Surface treatment process for implants made of titanium alloy

Abstract

A titanium 6 Al/4V alloy is provided with a surface topography that is similar to the Osseotite® surface produced on commercially pure titanium. Native oxide is removed from the Ti 6Al/4V alloy, followed by contacting the metal at ambient temperature with an aqueous hydrochloric acid solution containing a relatively small amount of hydrofluoric acid.

Claims

1. A method of producing a uniformly roughened surface on Ti 6/4 alloy for contact with living bone comprising: (a) removing the native oxide from said Ti 6/4 alloy to expose metal; (b) contacting said exposed metal with an aqueous solution of 0.005 to 1.0 wt % hydrofluoric acid and 10 to 30 wt % hydrochloric acid for a first period of time to create the desired surface topography having irregularities with peak-to-valley heights of less than 10 microns and average peak-to-peak distance of 1 to 3 microns.

2. The method of claim 1, wherein the first period of time is about 20 minutes.

3. The method of claim 1, wherein the aqueous solution includes 0.084 wt % hydrofluoric acid and about 20 wt % hydrochloric acid.

4. The method of claim 1, wherein the native oxide is removed by contacting the Ti 6/4 alloy with a second aqueous solution of hydrofluoric acid for a second period of time.

5. The method of claim 4, wherein the second aqueous solution contains about 7.9 to 9.0 wt % hydrofluoric acid.

6. The method of claim 5, wherein the second aqueous solution contains about 8.45 wt % hydrofluoric acid.

7. A method of producing a uniformly roughened surface on an implant formed of Ti 6/4 alloy for contact with living bone comprising: treating least a portion of an implant surface with a first aqueous solution including hydrofluoric acid for a first period of time to remove native oxide from the implant surface to create a first surface; and contacting the first surface with a second aqueous solution including hydrofluoric acid and hydrochloric acid for about twenty (20) minutes at ambient temperature to create a second surface having a topography for osseointegration of the implant with living bone, the topography having peak-to-valley heights of less than 10 microns and average peak-to-peak distance of 1 to 3 microns.

8. The method of claim 7, wherein the first aqueous solution contains about 7.9 to 9.0 wt % hydrofluoric acid.

9. The method of claim 7, wherein the second aqueous solution includes about 0.005 to about 1.0 wt % hydrofluoric acid and about 10 to about 30 wt % hydrochloric acid.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A shows a commercially pure titanium machined surface.

(2) FIG. 1B shows the surface of FIG. 1A after being treated with HF

(3) FIG. 1C shows the surface of FIG. 1B after being etched with HCl and H2SO4 so as to produce an Osseotite® surface on pure titanium.

(4) FIG. 1D is a surface map of the Osseotite® surface of FIG. 1C.

(5) FIG. 1E shows the effect of the treatment of FIG. 1A-C on Ti 6/4 alloy.

(6) FIG. 2A-E show several etching processes on Ti 6/4 alloy.

(7) FIG. 3A-E show the effect of etching with Keller's reagent and Kroll's reagent.

(8) FIG. 4A-B show the effect of etching with HCl alone.

(9) FIG. 5A-D show the effect of etching with HCl plus HF.

(10) FIG. 6 shows a typical dental implant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(11) Titanium and Titanium Alloys

(12) Although other metals, and ceramics have been proposed for use in dental implants, titanium has been generally used. Particularly commercially pure titanium, which contains trace amounts of carbon, iron, oxygen, hydrogen, and nitrogen. Titanium alloys have also been used since they are stronger than the commercially pure grades of titanium. One commonly used titanium alloy, Ti/6Al/4V, contains 6 wt % aluminum and 4 wt % vanadium, hereafter referred to as Ti 6/4.

(13) A characteristic of titanium and its alloys is the rapid formation of tenacious titanium oxide films on the surface, a feature which contributes to titanium's resistance to corrosion. This oxide film is considered to be a combination of various oxides of titanium, including TiO, TiO.sub.2, Ti.sub.2O.sub.3, and Ti.sub.3O.sub.4. It has been referred to the “native oxide” film. Measurement of the native oxide film by Auger spectrometer indicates that it typically has a depth of 70 to 150 Angstroms.

(14) As previously disclosed, removing the native oxide is important if a uniformly roughened surface is to be produced by acid etching. Experience has shown that most acids are not capable of removing the native oxide sufficiently so that a uniform roughness can be produced. Titanium surfaces are often pickled in mixtures of hydrofluoric acid and nitric acids to clean the surface. Aqueous solutions of hydrofluoric acid alone, without the addition of oxidizing acids such as nitric acid, are very aggressive toward titanium and its native oxide film. A relatively brief exposure to a dilute solution of hydrofluoric acid will remove the native oxide. Since after removing the native oxide, the hydrofluoric acid will begin to consume the metal as well, an undesirable result, the titanium implant is removed from the acid and rinsed to stop further attack. However, as is well known, the titanium metal surface will begin to oxidize quickly. Consequently, the exposed metal surface should be protected against oxygen exposure until the titanium implant is immersed in an acid bath to uniformly etch the surface, creating the desired surface topography. Other methods of removing the native oxide could be used, such as plasma treatment, but the use of hydrofluoric acid is preferred.

(15) The rate at which titanium is etched depends on the concentration of the hydrofluoric acid. A hydrofluoric acid solution containing about 15 vol. % of 49 wt % hydrofluoric acid was found to permit complete removal of the native oxide within about one-half minute, but with minimal consumption of the metal. This is illustrated in FIGS. 1A and B which show at 2000× magnification the surface of a commercially pure titanium metal dental implant after machining (producing macro-features such as threads or grooves) and then after being exposed to hydrofluoric acid to remove the native oxide. The machining marks have disappeared and the hydrofluoric acid has left the titanium grains exposed after the native oxide has been removed and some of the grain boundary material has been removed.

(16) In FIG. 1C, the surface of the commercially pure titanium (after native oxide has been removed) has been etched with a solution of 19.55 wt % hydrochloric acid and 72.29 wt % sulfuric acid at 60-70° C. for about 7 minutes. This desirable surface topography has been clinically demonstrated to achieve enhanced osseointegration. Implants having this surface are sold under the Osseotite® trademark by the assignee of the present invention. This desirable surface has a generally uniform set of sharp peaks with a maximum peak-to-valley height of 10 μm or less. The average peak-to-peak distance is about 1-3 μm. The result of a typical examination of an Osseotite® surface by surface mapping microscopy is shown in FIG. 1D.

(17) FIGS. 1A-1D illustrate the process and results produced on a commercially pure titanium dental implant. Clinical success of the Osseotite® surface in improving osseointegration of the implants has been confirmed and it is well accepted in the marketplace. Therefore, the present inventors had expected to create the same surface topography on titanium alloy Ti 6/4 using the same treatment. However, they were surprised to discover that the process providing uniform results on commercially pure titanium failed to produce the characteristic surface topography when applied to Ti 6/4 alloy.

(18) Other etching solutions were tested. In some instances, a surface similar to the Osseotite® surface was obtained, but in other cases, acid etching was ineffective. It was found also that the effect on Ti 6/4 alloy varied from batch to batch, so that each batch had to be tested to determine its suitability. After further investigation of this problem, the inventors found that certain acid etching solutions were capable of consistently producing the desired surface on Ti 6/4 alloy.

(19) Acid Etching of Ti 6/4 Alloy

(20) FIGS. 1E, and 2C-E, 3A-E, 4A, B, and 5A-D illustrate the results of some of the acids tested on Ti 6/4 E.L.I. alloy, as defined by ASTM B348 Grade 23 or ASTM F136. In each case, the implants had been given the same treatment in a hydrofluoric acid solution to remove the native oxide on the surface. In particular, the implants were immersed in 8.45 wt % hydrofluoric acid at room temperature. The results of the etching processes shown in FIGS. 1E, and 2C-E, 3A-E, 4A, B, and 5A-D can be compared with FIG. 1C, the Osseotite® surface produced on commercially pure titanium metal by an acid treatment with an initial mixture of 19.55 wt % hydrochloric acid and 72.29 wt % sulfuric acid at 60-70° C. for 7 minutes.

(21) Experiments were carried out with a series of acid compositions, the results being shown in Figures. The acid compositions and treatment conditions are summarized in the following table.

(22) TABLE-US-00001 TABLE 1 Native Oxide Removal Etching Treatment Acid Compostion.sup.(1) 8.45 Time, Time Temp FIG. Ti: wt % HF min. HF HC1 H.sub.2S0.sub.4 HN0.sub.3 min. ° C. 1C CP Yes 1.0 — 19.55 77.29 — 7 60-70 1E 6/4 Yes 1.0 — 19.55 77.29 — 7 60-70 2A 6/4 No — 0.284 1.062 — 2.297 1 61 2B 6/4 No — 0.284 1.062 — 2.297 8 61 2C 6/4 Yes 0.5 0.284 1.062 — 2.297 0.5 61 — 19.55 77.29 — 1.0 61 2D 6/4 Yes 0.5 0.284 1.062 — 2.297 0.5 61 — 19.55 77.29 — 7 61 2E 6/4 Yes  0.17 0.284 1.062 — 2.297 1.5 61 1.143 — — 1.923 1.5 ambient 3A 6/4 Yes 1.0 0.284 1.062 — 2.297 7 ambient 3B 6/4 Yes 2.5 0.284 1.062 — 2.297 7 ambient 3C 6/4 Yes 1.0 0.284 1.062 — 2.297 10 ambient 3D 6/4 Yes 2.5 0.284 1.062 — 2.297 10 ambient 3E 6/4 Yes 2.5 0.284 1.062 — 2.297 10 ambient 4A 6/4 Yes 1.5 — 20 — — 14 ambient 4B 6/4 Yes 1.5 — 20 — — 21 ambient 5A 6/4 Yes 1.0 0.26  20 — — 20 ambient 5B 6/4 Yes 1.0 0.175 20 — — 20 ambient 5C 6/4 Yes 1.0 0.09  20 — — 20 ambient 5D 6/4 Yes 1.0 0.09  20 — — 20 ambient .sup.(1)wt % acid, remainder water

(23) The above table generally follows the progress of experiments carried out to determine the acid etching needed to produce the desired surface topography on Ti 6/4 alloy. To produce the surface of FIG. 1C, the native oxide on the commercially pure titanium was removed by exposure to an 8.45 wt % HF solution for 1 minute at ambient temperature. After rinsing in deionized water containing baking soda to neutralize the residual acid and a further rinse in deionized water, the titanium was immersed in an aqueous solution of 19.55 wt % HCl and 77.29 wt % H.sub.2SO.sub.4 for 7 minutes at 60-70° C. to produce a uniformly roughened surface, i.e. the Osseotite® surface.

(24) FIG. 1E illustrates the surprising result when the same procedure was carried out on Ti 6/4 alloy. As will be seen in the photograph, the characteristic Osseotite surface was not obtained on Ti 6/4 alloy. The machining marks were still visible. It was concluded that a different etching process was needed for use with Ti 6/4 alloy if the Osseotite® surface was to be provided on the Ti 6/4 alloy.

(25) FIGS. 2A-E show the results obtained when two known etching acid mixtures were used. One was Keller's solution, containing HNO.sub.3, and HCl, and the second was Kroll's solution, containing HF and HNO.sub.3. The compositions used are shown in Table I above. FIGS. 2A and 2B show that Keller's solution alone did not produce the Osseotite surface, although some pitting can be seen. Since the pre-treatment with HF solution to remove the native oxide was not done, it is presumed that the native oxide interfered with the attempted etching with Keller's solution.

(26) FIGS. 2C to 2E show the results achieved when the native oxide was removed by pre-treatment with an HF solution and thereafter the titanium surface was exposed to two acid solutions in sequence. In FIGS. 2C and 2D, Keller's solution was used, followed by the mixture of HCl and H.sub.2SO.sub.4, known to be successful in etching chemically pure titanium. In FIG. 2E, Keller's solution was used first, followed by immersion of the Ti 6/4 alloy in Kroll's solution. None of these tests produced a surface topography like that shown in FIG. 1C on the Ti 6/4 alloy.

(27) FIGS. 3A-3E show the results obtained when the native oxide was removed with an HF solution, and Keller's solution was used for etching, hut at ambient temperature rather than at 61° C. previously used. It was found that this process was capable of providing a surface similar to FIG. 1C on some samples of Ti 6/4 alloy, but not on others (compare FIG. 3 D with FIG. 3 E). The difference in response of the samples appeared to be associated with the machining or the alloy heat (i.e., the conditions associated with a specific batch of titanium alloy). Therefore, additional experimentation was undertaken. However, it was concluded that etching with Keller's solution may be useful also, provided that control of the quality of the Ti 6/4 alloy can be achieved.

(28) FIGS. 4 A and B report the surfaces produced when the native oxide was removed by the usual method and then the surface was etched with an HCl solution. Although some pitting occurred, it was evident that HCl alone was not sufficient to produce a surface like that of FIG. 1C.

(29) FIGS. 5 A-D illustrate the improved results that were obtained when small amounts of HF were added to the 20 wt % HCl etching solution. It was concluded that a small amount of HF should be used if the desired surface topography was to be obtained. The surfaces of FIGS. 5C and 5D were given the same treatment and produced substantially the same surface, even though the C and D samples had different machining and heats. Thus, it was concluded that the process was broadly applicable to Ti 6/4 alloys.

(30) In the presently preferred process, Ti 6/4 alloy is immersed in an aqueous solution of hydrofluoric acid for the length of time required to remove the native oxide while not removing a significant amount of metal. A preferred solution, suitable for commercial application would contain about 7.9 to 9.0 wt % HF. However, more or less concentrated solutions could be used, with appropriate adjustment of the exposure time, provided that the native oxide was removed to prepare the surface for subsequent etching needed to create the desired surface topography.

(31) The etching step immerses the Ti 6/4 alloy, from which the native oxide had been removed, in an aqueous solution at room temperature containing about 0.053 to 0.105 wt % HF and 19-21 wt % HCl. Such solutions have been found to produce the desired surface topography on Ti 6/4 alloy within about 20 minutes and using only ambient temperatures. Again, some adjustment of the acid concentrations, temperature, and exposure time is believed to be possible, while still obtaining the desired surface. It is believed that equivalent results may be obtained within the broader range of 0.005 to 1.0 wt % HF and 10-30 wt % HCl.

(32) Dental Implants

(33) The etching process of the invention may be used to prepare the surface of various styles of dental implants. A typical example is illustrated in FIG. 6. The implant 10 will be placed in a pre-drilled hole in a patient's bone to replace the root of a missing tooth. The threaded portion 12 engages the bone, while at least some of the upper portion 14 contacts tissue. In many cases, the etching process will be applied to the threaded portion 12 of the implant 10, while the upper portion. 14, shown in FIG. 6 to include a head 16 portion for engaging dental prosthesis components and a neck portion 18, remains relatively smooth. In some cases, the roughened area may be extended upward into the neck and head regions, or even to the top of the implant 10. In other cases, only a portion of the threads will be roughened to improve osseointegration of the metal with bone, while the upper section of the threaded region will remain relatively smooth.