Patent Publication Number: US-2018049879-A1

Title: Bone capturing keel design for orthopedic implants

Description:
BACKGROUND 
     The disclosure relates to orthopedic implants, and more particularly, to orthopedic implants with keels. 
     For the most optimal surgical outcome, orthopedic implants must be accurately and reliably secured to the implantation site. The orthopedic implant will ideally include features to provide both immediate and long term securement and stabilization of the implant at the implant site, so that the patient can begin rehabilitation and return to semi-normal activities quickly and without delay, and so that the orthopedic implant will remain in place over the life of the implant. 
     SUMMARY 
     The present disclosure relates to a keel for an orthopedic implant, which keel extends outwardly from a bone contacting platform of the orthopedic implant, with the keel designed to be implanted into a patient&#39;s bone in order to help secure the implant to the bone and prevent movement of the implant relative to the bone. 
     In an exemplary embodiment, the keel has an outer wall that extends from the bone contacting platform and defines a cavity with a volume between the outer wall of the keel and the bone contacting platform wherein the cavity allows for bone ingrowth into the cavity of the keel. 
     In another exemplary embodiment, the keel has an outer wall that extends from the bone contacting platform, which keel has an open leading mouth and wherein a trailing end of the outer wall merges into the bone contacting platform, the outer wall thus defining a sloped, open mouthed cavity between the outer wall of the keel and the bone contacting platform, wherein the open mouth cavity allows for bone ingrowth into the tunnel of the keel. 
     In yet another exemplary embodiment, the keel has an outer wall that extends away from the bone contacting platform, which keel has an open leading mouth and an open trailing mouth which define a tunnel between the outer wall of the keel and the bone contacting platform wherein the tunnel allows for bone ingrowth into the tunnel of the keel. 
     In a further exemplary embodiment, the keel has a curved outer wall that extends away from the bone contacting platform, which keel has an open leading mouth and an open trailing mouth which define a tunnel between the outer wall of the keel and the bone contacting platform wherein the tunnel allows for bone ingrowth into the tunnel of the keel, wherein the open leading mouth may be the same size as the open trailing mouth, or the open leading mouth may be larger than the open trailing mouth. 
     In still yet a further exemplary embodiment, the keel has an outer wall with two side walls and a top wall, which outer wall extend from the bone contacting platform, which keel has an open leading mouth and an open trailing mouth, which open leading mouth and open training mouth define a tunnel between the outer wall of the keel and the bone contacting platform, wherein the tunnel allows for bone ingrowth into the tunnel of the keel, wherein the open leading mouth may be the same size as the open trailing mouth, or the open leading mouth may be larger than the open trailing mouth. Alternately, instead of having an open trailing mouth, a trailing end of the keel can merge into the bone contacting platform, in which case an open mouth cavity will be formed in the keel. As a further alternative, side windows can be formed on the two side walls to allow for bone growth ingress through the side windows. In still another alternative, protruding barbs can be formed to extend outwardly from the two side walls to provide for further fixation of the keel to the implantation site. 
     These and other features are described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of a prior art implant tray platform and keel, the keel having an arrow-shaped head on a vertical support rib spaced away from the implant tray platform. 
         FIG. 1B  is a detail perspective view showing of the keel of  FIG. 1A . 
         FIG. 1C  is a cross-sectional view through view lines  1 C- 1 C of  FIG. 1A . 
         FIG. 2  is perspective view of an exemplary keel shown on an implant tray platform. 
         FIG. 3A  is perspective view of another exemplary keel shown on an implant tray platform. 
         FIG. 3B  is a detail perspective view showing of the keel of  FIG. 3A . 
         FIG. 3C  is a cross-sectional view through view lines  3 C- 3 C of  FIG. 3A . 
         FIG. 4  is a diagrammatic cutaway side view showing the prior art implant tray platform and keel with a bearing portion attached, with the bearing portion being subjected to an anterior force and with the keel resisting rocking of the art implant tray platform and keel. 
         FIG. 5  is a diagrammatic cutaway side view showing the implant tray platform and keel of  FIG. 3A  with a bearing portion attached, with the bearing portion being subjected to an anterior force and with the keel resisting rocking of the art implant tray platform and keel. 
         FIG. 6  is a detail perspective view showing another embodiment of a keel with multiple side windows in its side walls. 
         FIG. 7  is a detail perspective view showing yet another embodiment of a keel with barbs formed on its side walls. 
         FIGS. 8A-C  are a posterior perspective view, top view, and posterior end view, respectively, of another exemplary embodiment of a keel. 
         FIGS. 9A-C  are a posterior perspective view, top view, and posterior end view, respectively, of another exemplary embodiment of a keel. 
         FIGS. 10A-C  are a posterior perspective view, top view, and posterior end view, respectively, of another exemplary embodiment of a keel. 
         FIGS. 11A-C  are a posterior perspective view, top view, and posterior end view, respectively, of another exemplary embodiment of a keel. 
         FIGS. 12A-C  are a posterior perspective view, top view, and posterior end view, respectively, of another exemplary embodiment of a keel. 
     
    
    
     DETAILED DESCRIPTION 
     Turning first to  FIGS. 1A, 1B, and 1C  there are shown, respectively, a perspective view of a prior art implant tray platform with keel  10 , a detail view of the keel  12  portion, and a cross-sectional view of  FIG. 1B  through view lines  1 C- 1 C. The keel  12  has an arrow-shaped head  14  on a vertical support rib  16 . The arrow-shaped head  14  has a narrowed posterior end  18  and a wider anterior end  20  with a width W, and has a length of L. The vertical support rib  16  is extends away from a bone facing surface  26  of an implant tray platform  24 . The vertical support rib  16  has a lower edge  22  that can extend from a base portion  32 , or can directly extend from the bone facing surface  26  of the implant tray platform  24 . The arrow-shaped head  14  of the keel  12  has a thickness H t  and a perimeter edge  28 . The arrow-shaped head  14  has an outwardly facing surface  15  and an inwardly facing surface  17 . A portion of the vertical support rib  30  beyond the anterior end  20  slants down and merges into the base portion  32 , or can directly extend from the bone facing surface  26  of the implant tray platform  24 . Referring to  FIG. 1B , at its wider anterior end  20 , the outwardly facing surface  15  and inwardly facing surface  17  of arrow-shaped head  14  are spaced distances H ea  and H ia , respectively, from the base portion  32 , and at its narrower posterior end  18 , the outwardly facing surface  15  and inwardly facing surface  17  of arrow-shaped head  14  are spaced distances H ep  and H ip , respectively, from the base portion  32 . This prior art keel has its arrow-shaped head  14  angled up by angle a about 30 degrees from the anterior to the posterior relative to the base portion  32 . Referring to  FIG. 1C , there is shown an underside view of the keel  12 . The arrow-shaped head  14  is divided by the vertical support rib  16  into two head halves  36 A and  36 B, which because of the space taken up by the vertical support rib  16 , have a smaller surface area than a surface area of the outwardly facing surface  15  of the arrow-shaped head  14 . The portion of the vertical support rib  30  forward the anterior end  20  of the arrow-shaped head  14  is not surrounded by any portion of the head. If an imaginary plane is projected straight down from around the perimeter edge  28  of the arrow-shaped head  14  to the base portion  32 , the volume between that imaginary line, taking out the volume of the vertical support rib  16 , defines a potential bone volume BV that can fill the space under the arrow-shaped head  14 . By way of example, with a prior art keel  12  having a keel head thickness H t  of 0.0787″ (2.0 mm), a length L of 0.394″ (10.0 mm), an anterior width W of 0.300″ (7.6 mm), a vertical support rib  16  thickness of 0.0787″ (2.0 mm), a distance H ia  of 0.63 (1.6 mm) from the inwardly facing surface of wider anterior end  20  of the arrow-shaped head  14  to the base portion  32 , with the distance H ip  of 0.300″ (7.6 mm) from the inwardly facing surface of narrow posterior end  20  of the arrow-shaped head  14  to the base portion  32 , and with the arrow-shaped head  14  angled about 30 degrees up from anterior to posterior, the cross-sectional surface area of the bottom of the arrow-shaped head  14  would be about 0.0386 square inches (24.9 mm 2 ), and the potential bone volume BV would be about 0.00372 cubic inches (60.9 mm 3 ). The relevance of the surface area of the bottom surface  17  of the arrow-shaped head  14  and of the potential bone volume BV will be discussed further below. 
       FIG. 2  is a perspective view of an exemplary implant tray platform with keel  40 . It includes an implant tray platform portion  41  and a keel  42  extending from a surface  58  of the implant tray platform portion  41 . The keel  42  has a top wall  44  and two opposite side walls  46  and  48  that extend from their bottom edges from the tray platform portion  41 , a posterior end  50  and an anterior end  52 , wherein the posterior end  50  is raised above the surface  58  of the implant tray platform portion  41  higher than the anterior end  52  leaving the top wall  44  slanted upwardly away from the surface  58  from its anterior end  52  to its posterior end  50 . The top wall  44  and the two opposite side walls  46  and  48  may be generally flat or can also be curved (not shown), and the two opposite side walls  46  and  48  may be either generally parallel to each other, or can slant inwardly toward each other from the posterior end to the anterior end of the keel (not shown.) The posterior end  50  thus defines a posterior keel opening  54  and the anterior end  52  thus defines an anterior keel opening  56  and a cavity or space S as well as a passageway is defined through the keel  42 . When the implant tray platform with keel  40  is fitted to a bone implant site that has been prepared to snugly receive the keel  41 , the close contact of the top wall  44  and side walls  46  and  48  with bone in the bone implant site will help to almost immediately secure the implant tray platform with keel  40  the bone implant site and prevent it from twisting or moving. Furthermore, over a relatively short period, bone will grow into a cavity inside of the keel  42  through the open ends  52  and  54  to thereby further lock the keel  42  in place at the implant site to better prevent movement of the implant tray platform with keel  40  at the bone implant site. The keel  62  and other portions of the implant tray platform with keel  40  that are to be locked into the bone preferably have surfaces that promote bone ingrowth and connection, such as by being surfaced to be rough. The keel  62  is preferably formed together (e.g. by casting, machining, etc.) as a single piece with the implant tray platform portion  41  and is made of strong material, including but not limited to titanium. 
       FIG. 3A  is a perspective view of another exemplary implant tray platform with keel  60 .  FIG. 3B  is a perspective detail view of keel  62 .  FIG. 3C  is a cross-sectional view of the keel  62  through view lines  3 C- 3 C of  FIG. 3A . The keel  62  extends from a surface  64  of an implant tray portion  61  has a slanted top wall  66  one two opposed side walls  68 , each with at least one opening  70  formed therethrough. The top wall  66  and the two opposite side walls  68  may be generally flat or can also be curved (not shown.) The two opposite side walls  68  may be either generally parallel to each other, or can slant inwardly toward each other from the posterior end to the anterior end of the keel (not shown.) The keel  62  has a higher posterior end  76  and a lower anterior end  78 , and the posterior end  76  thus defines a posterior keel opening  72  and the anterior end  78  thus defines an anterior keel opening  74  and a cavity or space S as well as a passageway is defined through the keel  62 . The slanted top wall  66  can be generally rectangular (but can have other shapes), and has a thickness Ht, a length L, an anterior external height H ea , an anterior internal height H ia , a posterior external height H ea , a posterior internal height H ip , an external width W e  and an internal width W. The slanted top wall  66  is preferably slanted up from anterior to posterior at angle  13  between about 10 to 20 degrees, and more preferably is slanted up by about 15 degrees. As shown, the openings  70  may be generally trapezoidal in shape, but can have other shapes, and there can be more than one window in the side walls  68 . The keel  62  of  FIGS. 3A, 3B, and 3C , by virtue of having openings  70  at its sides  68  as well as being open at its anterior end  74  and posterior end  72 , provides additional advantages over the exemplary keel  42  of  FIG. 2  because this keel design  62  will permit bone ingrowth through the opening  70  at the sides  68  in addition to at the open anterior end  74  and open posterior end  72 , which bone ingrowth can eventually infill into a potential bone volume BV inside the keel  62  between the slanted top wall  66  and side walls  68 , providing for tighter locking up of the keel  62  at the bone implant site. The top wall  66  is shown as being solid but can likewise have openings (not shown) formed therein if desired to permit bone ingrowth through the top wall  66  as well. By way of example, with a keel  62  having a top wall  66  thickness H t  of 0.0393″ (1.0 mm), a length L of 0.362″ (9.2 mm), an exterior width W e  of 0.394″ (10.0 mm), an interior width Wi of 0.315″ (8.0 mm), sidewalls  68  with a thickness of 0.0393″ (1.0 mm), distances H ea  (0.109″ (2.8 mm)) and H ia  (0.071″ (1.8 mm)) from the outwardly facing surface and the inwardly facing surface, respectively, of the anterior end  78  of the top wall  66  to the surface  64  of the implant tray platform  61 , distances H ep  (0.203″ (5.1 mm)) and H ip  (0.161″ (4.1 mm)) from the inwardly facing surface and inwardly facing surface, respectively, of the posterior end  77  of the top wall  66  to the surface  64  of the implant tray platform  61 , the cross-sectional surface area of the bottom of the top wall  66  would be about 0.1098 square inches (70.8 mm 2 ), and the potential bone volume BV would be about 0.01268 cubic inches (207 mm 3 ). Thus, when comparing both the cross-sectional areas and potential bone volumes of the prior art keel  12  of  FIGS. 1A-C  with the exemplary keel  62  of  FIGS. 3A-C , the difference is quite significant. The prior art keel  12  has a cross-sectional area of about 0.0386 square inches (24.9 mm 2 ) and potential bone volume of about 0.00372 cubic inches (60.9 mm 3 ). On the other hand, the exemplary keel  62  of  FIGS. 3A-C  has a much larger cross-sectional area of about 0.1098 square inches (70.8 mm 2 ) and potential bone volume BV of about 0.01268 cubic inches (207 mm 3 ); this represents about a 185% increase in cross-sectional surface area and about a 240% increase in potential bone volume BV. The relevance of the cross-sectional surface area of the head/top wall of a keel and its potential bone volume BV is that increasing both will help prevent implant tray platforms from shifting as forces are exerted thereon, which is explained below with reference to  FIGS. 4 and 5 . 
     Turning to  FIG. 4 , there is shown a diagrammatic cutaway side view showing the prior art implant tray platform and keel  10  with a bearing portion  80  attached to a top of the implant tray platform portion  24 , with the keel  12  extending into a space  82  formed in bone B at the implant site by having bone and other tissue removed to create a surface and space to receive the implant tray platform and keel  10 . Over time, bone at the implant site will grow around the keel  12  of the implant tray platform and keel  10  and lock the keel  12  in place to help secure the implant tray platform and keel  10  at the implant site. When the bearing portion  80  is subjected to a force F A  that bears downwardly and forwardly on its anterior end, a counteracting force Fp will act on the keel  14  to hopefully prevent rocking and shifting of the implant tray platform and keel  10 . It turns out that the amplitude of the counteracting force F p  will depend on several factors, including the cross-sectional surface area underside of the head  14  of the keel  12 , and the potential bone volume BV captured between the head  14  of the keel and the surface  24  of the implant tray platform portion  24 . With the arrow-shaped head  14  slanted at about 30 degrees, both the cross-sectional surface area underside of the head  14  and potential bone volume BV captured between the head  14  of the keel and the surface  24  of the implant tray platform portion  24  are relatively small, and the counteracting force F p  may not be able to counteract a strong force F A  that bears downwardly and forwardly on an anterior end of the implant. The average height of keel with an angled head is the sum of the height at the anterior end plus the height at the posterior end, divided by 2, and the potential bone volume BV will be this average keel height times the cross-sectional surface area of the head. Thus, in order to have a reasonably high average keel height, with a keel angled by 30 degrees, the posterior keel height must be relatively high since the anterior keel height will be relatively low. However, a high keel height may not be ideal for the following reason. In the process of creating surface(s) to support the implant, necrotic bone is removed to expose remaining healthy bone. Practitioners do their best to limit the amount of healthy bone removed since healthy bone supports the implant, and furthermore, implants wear out over time and must be replaced in revision surgeries. This requires that the old implant be removed, and that additional bone be removed at the implant site to provide a renewed surface for the replacement implant. It is desirable leave as much healthy and live bone during revision surgeries. In order to gain a reasonable high potential bone volume BV with the design of keel  12 , the height of the keel  12  at its posterior end H ep  needs to be relatively high to compensate for the relative low height of the keel  12  at its anterior end H ea . Thus, with the prior art implant tray platform and keel  10 , the counteracting force F p  generated by its keel  12  might be lower than desired and the amount of bone that may need to be removed during revision surgeries may be on a higher side. 
       FIG. 5  is a diagrammatic cutaway side view showing the implant tray platform and keel  60  of  FIG. 3A  with a bearing portion  80  attached, with the keel  62  extending into a space  84  formed in bone B at the implant site by having bone and other tissue removed to create a surface and space to engage with the implant tray platform and keel  60 . The space  84  is formed in the bone B by an angle Δ, which angle Δ is ideally be matched to the angle β of the top wall  66  relative to the surface  64  of the implant tray portion  61  so that there is a close fit of the of the space  84  to the top wall  66  of the keel. The space  84  in the bone B will preferably be likewise made to closely fit the side walls  68  of the keel  62 . In practice, the space  84  in the bone B is created by use of a punch that pushes a portion of bone aside, and creates the angled glide path space  84 . As can be seen, there will be an anterior portion  85  of the space  84  that is in front of the anterior end  78  of the keel  62  which remains unoccupied by the keel  62  when the implant tray platform and keel  60  are placed on the bone B. However, this anterior portion  85  is relatively small and bone will quickly grow into and fill this space, further preventing movement of the implant tray platform and keel  60  relatively to the bone B. When the bearing portion  80  is subjected to an anterior force F A , the keel  62  can generate a resisting counteracting force Fp that help prevent rocking and shifting of implant tray platform and keel  60  relative to the bone B. As with the prior art implant, over time, bone grows and bone around the implant site will grow around the keel  62  of the implant tray platform and keel  60  and lock the keel  62  in place to help secure the implant tray platform and keel  60  at the implant site. When the bearing portion  80  is subjected to a force F A  that bears downwardly and forwardly on its anterior end, a counteracting force F p  will act on the keel  62  to help prevent rocking and shifting of the implant tray platform and keel  60 . However, because of the significantly higher cross-sectional area and the significantly higher potential bone volume of the exemplary keel  62  of  FIGS. 3A-C , the resisting counteracting force Fp is also higher. To repeat, the prior art keel  12  has a cross-sectional area of about 0.0386 square inches (24.9 mm 2 ) and potential bone volume of about 0.00372 cubic inches (60.9 mm 3 ). In comparison, the exemplary keel  62  of  FIGS. 3A-C  has a much larger cross-sectional area of about 0.1098 square inches (70.8 mm 2 ) and potential bone volume BV of about 0.01268 cubic inches (207 mm 3 ); this represents about a 185% increase in cross-sectional surface area and about a 240% increase in potential bone volume BV. The relevance of this higher cross-sectional surface area of the top wall of the keel and its potential bone volume BV is that the implant tray platforms will be more resistant to shifting as forces are exerted thereon. Another advantage of the less angled top wall  66  (angled between about 10 to 20 degrees) is that the average top wall height off the bone facing surface of the implant try can be made lower, which will help reduce the amount of bone which may need to be removed during revision surgery. Accordingly, the keel  62  of  FIGS. 3A-C  has advantages compared to prior art keels. 
       FIG. 6  is a detail perspective view showing another embodiment of a keel  92  that extends above a surface  94  of a tray portion  95 . This keel  92  is very similar to the keel  62  of  FIGS. 3A-3C , and includes a slanted top wall  96  that slats upwardly from an anterior edge  108  to a posterior edge. There are multiple side windows  100  in its side walls  98  that are divided by side bolsters  102 , which can be included to further support the slanted top wall  96 . The top wall  96  and the two opposite side walls  98  may be generally flat or can also be curved (not shown.) The two opposite side walls  98  may be either generally parallel to each other, or can slant inwardly toward each other from the posterior end to the anterior end of the keel (not shown.) The anterior end  108  of the keel has a posterior keel opening  106  and the posterior end  110  has a posterior keel opening  104 . The keel  92  thus defines a cavity or space S as well as a passageway therethrough Like the keel  62  of  FIGS. 3A-C , the top wall  96  is preferably slanted upwardly from anterior to posterior by between about 10 to 20 degrees, and more preferably by about 15 degrees and has similar advantages as discussed above. 
       FIG. 7  is a detail perspective view showing yet another embodiment of a keel  122  with barbs  130  formed on its side walls  128 . The barbs  130  are formed by reliefs  138  leaving a fixed end  132  and a free end  134  that extends away from the side walls  128 , leaving a space  136 . The keel  122  has a slanted top wall  126  that slants upwardly from its anterior edge  140  to its posterior edge  142  up from a surface  124  of a tray portion  125 . The top wall  126  and the two opposite side walls  132  may be generally flat or can also be curved (not shown.) The two opposite side walls  128  may be either generally parallel to each other, or can slant inwardly toward each other from the posterior end to the anterior end of the keel (not shown.) The keel  122  preferably has an anterior keel openings  144  and a posterior keel opening  147 , with the keel openings  144  and  146  available for bone ingrowth into the space S formed inside the keel  122 . The barbs  130  have some flexibility so that when the keel is pushed into a space formed in the bone of an implant site (not shown), the barbs  130  will tend to flex out and make contact with the spaced formed in a prepared bone implant site and help retain the keel in place until bone can grow into and around the keel  122 . The top wall  126  is preferably slanted upwardly from anterior to posterior by between about 10 to 20 degrees, and more preferably by about 15 degrees and this keel  122  has similar advantages as discussed above, with the additional feature of the barbs  130 . 
       FIGS. 8A, 8B, and 8C  are a posterior perspective view, a top view, and posterior end view, respectively, of another embodiment of an exemplary keel  150  which extend above a surface  160  of a tray portion  161 . The keel  150  has a slanted top wall  152  joined to two opposing side walls  154  which slants down and intersect the surface  160  of tray portion  161  at an anterior end  158 . The top wall  152  and the two opposite side walls  154  may be generally flat or can also be curved (not shown.) The two opposite side walls  154  may be either generally parallel to each other, or can slant inwardly toward each other from the posterior end to the anterior end of the keel (not shown.) At a posterior end  156  there is a posterior keel opening  162  that leads into a space S. This keel  150  will allow bone ingrowth through its posterior opening  162  and into the space S to help secure the keel  150  when implanted at a bone B implant site (not shown.) If desired, windows and/or barbs (not shown) can be formed in the side walls  154 . This design is desirable when a particularly low profile keel is required with reasonably high bone volume and cross-sectional surface area to help retain the keel and associated tray portion  161 . 
       FIGS. 9A, 9B, and 9C  are a posterior perspective view, a top view, and posterior end view, respectively, of still another embodiment of an exemplary keel  170  which extend above a surface  180  of a tray portion  181 . The keel  170  has an arched wall  182  that arches up from two edges  184  that intersect the surface  180  of the tray portion  181 . The arched wall  182  is in the form of a section of a truncated cone, and has a posterior end  186  and an anterior end  188 , where the posterior end  186  is a larger arc section than the arc section of the anterior end  188 . The arched wall  182  is preferably symmetrical along a midline M. There is a posterior keel opening  190  at the posterior end  186  and an anterior keel opening  192  at the anterior end  188 . The arched wall  182  slopes downwardly at an angle of about 10 to 20 degrees from its posterior end  186  to its anterior end  188  towards the surface  190  of the tray portion  181  along the midline M and its two ends  184  angle inwardly. The arched wall  182  has an outer surface  194  and an inner surface  196  and the arched wall has a thickness “T”. There is a space “S” defined below the inner surface  196  of the arched wall  182  and the surface  180  of the tray portion  181  under the arched wall  182  which is available for bone ingrowth. Opening and/or barbs like shown in the embodiments of  FIGS. 3A-C , and  FIG. 7 , respectively, can be formed on the arched wall  182  to aid in greater bone ingrowth into the keel  170  and enhanced retention of the keel  170  at an implant site. 
       FIGS. 10A, 10B, and 10C  are a posterior perspective view, a top view, and posterior end view, respectively, of yet another embodiment of an exemplary keel  200  which extend above a surface  210  of a tray portion  211 . The keel  200  has an arched wall  202  that arches up from two edges  204  that intersect the surface  210  of the tray portion  211 . The arched wall  202  is in the form of a section of a truncated portion of an ellipsoid, and has a posterior end  206  and an anterior end  208 , where the posterior end  206  is a larger arc section than the arc section of the anterior end  208 . The arched wall  202  is preferably symmetrical along a midline M and bows out and is convexly curved from the posterior end  206  to the anterior end  208 . There is a posterior keel opening  220  at the posterior end  206  and an anterior keel opening  222  at the anterior end  208 . The arched wall  202  generally slopes downwardly at an angle of about 10 to 20 degrees from its posterior end  206  to its anterior end  208  towards the surface  210  of the tray portion  211  along the midline M and its two edges  204  curve inwardly. The arched wall  202  has an outer surface  224  and an inner surface  226  and the arched wall has a thickness “T”. There is a space “S” defined below the inner surface  226  of the arched wall  202  and the surface  210  of the tray portion  211  under the arched wall  202  which is available for bone ingrowth. Opening and/or barbs like shown in the embodiments of  FIGS. 3A-C , and  FIG. 7 , respectively, can be formed on the arched wall  202  to aid in greater bone ingrowth into the keel  200  and enhanced retention of the keel  200  at an implant site. 
       FIGS. 11A-C  are a posterior perspective view, top view, and posterior end view, respectively, of another exemplary embodiment of a keel  230  and has an unraised arched wall  232  in the shape of a hollow truncated catenoid that extends above a surface  240  of a tray portion  241 . It has an open posterior end  234  and has an open mouth  236 . It drops down at its anterior end  238  to join the surface  240  of the tray portion  241 . A space “S” is defined under keel wall  232  of the keel  230  which is available for bone ingrowth. Opening and/or barbs like shown in the embodiments of  FIGS. 3A-C , and  FIG. 7 , respectively, can be formed on the arched wall  232  to aid in greater bone ingrowth into the keel  230  and enhanced retention of the keel  230  at an implant site. 
       FIGS. 12A, 12B, and 12C  are a posterior perspective view, a top view, and posterior end view of still another embodiment of an exemplary keel  250  which extend above a surface  260  of a tray portion  261 . The keel  250  has an arched wall  252  that arches up from two ends  254  that join to the surface  260  of the tray portion  261 . The arched wall  252  is in the form of a section of a hollow cylinder, and has a posterior end  256  and an anterior end  258 , both of which are sized the same. There is an opening  270  at the posterior end  256  and an opening  272  at the anterior end  258 . The arched wall  252  has an outer surface  274  and an inner surface  276  and the arched wall  252  has a thickness “T”. The thickness “T” can be, but need not be consistent throughout the arched wall  252 . There is a space “S” defined below the inner surface  276  of the arched wall  252  and the surface  260  of the tray portion  261  which is available for bone ingrowth. Opening and/or barbs like shown in the embodiments of  FIGS. 3A-C , and  FIG. 7 , respectively, can be formed on the arched wall  252  to aid in greater bone ingrowth into the keel  250  and enhanced retention of the keel  250  at an implant site. This keel  250  design, unlike the other design, is not sloped down from its posterior end  256  to its anterior end  258  but can be useful in applications where the keel  250  and tray portion  261  can be dropped straight down onto a bone implant site that has been prepared to receive the keel  250  and tray portion  261 . 
     In embodiments, the keels can be formed together (e.g. by casting, machining, etc.) as a single piece with the implant tray platform portions and made of a strong material, such as titanium. However keels can be separate pieces that are permanently attached to the implant tray platform portions. 
     Any element in a claim that does not explicitly state “means” for performing a specified function or “step” for performing a specified function, should not be interpreted as a “means” or “step” clause as specified in 35 U.S.C. §112.