Abstract:
A dynamic tissue holding device for dynamically holding two tissue portions in contact with one another includes a resilient body having a spring and a band arranged for extending about the tissue portions to be held together. The band has a first end for attachment to a first attachment portion on the resilient body and a second end for attachment to a second attachment portion on the resilient body. 
     Stops are disposed on portions of the resilient body for limiting both the compression and expansion of the spring. The compression of the spring is limited by engagement of two of the stops with one another and the expansion of the spring is limited by engagement of at least one of the stops with the spring.

Description:
[0001]    This application claims the benefit under 35 U.S.C. 119(e) of the filing date of Provisional U.S. application Ser. No. 61/487,889, entitled Hybrid Medical Device Implant with Migration Management, filed on May 19, 2011, and expressly incorporated herein by reference, in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention is related to the general surgical repair of separated body tissues, and more particularly to internally fixating and stabilizing such body tissues, specifically bones. 
         [0003]    In the present state of the art, there are a number of systems available to repair biological tissues separated in surgery or by injury. These products serve to approximate and stabilize the tissues so that healing may commence and provide compression in the interface to promote healing. Compression and stability are critical for proper anatomical healing of tissue. With the correct amount of compression applied to the interface of the tissue portions to be joined, signals are sent to the tissue, thus allowing the tissue to remodel in proper anatomical position. The amount of compression applied to the tissue interface needs to be appropriate to the type of tissue that is being healed. 
         [0004]    Twisted wires are also typically used to keep bone fragments together so they may heal. Twisted wires only hold tension as long as the twisted wire pair remains stable. Often the wires untwist too soon failing to keep the bone fragments together so that they may heal. Wires can also cut into the bone fragments allowing them to separate so that healing is difficult. 
         [0005]    When it is necessary to access the thoracic cavity for a medical procedure, for example, it is required to cut the sternum into two pieces using a rib spreader. Once the procedure is completed within the thoracic cavity, the sternum must be repaired. For such repairs, it is known to use a dynamic compression device. Some of the drawbacks of this typical device, and others which are used include: 
         [0006]    1. Bulky spring materials, while occupying substantial space, often do not store much energy. Some use polymer elastic bands, while other use coiled springs; 
         [0007]    2. Wires are sometimes used to wrap the bones into position in compression with one another. However, wires can have sharp ends that can damage adjunctive tissues. Knot stacks in suture can interfere with the natural movement of surrounding tissues; and 
         [0008]    3. Current banding systems that incorporate a biasing mechanism to achieve dynamic compression put the biasing mechanism in line with the band or suture. This practice competes with precious space at the healing site. Suture or bands are used to approximate tissues so that they may heal. It is desirable to obtain the best purchase possible on the tissue, so that the binding mechanics offered by the suture may be utilized. The best purchase is optimized by ensuring that the suture has the greatest contact area with the tissue. If a biasing mechanism is interfering with this concept, the biasing mechanism may diminish the suture&#39;s ability to hold the tissues together. 
         [0009]    In addition, the current banding systems have stiff bands that are not compliant with bony undulations. Flat sutures are used, but are tedious to tie and do not hold reliably. 
         [0010]    The banding systems of the present invention are therefore attractive for use in sternal closure because they offer some distinct advantages over the twisted wires most commonly used in the procedure. 
         [0011]    Bands address the issues wires have, as noted in the preceding discussion. A band, by defmition, is wide. In being wide, a band distributes its forces over a wider surface area. This inhibits the band from digging into the bone. In being wide, a band affords a larger cross-sectional area whereby more material may be realized, thus presenting the opportunity to offer as much strength in the construct as is necessary to hold the bone fragments together. As such, bands address wires two main weaknesses, namely, digging into the bone fragments being held together and not having sufficient cross-sectional area. 
         [0012]    Bands ring in other attributes other than strength and reduced pressure on the bone. Some of these attributes are difficult to manage. With strength comes stiffness, as mentioned elsewhere herein. The larger cross-section of the band significantly increases the stiffness of the band. While stiffness and rigidity are good attributes in that they can stabilize the bone union, these attributes can also prevent the band from following the contours of the bone when inserted. This can lead to capturing tissues underneath the band that ultimately destabilize the union as the tissue continue to compress and disappear over time. 
         [0013]    Binding the band ends together can also impose some problems. Generally, this involves a mechanism on one band end that interfaces with holes or slots or contours on the other band end. This creates a tensioning system that is incremental in nature. As in the twisted wire system, this mechanical interface of the two ends is the weakest link in the system. This mechanical interface becomes stronger as the incremental steps become larger. But larger incremental steps are not conducive to fine tuning the tension, so this is problematic. Flat sutures have been used to tie tissues together, but the residual tension supplied in such a knotted structure is insufficient for optimum healing. There is a lot of fuss/time associated with trying to keep and hold a desirable tension with these flat sutures. What is needed is an attachment approach that provides variable tensioning. 
         [0014]    Another problem associated with banding systems is that their tension holding capabilities are insufficient for the environment in which they operate. Tension holding ability can be increased or enhanced by increasing friction at the binding interface of the band. What is needed, however, is a banding system with the ability to hold tension by selectively increasing friction at the binding interface during locking and/or after locking without increasing friction while tensioning. 
         [0015]    What is needed, therefore, are improved devices and techniques for holding two tissue portions in a state of compression and tension that address and overcome these shortcomings in an innovative way. 
       SUMMARY OF THE INVENTION 
       [0016]    There is disclosed, in one aspect of the invention, a dynamic tissue holding device for dynamically holding two tissue portions in contact with one another, which comprises a resilient body and a band adapted for extending about the tissue portions to be held together. The band has a first end for attachment to a first attachment portion on the resilient body and a second end for attachment to a second attachment portion on the resilient body. The band establishes a path of tension along its length and extends linearly between the two ends of the band. The resilient body comprises a spring, a first stop having a first opposing surface, and a second stop having a second opposing surface. The resilient body may be pre-compressed to a predetermined limit of compression wherein the first and second opposing surfaces of the first and second stops, respectively, are in substantial contact with each other, such that the pre-compression of the spring applies a predetermined level of tension to the band without strangulating the tissue. 
         [0017]    An additional stop is provided which contacts the spring when the spring is expanded a predetermined amount, so that the contact of the additional stop and the spring prevents further expansion of the spring, thereby establishing a predetermined limit of expansion of the spring. This additional stop preferably comprises a portion of one of the first and second stops, and more preferably a portion of each of the first and second stops. In one illustrated embodiment, the additional stop comprises a side surface of the first stop and a side surface of the second stop. 
         [0018]    The spring comprises a spring portion on each of opposing sides of the resilient body, and the first and second stops each comprise generally hemispherical or horseshoe-shaped structures, in the illustrated embodiment. The first stop comprises a pair of first opposing surfaces, and the second stop comprises a pair of second opposing surfaces. A gap is disposed between the first and second opposed stop surfaces when the surfaces are not in contact, the gap being at a maximum length when the spring is expanded to the aforementioned predetermined amount. Advantageously, both the stops and the gap may be sized to establish the predetermined limits of expansion and of compression of the spring, which may comprise a leaf spring in a currently preferred embodiment. 
         [0019]    Advantageously, the device of the invention is formed of two different materials. In one such embodiment, the resilient body comprises metal and the band bio-absorbable suture. Because of a desire to prevent the resilient body from floating within the patient&#39;s body once the suture is absorbed, in one embodiment the suture advantageously comprises a hybrid of materials, including an absorbable portion and a non-absorbable portion woven into the absorbable portion, wherein the non-absorbable suture portion will remain after absorption to provide sufficient anchoring for the resilient body to prevent migration. 
         [0020]    In another embodiment, rather than using a hybrid suture, an eyelet is disposed on the resilient body, for receiving a fastener for attaching the resilient body to adjacent bone. 
         [0021]    In another aspect of the invention, there is provided a dynamic tissue holding device for dynamically holding two tissue portions in contact with one another, which comprises a resilient body comprising a spring, and a band adapted for extending about the tissue portions to be held together. The band has a first end for attachment to a first attachment portion on the resilient body and a second end for attachment to a second attachment portion on the resilient body, and establishes a path of tension along its length and extending linearly between the two ends of the band. Stops are disposed on portions of the resilient body for limiting both the compression and expansion of the spring. Preferably, the compression of the spring is limited by engagement of two of the stops with one another and the expansion of the spring is limited by engagement of at least one of the stops with the spring. 
         [0022]    Preferably, as well, the resilient body comprises metal and the band comprises bio-absorbable suture. In one embodiment, the suture comprises a hybrid of materials, including an absorbable portion and a non-absorbable portion woven into the absorbable portion. In another, alternative embodiment, the resilient body comprises an eyelet disposed on the resilient body, for receiving a fastener for attaching the resilient body to adjacent bone. 
         [0023]    The invention, together with additional features and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying illustrative drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  illustrates a broken femur on which has been installed an implant constructed in accordance with the principles of the present invention; 
           [0025]      FIG. 1   a  is a detail view of the portion of  FIG. 1  denoted by the circle A, illustrating the inventive implant in its initial state; 
           [0026]      FIG. 1   b  is a detail view similar to  FIG. 1   a , illustrating the inventive implant in its expanded state; and 
           [0027]      FIG. 2  is a view showing a modified embodiment of the inventive implant. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0028]    There are various groups of materials from which medical device implants should be made. Most of these materials are either absorbable or permanent. There are some situations where it is desirable to make an implant out of a hybrid of these two materials. Such a device is disclosed herein, comprising a spring tensioner and a band. 
         [0029]    Referring now more particularly to the drawings, there is shown in  FIG. 1  a broken head of a femur  10  having a fracture  12 . A cerclage device  14 , comprising a tension band, holds two bone fragments  16 ,  18 , resulting from the fracture  12 , together. The tension band  14  comprises suture or other suitable banding substance  20  and a resilient body  22 . The tension band  14  is designed so that continuous tension is applied throughout the healing process. As bone fragments  16 ,  18  heal and fuse together, these fragments actually move and absorb into one another. As is desirable for optimum healing, tension band  14  supplies the forces necessary to push the bone fragments  16 ,  18  into one another. Optimal healing dictates that the initial forces imparted on the bone fragments  16 ,  18  are not as high as the physiological or native load bearing force that one would expect at the healing site. This imposes competing functions for the tensioning device  14  at its initial state. The inventive device  14  addresses the challenges presented by these competing functions by providing a mechanism that interferes with the resilient function of tensioning device  14  should excessive forces be realized. 
         [0030]    As healing progresses, the resilient nature of the tensioning device  14  diminishes as springs  24  convert their potential energy into energy directed to pulling the suture in, as bone fragments  16 ,  18  move into each other. As the springs  24  lose their potential energy, the compression they are able to impart on bone fragments  16 ,  18  diminishes. Optimal bone healing requires that some suture tension be maintained until the end of the healing cycle. Normally a resilient mechanism decreases its rate of energy release until it approximates zero at the end of the cycle. This is not desirable according to optimal healing parameters, but is desirable to the extent that zero forces need to be realized at the end of the healing cycle in order to avoid strangulation of the tissue. Tensioning device  14  will start to act as a tourniquet and to strangle the tissue just healed. Again, there are competing functions for the tensioning device  14 . The present invention addresses this problem by pre-loading the resilient mechanism with the amount of minimum energy needed to induce optimum healing. 
         [0031]      FIG. 1   a  illustrates the resilient body  22  in its initial state. Spring elements  24  within the resilient body  22  serve to pull on suture ends  26 ,  28 , which are attached to attachment portions on opposed ends of the resilient body  22  as shown, thereby tensioning the suture  20 , which acts to pull the bone fragments  16  and  18  together. The spring elements  24  may be compressing the resilient body  22  such that a first opposing surface  29  ( FIG. 1   b ) of a stop  30  is being pressed into a second opposing surface  31  ( FIG. 1   b ) of a stop  32 , on each side of the resilient body  22 . This initial compression effectively preloads the resilient body  22  to ensure that tension is applied to the suture  20 , which in turn compresses bone fragments  16 ,  18  at the end of the healing cycle. But, because stops  30 ,  32  come into contact with one another, via their respective opposing surfaces  29 ,  31 , tissue strangulation is avoided. The springs  24 , as illustrated, are leaf springs, and are designed to pull with the appropriate level of tension to generate optimal compression in the fracture  12 , so that the femur will heal as quickly as possible. 
         [0032]      FIG. 1   b  illustrates the springs  24  in their full extension, storing all of the potential energy needed to perform the functions required of the tensioning device  14 . Loading springs  24  requires the tensioning of the suture ends  26 ,  28  into the tensioning device  14 , and then locking or binding suture ends  26 ,  28  to the tensioning device  14  as indicated. A gap  34  is disposed between the respective opposing surfaces  29 ,  31  of the stops  30 ,  32 , indicating that the resilient mechanism has been loaded with energy. Gap  34  is also the distance to be traveled during the healing process. When the gap  34  widens, each spring  24  contacts and interferes with the outwardly adjacent surfaces of its adjacent stops  30 ,  32 , as shown in  FIG. 1   b . This interference between the springs and the adjacent respective stops constrains further widening of the gaps  34 , by increasing, at a much higher rate, the forces necessary to further widen the gaps because of the need to overcome the interference forces. The present invention enables this higher force rate to be matched with expected physiologic or native load bearing forces. In other words, the spatial relationships between the springs  24  and the stops  30 ,  32  can be arranged to predetermine the maximum effective size of the gaps  34  in accordance with the desired application. 
         [0033]    At this juncture, the tensioning device  14  is ready to provide compression between bone halves  16 ,  18 , while also providing greater resistance to expected physiologic or native load bearing forces and while also terminating all compressive forces once the fracture  12  has fully healed. 
         [0034]    One important feature of the invention is in the aspect of the design specification that requires the tensioning device  14  to not strangulate healing bone halves  16 ,  18 . The two components that can supply forces that result in strangulation are the spring  24  and the suture  20 . Should either of these components be made of a bio-absorbable material, that component may be designed to be absorbed before permanent strangulation is realized. In this case, the spring  24  is the likely candidate to be made out of a permanent material because of the high demands made on it (bio-absorbable materials afford inferior resiliency performance relative to a permanent material such as stainless steel). 
         [0035]    In the disclosed embodiment, the device  14  is made of a hybrid of two materials, bio-absorbable suture  20  and a metal resilient body  22 . As a result, the cerclage device  14  affords added protection against strangulation, in that its suture will absorb over time, thus limiting the ability of the device  14  to damage tissue. There is, however, another important aspect to the current invention that is realized. After the bio-absorbable suture  20  is absorbed by the body, the resilient body is free to move. This may be a serious problem, in that loose implants can migrate into joints or organs in a manner which may damage tissue. The present invention addresses this dynamic in two different embodiments. 
         [0036]    The first embodiment, shown in  FIGS. 1 ,  1   a , and  1   b , addresses the manner in which the suture absorbs. Specifically, in this embodiment, the suture  20  itself is preferably a hybrid structure. The absorbable portion, or base, of the suture  20  bears the stress or tension in the suture. A non-absorbable structure is woven into the absorbable base of the suture  20  in such a manner as to zig-zag across its profile. When the absorbable structure goes away, through absorption, the zig-zag structure expands to relieve strangulation, while still holding the resilient body in place so that it cannot migrate within the body. 
         [0037]      FIG. 2  illustrates a second embodiment of the invention, as well as a second approach for addressing the dynamic noted above with respect to the usage of bio-absorbable suture. In other words, the embodiment of  FIG. 2  shows another way of addressing the fixation of the resilient body  22  once the bio-absorbable suture  20  has absorbed. In this embodiment, the resilient body  22  includes an eyelet  40 , which is configured to accept a screw. The screw passes through the eyelet  40 , and into the bone, thus attaching the resilient body permanently to the bone. As thus configured, the resilient body is always fixed to the bone, and will not migrate throughout the body when the absorbable suture is absorbed by the body, thus mitigating the need to utilise the hybrid suture  20  discussed above in conjunction with the embodiment of  FIG. 1 . 
         [0038]    The concepts disclosed in conjunction with this invention can be applied to many configurations of cerclage devices conceived today, and may also be applied to address bone fixation in any part of the body, not just the femur, as disclosed herein as an example only. Another suitable use, for example, would be to repair the sternum after it has been cut for the purpose of accessing the thoracic cavity. Cerclage devices may have bands or cables, instead of the sutures described herein. Cerclage devices may also have buckles or clamps, instead of the resilient body described herein. Additionally, any implant whose hybrid nature causes it to become unstable and to move around the body as the body absorbs the absorbable portion of the implant may benefit from the principles of the present invention. 
         [0039]    Accordingly, although exemplary embodiments of the invention have been shown and described, it is to be understood that all the terms used herein are descriptive rather than limiting, and that many changes, modifications, and substitutions may be made by one having ordinary skill in the art without departing from the spirit and scope of the invention, which is to be limited only in accordance with the following claims.