Abstract:
A ball and socket joint prosthesis ( 10 ) for use in arthroplasty is provided. The prosthesis includes a body ( 32 ) for implantation at least partially within the medullary canal ( 24 ) of a long bone ( 12 ). The long bone defines trabeculae ( 60 ) in the proximal cancellous bone ( 22 ) and lamellae ( 71 ) in the cortical bone ( 65 ). The body includes a proximal portion ( 52 ) thereof and a distal portion ( 54 ). The proximal portion has a medial periphery ( 69 ) and includes surface features ( 64 ) on a substantial portion of its proximal portion. The surface features are positioned to optimally transfer load from the prosthesis to the long bone.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This Application is a Utility Application based upon U.S. Provisional Patent Application, Serial No. 60/255,644 filed Dec. 14, 2000, entitled PROSTHESIS WITH FEATURE ALIGNED TO TRABECULAE. 
     
    
     
       TECHNICAL FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to the field of orthopaedics, and more particularly, to an implant for use in arthroplasty.  
         BACKGROUND OF THE INVENTION  
         [0003]    The invention relates to implantable articles and methods for manufacturing such articles. More particularly, the invention relates to bone prosthesis and processes for manufacturing the same.  
           [0004]    There are known to exist many designs for and methods for manufacturing implantable articles, such as bone prosthesis. Such bone prosthesis include components of artificial joints, such as elbows, hips, knees, and shoulders. An important consideration in the design and manufacture of virtually any implantable bone prosthesis is that the prosthesis has adequate fixation when implanted within the body.  
           [0005]    Early designs of implantable articles relied upon the use of cements such as polymethylmethacrylate to anchor the implant. The use of such cements can have some advantages, such as providing a fixation that does not develop freeplay or does not lead to erosion of the joining bone faces postoperatively. However, the current trend is to use these cements to a lesser extent because of their tendency to lose adhesive properties over time and the possibility that the cement contributes to wear debris within a joint.  
           [0006]    Recently, implantable bone prosthesis have been designed such that they encourage the growth of hard tissue (i.e., bone) around the implant. The bone attachment usually occurs and growth is promoted when the surface of the implantable bone prosthesis is irregular or textured. The interaction of newly formed hard tissue in and around the textured surface of the implantable bone prosthesis has been found to provide a good fixation of the prosthesis within the body. A greater degree of bone fixation can usually be achieved where bone-engaging surfaces of an implantable bone prosthesis are more porous or irregular.  
           [0007]    Porous or irregular surfaces can be provided in implantable articles by a variety of techniques. In some instance, an irregular surface pattern or surface porosity is formed in an implantable bone prosthesis by embossing, chemical etching, milling or machining.  
           [0008]    Another problem which has been observed in the use of known hip joint systems relates to the proper distribution of stresses within the prosthesis and throughout the surrounding bone. If too little stress is applied to the bone, resorption can occur leading to atrophy of the affected area. Too much stress may also lead to resorption and atrophy, or may result in an undesirable hypertrophy of the affected area. In some prior art, femoral stem designs excessive forces are transmitted through the relatively rigid stem to the distal portion, resulting in hypertrophy of the bone surrounding the distal portion, and atrophy of the bone surrounding the proximal portion of the stem. Accordingly, there exists a need for an improved hip joint prosthesis which addresses these needs and other problems of prior hip joint designs.  
           [0009]    Attempts have been made to provide for proximal loading of the prosthesis within the bone. For example, in U.S. Pat. No. 5,004,075 to Vermeire a series of parallel spaced apart linear grooves  28  were positioned perpendicular to the longitudinal axis  22  of the neck of the prosthesis. A second set of parallel spaced apart linear grooves  29  were positioned generally perpendicular to the grooves  28 . These grooves serve to provide support in the proximal region of the stem of this prosthesis.  
           [0010]    In U.S. Pat. No. 4,865,608 to Brooker, Jr. a series of spaced apart parallel grooves  24  and  24 ′ were positioned along the outer periphery of the opposite sides of the proximal portion of the stem. The grooves were positioned at an angle of approximately 70 degrees with respect to the longitudinal axis of the stem.  
           [0011]    In total hip arthroplasty, initial and long term success are achieved through the use of a device which is designed to provide at least two features. The first of these features is the stable initial or immediate postoperative fixation within the femur. The second feature is the means to provide an optimal environment for a long-term stability in the femur. In the past, fixation has been achieved through the use of bone cement, porous coatings and bio-ceramics. Bio-ceramics includes such compositions as hydroxyapatite and tricalcium phosphates. Many of these cements, coatings and bio-ceramics have provided good clinical outcomes, however, none have addressed the biomechanics of load transmission through the proximal femur.  
           [0012]    Methods of achieving femoral fixation in the prior art have met with some success. These methods include simple press fit, surface roughness, porous coating, and bioceramics. Many devices have included texturing to transfer load in favorable mechanical modes. However, none of the prior art devices have designed the texturing (steps) to transfer load along the natural load paths of the proximal femur. The Brooker patent has angled steps on the anterior and posterior face, however, on the medial edge, the steps are longitudinal. This design will not appropriately transmit load to the medial calcar. The Vermeire patent has no steps on the medial edge, posing a similar problem.  
           [0013]    A commercially available product from Stryker Howmedica Osteonics known as the Omni Fit Femoral Stem has normalization features which transmit load directly vertical. This load path is not natural. This device has no medial steps. A commercially available product from DePuy Orthopaedics, Inc., the JMP S-ROM transmits axial loads, but again, does not follow the natural load path.  
         SUMMARY OF THE INVENTION  
         [0014]    Accordingly, a need has arisen for a prosthesis which achieves fixation to the long bone by designing features to transfer load along the natural load paths of the proximal long bone.  
           [0015]    The present invention includes a proximal long bone prosthesis which has been designed to provide initial stability and long term fixation through a series of features capable of transmitting load to the proximal long bone in a manner consistent with the natural load paths of the long bone. The long bone may be a femur, a humerus or any other long bone.  
           [0016]    The present invention allows reconstruction of the proximal long bone with a device that is specifically designed to provide stable initial fixation and long term stability by optimally transferring load along the natural load lines through the femur. The load paths through the proximal long bone are seen by both the alignment of the trabeculae in the proximal cancellous bone and by the direction of the layers or lamellae in the cortical bone.  
           [0017]    This device achieves initial fixation through a press fit. The press fit is achieved with a properly designed preparation instrumentation. Long term stability is achieved through a series of steps which are aligned normal to the trabeculae of the proximal femur cancellous bone and the lamellae of the proximal femoral cortex. The steps transmit load normal to their surface and hence along the natural femoral load lines. This replication of the natural femoral load paths lead to favorable remodeling of the proximal long bone. This fixation mode may be further enhanced with a bone in growth/on growth surface such as for example surface roughness, porous coating and/or bioceramics.  
           [0018]    According to one embodiment of the present invention, a ball and socket joint prosthesis for use in arthroplasty is provided. The prosthesis includes a body for implantation at least partially within the medullary canal of a long bone. The long bone defines trabeculae in the proximal cancellous bone and lamellae in the cortical bone. The body includes a proximal portion and a distal portion. The proximal portion has a medial periphery and includes surface features on a substantial portion of the periphery of the proximal portion. The surface features are positioned to optimally transfer load from the prosthesis to the long bone.  
           [0019]    According to another embodiment of the present invention, a hip-joint prosthesis for use in arthroplasty is provided. The prosthesis includes a body for implantation at least partially within the medullary canal of a long bone. The long bone has trabeculae in the proximal cancellous bone and has lamellae in the cortical bone. The body includes a proximal portion and a distal portion. The proximal portion has a medial periphery and includes a plurality of ribs extending from a substantial portion of the periphery of the proximal portion. The ribs are positioned so that the first direction of the ribs is from about 70 degrees to about 110 degrees with respect to the trabeculae in the proximal cancellous bone, the normal lamellae in the cortical bone or the medial periphery of the proximal portion of said body.  
           [0020]    According to yet another embodiment of the present invention, a joint prosthesis for use in arthroplasty is provided. The prosthesis includes a body for implantation at least partially within the medullary canal of a long bone. The long bone includes trabeculae in the proximal cancellous bone and lamellae in the cortical bone. The body includes a proximal portion and a distal portion. The proximal portion has a medial periphery and includes surface features on a substantial portion of the periphery of the proximal portion. The surface features are positioned to optimally transfer load from the prosthesis to the long bone.  
           [0021]    According to a further embodiment of the present invention, a stem for use in a joint prosthesis for implantation at least partially within the medullary canal of a long bone is provided. The long bone includes trabeculae in the proximal cancellous bone and lamellae in the cortical bone. The stem includes a proximal portion and a distal portion. The proximal portion has a medial periphery and surface features on a substantial portion of the periphery of the proximal portion. The surface features are positioned to optimally transfer load from the prosthesis to the long bone.  
           [0022]    According to another embodiment a method for producing a joint prosthesis for use in arthroplasty is provided. The method includes the steps of providing a body including a proximal portion and a distal portion, the proximal portion having a medial periphery thereof, placing surface features on a substantial portion of the periphery of the proximal portion of the body, positioning the surface features to optimally transfer load from the prosthesis to the long bone, and implanting the prosthesis at least partially within the medullary canal of a long bone.  
           [0023]    The technical advantages of the present invention include the ability to transmit loads to the proximal femur along the natural load lines. The load lines or load paths through the proximal femur are seen by both the alignment of the trabeculae in the proximal cancellous bone and by the direction of the lamellae in the cortical bone. This invention achieves initial fixation through a press-fit achieved with properly design preparation instrumentation. Long term stability is achieved through a series of steps which are aligned normal to the trabeculae of the proximal femoral cancellous bone and the lamellae of the proximal femoral cortex. The steps transmit load normal to their surface and hence along natural femoral load lines.  
           [0024]    Another technical advantage of the present invention includes the ability to provide long term stability and fixation by providing an environmental optimum for femoral bone remodeling. The long term stability achieved through the series of steps which are aligned normal to the trabeculae of the proximal femoral cancellous bone and the lamellae of the proximal femoral cortex transmit load normal to their surface and hence along the natural femoral load lines. This replication of the natural femoral load paths leads to favorable remodeling of the proximal femoral bone. This fixation mode may be further enhanced with a bone ingrowth or ongrowth surface, for example, by providing for surface roughness, porous coating and bioceramics.  
           [0025]    Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, descriptions and claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in connection with the accompanying drawings, in which:  
         [0027]    [0027]FIG. 1 is a plan view of a hip prosthesis in accordance with an embodiment of the present invention;  
         [0028]    [0028]FIG. 1A is a partial enlarged view of the hip prosthesis of FIG. 1 showing steps on the periphery of the prosthesis in greater detail;  
         [0029]    [0029]FIG. 1B is a partial enlarged view of the hip prosthesis of FIG. 1 showing steps with an alternate construction to those of FIG. 1A on the periphery of the prosthesis;  
         [0030]    [0030]FIG. 1C is a partial enlarged view of the hip prosthesis of FIG. 1 showing steps with an alternate construction to those of FIG. 1A on the periphery of the prosthesis;  
         [0031]    [0031]FIG. 1D is a cross-sectional view of FIG. 1 along the line D-D in the direction of the arrows illustrating one of many possible cross-sections;  
         [0032]    [0032]FIG. 2 is a lateral end view of a hip prosthesis in accordance with the embodiment of the present invention of FIG. 1;  
         [0033]    [0033]FIG. 2A is a cross-sectional view of FIG. 2 along the line A-A in the direction of the arrows illustrating one of many possible cross-sections;  
         [0034]    [0034]FIG. 3 is a medial end view of a hip prosthesis in accordance with the embodiment of the present invention of FIG. 1;  
         [0035]    [0035]FIG. 4 is a partial plan view of the hip prosthesis of FIG. 1;  
         [0036]    [0036]FIG. 5 is a partial plan view of the hip prosthesis of FIG. 4;  
         [0037]    [0037]FIG. 6 is a plan view of a hip prosthesis in accordance with another embodiment of the present invention;  
         [0038]    [0038]FIG. 7 is a plan view of a shoulder prosthesis in accordance with a further embodiment of the present invention;  
         [0039]    [0039]FIG. 7A is a partial plan view of the shoulder prosthesis of FIG. 7 showing an alternate stem-shoulder connection;  
         [0040]    [0040]FIG. 8 is a plan view of a hip prosthesis in accordance with a further embodiment of the present invention;  
         [0041]    [0041]FIG. 9 is a lateral end view of a hip prosthesis in accordance with the embodiment of the present invention of FIG. 8;  
         [0042]    [0042]FIG. 10 is a medial end view of a hip prosthesis in accordance with the embodiment of the present invention of FIG. 8;  
         [0043]    [0043]FIG. 11 is a plan view of a hip prosthesis in accordance with another embodiment of the present invention;  
         [0044]    [0044]FIG. 12 is a lateral end view of a hip prosthesis in accordance with the embodiment of the present invention of FIG. 11; and  
         [0045]    [0045]FIG. 13 is a medial end view of a hip prosthesis in accordance with the embodiment of the present invention of FIG. 11. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0046]    Embodiments of the present invention and the advantages thereof are best understood by referring to the following descriptions and drawings, wherein like numerals are used for like and corresponding parts of the drawings.  
         [0047]    According to the present invention and referring now to FIG. 1, joint prosthesis  10  is shown for use in arthroplasty. Arthroplasty is a well known procedure for the treatment of osteoarthritis. For a further explanation of arthroplasty may be found in Charnley, Sir John.  Low Friction Arthroplasty of the Hip . New York: Springer, Verlock, Berlin, and Heidelberg, 1979 incorporated herein by reference in its entirety.  
         [0048]    The joint prosthesis  10  is positioned in a long bone  12 . While the long bone  12  may be any long bone within the human anatomy, the present invention is particularly well suited for long bones which have a arcuate shape particularly adjacent the resected portion of the bone. For example, the long bone  12  may be in the form of a humerus or, as shown in FIG. 1, a femur.  
         [0049]    The femur  12  is resected along resection line  14  relieving the epiphysis  16  from the femur  12 . The epiphysis is shown as dashed line  11 .  
         [0050]    The prosthesis  10  is implanted in the femur  12  by positioning the prosthesis  10  in a cavity  20  formed by reaming a portion of cancellous bone  22  within medullary canal  24  of the femur  12 .  
         [0051]    The cavity  20  may be formed in the cancellous bone  22  of the medullary canal  24  by either broaching or reaming or other similar techniques to remove the cancellous bone  22  from the canal  24 . As shown in FIG. 1, the cavity  20  extends from metaphysis  26  into diaphysis  30  of the femur  12 .  
         [0052]    Any suitable combination of drilling, reaming or broaching can be used to form a cavity which corresponds closely to the periphery of the prosthesis. Typically, a broach (not shown) is driven into the medullary canal to form the cavity. This broach has a shape generally only slightly smaller than the portion of the implant that fits into the canal  24  so that the prosthesis is press fitted into the cavity  20 .  
         [0053]    Preferably and as shown in FIG. 1, the prosthesis  10  includes a body or stem  32 , a portion of which is positioned within the cavity  20  of the femur  12 , and a cup  34  which is connected to natural acetabulum  36 . The stem  32  is pivotally connected to the cup  34 . The stem  32  may be in direct contact with the cup  34  or may, as shown in FIG. 1, include a liner or bearing  40  positioned between the cup  34  and the stem  32 .  
         [0054]    The cup  34  may be made of any suitable, durable material which is compatible with the human anatomy. For strength and durability typically the cup  34  is made of a metal such as stainless steel, a cobalt chrome alloy or titanium or may be made of a ceramic.  
         [0055]    The liner  40  may be made of any suitable, durable bearing material and is often made of polyethylene for example ultrahigh molecular weight polyethylene.  
         [0056]    While the stem  32  may be made of unitary construction typically the stem  32  includes a stem portion  42  and a head portion  44 . The two-part construction of the stem  32  provides for easier manufacture and for providing varying offsets for the prosthesis by utilizing a plurality of head portions  44  and/or a plurality of stem portions  42 .  
         [0057]    The stem portion  42  may be connected to the head portion  44  in any suitable fashion. For example, the stem portion  42  may include a male taper portion  46  which mates with a female taper portion  50  on the head portion  44 .  
         [0058]    As shown in FIG. 1, the stem portion  42  includes a proximal stem portion  52 , a distal stem portion  54  extending downwardly from the proximal stem portion, and a neck portion  56  extending upwardly from the proximal stem portion  52 . The proximal stem portion  52  and the distal stem portion  54  are located within the cavity  20  formed within the cancellous bone  22  of the medullary canal  24 .  
         [0059]    Hip prosthesis are secured to the medullary canal of the femur typically either by a press-fit with the medullary canal or with the use of a cement mantel which is positioned between the prosthesis and the cancellous bone. In utilizing a cement mantel the cavity is broached or reamed slightly larger than the stem and a quantity of cement (for example, PMMA—polymethylmethacrylate) is placed within the cavity and the stem inserted therein. A small uniform layer of, for example, 1-4 mm of cement is formed between the stem portion  42  and the femur  12 . While the present invention may have some value for use in prosthesis having stems which utilize a cement mantel, the present invention is generally directed toward a prosthesis having a stem which is press-fitted into the cancellous bone.  
         [0060]    As body load or weight is transferred through the torso from the acetabulum  36  to the femur  12  the load is transmitted along trabeculae or load lines  60 . These trabeculae or load lines  60  are positioned in a direction generally conforming to the length of the femur and are curved in a direction toward the head of the femur.  
         [0061]    In the diaphysis  30  or the more distal portion of the femur  12 , the load lines  60  are generally linear and run parallel to longitudinal axis  62  of the femur  12 . This is mainly due to the fact that the femur  12  within the diaphysis has a generally circular cross-section in a generally cylindrical shape.  
         [0062]    On the other hand, within the metaphysis  26  the trabeculae or load lines  60  have a curved or arcuate shape or path and digress continually from the longitudinal axis  62  in the proximal direction.  
         [0063]    According to Wolff&#39;s Law, hypertrophy is defined as a thickening of the cortex with retention of normal cortical texture. According to Wolff&#39;s Law, the hypertrophy will occur at the area of highest stress surrounding an implant. The thickening of the cortex caused by the hypertrophy is a very desirable event in the postoperative patient. For many implants within a femur the location of hypertrophy is often at the distal end of the implant. This is caused by the artificially raised stress at the point of sudden transition from the flexible distal femur to the artificially stiffened proximal femur. This is true for both press-fit and cemented stems. This phenomenon of hypertrophy thus results in excellent adhesion in the diaphysis but results in a less than desirable condition between the implant and the femur in the metaphysis.  
         [0064]    To provide for the increased loading of the femur within the metaphysis and the resulted improvements caused by hypertrophy and Wolff&#39;s Law, according to the present invention surface features  64  are located on outer periphery  66  of the proximal stem  52 . The surface features  64  serve to increase the stress or load between the implant and the femur in the metaphysis  26  to thereby gain the benefit of Wolff&#39;s Law and hypertrophy within that portion of the femur.  
         [0065]    Preferably, as shown in FIG. 1, the stem  32  has a shape generally conforming to the shape of the femur  12 . Thus, typically, within the diaphysis  30 , the distal stem  54  is generally circular, having a shape generally similar to the circular shape of the femur within the diaphysis  30 . Similarly, within the metaphysis  26 , the proximal stem  52  has a generally oval cross-section and an arcuate orientation in the direction toward the acetabulum  32 .  
         [0066]    Further the proximal stem  52  becomes larger in the direction of the acetabulum  36 . This curving, oval and enlarging toward the acetabulum configuration of the proximal stem provides a shape generally conforming to the cancellous bone within the metaphysis  26  of the femur  12 .  
         [0067]    According to the present invention and referring now to FIGS. 1, 4 and  5 , the applicants have found that the surface features  64  should be positioned in an orientation to optimally transfer load between the stem  32  and the femur  12 .  
         [0068]    Applicants have further found that the surface features  64  should be positioned in an orientation relative to the load lines or trabeculae  60 . The load lines or trabeculae  60  pass through the proximal cancellous bone  22 . The load lines  60  also pass through cortical bone or cortex  65 . The cortical bone  65  has layers or normal lamellae  71  through which the load lines pass and which are concurrent therewith.  
         [0069]    The orientation of the surface features  64  to the load lines  60  is defined by angle α. Applicants have further found that the surface features  64  should be optimally positioned in an orientation generally normal to the load lines or trabeculae  60  or that the angle α is optimally around about 90 degrees.  
         [0070]    While the benefit of positioning the steps in relationship to the load lines or trabeculae are optimized when the steps are positioned generally normally or perpendicular to the load lines. It should be appreciated that the invention may be practiced where the steps  64  are positioned less than an ideal 90 degrees or normal to the load lines. For example, the steps may be positioned from about 70 degrees to about 110 degrees with respect to the trabeculae or load lines.  
         [0071]    While the steps are optimally positioned generally normally or perpendicular to the load lines  60 , it should be appreciated that every long bone in every person&#39;s anatomy has a different anatomical shape. For example, referring to FIG. 1, the long bone may have a shape other than that of long bone  12 . The long bone may have a shape as shown in long bone  13  or as shown in long bone  15 , both shown as dashed lines.  
         [0072]    While it might be ideal to make an individual, customized prosthesis with surface features designed and manufactured optimally normal to the load lines, this is probably not economically feasible. Applicants have thus found that the invention may, thus, be commercially practiced by designing the surface features  64  to be selected to be optimally positioned generally normal to the load lines or to have at the surface features designed to be aligned around 70 to 110 degrees from the load limes for a average or normal femur or long bone. The outer periphery  66  of the proximal stem  52  is typically designed to be positioned within and to be spaced from and to conform generally to the inner periphery  67  of the cortical bone  65  of an average femur or long bone. The outer periphery  66  thus, preferably, generally conforms to inner periphery  67  of the cortical bone  65  of the long bone to which it was designed.  
         [0073]    Referring again to FIG. 1, since the load lines  60  pass through normal lamellae of the cortex  65  and are concurrent therewith, the inner periphery  67  of the cortex  65  is generally in alignment with the load lines  60 . As stated earlier, to optimized the positioning of the surface features  64 , the features  64  are positioned normal to the load lines and the inner periphery  67  of the cortex  65 .  
         [0074]    Thus, for an average long bone to which a prosthesis  10  is designed, the outer periphery  66  of the proximal stem  52  conforms generally to the load lines  60 . Applicants have thus found that in commercially utilizing this invention, the prostheses may be designed and manufactured with the surface features positioned with respect to the outer periphery  66  of the proximal stem  52  of the prosthesis  10 . Since the load exerted on the prosthesis is large around the proximal stem  52  at the center of the inner periphery of the medial portion of the proximal stem also known as medial periphery  69  of the outer periphery  66 , the Applicants have discovered that the surface features  64  may be positioned with respect to the medial periphery  69  of the outer periphery  66   
         [0075]    The surface features  64  form an angle β with medial periphery  69 . For example, the surface features may be positioned from about 70 degrees to about 110 degrees with respect to the medial periphery  69  of the proximal stem  52  of the prosthesis  10 . The surface features  64  may optimally be positioned in an orientation generally normal to the medial periphery  69  or the angle β may optimally be around about 90 degrees.  
         [0076]    Thus, as shown in FIG. 1, in the portion of the metaphysis  26  next to the diaphysis  30 , the surface features  64  run generally perpendicular to the load line  60  and also nearly perpendicular to the longitudinal axis  62 . Conversely in the portion of the metaphysis  26  further from the diaphysis  30 , the surface features  64  run generally perpendicular to the load line  60 , but far from being perpendicular to the longitudinal axis  62 .  
         [0077]    The surface features  64  are generally in the form of grooves, ribs or ridges extending inwardly or outwardly from the surface  66 . The surface feature  64  generally has a uniform cross-section as shown FIGS. 1A through 1C.  
         [0078]    Applicants have found that by positioning the surface feature  64  in an orientation generally perpendicular to the load line  60  the supporting ability of the surface features  64  may be optimized. By optimizing the load capacity of the surface feature  64 , the stress imparted from the stem  32  to the femur  12  may maximize the stress at that position. Further, because Wolff&#39;s Law encourages hypertrophy or the thickening of the cortex in the metaphysis  26  of the femur  12 , the adherence and bone growth around the implant within the metaphysis area  26  is thereby improved.  
         [0079]    The applicants have found that a large portion of the load transferred by the stem is concentrated in that portion of the stem adjacent the more curved portion of the femur  12 .  
         [0080]    For example, referring now to FIG. 2A, a typical cross section of the proximal stem  52  of the prosthesis  10  is shown. It should be appreciated that the proximal stem  32  may have any suitable cross section. Since the cross section of the proximal portion of the long bone  12  is typically oval or non-circular, a non-circular prosthesis cross section is preferred. The shape of FIG. 2A is pentagonal or five sided with a large semicircular portion on the medial side.  
         [0081]    The surfaces  70 ,  72  and  74  which approximate the curved portion of the femur  12  transfer a major portion of the load between the femur  12  within the metaphysis  26 . Applicants have found that if the surface features  64  are positioned generally normal or perpendicular to the load lines  60  on surfaces  70 ,  72  and  74  a large majority of the benefit of providing the surface features generally normal to the load lines may be accomplished. Thus the surface features  64  located on other surfaces, for example, surfaces  76 ,  80  and  82  may be oriented in directions other than normal to the load lines or surface features  64  may be omitted from the surfaces  76 ,  80  and  82 .  
         [0082]    Referring now to FIG. 1A, to optimize the load carrying or stress increasing capacity of the surface features  64 , the surface features as shown in FIG. 1A may be in the form of steps or terraces. Such steps or terraces are more fully shown in U.S. Pat. No. 4,790,852 to Noiles and incorporated herein by reference in its entirety. The terraces  64  have an inner edge  84  and an outer edge  86 . A ledge  90  is formed between outer edge  86  and inner edge  84 . The ledge is positioned distally and serves to provide optimum support or stress for the stem  32 . The terraces  64  has a vertical spacing -V- between terraces of approximately 0.50 to 3.0 mm and a depth -D- of approximately 0.2 mm to 1.5 mm.  
         [0083]    It should be appreciated that while the terraces  64  as shown in FIG. 1A are preferred, the invention may be practiced with other types of surface features. For example, as shown in FIG. 1B, the surface features may be in the form of ribs  164  which provide an angled support surface  190 .  
         [0084]    Alternatively referring to FIG. 1C, the surface features may be in the form of grooves  164 ′ which extend inwardly from the surface.  
         [0085]    To further promote bone growth between the stem and the femur and referring again to FIG. 1A, the surface  66  of the surface features  64  may be coated by a coating  92 . The coating  92  may be any coating which promotes bone growth and/or interconnections between the prosthesis and the femur. For example the coating  92  may be a bioceramic. Such suitable bio-ceramics include hydroxyapatite or tricalcium phosphates. Alternatively, the coating  92  may be a porous coating. Alternatively, the coating may be a porous coating and a bioceramic coating in combination.  
         [0086]    Various porous coatings have found to be very effective. One particularly effective coating is sold by the Assignee of the instant application under the tradename Porocoat. The Porocoat coating is more fully described in U.S. Pat. No. 3,855,638 to Pilliar and hereby incorporated herein by reference in its entirety.  
         [0087]    This porous coating consists of a plurality of small discreet particles of metallic material bonded together at their points of contact with each other to define a plurality of connected interstitial pores in the coating. The particles are of the same metallic material as the metallic material from which the substrate is formed. Examples of suitable material include austenitic stainless steel, titanium, titanium alloys and cobalt alloys.  
         [0088]    The stem  32  may be made of any suitable durable material and, for example, may be made of a titanium, a cobalt chrome molybdenum alloy or stainless steel. The applicants have found that titanium TI-6AL-4V is well suited for this application.  
         [0089]    It should be appreciated that while, as shown in FIG. 1, the proximal stem  52  has a taper design, the aligning of surface features with respect to the load lines of the present invention may be practiced with the taper design or with a non-taper design. Further it should be appreciated that while, as shown in FIG. 1, the prosthesis  10  is shown with a coating  92 , the invention may be practiced without the porous coating  92 .  
         [0090]    The terraces  64  are aligned in a direction generally normal to the medial curve or load line  64  on the anterior face  70 , the medial arcuate surface  74  and the posterior surface  72 . The terraces  64  become horizontal as they approach the lateral aspect of the implant, (surfaces  76 ,  80  and  82 ) (see FIG. 2A) to align roughly normal to the lateral face of the implant.  
         [0091]    Referring now to FIG. 2, the stem  32  is shown in an anterior/posterior view. The stem  32  is shown with the distal stem  54  not including the surface features or terraces  64 . The proximal stem  52  however includes the terraces  64  on posterior lateral surface  76  and on anterior lateral surface  80 . As shown in FIG. 2, the proximal stem  52  does not have terraces  64  in the lateral surface  82 .  
         [0092]    As shown in FIG. 2 the terraces  64  on the posterior lateral surface  76  and the anterior lateral surface  80  are generally perpendicular to longitudinal axis  62 . It should be appreciated that the terraces  64  on surfaces  76  and  80  may be positioned normal to the load lines  60 . However, since most of the benefit of the positioning of the surface features  64  normal to the load line  60  is accomplished on surfaces  70  and  72 , for simplicity of design and manufacture, the terraces  64 , as shown in FIG. 2, may be positioned normal to the longitudinal axis  62 . Further, for simplicity and ease of manufacture, the lateral surface  82 , as shown in FIG. 2, may be made without terraces  64 .  
         [0093]    Referring now to FIG. 3 the stem  32  is shown in a posterior/anterior position. The medial surface  74  is shown with terraces  64  on surface  66  in the proximal stem  52 . The terraces  64  are positioned normal to load lines  60 .  
         [0094]    As shown in FIG. 3 the distal stem  54  may include a polished tip  94  extending a distance of, for example, one-half to one inch from the distal end of the stem  32 . The distal stem  54  may, for example, be grit blasted in the remaining portion  96  of the distal stem  54 .  
         [0095]    Referring now to FIG. 6, an alternate embodiment of the present invention is shown as prosthesis  210 . Prosthesis  210  is similar to prosthesis  10  of FIG. 1 except that, whereas prosthesis  10  of FIG. 1 includes a separate stem and head which are connectable together, the prosthesis  210  includes a head portion  244  which is integral with stem portion  242 . Prosthesis  210  includes stem  232  which is pivotally connected to cup  234  and includes a bearing or liner  240  placed therebetween.  
         [0096]    As with prosthesis  10 , prosthesis  210  includes steps  264  similar to steps  64  of prosthesis  10  which steps  264  are positioned generally normal or perpendicular to load lines or trabeculae  260 . As in the prosthesis  210  the steps  264  are positioned on the proximal stem  252  of the stem  232 . The steps  264  are preferably similar to the steps  64  of the prosthesis  10  of FIG. 1.  
         [0097]    Referring now to FIG. 7 an alternate embodiment of the present invention is shown as shoulder prosthesis  310 . The shoulder prosthesis  310  includes a stem  332  which is implanted into a humerus (not shown). The prosthesis  310  also includes a head portion  344  attached to the stem  322 . The head portion  344  may be secured to the stem  322  in any suitable manor and may alternatively be integral therewith. The head portion may have a external taper  346  extending therefrom which mates with an internal taper  350  in the stem  332 .  
         [0098]    Such a configuration is shown in U.S. Pat. No. 5,314,479 to Rockwood et al. incorporated by reference herein in its entirety. The stem portion  342  of the stem  332  includes a proximal stem  352  and a distal stem  354 . For the same reasons expressed with regard to the prosthesis  10  of FIG. 1, the prosthesis  310  includes steps  364  similar to the steps  64  of the FIG. 1 prosthesis. The steps  364  are aligned generally perpendicular or normal to the trabeculae or load lines  360 . For the same reasons expressed with regard to the FIG. 1 prosthesis  10 , the steps  364  are preferably positioned on the proximal stem  352 .  
         [0099]    Referring now to FIG. 7A, a alternate securing arrangement is shown for connecting the head portion to the stem. In this arrangement the stem  332 ′ may have a external taper  346 ′ extending therefrom which mates with an internal taper  350 ′ in the head portion  344 ′. Such a configuration is shown in U.S. Pat. No. 6,120,542 to Camino et al. incorporated by reference herein in its entirety.  
         [0100]    Another embodiment of the present invention is shown in FIGS. 8 through 10 as stem portion  432 . Stem portion  432  is similar to stem portion  32  of the FIG. 1 prosthesis except that the proximal stem  452  of the stem portion  432  includes steps  464  similar to the step  64  of the prosthesis  10  which steps  464  are positioned completely around the periphery of the proximal stem  452 .  
         [0101]    Referring now to FIG. 8, the stem portion  432  includes the distal stem  454 , the proximal stem  452  and neck portion  456 . The steps  464  are positioned completely around the periphery of the proximal stem  452 . In fact the steps  464  are positioned on the anterior face  472 , the anterior lateral face  480  and the posterior face  470 .  
         [0102]    Referring now to FIG. 9 the steps  464  are positioned on the posterior lateral face  476 , on the lateral face  482  and on the anterior lateral face  480 .  
         [0103]    Referring now to FIG. 10 the steps  464  are also positioned on the medial face  474  of the proximal stem  452 .  
         [0104]    Referring now to FIGS. 11, 12 and  13  a further embodiment of the present invention is shown as a stem portion  532 . Stem portion  532  is similar to stem portion  32  of the FIG. 1 prosthesis except that steps  564 , which are similar to steps  64  of the FIG. 1 prosthesis, are positioned only on the anterior, posterior and medial faces.  
         [0105]    Referring now to FIG. 11, the stem portion  532  includes a distal stem  554 , a proximal stem  552  and a neck portion  556 . The steps  562 , similar to the steps  64  of the FIG. 1 prosthesis  10 , are positioned only on the proximal stem of  552 . The Applicants have found since the loading on the stem portion  532  is primarily on the anterior, posterior and medial faces, the invention may be practiced with steps  562  positioned only on these faces. In fact, the invention may be practiced with the steps on perhaps less than these three faces.  
         [0106]    As shown in FIG. 11 the steps  562  are located on the medial face  574 , the posterior face  570  and the anterior face  572 . The anterior lateral face  580 , as shown in FIG. 11, does not include the steps  564 .  
         [0107]    Referring now to FIG. 12, no steps  562  are positioned on the posterior lateral face  576 , on the lateral face  582  and on the anterior lateral face  580 .  
         [0108]    Referring now to FIG. 13 the medial face  574  of the proximal stem of  552  includes these steps  564 .  
         [0109]    By providing a prosthesis which has a stem with steps which are aligned in a direction generally normal to the load lines or trabeculae of the prosthesis load carrying capacity of the proximal femur may be optimized. By optimizing the loading of the proximal femur, a manifestation of Wolff&#39;s Law can occur which causes the raised stresses at the greatest loading to create a thickening of the cortex and improvement of the bone growth and adherence of the prosthesis to the proximal femur.  
         [0110]    By providing a prosthesis having surface features in the form of steps which are positioned generally normal to the load lines of the prosthesis, the prosthesis may benefit from a long term stability and fixation by providing an environment optimum for femoral bone remodeling.  
         [0111]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.