WO1991016013A1

WO1991016013A1 - Improved surgical implants and method

Info

Publication number: WO1991016013A1
Application number: PCT/US1991/002736
Authority: WO
Inventors: Piran Sioshansi; Richard W. Oliver; Eric J. Tobin
Original assignee: Spire Corporation
Priority date: 1990-04-25
Filing date: 1991-04-22
Publication date: 1991-10-31
Also published as: JPH05507425A; EP0526581A4; US5123924A; EP0526581A1

Abstract

An improved surgical implant, including dental implant (10) formed as one of a group consisting of cobalt-chromium and its alloys, ant Ti and its alloys (14, 15) in contact with a bearing surface formed by UHMWPE (18) and a process of its manufacture are disclosed. The improved surgical implant is designed to withstand fretting wear and abrasion by vibrating micromotion of the implant against the surrounding bone structure and to reduce the wear of the UHMWPE component (18) of the surgical implant (10, 12) enhancing its useful life. Such micromotion has caused undesirable blackening in the surrounding tissue and has required premature replacement of the implant. The process essentially includes the ion implantation of the metallic component, with a resultant increase in its micro-hardness and a decrease in its coefficient of friction, particularly when articulating against the UHMWPE component.

Description

IMPROVED SURGICAL IMPLANTS AND METHOD

This is a PCT application incorporating the disclosures of the pending U.S. applications, to wit, application Serial No. 07/514,503, filed April 25, 1990, and a continuation-in- part application thereof, Serial No. 07/619,929, filed November 28, 1990, both assigned to a common assignee, Spire Corporation, Bedford, Massachusetts.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to surgical implants, including dental implants and, more particularly, to improved surgical implants formed of cobalt-chromium or titanium and their respective alloys, and a process of ion implanting the same.

2. The Prior Art

Surgical implants are widely used today in total joint replacements involving deteriorating or damaged hips, knees, shoulders, toes, fingers, elbows and in dental implants.

Surgical implants essentially comprise two parts: a metal part formed with an articulated surface designed to be received in and rub against a complementary load-bearing plastic surface of either an all-plastic part or a metal part with a plastic surface. The choice of metal for the metal part is either titanium and its alloys or cobalt-chromium and its alloys. The choice of plastic for the plastic part is, for the most part, ultra-high molecular weight p_olyethylene (UHMWPE) . In use, both the metal part and its complementary plastic part experience abrasion and wear, but such abrasion and wear are more pronounced with respect to the plastic part. Until recently, the excessive wear of the plastic part has been tolerated and was considered acceptable eventually to wear away in a sacrificial manner. This was so since problems associated with the implant's attachment (or rather the lack of it) to bone to bone and with the rate of rejection of the implant by the body have been considered more significant than the wear of the plastic part. In fact to some extent, the wear representing the implant's useful life, has for the most part been designed into the implant. Recently, the problem of implant-attachment to the surrounding bone has been quite successfully addressed, inter alia, by applying porous coatings to the implant's surface. The problem of body-rejection has been countered with anti-rejection drugs and medication. These advances have caused the wear problem in the plastic component (UHMWPE) to be reassessed. Consequently, there is now a great deal of concern in the medical device industry focusing on the wear of the plastic component in an implant, since the wear has now become the determinant factor for the useful life of the implant in a patient.

In U.S. Letters Patent No. 4,743,493, entitled "Ion Implantation of Plastics" and assigned to said common assignee, Spire Corporation of Bedford, Massachusetts, this wear problem was addressed with some success by ion implanting the plastic surface to a depth from about 0.1 to about 5 micrometers so as to increase its surface hardness and its resistance to chemical attack. A process for preventing surface discoloration of implants formed of titanium and its alloys, which titanium implants have been ion implanted to improve their wear performance, is disclosed in U.S. Letters Patent No. 4,693,760, entitled "Ion Implantation of Titanium Workpieces Without Surface Discoloration," also assigned to the said common assignee, Spire Corporation of Bedford, Massachusetts. Implants made from cobalt-chromium and its alloys, while exhibiting good wear resistance, suffer from poor biocompatibility. A process of passivating the electro-chemically active surface of such cobalt-chromium alloys, hence inhibiting their corrosion, is disclosed in U.S. Letters Patent No. 4,743,308, entitled "Corrosion Inhibition of Metal Alloys," also assigned to the said common assignee, Spire Corporation of Bedford, Massachusetts. This process essentially comprises the forming of a coating of biocompatible element from either platinum, gold or palladium on the surface of the cobalt-chromium implant, preferably by physical vapor deposition and exposing the thus coated surface to ion implantation.

The said copending application Serial No. 07/514,503, filed August 25, 1990, of which the present application is a continuation-in-part, has addressed the improving of the wear resistance of surgical implants made from titanium and its alloys by ion implantation. The present invention is intended, in the first instance, to improve the wear performance of the plastic component of a surgical implant by ion implanting, not the plastic component but rather the metallic component of the implant, with the metallic part of the implant made, not from titanium and its alloys but rather from a cobalt-chromium alloy.

As stated, inter alia, in said copending application Serial No. 07/514,503, in implants that are loosened over time, there develops a micromotion between the implant and the bone or between the implant and the cement interface. One such micromotion i.e., vibration, has developed, blackened f tissue has been observed in the affected area. Mircomotion can occur in both cemented and cementless fixation of implants. Cementless fixation relies on securing the implant in and to the surrounding bone structure with frictional contact, with or without the aid of auxiliary fixation devices implanted adjacent to and concurrently with the surgical implant. The observed blackened tissue is caused by fretting wear and abrasion of the titanium alloy against the surrounding bone structure. This fretting wear and resulting blackening of tissue is and remains a vexing problem in bone and dental surgery. The present invention also is intended to address this fretting wear and the resultant blackening of surrounding tissue.

SUMMARY OF THE INVENTION

This is a PCT application incorporating the disclosures of two pending U.S. applications, to wit, application Serial No. 07/514,503, filed April 25, 1990, and a continuation-in- part application thereof, Serial No. 07/619,929, filed November 28, 1990, both assigned to a common assignee, Spire Corporation, Bedford, Massachusetts.

The present invention is, inter alia, an improvement of a process disclosed in U.S. Patent No. 4,693,760 granted on September 15, 1987 to Piran Sioshansi, one of the co-inventors herein, and assigned to a common assignee, Spire Corporation of Bedford, Mass. , the disclosure of which is incorporated herein by reference.

It is a principal object of the present invention to overcome the above disadvantages by providing an improved surgical implant made of cobalt-chromium and its alloys or titanium and its alloys and a process of making the same.

More specifically, it is an object of the present invention to provide a surgical implant, such as bone or dental implant, formed of cobalt-chromium and its alloys or titanium and its alloys and including a metal part formed of a cobalt-chromium alloy or Ti and its alloys and a plastic part formed of UHMWPE, with the metal part of the implant being ion implanted with one of a group consisting of N +, N~+, C+, Ti+, Ar + , B+ , Ne+, Kr+, He+ , P+ and 0+. The implant preferably is one of a group including prostheses for artificial hips, knees, shoulders, wrists, elbows, fingers and toes, as well as any and all types of dental implants, including endosteal blade and cylindrical implants. Where the bone fixation portion is a femoral hip stem, the stem preferably tapers at an angle of about 2° toward its tip. Preferably, a member formed with an axial opening is designed to fit about the stem, and the axial opening of the member is tapered substantially to parallel the taper of the hip stem. Where the bone fixation portion is an endosteal blade implant, its stem preferably also tapers at an angle of from about 2° to about 4° toward its tip. Where the bone fixation portion is a cylindrical dental implant, its stem is either formed as a threaded cylinder or is of a hollow perforated design, which a partially threaded exterior. Preferably, the ion implantation is effected with an ion beam possessing an energy between about 20 keV and about 400 keV, a current density between 1 c about 0.1 and about lOOuA/cm , and a dose between about 1x10

18 2 and about 1x10 ions/cm .

The ion implantation is designed to create a surface region in the ion implanted cobalt-chromium or titanium surfaces that is characterized by: (1) a decrease in its coefficient of friction when rubbing against its complementary plastic part, (2) improved resistance to chemical attack and

(3) a surface region with a microhardness from at least about

500 Knoop for a 2 grams load to about 1200 Knoop for a 10 grams load. Preferably, the cobalt-chromium part of the surgical implant is a Co-Cr-Mo alloy, such as ASTM F-75 or

F-799 alloy, and the Ti part of the implant is commercially pure Ti or Ti-6A1-4V. Preferably, the plastic material of choice for the vast majority of total joint replacements is ultrahigh molecular weight p_olyethylene (UHMWPE) articulating against a mating surface formed of either a Ti-6A1-4V alloy, or a cobalt-chromium alloy. Note R.M. Rose et al.,

"Exploratory Investigations on the Structure Dependence of the

Wear Resistance of Polyethylene," Wear, 77 (1982) , pp. 89-104;

R.M. Rose et al. , "On the Pressure Dependence of the Wear of Ultrahigh Molecular Weight Polyethylene," Wear, 92 (1983), pp. 99-111; R.M. Rose et al. , "Radiation Sterilization and the Wear Rate of Polyethylene," Journal of Orthopaedic Research, 2: 393-400; and I.C. Clarke et al. , "Wear of Ti-6Al-4V Implant Alloy and Ultrahigh Molecular Weight Polyethylene Combinations," Titanium Alloys in Surgical Implants. ASTM, STP 796 (1983) , p. 136.V.

The process of forming an improved surgical implant according to the invention essentially includes forming an implant of two parts: a first metallic part formed of either cobalt-chromium and its alloys or Ti and its alloys and a second complementary plastic part formed of UHMWPE, exposing all aluminum fixtures and shields mounted within an implant chamber to an ion beam so as to cleanse them of surface contamination and to form a surface layer thereon having a sputtering coefficient lower than that of cobalt-chromium, creating a vacuum within the ion implantation chamber of about 3x10 torr, introducing the first metallic part into the ion implantation chamber to be secured therein by the cleansed and surface layer coated or aluminum fixtures, and exposing the articulating surfaces of the first metallic part of the implant to a direct line of the ion beam, with the ion beam having an ion beam power density on the surface of the implant not exceeding about lOOuA/cm 2 at 360 kV, i.e. 36 watts/cm2, the ion beam incorporating one of a group of elemental species consisting of nitrogen, carbon, titanium, argon, boron, neon, krypton, helium, phosphorus and oxygen, exposing the first metallic part of the implant to the ion beam for a period of about four to about forty hours, with an ion beam particle energy from about 20 keV to about 400 keV so as to implant a dose of about 3x10 17 ions/cm2, and wherein the ion beam

2 current density is between about 0.1 and about lOOuA/cm .

The ion implantation increases the microhardness of the Co-Cr-Mo alloy's of Ti surfaces more than two-fold and decreases their coefficients of friction when rubbing against the complementary UHMWPE part from about 0.138 to about 0.103.

Other objects of the present invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the surgical implant of the present disclosure, its components, parts and their interrelationships, and the method of making the same, the scope of the which will be indicated in the appended claims.

Brief Description of the Drawings For a fuller understanding of the nature and objects of the present invention, reference is to be made to the following detailed description, which is to be taken in connection with the accompanying drawings, wherein:

FIG. 1 is a perspective view of an artificial knee joint prosthesis made of two parts: a metal part and a complemen¬ tary part whose load bearing surface is formed of UHMWPE, with the metal part treated according to a previous process; to

FIG. 2 is a view of a metal alloy part of an artificial knee joint prosthesis, like the one shown in FIG. 1, but not treated according to the previous process;

FIG. 3 is a perspective view of an artificial hip-joint prosthesis made partly of metal and treated according to the previous process;

FIG. 4 is a fragmentary view of a metal component of an artificial hip-joint prosthesis like the one shown in FIG. 3, but not treated according to the previous process; FIG. 5 depicts measurement curves helpful in understanding the previous process;

FIG. 6 is a schematic view of an ion implanter useful in carrying out the process of the invention;

FIG. 7 is a side elevation, partly in section and on an enlarged scale, of a workpiece exposed to the process of the invention according to FIG. 6;

FIG. 8 is a schematic cross section of an artificial hip-joint prosthesis with a sleeve component used for centering a hip stem within a femoral cavity and made according to and incorporating the present invention; FIG. 9 is a schematic of a component part of the hip-joint prosthesis illustrated in FIG. 8;

FIG. 10 is a view of the sleeve component part of the hip-joint prosthesis illustrated in FIG. 8; FIG. 11 is a view similar to FIG. 8 but showing a distal tip component used for centering a hip stem within a femoral cavity;

FIGS. 12 and 15 are views similar to FIG. 9;

FIG. 13 is a view similar to FIG. 10 but illustrating the distal tip component of FIG. 11;

FIG. 14 is a view similar to FIGS. 8 and 11 but illustrating the press fitting of a hip stem within a femoral cavity;

FIG. 16 is a view, on an enlarged scale, of a representative endosteal dental blade implant made according to and incorporating the present invention;

FIG. 17 is a side elevation, on an enlarged scale, of a representative dental implant of threaded cylinder design incorporating the invention;

FIG. 18 is a side elevation, on an enlarged scale, of a representative dental implant of a hollow perforated design, with a partially threaded exterior, and incorporating the present invention;

FIG. 19 is a side elevation, partly in section and on an enlarged scale, of an implant exposed to the process of the invention according to FIG. 6;

FIGS. 20 and 21 depict pictorially, respectively, the results of corrosion resistance testing of an implanted vs. a non-implanted metal part; /__

FIGS. 22 and 23 are tables, indicating the coefficient of friction of an implanted vs. a non-implanted metal part rubbing against UHMWPE and, the microhardness of an implanted vs. a non-implanted metal part, respectively;

FIG. 24 is a pictorial comparison of wear track volumes from pinion disk tests on an implanted and a non-implanted metal part, per three indicated cycles; and

FIGS. 25, 26 and 27 depict curves representing a comparison of wear track profiles of implanted vs. non-implanted metal parts at the three cycles indicated in FIG. 24.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This is a PCT application incorporating the disclosures of two pending U.S. applications, to wit, application Serial No. 07/514,503, filed April 25, 1990, and a continuation-in- part application thereof. Serial No. 07/619,929, filed November 28, 1990, both assigned to a common assignee, Spire Corporation, Bedford, Massachusetts.

In general, the present invention pertains to an improved surgical implant formed of two parts: a metal part, formed of Ti and its alloys or cobalt-chromium and its alloys, and designed to wear against a complementary second plastic part preferably formed of ultra-high molecular weight polyethylene (UHMWPE) . Cobalt-chromium and its alloys, ion-implanted according to the invention, are designed significantly to reduce the wear and abrasion of the UHMWPE part in a wide variety of orthopaedic implants. And an implant formed of titanium and its alloys is designed effectively to withstand fretting wear and abrasion that may be occasioned by mircomotion between the implant and adjacent bone structure.

Titanium-based alloys and cobalt-based alloys have come to be the preferred metals for use as surgical implants.

Advantageous features of titanium-based alloys which make them preferable include excellent tensile strength, high fatigue strength, low density, high corrosion resistance, substantial ductility, a low modulus of elasticity compatible with bone structure that facilitates good adhesion between the bone structure and the alloy and, most importantly, excellent biocompatibility. Titanium and its alloys are strong, light metals that are easily malleable when heated and are ductile, particularly pure titanium. For surgical implantations, the titanium-based alloy, Ti-6A1-4V, has become the most widely used and accepted. Only one undesirable feature of titanium-based alloys has manifested itself over the years, and that has proven to be their poor wear performance.

Such poor wear performance most dramatically has manifested itself in implanted devices that become loose over time. In such loosened implants, either before or after the loosening, micromotion (i.e., vibrating motion) between the implant and the surrounding bone structure is present. Such micromotion of implant against bone subjects the implant to fretting wear and abrasion and blackens the tissue in the affected area.

In use, both the metal part and its complementary plastic part experience abrasion and wear, but such abrasion and wear are much more pronounced with respect to the plastic part. Until recently, the excessive wear of the plastic part has been tolerated and was considered acceptable. This was so since problems associated with the implant's attachment (or rather the lack of it) to bone and with the rate of rejection of the implant by the body have been considered more significant than the wear of the plastic part. Recently, the problem of implant-attachment to the surrounding bone has been quite successfully addressed, inter alia, by applying porous coatings to the implant's surface. The problem of body- rejection, on the other hand, has been countered with anti- rejection drugs and medication. These advances have caused the wear problem in the plastic component (UHMWPE) to be reassessed. There is thus a great deal of concern in the medical device industry focusing on the wear of the plastic component in an implant, since that wear has now become the determinant factor for the useful life of the implant in a patient. The present invention addresses the problems occassioned by fretting wear and abrasion of the metal implant and of the wear rate of the UHMWPE part in an implant by ion implanting the metal part of the implant rubbing against the plastic part.

In FIGS. 1 and 3 are illustrated artificial prostheses for a knee joint 10 in the former and for a hip joint 12 in the latter. Each prosthesis 10 and 12 comprises at least one element formed of metal and a complementary mating element formed of plastic. The illustrated knee joint prosthesis 10 thus is formed of two metal parts 14 and 15, separated by a plastic part 18. The parts 14 and 15 preferably are formed of a cobalt-chromium alloy, such as the surgical ASTM F-75 or F-799 alloy, or of a titanium alloy, such as surgical Ti-6Al-4V alloy. The plastic part 18 on the other hand is preferably formed of ultrahigh molecular weight polyethylene (UHMWPE) . In like fashion, the hip joint prosthesis 12 is formed of a metal part 20 having a hemispherical ball portion 16, preferably formed of surgical ASTM F-75 alloy or surgical Ti-6Al-4v alloy, and a plastic part 22, also preferably formed of UHMWPE. It is understood that the metal part 20 is placed into the femur, either by a cemented or cementless process. During walking, the alloy ball portion 16 articulates against the UHMWPE cup part 22. In like fashion, the metal part 15 of the knee joint prosthesis 10 works against the UHMWPE part 18 during walking. The conditions of loading, sliding velocity and body chemistry that obtain in and about the respective knee and hip prosthesis 10 and 12 in the body are such as tending to produce corrosion and wear in the Co-Cr alloy and/ or the titanium alloy, and wear to a more pronounced extent in /h the UHMWPE component. Looseness may be caused by micromotion between the implant and the bone, or the vibrating micromotion may be the result of the loosening.

U.S. Patent No. 4,693,760 addressed an undesirable by-product of ion implantation, namely surface discoloration in the ion-implanted areas. There, ion implantation was directed at improving wear performance of the cooperating parts, such as the alloy ball portion 16 working against the cup part 22. Little if any attention has been directed at fretting wear and abrasion of the metallic implant against the surrounding bone structure. Perhaps, this is so since this vibrating micromotion between the implant and the bone has not been appreciated or even recognized. The blackened tissue found in the affected area has caused a study of this phenomenon. As a consequence of the study, a theory of micromotion was born. It is the undesirable fretting wear and abrasion of metallic implant against bone, inter alia, that is being addressed herein.

As mentioned, the ion implantation of the metallic parts 15 and 20 may cause surface discoloration of the parts, which make them aesthetically less than desirable both to orthopaedic surgeons and their patients slated for prosthetic implants. FIGS. 2 and 4 illustrate the problem that the process of the previous invention was designed to prevent. FIG. 2 is a view of a part 30 formed of a titanium alloy, such as surgical T1-6A1-4V alloy, which part 30 has been ion / 7 implanted with nitrogen ions, producing a concentration of about 20at. % N to a depth of about 100 nm below the part's 24 surface, as required for improving its wear performance. FIG. 4 is a fragmentary view of a ball part 26 formed of a titanium alloy, such as surgical Ti-6A1-4V alloy, which part 26 also has been ion implanted with nitrogen ion so as to produce a concentration of about 20 at. % N to a depth of about 100 nm below its surface, as required for improving its wear performance. The surfaces 25 and 27 of both parts 15 and 26 exhibit unwanted discolorations 28. For the most part, these discolorations 28 are goldish-yellow or bluish-yellow and, like tarnishing of the metal, appear at certain locations.

In FIG. 5 are depicted RBS measurement curves of an unimplanted sample 36 versus an implanted sample 38 formed of titanium. The sample 38 implanted with nitrogen ion has been implanted with a dose of at least about 2 x 10 17 nitrogen

2 17 ions/cm and, preferably with a dose of about 3 x 10

2 nitrogen ions/cm . The measured data show that the implanted sample 38 has a high concentration of nitrogen and oxygen on its surface (titanium oxynitride) , as indicated by the curve

40. These titanium oxynitride compounds on the surfaces of the workpieces 24 and 26 are mainly responsible for the discolorations 28 thereon.

The process of the invention, producing orthopaedic implants, formed either of titanium and its alloys that are /S capable to withstand fretting wear and abrasion against bone or of a Co-Cr alloy to withstand corrosion and excessive wear, is preferably carried out in a suitable implant chamber 44 of a specially designed endstation 46 of a suitable high current iori implanter, such as a Varian-Extrion 200 kV implanter, an Eaton-Nova implanter or a like instrument. The endstation 46 is illustrated in FIG. 6.

Within the ion implantation chamber 44, a suitable fixture 48 is mounted on a base 50 designed for rotating and cooling a titanium or aluminum base plate 52. On the base plate 52 are mounted a plurality of appropriately shaped workpiece holders 54, made of aluminum or titanium. The work¬ piece holders 54 are designed to hold securely a plurality of workpieces 58 and directly expose these workpieces 58 to an incoming ion beam 56. The illustrated workpieces 58 are the cobalt-chromium alloy parts 15 of the knee joint prosthesis illustrated in FIG. 1. The workpieces also can comprise the femoral components illustrated in FIGS. 8-14. It is to be understood that the shape of the particular workpiece holders secured to the base plate 52 will depend upon the shape of the particular workpieces being processed at that time. In FIG. 19 is illustrated one such workpiece, a cobalt-chromium alloy part 15 and secured to one of the workpiece holders 54. In FIG. 7 is illustrated another workpiece, a member 59 made of titanium or aluminum alloy and secured to one of the workpiece holders 54. Such securing of member 59 to the workpiece holder 54 preferably is effected with the aid of a shaft 70 rotatably mounted within ho-lder 54 and rotatable by means of a suitable motor 72.

The fixture 48 is so designed as to expose, at one time or another, all surfaces of the workpieces directly to the ion beam 56. Any surface of the workpiece which cannot be exposed directly to the ion beam 56 must be shielded by an aluminum shield 60. It is imperative that the shield 60 fit flush with the edges of the part 15 in the back, as at 62. This flush- fitting is important to prevent the ion beam 56 from sputter depositing material around the corners of the part 15 and thus discolor the part 15 in the back.

As will be noted in FIGS. 8 and 10, member 59 is essentially of cylindrical shape and is formed with an axial opening 74. Preferably, the axial opening 74 is tapered to follow the taper of the stem portion 76 so as to center the hip stem within the femoral cavity, please observe FIG. 8. The free end of the mounting shaft 70 is provided with means 79 fictionally to engage, and thus secure, the tapered axial opening 74 of member 59 during its ion implantation by the ion beam 56.

Again and as evident from viewing FIG. 7, the fixture 48 is so designed as to expose, at one time or another, all surfaces of the workpieces directly to the ion beam 56. Preferably and in order to achieve this, the workpieces 58 and zo

59 are mounted normal to the base plate 52. Thus, not only is the base plate 52 rotatable, but in addition, when the shape of the particular workpiece so requires, the workpiece 59 itself also is mounted for rotation about its longitudinal axis by a second means 72, whose rotation is independent of that of the base plate 52.

In FIGS. 16-18, there is illustrated, on an enlarged scale, a representative endosteal dental blade implant 100, a representative cylindrical endosteal dental implant 110, and a representative hollow perforated endosteal dental implant 120, respectively. The dental implant 100 is shown essentially comprising a stem 102 designed to be implanted into the jawbone of a patient and a header 104, designed to have a crown (not shown) fitted thereto. The dental implant 110 essentially comprises a threaded, for the most part solid cylindrical stem 112 serving as the bone fixation portion, and a head screw 114 serving as the crown fixation portion. The dental implant 120, as illustrated, comprises a hollow, partially perforated cylinder, also featuring an externally- threaded part 122. A crown fixation portion, not shown, which may be like the head screw 114, serves as the crown fixation portion, as known. Dental implants, specifically their bone fixation portion, i.e., their stem 102, when implanted into the jawbone, are exposed to essentially identical wear condition as implanted hip stems of the press-fitted type, that is where no cement is used. These conditions include > / fretting wear and abrasion of the metallic implant part against the surrounding bone structure. Dental implants, as a consequence, experience similar blackening of the surrounding tissue. Patient concern is, however, now heightened due to the fact that the blackening occurs in the patient's mouth and is readily observable by the patient. The dental implants, as illustrated by the herein depicted endosteal dental blade implant 100, the cylindrical endosteal dental implant 110 and the hollow perforated dental implant 120, are subjected to the same ion implantation procedure as are the other workpieces 58 described above.

In the practice of the process of the invention, it is important that first all fixtures 48 and shields 60 in the chamber 44 be conditioned or seasoned by being exposed to a full ion implantation dose before performing any ion implantation on the cobalt-chromium alloy parts within the implantation chamber 4 . Such a full ion implantation dose preferably is about 3 x 10 17 ions/cm2 at the surfaces of the fixtures and shields, and extending about 100 nm below those surfaces. Such a dose preferably is effected with the ion beam 56 applied to the surfaces for a period of about three and a half hours, with an ion beam particle energy from about

10 keV to about 200 keV. The ion beam 56 preferably incorporates one of a group of elemental species, including nitrogen, oxygen, carbon, titanium, beryllium, neon, krypton, helium, phosphorus, argon and other noble gases. 2.2.

The conditioning or seasoning of the surfaces of all fixtures and shields within the implantation chamber 44 achieves two important functions: first it serves to remove any surface contamination and that may be present on the surfaces of these fixtures and shields and, second it serves to form an appropriate surface layer on the fixtures and shields. The composition of this surface layer will, of course, depend which one of the elemental species, mentioned above, is incorporated in the ion beam 56. This newly formed surface layer, such as a titanium nitride (TiN) or aluminum oxide (A1₂0₃) or aluminum nitride (A1N) surface layer, possesses a considerably lower sputtering coefficient, i.e., between about 0.06 and 0.09 at 50 keV for N than does pure aluminum or pure titanium both of whose sputtering coefficient is about 0.3 for 50 keV N . It is the sputtering of the titanium or aluminum compounds during the ion implantation of the parts 15 and 20 which is one of th causes of the discoloration 28. The seasoning of these pure titanium or aluminum fixtures 48 and shields 60 thus effectively removes this source of potential discoloration, i.e., any sputtering of titanium or aluminum compounds from those fixtures 48 and shields 60 onto the parts 15 and 20, during their ion implantation.

The next step of the process of the invention involves the creation of a proper vacuum environment within the implantation chamber 44, another potential cause of the discoloration 28. To this end, a vacuum within the implant

—6 chamber 44 must be created which is less than about 5x10 and preferably is about 2x10 torr, averaged during the ion implantation period of the parts 15 and 20. With the proper vacuum established within the implant chamber 44, with the aid of a suitable vacuum pump 66, a plurality of the workpieces 58 are introduced within the chamber 44. Preferably, the vacuum pump 66 should be of an oil-free type so as to avoid the possibility of introducing surface contamination onto the part to be ion implanted. The actual sequence of the two steps preferably is reversed, i.e., the workpieces 58 first are introduced into the chamber 44 and mounted therein in the fixtures 48, followed by the pump-down of the proper vacuum therein, it being of importance only that during the ion implantation step itself the proper average vacuum prevails, as above specified.

With the cobalt-chromium or titanium and their respective alloy workpieces 58 secured in the fixtures 48 within the chamber 44, the workpieces 58, in particular their respective articulating surfaces, are exposed to a direct line of the incoming ion beam 56. In order to achieve such a direct line, the fixture 48 is caused to rotate on its base 50 by motors not shown, as indicated by an arrow 68. Careful attention must also be paid to having the proper ion beam power density acting on the surfaces of the workpieces 58. This ion beam power density acting on the surfaces of the workpieces cannot 2- exceed about 6.0 watt/cm 2 and preferably is about 1.0

₂ watt/cm . Consequently, the peak ion beam power density of an

80 keV beam should not exceed about twelve microamperes per square centimeter.

The control of the ion beam power density can be achieved in a number of ways. Preferably, and as herein illustrated, this low power ion beam current density is effected by expanding the spot size of the incoming ion beam 56 (observe FIG. 6) by a magnetic quadrupole or an electro-static lens system 64. The surfaces of the workpieces 58, now secured in the fixture 48 within the implant chamber 44, are then exposed to the incoming ion beam 56, properly modified, if need be, by the lens system 64, for a period from about four hours to about forty hours, with a preferred ion beam particle energy from about 20 keV, to about 400 keV, so as to implant a dose from about 3 x 10 17 to about 5 x 1017 ions/cm2 and wherein the ion beam current density is between about 0.1 and about lOOuA/cm .

The workpiece 58 illustrated in FIGS. 8-15 and the representative endosteal dental blade implant 100 illustrated in FIG. 16 are respectively formed of Ti or Co-Cr and their respective alloys. The workpiece 58 includes a bone-fixation portion 80 and an articulating-surface portion 82. The dental implant 100 is formed of the stem 102, which functions as the bone-fixation part. Preferably, these bone-fixation parts 80 and 102 are designed so as to possess a modulus of elasticity ≥ sr of about 15 (±1) x 10 p.s.i., which is closely akin to that of human bone 90, whose modulus of elasticity is about 10-30 x

5 10 p.s.l.

As mentioned, and unlike the hip-joint prosthesis 12 of FIG. 3, the bone fixation portion 80 of the workpiece 58 is thinner than that of prosthesis 12 and is tapered along its axial length toward its distal tip 84. This bone-fixation portion 80, i.e., the femoral hip stem, preferably has a length 86 of about 25 cm from an extraction hole 88 to its distal tip 84, and a preferred thickness of about one to two cm about midway therebetween. Hole 88 is an extraction hole used for removing, if need be, the bone-fixation portion 80 from the intermedullary canal 78. Further, the hip stem also tapers at a preferred angle of about one to two degrees at this midway section toward its tip 84. It will be noted that the axial opening 74 of the centering sleeve member 59 also tapers at substantially the same angle so as closely to parallel the taper of the bone-fixation portion 80.

Preferably, both the member 59 and the workpiece 58 are formed of the same Ti and its alloy so that they can be compatible as much as possible. The entire outside surface of the bone-fixation portion 80 and the entire outside surface of the member 59 are ion implanted with an ion beam possessing an energy between about 20 keV and about 360 keV, a current density between about 0.1 and about lOOuA/cm , and a dose

_ fi 18 _? between about 5x10 and about 3x10 ions/cm . Preferably, the ion implantation creates a surface region of the ion implanted surfaces characterized by improved resistance to chemical attack and a surface region with a microhardness of at least about 500 Knoop. Knoop hardness is determined by the Knoop indentation test in which a diamond indenter is used to penetrate the surface. Preferably, the implant 58 and the member 59 are formed of one of a group consisting of commercially pure titanium and Ti-6A1-4V.

As may be observed in FIG. 8, the bone-fixation portion 80 of the implant workpiece 58 is positioned within the intermedullary canal 78, which is wider than the tapering bone-fixation portion 80 of the implant workpiece 58. So as to securely center and fix the implant workpiece 58 within this canal 78, the centering sleeve member 59 is used to fill the void between the the femoral stem and the canal 78. The fixation of these two parts 58 and 59 to the bone can be effected with or without the use of cement 92, such as polymethyl methacrylate (PMMA) cement, at the discretion of the surgeon. If cement 92 is used, it will occupy the free space between the hip stem 58 and the intermedullary canal 78, both above and below the member 59, as shown. The articulating - surface portion 82 is then secured to the stem 80, and the artificial hip joint is completed in the usual manner.

In FIGS. 11-13, there is illustrated a different embodiment of securing the implant workpiece 58. In lieu of z n the centering sleeve member 59, positioned about midway between the proximal and distal parts of the hip stem 58, a distal tip member 94 is employed to center the hip stem 58. It will be noted that the distal tip member 94 is provided with a central bore 96 designed to accommodate therein the distal tip 84 of the hip stem 58. Again and at the discretion of the surgeon, the space between the hip stem 58 and the intermedullary canal 78 may be filled with PMMA cement 92, extending both above and below the member 94.

In FIG. 14 is illustrated yet another embodiment of securing the implant workpiece 58, i.e., the hip stem, within the femoral cavity, i.e., the intermedullary canal 78, namely by being press-fitted therein. In press-fitting an implant within a bone cavity, no cement is used around the implant and within bone cavity. Some surgeons in the field, including the dental field, are concerned that the cement layer surrounding the implant inhibits further bone growth about the implanted part. Some surgeons even fear that the presence of PMMA cement might actually cause bone tissue surrounding the implant to die. Lately, especially in dental practice, cementless fitting closely against the surrounding bone structure is preferred. In either event, loosening of the implant will occur over time. When loosening of an implant occurs, revision surgery is the only remedy to remove the old device and install a new one. Revision surgery entails the scraping out the cement, in addition to removing the old 2- _? device, followed by preparing the femur for the new implant. This requires the removal of even more bone tissue, a further undesirable consequence. When revision surgery on a cementless implant is required, at least the loss of surrounding bone tissue is not as great as with a cemented implant. As may be noted in FIG. 14, the intermedullary canal 78 closely follows the taper 76 of the hip stem 58, with a wider bone structure 98 remaining in the femur, following implant.

The ion implantation of the hip stem 58, the fixation members 59, 94 and 114, and of the stems 102 and 112 of the endosteal dental blade implant 100 and the cylindrical dental implant 110, as well as the hollow cylindrical dental implant 120 and any crown fixation member thereof assure that the fretting wear and abrasion of those implants against the surrounding bone structure are minimized, reducing patient stress, discomfort and the need of premature repair and/or replacement of the implants.

In FIGS. 20 and 21, the results of a corrosion test on cobalt-chromium alloy are depicted in pictorial form. The pictures show the surfaces of two samples, magnified fifty times, namely that of an ion-implanted sample 70 versus a non-implanted sample 72, respectively both formed of a cobalt-chromium alloy known as ASTM F-75 alloy. Sample 70 has been ion-implanted, preferably with nitrogen according to the process of the invention, while sample 72 has not been so implanted. Both samples 70 and 72 were etched by immersion in aqua regia (3 parts of HCl and 1 part Nitric, combined for 25 minutes before the test) for about 45 minutes. After drying, the surface of the implanted sample 70 indicated that it has remained virtually unaffected by the etch, revealing no evidence of corrosion. In contrast, the sample of the non-implanted sample 72 clearly shows evidence of severe pitting corrosion.

FIG. 22 and FIG. 23 indicate two important physical properties of the cobalt-chromium alloy ASTM F-75 modified by ion implantation according to the invention: namely its coefficient of friction in rubbing against a surface formed of UHMWPE and, its micorhardness expressed by the Knoop Hardness Number. (The Knoop Hardness Number serves to indicate the relative microhardness of a material, such as the Co-Cr metal alloy herein investigated, as determined by the Knoop indentation test. The higher the Knoop Number, the higher is the microhardness of the tested surface.) As is evident from the table of FIG. 22, the coefficient of friction of the implanted sample alloy rubbing against an UHMWPE surface has been reduced from about 0.138 to about 0.103. The table of FIG. 23 indicates that the microhardness of the implanted sample alloy has increased to about 1773 Knoop Number at a 10 gram load when compared to a microhardness of about 886 Knoop Number at the same 10 gram load of the non-implanted sample alloy. So FIGS. 24-27 illustrate comparisons of wear track volumes and of wear track profiles respectively of an ion implanted alloy sample versus a non-implanted sample, after correction for compression and cold flow. The test involved a pin-on-disk testing conducted in bovine blood serum in which two kinds of pins were held against a revolving disk formed of

UHMWPE, first an ion-implanted Co-Cr pin working against one

UHMWPE surface and, second a non-implanted Co-Cr pin working against another UHMWPE surface.

In the track volume comparison test illustrated in FIG.

24, the relative wear at the indicated three cycles was

3 determined using profilometry to measure the volume in MM of the circular "track" left by revolving the pin on the disk.

The sample UHMWPE disks which were wearing against the ion implanted Co-Cr pins exhibited an insignificant wear volume of

3 about 0.02 MM in tests conducted up to one million cycles in duration. In sharp contrast, the sample UHMWPE disks which were wearing against the non-implanted Co-Cr pins suffered

3 considerable wear volume of about 0.04 MM at about 123,000 cycles, about 0.32 MM at about 370,000 cycles and about 0.34 MM 3 at about one million cycles. Thus, the ion implantation of the Co-Cr alloy pin demonstratably and effectively reduced the wear in the UHMWPE part by over 90% even after one million cycles. A comparison of the wear track profiles of FIGS. 25-27 at the respective 123,000 cycles, 370,000 cycles and 1,000,000 cycles bears that out, respectively plotting the depths of penetration of the UHMWPE disks by the ion implanted pins versus the control, non-implanted pins.

Thus it has been shown and described an improved orthopaedic implant made from cobalt-chromium or from titanium and their respective alloys working against a plastic part and a process designed to improve the wear performance of the metal part against the plastic part and to improve fretting wear performance against bone, which product and process satisfy the objects and advantages set forth above.

Since certain changes may be made in the present disclosure without departing from the scope of the present invention, it is intended that all matter described in the foregoing specification or shown in the accompanying drawings, be interpreted in an illustrative and not in a limiting sense.

Claims

3 . What is claimed is:

1. A surgical implant comprising:

(a) an implant formed of two parts: a first part formed of metal and, a second complementary part formed of plastic;

(b) said first metal part is formed of one of a group consisting of a cobalt-chromium alloy and Ti and its alloys and said second plastic part formed of UHMWPE;

(c) said first metal part being ion implanted with one

+ + + + + + of a group consisting of N , N„ , C , Ti , 0 , P ,

Ne , He , Kr , Ar and B ;

(d) said ion implantation being effected with an ion beam possessing an energy between about 20 keV and about 400 keV, a current density between about 0.1

2 and about 100 uA/cm and a dose between about

1 fi 18 _?

1x10 and about 1x10 ions/cm .

2. The surgical implant of claim 1 wherein said implant is one of a group consisting of prostheses for artificial hips, knees, shoulders, elbows, wrists, fingers, toes, and dental implants. 3. The surgical implant of claim 1 wherein said ion implanted first metal part is characterized by an increase in its microhardness and a decrease in its coefficient of friction when rubbing against said second plastic part.

4. The surgical implant of claim 3 wherein said decrease in said coefficient of friction of said implanted metal part articulating against said UHMWPE part is from about 0.138 to about 0.103.

6. The surgical implant of claim 3 wherein said second complementary plastic part is used as a load bearing surface supporting said first ion implanted metallic part constantly articulating against it, and wherein said constant articulating does not adversely affect said load bearing surface.

7. The surgical implant of claim 3 wherein said ion implanted first metal part is characterized by improved surface homogeneity and resistance to chemical attack and wherein said microhardness of said metal part is at least about 1300 Knoop for a 10 gram load. 3Y

8. The surgical implant of claim 1 wherein said cobalt-chromium alloy is a cobalt-chromium-molybdenum metal alloy, and said Ti and its alloys are formed of one of a group consisting of commercially pure titanium and Ti-6A1-4V.

9. The surgical implant of claim 1 wherein said first metal part is a femoral hip stem of at least 8 cm length and a thickness not exceeding about 3 cm; wherein said first metal part tapers at an angle of about 2° toward its tip, and further including a member formed with an axial opening designed to fit about a part of said first metal part and designed for centering said implant; and wherein said axial opening of said member is tapered to parallel said taper of said first metal part.

10. The surgical implant of claim 1 wherein said implant is a dental implant and wherein said first metal part is a member designed to be press fitted into a cavity formed in the jaw bone of a patient.

11. A process of forming a surgical implant comprising: (a) forming an implant of two parts: a first metallic part and a second complementary plastic part, said first part formed of one of a group consisting of a cobalt-chromium alloy and Ti and its alloys, and said second part formed of UHMWPE;

(b) exposing all metallic fixtures and shields mounted within an implant chamber to an ion beam so as to cleanse them of surface contamination and to form a surface layer thereon having a sputtering coefficient lower than that of said first metallic part;

(c) creating a vacuum within said implant chamber of about 3x10 torr;

(d) introducing said first metallic part into said implant chamber, to be secured therein by said cleansed and surface layer coated fixtures and, exposing said first metallic part to a direct line of said ion beam;

(e) said ion beam having an ion beam power density on the surface of said first metallic part not

2 exceeding about 6.0 watt/cm , said ion beam incorporating one of a group of elemental species consisting of nitrogen, carbon, titanium, argon, boron, neon, krypton, helium, phosphorus and oxygen;

(f) said exposing said first metallic part of said implant to said ion beam being effected for a period of about four to about fifty hours, with an ion beam particle energy from about 20 keV to about 400 keV so as to implant a dose of about 3x10 17 ions/cm2, and wherein the ion beam current density is between

2 about 0.1 and about 100 uA/cm . 3l_*

12. The process of claim 11 wherein said implant is one of a group consisting of prostheses for artificial hips, knees, shoulders, elbows, wrists, fingers, toes, and dental implants.

13. The process of claim 11 wherein said ion implantation of said first metallic part increases its microhardness and decreases its coefficient of friction articulating against said UHMWPE part; and wherein said second complementary plastic part is used as a load bearing surface supporting said first ion implanted metallic part constantly articulating against it, and wherein said constant articul¬ ating does not adversely and materially affect said load bearing surface.

14. The process of claim 13 wherein said ion implanted first metal part is characterized by improved resistance to chemical attack and wherein said microhardness of said metal part is at least about 1300 Knoop for a 10 gram load.

15. The process of claim 11 wherein said cobalt-chromium alloy is a cobalt-chromium-molybdenum metal alloy.