while the invention is susceptible to various modifications and alternative forms , specific embodiments thereof have been shown by way of example in the drawings and will be described in detail herein . it should be understood , however , that it is not intended to limit the invention to the particular forms disclosed , but , to the contrary , the intention is to cover all modifications , equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims . turning now to the drawings and referring first to fig1 - 3 , a dental implant has an externally threaded , substantially cylindrical main body 12 symmetrical around the longitudinal axis a -- a of the implant . the top end 14 of the implant 10 is fitted in a known way for receiving dental components useful for making a dental restoration , as well as a dental restoration itself . as can be seen in fig2 the illustrative implant 10 has an internal bore 16 which is internally threaded for receiving a machine - screw - like bolt ( not shown ) and , surrounding this bore , a hexagonal boss 18 which can be engaged for screwing the implant into a pre - drilled bore 19 in a patient &# 39 ; s jaw bone j . the hexagonal boss 18 also forms part of a non - rotational coupling between the implant 10 and the above - mentioned components and restoration . the boss 18 is surrounded by an annular shoulder 20 against which such components and restoration may abut when attached to the implant 10 . the bottom end 21 of the implant 10 has four axially elongated recesses or cut - outs 22 , 24 , 26 and 28 , spaced symmetrically 90 degrees apart around the longitudinal axis of the implant 10 . these four recesses form four cutting edges 32 , 34 , 36 and 38 which are effective to self - tap the implant 10 into the pre - drilled bore 19 when the implant 10 is turned counterclockwise around the longitudinal axis as viewed in fig4 ( in the direction indicated by the curved arrow b ). as seen most clearly in the enlarged fig4 the cutting edges 32 , 34 , 36 and 38 are all within the circular locus 40 of the main body 12 , so that the threads on the main body following the self - tapping threads into the bore 19 will engage in the side wall of the bore 19 and thereby prevent wobbling of the implant 10 as it is screwed into the patient &# 39 ; s jaw bone j . the main body portions 42 , 44 , 46 and 48 that remain between adjacent pairs of the recesses 22 - 28 have elongated circumferential thread segments 52 , 54 , 56 and 58 , respectively . these thread segments are actually milling tools in that they are used to cut internal threads into the preformed bore of the bone . as such , thread segments 52 , 54 , 56 , and 58 are actually modified thread segments because of the additional function they perform . if these modified thread segments had constant major and minor radii , they could rub on the interior wall of the bore 19 as the implant 10 is threaded into that bore 19 . to reduce such rubbing friction between the segments 52 - 58 and the jaw bone j , the major and minor radial dimensions of each of these segments are progressively reduced substantially immediately following the relevant cutting edge , and this reduction continues to the trailing edge of the segment . for example , looking at the body portion 42 , the major radius of each segment 52 is progressively reduced in size from the recess 22 to the recess 24 so that the outer surface of this segment takes on an inwardly - curving contour . fig4 shows a set of such contours , on successively smaller diameters proceeding toward the bottom end of the implant 10 , due to the fact that the bottommost portion 30 of the self - tapping segment of the implant tapers to a smaller size toward the extremity 23 of the bottom end 21 . accordingly , each tapping thread segment , regardless of the diameter of its circumferential locus , is provided with this friction - reducing structure . as illustrated by the innermost broken - line arc in fig4 the major radius of curvature of each thread segment is smaller than that of the implant body 12 and the bore hole 19 . as can be seen in fig4 this configuration confines the engagement between the peak of the self - tapping thread and the bone to the cutting edges of the tapping thread segments . thus , the torque applied to the implant 10 to thread it into the bone is utilized primarily to cut a thread into the bone , and not to overcome friction between the bone and non - cutting major diameter surfaces of the implant . consequently , the torque required to be applied to the implant 10 to effect self - tapping is reduced , as is the time required to install the implant 10 . fig5 is a helical section view of the implant 10 showing the peaks and the troughs of four successive thread segments 52 , 54 , 56 and 58 corresponding to one thread turn , and their associated four body portions 42 , 44 , 46 and 48 . in fig5 the reference numerals 52a , 54a , 56a and 58a indicate the peak , or major diameter , of the thread segments following the respective relevant cutting edges 32 , 34 , 36 and 38 . reference numerals 52b , 54b , 56b and 58b indicate the trough , or minor diameter , of each of those respective thread segments . in accordance with the invention , the troughs 52b , 54b , 56b and 58b , as well as the peaks 52a , 54a , 56a and 58a , of these thread segments move toward the implant axis thereby reducing the minor diameter as well as the major diameter of each segment , immediately following the relevant cutting edge . the circular dashed line 64a shows the cutting locus of the peak of the cutting edge 32 , while the circular dashed line 66a of relatively smaller radius shows the cutting locus of the trough of the same cutting edge 32 . as is evident from these locus lines , both the peak 52a and the trough 52b of the thread segment 52 are &# 34 ; friction relieved &# 34 ; away from the interior of the bore immediately following the cutting action of the cutting edge 32 . similarly the peak 54a and the trough 54b of the thread segment 54 are &# 34 ; friction relieved &# 34 ; away from the interior of the bore immediately following the cutting action of the cutting edge 34 , as is apparent from the major and minor locus lines 64b and 66b . it is also apparent from the major and minor locus lines 64b and 66b of the second cutting edge 34 are larger than the major and minor radii 64a and 66a of the first cutting edge 32 . the third thread segment 56 includes a peak 56a and trough 56b with still larger radii and its cutting edge 36 cuts on these larger radii as is indicated by the major and minor locus lines 64c and 66c . thread segment 56 is similarly &# 34 ; friction relieved &# 34 ; at both its peak 56a and its trough 56b the fourth thread segment 58 shown in fig5 has similar properties , as do the thread segments of successive thread turns which follow on increasingly larger radii in the bottom end 21 of the implant 10 . it should be noted that the amount of reduction in the major radius is independent of the reduction in the minor radii . thus , the major radii may be reduced more or less than the minor radii . although the frictional relief is shown starting immediately after the cutting edges , the frictional relief may also start a short distance behind the cutting edge . furthermore , the relief may be accomplished by other methods than by reducing the major and minor radii . for example , the reduction in the pitch radius which has been discussed thus far is typically measured from the longitudinal axis of the body of the implant . after the cutting edge , the major and minor radii measured from the longitudinal axis are reduced . however , if the axis from which the radius is measured is offset from the longitudinal axis , then relief may accomplished in this manner as well . in this way , the radius after the cutting edge is not reduced , but instead is merely measured from an axis offset from the longitudinal axis of the implant to achieve the desired relief . the invention may be further understood with the aid of fig6 illustrating the implant 10 with an external thread 90 having a major radius 92 and a minor radius 94 . the line a -- a represents the longitudinal axis of the implant 10 . by definition , the &# 34 ; pitch radius &# 34 ; of the thread 90 is located on the line 96 on which the thread width w of each turn is equal to the space s between adjacent turns . this property is not affected by changing the major radius 92 alone , as is indicated by the sloped line b -- b , which shows a progressive reduction of the major radius from one thread turn to the next . on the other hand , if both the major radius 92 and minor radius 94 of the thread 90 are reduced together , no other change in the thread parameters being made , the pitch radius 96 is also reduced as can be seen if the minor radius is reduced along line c -- c . thus , as is shown in fig1 and 4 , the bottom portion 30 of the implant 10 tapers to a smaller size toward the extremity 23 in a shape which reduces both the major and the minor radii of the thread , thereby reducing the pitch radius as well . referring again to fig5 the progressive reduction of both the major and the minor radii of each thread segment 52 , 54 , 46 and 58 brings about a progressive reduction of the pitch radius of that thread segment . consequently , in the illustrated embodiment , the pitch radius of each thread segment is gradually reduced from the cutting edge to the trailing edge of that thread segment . as can be seen most clearly in fig1 the lower portion 30 of the implant 10 is tapered to a smaller diameter than the main body 12 over an axial distance encompassing at least the last four thread turns , which are numbered ( 1 ), ( 2 ), ( 3 ) and ( 4 ), respectively , beginning at the apical end surface 23 . in order to maintain a substantially constant thread depth along the length of the tapered portion 30 , both the minor and major radii of the thread ( from the axis of the implant ) increase progressively between the narrow end and the wide end of the tapered portion 30 ( see fig1 and 4 ). as can be seen in fig1 the bore 19 has an internal radius which is smaller than the major ( outer ) radius of the first thread turn ( 1 ). the first thread turn ( 1 ) therefore starts cutting into the sidewalls of the bore 19 immediately upon introduction of the implant 10 into the bore by rotating the implant about its axis while pressing it axially into the bore . as the implant 10 is advanced into the bore 19 , the successive cutting edges 32a , 34a , 36a and 38a formed along the first turn ( 1 ) of the thread cut progressively deeper into the bore wall . the depth of the cut made by the first cutting edge 32a is illustrated by the broken line emerging from the tip of that cutting edge in fig4 . as can be seen in fig4 because of the taper of the lower portion 30 of the implant 10 , each of the cutting edges 32b , 34b , 36b and 38b in the second turn ( 2 ) of the implant thread has a larger radius than the corresponding cutting edge in the first turn ( 1 ). similarly , the cutting edges 32c - 38c in the third turn ( 3 ) and edges 32d - 38d in the fourth turn ( 4 ) all progressively increase in radius so that they cut progressively deeper into the bore wall as the implant 10 is threaded into the bore 19 . fig7 a - 7c illustrate three sectional cuts through two longitudinally adjacent modified thread segments 101 and 102 in the tapered end of the implant . fig7 a is a sectional view immediately behind the cutting edge of the modified thread segments 101 and 102 . fig7 b is a sectional view between the cutting edge and the trailing end of the thread segments 101 and 102 . fig7 c is a sectional view near the trailing end of the thread segments 101 and 102 . in essence , fig7 a - 7c progressively show the thread segments 101 and 102 at different circumferential locations from a point near their cutting edges to a point near their trailing ends . line d -- d represents the longitudinal axis of the implant . line 104 is the major radius of thread segment 101 immediately following its cutting edge . line 106 is the minor radius of thread segment 101 immediately following its cutting edge . line 106 coincides with the radius of trough 112 . thread segment 102 is closer to the apical end of the implant than thread segment 101 . this is shown by the relative radii of troughs 110 , 112 , 114 with respect to longitudinal axis d -- d . trough 114 has the smallest radius while trough 110 has the largest radius with the radius of trough 112 falling therebetween . this is also shown by the relative radii of the peaks of thread segments 101 and 102 . the dashed lines represent the current invention . as can be seen by the solid - line trough 112 , the radius of the floor of the trough 112 progressively increases due to the fact that each thread segment in the tapered portion is spiraling outward away from the longitudinal axis d -- d of the implant so as to eventually merge with the threads in the non - tapered body of the implant which typically have larger major and minor radii . the present invention offsets this rising floor of the trough 112 by cutting further into the implant body to produce trough 112 &# 39 ; shown in dashed lines in fig7 b - 7c . this cutting produces additional relief and effectively modifies the pitch radius such that it remains constant or even decreases in the tapered end portion . thus , as bone tissue is initially cut by the cutting edge of the present invention , it does not continuously encounter a progressively rising trough 112 which would then compact the bone into the sidewalls of the bore of the bone . instead , it encounters reduced - radius trough 112 &# 39 ;. continuous compression of the bone tissue can lead to trauma of that bone tissue . the present invention minimizes the likelihood of such trauma . moreover , as an implant lacking the inventive reduction in minor radius is turned into the bore , bone tissue entering trough 112 between the two longitudinally adjacent thread segments 101 and 102 would confront not only a rising floor due to the normally increasing minor radius associated with the taper , but it also would encounter a reduced area through which it must pass . on the other hand , as the minor radius decreases to produce trough 112 &# 39 ;, the tool which cuts the progressively deeper trough 112 &# 39 ; extends further inward toward the longitudinal axis d -- d . consequently , the sides of the thread segments 101 and 102 are also cut by the tool to produce additional area between two longitudinally adjacent thread segments 101 and 102 as is shown by the dashed lines . thus , the area between the two longitudinally adjacent threads segments 101 and 102 increases as does the volume therebetween . this increase allows for the bone tissue to adequately occupy this volume , but not so much so that the bone tissue is compressed to a point at which trauma may occur . in the preferred embodiment of this invention , the minor radius at trough 112 &# 39 ; decreases in a manner to at least offset the reduction in area due to the normal increasing minor radius associated with the tapering at the lower portion of the implant . in each thread turn the four cutting edges 32 , 34 , 36 , 38 are equally spaced around the longitudinal axis of the implant 10 , which contributes to axial stability from the start of the threading of the implant 10 into the bore 19 . to further contribute to such stability , each succeeding cut increases the depth by only a small increment ( typically 0 . 001 inch ), which correspondingly restrains the increase of friction to small incremental increases . the initial cut made by the first cutting surface to engage the bone ( cutting surface 32a in the implant of fig1 - 4 ) is slightly deeper than the incremental increase in the cutting depth effected by each successive cutting surface . by the time the entire tapered portion 30 has entered the bore 19 , the path of the implant 10 into the bore is stably established . it should be noted that the successive increases in the cutting radius occur not only in the first thread turn , but also in each successive thread turn . thus , in the illustrative implant 10 in which the taper extends over four thread turns , and in which the four recesses provide four incremental increases in the cutting radius in each thread turn , there are a total of 16 incremental increases in the cutting radius along the entire length of the taper . of course , the tapered portion of the implant 10 may extend over any desired number of thread turns , either greater or fewer than the four - thread turns in the tapered region of the implant of fig1 - 4 . also , the thread itself may be either a right - hand thread or a left - hand thread , and may be either a single - lead thread ( as illustrated ) or a multiple - lead thread . it will also be appreciated that the number of recesses included in the implant may be greater or fewer than the four recesses included in the implant of fig1 - 4 . for example , fig8 and 9 illustrate a narrow implant 67 that has only two recesses 68 and 69 , diametrically opposed , because of the smaller diameter of the implant . fig1 and 11 illustrate a wide implant 70 that includes six recesses 71 - 76 equally spaced around the axis of the implant , because of the larger diameter of the implant . fig1 and 13 illustrate an implant 80 in which the recesses 81 - 84 are at least partially closed at both ends rather than extending through to the apical end of the implant as in the implant 10 of fig1 - 6 . the closed - end recesses 81 - 84 direct the bone particles from the tapping operation to the side wall of the pre - drilled hole rather than to the end of the hole , to perhaps facilitate bonding of the bone to the implant 80 . although the cutting surface formed by each of the recesses described above lies in a radial plane extending parallel to the axis of the implant , the plane of the cutting surfaces may be tilted as illustrated in the fig1 to form an acute angle α in transverse cross - section , forming a chisel - like corner c at the outermost edge of the cutting surface . that is , the recess wall r that forms the cutting edge forms an acute angle α with the thread segment t that trails that cutting edge . this acute angle α concentrates the force applied to the bone during the self - tapping operation in a smaller area of the bone , thereby reducing the amount of torque required to be applied to the implant to effect self - tapping . to concentrate the applied force in an even smaller area of the bone , the plane of the cutting surface s may also be tilted as illustrated in fig1 to form an acute angle β in longitudinal cross - section or side elevation , forming a sharp point p at the distal end of the outermost edge of the cutting surface . if desired , the cutting surface may be made curvilinear in either or both of the transverse and longitudinal sections , as illustrated by the broken lines in fig1 and 15 , to even further concentrate the force applied to the bone by the implant during self - tapping .
a self - tapping dental implant comprises a generally cylindrical body with a threaded outer surface for securing the implant to the wails of a preformed hole in a jaw bone . the cylindrical body has a plurality of longitudinal recesses formed in the threaded surface at one end and extends longitudinally through a plurality of turns of the thread to form a self - tapping cutting edge at each interruption of the thread by one of the recesses . each thread segment extends between a pair of adjacent recesses and diminishes in radius between the leading and trailing ends of the thread segment .