Abstract:
A method for producing a prosthetic having metal articulating surfaces is disclosed. The method includes impinging at least a portion of the articulating surface with high energy laser electromagnetic radiation, to form a lasershot peened surface. A hardened portion of the surface is then post-processed to form a layer having a predetermined finish.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to orthopedic implants and particularly to a method of surface hardening of bearing surfaces of the implant.  
       BACKGROUND OF THE INVENTION  
       [0002]     It is generally known in the art to use prosthetic devices to replace portions of the human anatomy that have been damaged due to injury or age. Often these prosthetic devices are formed of materials that are inherently strong yet easily formable. Many modular prosthetic devices are formed of at least metal stem portions that are inserted into long bones to provide a base for an external portion that extends from the boney portion. A taper or neck often interconnects the portion that extends from the bone, such as a head of a humerus or a femur, and the stem that is inserted in the bone. A taper may also be used to interconnect modular positions that are disposed within the bone after implantation. It is also known to provide bearing surfaces that must interact with one another while not wearing quickly or producing much wear debris.  
         [0003]     The taper or neck that interconnects the two portions of the prosthesis, sometimes referred to as a Morse taper, must be strong enough to withstand cyclic loads that will be seen in a wide variety of anatomies, patient activity levels, and compromised boney constructs. The neck must also allow a range of movement that closely simulates the natural human anatomy. Other types of prosthetic devices are also modular and are formed from multiple interconnecting components. These components may also be interconnected by way of a Morse taper.  
         [0004]     While materials generally used in these devices are inherently strong and have high tensile strengths, they require a particular thickness or mass to provide enough support for the portion of the anatomy that is being replaced. Due to this, the Morse taper is often larger and does not provide a full or natural range of motion. If the taper is for internal bone connection, a strong enough connection may produce a taper that is too big to fit into smaller bones. Due to this, it is desirable to produce prosthetic devices that include neck or interconnection portions that are small enough to fit into smaller bones and allow a full range of motion while being strong enough to support the stresses which the prosthetic will encounter.  
         [0005]     One solution has been to provide new metal alloys that are particularly strong. These metal alloys may be formed into a myriad of shapes while still providing much of the support necessary for the prosthetic device. These new metal alloys, however, are still required to have large enough interconnection portions to provide the necessary strength to the materials.  
         [0006]     Other known methods include the cold working or work hardening of prosthetics, such as that disclosed in the commonly assigned U.S. Pat. No. 6,067,701 entitled “Method for Forming a Work Hardened Modular Component Connector,” which is hereby incorporated by reference.  
         [0007]     Cold working induces residual stresses within the component and may also produce the required or different residual stresses within the component. Particular residual stresses can be either compressive or tensile depending upon their nature. Compressive residual stresses are particularly desired. In particular, compressive residual stresses inhibit or stop cracks which may form in the prosthetic device. Furthermore, compressive stresses inhibit the initiation of a crack within the area which is loaded by external forces.  
         [0008]     Compressive stress, especially near the surface of the component, also provides additional benefits. In particular, compressive stress near the surface can decrease fatigue and stress corrosion failures. In particular, these fatigue and stress corrosion failures originate at the surface and the compressive stress help inhibit such failures. In addition, the compressive residual stresses increase resistance to other undesired events such as fatigue failures, corrosion failure, stress corrosion cracking, hydrogen assisted cracking, fretting, galling, and corrosion caused by cavitation. Additionally, work hardening, which produces the compressive stresses, increases intergranular corrosion resistance, surface texturing, and closing of surface porosity.  
         [0009]     Although compressive stresses, or other particular residual stresses, provide these many benefits, it is more beneficial to precisely create the desired residual stresses within the prosthetic device. Although exploratory cold working a component may produce the desired residual stresses, predetermining and work hardening components to produce predetermined residual stresses is preferable. Therefore, it is desired to provide a known process to produce within the prosthetic device, known and predetermined residual stresses that will provide compressive and tensile stresses, that are desired in a component.  
         [0010]     Thus, it is desirable to have a method of producing prosthetic devices that leave no uncertainty to the strength being introduced into the prosthetic device. This would allow for more efficient manufacturing and an increase in prosthetic strength that survive the testing phase. That is, it is desirable to produce a prosthetic device that needs not be tested as often while still assuring that the prosthetic device will be able to handle the loads after being implanted.  
       SUMMARY OF THE INVENTION  
       [0011]     A method for producing a prosthetic having metal articulating surfaces is disclosed. The method includes impinging at least a portion of the articulating surface with high energy laser electromagnetic radiation, and allowing the surface to cool to form a layer having a predetermined hardness. A hardened portion of the surface is then post-processed to form a layer having a predetermined finish. In another embodiment of the present invention, a method for producing an acetabular cup is disclosed. The process subjects the hemispherical articulating surface of the acetabular cup to lasershot peening to form a surface having a predetermined hardness. Honing is performed on the surface to provide an articulating surface having a predetermined hardness and surface finish.  
         [0012]     In another embodiment to the present invention, a implantable femoral prosthetic is provided having an articulating head portion. The surface of the head portion is subjected to lasershot peening to form a hardened surface. As with the previously described bearing, the head surface is machined to provide a articulating surface having a suitable surface finish.  
         [0013]     In another embodiment to the present invention, a method of producing a Morse taper joint for a prosthetic is disclosed. At least one surface of the Morse taper is subjected to pulses of high intensity laser electromagnetic radiation to cause surface hardening of the Morse taper joint. Post-processing is conducted on the treated surface to produce the proper surface finish.  
         [0014]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0016]      FIG. 1  represents an articulating acetabular prosthetic having a hardened surface;  
         [0017]      FIG. 2  represents a cross-sectional view of a prosthetic assembly utilizing the prosthetic shown in  FIG. 1 ;  
         [0018]      FIGS. 3 and 4  represents the hardening of an articulating surface of the prosthetic shown in  FIG. 1 ; and  
         [0019]      FIGS. 5 and 6  represent the hardening of various surface of a femoral prosthetic. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     The following description of the embodiments are merely exemplary in nature and are in no way intended to limit the invention, its application, or uses.  
         [0021]     Referring to  FIG. 1 , an acetabular prosthesis  20  according to the teachings of a first embodiment of the present invention is shown. The acetabular prosthesis  20  has a inner surface  22  defining a first metal bearing surface  24  and an outer surface  26  capable of being coupled or impacted into a prepared acetabulum. As further descried below, the first metal bearing surface is hardened using a laser peening process. Defined between the outer surface  26  and the inner surface  22  is a peripheral surface or rim  30 . Disposed on the peripheral surface  30  is a locking mechanism  34 . The locking mechanism  34  is capable of coupling a second prosthetic such as an optional constraining ring  36 , or an optional insert bearing  38  into a bearing cavity  40  which is defined by the inner surface  22  of the acetabular prosthesis  20 . The optional bearing inert  38  can be biocompatible polymer, ceramic or hardened metal. The second prosthetic substantially surrounds a head of a femoral component (as further described later), were substantially encloses includes for example a slotted constraining ring.  
         [0022]     The outer surface  26  of the acetabular prosthesis  20  defines a plurality of locking projections  42  for coupling the acetabular prosthesis  20  to the prepared acetabulum. It is envisioned that the outer surface  26  can be surface treated to facilitate bone ingrowth or fixation to bone cement, such as by porous coating.  
         [0023]      FIG. 1  further illustrates that the acetabular prosthesis  20  can have a plurality of through holes  44  to assist in fixation to the prepared acetabulum. Defined in the peripheral surface  30  is the plurality of through holes  44  for mounting the acetabular prosthesis  20  to the prepared acetabulum  28  using standard bone coupling fasteners or screws.  
         [0024]     Although there are many ways to couple the constraining ring  36  or insert bearing  38  into the bearing cavity  40  defined by the inner surface  22  such as fasteners or tabs, shown is the locking mechanism  34  is formed by a locking flange  35  which defines a coupling groove  46 . The coupling groove  46  is designed to accept the locking ring  48 . The locking flange  35  has a plurality of alignment notches  50  disposed therein to facilitate the acceptance of the locking ring  48  and alignment of the acetabular prosthesis  20 , further discussed herein.  
         [0025]     FIGS.  2  illustrates a cross-sectional view of head  67  of a femoral prosthesis  68  into the acetabular prosthesis  20 , along with the use of the optional constraining ring  36 . It should again be noted that as the inner surface  22  of the acetabular prosthesis  20  is a highly polished hardened metal bearing surface formed from a bio-compatible material such as titanium, cobalt chrome, stainless steel, etc. The first head  67  of the femoral prosthesis  68  will articulate within the bearing cavity  40  defined by the inner surface  22  of the acetabular prosthesis  20 . As can be seen, the first head  67  is inserted into the bearing cavity  40 . Next, the constraining ring  36 , which was previously disposed about the neck  102  of the femoral implant  68 , is positioned adjacent to the-peripheral surface  30  of the acetabular prosthesis  20 .  
         [0026]     The locking ring  48  is inserted into the coupling groove  46  defined by the locking flange  35  and the constraining ring  36  is released to affix the constraining ring  36  onto the acetabular prosthesis  20 . In this way, a metal-metal articulating bearing surface is formed between the inner surface  22  and the femoral head  67 . As is shown, the first femoral head  67  engages the metal bearing surface  24 . The locking ring  48  is positioned within the constraining ring groove  56  to fix the constraining ring  36  to the acetabular prosthesis  20 , thus locking the first head  67  into its proper orientation.  FIG. 2  depicts the location of the locking ring  48  with respect to the acetabular prosthesis  20  and constraining ring  36 . Further shown is the location of the fastener or screw to the locking mechanism  34 .  
         [0027]     As shown in  FIG. 3 , to counter fatigue failure of portions of the bearing elements from deformities that can develop in the bearing articulating surfaces  22 , the present invention provides a laser shock peened curved surface  23  on at least a portion of the contact surfaces of the bearing elements. The pre-stressed laser shock peened curved regions  23  have deep compressive residual stresses which are imparted by laser shock peening. The compressive residual stresses extend into the bearing elements from the laser shock peened surfaces  23 . Preferably, the entire curved spherical surface of the femoral head  67  are so laser shock peened as are the curved contact semi-spherical outer and inner circumferential surfaces of the acetabular and femoral bearing surfaces, respectively.  
         [0028]     The laser beam shock induced deep compressive residual stresses in the compressive pre-stressed regions are generally about 50-150 KPSI (Kilo Pounds per Square Inch). These pre-stressed regions extend from the laser shock surfaces to a depth of about 20-50 mils into laser shock induced compressive residually pre-stressed regions. The laser beam shock induced deep compressive residual stresses are produced by repetitively firing a high energy laser beam from the laser  70  that is focused on surface which is covered with paint to create peak power densities having an order of magnitude of a gigawatt/cm 2 . As is known, the laser beam is fired through a curtain of flowing water that is flowed over the surface and the surface is ablated generating plasma which results in shock waves on the surface of the material. These shock waves are re-directed towards the surface by the curtain of flowing water to generate traveling shock waves (pressure waves) in the material below the surface. The amplitude and quantity of these shock waves determine the depth and intensity of the compressive stresses. The surface is used to protect the target surface and also to generate plasma. Ablated surface material is washed out by the curtain of flowing water.  
         [0029]      FIGS. 4   a  and  4   b  show the hardening of the acetabular component  20 . The laser peened surface  23  has a surface roughness which would prevent its use as an orthopedic implant bearing. As such, further surface honing and polishing of the bearing surface using a honing instrument  73  provides a prosthetic having a bearing surface with a predetermined hardness and surface finish.  
         [0030]      FIGS. 5 and 6  represents the hardening of surfaces of the femoral prosthetic. As seen in  FIG. 5 , the exterior surface  72  of the articulating head and Morse taper is hardened as previously described by impinging the surface with a high intensity laser. The stem portion  102  of the femoral component  68 , which is configured to be coupled to a bone, has a tapered neck with a male portion  74  of a Morse taper joint  76 . A portion of a coupling surface of the male portion of the Morse taper joint is hardened using overlapping spots of laser shock peening which induces residual stress that extends into the prosthetic from the laser shock peened surface.  FIG. 6  represents a cross-sectional view of the machined femoral prosthetic shown in  FIG. 5 . Shown is the female portion  78  of the Morse taper  76  which has a coupling surface  78  which has been hardened.  
         [0031]     In general, a method for implanting a medical device, for example a femoral prosthetic  68 , into a patient is described. Prior to implantation, at least one surface of a femoral prosthetic  68  is subjected to laser shock peening. The surface can be an internal coupling surface such as a Morse taper joint or can be the bearing surface of the femoral head. The femoral prosthetic  68 , which can be a single piece or a modular component, is then implanted into a prepared joint. In this regard, the stem of the femoral prosthetic  68  is inserted into a resected femur. In the case of a modular femoral prosthetic  68 , the surface hardened head  67  is coupled to the stem portion  102  of the femoral component  68 .  
         [0032]     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. For example, the teachings of the present invention are disclosed in a description of an acetabular and femoral prosthetic. It is however envisioned that the teaching are equally applicable to other joints, for example knee, ankle, or shoulder joints. Such variations are not to be regarded as a departure from the spirit and scope of the invention.