Abstract:
Implants and methods are presented for surgically repairing a hip joint with a proximal femoral prosthesis that comprises femoral head component and a femoral stem component. The femoral stem component comprises a neck portion, a flange portion, a transitional body region and an elongated stem. The femur is prepared for implantation of the femoral hip prosthesis by resecting the proximal femur and reaming a symmetric intramedullary cavity in the femur. The femoral hip prosthesis is then inserted the on the resected femur and in the intramedullary cavity. The femoral hip prosthesis elastically deforms when loaded during use to apply dynamic compressive loads and displacement to the calcar region of the resected proximal femur.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of the following, which is herein incorporated by reference: 
     U.S. patent application Ser. No. 12/429,882 filed Apr. 29, 2009, and is entitled FEMORAL HIP PROSTHESIS AND METHOD OF IMPLANTATION, which is a continuation of the following: 
     U.S. patent application Ser. No. 10/763,314 filed Jan. 22, 2004, now U.S. Pat. No. 7,534,271, and is entitled FEMORAL HIP PROSTHESIS AND METHOD OF IMPLANTATION. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to a femoral hip prosthesis for replacing a portion of proximal femoral bone during hip replacement and the methods of assembly and use thereof. 
     2. The Relevant Technology 
     Total hip arthroplasty using a metallic hip prosthesis has been successfully performed since the early 1960&#39;s and is now a routine procedure to address orthopedic diseases such as osteoarthritis, fracture, dislocations, rheumatic arthritis, and aseptic or avascular bone necrosis. During this procedure, the bone is prepared for the prosthesis by removing the damaged articulating end of the bone by resecting a portion of the bone including the femoral head. This exposes the inside, of the metaphaseal region of the intramedullary canal in the proximal femur. The surgeon then drills or reams a cavity in the femur approximately in line with the intramedullary canal. This cavity is used to align other tools such as reamers, broaches and other bone tissue removal instruments to create a roughly funnel shaped bone cavity that is smaller in cross-section as it extends down from the bone resection at the proximal end of the femur into the distal intramedullary canal. This funnel shaped cavity is typically also eccentric with more bone material removed from the medial calcar region of the proximal femur than the region on the lateral side of the canal. 
     Oftentimes a grouting agent commonly referred to as bone cement is then added to the funnel shaped cavity. Once the prosthesis is inserted into the cavity, this creates a bone cement mantle between the prosthesis and the bone. Sometimes the shape of the cavity is prepared to closely match the shape of the external surface of the prosthesis, and the prosthesis is press fit into the cavity without the use of bone cement. These press-fit prostheses typically have a textured bone-ingrowth surfaces place strategically at specific locations on their surface to help facilitate lone-term bone tissue growth into the prosthesis. This bone ingrowth into the porous structure on the implant creates a long lasting secure bond between the prosthesis and the proximal femur. 
     Once the bone cavity is prepared, the prosthesis is placed into the bone cavity and is supported directly by internal bone tissue in the case of a press fit implant or indirectly by the bone cement mantle in the case of the cemented implant. Then, the prosthesis is aligned such that the articulating end of the implant articulates with the opposite side of the natural joint in the case of a hemiarthoplasty, or articulates with a corresponding implant replacing the opposite side of the joint in the case of a total joint arthroplasty. 
     Current designs of proximal femur hip prosthesis have eccentric, non-symmetric cone shaped central body portions. The current methods of implant fixation allow for transfer of axial loads to the proximal femur mainly through shear stresses at the eccentric funnel shaped bone-prosthesis interface. The effective transfer of load is significantly dependent on the three-dimensional shape of funnel shaped cavity, the bone-prosthesis or bone-cement-prosthesis interface as well as physiological loading of the proximal end. Partly because of the eccentrically shaped cross-section of the central body portion, these currently available prostheses transmit radial expansion forces on the proximal femoral cavity as the implant is loaded in compression. The funnel shape of the cavity and the matching shape of the implant or bone cement result in circumferential hoop stresses and radial expansion stresses are distributed to the bone as the femoral component is axially loaded. This results in complex axial and shear stresses at the bone-implant interface. Consequently, the distribution of the loads that transmit from the femoral head axially through the proximal femur is altered after THA. 
     A potential cause of failure of currently used prosthesis is associated with the possible resorption of the bone surrounding the implant. The bone resorption can be the result of an altered distribution of shear stresses on the remaining proximal femoral tissue. In time, the lack of adequate stress transfer from the metal stem to the surrounding bone may cause a loss of bone density, resulting in the increased possibility of bone failure or loosening of the bone-prosthesis interface. The gradual loss of bone support in the calcar region of the eccentric cavity increases the bending load that must be borne by the prosthesis. This increase in bending load on the prosthesis can lead to stress shielding by the prosthesis resulting in prosthesis fatigue and potentially to eventual clinical failure. 
     SUMMARY 
     The present invention is directed to a femoral hip prosthesis that satisfies the need for anatomically distributing the dynamic compressive loads on the hip joint to the proximal femoral bone. The femoral hip prosthesis is adapted for implantation against a resected surface on a proximal end of a femur, and also in an intramedullary cavity of the femur. The femoral hip prosthesis comprises femoral head component and a femoral stem component. The femoral stem component comprises a neck portion, a flange portion, a transitional body portion, and an elongated stem portion. The neck portion comprises a proximal male friction fit portion and a distal neck body. The flange portion is distal and adjacent to the neck portion and is attached to the distal neck body. The flange portion comprises an upper portion and a bottom surface. The transitional body region is adjacent to the bottom surface of the flange portion and also extends from the distal neck body. The elongated stem portion extends distally from the transitional body region and is aligned with a longitudinal axis. The longitudinal axis is oriented at an acute angle relative to the bottom surface of the flange portion. The elongated stem portion comprises a uniform envelope that may contain rotation-restricting splines, a tapered portion or a transverse slot. The femoral hip prosthesis may also alternatively contain a rotation-restricting boss that is attached to the bottom of the flange portion. The femoral hip prosthesis also comprises a distal end tip portion on the distal end of the elongated stem portion. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the present invention will now be discussed with reference to the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. 
         FIG. 1  is a perspective view of the show from the anteriomedial direction showing the femoral hip prosthesis including the before it is inserted into the resected proximal femur; 
         FIG. 2  is a perspective view shown form the anterioromedial direction showing the femoral hip prostheses before it is inserted and a cross-sectional view of the resected proximal femur with the intramedullary cavity and the boss cavity prepared; 
         FIG. 3  is a perspective view shown from the posteriorolateral position showing the before it is inserted into the resected proximal femur; 
         FIG. 4  is an anterior side view of the femoral hip prosthesis shown outside of the femur; 
         FIG. 4   a  is an embodiment of a substantially circular cross-section of the elongated stem portion; 
         FIG. 4   b  is an embodiment of a substantially square cross-section of the elongated stem portion; 
         FIG. 4   c  is an embodiment of a substantially triangular cross-section of the elongated stem portion; 
         FIG. 4   d  is an embodiment of a substantially hexagonal cross-section of the elongated stem portion; 
         FIG. 4   e  is an embodiment of a substantially star shaped cross-section of the elongated stem portion; 
         FIG. 5  is an isometric view of the medial side of the femoral stem component as it appears outside of the femur; 
         FIG. 6  is a cross-sectional view of the proximal femur from the anterior side showing the positioning of the femoral stem component inside of the proximal femur; 
         FIG. 7  is a medial view of the femoral hip prosthesis inside of the proximal femur. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Depicted in  FIGS. 1 through 7  are different embodiments of an implantable proximal femoral hip prosthesis  50  and methods of implantation therein. These embodiments of a hip joint replacement procedure and the design of the femoral hip prosthesis  50  that is meant to restore the biomechanical function of the hip joint while maintaining a secure interface with the proximal femur  10  and help to preserve anatomical loading of the remaining bone that surrounds the femoral hip prosthesis  50  once it is implanted. This allows the loads on the hip joint to be distributed optimally to the proximal femur  10 . 
     The femoral hip prosthesis  50  comprises a femoral head component  700  and a femoral stem component  100 . The femoral stem component  100  comprises a neck portion  150 , a flange portion  200 , a transitional body portion  300 , an elongated stem portion  400 , and a distal tip end  500 . The non-eccentric symmetrical shape of the interface between the elongated stem portion  400  of the femoral stem component  100  and a cavity  25  along with the contact at the interface between a proximal resection  20  and the femoral stem component  100  helps to stabilize the femoral hip prosthesis  50  and transfer more anatomic loads from the prosthesis  50  to the bone efficiently. 
     To prepare the patient for a proximal femoral hip prosthesis  50 , the surgeon first makes an incision or incisions near the hip joint, then the surgeon cuts though some of the tissue near the articulating joint, and retracts these tissues apart to visualize and access the diseased bone structures that are to be replaced by the hip joint replacement prostheses.  FIG. 1 , is a simplified perspective view from the anteriomedial direction showing the proximal femur  10  and the femoral hip prosthesis  50 , showing the femoral head component  700 , the femoral stem component  100 , and the proximal femur  10 . For clarification, all of the other tissues necessary to provide function to the hip joint are not shown in this simplified view. The surgeon aligns bone tissue removal tools, such as drills, reamers, or broaches (not shown) with alignment instrumentation (not shown) to form a substantially non-eccentric, symmetric intramedullary cavity  25  in the cancellous bone  3  of the proximal femur  10  that is in longitudinal alignment with the shaft  45  of the proximal femur  10 . The intramedullary cavity  25  is formed with a cross-sectional shape, such as a diameter  46  as shown in an embodiment of the intramedullary cavity  25  shown in  FIG. 2 . The diameter  46  is the measurement of the diameter of the maximum circular periphery that encompasses an envelope that the cross-sectional shape of the intramedullary cavity  25  comprises. The shape of the intramedullary cavity  25  can also be substantially non-eccentric, symmetric, non-circular shapes such as a square (not shown), star shape, (not shown), hexagon (not shown), or other parallelogram shape. Matching non-circular shapes of both the femoral stem component  100  and the intramedullary cavity  25  are potentially more efficient at restricting torsional movement between the femoral stem component  100  and the proximal femur  10  than circular cross-sectional shapes. The intramedullary cavity  25  also is formed to a length  47  as also shown in  FIG. 2 . The cross-sectional shape, diameter  46  and length  47  of the intramedullary cavity  25  in the proximal femur  10  is dependent on the morphology, structure, and pathology of the patient anatomy and the anticipated biomechanical in vivo loads resulting from the use of the femoral stem component  100 . 
     The intramedullary cavity  25  may have a multiple diameters, or in the case of non-circular cross-sectionally shaped cavities multiple sizes, to approximately match the shape of the femoral stem component  100 .  FIG. 2 , which illustrates a cross-sectional view of the proximal femur  10 , shows that both a first diameter  46  and a second diameter  48  can form an intramedullary cavity  25  is shown in  FIG. 2 . The second diameter  48  in the embodiment of  FIG. 2  is smaller than the first diameter  46 . Additionally, a third, forth, fifth, or more diameters (all not shown) can form the intramedullary cavity  25 . For the purposes of clarity of illustration, the cross-section of the intramedullary cavity on  FIG. 1  and  FIG. 2  are circular, resulting in a substantially cylindrical opening. Correspondingly different size or shaped tissue removal tools (not shown) are used to prepare an intramedullary cavity  25  with more than one size diameter  46 . The surgeon may also find it advantageous to form different or alternating shapes of cross-sections in the symmetric, non-eccentric intramedullary cavity  25 . For example, the cavity may be first diameter circular, and then square, then a second diameter circle, then a star shape, then cone shaped, then finally spherically shaped at its deepest, most distal end. 
     After the basic intramedullary cavity  25  is formed, instrumentation (not shown) is used to align cutting guides for bone cutting instruments (not shown) to form a proximal resection  20  on the proximal femur  10 . The proximal resection  20  may have different surfaces such as a calcar resection surface  12  that is formed when the femoral calcar  11  is transverely cut through the proximal femur  10 . The calcar resection surface  12  is cut at an acute angle  22  with respect to the longitudinal axis  21  of the proximal femur  10 . This acute angle is typically between 10° and 80°. Although the proximal resection  20  may be simply one continuous transverse cut that passes from the medial to the lateral side of the proximal femur in the direction and plane defined by a the plane outlined by the dashed line  16  shown in  FIG. 1 . This alternative resection  17  is formed by extending the calcar resection  12  from medial to lateral though the entire proximal femur  10 . 
     More bone conserving cuts may also be formed in to the proximal femur  10  as shown in  FIG. 1 . These cuts may include a formed concentric region  15  that is larger in size but concentric to or aligned with the intramedullary cavity  25 . These cuts may also include a transverse resection  13  that is cut relatively perpendicular to the intramedullary cavity  25 . To simplify the surgical procedure, the resections shown in  FIG. 1  can all be formed by a single reamer (not shown). This reamer has a cutting surface formed in the shape of the combined profile of all of the resection cuts. It can be rotated or oscillated about the longitudinal axis  21  of the intramedullary canal  25 , until the desire bone tissue is removed. As shown in  FIG. 2 , the various cuts that together form the proximal resection  20  pass through portions of both the relatively dense cortical bone  44  and the more porous cancellous bone  3 . Thus, the cutting surfaces of the tissue removal tools are designed to cut both dense cortical bone  44  and less dense cancellous bone  3 . 
     After the intramedullary cavity  25  and the proximal resection  20 , including the calcar resection  12  and when applicable other bone tissue removal cuts are formed, the femoral stem component  100  can be inserted to mate with the exposed bone surfaces. The femoral stem component  100  comprises a proximal male friction fit portion  150 , a distal neck body  160 , a flange portion  200 , a transitional body portion  300 , an elongated stem portion  400 , and a distal end tip portion  500 . These portions will be discussed in detail below. 
     The femoral stem component  100  has a proximal male friction fit portion  150  on its most proximal end that is shaped to accept partially hemispherical femoral head component  700 . One shape of the proximal male friction fit portion  150  is a cylindrical taper shape with the smaller diameter on the male friction fit portion proximal section  151 , a tapered male friction fit portion  152  distal to the male friction fit portion proximal section  151 , and a larger diameter male friction fit portion taper maximum cross-section bottom end  153  on the distal end of the male friction fit portion  152 . The proximal male friction fit portion  150  could also be a straight cylindrical shape without a taper, or a series of successively larger diameter cylindrical shapes. 
     A femoral head component  700  has a male cavity  720  that is dimensioned to fit over and mate with the friction fit portion  152  of the proximal male friction fit portion  150  when the femoral head component  700  is assembled on the proximal male friction fit portion  150 . The femoral head prosthesis  700  has an external bearing surface portion  710  on its external surface that is substantially on its proximal side when implanted. The external bearing surface portion  710  of the femoral head prosthesis  700  is substantially hemispherical shaped on a portion of its load bearing external bearing surface. This hemispherical shape is designed to mate with either an artificial prosthetic acetabular cup surface (not shown) as is the case for a total hip arthroplasty or a natural acetabular surface as is the case for a hip femoral hemiplasty. 
     The proximal male friction fit portion  150  has a male friction fit portion neck  154  that is distal to the male friction fit portion portion  152  and adjacent to the male friction fit portion taper bottom end  153 . This male friction fit portion neck  154  functions as an undercut relief for the femoral head component  700  when assembled. Because the male friction fit portion neck  154  is smaller in diameter than the male friction fit portion portion  152 , the femoral head component  700  can be pressed onto the proximal male friction fit portion  150  with the only direct contact between the two on the friction fit portion  152  of the femoral stem component  100  and the male friction fit portion  720  of the femoral head component  700 . 
     The male friction fit portion neck  154  is proximal to and attached directly to a more bulky distal neck body  160 . The distal neck body  160  is shaped to distribute the loads transmitted through the proximal male friction fit portion  150  from the femoral head component  700  through a flange portion  200  and a transitional body portion  300 . The shape of the distal neck body  160  transitions from a simple symmetric shape similar to the cross-section of the male friction fit portion neck  154  to a more complex asymmetric shape that is similar to the combined shape of the flange portion  200  and the transitional body portion  300 . In the embodiment shown in  FIG. 1  through  FIG. 7 , the cross-sectional shape of the distal neck body  160  at the proximal section is round because the male friction fit portion is a conical tapered and the male friction fit portion neck  154  is a cylindrical hourglass shape. However, the shape of the distal neck body  160  at the proximal section can be other shape to correspond with the shape of the proximal male friction fit portion  150 . 
     The flange portion  200  has an upper portion  210  on its proximal side that contacts at least a part of the distal neck body  160 . In the embodiments shown in  FIGS. 1 ,  2 ,  3 ,  6  and  7  the flange portion is angled to match the at the same angle as the calcar resection  12  made by the surgeon on the proximal femur  10 . The angle and the size of the flange portion  200  are dependent on the anatomy of the patient and the morphology of the calcar resection  12 . The flange portion  200  has an anterior-posterior flange portion width  240  that is wide enough to cover at least a portion of the cortical bone tissue  44  that has been resected. The cortical bone tissue  44  is more rigid than the cancellous bone tissue  3 . In a healthy hip joint, the compressive loads are transmitted through both the cancellous bone tissue and the cortical bone tissue  44  of the proximal femur  10 . Because the cortical bone tissue  44  is more dense and rigid, and can sustain a higher load per square unit area without fracture than the cancellous bone tissue  3 , cortical bone tissue  44  is a more efficient distributor of compressive loads than cancellous bone tissue  3 . Thus, the flanged portion  200  is shaped to cover both the resected cancellous bone  3  and the resected cortical bone  44  so that the compressive loads transmitted through the flange portion  200  are distributed as anatomically close as possible to how they were distributed when the proximal femur  10  was healthy and intact. 
     The flanged portion  200  is less thick than it is wide. As shown in the embodiment of  FIG. 4 , the flange portion thickness  221  is between 0.5 millimeters and 12 millimeters. The flange portion  200  is substantially thick enough to transmit loads from the hip joint to the calcar resection surface  12  of both the cancellous bone tissue  3  and the cortical bone tissue  44 . The flange portion  200  is also thin enough to limit the about of bone that must be resected to form the calcar resection  12 . 
     The transitional body region  300  is the portion of the femoral stem component  100  that transitions from the distal neck body  160  and the flange portion  200  to the distal elongated stem portion  400 . The transitional body region  300  is adjacent to both the distal neck body  160  and the flange portion  200  on its proximal side and adjacent to the elongated stem portion  400  on its distal side. The transitional body portion  300  has a maximum height  310  that is the linear distance measured between a plane tangent to the bottom surface  220  of the flange portion  200  and a plane through the most distal part of the transitional body portion  300 . In  FIG. 4 , these two planes are shown as lines since this is a side view. In the embodiment of the transitional body portion shown in  FIG. 4 , the transitional body portion  300  has a curved fillet  330  its medial side. Although this is shown as a round fillet in  FIG. 4 , the medial side of the transitional body portion  300  can be a chamfered fillet, a stepped fillet, or any other non-linear or linear shape that transitions from the shape of the elongated stem portion  400  to the shape of portions of the distal neck body  160  or the flange portion  200 . The maximum height  310  of the majority of the transitional body region  300 , when measured normal from the bottom surface  220  of the flange portion  200  to any part of the elongated stem portion  400  is less than thirteen millimeters or less. Both the physical structure of the femoral hip prosthesis  50 , and the mechanical properties of the material from which the prosthesis is fabricated, function together to determine the functional strength and elasticity of the femoral stem component  100 . 
     Conventional orthopedic alloys such as cobalt chrome, titanium and stainless steel alloys and orthopedic composite materials have proven to provide reasonable strength and rigidity to orthopedic implants and may also be used to fabricate the femoral stem component  100 . However, when conventional orthopedic alloys or composites are fabricated into the eccentric conical shape of a typical femoral stem component  100 , the resulting implant is more rigid than the proximal femoral  10  that the femoral stem component  100  is replacing. Flexibility of the stem component  100  is necessary to allow the flex and compliance desired to dynamically anatomically load the proximal femur  10  bone during biomechanical loading. The relatively small shape of the transitional body portion  300  allows for more flexion of the flange portion  200  when the proximal male friction fit portion  150  is loaded than is seen with the bulkier conventional eccentric cone shaped femoral prosthesis. The unique shape of the femoral stem component  100  allows for flexibility of the prosthesis even when fabricated from rigid orthopedic alloys such as such as cobalt chrome, titanium and stainless steel alloys. 
     This dynamic flexibility within the transitional portion  300  is desired since it allows the flange portion  200  of the femoral stem component  100  to transmit loads and displacements to the femoral calcar region  11  of the proximal femur  10 . When bone is loaded and allowed to deform, a piezoelectric effect within the tissue simulate the bone cells into further production. This phenomenon, sometimes called Wolfs Law, coupled with other physiologic and biochemical principles, helps to keep the bone surrounding the femoral hip prosthesis  50  healthy and vibrant. The femoral stem component  100  is designed to optimize the effects that a flexible, yet strong femoral hip prosthesis  50  will have on the surrounding loaded bone tissue. As the hip joint is loaded during clinical use, loads are transmitted through the male friction fit portion  154  and distal neck body  160  to the flange portion  200  and the transitional body portion  300  to the stem. Since the transitional body portion  300  is relatively flexible and not as bulky and rigid as a conventional femoral hip prosthesis, the transitional body portion  300  allows the femoral stem component  100  to flex and transmit the compressive load to the bone in the calcar region  11  of the proximal femur  10 . These loads on the bone may allow the dynamization necessary to keep the tissue surrounding the femoral stem component  100  healthy and help prevent bone resorption in the calcar region  11  of the proximal femur  10 . 
     Distal and adjacent to the transitional body portion  300  is the elongated stem portion  400 . The elongated stem portion  400  comprises some or all of the following portions and features; a tapered portion  450 , a splined section  420 , and transverse slot  480 . The elongated stem portion is encompassed within a cylindrically shaped envelope referred to as uniform envelope  410 . The cross-sectional shape and the area of the uniform envelope  410  remains substantially uniform throughout the longitudinal length of the elongated body. The uniform envelope  410  has a circular uniform cross-sectional periphery  902  that is defined by the maximum cross-sectional peripheral diameter  905  of the elongated stem portion  400 . The uniform envelope  410  is the same length as the elongated stem portion. The elongated stem portion is adjacent to the transitional body portion  300  on its proximal end and adjacent to a distal tip portion  500  on its distal end. 
     As shown in  FIG. 4 , the elongated stem portion  400  is longitudinally aligned with a longitudinal axis  425 . When the femoral stem component  100  is implanted in the proximal femur  10 , the longitudinal axis  425  is approximately in alignment with the longitudinal axis  21  of the intramedullary cavity  25 . All the possible features or portions of the elongated stem portion  400 , including the tapered portion  450 , the splined section  420 , and the transverse slot  480  have cross-sections perpendicular to the longitudinal axis  425  and are contained within a maximum diameter  905  of a cross-sectional periphery  902  that defines the cross-section of the uniform envelope  410 . Representative shapes of cross-sectional areas viewed from a cross-sectional view cut plane  900  are shown in  FIG. 4   a  through  FIG. 4   e . Included in these figures are the cross-sectional periphery  902  and the maximum diameter  905  of the cross-sectional periphery  902 . 
     Material may be removed from the elongated stem portion  400  to created features such as taper portions  450 , splines  460  or the transverse slots  480 . However, the basic substantial shape of the external periphery of the cross-section of the elongated stem portion  400  remains uniform and circular. Thus, the elongated stem portion and the uniform envelope  410  are both substantially symmetric and non-eccentric. The embodiment of the elongated stem portion  400  shown in  FIG. 4  is substantially cylindrical in shape  910 . The cross-section of this cylindrically shaped elongated stem portion is shown in  FIG. 4   a . However, for other embodiments of the femoral stem component  100 , the cross-sectional shape of the elongated stem portion  400  can be also non-circular shapes such as substantially square shape  920 , as shown in  FIG. 4   b ; a substantially triangle shape  930 , as shown in  FIG. 4   c ; a substantially hexagonal shape  940 , as shown in  FIG. 4   c , a substantially star shape  950  as shown in  FIG. 4   e , or any other substantially non-eccentric, symmetric shape such as a tube (not shown) that can functionally form the cross-section of the elongated stem portion  400 . 
     In the embodiments shown, the longitudinal axis  425  of the elongated stem portion  400  is a substantially straight axis throughout the length of the elongated stem portion  400 . However, to better match the anatomy of the proximal femur  10 , the longitudinal axis  425  can also be curved. The curve may be in the anterior-posterior plane, the medial-lateral plane or a compound curve that is seen in both the anterior-posterior plane and the medial-lateral plane. A flexible reamer (not shown) could be used to form the curved intramedullary cavity before the prosthesis  10  with a curved longitudinal axis  425  is implanted. 
     The elongated stem portion  400  may include a tapered portion  450  along its length. This is shown in  FIG. 2 . This tapered portion  450  may also include splines  460  or transverse slots  480  cut into it. The cross-sectional area of the tapered portion  450  in the embodiments shown decreases linearly along the longitudinal length of the tapered portion  450  as the tapered portion  450  transitions down the length of the elongated stem portion  400  from proximal to distal. The direction of the tapered portion  450  may also be in the opposite direction. The tapered area in the elongated portion  400  allows for greater flexibility in bending along the tapered portion  450  due to the reduced cross-sectional area and reduced cross-sectional bending moment of inertia. The tapered portion  450  also allows for an interference tapered wedge fit between the elongated stem portion  400  and the intramedullary cavity  25  in the proximal femur  10  when the cross-sectional size of the intramedullary canal  25  is less than the maximum diameter  905  of the periphery  902 . 
     Features such the splines  460  are cut into the elongated stem portion  400  for various structural and functional reasons such as to provide additional torsional resistance to the femoral stem component  100 . In the embodiments shown, the splines  460  are evenly spaced around the periphery  902  of the distal elongated stem portion  400 . The splines  460  are cut longitudinal around the periphery  902  of the elongated stem portion  400 . This allows the splines  460  to resist axial rotation between the femoral stem component  100  and the intramedullary cavity  25 . The splines  460  may also provide additional structural flexibility to the distal end of the femoral stem component  100 . 
     At the distal end of the femoral stem component  100 , an optional longitudinal transverse slot  480  may be cut transversely into the elongated stem portion  400  to provide additional flexibility and potentially additional torsional resistance to the femoral stem component  100 . The embodiment of the slot  480  that is shown is substantially uniform in cross-sectional and in shape though its length. The cross-sectional shape of the slot  480  may also be non-uniform. The cross-sections shape of the slot  480  may also change. For example the sides of the slot  481  may change from parallel planar surfaces to non-parallel or non-planar surfaces as the slot transitions from distal to proximal. The slot  480  also has a fillet  485  that takes the form of a rounded radius shape at its most proximal end. The shape of this fillet  485  may be other shapes that allow a relatively smooth transition from the slot  480  to the non-slotted cross-section. For example the slot  480  may be keyhole shaped. 
     Adjacent and distal to the elongated stem portion is the distal end tip portion  500 . The distal end tip portion  500  has a lead-in section  510  that reduces in cross-sectional area from proximal to distal. The lead-in section may be tapered as in the embodiment of  FIG. 3 , or spherical as in the embodiment of  FIG. 4 , or any shape that is successively smaller in cross-sectional area from proximal to distal. The distal end tip portion  500  helps to guide the femoral stem component  100  into the intramedullary canal  25 . The relatively smooth shape of the distal end tip portion  500  also functions to reduce the stress on the proximal femoral bone associated with discontinuity of terminating a rigid prosthesis in the intramedullary canal  25 . 
     The load distribution on the proximal femur  10  of an intact hip joint can be essentially resolved into an axial component, a bending moment in the medial and lateral direction, a bending moment in the anterior posterior direction, and a torsional moment with a rotational axis approximately in line with the longitudinal axis  21  of the proximal femur. The distribution of the magnitude and direction of these force components depend upon complex combinations of biomechanical factors such as leg stance, patient weight distribution, and patient gait. The femoral stem component  100  is designed to translate these forces to anatomic loads on the proximal femur  10 . As described above, the flange portion  200  helps to translate the compressive loads to the cancellous bone  3  and cortical bone  4  in the calcar region  11 . The elongated stem portion  400  helps to transmit the bending and torsional moments to the intramedullary canal  25 . In addition, a rotation-restricting boss  600  helps to transmit some of the torsional moments to the bone in the calcar region  11  of the proximal femur  10 . As shown in  FIG. 6 , the boss periphery  630  of the rotation-restricting boss  600  interfaces with the bone surrounding the boss cavity  14 . This structural interference between the femoral stem component  100  and the proximal femur  10  helps to restrict rotation, caused by the above-described resultant torsional moment with approximately in line with the longitudinal axis  21  of the proximal femur  10 , between the femoral stem component  100  and the intramedullary cavity  25 . 
     The size and location of the rotation-restricting boss  600  are factors that affect the amount that the rotation-restricting boss  600  restricts rotational movement of the femoral stem component  100 . Due to greater resistance from a rotation-restricting boss with a larger resultant moment arm, the further that the rotation-restricting boss  600  is located from the longitudinal axis  425  of the elongated stem portion  400 , the more effective it is in transmitting rotational loads and restricting rotational movement of the femoral stem component  100  to the proximal femur  10 . Also, the larger the cross-sectional area of the rotation-restricting boss  600 , the more effective it is in distributing torsion and restricting rotational movement of the femoral stem component  100 . 
     The embodiments of the rotation-restricting boss  600  that are shown by example are circular in cross-section resulting in a cylindrical shaped boss. However, other cross-sectional shapes such as square, rectangular, triangular or diamond shapes may be more practical to machine or may be better at distributing torsional loads from the femoral hip prosthesis  50  to the proximal femur  10  than the shown cylindrically shaped rotation-restricting boss  600 . The optimized shape of the rotation-restricting boss  600  may be more fin shaped than cylindrical shaped or longer than it is wide. This shape is partially dependent on the mechanical characteristics of the bone tissue where the rotation-restricting boss  600  is inserted. 
     The rotation restricting boss  600  has an axis of protrusion  620  with origin  621  substantially on a plane tangent or coincident with the bottom surface  220  of the flange portion  200 . The boss axis of protrusion origin  621  and the stem portion longitudinal axis  425  are spaced apart by a length this is more than the maximum length  905  of the cross-section of the periphery  902  of the uniform envelope  410  of the elongated stem portion  400 . 
     The rotation-restricting boss  600  in the embodiment of the prosthesis  10  shown in  FIG. 3  has an axis of protrusion that is substantially normal to the bottom surface  220  of the flange portion  200 . The rotation-restricting boss in the embodiment of the prosthesis  10  shown in  FIG. 4 ,  FIG. 5  and  FIG. 6  has an axis of protrusion  620  that is substantially parallel with the longitudinal axis  425  of the elongated stem portion  400 . As shown in cross-section of the proximal femur illustrated in  FIG. 6 , the corresponding boss cavity  14  is in line with the axis of protrusion  620 . 
     As shown in  FIG. 6  and  FIG. 7 , after the femoral stem component  100  is implanted in the proximal femur  10 , the flange portion  200  is pressed against a resected surface  20  on the proximal femur  10 , and the elongated stem portion  400  is pressed in the intramedullary cavity  25  aligned with the long axis of the proximal femur  10 . As the femoral head  700  is loaded by the hip joint, a substantial component of the axial compressive force is transmitted to the cancellous  3  and cortical  4  bone in the calar region  11  from the flange portion  200 . The principle torsional loads are transmitted to the splines  460 , slot  480  and rotational-restricting boss  600  and the principle bending loads are transmitted to the intramedullary canal  25  through the elongated stem portion  400 . Collectively, these feature and portions of the femoral hip prosthesis  10  contribute to distribute anatomical loads from hip joint to the remaining proximal femoral bone tissue. 
     While the present invention has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense, as numerous variations are possible. The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. No single feature, function, element or property of the disclosed embodiments is essential. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. The following claims define certain combinations and subcombinations that are regarded as novel and non-obvious. Other combinations and subcombinations of features, functions, elements and/or properties may be claimed through amendment of the present claims or presentation of new claims in this or related applications. Such claims, whether they are broader, narrower or equal in scope to the original claims, are also regarded as included within the subject matter of applicant&#39;s invention. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.