Patent Application
20080221688
The present invention relates generally to the field of preparing medical devices for implantation.
Some implants include roughened bone-engaging surfaces that increase friction between implant surfaces and bone structures. While potentially beneficial for maintaining the implant in its desired location relative to the mating bone structure, some surface roughening procedures also may introduce microscopic stress risers. Further, in some static and some dynamic applications, the implant may be subject to loading, including cyclic loading. Over time as a result of the loading, the implant may fatigue, and cracks may initiate at the stress risers. Further loading may exceed the limits of the device, and propagate cracks that lead to failure of the device. Increasing resistance to fatigue may extend the recommended life cycle of the implant.
The present disclosure is directed to a method of maintaining fatigue performance of an implant to overcome one or more deficiencies in the art.
In one exemplary aspect, the present disclosure is directed to a method including introducing a residual compressive stress into a bone-engaging portion of an implantable device configured for implantation in a body. The bone engaging portion may include a bone-engaging surface. The method also may include texturing the bone-engaging surface of the implantable device to increase a roughness of the bone-engaging surface.
In an exemplary aspect, the introducing a residual compressive stress is performed until the compressive stress is at a first depth in the bone engaging portion, and wherein texturing the bone-engaging surface is performed until the texturing is at a second depth in the implantable device, and wherein the second depth is less than the first depth.
In an exemplary aspect, the introducing a residual compressive stress includes shot peening the bone-engaging portion and the texturing the bone-engaging surface includes grit blasting.
In another exemplary aspect, the present disclosure is directed to a method of treating a bone-engaging portion of an implantable device to maintain fatigue resistance properties while providing surface texturing. The method may include peening the bone-engaging portion of the implantable device to introduce a residual compressive stress into the bone engaging feature. The residual compressive stress may have a first depth. The method also may include texturing the peened bone engaging portion to increase a surface roughness of the bone engaging portion. The texturing may have a second depth.
In an exemplary aspect, the second depth is less than the first depth.
In an exemplary aspect, the texturing comprises grit blasting the bone-engaging portion of the implantable device.
In another exemplary aspect, the present disclosure is directed to an implantable device. The device may include a bone-engaging portion having a bone-engaging surface and a thickness. The bone-engaging portion may have a residual compressive stress extending to a first depth. The bone-engaging surface also may have a roughened texture. The roughened texture may penetrate the bone engaging surface to a second depth that may be less than the first depth.
Further aspects, forms, embodiments, objects, features, benefits, and advantages of the present invention shall become apparent from the detailed drawings and descriptions provided herein.
For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments, or examples, illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described embodiments, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates.
The systems, devices, and methods described herein may be used to increase the projected useful life of bone-engaging implants. Some conventional implants include bone-engaging surfaces which have been treated to introduce surface irregularities, or treated to increase surface roughness. Increased surface roughness cooperates with adjacent bone to frictionally secure the implant in place. In addition, an implant may biomechanically engage an adjacent bone as the bone grows into irregularities and imperfections available on a roughened surface.
A roughened surface, although advantageous for securing an implant, may be detrimental to an implant's fatigue strength. Forces or loads repeatedly introduced during shifting of weight, such as during patient movement, impart cyclic stress on the implant. This cyclic stress presented over time may fatigue the implant, lowering its estimated useful life.
Stress typically concentrates at certain locations, such as at surfaces and at certain physical features. The physical features where stress concentrates may be intentional, such as at a sharp corner, or unintentional, such as at a crack. A microscopic, or near microscopic crack in the surface of a load bearing object may present a potential stress concentration as a stress riser. Cyclical loading of a surface impaired with such a stress riser may lead to a shortening of the product's useful life.
For example, in a conventional material, a tensile stress concentrated at a crack tends to pull the crack open. As the crack opens, the root of the crack travels deeper into the surface, thereby further reducing a cross-section of the material available to resist the tensile load. Thus, even more stress is concentrated into the crack. Each time the tensile load is applied, the crack deepens, and if unabated, the repetitive, or cyclic loading will drive the crack deeper until the load bearing object is rendered unusable by such fatigue cracking.
Increasing resistance to effects of cyclic loading, for example, as against tensile forces acting on the surface, may increase an implant's projected life. More particularly, using the systems, devices, and methods disclosed herein, increased resistance to fatigue failure may be achieved while still maintaining the implant's ability to cooperate with the bone to frictionally or biomechanically engage the bone.
Turning now to
In order to increase the estimated useful life of the implant, the bone-engaging surface 114 may be treated to maintain fatigue resistance properties while promoting bone integration and frictional resistance to displacement. In this example, the bone engaging surface 114 may be treated to first introduce compressive stress to the implantable device, followed by a treatment to roughen or texture the bone-engaging surface of the implant.
One exemplary process for treating the bone-engaging surface 114 is described with reference to
The surface of the implant 100 may be described as being comprised of many layers of atoms arranged in a lattice, or matrix. Spaces or voids, as well as out-of-place metal atoms or interstitials, are interspersed throughout the matrix. It is possible to force interstitials, along with otherwise aligned atoms, into these voids in a deeper surface layer. Plastic deformation occurs during the permanent dislocation of a metal atom, resulting in a breaking of existing atomic bonds followed by subsequent re-bonding in a new location. If a dent is limited to a dimple on only one side of a work piece, the atoms have been compressed into a smaller space. The area of the plastic deformation contains more atoms, hence more electrical bonds. As more and more atoms occupy the same space in a metal, that space's ability to deform plastically diminishes so that working more metal atoms in the same space creates a stronger metal, albeit with less ductility.
Thus, the compression of metal atoms en mass both hardens and strengthens the material. This strengthening of the metal is termed strain-hardening, or work-hardening, and it is accomplished through plastic deformation. In addition, work-hardening results in a residual compressive stress. Essentially, any applied tensile forces may be countered by the compressive force already existing in the surface layers. Shot peening is one method of work-hardening a material, such as the surface 114 of the implantable device 100.
Referring now to
Although some energy is dissipated as heat and other energy potentially lost through break-up of a shot particle, the remaining energy is transferred into the bone-engaging surface 114. The instantaneous transference of energy upon impact physically displaces a volume of metal at the point of impact, as shown in
Upon impact, an unconstrained portion of the displaced metal plastically deforms into free space on either side of the single shot 120, forming ridges 122 with a raised, rounded edge.
A constrained portion 124 of the area around the impact is bounded and unable to plastically deform into free space. The constrained portion 124 is work-hardened as a certain volume of metal atoms is compressed into a lesser volume. The work-hardened or constrained portion 124 has a thickness or depth 126 that corresponds to the amount of energy imparted upon impact of the single shot 120.
The depth of the compressive layer in shot peening is dependent on a number of controllable factors, including shot size, shot material, shot velocity, distance between the surface and the nozzle, angle of impact and time under shot peen. Other considerations include the repair status of the shot peen device, the degradation of the shot peen media over time and the internal degradation of the shot peen device over time.
While shot peening in general can change the appearance of a surface, only the deeper, plastically-deforming dimples result in improved mechanical properties. Therefore, it is useful to be able to determine the depth and consistency of coverage.
Generally, there are two measurements used to verify the shot peening process. “Coverage” refers to the degree of overlap of dimples that is attained. Coverage can be examined visually and directly. “Intensity” refers indirectly to the amount of plastic deformation imparted to the target material.
However, the intensity and consistency of coverage cannot be directly equated to desired mechanical conditions without resorting to destructive test methods. Non-destructive test methods such as X-ray radiography, mag-particle inspection, ultrasonic testing, visual inspection, dye penetrant inspection, eddy current testing, and coupon testing and correlation, among others, can be used as indirect measures of depth and consistency.
One method of verifying coverage and intensity employs Almen strips. These uniform steel test coupons physically deform under peening, indicating the coverage and intensity. These may be used in test experiments that subject the same implantable device to increasing amounts of peening time. Other factors are held constant throughout the experiment such as shot velocity, location of the implant, shot size and quality, angle of impact, material and shape of the implant and Almen strip manufacturing lot, among others. Subsequent to, or in conjunction with the shot peening of the sample implant, an Almen strip is shot peened under the same controlled conditions. The peening time is then increased, and the test is repeated. As residual compressive stresses accumulate, the Almen strip test coupon begins to curve. At each setting or parameter, the curvature of the Almen strip is measured and the corresponding implant is destructively tested by metallographic sampling. When the metallurgical sample exhibits the desired depth and consistency of shot peening, the curvature of the corresponding Almen strip will be measured and charted. The correlation between Almen strip curvature and actual surface compression produces a reliable and repeatable verification method. Hence, the shot peen process can then be manipulated as desired while ensuring that the process imparts a consistent depth of compression to the shot-peened surface.
Various implant features and base materials require varying process controls to obtain a sufficient compressive depth. One variable is the type and geometry of the shot media, which must not have an adverse effect on the target material's metallurgy or surface strength. The media, or shot may be made from cast steel, conditioned cut wire steel, glass, and ceramic, among other materials. The shape of shot may be approximately round as in the case of conditioned cut wire, or actually spherical as in the case of ball bearings.
One experienced in the art of shot peening will be familiar with other variations in establishing the correlation data for verification of the process and the best parameters and machinery to use for a particular implant. In some applications an implant may require partial masking to protect sensitive portions.
The resulting surface after shot peening may include small rounded ridges and dimples. In order to further improve the surface 114 to promote even additional surface bone-engaging texturing, the surface 114 in some embodiments, may be exposed to additional processing. This additional processing may create a surface that even more fully promotes bone integration and frictional resistance to displacement.
A processing step following shot peening applies a further more random roughening of the surface. In this embodiment, the bone-engaging surface 114 is textured, or roughened, beyond what is attainable through shot peening alone.
Turning to
The grit blasting process may include any known grit, which is selected based on a survey of the target material used for an implant. Grit particles may be formed of glass, sand, metal, polymers, slag, alumina oxide, among others. Typically, though not always, the selected grit particles are harder than the implant material.
Grit blasting alone, while useful for improving coating adhesion, can create stress risers leading to a shortened useful life.
Thus, as shown in
Returning to
Although the above example uses shot peening for the work-hardening process, other work-hardening processes also may impart a suitable compressive layer to the implant. For example, in some exemplary processes, the compressive stress layer is introduced to the implant using a forging process, a pressurization process, a water jet process, a drawing process among other processes and treatments. Cold-working treatments may be used to work harden the implant 100. Some of these may include, for example, cold rolling, roll forming, drawing, deep drawing, pressing, bending, cold forging, cold extrusion, hammering, and shearing, among others.
Alternatively, other forms of work-hardening via peening processes other than shot peening can be used. For example, laser peening uses shock waves to induce residual compressive stress. This may be useful when a very deep, or tightly controlled compressive layer is desired. Strain peening also may be used, whereby the implant is pre-strained below its elastic limit so that the bone-engaging surface is in tension is followed by shot or laser peening the surface to create a compressive layer, and then releasing the implant to impart further compression as it returns to its original form. Dual peening may be used to introduce additional compression by shot peening a second time with a smaller-sized shot.
Also, it is noted that grit blasting is just one example of a texturing process that may be used to promote bone integration and frictional resistance to displacement. Other suitable processes include, for example, chemical or electrical etching, sanding, electrical discharge, grit-blasting, abrading, plasma etching, or embedding particles within the surface. Yet other processes for texturing or roughening the surface of the implant 100 are contemplated.
Also, it should be noted that treatment of the entire bone contacting surface or a portion of the bone contacting surface may be suitable to impart the strength and surface texture desired. For example, in
The upper portion 202 includes an upper surface 206 and a keel 208, while the lower surface includes a lower surface 210 and a keel 212. These surfaces 206, 210, along with surfaces of the keels 208, 212 act as bone engaging surfaces that interface with the bone tissue of the adjacent vertebrae. The bone engaging surfaces may be treated by a work-hardening process to increase fatigue resistance and then by a texturing process to increase the capacity of the bone engaging surfaces to mechanically engage adjacent bone tissue. In some embodiments, only the upper and lower surfaces are treated, while in other embodiments only the keels are treated. In yet other embodiments, the keels and the outer surfaces are treated. Some embodiments may include only a portion of a surface to be treated with one or both of the blasting and texturing processes.
In some embodiments, the ball and socket joint components may be highly polished and any imperfections may be undesirable. Therefore, a manufacturer may desire to protect the ball and socket joint 205 from either the ball peening or from the grit blasting material. Accordingly, prior to the ball peening and blasting processes, the ball and socket joint components may be masked so as to protect them from accidental peening and blasting.
The implants need not be under cyclic load to benefit from the process disclosed herein. Accordingly, any bone engaging surface may be benefited from the processes disclosed herein. For example, in addition to the implants mentioned herein, the process may be used to increase the fatigue resistance and the bone engaging properties of implantable devices, such as bone pins and bone screws. As described above, these processes may find particular utility when used on spinal implants that may be subject to cyclic loading.
It is understood that all spatial references, such as “top,” “inner,” “outer,” “bottom,” “left,” “right,” “anterior,” “posterior,” “superior,” “inferior,” “medial,” “lateral,” “upper,” and “lower” are for illustrative purposes only and can be varied within the scope of the disclosure.
While embodiments of the invention have been illustrated and described in detail in the disclosure, the disclosure is to be considered as illustrative and not restrictive in character. All changes and modifications that come within the spirit of the invention are to be considered within the scope of the disclosure.
