Bioactive device having surface with alloyed layer of calcium phosphate compounds and method of making

Abstract

A dental or orthopedic implantable prosthetic device (1) which has a bioactive surface of an alloyed layer of material having calcium phosphate compounds. The device is formed by placing a suitable substrate of biocompatible material in a vacuum chamber (10), the substrate is cleaned by ion beam sputtering (18a) and then ion beam sputtering (14a) evolves and deposits (16a) bioactive material onto the surface of the device. The bioactive layer is mixed into the surface forming an alloyed zone by augmenting ion beam (18a) and is grown out to a selected thickness while being continuously bombarded by the augmenting ion beam.

Description

FIELD OF THE INVENTION

[0002] This invention relates generally to prosthetic devices including orthopedic, dental and other implantable devices and more particularly, to devices, such as implants, having a surface formed with improved bioactive characteristics.

BACKGROUND OF THE INVENTION

[0003] Plasma sprayed hydroxylapatite (hereinafter also referred to as HA) coatings have been successfully used since at least the early 1980s to enhance the load transmitting capabilities of orthopedic and dental prosthetic implants placed into bone. Biocompatible materials, such as HA, have a unique attribute-compared to most so-called biomaterials in that they are “bioactive” and react compatibly with bone which forms a tenacious bond with HA, a phenomenon commonly known as biointegration. HA also has been demonstrated to enhance the speed of bone healing around implants. From the beginning, however, although widely used clinically, plasma sprayed HA coatings have been subject to a number of physical and biological phenomena that often compromise the health and even survival of the implant. A brief discussion of exemplary problematic areas follows.

[0004] Delamination of HA Coatings

[0005] The tenacity of the bond between plasma sprayed HA and titanium implant substrates can vary considerably due to processing variables. Even if the bond is good, the coating is still subject to chipping during surgical placement if the surgeon is not careful. Plasma sprayed HA implant surfaces exposed by chipping, or other processes of HA degradation, invariably appear to be grayish-black and rough as if burned.

[0006] Sub-Crestal Infections With Concomitant Bone Loss

[0007] Dental implants with plasma sprayed HA coating that extend supra-crestally into the gingival tissue appear to be more subject to infection than uncoated implants and can cause severe crestal bone loss as well as delamination and dissolution of the HA coating.

[0008] Infection often ensues, usually resulting in rapid degradation and loss of the HA coating in the vicinity of the infected area. These implants sometimes can be saved by reopening the implant site, debriding the infected area and abrading the exposed portions of the implant to remove the remaining supra-crestal HA coating down to a clean, bright titanium surface. If this salvage procedure is not attempted, the implant will probably be lost with a substantial loss of the surrounding bone.

SUMMARY OF THE INVENTION

[0009] It is an object of the invention to overcome the prior art limitations noted above. Another object of the invention is to provide an orthopedic and dental prosthetic implant having improved bioactive characteristics.

[0010] Briefly stated, a prosthetic device made in accordance with a preferred embodiment of the invention has a surface formed with improved bioactive characteristics. According to a feature of the invention, an implantable device has a substrate of titanium alloy or other suitable biocompatible material with a layer of inorganic material comprising calcium phosphate containing compounds applied to the surface of the device. A preferred inorganic material for application to the surface is hydroxylapitite (HA). According to another feature of the invention, the layer is bombarded into the substrate using inert ions to form an alloy or intimate mixture of the substrate and inorganic materials. The alloyed surface can be overlaid with an inorganic surface layer continuously bombarded while grown to the alloyed surface, such as an HA type surface layer bonded to an apatitic titanium alloyed surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The Accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention, and together with the description serve to explain the objects, advantages and the principles of the invention.

[0012] FIG. 1 is a schematic view of apparatus used in making a prosthetic device according to the invention,

[0013] FIG. 2 is a chart showing the sequence of steps in forming the prosthetic device, and

[0014] FIG. 3 is a schematic elevational view of a broken away prosthetic device made in accordance with the invention.

[0015] Additional objects and features of the invention will be set forth in part in the description which follows and in part will be obvious from the description. The objects and advantages of the invention may be realized and attained by means of the instrumentalities pointed out in the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] In accordance with the invention, an orthopedic or dental prosthetic implant device is provided with a surface layer having a bioactive apatitic attribute permitting bone to bond directly and tenaciously to the implant. A preferred substrate material is commercially pure (C.P.) titanium or a titanium alloy such as Ti6Al4V and an implant made in accordance with the invention has a bioactive apatitic type titanium alloy (BATA) surface. However, other biocompatible implant material substrates can be used, such as other metals, ceramic and plastic. Alloy is used in the sense defined in Merriam-Webster's Collegiate Dictionary of a compound, mixture, or union of different things. Application parameters can vary the ratio of HA or other substances imbedded into the surface microstructure of the implant substrate. Further, the alloyed substrate surface, for example apatitic type titanium, can be overlaid with an HA type surface layer alloyed thereto. It is also within the purview of the invention to alloy other substances, alone or in combination, to alter or otherwise enhance the bioactive attributes and load transmitting abilities of the implant.

[0017] A preferred method of making an implant device in accordance with the invention employs a vacuum deposition process. The surfaces of an implant device are layered with a thin film of HA type material by means of a sputtering or any other thin film deposition technique. Simultaneously or sequentially, the HA type layered implant surface is bombarded with inert ions, such as argon, by means of a powerful ion beam accelerator device. The HA type material is impacted into the implant surface with such force by the inert material ions that it is driven into the interstitial spaces present in the microstructure of the implant substrate material, such as titanium. The process parameters are completely controllable. The depth of the alloyed apatitic type material titanium layer can be predetermined and the process additionally allows for an overlay application of an HA type layer, preferably between approximately 500 and 10,000 angstroms in thickness, intimately bonded to the underlying alloyed apatitic type material titanium alloy surface if desired. Thus, the HA type material literally becomes integrated with the implant material. One such process for obtaining the desired surface preparation can be adapted from the processes disclosed in U.S. Pat. No. 5,055,318, the subject matter of which is incorporated herein by this reference. Dimensionally, depth wise, these bioactive surfaces are measured in angstroms, with a suitable layer extending up to 5000 angstroms into the substrate material. Many types of materials can be alloyed with a variety of substrates using this process or variations of it. For example, a fluoroapatitic type titanium surface can be applied if desired. Another modification is to apply the BATA process onto a titanium plasma sprayed (TPS) particulate coated implant surface.

[0018] According to the preferred method of forming the improved prosthetic device surface, a dual ion beam process is employed and carried out in a vacuum chamber 10 indicated in a dashed line in FIG. 1. Substrates 1 to be treated are attached to a parts platen 12. A sputter ion source 14 directs a sputter beam 14a of inert gas ions toward target platen of bioactive material. Sputtered bioactive material 16a is directed toward devices 1 along with an augmenting inert gas ion beam 18a from augmenting ion source 18. A film thickness sensor 20 allows precise measurement of the thickness of the bioactive layer deposited in and on the surface of the devices.

TABLE I
Feature       Function
Vacuum        Process is carried out in a high vacuum (allows control
Chamber       over the quality of the bioactive alloy formed in and
              on the surface of the device).
Sputter Ion   Inert gas ion beam sputters bioactive material from the
Beam          target platen
Target Platen Bioactive material located on the target platen
Part Platen   Devices to be treated attached to the part platen
Augmenting    Inert gas ion beam used to first sputter clean the surface
Ion Beam      of the devices, next to mix the bioactive material into
              the surface of the devices forming the ballistically alloyed
              zone, then to control structure of the bioactive layer as
              it is grown out from the ballistically alloyed zone.
Film          Allows precise measurement of the thickness of the
Thickness     bioactive alloy layer deposited in and on the surface of
Sensor        the device.

[0019] As noted above, the bioactive surfaces comprise alloyed layers of calcium phosphate compounds. Table I describes the general dual beam deposition process utilizing the FIG. 1 apparatus and FIG. 2 shows the processing sequence. As shown in FIG. 2, the devices are placed in a vacuum chamber at step 1; the surfaces of the devices are cleaned by ion beam sputtering at step 2; bioactive material is evolved and deposited on the surface of the devices at step 3; the initial layer of bioactive material is alloyed into the surface of the devices at step 4; and the bioactive layer is grown and continuously augmented by an ion beam at step 5. In FIG. 3 the device subsurface is shown schematically at 1a and the original surface of the device is indicated in a dashed line at 1d. The bioactive outer layer grown from a ballistically alloyed zone 1b is shown at 1c.

[0020] Table II includes specific individual steps in the processing sequence and identifies typical process parameters and ranges of parameters suitable for the process.

TABLE II
Step	Description	Typical Process Parameters	Range Process Parameters
1	Device placed in vacuum chamber on	Vacuum: 1.0E(−07) Torr	Vacuum: 1.0E(−08) to
	an articulated fixture which allows		1.0E(−05) Torr
	programmed orientation of the device
	during the process.
2	Surface of device cleaned by ion beam	Ion Species: Ar	Ion Species: He, Ne, Ar, Kr
	sputtering with the ion beam from the	Beam Energy: 500 eV	or Xe
	augmenting ion source.	Beam Current: 1.0 mA/cm2	Beam Energy: (0.1-100) keV
		Time: 50 minutes	Beam Current: (0.01-1500) mA/cm2
			Time: (0.033-5000) minutes
3	Sputter ion beam use to ion beam	Ion Species: Ar	Ion Species: He, Ne, Ar, Kr or
	sputter hydroxylapatite or other	Beam Energy: 1000 eV	Xe
	bioactive material from the target plate	Beam Current: 2.5 mA/cm2	Beam Energy: (0.1-100) keV
	onto the surface of device.	Material: hydroxylapatite	Beam current: (0.1-1500) mA/cm2
		Evolution Rate: 0.2 angstom/sec	Material: apatitic minerals
			including calcium and/or
			phosphorous containing
			compounds, or fluoride
			containing compounds
			including Ca2F
			Evolution Rate: (0.008-120) angstoms/sec.
4	Augmenting ion beam used to	Ion Species: Ar	Ion Species: He, Ne, Ar, Kr or
	ballistically alloy first few layers of	Beam Energy: 1000 eV	Xe
	suttered bioactive material into device	Beam Current: 1.0 mA/cm2	Beam Energy: (0.1-100) keV
	surface.	Thickness: 2,000 angstoms	Beam Current: (0.1-1500) mA/cm2
5	Bioactive layer is grown out from the	Ion Species: Ar	Ion Species: He, Ne, Ar, Kr or
	ballistically alloyed layer as ion beam	Beam Energy: 200 eV	Xe
	sputtering of the target continues.	Beam Current: 0.05 mA/cm2	Beam Energy: (0.1-100) keV
	Augmenting ion beam used to control	Thickness: 2,000 angstoms	Beam Current (0.01-1500) mA/cm2
	the structure of the bioactive layer as is		Thickness: (100-100,000)
	grown.		angstroms

[0021] Table III includes the Table II steps and step descriptions along with actual parameters for examples of carrying out the process identified as Run #1 and Run #2, resulting in a coating thickness of 2,265 angstroms for the devices of Run #1 and 2,812 angstroms for the devices of Run #2.

TABLE III
Step	Step Description	Run #1	Run #2
1	Device placed in vacuum	Vacuum: 5.0E(−05) Torr	Vacuum: 7.0E(−05) Torr
	chamber on an articulated fixture
	which allows programmed
	orientation of the device during
	the process.
2	Surface of device cleaned by ion	Ion Species: Ar	Ion Species: Ar
	beam sputtering with the ion	Beam Energy: 500 eV	Beam Energy: 500 eV
	beam from the augmenting ion	Beam Current: 7.0 mA/cm2	Beam Current: 6.0 mA/cm2
	source.	Time: 50 minutes	Time: 50 minutes
3	Sputter ion beam use to ion	Ion Species: Ar	Ion Species: Ar
	beam sputter hydroxylapatite or	Beam Energy: 1000 eV	Beam Energy: 1000 eV
	other bioactive material from the	Beam Current: 1.0 mA/cm2	Beam Current: 1.0 mA/cm2
	target platen onto surface of	Material: hydroxylapatite	Material: hydroxylapatite
	device	Evolution Rate: 0.2 angstom/sec	Evolution Rate: 0.2 angstom/sec
4	Augmenting ion beam used to	Ion Species: Ar	Ion Species: Ar
	ballistically alloy first few layers	Beam Energy: 1000 eV	Beam Energy: 1000 eV
	of sputtered bioactive material	Beam Current: 1.0 mA/cm2	Beam Current: 1.0 mA/cm2
	onto device surface.	Time: 12 minutes	Time: 15 minutes
5	Bioactive layer is grown out	Ion Species: Ar	Ion Species: Ar
	from the ballistically alloyed	Beam Energy: 200 eV	Beam Energy: 200 eV
	layer as ion beam sputtering of	Beam Current: 0.05 mA/cm2	Beam Current: 0.05 mA/cm2
	the target continues.	Thickness: 2,265 angstoms	Thickness: 2,812 angstoms
	Augmenting ion beam used to
	control the structure of the
	bioactive layer as it is grown

[0022] Virtually all of the previously mentioned problems associated with plasma sprayed HA implants are resolved by use of the BATA surface technique described above for the following reasons. There is no oxidation or other high temperature deterioration (blackening) of the implant substrate from the BATA process. The previously noted problems are eliminated because no appreciable heat is applied during the BATA surface application process. Delamination or chipping is eliminated due to the alloyed nature of the BATA surface which is integrated into the implant substrate. However, abrasion or other marring of the BATA surface is still possible through careless handling during surgical placement. The clinical significance however, is of no greater importance than the same type of abrasion on a conventional uncoated or TPS coated titanium implant.

[0023] Preliminary in-vivo studies in dog femurs indicates histologically that advanced healing of surrounding bone abutting the BATA surface test specimens was evident at six weeks when compared to uncoated control specimen seen at twelve weeks.

[0024] X-ray diffraction has been classically used for crystallographic assessment of bulk materials. This technique has also been modified in order to enable evaluation of the top outermost layers of materials and has been widely utilized for surface characterization. Devices made in accordance with the invention were subjected to this modified technique and the result of the procedure revealed an amorphous calcium and phosphate based layer along with alpha titanium.

[0025] This surface modification, that is, the alloyed surface of intermixed amorphous calcium and phosphate with titanium, results in a higher dissolution rate and enhanced bioactivity than that of crystalline phases.

[0026] Although the invention has been described with regard to specific preferred embodiments thereof, variations and modifications will become apparent to those skilled in the art. For example, additional inorganic material, such as calcium fluoride, can be alloyed into the HA overlayer coating in accordance with the teaching of the invention, it is, therefore, the intent that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.

Claims

1. A device for implantation in a living body, the device having enhanced bioactivity to promote early osseointegration with bone structure into which the device is implanted comprising: a substrate of biocompatible material, the substrate having an outer surface and a thin layer of calcium and phosphate in the amorphous phase alloyed with the biocompatible material of the substrate to a selected depth.

2. A device according to claim 1 in which the substrate material is selected from the group consisting of titanium alloy and commercially pure titanium.

3. A device according to claim 2 in which the calcium and phosphate is alloyed with the substrate material to a depth of approximately 5,000 angstroms.

4. A device according to claim 1 further comprising an outer layer of a calcium phosphate containing compound.

5. A device according to claim 4 in which the outer layer has a thickness within the range of approximately 2,000 to 3,000 angstroms.

6. A method of enhancing the bioactivity of a device for implantation in a living body to promote early osseointegration comprising the steps of taking a substrate of biocompatible material, applying a thin layer of a calcium phosphate containing compound to the substrate, alloying the thin layer with the biocompatible material up to a depth of approximately 5,000 angstroms in which the thin layer is in the amorphous phase thereby promoting early osseointegration.

7. The method of claim 6 in which the substrate material is selected from the group consisting of titanium alloy and commercially pure titanium.

8. The method of claim 7 in which the thin layer is alloyed with the biocompatible material by bombarding the substrate with inert ions.