Abstract:
A method of forming an implant to be implanted into living bone is disclosed. The method comprises the act of roughening at least a portion of the implant surface to produce a microscale roughened surface. The method further comprises the act of immersing the microscale roughened surface into a solution containing hydrogen peroxide and a basic solution to produce a nanoscale roughened surface consisting of nanopitting superimposed on the microscale roughened surface. The nanoscale roughened surface has a property that promotes osseointegration.

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/318,641, filed Mar. 29, 2010. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to implants and, in particular, to a dental implant having a nanometer-scale surface topography and methods of making same. 
     BACKGROUND OF THE INVENTION 
     It is becoming more common to replace a missing tooth with a prosthetic tooth that is placed upon and attached to a dental implant. Dental implants are often comprised of metal and metal alloys, including titanium (Ti) and titanium alloys. The dental implant serves as an artificial root that integrates with the gingiva and the bone tissue of the mouth. 
     For the dental implant to function successfully, sufficient osseointegration is required. In other words, a bond between the implant and the bone must be formed and retained. The surface of the implant may be roughened to help enhance the osseointegration process. Non-limiting examples of processes for roughening an implant surface include acid etching and grit blasting, which impart roughness on the surface. 
     Other existing techniques involve forming a generally thin (e.g., generally less than 10 microns) coating of osseointegration materials, such as hydroxyapatite (HA), other calcium phosphates, or other osseointegration compounds, for forming a direct chemical compound between the implant and the bone. Plasma spraying and sputtering are two major techniques that have been used to deposit, for example, HA, onto an implant. 
     U.S. Pat. App. Pub. Nos. 2008/0220394, 2007/0110890, and 2007/0112353 disclose methods of discrete deposition of hydroxyapatite crystals to impart a nano-scale topography. Although effective, the disclosed processes require that a residual substance (i.e. HA crystals) be left on the surface post-processing in order to impart a nano-scale topography into the surface. 
     The present invention is directed to an improved implant having nanometer-scale surface topography directly imparted into the surface for improving the rate and extent of osseointegration, and methods of making the same. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method of forming an implant to be implanted into living bone. The method comprises the acts of roughening at least a portion of the implant surface to produce a microscale roughened surface. The method further comprises the act of immersing the microscale roughened surface into a solution containing hydrogen peroxide and a basic solution to produce a nanoscale roughened surface consisting of nanopitting superimposed on the microscale roughened surface. 
     In another aspect, another method of forming an implant to be implanted into living bone is disclosed. The method comprises the act of removing a native oxide layer from at least a portion of the implant surface. The method further comprises the act of roughening at least the portion of the implant surface to produce a microscale roughened surface. The method further comprises the act of rinsing the microscale roughened surface in deionized water. The method further describes the act of immersing the microscale roughened surface into a solution containing hydrogen peroxide and a basic solution at a high pH level to produce a nanoscale roughened surface consisting of nanopitting superimposed on the microscale roughened surface. The method further comprises the acts of passivating the nanoscale roughened surface with nitric acid, and rinsing the nanoscale roughened surface in deionized water. 
     The above summary of the present invention is not intended to represent each embodiment, or every aspect, of the present invention. This is the purpose of the figures and the detailed description which follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings. 
         FIG. 1  is a side view of an implant according to one embodiment. 
         FIGS. 2   a ,  2   b , and  2   c , are a side view, an insertion end view, and a gingival end view, respectively, of an implant according to a second embodiment. 
         FIGS. 3   a ,  3   b , and  3   c , are a side view, an insertion end view, and a gingival end view, respectively, of an implant according to a third embodiment. 
         FIGS. 4   a  and  4   b  are a side view, an end view, and a cross-sectional view, respectively, of an implant according to a fourth embodiment. 
         FIG. 5  is a flow diagram detailing a method of forming an implant according to an embodiment of the present invention. 
         FIG. 6  is a side view of the implant in  FIG. 1  with a roughened outer surface. 
         FIG. 7   a  is a flow diagram detailing a method of forming an implant according to another embodiment of the present invention. 
         FIG. 7   b  is a flow diagram detailing a method of forming an implant according to yet another embodiment of the present invention. 
         FIG. 8   a  is a scanning electron microscope (SEM) image showing a commercially pure titanium implant post-acid etching at 2 kX. 
         FIG. 8   b  is a field emission scanning electron microscope (FESEM) image showing a commercially pure titanium implant post-acid etching at 30 kX. 
         FIG. 9   a  is an FESEM image showing a commercially pure titanium implant post-KOH/H 2 O 2  treatment at 2 kX using a method of the present invention. 
         FIG. 9   b  is an FESEM image showing a commercially pure titanium implant post-KOH/H 2 O 2  treatment at 30 kX using a method of the present invention. 
         FIG. 9   c  is an FESEM image showing a commercially pure titanium implant post-KOH/H 2 O 2  treatment at 100 kX using a method of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to implants having a nanometer scale surface topography consisting of irregular shaped pitting and methods of making the same. An implant in the context of the present invention means a device intended to be placed within a human body such as to connect skeletal structures (e.g., a hip implant) or to serve as a fixture for a body part (e.g., a fixture for an artificial tooth). Although the remainder of this application is directed to a dental implant, it is contemplated that the present invention may also be applied to other (e.g., medical) implants. 
       FIG. 1  shows a standard dental implant  10  that includes an head portion  12 , a lowermost end  14 , and a threaded bottom portion  16 . The implant  10  may, for example, be made of titanium, tantalum, cobalt, chromium, stainless steel, or alloys thereof.  FIGS. 2   a - c ,  3   a - c , and  4   a - b , which are discussed below, describe alternative implant designs that may also be used with the present invention. 
     In the implant  10  of  FIG. 1 , the head portion  12  includes a non-rotational feature. In the embodiment shown, the non-rotational feature includes a polygonal boss  20  that may be engageable with a tool that screws the implant  10  into bone tissue. In the illustrated embodiment, the polygonal boss  20  is hexagonal. The polygonal boss  20  may also be used for non-rotationally engaging a correspondingly shaped socket on a restorative or prosthetic component that is attached to the implant  10 . 
     The exterior of the threaded bottom portion  16  facilitates bonding with bone or gingiva. The threaded bottom section  16  includes a thread  18  that makes a plurality of turns around the implant  10 . The threaded bottom portion  16  may further include a self-tapping region with incremental cutting edges  17  that allows the implant  10  to be installed without the need for a bone tap. These incremental cutting edges  17  are described in detail in U.S. Pat. No. 5,727,943, entitled “Self-Tapping, Screw-Type Dental Implant,” which is incorporated by reference in its entirety. 
       FIGS. 2   a - c  disclose an implant  36  that differs from the implant  10  of  FIG. 1  in the details of the cutting edges  17 ′ and the contours of the threads defining the exterior of the threaded bottom portion  16 ′. When viewed in the cross-section (see  FIG. 1   b ), the threaded outer surface  16 ′ is non-circular in the region of the threads and/or the troughs between the threads. This type of thread structure is described in detail in U.S. Pat. No. 5,902,109, entitled “Reduced Friction, Screw-Type Dental Implant,” which is incorporated by reference in its entirety. 
     In  FIGS. 3   a - c , an implant  41  having a wide diameter in the region of the threaded bottom portion  42  is illustrated. The diameter is in the range of from about 4.5 mm to about 6.0 mm with the diameter of 5.0 mm being a fairly common dimension for a wide diameter implant. Such an implant  41  is useful to engage one or both cortical bones to provide enhanced stability, especially during the period of time after installation. 
       FIGS. 4   a - b  illustrate an implant  110  according to another embodiment that may be used with the present invention. The implant  110  includes a middle section  114  designed to extend through the gingiva. Preferably, it is a smooth surface that includes a titanium nitride coating so the underlying titanium or titanium alloy is not readily seen through the gingiva. The implant  110  also includes a threaded portion  120  that may include various thread structures and is preferably roughened to increase the osseointegration process. It is contemplated that implants other than those illustrated in  FIGS. 1-4  may be used with the present invention. 
     According to embodiments of the present invention, a nanoscale roughened surface is superimposed onto a microscale roughened surface on at least a portion (e.g., the threaded bottom portion) of the surface of an implant. In one embodiment, the nanoscale roughened surface is created by immersing the microscale roughened surface into a solution containing hydrogen peroxide and a basic solution. Non-limiting examples of suitable basic solutions include potassium hydroxide solutions and sodium hydroxide solutions. 
     Turning now to  FIG. 5 , a general method of producing a nanoscale roughened surface on an implant is set forth according to one embodiment of the present invention. At step  500 , an implant is provided. At least a portion of the implant surface is roughened to a microscale roughness at step  501 , for example, by machining, acid etching and/or grit blasting the implant surface. As an example,  FIG. 6  shows the implant  10  of  FIG. 1  having a roughened surface  630 . Nanopitting is then created on the microscale roughened surface by immersion into a solution containing hydrogen peroxide and a basic solution, to produce a nanoscale roughened surface on the implant at step  502 . 
     Referring now to  FIG. 7   a , another general method of forming an implant according to another embodiment of the present invention is illustrated. An implant comprised of titanium, a titanium alloy (e.g. titanium 6 AL-4V ELI alloy), stainless steel, or the like is provided at step  750 . At step  754 , nanopitting is created on a microscale roughened surface to produce a nanoscale roughened surface on the implant. At step  756 , the implant is passivated with nitric acid. The implant may then be rinsed in reverse osmosis/deionized (RO/DI) water to remove residual solvents and hydroxyapatite at step  758 . The implant is then dried at step  764  and sterilized at step  766  using, for example, gamma sterilization techniques. 
     Referring to  FIG. 7   b , a more detailed method of producing a nanoscale roughened surface on an implant is illustrated according to another embodiment of the present invention. A threaded dental implant comprised of titanium, a titanium alloy (e.g. titanium 6 AL-4V ELI alloy), stainless steel, or the like is provided at step  700 . The surface of the implant is generally clean and dry. A threaded bottom portion of the implant is etched to remove a native oxide layer from the implant surface at step  701 . The native oxide layer may be removed by a first acid solution, which may include aqueous hydrofluoric acid. The threaded bottom portion is then acid etched form a microscale roughened surface at step  702 . “Microscale,” as used herein, should be understood to describe an article or feature generally measured in microns such as, for example, 1 micron to 100 microns. Acid etching may result from immersion in a mixture of sulfuric and hydrochloric acids, creating peak-to-peak and peak-to-valley irregularity distances in the microscale roughened surface of about 1 micron to 3 microns. This type of roughening method utilized on commercially pure (CP) titanium is described in detail in U.S. Pat. No. 5,876,453, entitled “Implant Surface Preparation,” which is incorporated by reference in its entirety. An additional roughening method utilized on Titanium 6 AL-4V ELI alloy is described in detail in U.S. Pat. App. Pub. No. 2004/0265780, entitled “Surface Treatment Process for Implants Made of Titanium Alloy,” which is also incorporated by reference in its entirety. It is contemplated that other surface roughening techniques including, but not limited to, grit blasting, titanium plasma spraying, and combinations thereof, may be used. Grit blasting the threaded bottom portion to form a microscale roughened surface generally results in peak-to-peak and peak-to-valley irregularity distances of about 10 microns to 30 microns. Grit blasting and acid etching the threaded bottom portion to form the microscale roughened surface generally results in both levels of topographies, i.e., with about 1 micron to 3 microns peak-to-peak and peak-to-valley irregularity distances superimposed on 10 microns to 30 microns peak-to-peak and peak-to-valley irregularity distances on the microscale roughened surface. 
     At step  703 , the microscale roughened surface is immersed into a solution containing hydrogen peroxide and a basic solution to produce a nanoscale roughened surface consisting of nanopitting superimposed on the microscale roughened surface. The basic solution can be any base with a pH in the range of about 7 to about 14, and preferably about 14, such as potassium hydroxide or sodium hydroxide. “Nanoscale,” as used herein, should be understood to describe an article or feature generally measured in nanometers such as, for example, 1 nanometer to 500 nanometers. Generally, immersion into the hydrogen peroxide/basic solution results in nanopitting of about 1 nanometer to about 100 nanometers. 
     Immersion time, hydrogen peroxide concentration, and basic solution concentration are among several factors that affect the rate and amount of nanopitting superimposed onto the microscale roughness of the implant surface. For example, immersing a commercially pure titanium implant in a solution of 3-5% potassium hydroxide and 13-22% hydrogen peroxide for 1 minute at 50 degrees Celsius typically results in an acceptable nanoscale roughness of the implant surface. Longer immersion times may impact the micron level topographies, while potassium hydroxide concentrations of less than 3% and/or hydrogen peroxide concentrations of less than 13% may result in the nano-topography not being adequately formed. 
     Another factor affecting the rate and amount of nanopitting onto the microscale roughness of the implant surface is the processing temperature. At temperatures of higher than about 60 degrees Celsius, for example, the etching is accelerated and can begin to impact the micron level topographies. Thus, it may be desirable for the processing temperature to be maintained at or below about 60 degrees Celsius. 
     Processing temperature, immersion time, and/or chemical concentration may be adjusted to compensate for one or more of these variables being within an otherwise unacceptable range, in order to nevertheless produce acceptable nano-topography. For example, potassium hydroxide concentrations of less than 3% may be adjusted by increasing immersion time and/or processing temperature in order to produce an acceptable amount of nanopitting on the microscale roughness of the implant surface. 
     Post-processing, the implant is passivated with nitric acid at step  704 . At step  705 , the implant is rinsed in hot deionized water (e.g. 70 degrees Celsius to 100 degrees Celsius) to remove any acid residuals and to potentially enhance titanium hydroxide groups on the surface. 
     Hydroxyapatite (HA) nanocrystals may then optionally be deposited on the nanoscale roughened surface of the implant at step  706 . The HA nanocrystals may be introduced onto the nanoscale roughened surface of the implant in the form of a colloid. A representative amount of HA in the colloid is typically in the range of about 0.01 weight percent to about 1 weight percent (e.g., 0.10 weight percent). To form the colloid, HA nanocrystals may be combined in solution with a 2-methoxyethanol solvent and ultrasonically dispersed and deagglomerated. The pH of the colloidal solution may be adjusted with sodium hydroxide, ammonium hydroxide, or the like on the other of about 7 to about 13. As such, the colloidal solution may include HA nanocrystals, 2-methoxyethanol, and a pH adjuster (e.g. ammonium hydroxide, and/or sodium hydroxide). This type of HA deposition is described in detail in U.S. Pat. App. Pub. Nos. 2007/0110890 and 2007/0112353, both entitled “Deposition of Discrete Nanoparticles on an Implant Surface,” which are incorporated by reference in their entireties. The implant may then be rinsed in reverse osmosis/deionized (RO/DI) water to remove residual solvent and HA at step  708 . 
     Alternatively or in addition to the acts of depositing HA nanocrystals at step  706  and rinsing at step  708 , a sodium lactate coating may be applied on the nanoscale roughened surface of the implant at step  707  and the implant rinsed at step  708 . In either embodiment, the implant may then be dried (e.g., oven dried), at step  714 , and sterilized at step  716  using, for example, gamma sterilization. 
     The implant surface may be characterized utilizing Field Emission Scanning Electron microscopy (FESEM). Depending upon the resolution of the instrument, the nanopitting may typically be witnessed at magnifications of 30 kX or higher. As discussed above, the nanopitting generally has a distribution in the range of about 1 nanometer to about 500 nanometers, and typically between about 1 nanometer and about 100 nanometers. 
     Example 1 
       FIGS. 8   a  and  8   b  are scanning electron microscope images showing a micron-level roughness imparted by an acid etching process on a commercially pure titanium implant. The image of  FIG. 8   a  was taken at 2 kX utilizing an SEM. The image of  FIG. 8   b  was taken at 30 kX utilizing an FESEM. 
     The implant shown in  FIGS. 8   a  and  8   b  was machined, cleaned, and acid etched to impart a microscale roughness on the surface of the implant using a process similar to that described in U.S. Pat. No. 5,603,338, herein incorporated by reference in its entirety. The native oxide layer of the implant was removed via immersion in a hydrofluoric acid solution of about 5% v/v (about 8.5% w/w) for about 60 seconds at about 20-25 degrees Celsius. The acid etching was accomplished by immersion in an H 2 SO 4 /HCl solution for about 7 minutes at about 60-70 degrees Celsius.  FIG. 8   a  demonstrates the micron-level topography imparted by this acid etching at a magnification of 2 kX. Characteristic 1-3 micron peak-to-peak micropitting is clearly defined.  FIG. 8   b , which is an FESEM image of the surface at a magnification of 30 kX, demonstrates the general lack of nanometer-scale surface roughness features after this level of processing. 
     The implant was then immersed in about 4% w/w potassium hydroxide and about 16% w/w hydrogen peroxide at a starting temperature of about 50 degrees Celsius for about 1 minute, according to one embodiment of the invention. Post-processing, the implant was thoroughly rinsed in de-ionized water, then passivated through 40 kHz ultrasonic immersion in about 22% w/w nitric acid for about 10 minutes at about 60 degrees Celsius, followed by additional rinsing in de-ionized water, and oven drying at about 100-150 degrees Celsius. 
     The additional processing imparted a nanometer level topography, as demonstrated in the FESEM images of  FIGS. 9   a - c .  FIG. 9   a , which is an FESEM image at a magnification of 2 kX, demonstrates the micron-level roughness imparted by the acid etching remains on the implant, including the characteristic 1-3 micron peak-to-peak micropitting. The nanoscale roughness cannot be witnessed at this magnification. 
       FIG. 9   b , which shows the surface of  FIG. 9   a  at a magnification of 30 kX, demonstrates the nanoscale roughness features of the implant surface. Nanopitting in the 1-100 nanometer range can be witnessed at this magnification.  FIG. 9   c , which is a magnification of the surface of  FIGS. 9   a  and  9   b  at 100 kX, more clearly demonstrates the resultant nanoscale roughness. 
     The implant shown in  FIGS. 9   a - c  was then evaluated for surface chemistry utilizing Electron Dispersion Spectroscopy. A spot size of approximately 275×375 microns was analyzed for chemistry. The freshly processed and passivated sample demonstrated a 100% titanium surface chemistry, indicating that no residuals were present at the detection limit of the instrument. 
     Example 2 
     All of the solutions containing the concentrations of KOH and H 2 O 2  provided in Table 1 below resulted in acceptable nano-topography on commercially pure titanium with about 1 minute exposure to a hydrogen peroxide and potassium hydroxide solution at about 50 degrees Celsius: 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Concentrations of KOH and H 2 O 2   
               
             
          
           
               
                   
                 % KOH 
                 % H 2 O 2   
               
               
                   
                   
               
             
          
           
               
                   
                 4 
                 16 
               
               
                   
                 4 
                 19 
               
               
                   
                 4 
                 22 
               
               
                   
                 3 
                 16 
               
               
                   
                 3 
                 19 
               
               
                   
                 5 
                 13 
               
               
                   
                 5 
                 19 
               
               
                   
                   
               
             
          
         
       
     
     A solution having about 4% w/w KOH and about 16% w/w H 2 O 2  resulted in acceptable nano-topography on titanium 6 AL-4V ELI with 1 minute exposure to a hydrogen peroxide and potassium hydroxide solution at about 50 degrees Celsius. 
     A solution having about 4% w/w NaOH and about 16% w/w H 2 O 2  resulted in acceptable nano-topography on titanium 6 AL-4V ELI with about 1 minute exposure to a hydrogen peroxide and sodium hydroxide solution at about 50 degrees Celsius. 
     Example 3 
     All of the solutions containing the concentrations of KOH and H 2 O 2  provided in Table 2 below, with KOH and  H 2 O 2    concentrations ranging from about 1% w/w to about 6% w/w resulted in acceptable nano-topography on grit-blasted and acid-etched commercially pure titanium with about 4 minute exposure to a hydrogen peroxide and potassium hydroxide solution at about 33 degrees Celsius. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Concentrations of KOH and H 2 O 2   
               
             
          
           
               
                   
                 % KOH 
                 % H 2 O 2   
               
               
                   
                   
               
             
          
           
               
                   
                 4 
                 1 
               
               
                   
                 4 
                 2 
               
               
                   
                 4 
                 3 
               
               
                   
                 4 
                 4 
               
               
                   
                 4 
                 5 
               
               
                   
                 4 
                 6 
               
               
                   
                 1 
                 4 
               
               
                   
                 2 
                 4 
               
               
                   
                 3 
                 4 
               
               
                   
                 5 
                 4 
               
               
                   
                 6 
                 4 
               
               
                   
                   
               
             
          
         
       
     
     The solutions containing the concentrations of KOH and H 2 O 2  provided in Table 2 slow down the method of forming acceptable nano-topography, thus improving process control in the production environment. As the base or peroxide concentration approached 0% w/w, the desired surface topography was not formed. 
     It is contemplated that various combinations of variables (e.g., concentration of basic solution, concentration of hydrogen peroxide, exposure times, temperatures) may be used to forms the desired surface attributes. According to one non-limiting example, the desired surface may be obtained using 4.1% KOH, 3.85% H 2 O 2 , 3 minute exposure time, and 31 degrees Celsius. 
     While the present invention has been generally described relative to the part of the implant contacting bone tissue, it is contemplated that the acts of etching, acid etching, roughening, nanopitting, and depositing herein described may be performed on the entire implant. 
     While the present invention has been described with reference to one or more particular embodiments, those skilled in the art will recognize that many changes may be made thereto without departing from the spirit and scope of the present invention. Each of these embodiments and obvious variations thereof is contemplated as falling within the spirit and scope of the claimed invention, which is set forth in the following claims.