Abstract:
A medical device implant made with a shape-memory material is originally produced in a “permanent” configuration. The implant is then “programmed” into a temporary (typically smaller sized) configuration to facilitate implantation, after which, an external stimulus activates the implant to return to its permanent configuration. Apparatus and methods include inserting a first portion of the implant into an aperture of a compression fixture base, heating the implant, and then driving and compressing the remaining portion of the implant, which has a different, typically larger profile than that of the first portion, into the aperture with a compression fixture cover to “program” the remaining portion. The resulting programmed implant has first and remaining portions that have identical, or substantially identical, profiles.

Description:
FIELD OF THE INVENTION 
     The invention relates to medical device implants made from a shape memorizing polymer material that can be used as a base in osteo-implants and other in vivo surgical procedures requiring a stable locking base. More particularly, the invention relates to apparatus and methods of “programming” the medical device implant from a first permanent state into a second temporary state to facilitate deployment of the implant. After deployment, the medical device implant can be activated to change back into the first state. 
     BACKGROUND OF THE INVENTION 
     Shape-memory materials have the ability to change from a permanent or desired shape into a temporary transitional shape and then back into the permanent or desired shape. Shape-memory materials are stimuli-responsive in that they can change shape upon application of an external stimulus. These solid materials are initially formed into a “permanent” shape or configuration suited for their ultimate use. These materials can then be transformed into a transitional shape to facilitate, for example, implantation. Once implanted, an external stimulus (e.g., heat, light, chemical) can be applied to the material to transform the material back into its permanent shape or configuration. This process involves “programming” the shape-memory polymer from its permanent shape into a temporary shape and then “recovering” the permanent shape from the temporary shape. 
     In view of the advantages such materials provide, medical device implants are increasingly made with shape-memory materials. Typically, shape-memory materials are used so the implant can be temporarily reduced in size, thus requiring a smaller surgical entry site, and in the case of bone implants, smaller drilled holes in the bone. Smaller surgical entry sites and drilled holes lessen the invasiveness of the procedure and shorten recovery time. 
     When mass producing medical device implants made with shape-memory materials, a programming process should be conducted such that the devices are uniformly produced in their temporary shape without adversely affecting the devices&#39; ability to transition back from the temporary shape to the permanent shape. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide apparatus and methods of programming a medical device implant manufactured from a shape-memory polymer that results in consistently uniform products that retain their ability to effectively transition from the temporary shape to the permanent shape. 
     An example of a medical device implant manufactured from a shape-memory polymer that can be programmed by the invention has a first section or main body defining a central longitudinal axis there through. A second section comprising a pair of legs extends outward from the main body at respective angles to the central axis, forming a winged medical device implant. An interior channel is defined by the first and second legs and the main body. The interior channel opens to the ambient environment at the distal end of the first and second legs. 
     A compression fixture constructed in accordance with the invention is used to program the medical device implant. The compression fixture includes a base and a cover. The base has an aperture for receiving the medical device. The aperture has a cross section generally equal to the cross section of the main body of the medical device. The main body is positioned in the aperture such that the legs extend outward from the top of the aperture. The cover of the compression fixture is then positioned over the base. The cover has an engaging member that contacts the main body through the channel between the first and second legs. The compression fixture is then heated to a predetermined temperature for a predetermined period of time such that the polymeric material can be deformed without fracturing. The cover and base of the compression fixture are then compressed together (i.e., moved towards each other), causing the engaging member to drive the medical device further down into the aperture. This forces the first and second legs towards each other and into the aperture such that the legs are ideally, or at least substantially, parallel to the longitudinal axis. The compression fixture is then cooled, whereupon the cover and base are separated from each other. The “programmed” medical device can then be removed from the aperture in the base. The programmed medical device will retain the temporary “programmed” shape of the first and second legs compressed toward the longitudinal axis until acted upon by an external stimulus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
         FIGS. 1A ,  1 B, and  1 C illustrate elevational, top, and bottom views, respectively, of an embodiment of a medical device implant in its permanent state prior to being programmed; 
         FIG. 2  illustrates an elevational view of an embodiment of the medical device implant in its temporary state after being programmed; 
         FIG. 3  illustrates an embodiment of a base of a compression fixture loaded with medical device implants prior to being programmed according to the invention; 
         FIGS. 4A and 4B  illustrate an embodiment of a cover of a compression fixture according to the invention; 
         FIG. 5  illustrates a loaded compression fixture prior to programming of the medical device implant according to the invention; 
         FIG. 6  illustrates the loaded compression fixture placed between compression plates of a compression machine within a thermal chamber according to the invention; 
         FIG. 7  illustrates the compression fixture after the cover and base have been compressed together according to the invention; 
         FIG. 8  illustrates the compressed compression fixture after removal from the compression machine and thermal chamber; 
         FIG. 9  illustrates the base of the compression fixture containing the programmed medical devices implants within respective apertures of the base after removal of the cover; 
         FIG. 10  illustrates the bottom surface of the base of the compression fixture; and 
         FIGS. 11A and 11B  illustrate alternative embodiments of the medical device implant in its temporary state after being programmed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention relates to apparatus and methods of programming a medical device implant made with a shape-memory material. In particular, the medical device implant is manufactured from a shape-memory polymer and one example is a cross-linked methyl methacrylate (MMA) polymer that uses Memori™ 7111. 
     In one embodiment, the invention is used to program a medical device implant that is a “push-in” bone suture anchor. The suture anchor is used in various procedures for fixation of suture to bone in the shoulder, foot/ankle, knee, hand/wrist, and elbow. Such procedures include rotator cuff repair in the shoulder, medial collateral ligament repair in the knee, ulnar collateral ligament reconstruction in the hand/wrist, and tennis elbow repair. In each of these procedures, a hole is drilled into the bone and the medical device implant is positioned into the bone while in its programmed state. A stimulus (e.g., the heat from the body) activates the implant to return (e.g., expand) to its permanent shape, conforming the implant to its surroundings, which creates a stable locking base. This provides stronger fixation with less tissue damage than conventional means, such as, for example, surgical nails. 
       FIGS. 1A-1C  show winged medical device implant  10 , which is a bone suture anchor that can be used in orthopedic procedures. Medical device  10  has a left leg  12  and right leg  14  which both extend from a main body  16 . Main body  16  has a central longitudinal axis  20  and a width  22 . Extending through main body  16  is an optional through-bore  201 . Left leg  12  and right leg  14  extend downward and outward from main body  16  at respective angles Ø 1  and  0 Ø 2  formed with axis  20 . The angles are preferably equal and preferably each about 35°, but may range from about 30° to 40°. Each leg extends outward beyond the peripheral profile of main body  16  as viewed from the top of main body  16  in the direction of axis  20 . A channel  17  and interior space  19  are defined between left leg  12 , right leg  14 , and main body  16 . As shown in  FIG. 1C , each leg  12 ,  14  has an optional semi-circular groove  125 ,  145  on its inner face in channel  17  that extends from the end of the leg to interior space  19 . This configuration of medical device implant  10  illustrates its permanent shape which, as shown, forms an inverted Y-shaped structure. Alternatively, other shapes are possible depending on the application. 
       FIG. 2  illustrates medical device  10  in its programmed state. In this state, left leg  12  and right leg  14  have been programmed to be generally aligned with main body  16  such that the outer periphery of main body  16  and left leg  12  form a substantially, if not completely, common linear profile  13  on a left side of medical device  10 , while the outer periphery of main body  16  and right leg  14  form a substantially, if not completely, common linear profile  15  on a right side of medical device  10 . By providing such common profiles, deployment of medical device  10  in vivo can be made with a smaller incision and a smaller drilled hole in bone than if made with the legs of device  10  extended in their permanent position. In the programmed configuration, legs  12  and  14  are compressed together such that grooves  125  and  145  meet to create a through-bore  202  having a diameter preferably equal to the diameter of through-bore  201  in main body  16 . This creates a through-bore extending the entire length of programmed medical device  10 , which is used to load the programmed device onto or into certain implant/deployment tools. Note that all embodiments of medical device  10  do not require such a through-bore. 
     In the programmed state, a lateral through-hole  23  passing through medical device  10  is formed at interior space  19 . Through-hole  23  is preferably defined within medical device  10  by legs  12  and  14  and main body  16  and, in a preferred embodiment, is laterally aligned where legs  12  and  14  extend from main body  16  (i.e., at the crotch of legs  12  and  14 ). Alternatively, through-hole  23  may be positioned solely within, and may be defined solely by, main body  16 . Lateral through-hole  23  is used during the programming process (see further below). 
       FIG. 3  shows an embodiment of a base of a compression fixture for programming medical device implant  10  in accordance with the invention. Base  30  has a plurality of apertures  32  extending inward from top surface  34 . Each aperture is sized to mirror the cross section of main body  16  of medical device  10 . The depth of each aperture should be longer than medical device  10  as measured along longitudinal axis  20 . Depending on the pre-programmed shape of medical device  10 , apertures  32  may be cylindrical bores. Alternatively, apertures  32  may be of other shapes or configurations in accordance with the particular pre-programmed shape of medical device  10 . Apertures  32  also include slots  36  that extend down from top surface  34  and extend from one side of base  30  through to the opposite side. The depth of slots  36 , as measured down from top surface  34 , can be equal to or greater than the depth of aperture  32 , but in no case should the slots be less than that depth by more than the length of main body  16  as measured along the longitudinal axis. 
       FIG. 3  also shows the main body  16  of each medical device implant  10  inserted at least partially into a respective aperture  32 . In a preferred embodiment, apertures  32  are configured such that main body  16  engages the aperture walls (i.e., the inner surface of base  30  forming the perimeter of aperture  32 ). While tolerances may vary in the manufacturing of base  30  and apertures  32 , a tight fit is preferred between main body  16  and base  30  when main body  16  is received within aperture  32 . With main body  16  received within aperture  32  as shown, legs  12  and  14  of medical device  10  (in its permanent state) abut top surface  34  of base  30  at opposite perimeter edges of aperture  32  and extend outward and above top surface  34 . In this position, channel  17  is open to the ambient environment at the top of base  30 . 
       FIGS. 4A and 4B  show an embodiment of a compression fixture cover for programming medical device  10  in accordance with the invention. Cover  40  is a generally U-shaped structure having a left side  42 , a right side  44 , and a connecting top side  46 . The left and right sides are spaced apart from each other to form a channel  48  for receiving base  30 . Traversing the space between the left and ride sides of cover  40 , and carried by the left and right sides, are a plurality of engaging members  50 . In this embodiment, engaging members  50  are rods or pins that extend across and through the distal ends of left side  42  and right side  44 . Depending on the configuration of the medical device implant to be programmed, engaging members  50  alternatively may be other types of structures and their position across channel  48  may vary. Each engaging member  50  is received within a hole  43   a  in left side  42  and a corresponding hole  43   b  in right side  44 . Corresponding pairs of holes  43   a,b  are preferably aligned, and engaging members  50  preferably extend parallel to top side  46 . Each engaging member  50  is individually removable through holes  43   a,b.    
     Cover  40  also preferably includes a plurality of gage members  52  that extend into channel  48  from top side  46 . In this embodiment, gage members  52  are inserted into respective through-holes  45  in top side  46  and are pins or rods, but alternatively may be other types of structures depending on the configuration of the medical device to be programmed. Gage members  52  are received in through-holes  45  such that they do not protrude above top surface  47  of cover  40 . Each gage member  52  is long enough to contact and abut (i.e., sit on) a respective engaging member  50  without protruding above top surface  47 . Gage members  52  are not permanently attached to engaging members  50 , but may be permanently attached to cover  40  by any known means and, accordingly, may not require through-holes  45 . In this embodiment, each gage member  52  is oriented preferably perpendicularly to a respective engaging member  50 , forming an inverted T-shaped structure within channel  48 . 
       FIG. 5  shows cover  40  positioned over base  30  with channel  48  partially receiving base  30 . Engaging members  50  are received within respective channels  17  of medical device implants  10  (which were previously loaded into apertures  32  of base  30 ). In this position, engaging members  50  are preferably received within channels  17  deep enough to contact main body  16  at interior space  19  (i.e., the crotch of left and right legs  12  and  14 ). This allows cover  40  to maintain the position shown with respect to base  30 , wherein no part of cover  40  contacts any portion of left and right legs  12  and  14  extending above aperture  32 . The assembly of cover  40  and base  30  as shown may be referred to as a loaded compression fixture  55 . 
     To program the medical device, loaded compression fixture  55  is placed in a compression machine, which is positioned in a thermal chamber.  FIG. 6  shows compression fixture  55  placed in a compression machine  60  between compression plates  62  and  64 . A suitable compression machine is a Model No. 5567-Q5039 by Instron® of Norwood, Mass. Compression machine  60  is positioned in a thermal chamber  70 , which is better seen in  FIG. 7 . A suitable thermal chamber is an Instron® SFL Model No. 3282. Thermal chamber  70  should be capable of producing a maximum internal temperature of about 190 degrees Celsius. However, depending on the type of shape-memory material used, this maximum temperature may be much lower. 
     Compression fixture  55  should be allowed to equilibrate in thermal chamber  70  to a set temperature. The set temperature can vary widely depending on the shape-memory material used. For example, the set temperature for a cross-linked methyl methacrylate (MMA) polymer can range from about 30 degrees Celsius to about 190 degrees Celsius, depending on the specific composition. Equilibration time can range from about one minute to about 30 minutes, again depending on the shape-memory material and the set temperature. 
     Once the temperature of compression fixture  55  has equilibrated, compression plate  62  is brought towards compression plate  64  (alternatively, depending on the compression machine, both plates may be brought together simultaneously, or plate  64  may be brought towards plate  62 ). As plates  62  and  64  are brought together, cover  40  moves down over base  30 , which in turn causes engaging members  50  to drive medical devices  10  further into apertures  32 . As device  10  is driven further into aperture  32 , left leg  12  and right leg  14  are compressed towards each other (i.e., inwards) as they are forced into aperture  32 . This movement aligns left leg  12  and right leg  14  with main body  16 . Gage members  52 , which move with cover  40  and engaging members  50 , maintain a uniform diameter through-bore  202  between and along the entire length of compressed legs  12  and  14  as grooves  125  and  145  are compressed towards each other and around gage member  52 . Through-bore  202  replaces channel  17 . Preferably, engaging member  50  contacts main body  16  across the entire width of main body  16  as cover  40  is driven downward by compression plate  62 . By contacting the entire width of main body  16 , the downward force applied by engaging member  50  is preferably evenly distributed across medical device  10  such that left leg  12  and right leg  14  descend into aperture  32  at an equal rate. This equal rate of descent along with gage member  52  results in a symmetrical programmed device. As the entire medical device  10  is received within aperture  32 , left leg  12  and right leg  14  are each compressed and subjected to a strain of at least 15 percent and preferably 22 to 30 percent as measured from the permanent state. “Strain” may be defined as a forced change in the dimensions of a body. 
       FIG. 7  shows the compression fixture fully compressed within the compression machine and thermal chamber. That is, inside surface  49  of top side  46  of cover  40  butts against top surface  34  of base  30 . Accordingly, the distance between inside surface  49  and the engaging members  50  should be such that the entire medical device implant  10  is fully received within aperture  32  when inside surface  49  butts against top surface  34  of base  30 . Note that for other medical device implants and/or embodiments of the invention, cover  40  need not butt against base  30 . Cover  40  need only move toward base  30  enough to ensure that either the entire medical device implant or a sufficient amount or length of the medical device implant has been driven into aperture  32  such that the desired programming can be accomplished. 
     Once medical device implants  10  have been heated and compressed, compression fixture  755  is removed from compression machine  60  and thermal chamber  70  and allowed to cool to preferably 20° to 30° Celsius and more preferably to about 27° Celsius. Compression fixture  755  may be placed in a freezer to accelerate cooling. The time in the freezer may be about 20 minutes, depending on the temperature in the freezer.  FIG. 8  shows compression fixture  755  removed from the compression machine and thermal chamber. 
     After the compression fixture and medical devices have cooled sufficiently, engaging members  50  are preferably first removed from cover  40 . In this embodiment, engaging members  50  are rods or pins that are pushed through holes  43   a,b  in right and left sides  42  and  44  and through-hole  23 , which formed at interior portion  19  of medical device implant  10  when legs  12  and  14  were compressed together. Through-hole  23  provides a space for engaging member  50  so the legs can be compressed together without damaging the implant. Removal of engaging members  50  allows cover  40  to be removed from base  30  without disturbing the medical device implants within apertures  32 . 
     Alternatively, cover  40  may be removed from base  30  without first removing engaging members  50 . However, much more force is required to separate cover  40  from base  30 , and there is a risk of bending engaging members  50  and/or damaging or disrupting the symmetry of programmed legs  12  and  14 . Thus, removing cover  40  without first removing engaging members  50  is not recommended. 
     Upon removal of cover  40  from base  30 , most, if not all, vertical gage members  52  will be held between the compressed legs  12  and  14  of the medical device implant. Gage members  52  may then be pulled out from between the legs through the newly created through-bore  202 . Note that some gage members  52  may remain frictionally attached to cover  40  upon cover  40 &#39;s removal from base  30 , depending on the respective diameters of the gage member and through-hole  45  in top side  46 . These tight-fitting gage members are thus pulled out from the programmed medical device implants as cover  40  is separated from base  30 . The same is true for those gage members  52  that are permanently attached to cover  40 . 
       FIG. 9  shows base  30  with programmed medical device implants  10  completely contained within respective apertures  32 . Because apertures  32  have a depth greater than the length of medical device  10 , each medical device  10  is driven completely into a respective aperture upon the cover&#39;s inner surface  49  engaging the base&#39;s top surface  34 . The medical devices  10  are driven into the apertures by engaging members  50 , which also travel downward as cover  40  travels downward over base  30  during the compression process. 
       FIG. 10  shows the bottom surface  102  of base  30 . Bottom surface  102  has removal apertures  104  respectively aligned with and extending through to apertures  32 . Medical device implants  10  can be removed from base  30  with a removal device, such as, for example, a pin gage or an appropriately sized rod, inserted into a removal aperture  104  to urge a programmed medical device implant  10  from a respective aperture  32 . The medical device implant is removed in its programmed state, as shown in  FIG. 2 . Preferably, the deflection of the legs in the programmed state represents a strain of at least 15 percent and preferably 22 to 30 percent as measured from the permanent state. 
     Base  30  and cover  40  are each preferably made of aluminum 6061 but, alternatively, may be made of any equally strong or stronger alloy/metal or other material capable of withstanding (i.e., maintaining their shape and structural integrity when subjected to) the heating, compression, and cooling described above. 
     Engaging members  50  and gage members  52  are each preferably made of heat treated tool steel but, alternatively, may be made of any equally strong or stronger metal or other material capable of withstanding (i.e., maintaining their shape and structural integrity when subjected to) the heating, compression, and cooling described above. 
     While a preferred embodiment of the invention has been described and disclosed, modifications to the apparatus and methods described herein are possible within the scope of the invention. For example, although base  30  has been shown as a generally elongated rectangular block, the shape of base  30  is not limited to that shape. For example, base  30  may have a cylindrical, cubical, ellipsoidal, or trapezoidal shape. The configuration of base  30  may depend at least in part on the type and/or size of medical device implant to be programmed, and the manner in which the implant is to be programmed. 
     Similarly, cover  40  has been shown as a generally elongated U-shaped structure. Cover  40 , however, is not limited to such a shape. So long as cover  40  mates with base  30  in a manner that allows cover  40  and base  30  to be compressed together by a compression machine to accomplish the desired programming of a medical device implant, cover  40  may be of other shapes or configurations. 
     Note also that although the compression fixture has been shown and described such that a plurality of identical medical device implants  10  can be simultaneously programmed by base  30  and cover  40 , the invention is not limited in this way. For example, a compression fixture in accordance with the invention may only accommodate a single medical device implant for programming, or may alternatively accommodate a plurality of different medical device implants for simultaneous programming wherein individual apertures of base  30  and corresponding engaging members of cover  40  may be customized in accordance with the particular medical device implant to be programmed. 
     Furthermore, while legs  12  and  14  are programmed to define a common plane or profile (e.g., profiles  13  and  15 ) with main body  16  along preferably the entire length of medical device  10  from one end of the main body to the distal ends of the respective legs as shown in  FIG. 2 , legs  12  and  14  may be compressed from their permanent position to their programmed position to a degree which does not render the legs fully coplanar with the main body or perfectly parallel with longitudinal axis  20  either for the entire length of the legs or for only a portion of the length of the legs. For example,  FIG. 11A  shows an embodiment of a programmed medical device implant  1110 A, wherein left leg  1112 A and right leg  1114 A each forms a common plane or profile with main body  16  for only a portion of the respective lengths of the legs. A short distal portion  118  of each leg is bent slightly outwards from the main body. That is, most, but not all, of the length of the left and right legs are parallel to axis  20 . 
     Similarly,  FIG. 11B  shows another embodiment of programmed medical device implant  1110 B, wherein left leg  1112 B and right leg  1114 B are only substantially coplanar with main body  16  or substantially parallel to axis  20 . That is, angles α 1  and α 2  are small and substantially less than angles Ø 1  and Ø 2  (see  FIG. 2 ) and may range, for example, from 0+° to 5°. 
     Another way to describe the slightly less than ideal programmed state of the medical device implant embodiments shown in  FIGS. 11A and 11B  is to define the widths  1122 A and  1122 B of the implants at the respective distal ends of the legs as being no more than 10% greater than width  22  (see  FIGS. 1A and 1B ). In any case, the programmed legs of the medical device implant are subject to a strain which is released when the programmed medical device implant is activated to return to its permanent state. Preferably, the left and right legs are subjected to a strain of at least 15 percent and preferably 22 to 30 percent as measured from their permanent state. 
     Thus it is seen that apparatus and methods for programming a shape-memory medical device implant are provided. One skilled in the art will appreciate that the invention can be practiced by other than the described embodiments, which are presented for purposes of illustration and not of limitation, and the invention is limited only by the following claims.