The patent's assignee is
News editors obtained the following quote from the background information supplied by the inventors: "The present invention relates to orthopedic implants and, more specifically, the preventions of fretting corrosion in the modular taper connections of orthopedic implants.
"Corrosion of total joint replacements like hip prostheses is a serious and significant problem in today's designs. One main location where this serious form of degradation takes place is at the modular taper junctions that are present in virtually all joint replacements, but particularly with respect to hip replacements.
"Modularity allows implants to have several significant advantages in terms of materials selection, geometry, ease of insertion, and ease of revision. In modular implants, portions of the implant are made from separate components which can be mixed and matched at the time of surgery to provide the best combination of materials and geometries for a specific patient's joint replacement. These modular designs require the presence of a junction or interface between components that serves as the location where the individual parts of the implant are joined together. These junctions are typically modular taper interfaces, conical (or elliptical) in shape, that consist of a taper of low angle (e.g., 5 degrees 40 minutes, or other shape) that provides a tight, secure junction to hold the two adjacent components in place.
"In a total hip replacement, for example, there may be modular taper junctions between the head of the femoral component and its neck, between the neck and the stem, and between the top (proximal) and bottom (distal) portions of the stem. In the acetabular component, there may be taper interfaces between the cup and the acetabular shell in which is it housed. Other modular taper interfaces may also be present. These modular components may be present between adjacent metal parts, or between metal and ceramic parts. In all of these cases, the tapers have characteristics that make them prone to a type of corrosion attack known as fretting crevice corrosion. In this degradation process, the taper interfaces result in a very tight crevice-like junction in which fluid from the body can penetrate and they are subject to very high cyclic mechanical loading due to the activities of daily living (e.g., walking). The metal alloys used in these joint replacements have multiple requirements that include very high strength and fracture resistance and very high corrosion resistance. The alloys used are typically from one of several alloy systems that include titanium alloys, cobalt-chromium alloys, surgical grade stainless steels, zirconium alloys, and tantalum alloys. (The first three representing a preponderance of all total hip replacements).
"When these modular junctions are created by bringing components together in the body, the combination of loading, crevice geometry and solution within the crevice can lead to a severe fretting corrosion attack at the modular taper junction. When the metal alloy surfaces are exposed to aqueous solutions and subjected to fretting motion (small scale cyclic rubbing-like motion at the interface in the range of 100 um or less), the surface oxide thin films that naturally occur on the surfaces of these metal alloys and which provide the corrosion resistance of these alloys, are abraded and the underlying metal is rapidly corroded until the passive oxide films can re-establish themselves (a process which occurs within milliseconds). With activities of daily living, fretting abrasion can occur at modular taper interfaces in the range of 3 to 30 um between the two opposing sides and result in repetitive damage of the oxides. The water present within these crevices can then react with the bare, unoxidized metal to reform the oxide film on the surface, but the byproduct of these reactions can include metal ions and metallic-based particles being released into the crevice and the generation of hydrogen ions (i.e., acids) into the crevice and electrons into the metal. Repeated cyclic loading of these interfaces can set up a process that can lead to severe local crevice solution chemistries of very low pH and very high oxidizing conditions and which can release large amounts of corrosion debris and corrosion byproducts that can damage the taper itself and can induce adverse biological reactions.
"There are several consequences of this form of corrosion attack that includes loss of mechanical function of the device, and release of ions and particles into the body which can cause several significant adverse biological responses that include osteolysis and pseudotumor formation."
As a supplement to the background information on this patent application, NewsRx correspondents also obtained the inventor's summary information for this patent application: "It is therefore a principal object and advantage of the present invention to prevent the processes of mechanically assisted crevice corrosion in modular taper connections of orthopedic implants.
"In accordance with the foregoing objects and advantages, the present invention minimizes or eliminates the corrosion attack of these modular tapers by addressing one or more of the underlying conditions at the taper that causes the mechanisms of fretting crevice corrosion. The present invention seeks to minimize the crevice-like geometry within modular taper interfaces, to minimize or eliminate the fretting motion between taper parts, and to introduce materials and geometries that prevent hard-on-metal contacts that cause fretting corrosion damage by providing materials and a topographic design approach that alleviates or minimizes the conditions that allow fretting crevice corrosion of modular taper interfaces in total joint replacements to occur. The present invention applies to all total hip replacements used today, and may be applicable to other tapered or crevice-like interfaces in medical devices where fretting crevice corrosion (otherwise known as mechanically assisted crevice corrosion) can occur.
"The present invention addresses fretting crevice corrosion by addressing crevice solution chemistry changes due to fretting crevice corrosion that adversely impact the tapers. The present invention also addresses asperity-asperity contacts that result in oxide film fracture and repassivation, and which results in highly accelerated corrosion attack. The present invention further addresses material contact across the taper interface that can reduce or eliminate oxide film disruption (and the associated accelerated corrosion attack).
"The present invention accomplished these tasks by creating a taper interface topography that can provide load transfer across the interface, distribute the contact stresses throughout the taper and not compromise the fatigue resistance of the taper junction, while allowing extensive fluid exchange between the taper region and the outside solution. In the design approach of the present invention, specific interface geometries are introduced, such as grooves or pillars on one side of the taper junction that provide sufficient contact and load transfer, while also having an open solution region where buildup of hydrogen ions, or other degradation products will not dramatically alter solution chemistry and lead to adverse run-away corrosion attack as is seen in current taper designs.
"The present invention also accomplishes these tasks by creating a taper interface material and topography that results in a sticky and compliant contact between sides of the taper junction such that there is no relative motion between sides of the interface (i.e., sticky), but the tapers can deform to accommodate the stresses and strains transmitted across the interface. The present invention also uses high strength polymer fiber-like or film-like geometries that can withstand the loading across the interface, inhibit metal-on-metal (or ceramic on metal) contact, while also providing a sticky-compliant junction with space for fluid exchange. Acceptable candidate fiber and film types for use with the present invention include highly oriented, ultra high molecular weight polyethylene (UHMWPE), fibers of poly ether-ether ketone (PEEK), as well as self-reinforced composite constructs of these fibers. Thus, the present invention addresses fretting crevice corrosion behavior by designing interfacial geometries that maximally resist fretting motion while preserving good adhesion and transfer of load by taking advantage of sticking-compliance relationship (porous, columnar surfaces) and open access of fluid to the crevice geometry. The present invention also addresses surface modification approaches to enhance the surface oxide's resistance to fretting corrosion damage by adjusting the surface characteristics (hardness, inertness) by, for example, interposing appropriate films (e.g., PEEK thin film) to inhibit oxide abrasion.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
"The present invention will be more fully understood and appreciated by reading the following Detailed Description in conjunction with the accompanying drawings, in which:
"FIG. 1 is a schematic of surface geometric structures to provide high interfacial adhesion according to the present invention;
"FIGS. 2(a) and (b) are graphs of fretting corrosion test results for PEEK fibers placed between stainless steel pin and disk, including a comparison of stainless steel on stainless steel without PEEK fibers for comparison;
"FIGS. 3(a) and (b) are graphs of the coefficient of friction and work per cycle for fretting corrosion tests with PEEK fibers interposed, where frictional measurements are consistent between fiber types and work results show the fibers allow continued sliding at high loads (and high contact stresses).
"FIG. 4 shows a scanning electron microscopic image of two PEEK fibers after fretting corrosion testing with the stainless steel disk below;
"FIG. 5 is a plot of coefficient of friction over cycles for pin on disk testing of the present invention;
"FIG. 6 is a graph of tangential load verses displacement for pin on disk testing of the present invention;
"FIG. 7 is a graph of nominal current density over time for pin on disk testing of the present invention;
"FIG. 8 is a graph of relative normalized intensity verses energy for pin on disk testing of the present invention;
"FIG. 9 is a graph of relative normalized absorbance verses wavenumber for pin on disk testing of the present invention;
"FIG. 10 is a post-test SEM image of pin on disk testing of the present invention;
"FIG. 11 is a post-test SEM image of pin on disk testing of the present invention;
"FIG. 12 is a post-test FTIR spectra of pin on disk testing of the present invention;
"FIG. 13 is a post-test FTIR spectra of pin on disk testing of the present invention;
"FIG. 14 is a post-test optical microscope image of pin on disk testing of the present invention;
"FIG. 15 is a post-test optical microscope image of pin on disk testing of the present invention;
"FIG. 16 is a post-test optical microscope image of pin on disk testing of the present invention;
"FIG. 17 is a graph of height verses distance as indicated in FIG. 15 of pin on disk testing of the present invention;
"FIG. 18 is a graph of height verses distance as indicated in FIG. 16 of pin on disk testing of the present invention;
"FIGS. 19(a) and (b) are graphs showing cyclic loading fretting corrosion test results for a CoCr-CoCr modular head-neck couple subjected to increasing cyclic loads from 100 N to 3200 N;
"FIG. 20 is a graph of typical fretting current response for a similar head-neck taper interface, where the test is identical to the tests reported above, except the test was performed on a couple that had been previously subjected to one million cycles of fretting corrosion loading and then retested for its onset load;
"FIG. 21 is a graph of current over time during in vitro testing of an embodiment of the present invention;
"FIG. 22 is a graph of current over time during in vitro testing of an embodiment of the present invention;
"FIG. 23 is a graph of current over time during in vitro testing of an embodiment of the present invention;
"FIG. 24 is a graph of average current verses nominal maximum load during in vitro testing of an embodiment of the present invention;
"FIG. 25 is a series of SEM images of a laterally positioned self-reinforced composition after in vitro testing of an embodiment of the present invention;
"FIG. 25 is a series of SEM images of a medially positioned self-reinforced composition after in vitro testing of an embodiment of the present invention;
"FIG. 27 is a digital optical microscopy image taken after in vitro testing of an embodiment of the present invention;
"FIG. 28 is a digital optical microscopy image taken after in vitro testing of an embodiment of the present invention;
"FIG. 29 is a graph of height verses distance measured as indicated in FIG. 27 after in vitro testing of an embodiment of the present invention.
"FIG. 30 is a graph of height verses distance measured as indicated in FIG. 28 after in vitro testing of an embodiment of the present invention."
For additional information on this patent application, see: Gilbert, Jeremy. Prevention of Fretting Crevice Corrosion of Modular Taper Interfaces in Orthopedic Implants. Filed
Keywords for this news article include: Alloys, Chemicals, Rehabilitation, Stainless Steel, Solution Chemistry,
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