1120  on the underside  64  of the top shell  52  of the implant  50 . The linear portion  1112  extends across the width of the underside  64  of the top shell  52  of the implant  50  at a point generally between the center and trailing end  56  of the implant  50 . The band  1110  further includes a curvilinear portion  1140  that continues from one end  1124  of the linear portion  1112  and extends around the outer perimeter of a portion of the top shell  52  to the other end  1126  of the linear portion  1112 . The curvilinear portion  1140  preferably extends around approximately two-thirds of the perimeter of the top shell  52 , including the leading end  58  of the implant  50 , following the oval-shaped contour of the top shell  52  of the implant  50 . 
     A perimeter groove  1130  is formed around a portion of the perimeter of the top shell  52  to accommodate the curvilinear portion  1140  of the band  1110 . The curvilinear portion  1140  nests in the groove  1130  such that the curvilinear portion  1140  preferably does not extend beyond the lateral sides  60 ,  62  of the top shell  52 . Preferably, opposing indentations  1104  are formed in the top shell  52  to accommodate the prongs  1108  of the implant retention device  1100 . The indentations  1104  receive an end  1118  of the prong  1108  such that the prong  1108  mates with the top shell  52  and the end  1118  of the prong  1108  sits flush against an exterior surface of the top shell  52 . As shown in  FIG. 6 , the prongs  1108  curve away from the implant  50  and have a greater height than the curvilinear portion  1140  and linear portion  1112  of the band  1110 . The prongs  1108  are sized to extend beyond the trailing end  52  of the implant  50  once mounted to the implant  50 . Prior to insertion of the implant  50 , the resilient prongs  1108  are compressed in at the ends  1116  such that the prongs  1008  move toward each other to reduce the span of the prongs  1108  such that the implant  50  can be inserted through the annular opening  30 . Upon insertion, the prongs  1108  extend from the trailing end  56  of the implant  50  toward the annular opening  30 . When the prongs  1108  are released from the compressed position, the prongs  1108  have a span greater than the size of the annular opening  30  to prevent the implant  50  from being expelled through the annular opening  30 . Again, any forces exerted on the prongs  1108  from the inner wall of the annulus  22  are incapable of recompressing the prongs  1108 . 
     By another approach, the resilient prongs may be part of a component which snaps onto an underside of the top shell. Referring now to  FIGS. 11-17 , such an embodiment is shown. The implant retention device  1200  is generally comprised of a pair of opposed prongs  1208  that are integral with a generally rectilinear band  1210 , with the band  1210  snapping around a corresponding rectilinear feature  70  on the underside  64  of the top shell  52 . The prongs  1208  and band  1210  may be formed of a resilient material, such as a polymer or a resilient metal, but may be made of any other known metals, ceramic, plastic, composite material, or elastomer. 
     The concave recess  72  on the underside  64  of the top shell  52  is formed within a generally rectilinear projection  70  from the underside  64  of the top shell  52 . The rectilinear projection  70  includes grooves  1212 ,  1224  on opposing sides  72 ,  74  to retain opposing sides  1214 ,  1216  of the rectilinear band  1210 . The other pair of opposing sides  1218 ,  1228  of the band  1210  are positioned within a groove  1222 ,  1242  formed between an edge  76 ,  78  of the rectilinear projection  70  and opposing ridges  1220 ,  1240  extending along the lateral sides  60 ,  62  of the top shell  52 . The lateral sides  60 ,  62  of the top shell  52  have a pair of opposing indentations on either side of the opposing ridges  1220 ,  1240 , with a first pair of opposing indentations  1230  adjacent the leading end  58  of the implant  50  and a second pair of opposing indentations  1232  adjacent the trailing end  56  of the implant  50 . The indentations  1230 ,  1232  accommodate the prongs  1208 , such that exterior surfaces of the end portion  1236  of the prongs  1208  are secured in mating contact against the indentation portion  1230 ,  1232  of the top shell  52 . When the band  1210  is snapped into the grooves  1212 ,  1224 ,  1222 ,  1242 , the prongs  1208  are positioned along opposing lateral sides  60 ,  62  of the top shell  52  and extend beyond the trailing end  56  of the implant  50 . The arcuate prongs  1208  are positioned such that they curve away from each other and have a span greater than the width of the annular opening  30  to prevent the implant  50  from backing out of the nuclear space  24 . Again, the prongs  1208  are resilient such that they can be compressed to reduce the span for insertion through the annular opening  30  and into the nuclear space  24 . The prongs  1208  are then released and allowed to expand to a span greater than the span of the annular opening  30  such that the implant is not expelled through the annular opening  30 . In this embodiment, the top shell  52 , including the features used to accommodate and secure the band  1210  and prongs  1208 , is preferably symmetrical, such that the band  1210  may be secured with the prongs  1208  positioned on either end of the implant  50 . As a result, the surgeon may attach the prongs  1208  to either end of the implant  50 , depending on the side of surgical approach. 
     Referring now to  FIGS. 18-21 , another embodiment is shown in which the prongs may be secured to either of the lateral ends of the implant, with each end of the implant being configured to receive the resilient prongs. The implant retention device  1300  includes a linear portion  1310  extending between a pair of opposed prongs  1308 . The prongs  1308  and linear portion  1310  may be formed of a resilient material, such as a polymer or a resilient metal, but may be made of any other known metals, ceramic, plastic, composite material, or elastomer. A groove  1312  is formed on the underside  64  of the top shell  52  adjacent the trailing end  56  for receiving and retaining the linear portion  1310 , with the prongs  1308  extending past the trailing end  56  of the implant  50 . The linear portion  1310  extends across the width of the underside  64  of the top shell  52  and snaps into one of the grooves  1312  to secure the implant retention device  1300 . An identical groove (not shown) is formed on the underside  64  of the top shell  52  adjacent the leading end  52  for receiving the linear portion  1310 , such that the prongs  1308  can alternatively extend past the leading end  52  of the implant  50 . The top shell  52 , including the features used to accommodate and secure the linear portion  1310  and prongs  1308 , is symmetrical, such that the linear portion  1310  may be secured in either groove, with the prongs  1308  positioned on either end of the implant  50 . Although the implant retention device  1300  is shown mounted in groove  1312 , it is understood that the groove on the other side of the implant  50  may be utilized such that the prongs  1308  may attach to either end of the implant  50  depending on the side of surgical approach. The prongs  1308  are integrally formed with the linear portion  1310 , such that when the linear portion  1310  is secured in the groove  1312  the prongs  1308  are positioned on opposing sides  60 ,  62  of the top shell  52 , with the prongs  1308  adjacent the annular opening  30  and extending beyond the trailing end  56  of the implant  50 . Each prong  1308  has a through hole  1322  on the end  1318  adjacent the top shell  52 , with a corresponding receiving projection  1320  integral with the top shell  52  to facilitate attachment of the prongs  1308  to the top shell  52 . As shown, the receiving projections  1320  are located on indented portions  1314 ,  1324  in the lateral sides  60 ,  62  of the outer oval-shaped perimeter of the top shell  52  implant  50 . The top shell  52  includes two pairs of opposing indented portions  1314 ,  1324 , with a first pair of opposing indented portions  1314  located adjacent the leading end  58  of the implant and a second pair of opposing indented portions  1324  located adjacent the trailing end  56  of the implant. A receiving projection  1320  projects from the outer side surface of each of the pairs of indentations  1314 ,  1324 . The arcuate resilient prongs  1308  curve away from the implant  50  and have an uncompressed span greater than the span of the annular opening  30 . The prongs  1308  are then compressed for insertion of the implant  50  through the annular opening  30  and then allowed to expand within the nuclear space  24  to prevent the implant  50  from being expelled through the annular opening  30 . 
     As a further embodiment of an implant restraint device  1400 , resilient prongs  1408  may be machined into a shell of the implant, such that the prongs  1408  are integral with the body of the implant  50 . Referring now to  FIGS. 22-24 , the prongs  1408  are located in generally the same location as the above-described embodiments. Again, the prongs  1408  extend beyond the trailing end  56  of the implant  50  toward the annular opening  30 , the prongs  1408  having a span wider than the width of the annular opening  30  to substantially prevent any backout of the implant  50  through the annular opening  30 . The prongs  1408  are integral with the top shell  52  of the implant, rather than being formed from an additional component, such as described in the above embodiments. The prongs  1408  are generally formed from the same material as the implant  50 , and may be manufactured from any known metals, ceramic, plastic, composite material, or elastomer. 
     The implant retention devices and features as described herein may be adapted for use with a variety of artificial joint arrangements other than nuclear implants. In addition, the implant retention devices and features as described herein may be adapted for use with a variety of surgical approaches. Most of the surgical approaches shown in the illustrations are from an anterior or lateral approach but are easily adaptable for a posterior approach, for example. In a posterior approach, an incision portal is made in the posterior annulus. 
     Those skilled in the art will recognize that a wide variety of modifications, alterations, and combinations can be made with respect to the above described embodiments without departing from the spirit and scope of the invention, and that such modifications, alterations, and combinations are to be viewed as being within the ambit of the inventive concept.