Abstract:
This disclosure provides novel devices for controlling the unloading stress and recoverable strain of Nitinol devices and/or other shape memory material devices. The devices may be used to bring bone fragments into close proximity with each other, generate a compressive load between the bone fragments, and maintain that compressive load between the bone fragments while healing occurs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 62/238,472, which was filed on Oct. 7, 2015, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to approaches for controlling the unloading stress of Nitinol devices, and/or other shape memory material devices, in order to facilitate healing of diseased or damaged tissue. The present disclosure finds particular utility in the field of orthopedics, and specifically as an approach for creating and using devices for reducing fractures and generating and maintaining compression between bone fragments. While the present disclosure has application throughout the body, its utility will sometimes hereinafter be illustrated in the context of providing and using screws, staples, plates, and/or intramedullary devices for the repair of fractured or displaced bone tissue. 
       BACKGROUND 
       [0003]    In the field of orthopedic surgery, it is common to rejoin broken bones. The success of the surgical procedure often depends on the ability to reapproximate the fractured bone, the amount of compression achieved between the bone fragments, and the ability to sustain that compression over a period of time. If the surgeon is unable to bring the bone fragments into close contact, a gap will exist between the bone fragments and the bone tissue will need to first fill that gap before complete healing can take place. Furthermore, gaps between bone fragments that are too large can allow motion to occur between the fragments, disrupting the healing tissue and thus slowing the healing process. Thus, non-unions, mal-unions, and delayed-unions of fractures can occur when the gap between bone fragments is too large. Optimal healing requires that the bone fragments be in close contact with each other, and for a compressive load to be applied and maintained between the bone fragments. Compressive strain between bone fragments has been found to accelerate the healing process in accordance with Wolf&#39;s Law. 
         [0004]    Broken bones can be rejoined using screws, staples, plates, intramedullary devices, and other devices known in the art. These devices are designed to assist the surgeon with reducing the fracture and creating a compressive load between the bone fragments. 
         [0005]    Screws are typically manufactured from either titanium or stainless steel alloys and may be lag screws or headless screws. Lag screws have a distal threaded region and an enlarged head. The head contacts the cortical bone surface and the action of the threaded region reduces the fracture and generates a compressive load. Headless screws typically have a threaded proximal region and a threaded distal region. A differential in the thread pitch of the two regions generates compression across the fracture site. There also exist fully-threaded headless compression screws that have a thread pitch which differs over the length of the single continuous thread. 
         [0006]    Staples are formed from a plurality of legs (typically two legs, although sometimes more than two legs may be provided) connected together by a bridge. Staples are typically manufactured from stainless steel alloys, titanium alloys, or Nitinol, a shape memory alloy. The staples are inserted into pre-drilled holes on either side of the fracture site. 
         [0007]    Plates are also used to rejoin broken bones. These plates are generally formed from a sheet or ribbon of material having a plurality of holes formed therein. The plates are typically manufactured from either stainless steel alloys or titanium alloys. The plates are placed adjacent to a fracture so that the plate spans the fracture line, and then screws are inserted through the holes in the plate and into the bone fragments on either side of the fracture site to stabilize the bone fragments relative to one another. 
         [0008]    Intramedullary devices are often used for fractures of the long bones; however, they are also frequently used in the phalanges, and specifically for the treatment of “hammer toe”, which is a deformity of the proximal interphalangeal joint of the second, third, or fourth toe causing the toe to be permanently bent, e.g., bent upwards. Typical intramedullary devices used in the phalanges have opposing ends that are adapted to grip the interior wall of the intramedullary canal. These intramedullary devices are typically made of titanium alloys, stainless steel alloys, Nitinol and/or other materials, e.g., PEEK. The titanium alloy devices and stainless steel alloy devices often have barbs or threaded regions at their opposing ends to grip the interior wall of the intramedullary canal. The Nitinol devices may have a pair of radially-extending “legs” at their opposing ends that expand outward when warmed to body temperature, with the pair of legs at each end of the device being disposed in a common plane. 
         [0009]    While the foregoing devices (e.g., screws, staples, plates, and intramedullary devices) are designed to bring the bone fragments into close contact and to generate a compressive load between the bone fragments, the devices do not always succeed in accomplishing this objective. It is widely reported that the compressive load generated by these devices between the bone fragments dissipates rapidly as the bone relaxes and remodels around the device. 
         [0010]    Nitinol can be used to improve the functional performance of these devices by utilizing either the shape memory effect of Nitinol or the superelastic properties of Nitinol to pull together the opposing bone fragments; however, the recovery forces and recoverable strain generated by the Nitinol versions of these devices may be too great and may damage bone tissue and thus not provide a means to generate and maintain compression between the bone fragments. 
         [0011]    Thus there exists a clinical need for device that can be used to control the unloading stress and recoverable strain of Nitinol devices, and/or other shape memory material devices, so as to allow for devices that are able to bring bone fragments into close proximity with each other, generate a compressive load, and maintain that compressive load for a prolonged period of time while healing occurs without damaging the bone tissue. 
       SUMMARY 
       [0012]    This disclosure provides novel devices for controlling the unloading stress and recoverable strain of Nitinol devices/implants and/or other shape memory material devices/implants. The devices may be used to bring bone fragments into close proximity with each other, generate a compressive load between the bone fragments, and maintain that compressive load between the bone fragments while healing occurs. 
         [0013]    In an embodiment, a device is provided for controlling the recoverable strain and compression generated by a Nitinol staple. The device allows the surgeon to control to what extent the legs of the staple are opened to or beyond the parallel state. The more the staple is opened, the more recoverable strain and compression the staple will exert on the bone. This device is also beneficial because it allows the surgeon to control the rate of loading the compression onto the bone, and also provides a feedback for the surgeon to feel how much compression the bone is experiencing. 
         [0014]    In another embodiment, a device is provided for controlling the recoverable strain and compression generated by a Nitinol compression screw. The Nitinol compression screw is elongated and held in the elongated state with an internal retaining pin. It may be provided to the surgeon with the screw stretched to the maximum (˜8%). The surgeon can partially withdraw the internal retaining pin to decrease the compressive force and recoverable strain the screw will exert on the bone following implantation. The more the internal retaining pin is removed, the less recoverable stress and strain the screw will exert. This is beneficial because it allows the surgeon to have better control of the device, and may be especially beneficial for the treatment of low density bone fractures. 
         [0015]    In another embodiment, a device is provided for controlling the recoverable strain and compression generated by a Nitinol compression plate. The Nitinol compression plate is elongated and held in the elongated state with a delivery device. It may be provided to the surgeon with the plate fully elongated. The surgeon can use the delivery device to allow for partial stress/strain recovery of the device prior to implantation. This is beneficial because it allows the surgeon to have better control of the device, and may be especially beneficial for the treatment of low density bone fractures. 
         [0016]    In yet another embodiment, a device is provided for controlling the recoverable strain and compression generated by a Nitinol intramedullary implant. The Nitinol intramedullary implant is provided in an elongated state with an internal retaining pin. The surgeon can partially withdraw the internal retaining pin to decrease the compressive force and recoverable strain the intramedullary implant will exert on the bone following implantation. The more the internal retaining pin is removed, the less recoverable stress and strain the intramedullary implant will exert. This is beneficial because it allows the surgeon to have better control of the device, and may be especially beneficial for the treatment of low density bone fractures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The various features and advantages of this disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
           [0018]      FIGS. 1, 2, 3, and 4  schematically illustrate a staple delivery device that can be used to control the recoverable strain and compressive force generated by the staple. 
           [0019]      FIGS. 5 and 6  schematically illustrate a compression screw system that has an internal retaining pin that can be used to control the recoverable strain and compressive force generated by the screw. 
           [0020]      FIGS. 7, 8, and 9  schematically illustrate a compression plate delivery device that has the ability to be adjusted by the surgeon to control the recoverable strain and compressive force generate by the compression plate. 
           [0021]      FIGS. 10 and 11  schematically illustrate an intramedullary implant system that has an internal retaining pin that can be used to control the recoverable strain and compressive force generated by the intramedullary implant. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    A method for controlling an unloading stress of an orthopedic implant according to an exemplary aspect of this disclosure includes, inter alia, positioning an implant made of a shape memory material in a deformed state to generate a stress level and recoverable strain within the implant, holding the implant in the deformed state with a retaining mechanism, and adjusting the retaining mechanism to control a level of deformation of the implant and alter the stress level and recoverable strain of the implant. 
         [0023]    In a further embodiment, an implant is a compression screw. 
         [0024]    In a further embodiment, a compression screw is made of Nitinol. 
         [0025]    In a further embodiment, an implant is a compression staple. 
         [0026]    In a further embodiment, a compression screw is made of Nitinol. 
         [0027]    In a further embodiment, an implant is a compression plate. 
         [0028]    In a further embodiment, a compression plate is made of Nitinol. 
         [0029]    In a further embodiment, an implant is an intramedullary implant. 
         [0030]    In a further embodiment, an intramedullary implant is made of Nitinol. 
         [0031]    In a further embodiment, positioning an implant in a deformed state includes using superelastic or shape memory properties of the implant. 
         [0032]    In a further embodiment, a retaining mechanism is a delivery device. 
         [0033]    In a further embodiment, a delivery device includes a plunger. 
         [0034]    In a further embodiment, a retaining mechanism is an internal retaining pin. 
         [0035]    In a further embodiment, adjusting a retaining mechanism includes advancing a plunger to spread legs of an implant apart. 
         [0036]    In a further embodiment, adjusting a retaining mechanism includes partially unscrewing an internal retaining pin from a central hollow region of an implant. 
         [0037]    In a further embodiment, adjusting a retaining mechanism includes moving a restraining clip to change an amount that a central bridge of an implant bows outward. 
         [0038]    In a further embodiment, at least a portion of an implant is reversibly stretched prior to holding the implant with a retaining mechanism. 
         [0039]    In a further embodiment, an unloading stress of an implant is controlled with a retaining mechanism during implantation of the implant into bone. 
         [0040]    A method according to another exemplary aspect of this disclosure includes, inter alia, positioning an implant made of a shape memory material in a deformed state to generate a stress level and recoverable strain within the implant, adjusting a level of deformation of the implant to alter the stress level and recoverable strain of the implant, and implanting the implant into bone, wherein the implant exerts a compressive load on the bone that is proportional to the stress level and recoverable strain generated in the implant while adjusting the level of deformation 
         [0041]    Referring first to  FIGS. 1-4 , there is shown a novel device for controlling the recoverable strain and compressive force generated by a compression staple  10  made of a shape memory material such as Nitinol. The compression staple  10  is an exemplary type of orthopedic implant. A delivery device  5  (i.e., a retaining mechanism) holds the compression staple  10  on a pair of pin plates  15 . A plunger  20  is connected to a knob  25 . Turning the knob  25  causes the plunger  20  to advance and contact a bridge  19  of the compression staple  10 , and this contact spreads the legs  21  of compression staple  10  apart. The more that the knob  25  is advanced, the more the compression staple  10  legs  21  open, and thus the more compressive force and recoverable strain the compression staple  10  is capable of. The delivery device  5  also provides the surgeon with feedback as to how much compression the staple is going to exert. The more the knob  25  is turned, the more torque that is required. The torque required to turn the knob  25  is related to and may be directly proportional to the compressive force and recoverable strain the compression staple  10  will exert upon implantation into bone. 
         [0042]    Referring next to  FIGS. 5 and 6 , there is shown a compression screw system  30 . The compression screw system  30  is another exemplary type of orthopedic implant. The compression screw system  30  includes compression screw  35  made of a shape memory material, such as Nitinol, and an internal retaining pin  40  (i.e., a retaining mechanism). During manufacturing, a central hollow region  45  of the compressions screw  35  is reversibly stretched. The internal retaining pin  40  is inserted into the compression screw  35  and keeps the compression screw  35  stretched. Prior to implantation, the surgeon can partially unthread the internal retaining pin  40 . This will allow for partial recovery of the elongation of the compression screw  35 . The more that the internal retaining pin  40  is unscrewed, the less compressive force and recoverable strain the compression screw  35  will exert when the internal retaining pin  40  is fully removed following implantation. 
         [0043]      FIGS. 7, 8, and 9  illustrate a compression plate system  50 . The compression plate system  50  is yet another exemplary type of orthopedic implant. The compression plate system  50  includes a delivery device  55  (i.e., a retaining mechanism) and a compression plate  60  made of a shape memory material, such as Nitinol. The delivery device  55  has a knob  65  connected to a plunger  70  and a restraining clip  75 . Turning the knob  65  clockwise causes the restraining clip  75  to translate toward the knob  65 . As the restraining clip  75  translates upward, it rides on a chamfer  80  (see  FIG. 9 ). A cover  85  (see  FIG. 8 ) of the delivery device  55  is hinged so as to allow the cover  85  to articulate open as the restraining clip  75  travels up the chamfer  80 . As this happens, a central bridge of the compression plate  60  bows outward. This reduces the recoverable strain and compressive force that compression plate  60  will generate following implantation. 
         [0044]    Referring now to  FIGS. 10 and 11 , there is shown an intramedullary implant system  90 . The intramedullary implant system  90  is yet another exemplary type of orthopedic implant. The intramedullary implant system  90  includes an intramedullary implant  95  made of a shape memory material (e.g., Nitinol) and an internal retaining pin  100  (i.e., a retaining mechanism). During manufacturing, a central hollow region  105  of the intramedullary implant  95  is reversibly stretched. The internal retaining pin  100  is inserted into the intramedullary implant  95  and keeps the implant stretched. Prior to implantation, the surgeon can partially unthread the internal retaining pin  100 . This will allow for partial recovery of the elongation of the intramedullary implant  95 . The more that internal retaining pin  100  is unscrewed, the less compressive force and recoverable strain the intramedullary implant  95  will exert when the internal retaining pin  100  is fully removed following implantation. 
         [0045]    The foregoing description shall be interpreted as illustrative and not in any limiting sense. A worker of ordinary skill in the art would recognize that certain modifications could come within the scope of this disclosure. For these reasons, the following claims should be studied to determine the true scope and content of this disclosure.