POROUS METAL SURFACE AND METHOD OF PRODUCTION

Abstract: A porous surface (2) is selectively formed on a workpiece such as a medical implant (8) by pulsing a laser device in a controlled manner to produce a plurality of small, spaced-apart cavities (6) of uniform or variable depth. The porous surface provides a mechanical grip with bone cement as well as a medium for bone and tissue ingrowth.

Description: Porous Metal Surface And Method Of Production BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates generally to porous metal surfaces and methods for forming such surfaces, particularly for use in medical prostheses. BRIEF DESCRIPTION OF THE PRIOR ART It is well-known in the medical implant art to provide a porous surface on selected areas on a medical prosthesis to permit the bone cement, or ideally the bone itself, to penetrate the voids in the surface in order to establish and maintain a strong mechanical bond with the implant. A frequently employed technique for creating such an active implant surface area involves the selective placement of a porous coating on the implant device. The most commonly used porous coatings are gravity or pressure sintered spherical powders, diffusion bonded metal fibers and plasma sprayed powder coatings. Exemplary of such sintered metal powder coatings are those described in our U.S. Patent Nos. 4,612,160 and 4,854,496. Titanium and titanium alloys have experienced wide usage as medical implant materials, especially for medical prostheses such as orthopedic devices in the form of knee and hip joints. Diffusion bonded metal fiber coatings have been produced from titanium wire in the form of random porous fiber metal coatings. Likewise, in plasma sprayed coatings, it is also known to utilize either commercially pure titanium or titanium alloy powders. The desirability of producing porous surfaces on medical prosthetic devices is well-known as seen, for example, in U.S. Patent No. 3,855,638 to Pilliar, U.S. Patent No. 3,605,123 to Hahn, U.S. Patent No. 4,017,911 to Kafesjian and U.S. Patent No. 3,808,606 to Tronzo. The various medical factors involved in bone or tissue ingrowth, including those involved in bone cement adhesion, are documented and well-known to those in the medical implant art. The medical community has also clearly indicated that control of the pore size in porous coatings is highly desirable and that dimensional control of the implant itself is absolutely necessary for satisfactory prosthetic devices. In addition, careful control of the metal chemistry and the elimination of sources of contamination are also recognized to be essential for implanted prosthetic devices. As noted previously, it has been common practice to produce porous surfaces by applying a porous coating to the implant substrate. It has been found in some instances, however, that after prolonged periods of use, portions of the porous coating may break loose from the coating mass. When such an event occurs, the loose porous coating material becomes a contaminant in the surrounding tissue. Naturally, such contamination is highly objectionable since it may require surgical intervention in order to correct the problem. It is certainly desirable to prevent such contamination while still providing sufficient porosity in the implant surface to permit proper mechanical interlocking by cement adhesion and bone or tissue ingrowth. The present invention solves these prior shortcomings by providing an improved porous surface which eliminates the opportunity for surface breakaway and subsequent tissue contamination. The present invention provides a process for forming a porous surface on or in a workpiece such as a medical implant, having closely controlled porosity which may be uniformly dispersed or varied in spacing and in size in pre-selected areas over the implant surface. The process of the present invention further provides a porous surface which is more economical to produce than comparable processes such as the commonly used powd≤r .metallurgy sintering cr diffusion bonding processes of the prior art. The present invention also provides a medical implant device having extremely close dimensional tolerances with closely controlled porous - ¬ surface areas having pores or cavities of selected size and spacing. The present invention provides a porous surface and process for producing same in which the size of each cavity with respect to its diameter and depth may be closely controlled in either a constant or varying pattern across the workpiece to provide consistent high quality surfaces. In addition, the cosmetic appearance of the porous surface is enhanced. SUMMARY OF THE INVENTION The present invention is directed to the formation of porous surfaces, wherein a workpiece of, for example, commercially pure titanium, titanium alloy, or cobalt- chromium alloy has a plurality of cavities formed therein of a predetermined diameter, depth and spacing. The method of the present invention comprises the steps of providing a workpiece, such as a medical implant device, for example; mounting the workpiece in a fixture, preferably having positioning means associated therewith; providing a laser in spaced relation to said mounted workpiece; adjusting the power of the laser to provide a laser beam of a selected value whereby a cavity of pre-selected depth is drilled into the surface of the workpiece; and pulsing the laser at a selected frequency while simultaneously moving one or both of the laser and workpiece relative to one another, whereby a plurality of spaced-apart cavities of a pre-selected size and spacing are formed in the workpiece surface. By selecting the proper focal point, laser power level and indexing location, a surface with selected surface connected porosity characteristics is produced. The porosity may be produced in specific locations, with a pre¬ selected size and density of cavities to provide a surface area of closely controlled dimensions particularly suitable for medical implant devices. A workpiece having a porous ϋuilάcβ produced in accordance vitu tn£ present invention is also suitable for uses other than medical implants, such as, for example, titanium components bonded by epoxy to other structural elements. The porous surface provides an excellent base for obtaining a strong mechanical joint with castable epoxy materials. BRIEF DESCRIPTION OF THE DRAWINGS These, as well as other objects and advantages of the present invention, will become clearer when reference is made to the following description when taken with the drawings in which: Figure 1 is an enlarged plan view of a porous surface in accordance with the present invention; Figure 2 is a side cross-sectional view taken along line II-II of Figure 1; Figure 3 is a side elevation view of an acetabular cup having a porous surface formed in accordance with the present invention; Figure 4 is a side elevation view of a hip prosthetic device having a pattern of surface porosity produced in accordance with the present invention; Figure 4A is an enlarged view of a portion of the porous surface taken from area 4A of Figure 4; Figure 5 is an enlarged plan view of a porous surface in accordance with the present invention; Figure 6 is a cross-sectional side view of the surface taken along line VI-VI of Figure 5; Figure 7 is a schematic perspective view of a laser device and workpiece positioning table and associated controls useful in practicing a method of the present invention; Figure 8 is a photomicrograph of a titanium alloy workpiece showing a porous surface produced in accordance with the present invention at a magnification of 20 power; Figure 8A is a photomicrograph of a cobalt- chromium alloy showing a porous surface produced in accordance with the present invention at Ξ. magnification of 20 power; Figure 9 is a photomicrograph showing a cross- sectional side view of two cavities in a titanium alloy workpiece at a magnification of 50 power; Figure 10 is a photomicrograph showing a cross- sectional view of a cavity formed in a titanium alloy at 100 power magnification; Figure 11 is a top side perspective view of a tibial knee tray useful in a knee implant system; and Figure 12 is a top side perspective view of a femoral knee component useful in a knee implant system. DETAILED DESCRIPTION OF THE INVENTION It will be readily understood that the present invention utilizing a laser enables the production of medical implant devices of very close dimensional tolerances and without potential contamination since the conventional porous surface layer is eliminated. As is known, the term laser is an acronym for light amplification by stimulated emission of radiation. Further description of lasers may be found in Van Nostrand's Scientific Encyclopedia, Seventh Edition, Douglas M. Considine, Editor. There are a number of materials that are capable of being utilized as lasers, however, those which most often are used for industrial processing are either of the CO- or YAG type. Of particular usefulness in the present invention is the YAG laser utilizing a neodymium-doped yttrium aluminum garnet material operating at between about 25 to 50 watts of power. The power level is adjusted according to the size, depth, and repetition rate utilized for drilling into the surface layer of the implant substrate being treated. According to the process of the present invention, the workpiece, such as a medical implant or other workpiece whose surface is to be modified, is positioned in the near focal point of a suitable laser system. The laser may be pulsed, for example, at between about 8 to about 15 pulses per second at the desired power level to produce a cavity of desired depth and diameter. Either the laser beam is moved to another selected position and pulsed again, or the substrate is moved prior to the next laser pulse. Those skilled in the art will readily understand that a combination of concurrent movement of the substrate and laser beam will accomplish the same end result. Indexing movement of the workpiece or laser device is either continuous or intermittent. By selection of the focal point, laser power level and indexing parameters, a surface with any desirable porosity is easily produced and replicated with consistent quality. The porous surface formed by the present invention may be produced in specific locations on the workpiece and with a selected density of cavities. The porosity produced is easily distinguished from laser marking such as that utilized to mark metal and plastic items for identification or decorative purposes, as such marking is less than 0.005" deep. In order for porosity to be useful for either bone ingrowth or adhesive bonding, the depressions are significantly over 0.005* deep, and preferably on the order of about 0.030" deep. Perhaps the greatest advantage of the present invention over prior art methods resides in the fact that the porous surface is integral with the implant base material so as to avoid the flaking contamination problems present in prior coatings applied by sintering, plasma spraying, or by diffusion bonding techniques. In Figures 1 and 2, a thin workpiece 2 is depicted in idealized fashion having a flat upper surface 4 and a plurality of pores or cavities 6 drilled therein by means of a laser device such as that schematically shown in Figure 7. Of course, it is understood that the scale of Figures 1 and 2 is enlarged approximately 10 times actual size so that the details of the cavities 6 can be easily seen. Utilizing a pulsed laser beam with a controlled energy level, the cavities 6 are formed in the surface 4 of the workpiece 2 in a repetitive manner as shewn in Figure 1. Typically, the operating conditions are selected such that the cavities 6 have a controlled depth "a" which is less than the thickness of the workpiece 2 so as to avoid complete penetration thereof. A typical overall thickness of the workpiece 2 may be on the order of about 1/8". The cavity diameter "d" is likewise controlled as well as the cavity spacing "s" such that bone ingrowth and bone cement adherence is enhanced. EXAMPLE 1 A flat titanium workpiece of "CP" or commercially pure titanium was prepared having a planar surface 4 as in Figures 1 and 2. The titanium substrate 2 was positioned on a moveable work table spaced from a YAG laser. The alignment of the workpiece 2 was calibrated using a red ruby laser alignment beam produced by the laser machine in conventional fashion. After aligning the laser focusing lens approximately 5" from the titanium substrate 2, a shielding or inert cover gas of argon was turned on and the YAG laser was activated in a pulsating mode having a frequency of approximately 11 pulses per second. The table with the CP titanium substrate 2 mounted thereon was moved beneath the stationary pulsating laser beam and a series of cavities 6 were formed therein along a first row "A", Figure 1. The table and attached workpiece were moved at a constant rate; the laser, which also was pulsed at a constant frequency, formed cavities 6 in an equally spaced manner. Rather than moving at a constant rate, the table and workpiece can be intermittently moved. After the row "A" of cavities 6 was formed, the laser was turned off and the table returned to the starting position and indexed one cavity spacing "s" perpendicular to the first row "A" of the newly produced cavities. At this point, a second row "B" of cavities was formed. This procedure of row indexing was repeated for rows "C" through "H" until the desired modified area of surface porosity was produced. A typical diameter "d" for the cavities 6 is between about 0.020" and about 0.030", with a preferred depth "a" between about 0.020" and 0.030" and a center-to-center cavity spacing "s" of between about 0.025" and 0.030". A preferred range for the cavity spacing is greater than 1/2 a cavity diameter, but preferably less than 0.125". As seen in Figures 5 and 6, the rows of cavities 22 and 24 may be formed of different depths a, a' to provide a porous surface of varying yet controlled porosity across a predetermined width thereof. In this example, depth a is about 0.020" and a' is about 0.030". Photomicrographs of actual porous surfaces and individual cavities formed by the invention are depicted in Figures 8, 8A, 9 and 10. It will be readily observed from the photomicrographs that a typical cross-sectional cavity geometry is not completely concentric as indicated in Figures 1, 2, 5 and 6 of the drawings. Rather, the cavities are slightly skewed from the vertical axes in the direction of the travel direction of the positioning table. This slightly skewed configuration is actually beneficial in that it presents a greater opportunity for stronger mechanical bonding between the cement and/or bone structure and the implant than is the case with a smooth, concentrically formed cavity surface. The small melted and frozen protrusions at the top surface of the cavities can also be readily removed for cosmetic purposes through a simple tumbling operation after the laser drilling step is completed. The photomicrographs of Figures 8, 9 and 10 show a plurality of cavities having a diameter of about 0.024", a depth of about 0.025", with a spacing of about 0.030" from cavity center to cavity center. EXAMPLE 2 A femoral stem of a hip prosthesis 8 constructed, for example, of a Titanium-6 Aluminum-4 Vanadium ELI alloy is depicted in Figure 4 having a porous surface portion 10 formed in accordance with the present invention. The pore pattern 10, as seen in Figure 4A, is triangular in plan view configuration and was applied to selected areas to enhance bone ingrowth on the anterior and posterior surfaces of the mplantable device 0. As seen in the actual photomicrographs of Figures 8 and 9, and in Figure 4A, cavities 12 were formed in surface 10 having a diameter of 0.024" and a depth of 0.025" with a center-to-center cavity spacing of 0.03.0" average. The cavities 12 were formed using a C0_ laser with an argon cover gas to protect the surface of the oxygen reactive titanium alloy during the laser drilling operation. Curved or other complex surface geometries may also be utilized in the method of the invention. As seen in Figure 3, an acetabular hip cup 14 in the shape of a thin- walled hemisphere has its outer surface treated according to the present invention wherein a porous surface 16 is formed around a selected portion thereof. A typical acetabular hip cup 14 includes spaced screw holes 18, 18' formed therein for securement of screws directly to the bone. The cup 14 possesses a thin wall, on the order of about 0.125" thick, which makes laser drilling of the cavities to a controlled depth of about 0.030" particularly desirable. In this manner, a porous surface 16 of controlled depth is formed on the outer surface of the cup 14 without drilling through the thin wall and without impairing the load bearing structural integrity of the cup. The danger of tissue contamination potentially present in prior art porous surface coating techniques is, likewise, eliminated. Examples of other complex surface geometries are shown in Figures 11 and 12. A tibial knee tray 50 having a porous surface 52 formed in accordance with the invention is shown in Figure 11. A femoral knee component 60 having a porous surface 62 formed in accordance with the present invention is depicted in Figure 12. The femoral knee component 60 and the tibial knee tray 50 are useful components in a knee implant system. EXAMPLE 3 A substrate 20 of a cobalt-chromium medical implant alloy designated ASTM F-75-87 was treated in accordance with the present invention. The cobalt-chromium «tυ .j. .6Cc as pu5i _.iuiicu _ιι <-ι « - ι_<_ι.ι_/ ___.£; spaced from a YAG laser and rows of cavities were formed in the surface thereof. Figure 8A depicts a photomicrograph of the cobalt-chromium medical implant alloy at 20 power magnification showing controlled porosity produced with the YAG laser beam. The cavities are typically 0.027" in diameter and 0.021" average depth with a center-to-center spacing of about 0.030". The ASTM type F-75-87 cobalt- chromium alloy is a cast material and it is understood that the wrought cobalt-chromium alloy designated ASTM F-799-87 is also suitable. A single cavity formed in a Titanium-6 Aluminum-4 Vanadium ELI alloy is depicted in the photomicrograph of Figure 10, at a 100 power magnification. It is observed in Figure 10 that the laser drilling technique of the present invention utilizing a quickly pulsed beam produces a very thin heat affected zone surrounding each cavity. The heat affected zone is shown as a white layer in the photomicrograph, with the balance of the micro structure unaffected by the highly localized melting effected by the pulsed laser beam. Thus, the metallurgical structure and mechanical properties of the adjacent titanium alloy material are substantially unaffected by the laser drilling technique of the present invention. One presently preferred arrangement of the basic equipment for practicing the method of the present invention is depicted in Figure 7. A conventional laser electrical control system 26 for regulating the power and pulse frequency of the laser beam is operably coupled by conduits 28 to a conventional laser beam generator 30. The laser beam generator 30 is mounted on a cross brace 31 carried by a support stand 32 to position the laser beam generator in a spaced location above a workpiece positioning unit 34. The laser beam generator 30 is conventional and may include a YAG, C0_, or like laser source for producing a coherent laser beam 36 useful for causing localized melting in metal or other materials. A conventional lens 37 is mounted on an arm 38 carried by the cross brace 31 for focusing the laser beam 36 on a surface of a workpiece 40, suc as the medical implant acetabular cup 14 or femoral stem 8, as previously described. The -workpiece positioning unit 34 includes a moveable work table 42 carrying a rotatable platen 44 which together are moveable in the three x, y and z axes and rotatable about the y and z axes, as shown in the representation of the x, y and z axes identified by reference numeral 45 in Figure 7. In this manner, the work positioning unit 34 provides five degrees of freedom for positioning the workpiece 40 relative to the laser beam 36 whereby surfaces of any contour can be treated in accordance with the invention. Movement of the workpiece positioning unit 42 is controlled by a conventional numerical controlled or "NC" positioning system control 46 of the type commonly used in the computer controlled machine tool art. The positioning system control 46 is operably coupled to the workpiece positioning unit 42 by way of conduits 48. Through appropriate programming of the positioning system control 46, the workpiece 40 is moved a proper distance and/or at a proper speed relative to the pulse rate of the laser beam 36 controlled by the laser control system 26 to provide proper spacing between the cavities formed in the workpiece 40 by the laser beam 36. Likewise, when drilling non-planar surfaces, a proper focal length is maintained by vertical movement of the workpiece positioning unit effected by the pre-programmed commands supplied by the positioning system control 46 which also controls the horizontal indexing movement of the workpiece positioning unit 34. As a result, high quality and consistent dimensional accuracy is assured. Of course, it is understood that the workpiece can remain stationary and the laser beam generator 30 can be moved to accomplish similar results. Also, while the invention has been described in connection with only one laser beam, it will occur to those skilled in the art that multiple laser beams could be employed simultaneously to form a porous surface according to the invention. Having described presently preferred embodiments, it is to be understood that it may be otherwise embodied within the scope of the appended claims.

Claims: -12- Having described the invention, what is claimed is: 1. A method of forming a porous surface on a workpiece comprising the steps of: (a) providing a laser device for generating a laser beam; (b) providing means for orienting said workpiece relative to said laser beam at a first location; (c) activating said laser device to direct the laser beam to the workpiece to form a cavity in said workpiece at said first location; (d) interrupting said laser beam and simultaneously moving one of said workpieces or laser devices to a second location; (e) activating said laser device to direct the laser beam to the workpiece to form a cavity in said workpiece at said second location; and (f) pulsing said laser device to alternately activate and interrupt said laser device and indexing said workpiece in accordance with steps (c) , (d) and (e) to form a plurality of spaced- apart cavities along a selected surface area. 2. The process of claim 1 wherein the workpiece is a medical implant device of a material selected from the group consisting of commercially pure titanium, alloys of titanium, and alloys of cobalt-chromium. 3. The process of claim 1 wherein the cavities formed are generally cylindrical in shape, having a diameter of between about 0.020" to about 0.030", a depth of between about 0.020* to about 0.030" and an index spacing of about 0.025" to about 0.030" as measured between respective centers of an adjacent pair of cavities. 4. The prccccG cf claim 1 including the step of selectively regulating a power output of said laser device whereby cavities of different size are selectively formed within said porous surface. 5. The process of claim 1 wherein the workpiece is a medical implant device in the form of an acetabular cup. 6. The process of claim 1 wherein the workpiece is a medical implant device in the form of a femoral stem hip prosthesis. 7. The process of claim 1 wherein the workpiece is a medical implant device in the form of a tibial component of a knee implant system. 8. The process of claim 1 wherein the workpiece is a medical implant device in the form of a femoral component of a knee implant system. 9. The process of claim 1 wherein the means for orienting the workpiece includes a workpiece positioning means moveable in at least three axes and a control means for selectively moving said workpiece positioning means relative to said laser beam. 10. The process of claim 1 including the step of moving the workpiece along a vertical axis to maintain a selected laser beam focal length when indexing in a horizontal axis along a non-planar workpiece surface. 11. The process of claim 1 wherein the laser device is one selected from the group consisting of YAG and CO- lasers. 12. The process of claim 11 wherein said laser device is a YAG laser activated at a power level of between about 25 to about 50 watts. 13. The process of claim 1 wherein the laser device is moved relative to the workpiece to achieve movement between pulses. 14. The process of claim 1 wherein the workpiece is moved relative to the laser device to achieve movement between pulses. 15. The process of claim 1 wherein both the wcrkpisce and the laser device are moved simultaneously to achieve movement between pulses. -14- 16. The process of claim 1 wherein the workpiece is a medical implant made from a titanium alloy and wherein said laser device is a YAG laser and wherein a spacing, as measured between centerlines of adjacent pairs of cavities, is more than one-half a diameter of a cavity and less than about 0.125". 17. A workpiece produced in accordance with the process of claim 1. 18. The workpiece of claim 17 wherein the porous surface is adapted to be joined to an epoxy type material. 19. A process for producing a porous surface on a workpiece comprising the steps of: (a) providing a laser device; and (b) pulsing said laser device and indexing said workpiece to form a plurality of spaced-apart cavities within said workpiece having a controlled depth less than a thickness of said workpiece. 20. The process of claim 19 wherein the workpiece is a thin-walled medical implant device in the shape of a hemispherical shell for forming an acetabular cup in a hip implant system. 21. The process of claim 20 wherein the hemispherical shell has a wall thickness of about 0.125". 22. A medical implant constructed of a base material comprising at least one porous surface area defined by a plurality of spaced-apart, cylindrically-shaped cavities formed in said base material adapted to provide a mechanical bonding surface between said implant and a surrounding environment. 23. The medical implant device of claim 22 wherein the cavities are formed in a predetermined array by a laser beam. 24. The medical implant of claim 23 wherein the base material is one selected from the group consisting of commercially pure titanium, alloys of titanium and alloys of cobalt-chromium.

Inventors: BUGLE CLIFFORD M, DONLEVY ALFRED L, BUGLE, CLIFFORD, M., DONLEVY, ALFRED, L.
Applicants: DYNAMET INC, DYNAMET, INC.
Application year: 1991
Main classification: A61F2
IPC codes: A61F2, B23K26