Microminiature chainmail interface between skin and a transcutaneous prosthetic device and a method of manufacture

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.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

BACKGROUND OF THE INVENTION

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.

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.

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.

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.

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.

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.

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.

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'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.

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

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.

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.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, a direct skeletal attachment (DSA) device10comprises an intramedullary stem20, a post30, and an interface40positioned between the stem20and post30. The intramedullary stem20is shaped to be inserted in the intramedullary space in bone (such as the femur) at the site of an amputation. The post30is shaped for attachment to an artificial limb or other prosthesis. As shown inFIG. 1, the post30is 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 post30.

The interface40comprises a series of interconnected links41, which form a micro-miniature chainmail scaffolding42. Because the links41are in direct contact with tissue, they are made of a biomaterial that is both biocompatible and durable. In one configuration, the links41are constructed from a titanium alloy. In alternative embodiments, a cobalt-chrome alloy is used. The inner diameters of the chainmail links41are approximately 1 mm or less.

As shown inFIG. 2, the interface40is dome-shaped, with the interface40centered around a common axis shared with the stem20and post30. However, the interface40is 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 interface40can have a variety of shapes, which can be achieved by varying the spacing and size of the links41. As shown inFIG. 3, skin grows on the outside surface45of the interface40up to the post30.

Referring again toFIGS. 1-3, the interface40comprises a series of interconnecting links41. To address the need to overcome problems related to the mismatches in mechanical compliance between skin and the main body of the DSA device10, the interface40incorporates 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-4Cshow different regions of the interface40with varying flexibility, mobility, and porosity.FIG. 4Ashows a pliant portion of the interface40, where the links41are free to move relative to each other because of the large spacing within the inside diameter of any one link41.FIG. 4Bshows a less compliant region, where the space at the center of a link41is decreased by either increasing the thickness of a link41or decreasing the inner diameter of the link41.FIG. 4Cshows portions of the interface40that are rigid, with neighboring links fused to each other.

A gradient provides for a transition in compliance or mobility between pliable skin and the rigid prosthetic attachment post30and the intramedullary stem20. In one configuration, the transition is gradual with the interface40rigid at a first end43adjacent to the stem20and post30, but pliable at a second end44furthest from the stem20and post30.

Referring again toFIG. 3, the links41form a chainmail scaffolding42to provide sites for ingrowth of skin cells and vascular tissue. In this manner, skin grows into and locks onto the chainmail scaffolding42from the outside surface45, whereas vascularized soft tissues, needed to support normal skin homeostasis, grow into from the opposite (i.e. inside) surface46of the chainmail scaffolding42. In one embodiment, the porosity of the scaffolding42becomes fully dense at the main DSA body10to minimize entry points for bacterial transmission. The change from porous to fully dense can be gradual or abrupt.

In an alternative configuration, a gradual change in pore size stops at a minimum pore size, then the remainder of the chainmail scaffolding42from this point towards the stem20and post30is 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.

As previously stated, the pliant portion of the chainmail42consists of interconnected links41, with each link41free to move relative to neighboring links41. Because the range of movement for a link41is determined by the amount of free space within the interior portion (or inner diameter) of a link41to 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 interface40. For instance, portions of the chainmail scaffolding42can be made less compliant by decreasing an inner diameter of the link41, thereby limiting the movement of the links41within that portion of the interface40. That is, there is less open space in the interior of a link41having a reduced inner diameter; thus, interconnected links41which pass through this space are restricted in their movement. Alternatively, the thickness of each link41can be increased, which also restricts the range of motion between interconnected links41.

Yet other portions of the chainmail42can be made stiff by building links41such the walls47of adjacent links41intersect and join, as shown inFIG. 4C. The structural stiffness of these joined links41can be varied by adjusting the link-to-link spacing, or the link41inner diameters, or the link41outer diameters, or the link41thicknesses, 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 link41dimensions and spacings throughout the chainmail structure42.

In the embodiment shown inFIGS. 1-3, the size of each link41start at 350 micron inside diameter×420 micron outside diameter×35 micron thickness at the second end44(furthest from the stem20and post30) of the interface40. The inside diameter of the links41progressively decreases and the thickness of the links41progressively increase, each by 12.5 microns, until the links41become solid discs at the first end43of the interface40next to the stem20and post30. In this preferred embodiment, the center-to-center link spacing as the interface is being manufactured is 230 microns. The links41are radially distributed relative to a point on the axis of the stem20and post30. While these dimensions are provided as examples, a person having ordinary skill in the art will appreciate that a suitable interface40conducive to cell growth can have different dimensions.

In alternative embodiments, the interface40is manufactured with different interface40shapes, gradient distributions, link41sizes, and number of layers of links41(i.e., number of interconnecting links41along radial or axial direction from the central stem20). In addition, the cross section of the links41may be circular, ellipsoidal, square, rectangular, hexagonal, or other shapes. The surface of the links41may be micro-textured or smooth.

In yet another alternative embodiment, the chainmail42can embed other parts or features48with non-link geometries. For one example,FIG. 5shows an interface40with chainmail42having through-hole features48distributed though the interface40. The through-holes48provide another feature for skin and underlying tissue to interconnect and lock into the interface40.

The chainmail interface40thus 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 devices10that incorporate the chainmail-based skin-to-DSA interface40. Additionally, the method is suitable for manufacturing micro-miniature chainmail100, which has smaller dimensions than traditional chainmail. The method is based on conventional additive manufacturing (AM) processes, but incorporates a modified chainmail100design 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.

FIG. 6is a flowchart showing the steps of the process. At step S601of 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 interface40and DSA device10, customized for a recipient, could first be created using data from measurements and reconstructions from MRI, CT, and/or laser scans of the recipient'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 link41diameter or thickness.

At step S602, 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 chainmail100or interface40and DSA device10can 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 S601.

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 chainmail100or chainmail scaffolding42of interface40, adjacent interconnected links41are free to move relative to each other. However, during construction the movable links41will require support structures because the links41would otherwise move as the powder is being deposited and leveled during the build operation at step S602. Additionally, freely moving links41might warp if a heat treatment is used as part of the AM process to eliminate residual stresses in the part.

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 chainmail100because the sacrificial supports would be interwoven with the links41of the chainmail100. As such, the method of the present invention reduces or eliminates the need for such support structures by modifying the pre-build chainmail100design such that the links41are self-supporting or have minimal supporting structure.

In one configuration, links41ultimately intended to freely move are made thicker and forced to intersect with neighboring links41at their outer perimeters, thus making them self-supporting. Links41can be made self-supporting by reducing the link41inner diameter, increasing the link41outer diameter, or reducing the link-to-link spacing, or a combination thereof. While fusing adjoining links41aids the build process at step S602, the final product must include freely movable links41. To free the fused links41, at step203the part is exposed to an etchant and the outside surface of the link41is eroded until the links41have the desired dimensions and no longer intersect, freeing them to be able to move relative to their neighbors.FIG. 8depicts a DSA device10, with fused links41, in an etchant.

In an alternative embodiment, a minimal support structure301is provided to support the links41during the AM build process. As shown inFIGS. 9A-9B, the support structure301is in the shape of a lattice. In this configuration, it is important for the dimensions of the support structure301to remain within a range that can be removed during the etching step S603since physical removal is not practical. Stated differently, if the etching step S603removes 35 microns from the outside surface of the link41, then the width of each beam in the support structure301should be around 35 microns or less so that it is completely removed during etching at step S603. Of course, the required dimensions of the support structure301depends on its dissolve rate compared to the links41, which can be influenced by surface area and other factors.

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 inFIG. 10, where the link41dimensions at the interface40are enlarged and start at 315 micron inside diameter×455 micron outside diameter×70 micron thickness. These dimensions are for a link41having 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 links41. The remaining link41dimensions are not changed. In this particular example, the part is inverted and built on a base300(as shown inFIG. 7) to provide support for the first layer of the first row of links41at the first end43of the interface40. After the part is completely built-up, the support base300on which the part is built is cut-off.

As with the previous examples, at step S603the intersecting links41of the part are submerged in an etchant, for example hydrofluoric acid if the chainmail100were made out of Ti6A14V. The part remains in the etchant until the links41are eroded by 35 microns per exposed surface. At this point, the desired dimensions of the links41are 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.

In an alternative embodiment, the pre-build CAD model is designed with all of the links41the 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 link41dimensions.FIG. 7depicts a DSA device10with only a portion of the links submerged in the etchant.

Similarly, in yet another alternative embodiment, a pre-build CAD model is first created where the links41are sized to the minimum feature size allowed by the DMLS process. The links41are 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 chainmail100or interface40.