Abstract:
The disclosure describes a direct skeletal attachment (DSA) device including a micro-miniature chainmail skin-to-DSA interface. The interface comprises various porous architectures for skin ingrowth and integration as barriers against pathogens. Failure of skin-to-DSA interfaces can occur due to mismatches in mechanical compliance between pliable skin and more rigid DSA interfaces. To address this problem, in embodiments disclosed herein is an interface having a gradient in mechanical compliance or link mobility, ranging from fully flexible, to less compliant, to rigid where it attaches to the main DSA body.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. §119 of Provisional Ser. No. 62/125,162, filed Jan. 14, 2015, which is incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    The invention relates generally to transcutaneous prosthetic devices. More specifically, the invention relates to an interface between a transcutaneous prosthesis and the skin of a patient. 
         [0004]    Approximately two million persons were living with limb loss in the United States in 2007. The main causes of limb loss are vascular disease (54%), including diabetic vasculopathy and peripheral arterial disease, trauma (45%), and cancer (less than 2%). As a result, approximately 185,000 amputations occur in the United States each year. 
         [0005]    Limb prostheses, which are used to recover some functionality, are typically mated to the residual stump (residuum) of amputated limbs using custom conformal sockets. Socket attachment can be achieved by creating a vacuum between the residuum and the prosthesis. As the patient dons the prosthesis, air is expelled from the socket through a one-way valve. The negative pressure around the residuum holds the prosthesis in place until the user releases it by opening the valve. The socket attachment method is not an ideal solution. Problems include: phantom pain due to loss of osseoperception; difficulty in properly attaching the prosthesis from changes in skin condition and/or residuum volume; difficulty fitting short residuums; skin irritation; lack of robust stabilization between the prosthesis and residual limb; and, in general, difficulties from frequently donning and doffing the socket. 
         [0006]    Direct skeletal attachment (DSA) is an alternative method of prosthesis attachment that can provide osseoperception, improved locomotor activities of a patient, and elimination of other problems associated with donning and using a socket. In the DSA approach an intramedullary stem integrates with intact bone, and a percutaneous pylon attached to the stem acts as a mounting post for the prosthesis. See, for example, U.S. Pat. No. 3,947,897, which describes an apparatus for connecting a prosthesis to a bone of a residuum. 
         [0007]    Because the DSA implant protrudes through the skin of the patient, DSA implants are susceptible to infections. To address this issue, DSA implants incorporate skin-to-DSA interfaces comprising various porous architectures for skin ingrowth and implant cutaneous integration as barriers against pathogens traveling down the pylon down to the stem and the surrounding tissues, in particular bone. However, current skin-to-DSA interfaces often fail, leading to infection and implant instability, requiring DSA device removal and replacement either with another DSA implant or more conventional socket suspension system. 
         [0008]    Skin-to-DSA interface failures can occur due to the mismatch in mechanical compliance between pliable skin and the more rigid DSA interface or the DSA device itself, which are often composed of titanium alloys such as Ti-6AI-4V. This mismatch can lead to stress risers that cause the skin to tear away from the interface as the skin moves relative to the bone during normal motion or as the recipient gains or losses weight. To minimize tearing, it is thought that the mobility of skin around the implant should be minimized; both surgical techniques and devices for this purpose have been developed. 
         [0009]    Devices attempting to solve this problem include a percutaneous bar with a flexible mesh collar, holes at the subcutaneous perimeter of a flange, and a collar made of a stainless steel spring or nylon hooks. Animal studies with these devices produced promising results, however, many of the implants are sensitive to its positioning relative to the dermal and subcutaneous tissues and do not tolerate junction shifting when the distance from the bone to the skin-binding junction changes. Another approach was positioning of a bar with a porous flange in the dermal tissues immediately below the epithelium. While this may reduce the mobility of skin in the plane parallel to the flange, the attachment to the solid bar still remained fragile. In another device, an interface design provides a dome-shaped device with holes for skin attachment; however, the interface is rigid and therefore does not address the problem of compliance mismatch. 
         [0010]    When the skin at the skin-to-DSA interface tears, it creates entry points for bacteria and other pathogens into the body. Tears can self-repair by reepithelization, but the repairs are weaker after each tear. For example, recurring atrophic or hyper-trophic scarring and callus formation at the skin-to-implant interface will incrementally reduce the strength of the tissue adhesion in subsequent repairs, thus spiraling into weaker dermal and epidermal integration and thereby increase the risk of further tears and infection. While the initial clinical studies using DSA limb prostheses in humans were conducted in the U.S. in the mid 1970&#39;s, the FDA does not currently allow DSA procedures, in part because of a lack of compelling evidence for a solution to the skin seal problem. 
         [0011]    Despite these problems, DSA prosthetic devices are permitted in other countries. Over 150 patients in Sweden, Germany, the Netherlands, and Australia have received DSA devices, and analysis and in-depth interviews with patients living with osseointegrated prostheses objectively confirmed functional improvements. Participants described their experience with DSA prostheses as making a revolutionary change to the quality of their lives. However, improved DSA interfaces are still required to minimize infection and reduce the need or surgical removal or periodic replacement of the DSA device. It would therefore be advantageous to develop a DSA interface that reduces skin tearing. 
       BRIEF SUMMARY OF THE INVENTION 
       [0012]    According to embodiments of the present disclosure is an improved skin-to-DSA interface. In one embodiment, the interface comprises chainmail having a gradient in mechanical compliance, mobility, and porosity. Interconnected links of the chainmail have varying inner diameters and spacing, which affect the compliance, mobility, and porosity characteristics. 
         [0013]    Further disclosed is a method of manufacturing the interface using additive manufacturing techniques. In one embodiment, neighboring links are fused to create a stable base on which to create additional layers of the interface. In this embodiment, an etching step is used to free the links upon completion of the additive manufacturing steps. Etching can also be used to create the differentiations in mechanical compliance, mobility, and porosity. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0014]      FIG. 1  is a rendering of the interface as part of a DSA device according to one embodiment. 
           [0015]      FIG. 2  is an alternate view of the device showing the interface, including the inner surface and outer surface of the interface. 
           [0016]      FIG. 3  is a detailed view of the chainmail structure of the interface according to one embodiment. 
           [0017]      FIGS. 4A, 4B, and 4C  show the relative dimensions of the links in different regions of the interface. 
           [0018]      FIG. 5  shows features incorporated into the interface. 
           [0019]      FIG. 6  is a flow diagram depicting the method of the present invention. 
           [0020]      FIG. 7  shows a part built on a base using an additive manufacturing process, according to one embodiment of the invention. 
           [0021]      FIG. 8  shows a post-build etching process, where intersecting links of the interface, which may be fused after the additive manufacturing build process, are submerged in an etchant in order to free those links to move relative to their neighboring links. 
           [0022]      FIGS. 9A-9B  show a support structure used during the additive manufacturing process to support links that are ultimately free moving. 
           [0023]      FIG. 10  is a detailed view of the interconnected links. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    In one embodiment, a direct skeletal attachment (DSA) device  10  comprises an intramedullary stem  20 , a post  30 , and an interface  30  positioned between the stem  20  and post  30 . The intramedullary stem  20  is shaped to be inserted in the intramedullary space in bone (such as the femur) at the site of an amputation. The post  30  is shaped for attachment to an artificial limb or other prosthesis. As shown in  FIG. 1 , the post  30  is round and relatively short. However, the shape and size can be configured differently depending on the attachment mechanism of the artificial limb being attached to the post  30 . 
         [0025]    The interface  40  comprises a series of interconnected links  41 , which form a micro-miniature chainmail scaffolding  42 . Because the links  41  are in direct contact with tissue, they are made of a biomaterial that is both biocompatible and durable. In one configuration, the links  41  are constructed from a titanium alloy. In alternative embodiments, a cobalt-chrome alloy is used. The inner diameters of the chainmail links  41  are approximately 1 mm or less. 
         [0026]    As shown in  FIG. 2 , the interface  40  is dome-shaped, with the interface  40  centered around a common axis shared with the stem  20  and post  30 . However, the interface  40  is not limited to a dome shape and can be adjusted to conform to the contours of the end of the residuum. As such, a person having skill in the art will appreciate that the interface  40  can have a variety of shapes, which can be achieved by varying the spacing and size of the links  41 . As shown in  FIG. 3 , skin grows on the outside surface  45  of the interface  40  up to the post  30 . 
         [0027]    Referring again to  FIGS. 1-3 , the interface  40  comprises a series of interconnecting links  41 . To address the need to overcome problems related to the mismatches in mechanical compliance between skin and the main body of the DSA device  10 , the interface  40  incorporates gradients in: 1) mechanical compliance, ranging from a fully flexible interface, to less compliant, to a rigid interface; 2) link mobility, ranging from freely mobile to constrained; and/or, 3) porosity, ranging from highly porous to fully dense. For example,  FIGS. 4A-4C  show different regions of the interface  40  with varying flexibility, mobility, and porosity.  FIG. 4A  shows a pliant portion of the interface  40 , where the links  41  are free to move relative to each other because of the large spacing within the inside diameter of any one link  41 .  FIG. 4B  shows a less compliant region, where the space at the center of a link  41  is decreased by either increasing the thickness of a link  41  or decreasing the inner diameter of the link  41 .  FIG. 4C  shows portions of the interface  40  that are rigid, with neighboring links fused to each other. 
         [0028]    A gradient provides for a transition in compliance or mobility between pliable skin and the rigid prosthetic attachment post  30  and the intramedullary stem  20 . In one configuration, the transition is gradual with the interface  40  rigid at a first end  44  adjacent to the stem  20  and post  30 , but pliable at a second end  43  furthest from the stem  20  and post  30 . 
         [0029]    Referring again to  FIG. 3 , the links  41  form a chainmail scaffolding  42  to provide sites for ingrowth of skin cells and vascular tissue. In this manner, skin grows into and locks onto the chainmail scaffolding  42  from the outside surface  45 , whereas vascularized soft tissues, needed to support normal skin homeostasis, grow into from the opposite (i.e. inside) surface  46  of the chainmail scaffolding  42 . In one embodiment, the porosity of the scaffolding  42  becomes fully dense at the main DSA body  10  to minimize entry points for bacterial transmission. The change from porous to fully dense can be gradual or abrupt. 
         [0030]    In an alternative configuration, a gradual change in pore size stops at a minimum pore size, then the remainder of the chainmail scaffolding  42  from this point towards the stem  20  and post  30  is fully dense. Below a certain pore size, skin may not be able to effectively grow into the pore, yet the pore is still large enough for pathogens to pass through. As such, a pore size below a critical limit is not created to prevent infiltration of pathogens in areas where skin may not be able to effectively grow to provide a protective barrier. 
         [0031]    As previously stated, the pliant portion of the chainmail  42  consists of interconnected links  41 , with each link  41  free to move relative to neighboring links  41 . Because the range of movement for a link  41  is determined by the amount of free space within the interior portion (or inner diameter) of a link  41  to which it is connected, the level of link mobility can be decreased by decreasing this space. Link mobility, in turn, influences the mechanical compliance of the interface  40 . For instance, portions of the chainmail scaffolding  42  can be made less compliant by decreasing an inner diameter of the link  41 , thereby limiting the movement of the links  41  within that portion of the interface  40 . That is, there is less open space in the interior of a link  41  having a reduced inner diameter; thus, interconnected links  41  which pass through this space are restricted in their movement. Alternatively, the thickness of each link  41  can be increased, which also restricts the range of motion between interconnected links  41 . 
         [0032]    Yet other portions of the chainmail  42  can be made stiff by building links  41  such the walls  47  of adjacent links  41  intersect and join, as shown in  FIG. 4C . The structural stiffness of these joined links  41  can be varied by adjusting the link-to-link spacing, or the link  41  inner diameters, or the link  41  outer diameters, or the link  41  thicknesses, or a combination thereof. Therefore, a functional gradient in flexibility and compliance—ranging from fully flexible, to less flexible, to stiff, to rigid—can be created by selectively varying link  41  dimensions and spacings throughout the chainmail structure  42 . 
         [0033]    In the embodiment shown in  FIGS. 1-3 , the size of each link  41  start at 350 micron inside diameter×420 micron outside diameter×35 micron thickness at the second end  44  (furthest from the stem  20  and post  30 ) of the interface  40 . The inside diameter of the links  41  progressively decreases and the thickness of the links  41  progressively increase, each by 12.5 microns, until the links  41  become solid discs at the first end  43  of the interface  40  next to the stem  20  and post  30 . In this preferred embodiment, the center-to-center link spacing as the interface is being manufactured is 230 microns. The links  41  are radially distributed relative to a point on the axis of the stem  20  and post  30 . While these dimensions are provided as examples, a person having ordinary skill in the art will appreciate that a suitable interface  40  conducive to cell growth can have different dimensions. 
         [0034]    In alternative embodiments, the interface  40  is manufactured with different interface  40  shapes, gradient distributions, link  41  sizes, and number of layers of links  41  (i.e., number of interconnecting links  41  along radial or axial direction from the central stem  20 ). In addition, the cross section of the links  41  may be circular, ellipsoidal, square, rectangular, hexagonal, or other shapes. The surface of the links  41  may be micro-textured or smooth. 
         [0035]    In yet another alternative embodiment, the chainmail  42  can embed other parts or features  48  with non-link geometries. For one example,  FIG. 5  shows an interface  40  with chainmail  42  having through-hole features  48  distributed though the interface  40 . The through-holes  48  provide another feature for skin and underlying tissue to interconnect and lock into the interface  40 . 
         [0036]    The chainmail interface  40  thus described has unique features that cannot be manufactured using traditional techniques. Therefore, another aspect of the present invention disclosed herein is a practical process to manufacture DSA devices  10  that incorporate the chainmail-based skin-to-DSA interface  40 . Additionally, the method is suitable for manufacturing micro-miniature chainmail  100 , which has smaller dimensions than traditional chainmail. The method is based on conventional additive manufacturing (AM) processes, but incorporates a modified chainmail  100  design to accommodate a post-processing etching step. In general, a person having ordinary skill in the art will recognize that AM processes can build-up structures of arbitrarily complex geometries in a layer-by-layer fashion. 
         [0037]      FIG. 6  is a flowchart showing the steps of the process. At step S 601  of the process, a 3D computer-aided design (CAD) model of the part to be built is decomposed into simpler 2D cross-sectional layer descriptions to define each build layer. For one example, a CAD model of the interface  40  and DSA device  10 , customized for a recipient, could first be created using data from measurements and reconstructions from MRI, CT, and/or laser scans of the recipient&#39;s residuum. As will be discussed in greater detail, the model includes dimensions that are larger than the desired final dimensions because a post-build step reduces the link  41  diameter or thickness. 
         [0038]    At step S 602 , the part is manufactured using an AM process. In one embodiment, the part is constructed using a direct metal laser sintering (DMLS) process, such as the process provided by 3D Microprint GmbH. Using a DMLS process, the chainmail  100  or interface  40  and DSA device  10  can be constructed of a titanium alloy, among other materials. In this manner, each layer of the part is formed sequentially, first by depositing a thin layer of titanium alloy (Ti-6A1-4V) powder, for example, then using a high-power, highly-focused laser beam to selectively sinter or melt those regions in the layer defining the part. The process of depositing and melting the metal powder is repeated for each layer as instructed by the model created in step S 601 . 
         [0039]    As is typical in AM processes, some part features may include sacrificial support structures that are simultaneously constructed with the part. For example, for the chainmail  100  or chainmail scaffolding  42  of interface  40 , adjacent interconnected links  41  are free to move relative to each other. However, during construction the movable links  41  will require support structures because the links  41  would otherwise move as the powder is being deposited and leveled during the build operation at step S 602 . Additionally, freely moving links  41  might warp if a heat treatment is used as part of the AM process to eliminate residual stresses in the part. 
         [0040]    With typical parts constructed in an AM process, the sacrificial support is easily removed by breaking or cutting the support away from the part at the end of the build. In this example, the support typically remains on the exterior of the part being built or is otherwise accessible for removal. On the other hand, cutting or breaking supports would not be practical or even feasible with chainmail  100  because the sacrificial supports would be interwoven with the links  41  of the chainmail  100 . As such, the method of the present invention reduces or eliminates the need for such support structures by modifying the pre-build chainmail  100  design such that the links  41  are self-supporting or have minimal supporting structure. 
         [0041]    In one configuration, links  41  ultimately intended to freely move are made thicker and forced to intersect with neighboring links  41  at their outer perimeters, thus making them self-supporting. Links  41  can be made self-supporting by reducing the link  41  inner diameter, increasing the link  41  outer diameter, or reducing the link-to-link spacing, or a combination thereof. While fusing adjoining links  41  aids the build process at step S 602 , the final product must include freely movable links  41 . To free the fused links  41 , at step  203  the part is exposed to an etchant and the outside surface of the link  41  is eroded until the links  41  have the desired dimensions and no longer intersect, freeing them to be able to move relative to their neighbors.  FIG. 8  depicts a DSA device  10 , with fused links  41 , in an etchant. 
         [0042]    In an alternative embodiment, a minimal support structure  301  is provided to support the links  41  during the AM build process. As shown in  FIGS. 9A-9B , the support structure  301  is in the shape of a lattice. In this configuration, it is important for the dimensions of the support structure  301  to remain within a range that can be removed during the etching step S 603  since physical removal is not practical. Stated differently, if the etching step S 603  removes 35 microns from the outside surface of the link  41 , then the width of each beam in the support structure  301  should be around 35 microns or less so that it is completely removed during etching at step S 603 . Of course, the required dimensions of the support structure  301  depends on its dissolve rate compared to the links  41 , which can be influenced by surface area and other factors. 
         [0043]    To accommodate the loss of material in the etching process, the pre-etch build must have larger dimensions than the desired dimensions of the finished product. By way of example, a modified pre-build computer aided design (CAD) model of the preferred embodiment is shown in  FIG. 10 , where the link  41  dimensions at the interface  40  are enlarged and start at 315 micron inside diameter×455 micron outside diameter×70 micron thickness. These dimensions are for a link  41  having dimensions of 350 micron inside diameter×420 micron outside diameter×35 micron thickness after etching. To create a gradient, the inside diameters are progressively decreased and the thicknesses progressively increased, each by 12.5 microns, for the next 11 rows of links  41 . The remaining link  41  dimensions are not changed. In this particular example, the part is inverted and built on a base  300  (as shown in  FIG. 7 ) to provide support for the first layer of the first row of links  41  at the first end  43  of the interface  40 . After the part is completely built-up, the support base  300  on which the part is built is cut-off. 
         [0044]    As with the previous examples, at step S 603  the intersecting links  41  of the part are submerged in an etchant, for example hydrofluoric acid if the chainmail  100  were made out of Ti6A14V. The part remains in the etchant until the links  41  are eroded by 35 microns per exposed surface. At this point, the desired dimensions of the links  41  are attained and become free to move relative to each other. The part is then quickly removed from the etchant and quenched, for example, in a series of water baths. 
         [0045]    In an alternative embodiment, the pre-build CAD model is designed with all of the links  41  the same size and intersected by the same amount of overlap. After the part is built, the entire part is submerged in an etchant and gradually removed to create a gradient in the link  41  dimensions.  FIG. 7  depicts a DSA device  10  with only a portion of the links submerged in the etchant. 
         [0046]    Similarly, in yet another alternative embodiment, a pre-build CAD model is first created where the links  41  are sized to the minimum feature size allowed by the DMLS process. The links  41  are then submerged in etchant to erode the links to the desired sizes, which can be smaller than the minimum size produced by the DMLS process. This embodiment is useful to accommodate DMLS processes with minimum feature sizes that are larger than those needed to make the chainmail  100  or interface  40 . 
         [0047]    While the disclosure has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modification can be made therein without departing from the spirit and scope of the embodiments. Thus, it is intended that the present disclosure cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.