Abstract:
A bone implant having a core section to which a prosthesis is eventually secured, and rib-like projections or anchoring prongs enhancing the hold of the implant in the bone. The invention is particularly, but not exclusively, intended as a dental implant. The socket for the implant is drilled in a bone using a suitable template which is subsequently inserted in an original pre-drilled bore and which serves as a guide for subsequent drilling of bores for the anchoring prongs or rib-like projections. Several different types of the templates are disclosed together with a number of compatible implants. A specific tool is also disclosed which reduces the damage to the bone by reducing the volume of the initial portion of the socket adapted to receive the associated template.

Description:
BACKGROUND 
   The present invention relates to the art of bone implants and devices and methods for forming, in a bone, a socket for receiving same. 
   It is known to provide various types of bone implants used in dentistry, joint prostheses installation or other branches of medicine. In most cases, the bone implant is used as an anchor in the bone for an extension secured to the top surface of the implant. A typical example of such implant is presented by implants used in dentistry but the present invention is not limited to that particular field of medicine. 
   The known bone implants are of two basic types. First, an implant is simply screwed into a pre-drilled bore by self-tapping action during which the thread of the implant taps into the surrounding bone often causing substantial damage to the bone tissue surrounding the implant and thus retarding the healing process. Another implant, used often in installation of the artificial joint head to a femur, is simply an elongated, slightly tapered stem driven by impact force into the marrow of the femur destroying a considerable part of the marrow. It is also known to provide what basically amounts to a bolt-and-nut securement of the implant used in supporting the head of an artificial hip joint. Such arrangement again uses a considerable space and its installation often results in substantial destruction of the bone and marrow tissue in the vicinity of the implant. 
   It is an object of the present invention to provide a bone implant which would provide a firm securement to the bone tissue without subjecting the bone or the marrow to undue damage caused by the installation process. 
   SUMMARY OF THE INVENTION 
   In general terms, the invention provides, in one aspect thereof, a bone implant which comprises (1) a core having a first end, a second end and defining a centerline extending centrally of and between said ends, and a circumferential side wall portion extending between said ends and being parallel with the centerline; (2) at least two elongated projections extending from said core and having a rounded contour when viewed in axial direction of said centerline. 
   The rib-like projections may take the shape of ribs extending from the circumferential side wall. If desired, the ribs may extend beyond the second end of the core by way of anchoring stems. Another modification is devoid of the ribs. The rib-like projections are stems projecting from the second end of the core at locations slightly spaced radially inwardly from the circumference of the second end. The projections may be formed by a plurality of fins disposed in a row parallel with the axis of the core or with elongation of the respective stem. The stems projecting from the core can either be parallel with the axis of the core or at least one of the stems may extend obliquely away from the core. The core may have a circular cross-section perpendicular to its axis, or it may be of a radially elongated shape, for instance, oval, or even multilateral, particularly rectangular with rounded corners. 
   In another aspect, the invention provides template means for use in forming a socket in a bone into which the respective implant may be tapped. The template means may comprise a template formed by a sleeve having an axis, a first end, a second end, an inner wall and an outer wall. The outer wall corresponds in shape and size to the circumferential side wall of the respective implant. At least two axially elongated channels or grooves of a semi-circular cross-section are formed in the outer wall to serve as drill guides. They extend parallel with the axis of the template. The template means further includes a generally cylindric locking pin discrete from the sleeve and compatible in radius with one or all of said grooves. 
   In another embodiment of the template means, the axially elongated channels are bores extending from the first end to the second end and disposed inwards of the circumferential wall, to guide drill bits forming bores in the bone for receiving the stems. 
   Yet another embodiment of the template means comprises, in addition to the previously mentioned template, a template having an oblique guide channel extending through a core between the first and second ends thereof. At the same time, this template has a locking stem projecting from the second end to prevent inadvertent turning of the template about its axis. 
   In a particularly preferred embodiment, it is proposed that the core of the implant and of the template be concavely curved at the second end and that the corresponding bore in the bone be formed with a convex bottom using new tools which are also subject to the present invention. 
   In particular, the tools for drilling the last mentioned bore for the core of the implant are shaped drill bits having each a centrally disposed cylindric stem at a free end and a pair of radially projecting, opposed cutting plates which are provided with a concavely curved cutting plate turned towards the free end of the tool. The plates of the formed tool thus resemble the cutting wings of what is known as a spade bit. The cylindric stem has a small diameter corresponding to that of a drill for making a pilot bore for guiding the shaped drill bit or bits. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the different embodiments of the invention will become apparent from the following description, referring to the accompanying diagrammatic, simplified and not-to-scale drawings. The drawings depict: 
       FIG. 1  a cross-sectional view, taken along the section line A-A of  FIG. 2 , depicting a first and second embodiment of the implant of the present invention, the features of the second embodiment additional to those of the first embodiment being diagrammatically shown in broken lines; 
       FIG. 2  a top plan view of  FIG. 1 ; 
       FIG. 3  a side view of a template of template means used in forming a socket for receiving the implant shown in  FIGS. 1 and 2 ; 
       FIG. 4  a top plan view thereof; 
       FIG. 5  a side view of a locking pin of the template means; 
       FIG. 6  a top plan view of  FIG. 5 ; 
       FIG. 7  a diagrammatic section of a bone with a pre-drilled bore for the core of an associated implant; 
       FIG. 8  a section similar to that of  FIG. 7  but with the template partly inserted; 
       FIG. 9  a section similar to  FIG. 8  but showing the template in place and one of retaining grooves being drilled; 
       FIG. 10  a section similar to  FIG. 9  showing the socket for the first embodiment of  FIG. 1  in its final shape; 
       FIG. 11  a top plan view of  FIG. 10 ; 
       FIG. 12  a section view showing the implant tapped in the socket; 
       FIG. 13  a diagrammatic side view of another embodiment of the implant of the present invention; 
       FIG. 14  a bottom plan view thereof; 
       FIG. 15  a sectional view, taken along the section line B-B of a template for forming a socket for the implant shown in  FIG. 13 ; 
       FIG. 16  a top plan view of the representation of  FIG. 15 ; 
       FIG. 17  a top plan view of a bone implant according to a yet another embodiment of the present invention; 
       FIG. 18  a simplified sectional view taken along the section line C-C of  FIG. 17 ; 
       FIG. 19  a top plan view of a first template of template means for forming a bore of a socket for one anchoring stem of the implant of  FIGS. 17 and 18 ; 
       FIG. 20  section D-D of  FIG. 19 ; 
       FIG. 21  a top plan view of a second template of template means for forming an oblique bore for an oblique stem of the implant of  FIGS. 17 and 18 ; 
       FIG. 22  a sectional view taken along the section line E-E of  FIG. 21 ; 
       FIG. 23  a sectional view showing the socket finalized by use of the templates of  FIGS. 19 ,  21 ; 
       FIG. 24  is a sectional view showing the core of the implant of  FIG. 18  tapped in the corresponding socket; 
       FIG. 25  a sectional view similar to that of  FIG. 24  but showing the entire implant with an oblique stem in place; 
       FIG. 26  a sectional view showing the use of the invention in providing an anchor for the head of an artificial hip joint in a femur; 
       FIG. 27  a perspective view of yet another embodiment of the implant; 
       FIG. 28   a  a top plan view of a pilot bore drilled in a bone, in preparation of forming the socket for the implant of  FIG. 27 ; 
       FIG. 28   b  a diagrammatic cross-section F-F of  FIG. 28   a;    
       FIG. 29   a  top plan view of a first template of template means for forming a socket for the implant of  FIG. 27 ; 
       FIG. 29   b  is a cross-sectional view taken along section line G-G of  FIG. 29   a  and showing the template of  FIG. 29   a  placed in a pilot bore of  FIG. 28   a , with an indication of retaining bores eventually drilled through drill leading bores of the first template; 
       FIG. 30   a  a top plan view of a second template of template means for forming a socket for the implant of  FIG. 27 , the second template being shown inserted in the pilot bore of  FIG. 29 ; 
       FIG. 30   b  a cross-section H-H of  FIG. 30   a;    
       FIG. 31   a  is a view similar to that of  FIG. 30   a  but showing the pilot bore enlarged to correspond in shape and size to the circumference of the core of the implant of  FIG. 27 ; 
       FIG. 31   b  a section I-I of  FIG. 31   a;    
       FIG. 32   a  a top plan view of a finished socket for the implant of  FIG. 27 ; 
       FIG. 32   b  section J-J of  FIG. 32   a;    
       FIG. 33  a side view of a drill bit for preparing a pilot bore for the implant of  FIG. 44 ; 
       FIG. 34  a section view showing the drill of  FIG. 33  at the time of a partial withdrawal from a finished pilot bore; 
       FIG. 35  a section view similar to  FIG. 34  but showing only the pilot bore; 
       FIG. 36  a side view of a first shaped bit for forming a socket for the core of the implant  FIG. 44 ; 
       FIG. 37  a bottom plan view of  FIG. 36 ; 
       FIG. 38  a section view similar to  FIG. 34  showing the first shaped bit in a terminal inward position; 
       FIG. 39  a view similar to  FIG. 35  but showing the stage where the implant socket has been partly formed by the first shaped bit; 
       FIG. 40  a view similar to  FIG. 36  but showing a second shaped bit for forming the implant socket; 
       FIG. 41  a bottom plan view of  FIG. 40 ; 
       FIG. 42  a section view similar to that of  FIG. 38  showing the second shaped bit in a terminal inward position; 
       FIG. 43  a view similar to  FIG. 39  but showing a portion of the socket for receiving a core of the implant of  FIG. 44 ; 
       FIG. 44  a side view, not to scale, of an implant for eventual placement in the socket of  FIG. 43 ; 
       FIG. 45  a bottom plan view of  FIG. 44 ; 
       FIG. 46  a section view showing a template for forming bores for anchoring stems of the implant of  FIG. 44 ; and 
       FIG. 47  a top plan view of the template of  FIG. 46 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Turning now to  FIG. 1 , reference number  20  designates the first and second embodiment of the inventive implant, with additional structural features of second embodiment shown in broken lines. The implant is made of titanium or any other material suitable for the purpose. The description of the first embodiment relates to a dental implant but it will be appreciated that other than dental applications of the implant may exist. 
   The implant  20  includes a core  22 . The core  22  is solid and defines a first end  24 , a second end  26  and a centerline  38  extending centrally of and between the ends  24 ,  26 . In the embodiment shown, two rib-like projections  30 - 32  having an axial elongation, parallel with the axis  38 , project radially outwards from the circumferential wall  34  of the core  22 , and, longitudinally from the first end  24  to the second end  26 . In a modified embodiment, the projections extend beyond the second end, defining two anchoring extensions or stems  31 ,  33 . The circumferential wall  35  is parallel with the axis  38 . In the embodiment shown, the wall  35  is cylindric, having a circular cross-section but other shapes, e.g. an oval or multilateral, cross-sectional configurations of the wall  35  are possible. 
   The first end  24  of the core  22  is provided with a securement means, for instance a threaded bore  28  for fixing a respective prosthesis or healing cap to the core  22 . For clarity, the projections  30 - 32  may also be designated as a first and second, projection, respectively. 
   The projections  30 - 32  may be solid ribs of rounded contour as shown in  FIG. 2 , integrally formed with the core  22 . It is preferred, however, that the ribs be each formed, at least over a part of their overall length, by a straight row of fins  40 ,  42  having an inclination shown in  FIG. 1 . In other words, the fins  40 ,  42  slope at a slight angle of inclination in a direction radially inwards and axially away from the first end  24 . This provides the fins with a back biting feature which impedes the withdrawal of the implant from its pre-formed cavity, once the implant has been tapped in. 
   As already mentioned, one of the first embodiment versions has two ribs  30 ,  32  at radially opposed locations, even though the number of the ribs is arbitrary depending upon the particular application of the implant  20 . In most cases, however the spacing of the ribs about the axis  38  is uniform, i.e. in case of two ribs, 180°, in case of four ribs, 90° etc. 
   The size of the core  22  and of the ribs  30 - 32  is compatible with the shape and size of a socket formed in the respective bone, so that the individual parts of the implant  20  contact the walls of the socket when the implant has been tapped in, as will be described later. 
   In summary, therefore, the anchoring projections or ribs  30 - 32  may extend only over the axial length of the core  22  or, in a second embodiment, may extend beyond the second end  26 . The ribs may have the configuration of a generally solid rib-like projections or may have a series of back biting plate-like fins over at least a portion of their length. 
   The socket configured to receive the implant described above is produced by utilizing template means which includes a template means  50  an exemplary embodiment of which is depicted in  FIGS. 3 to 6 . It includes a generally template  50  formed by a cylindric sleeve  52  which has a first end  54 , a second end  56 , an inner wall  58  and an outer, circumferential wall  60 . The outer diameter  62  of the outer or circumferential wall  60  corresponds to the diameter of the core  22  of the implant and also of an associated pre-drilled bore  76  to which a further reference will be made later. Provided in the cylindric outer wall  60  are two axially elongated channels  64 ,  66 ,  68 ,  70 , also described, for convenience, as a first, second, third and fourth channel, respectively. The channels are spaced from each other at an angle of about 90° about the axis  53  of the sleeve  52 , it being understood that, in certain applications, only two of the channels disposed at diametrally opposite locations, such as channels  64 - 66  may suffice. It will also be appreciated that a template (not shown) having three channels at a 120° spacing from each other could be provided. The actual number of channels in the outer wall  60  is arbitrary and depends on the application of the present invention which normally determines the diameter  62  of the sleeve  52 . The channels  64 - 70  are concavely rounded as best seen from  FIG. 4 . The central area of the sleeve  52  defines an opening  58  extending, like the channels  64 - 70 , full length of the sleeve  52 , from the first end  54  to the second end  56  thereof. A handle  74  facilitating the handling of the sleeve  52  projects radially from the first end  54  at a location between two adjacent channels  66 ,  68  so that it does not obstruct the equidistant circumferential spacing between the channels as described. The preferred embodiment of the inventive template means includes, in addition to the template  50 , a cylindric locking pin  72  having an outer radius corresponding to that of the concave rounding of the channels  64 - 70 . The length of the pin  72  is greater than the overall length of the implant. 
   The forming of a socket receiving the first embodiment of the implant will now be described with reference to  FIGS. 7-12 . The implant  20  only has two rib-like projections  30 ,  32  and the length of the projections  30 ,  32  is the same as that of the core  22 . 
   First ( FIG. 7 ), a generally cylindric pre-drilled bore  76  is formed in the bone  78 . The bore  76  is compatible in shape with the circumferential wall of the core  22 . In the second stage, the template  50  is inserted in the pre-drilled bore  76  as shown in  FIG. 8 . The circumferential positioning of the channels  64 ,  66  is set using the lever  74  for adjustments. When the desired location is achieved, a drill is driven into the bone  78  (using the first channel  64  as a guide) to a desired depth. In the case of making the socket for the implant shown only in solid lines of  FIG. 1 , the depth would only be to the bottom  77  of the bore  76  even though, obviously, as already mentioned, the depth could go beyond the bottom  77  to accommodate the modified version of the implant of  FIG. 1  including the parts shown both in the solid and broken lines. This results in the forming of an inwardly open, concavely rounded retaining groove  80  formed in the wall of the bore  76 . The shape of the groove  80  viewed in axial direction corresponds to the rounded shape of the rib-like projection  30  of  FIG. 1 . 
   In the next step (not shown in the drawings), the drill  79  is withdrawn and, with the template  50  still in, the locking pin  72  is inserted in the circular opening defined partly by the retaining groove  80  and partly by the first channel  64  of the template  50 . This locks the template  50  circumferentially of the axis of the bore  76 . With the template  50  locked in place, the drill  79  is driven (not shown in the drawings) along the second channel  66  forming a second retaining groove  82 . The locking pin  72  and the template  50  are then withdrawn and the shaped socket  84  is ready to receive the implant  20 . If desired, the implant  20  may be provided with a growth enhancing coating. The implant  20  is tapped into the socket  84  with the rib-like projections  30 ,  32  engaging the retaining grooves  80 ,  82  thus holding the implant  30  in a desired circumferential position, without any excessive radial forces being present. 
   It will be appreciated that the operation described can be effected in the corresponding fashion using an implant with more than two rib-like projections and correspondingly more retaining grooves formed in the bone or using two or more ribs having free ends projecting beyond the second end  26  of the implant. 
     FIGS. 13 ,  14  depict a side view of a modified implant  120 . Here, instead of the sidewise disposed anchoring projections  30 - 32 , at least two, but possibly four or even more, anchoring stems  130 ,  132 ,  134 ,  136  project downwards from the second end  126  of the core  122  at a radially inward spacing from the diameter of the core  122  of the implant. Otherwise, there is a substantial similarity between the first and second embodiments. As in the first embodiment, the embodiment of the implant  120  comprises a core  122  having a first end  124 , a second end  126 , a threaded bore  128  and rib-like projections formed in the stems  130 ,  132 ,  134  and  136 . Similarly, the stems  130 - 136  are parallel with centreline  138  of the core. 
   As shown in  FIGS. 15 ,  16 , the template means for forming an implant socket for the implant of  FIGS. 13 ,  14  differs from the first embodiment of the template means by a modified template  150  wherein the drill guiding channels have the shape of cylindric bores or passages  164 ,  166 ,  168 ,  170  through the sleeve  152 . The sleeve  152  may be substituted by a solid core of the template  150  even though, the shape of a sleeve  152 , having a central opening with an inner wall  158  is preferred as it facilitates the removal of blood and air as the socket is being formed. The template  150  is likewise provided with a handle  174 . 
   The use of the template  150  is analogical to the previously described use of the template  50  and therefore will not be described in detail. Briefly, as in the first embodiment, a bore (not shown) corresponding in size to the generally cylindric core  122  of the implant  150  is pre-drilled, whereupon the template  150  is inserted in the pre-drilled bore and appropriate retaining channels (corresponding, in function, to the retaining grooves  80 ,  82 ) are formed for receiving the projections  130 ,  136 . The second embodiment of the template means also includes a locking pin (corresponding to pin  72  and therefore not shown in the drawings). When the first retaining channel, e.g. the one for the projection  130 , has been drilled, the locking pin is inserted, much in the fashion of the first embodiment, to circumferentially lock the template  150  in the bore. The remaining retaining channels are then drilled in precisely determined locations of the bottom determined by the location of the channels, such as channels  168 ,  170  etc. 
   The embodiments thus far disclosed have a common feature in that the circumferential wall of the core of the implant and the retaining members  30 ,  32  etc. and  130 ,  132  etc. are all generally parallel with the centreline of the implant core. In some instances it may be desirable to secure the implant in the bore in a positively locked fashion. This is accomplished by yet another embodiment which will now be described in detail with reference to  FIGS. 17-25 . 
   The implant  200  is comprised of two distinct parts: a core  202  and a threaded retainer  204 . The core  202  has a first end  206  and an opposite second end  208 , both having generally identical shapes of their circumference, so that a circumferential wall  210  extending between the ends  206 ,  208  is parallel with the centreline  212 . 
   The circumferential wall  210  may be provided, if desired, with back biting fins disposed about the circumference of the wall  210  as diagrammatically shown in  FIG. 18 . The fins can be each continuous about the circumference of wall  210  or may be in the form of a number of discrete projections uniformly distributed about the circumference and/or axial length of the core  202 . For simplicity, the fins are only shown in  FIG. 18 . It will be understood that the size of the core receiving portion of the socket would be selected to match the maximum diameter of the core. 
   A threaded bore  214  extends through the core  202  at an acute angle α relative to the centreline  212 . The implant  200  is provided with an integrally formed retainer pin  215  projecting from the second end  208  in a direction parallel with the centreline  212 . 
   The threaded retainer  204  has a first portion provided with a thread  216  compatible with the thread formed in the bore  217 . The second portion  218  of the retainer  204  has a diameter smaller than that of the threaded portion so that it can freely pass through the threaded bore  214 . The free end of the first portion is provided with a suitable screwdriver engaging means, for instance a radial slot  220 . 
   When the implant  200  is fully assembled, the thread  216  engages the thread of the bore  214 , while the second portion  218  of the retainer  204  projects from the core  202  at the angle α as shown in broken lines of  FIG. 18 . In the embodiment shown, the portion  218  projects from the second end  208  of the core  202 . It will be understood, however, that in certain applications, depending on the angle α and the axial length of the core  202 , the second portion  218  could at least partly extend from the circumferential side wall  210 . It will also be appreciated that in certain applications it is possible to provide more than one angled threaded bore such as bore  214 . The additional angled bores, of course, would have to be designed such as not to intersect each other. 
   Thus, in an assembled form, the implant  200  comprises, in addition to the core  200  at least one angled retainer  204  projecting from the core  202  at an acute angle relative to the centreline  212 . The inclined position of the retainer  204  secures a positive lock of the implant  200  in a complementary socket. 
   The forming of the socket complementary with the implant of  FIGS. 17 ,  18  is effected by means of template means comprising a first template  222  and a second template  224 . The first template  222  corresponds in shape and size to the size and shape of the core  202 . It has a first end  226 , a second end  228  and a circumferential side wall  230  extending between the two ends in a direction parallel with the centreline  232 . In the embodiment shown in the drawings, and for the sake of simplicity, the side wall  230  (and thus also the side wall  210 ) is cylindric. An offset drill guide channel  234  extends between the first and second ends  226 ,  228  in a direction parallel with the centreline  232  of the first template  222 . 
   The second template  224  has core portion  236  and an integrally formed locking pin  238 . An inclined drill guide channel  240  extends between a first end  242  of the core portion  236  and the second end  244  thereof, at the same angle α as referred to above. 
   In operation, a bore  246  having a bottom  248  is first formed in a bone  250 . Then, using the first template of  FIG. 20 , which may be provided with a handle  217  similar to the previously described embodiments, a drill bit (not shown) is guided through and by the channel  234  to form, in the bottom  248 , a retainer bore  252  complementary with the lock pin  238  and with the retainer pin  215  of the implant  200 . Next, the second template  224  is inserted with its locking pin  238  in the retainer bore  252 . A drill is inserted into and guided by the channel  240 , and driven into the bone  250 , forming the oblique retainer bore  254  compatible with the second end  218  of the retainer  204  ( FIG. 24 ). When the retainer bore  252  is formed, the drill is withdrawn, and the core portion  236  withdrawn from the bore  246 , leaving an implant receiving socket shown in  FIG. 23 . The core  202  with the retainer pin  215  are now inserted into the socket. With the core  202  and retainer pin  215  in place, the threaded bore  214  is automatically aligned with the oblique retaining bore  254 , ready to receive the retainer  204 . The second portion  218  of the retainer  204  may be provided with a growth enhancing composition and the thread  216  with a suitable sealant, whereupon the retainer  204  is simply threaded into the core  202 , whereby the implant  200  is positively locked in the preformed socket, as shown in  FIG. 25 . 
   As already mentioned, the exemplary embodiments thus far described are dental implants, it being understood that the inventive method, implant and template means can also be utilized with advantage in a different field, as shown by way of an example, in  FIG. 26 . 
   Here an implant  320  is shown in a femur. It has the configuration similar to but correspondingly different in size from, the implant of  FIGS. 1 and 2 . The implant  320  includes extensions  328 ,  330  of rib-like projections  332 ,  334 . Additional ribs, not shown but corresponding in location to ribs  34 ,  36  and their respective extensions can also be provided. The implant shown provides a securement to the femur- 322  of an artificial hip joint head  324 . The socket for the implant  320  is drilled in the same fashion as described with reference to the template of  FIGS. 3 and 4 . It will be observed, however, that the rib-like projections and their extensions are embedded in the bone tissue  336 , not in the marrow  338  as is the case of most femoral prostheses. This results in a reduced damage to the bone marrow  338  and thus to the red blood cell formation in the femur  322 . Also, the tapping of the implant into the bone marrow does not produce excessive radial forces as the socket for the implant is pre-formed before the insertion. 
   The embodiment described in the preceding paragraph is intended to demonstrate that the present invention can be used in fields other than dental implants. 
   For simplicity, the core  22 ,  122 ,  202  and  326  are all shown as having a cylindric configuration which has the advantage of the formation by simple pre-drilling of the bore such as bore  76  in  FIG. 7 . The circular cross-section of the bore necessitates the locking of the correspondingly cylindric template at an exact circumferential position by the locking pin  72 ,  238  or by some more complex and not preferred or described securement. While the locking of the cylindric core as described, using the locking pin mechanism, provides the required accuracy, the resistance of an implant to an undesired subsequent circumferential displacement or rotation of the core can be further enhanced by utilizing a core having other than circular cross-section, for instance oval or rectangular with rounded corners. 
     FIG. 27  shows a perspective view of still another embodiment of an implant  400  of the present invention, in which the core  402  has a first end  404  and a second end  406 . The cross-sectional configuration of the core  402  is rectangular with rounded outer corners  408 ,  410 ,  412 ,  414  between generally rectangular side walls  416 ,  418 ,  420 ,  422  all of which extend between the ends  404 - 406  in a direction parallel with the centreline  424  of the core  402 . Two retainer stems  425 ,  426  extend from the second end  406 . 
   The template means for forming a socket for the implant of  FIG. 27  includes a first template and a second template. 
   The first template ( FIGS. 29   a ,  29   b ) is similar to the template of  FIG. 16 . It is provided with two drill guiding channels  432 ,  434  having the shape of cylindric bores or passages through the sleeve  436 . As in the previous embodiment of  FIG. 16 , the sleeve  436  may be substituted by a solid cylinder. However, the shape of a sleeve  436  having a central opening defining an inner wall  437  is preferred as it facilitates the removal of blood and air as the socket is being formed. 
   The second template is somewhat similar to the structure of the implant of  FIG. 27 . It mainly differs in that the core  438 , while also having the shape of a prism, is of a smaller size when viewed in the direction of the centreline  440  ( FIG. 30   b ). Projecting from a second end  442  of the core  438  is a pair of anchor stems  444 ,  446 . The core  438  of the second template has a rectangular circumference having four side walls  448 ,  450 ,  452  and  454  which adjoin each other at rounded corners. 
   The socket is formed by first drilling in a bone  428  a cylindric pilot bore  430  ( FIG. 28   a ,  28   b ) of a relatively small diameter which corresponds to the outer diameter of the first template. The first template is then inserted into the bore  430  and, utilizing the guide channels  432 ,  434 , retaining bores  456 ,  458  are drilled to extend from the bottom of the pilot bore  430  at a spacing from each other corresponding to that of the guide channels  432 ,  437  which, in turn, corresponds to the spacing between the retainer pins  424 ,  426  of the implant. A locking pin similar to that previously described can be used after one of the two bores, to prevent undesired turning of the first template while forming the second bore. 
   Upon withdrawal of the first template from the pre-formed socket, the second template is inserted in the pilot bore  430 . This is shown in  FIGS. 30   a  through  31   b . It will be noted from  FIG. 30   a  that the diameter of the pilot bore  430  equals the length of the diagonals of the rectangular core  438  of the second template, as viewed in axial direction. The stems  444 ,  446  are disposed in the retaining bores  456 ,  458  preventing the second template from turning within the pilot bore  430 . 
   The circumferential surface of the core  438  provides a guide for a router  445  ( FIG. 30   a ) cutting a rectangular bore  460  ( FIG. 32   a ). When the circumferential shape of the rectangular bore  460  is finished, the second template is withdrawn and the socket is ready for tapping in the implant of FIG.  27 , the core  402  in the rectangular bore  460  and the retainer stems  424 ,  426  in the retaining bores  456 ,  458 . 
   The rectangular cross-sectional configuration of the core  402  compatible with the bore  460  provides additional resistance of the implant to torque stress. 
   As mentioned at the outset, the drawings of the present application present diagrammatic, simplified representations of different embodiments of the present invention thus far described. Thus, for the sake of simplicity, the bores accommodating the core of the associated implant are indicated as having a flat bottom while, in an actual shape, the bottom of the bores would often be shallowly concavely conical, depending on the tool used in forming same. 
   In a further development of the present invention, it has been realized that the actual shape of the bores can be advantageously modified by using different tools for the formation of the socket. This development will now be described in detail with reference to  FIGS. 33-47 , by way of an exemplary embodiment of the tools, an implant and a method of preparing a socket for a dental implant. 
   The tools for the forming of the socket include at least one drill bit, at least one but preferably two core forming shaped bits and a template. 
   Turning firstly to  FIG. 33 , reference number  500  designates a drill bit. As is well known the drill bit comprises a stem  502  provided, at a free end thereof, with a spiral convolute  504  extending all the way to the free end tip  506 . The opposite, root end of the stem  502  is provided with an integrally formed latch fitting  508 . As is well known, the fitting  508  serves the purpose of latching the bit  510  to a dental drill. Typically, in the art of dental implants the diameter of the bit  500  would be about 1.5 mm. The bit is provided with an abutment ring  509  which may be axially adjustably secured to the stem  502 . For instance, a setscrew in a radial threaded bore (not shown in the drawing) can be provided to firmly connect the ring  509  at a selected axial location of the stem  502 . Many other alternatives are well known of the abutment means for limiting the maximum depth of the bore produced by the bit  500 . 
   Reference should now be had to  FIGS. 36 and 37  wherein the numeral  510  designates a first shaped bit  510 . The bit  510  is somewhat similar in shape and function to what is commonly known as a spade bit. The structure of the bit  510  includes a shank  512  and a smooth cylindric free end stem  513  terminating in a rounded free end tip  514 . As in the bit of  FIG. 33 , there is a latch fitting  516 . Preferably, the stem  512  and shank  513  are integrally formed. Two cutting plates  518 ,  520 , having the shape of a mirror image of each other, disposed between the shank  512  and stem  513  project from radially opposite sides at a location between the shank  512  and the stem  513 . Preferably, the plates  518 ,  520  are integrally formed with the shank  512  and stem  513 . Each plate  518 ,  520  has a sharpened, concavely curved cutting edge  522 ,  524  turned towards the free end tip  514 , and a side edge  526 ,  528 , generally parallel with the axis of the shank  512 . As already mentioned, the plates  518 ,  520  are a mirror image of each other. Accordingly the corners between the respective cutting edges and the side edges  522 - 526 ,  524 - 528  are equidistantly spaced from each other. The axial distance from the tip  514  to the cutting edges  522 ,  524  is predetermined as will be mentioned. 
     FIGS. 40 ,  41  show a second shaped bit  530 . The overall configuration of the second shaped bit  530  corresponds to that of the first shaped bit  510 . However, the cutting plates  532 ,  534  have a different shape and disposition. Each cutting plate has a generally identically, concavely shaped first cutting edge  536 ,  538 . However the edges are spaced from the rounded tip  540 , a distance which is smaller than in the embodiment of  FIG. 36 . The opposed side edges  542 ,  544  of the plates  532 ,  534  are spaced radially from each other the same distance as the side edges  526 ,  528  of  FIG. 36 . However, in axial direction, they are longer than the edges  526 ,  528 . Furthermore, the ends of the side edges  542 ,  544  terminate each in a short oblique by outwardly directed straight, cutting edge  546 ,  548 . The shank  550 , the plates  532 ,  538  and the stem  551  otherwise correspond to the structure of the corresponding parts of the shaped bit of  FIG. 36 . 
   Both the stem  513  and  551  have the same diameter corresponding to that of the drill bit  500 . 
     FIGS. 46 and 47  show a template  552  for finishing the socket pre-formed by the above tools. In particular, the template  552  serves the purpose of guiding a drill bit (not shown) to provide bores for projections of the implant which will be referred to hereinafter. The template  552  is very similar, but not identical, to that of  FIGS. 15 ,  16 . It has the drill guiding channels having the shape of cylindric bores or passages  554 ,  556 ,  558 ,  560  through the sleeve  562 , and a central opening  564 . And there is a handle  566  as in the previously described embodiment. The only difference is in the shape of the second end of the sleeve remote from the handle  566 . The second end is concavely shaped at  568  at a radius corresponding to the radius of the cutting edges  536 ,  538  ( FIG. 40 ). 
     FIGS. 34 and 35  show the forming of a small diameter pilot bore  570  in a bone  572 . In  FIG. 34 , a stage is shown where the bore  570  has been bored by the drill  500  to a predetermined depth determined by the axial distance of the ring  509  from the tip  506  of the drill. In  FIG. 34 , the drill bit  500  is being withdrawn from a pilot bore  570 .  FIG. 35  shows the finished pilot bore  570  ready for further forming of the implant socket. 
     FIG. 38  shows the use of the first shaped bit  510 . Using the previously formed ( FIG. 35 ) pilot bore  570  as a guide for the stem  513 , the cutting edges  522 ,  524  of the plates  518 ,  520  form an upper portion of an enlarged bore  574  ( FIG. 39 ). As in most of the preceding embodiments, the diameter of the bore  574  is compatible with the diameter of an associated template, in this case template  552 . It is noteworthy that the bottom  576  ( FIG. 39 ) is convexly shaped thus saving a volume of bone tissue which, in the previously described embodiments, is cut off and thus wasted. Furthermore, it is shown in  FIG. 38  that the depth of the upper portion of the bore  574  is determined by the depth of the pilot bore  570 . When the tip  514  reaches the bottom of the pilot bore, the desired depth of the upper portion of the bore  574  has been reached. 
   In the next step, the rotating second shaped bit  530  is inserted in the preformed upper part of the bore  574  and the drilling of the bore  574  continued. Eventually, the free end  540  of the stem  551  (which is shorter than its counterpart of bit  510 ) reaches the bottom ( FIG. 42 ) of the short remainder of the pilot bore  570  whereby the desired depth of the enlarged bore  574  is reached. Preferably, the concave curvature of the cutting edges  536 ,  538  of the second shaped bit  530  is the same as that of edges  522 ,  524  of the first shaped bit. It is apparent from  FIG. 42  that when the tip  540  reaches the bottom of the pilot bore  570 , the oblique cutting edges  546 ,  548  will have cut a small bevel  578  at the opening of the bore  574  which facilitates the insertion of an associated template such as template  552  and, eventually, of an associated implant such as implant  580  shown in  FIG. 44 . 
   The exemplary implant  5  of  FIGS. 44 ,  45  corresponds in overall configuration to that of  FIG. 13 . At least two, in the embodiment shown, four projections  582 ,  584 ,  586 ,  587  project axially away from the lower end  590  of the core  592  at a radially inward spacing from the diameter of the core  592 . The important difference is in that the lower end  590 , instead of being generally flat, is concavely shaped in conformity with the shape of the bottom  576  of the finished enlarged bore  574 . As diagrammatically indicated in  FIG. 44 , if desired, the projections  582 - 588  may be provided with oblique, back biting fins previously described. This also applies to any of the corresponding projections of the implants previously described. 
   The bores of the socket for receiving the projections  582 - 588  are drilled using the guide channels  554 - 560  of the template of  FIG. 46 ,  47  and, possibly, also using a locking pin such as the pin  72  when the first of the four bores has been formed. Once the projection bores are drilled, the formation of the socket is finished and the socket is ready for the implant to be tapped in. 
   The advantage of the last described embodiment is seen in that, firstly, it avoids unnecessary removal of bone tissue. Also, the plate-like shape of the cutting plates allows for a much better access for the cooling water or other coolant while drilling the opening for the core of the implant. This further reduces damage to the bone tissue by burning which may occur in case of a relatively limited access of the coolant through the spiral grooves of the regular drill bit. 
   Those skilled in the art will readily appreciate that in the last described embodiment of the forming of the socket, the template of  FIGS. 46 and 47  is to be considered as an exemplary embodiment since other described templates with the correspondingly modified core of the implant can be used, all utilizing the reduced loss of bone tissue. Also, only one shaped bit  530  can be used eliminating the use of the bit  510 , even though the use of two separate bits is preferred. 
   The numerous embodiments of the implants and templates disclosed are to be considered as presently preferred embodiments of the invention which, however, can be modified to differ from what has been described, without departing from the scope of the present invention as set forth in the accompanying claims.