Abstract:
A method for forming an acetabular shell includes providing a working surface such as a mandrel, and spraying the working surface with a first layer of material having a first composition such as aluminum oxide. After a suitable thickness is generated, the spray composition is gradually changed to other compositions having desired particle sizes and distribution. In one example, the composition is changed to a mixture of aluminum oxide and titanium oxide and/or titanium. As thickness builds up, the relative amount of aluminum oxide is decreased such that the composition is all titanium and titanium oxide. After a desired thickness is generated, the acetabular shell is extracted off the mandrel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/366,727 filed Mar. 2, 2006, which claims the benefit of a U.S. Provisional Application 60/658,407 filed on Mar. 3, 2005. The disclosure of the above application is incorporated herein by reference. 
     
    
     FIELD 
       [0002]    This invention relates to acetabular shells and more particularly to a method of making an implant such as an acetabular shell by free form plasma (thermal) spray technology. 
       INTRODUCTION 
       [0003]    In replacement hip surgery a femoral component can be inserted into the prepared femur. The femoral component can include a stem portion which projects into the femoral canal of the prepared femur and has an integral or separate modular head of substantially spherical shape. The ball-like head of the femoral component is received within an acetabular cup component which is implanted in the patient&#39;s hip socket, i.e., the acetabulum. The acetabular cup has a substantially hemi-spherical bearing surface for movement of the ball head of the femoral component during action of the joint. 
         [0004]    Various designs of acetabular cups are available and it is often a multi-piece component having at least a separate outer shell and an inner liner. Where the acetabular cup has an inner liner, that inner liner is generally press-fitted into the outer shell. In some designs of hip prostheses the material of the bearing surface of the acetabular cup, e.g. its inner liner where present, is of the same material as that of the ball head, e.g. for a ceramic head, a ceramic bearing surface is provided (a so-called ceramic-on-ceramic prosthesis) and for a metal head, a metal bearing surface is provided (a so-called metal-on-metal prosthesis). In some other designs, the acetabular bearing surface is of polyethylene, as the acetabular cup is either provided with a polyethylene inner liner or the acetabular cup is a single component made entirely from polyethylene. The shape of the bearing surface into which the ball head is received affects the degree of movement available after implantation of the joint. 
       SUMMARY OF THE INVENTION 
       [0005]    A method for forming an implant includes providing a working surface such as a mandrel, and spraying the working surface with a first layer of material having a first composition such as aluminum oxide. After a suitable thickness is generated, the spray composition is gradually changed to other compositions having desired particle sizes and distribution. In one example, the composition is changed to a mixture of aluminum oxide and titanium oxide and/or titanium. As thickness builds up, the relative amount of aluminum oxide is decreased such that the composition is all titanium and titanium oxide. After a desired thickness is generated, the acetabular shell is extracted off the mandrel. 
         [0006]    A method of making an implant according to various features includes forming a ceramic shell having a first surface and a second surface. A first surface of the ceramic shell is located onto a mounting instrument. A layer of material is sprayed onto the second surface of the ceramic shell. The ceramic shell is then sintered. 
         [0007]    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 embodiments 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 
         [0008]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0009]      FIG. 1  illustrates an exemplary thermal spraying device spraying a first layer of material onto a mandrel; 
           [0010]      FIG. 2  illustrates a cross-sectional view of the mandrel of  FIG. 1 ; 
           [0011]      FIG. 3  illustrates a partial cross-sectional view of the spraying device and mandrel of  FIG. 1  shown during an initial spraying step; 
           [0012]      FIG. 4  illustrates the cross-sectional view of  FIG. 3 , shown during an intermediate spraying step; 
           [0013]      FIG. 5  illustrates the cross-sectional view of  FIG. 4  shown during spraying of an outboard porous layer; 
           [0014]      FIG. 6  illustrates the acetabular shell removed from the mandrel; 
           [0015]      FIG. 7  illustrates a cross-sectional view of the acetabular shell of  FIG. 6 ; 
           [0016]      FIG. 8  illustrates a perspective view of a ceramic casting according to a another embodiment; 
           [0017]      FIG. 9  illustrates a perspective view of a ceramic shell removed from the casting of  FIG. 8 ; 
           [0018]      FIG. 10  illustrates a perspective view of the ceramic shell arranged on a mandrel and a spraying device spraying a layer of material onto an outer concave surface of the ceramic shell; 
           [0019]      FIG. 11  illustrates a cross-sectional view of the ceramic shell having the layer of material sprayed thereon and defining an acetabular shell; and 
           [0020]      FIG. 12  illustrates the acetabular shell of  FIG. 11  shown during a sintering process. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    The following description of the embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. Moreover, while the description below is directed to an acetabular shell, the teachings described herein may similarly be employed to form other implants, such as, but not limited to femoral implants, tibial implants, tibial trays, and glenoid implants. 
         [0022]    With initial reference now to  FIGS. 1 and 2 , a thermally stable mandrel and plasma spraying device are shown and identified at reference  10  and  12  respectively. The mandrel  10  generally defines a semi-hemispherical dome  14  and a longitudinal support portion  16 . While the mandrel  10  and spraying device  12  are shown having specific geometries, it is appreciated that they are merely exemplary and other mandrels and spraying devices may be similarly employed. 
         [0023]    The mandrel  10  is generally shaped inversely to a desired acetabular shell. More specifically, the mandrel  10  defines the semi-hemispherical dome  14  providing an outer convex surface  20  that corresponds to an inner concave surface of the acetabular shell. In one example, the thermally stable mandrel  10  may be made of tungsten. The outer convex surface  20  may be polished to a roughness acceptable for articulating surfaces of acetabular shells. The thermally stable mandrel  10  allows the generation of multiple acetabular shells without the need of individual grinding and polishing operations between subsequent shell formations. 
         [0024]    With specific reference to  FIG. 2 , the mandrel  10  may be mounted on a holding fixture  22 . The holding fixture  22  may provide a bearing set  26  for rotational capability. Other arrangements may also be provided. 
         [0025]    The mandrel  10  may define a coolant channel  28  for communicating a coolant (not specifically shown) from an inlet  30  defined on the longitudinal portion  16  to an outlet  32  defined on the longitudinal portion  16 . As shown, the coolant channel  28  is adapted to communicate fluid through the longitudinal portion  16 , around the semi-hemispherical portion  14  and back through the longitudinal portion  16 . The coolant port arrangement on the mandrel  10  helps draw away heat to maintain an acceptable temperature on the outer convex surface  20  and the mandrel  10  as a whole during a spray event. It is appreciated that the coolant channel  28  may be arranged differently on the mandrel  10  and/or other cooling mechanism or techniques may be employed to maintain acceptable mandrel temperatures during use. In one example, the mandrel  10  is maintained under 200° C. Other temperatures are contemplated. In addition, while not specifically shown, heat removal may be accomplished by flow of external gases over the acetabular shell being created. 
         [0026]    With continued reference to  FIGS. 1 and 2  and additional reference to  FIG. 3 , a method of making an acetabular shell according to the present teachings will be described. In a controlled atmosphere, very fine powder of aluminum oxide  38  is sprayed onto the mandrel  10  such that a continuous layer of aluminum oxide  38  is generated on the mandrel  10  on the outer convex surface  20 . The very fine powder exhausted from the spraying device  12  may be in the molten, or semi-molten state. In one example, prior to applying the aluminum oxide  38 , a mold relief agent may be applied to the mandrel  12  to facilitate removal of the acetabular shell once completed. 
         [0027]    Once a suitable thickness (such as, but not limited to, less than 5 mm) of aluminum oxide  38  is sprayed onto the mandrel dome  14 , the spray concentration changes to a mixture of aluminum oxide and titanium oxide and/or titanium collectively identified at reference  40  ( FIG. 4 ). After a suitable thickness is generated (such as, but not limited to, 5 mm), the proportions of aluminum oxide is reduced causing the composition of titanium oxide and/or titanium to increase. This layer is identified at reference  42  ( FIG. 5 ). The thickness of layer  42  may be 3 mm, although other thicknesses are contemplated. 
         [0028]    Concurrently, the particle size of the titanium oxide and/or titanium may be increased to generate a porous outer surface. It is understood that while unique reference numerals  38 ,  40  and  42  have been used to identify sequential layers of sprayed material, the relative proportions of aluminum oxide, titanium oxide and/or titanium in the sprayed material gradually change as desired. As a result, there is not necessarily any identifiable transition boundaries across the thickness of the sprayed material. 
         [0029]    Turning now to  FIG. 6 , once a suitable thickness is achieved (such as, but not limited to, 13 mm), a newly formed acetabular shell  50  is extracted off the mandrel  10 . The resulting acetabular shell  50  provides an inner portion  52  having a concave inner surface or articulating surface  54  of aluminum oxide, an intermediate portion  58  comprising a ceramic composition changing from aluminum oxide (from the articulating surface  54 ) to titanium and titanium oxide and finally to an outer portion  60  comprising porous titanium. The articulating surface of the acetabular shell  50  may then be polished to a desired roughness to serve as an articulating surface. It is appreciated that other materials may be used. For example, zirconia and/or other materials may be used for the aluminum oxide. Likewise, any composition of cobalt, chromium and/or molybdenum may be used for the titanium oxide and/or titanium. In addition, the acetabular shell may alternatively be made exclusively of one material. 
         [0030]    In another embodiment, an implant such as a stem may take the place of the mandrel  10 . As a result, the sequential layers may be sprayed directly onto the implant and remain on the implant as an integral feature. 
         [0031]    Turning now to  FIGS. 8-12 , a method of making an acetabular shell according to an additional embodiment will be described. In this embodiment, rather than forming all layers of an acetabular shell by way of plasma spraying, a prefabricated ceramic shell  68  ( FIG. 9 ) is used to define a template for receiving a subsequent plasma spray. 
         [0032]    As shown in  FIG. 8 , a ceramic shell  70  may be formed by way of a casting process. In one example, a first and second die  72  and  74  are used to define a desired outboard surface  76  and an inboard articulating surface  78  of the ceramic shell  70  ( FIG. 9 ). As illustrated in  FIG. 8 , the first die  72  defines a concave cavity  80  and the second die  74  defines a convex extension surface  82 . The concave cavity  80  defines a plurality of outward knobs  86 . The outward knobs  86  are operable to define a textured surface, represented as dimples  90  on the outboard surface  76  of the cast ceramic shell  68  ( FIG. 9 ). The convex extension surface  82  is operable to define the inboard articulating surface  78  of the cast ceramic shell  68 . In one example, the walls of the concave cavity  80  are coated with titanium powder such that a layer of titanium is defined on outboard surface  76  of the cast ceramic shell  68 . 
         [0033]    During the casting process, a slurry of ceramic  92  is delivered to the first and second die  72  and  74 . In one example, the slurry of ceramic  92  may be created in a fluid with binders and deflocculating agents as desired. The particle size of the ceramic, the quantity of binder and deflocculating agents and the ratio of various components in the slurry may be adjusted to achieve a slurry providing favorable casting properties. While the respective die cavities  72  and  74  are shown in an open position in  FIG. 8 , it is appreciated that the slurry of ceramic  92  may be delivered through a port to a closed die cavity. 
         [0034]    As shown in  FIG. 9 , the ceramic shell  68  defines a hemispherical dome  96 . It is appreciated that other shapes may be alternatively formed. It is further appreciated that while the textured surface is depicted as dimples  90 , the first die  72  may be configured to define any textured surface including, but not limited to, ridges, notches and other configurations. Furthermore, while the formation of the ceramic shell  68  has been described by way of a casting process, other fabrication techniques may be used. Once the ceramic shell  68  has been cast, the ceramic shell  68  is dried or semi-dried into a stable dome  98 . In one example, the process of converting the ceramic shell  68  to a semi-dried, stable dome  98  may be achieved in a baking oven  100 . The baking oven  100  is operable to drive off any excess fluid slurry. Furthermore, any binders and/or deflocculating agents may or may not be removed. 
         [0035]    Turning now to  FIG. 10 , the stable dome  98  may be placed onto a mandrel  110 . The mandrel  110  may cooperate with a longitudinal support portion  116 , a holding fixture  122  and a bearing set  126 . Furthermore, the mandrel  110  may include a coolant channel  128 . Again, other configurations may be employed. 
         [0036]    Next, a layer of titanium oxide  140  is sprayed with a plasma sprayer  12  to a desired thickness. As best illustrated in  FIG. 11 , mechanical interlocking is achieved between the titanium oxide  140  and the outboard textured surface  90  of the ceramic dome  98 . The interface between the textured surface  90  and the titanium oxide  140  resists torsional slippage and radial slippage of the titanium oxide  140  relative the outboard surface  76  of the ceramic dome  98 . 
         [0037]    With reference now to  FIG. 12 , a newly formed acetabular shell  150  is then placed into a baking oven  100  for a sintering process. In one example, a sinter cycle may be performed as follows. First, the oven temperature may be raised to 175° C. at 5° C./min. The 175° C. may be maintained for 4 hours. The temperature may be ramped to a peak temperature such as 1650° C. at 5° C./min (optionally lower temperatures may be used i.e. 1300° C.-1450° C.). The peak temperature may be maintained for 8 hours. It is appreciated that a peak temperature of 1300° C.-1450° C. may be maintained for longer periods than higher temperatures (such as 1650° C.). The temperature may be ramped down to 600° C. at 5° C./min. The 600° C. temperature may be held for 30 minutes. 
         [0038]    It is appreciated that the sintering procedure explained above is merely exemplary. As such, ramp rate, dwell time and dwell temperatures (collectively referred to as variables) of the sinter cycle may define other ranges. The variable assigned during the sinter process may be chosen to discourage crack formation in the structure. In one example, a thermally induced compressive stress may be generated in the ceramic structure to discourage premature failure. 
         [0039]    Once the acetabular shell  150  is sintered, the concave inner surface  78  may be polished to a desired roughness to serve as an articulating surface. 
         [0040]    While the invention has been described in the specification and illustrated in the drawings with reference to various embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. Furthermore, the mixing and matching of features, elements and/or functions between various embodiments is expressly contemplated herein so that one of ordinary skill in the art would appreciate from this disclosure that features, elements and/or functions of one embodiment may be incorporated into another embodiment as appropriate, unless described otherwise above. Moreover, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the foregoing description and the appended claims.