Abstract:
A dental implant that includes a cylindrical body which can be positively secured against micromotion within a bore in a jawbone by a spindle-shaped expansion mechanism and further secured against the contamination by microorganism through a gap in an internal channel of tubular portion by a compressive contact mechanism.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a dental implant for preparing tooth prosthesis. More specifically, this invention relates to a dental implant having the function for immediate loading after implant placement. 
     2. Description of the Prior Art 
     Modern dental implants are designed based upon the biological fact that a titanium-alloy implant and a bone integrate with each other very strongly, bearing a tensile strength more than 100 kg. This fact was found by Branemark of Sweden in 1969, and is called Osseointegration. Typical such osseointegrated implants comprise a tubular body portion called a fixture which is emblaced within a bore drilled in the bone. 
     As shown in FIG. 9, the dental implant  110  developed by Branemark is composed of a fixture  111 , an abutment  112 , a gold cylinder  113 , an abutment screw  114 , and a gold screw  115  (totally five pieces). The fixture  111  has a tublar body and is threaded externally, and when implanted in bone it is perfectly osseointegrated with alveolar bone after three to six months. The abutment  112  connects the fixture  111  to the gold cylinder  113  through soft tissue. The gold cylinder  113  connects the abutment  112  to a prosthesis (called an upper structure). The abutment screw  114  fixes the abutment  112  to the fixture  111 , and the gold screw  115  fixes the upper structure to the abutment  112  through the gold cylinder  113 . 
     In the method developed by Branemark for using the dental implant  110 , it has been ruled out to load by connecting the prosthesis to the implant until three to six months after placing an implant in the alveolar bone under mucous membrane, in order to keep the implant at rest. This is called delayed loading protocol. It was reported that micromotion of the implant was produced by omitting the delayed loading period and when the amount of micromotion exceeded 100 μm, some disorder was caused on the surface between the implant and the bone. On the other hand, in Branemark&#39;s method, even solid denture is contraindicated during the delayed loading period of three to six months after implant placement, as its protocol attaches importance for keeping the implant at rest during the period. Since this means to lengthen the necessary period for operation and to bring upon patient&#39;s pain, it has been regarded as a problem to be solved. 
     In order to overcome the drawbacks, it is indispensable to load immediately after implant placement without rest period. Hence, an effective countermeasure to prevent micromobility of the implant becomes necessary. Several types of implants using the mechanical locking means for securing the implant in place within the bore in the jawbone have been suggested. These types of implants have an expansion screw in the internal channel of the fixture, the lower half of the fixture being cut in to the end and divided into several legs, and when the expansion screw is rotated the legs are expanded radially and outwardly, causing the anchoring effect of the fixture in bone. The method to load immediately after implant placement is called immediate loading, and the design to secure the anchoring effect in bone by expanding the legs of the implant in immediate loading is called apical expansion design. 
     Referring to FIG. 10, the first dental implant  120  of apical expansion design issued in U.S. Pat. No. 2,721,387 to Ashuckian (1955) has spindle-like form imitating the socket of the extracted tooth. In the internal channel of the fixture  121  of the dental implant  120 , an expansion screw  122  is inserted. By rotating the expansion screw  122 , an expansion nut  123  is drawn upwards, thereby spreading apart two legs which are formed by dividing the lower half of the fixture  121 . Thus, the fixture  121  is anchored in bone so as to prevent micromotion of the implant. 
     As shown in FIG. 11, the dental implant  130  of apical expansion design issued in U.S. Pat. No. 3,708,883 to Flander (1973) has the structure that the lower half of a cylindrical fixture  131  is divided into two legs and a frustoconical head  132  is attached on one end of an expansion screw  133  which is inserted into the internal channel of the fixture  131 . By rotating a square nut  134  threaded into the other end of the expansion screw  133 , the expansion screw  133  is drawn upwards together with the head  132  and the legs of the fixture  131  are expanded. 
     As shown in FIG. 12, the dental implant  140  of apical expansion design issued in U.S. Pat. No. 5,087,199 to Lazarof (1992) has the structure that the lower half of a cylindrical fixture  141  is divided into plural legs and an expansion screw  142  having a conical head  143  is inserted into the internal channel of the fixture  141 . By rotating the expansion screw  142 , the point of the conical head  143  goes down into the hollow  144  at the internal center of the lower portion of the fixture  141 , and the legs of the fixture  141  are expanded. 
     Further, as shown in FIG. 13, the dental implant  150  of apical expansion design issued in U.S. Pat. No. 5,489,210 to Hanosh (1996) has the structure resembling closely to the dental implant  140  in that the lower half of a cylindrical fixture  151  is divided into plural legs and an expansion screw  152  having a conical head  153  is inserted into the internal channel of the fixture  151 . By rotating the expansion screw  152 , the point of the conical head  153  goes down into the hollow  154  at the internal center of the lower portion of the fixture  151 , and the legs of the fixture  151  are expanded. 
     These types of implants of apical expansion design, as described above, have codimon mechanism that they have an expansion screw in the internal channel from the head to the lower end of the cylindrical fixture, the lower half of the fixture being cut in to the end and divided into plural legs, and when th e expansion screw is rotated are expanded radially and outwardly, causing anchoring effect in bone preventing micromotion of the implant. 
     However, these designs have the risk of micro-leakage of bacteria called microorganisms from the head of the implant exposed in patient&#39;s intraoral cavity to the bottom of the implant placed in bone, passing through the micro-gap between the male threads of the expansion screw and the female threads of the internal channel of the fixture. Therefore, the dental implants of these types of apical expansion design may fail unless precautions attention is paid to potential contamination in the apical region, which could cause serious damage to periimplant bone crucial to achieve osseointegration, so that they do not have any practical value. 
     An improved dental implant  160  of apical expansion design is described in U.S. Pat. No. 5,681,187 issued to Lazarof (1997). The dental implant  160 , as shown in FIG.  14 (A), has a cylindrical fixture  161 , the lower half of which is divided into plural (four) blade-like legs. The fixture  161  has a circular ring-shaped internal shoulder  162  at the upper middle in the internal channel of the fixture  161 . A screw head  163 , the outer diameter of which is larger than the inner diameter of the internal shoulder  162 , and a male screw portion  164  which is inserted through the internal shoulder  162  constitute an expansion screw  165 . 
     When the expansion screw  165  is rotated, a frustoconically-shaped expansion nut  166  is drawn toward the middle of the implant with the result that, as shown in FIG. 14 (B), the legs  167  of the fixture  161  located within the socket of the extracted tooth are spread apart, thereby anchoring the dental implant  160  in bone so as to prevent micromotion. 
     By means of the structure of the dental implant  160  of apical expansion design described above, when the expansion screw  165  is rotated and the frustoconically-shaped expansion nut is drawn upwards, then the legs  167  of the fixture  161  are expanded and reactive forces to return the legs  167  to their original form work against active forces to expand the legs  167  outwardly. These reactive forces work so as to compress the screw head  163  of the expansion screw  165  to the internal shoulder  162  of the fixture  161 , causing a close contact between the both. This is said to be effective for preventing microleakage of bacteria from the head portion of the dental implant  160  to the other end through the internal channel of the fixture  161 . 
     In the actual product of the dental implant  160 , an additional structure is added to reinforce the effect described above (Lazarof S, Hobo S, Nowzari H: “The immediate load implant system”, Quintessence Pub. Co., 1988, Tokyo). As shown in FIG. 15, the expansion screw  165  has a 15° reverse bevel  168  on the periphery of the apical side of its crew head  163 , forming a circular incisal edge  169 . When the expansion screw  165  is tightened to perform apical expansion, the incisal edge  169  of the reverse bevel  168  is compressed in close contact against the upper surface of the internal shoulder  162  of the fixture  161  and deformed by strong compressive force to thereby cause a cold welding effect, so that the passway for micro-leakage of bacteria is sealed. As a result, the effect to prevent microorganisms is reinforced. 
     However, according to recent clinical experiences, it has become clear that the effect of the dental implant  160  of apical expansion design, shown in FIG. 14, involves the following drawbacks. 
     As shown in FIG.  14 (B), when the expansion screw  165  of the dental implant  160  is rotated and the expansion nut  166  is drawn upwards, the blade-shaped legs  167  formed by dividing the lower half of the fixture  161  are expanded and the points of the blade-shaped legs  167  cut in deep into peripheral bone and fixed. In that case, it cannot be avoided that mechanical stresses are concentrated on the points of the blade-shaped legs  167  and partly damage peripheral bone. Further, in the similar condition, when a strong torque is applied on the head  163  of the dental implant  160 , twisting force is produced between the base and the point of each blade-shaped leg  167  of the fixture  161 , and the legs  167  sometimes are broken at their bases. Still further, horizontal micromotion of the implant  160  is apt to be produced by a horizontal force applied on the head of the fixture  161  in case that the points of the blade-shaped legs  167  are fixed and the upper body of the fixture  161  is unstable. 
     On the other hand, during the functioning of the dental implant such as mastification, an occlusal force of 60 kg at maximum is applied downwards on the head  163  of the dental implant  160  exposed in the oral cavity. Since the sum of the cross sectional areas of the threaded shank  164  of the expansion screw  165  and the expansion nut  166  is far larger than that of the legs  167  of the fixture  161 , when an occlusal force is applied, the expansion screw  165  and the expansion nut  166  remain at the almost initial position, while the legs  167  of the fixture  161  cut in deep into the bone and move downwards. As a result, the expansion screw  165  moves upwards relative to the fixture  161 , making the base of the head  163  of the expansion screw  165  and the upper surface of the internal shoulder  162  of the fixture  161  set apart from each other and destroying the cold welding effect. Consequently, micro-leakage of bacteria through the slight gap between the both is invited. A clinical statistics showed that contamination by microorganisms of the dental implant  160  has been observed with a 3% occurrence rate. 
     Therefore, to overcome the drawbacks of the dental implant  160  described above, the following problems have to be resolved. 
     (1) Stresses concentrate on the points of the legs  167  when the legs  167  are expanded, and a twisting force is produced between both ends of each leg  167  when a torque is applied on the head of the fixture  161 , while horizontal micromotion of the fixture  161  occurs when a horizontal force is applied on the head of the fixture  161 . 
     (2) Micro-leakage of bacteria occurs when an occlusal force is applied downwards on the head of the implant. 
     OBJECT OF THE INVENTION 
     To overcome the drawbacks of the prior art described above, the object of the present invention is to provide a dental implant which has necessary and sufficient features as follows. 
     1) Mechanical stresses do not concentrate on the points of the legs of the dental implants when the legs are expanded; and any twisting force is not produced between both ends of each leg when a torque is applied on the head of the dental implant, and further horizontal micromotions do not occur when a horizontal force is applied on the head of the dental implant. 
     2) Micro-leakage of bacteria does not occur when an occlusal force is applied downwards on the head of the dental implant. 
     SUMMARY OF THE INVENTION 
     In the present invention, there is provided a dental implant comprising: 
     a cylindrical body receivable within a bore provided in a jawbone of a patient and having an internal channel from an upper end to an under end of the cylindrical body, including an annular internal shoulder at an upper middle of the internal channel and a skirt portion on an under half of the cylindrical body, plural slits being cut in on the skirt portion so as to form plural blade-like portions in the axis direction of the cylindrical body, female threads being formed on an internal surface of upper portion of the cylindrical body, 
     a nut body including a cylindrical portion inserted into a circular edge of the skirt portion of the cylindrical body and an inner cavity having female threads, 
     a first bolt body including on its upper end a head whose outer diameter is larger than an inner diameter of the internal shoulder of the cylindrical body and including on its under side a shank whose outer diameter is smaller than the inner diameter of the internal shoulder of the cylindrical body, the shank having male threads externally and being engaged with the female threads of the nut body, 
     a connecting body having an internal channel from an upper end to an under end of the connecting body, including an internal shoulder near the under end of the connecting body, the connecting body having an inner diameter smaller than an outer diameter of head of the cylindrical body and larger than an inner diameter of the internal channel of the cylindrical body, the head of the cylindrical body being slided into an under end of the internal channel of the connecting body, head of the connecting body being embodied with a prosthesis, and 
     a second bolt body including a head whose diameter is larger than an inner diameter of the internal shoulder of the connecting body and smaller than an inner diameter of portion of the internal channel of the connecting body positioned above the internal shoulder of the connecting body, and including a shank having male threads externally and having an outer diameter which is smaller than the inner diameter of the internal shoulder of the connecting body, the male threads of the second bolt body being engaged with the female threads provided on the internal surface of upper portion of the cylindrical body, 
     so that inserting the first bolt body into the internal channel of the cylindrical body through the internal shoulder of the cylindrical body and rotating the first bolt body while engaging the male threads of the shank of the first bolt body with the female threads of the nut body causes swelling of the skirt portion of the cylindrical body into a spindle-like shape and causes close contact between a lower surface of the head of the first bolt body and an upper surface of the internal shoulder of the cylindrical body, and 
     so that inserting the second bolt body into the internal channel of the connecting body through the internal shoulder of the connecting body, rotating the second bolt body while engaging the male threads of the shank of the second bolt body with the female threads of the internal channel of the cylindrical body and producing close contact between an under surface of the head of the second bolt body and an upper surface of the internal shoulder of the connecting body causes embodiment of the cylindrical body with a prosthesis through the connecting body. 
     As apparent from the above, the present invention is not based on the apical expansion design of the prior art, and is based on the spindle-shaped expansion mechanism. By inserting the nut body into the circular edge of the skirt portion of the cylindrical body (hereinafter often referred to as “fixture”), the circular edge of the skirt portion of the fixture is fixed. By rotating the first bolt body fitted into the nut body, its rotating force (torque) is converted to a compressive force applied between both ends of the blade-like portions. In the prior art, the legs of the fixture are expanded by the rotation of the expansion screw. On the other hand, in the present invention, the blade-like portions are swelled into spindle-like shape by rotating the first bolt body (hereinafter occasionally referred to simply as “bolt body”) and applying the compressive force in the direction to shorten the distance between both ends of the blade-like portions. This is a remarkable difference between the prior art and the present invention. 
     In the present invention, the blade-like portions compress peripheral bone by swelling into a spindle-like shape, and its reactive force prevents the micro-motion of the dental implant. At that time, since the compressive force acting on peripheral bone works with the outwardly facing planar abdominal region of the blade-like portions which form a spindle-like shape, the blade-like portions would not cut into bone and mechanical stresses would not be concentrated anywhere. Therefore, the damaging effect on peripheral bone which occurs in the prior art by the concentration of stresses can be avoided. In addition, since the lower ends of the blade-like portions are linked by the circular edge of the skirt portion of the cylindrical body and fixed by the nut body, they constitute a rigid spindle-shaped structure. Therefore, the break-down of blade-like portions at their bases by twisting force does not occur. Finally, since the spindle-like structure is held firmly in peripheral bone as one body, horizontal micromotion can also be avoided. 
     During the functioning of the dental implant such as mastication, as previously described, an occlusal force of 60 kg at maximum could be loaded downwards on the exposed head of the dental implant in the oral cavity. In the present invention, the stress is applied on the both ends of each blade-like portion in the direction to shorten the length of a string of blade-like portion which is bent like a bow by rotating the bolt body. Comparing under the same strength of occlusal force, the relative displacement of the head of the bolt body to the cylindrical body (fixture) is far smaller in the present invention than in the prior art. As a result, the possibility that the base of the head of the bolt body and the upper surface of the internal shoulder of the fixture are set apart from each other is far smaller in the present invention than in the prior art. Therefore, the preventive effect against the micro-leakage of bacteria through the micro-gap in the internal channel of the fixture is remarkably reinforced in the present invention. 
     Preferably, the nut body may be furnished with a head having an outer diameter larger than that of the cylindrical portion of the nut body, the nut body having a maximum diameter substantially equal to an outer diameter of the circular edge of the skirt portion of the fixture. Further, the outer diameter of the cylindrical portion of the nut body is slightly larger than an inner diameter of the circular edge of the skirt portion of the fixture, so that the cylindrical portion of the nut body can be pressed into a cavity of the skirt portion of the fixture to thereby enable fixing the nut body to the skirt portion of the fixture. 
     Since the compressive force is applied in the longitudinal direction of the blade-like portions by tightening the first bolt body and then the blade-like portions could be deformed irregularly, it is preferred that an outward bulge, an inner concave dent or a narrowed portion be formed in the middle of the blade like portions of the fixture. As a result, the blade-like portions can be guided outwardly by the outward bulge, the inner concave dent or the narrowed portion and deformed in an approximately uniform bow-like shape each to thereby swell into a spindle-like shape as a whole. 
     Preferably, a circular washer composed of a metal softer than the fixture or the first bolt body may be interposed between an upper surface of the internal shoulder of the fixture and a lower surface of the head of the first bolt body so as to cause a cold welding effect on the both surfaces of the circular washer. Thus the passway of bacteria is more effectively sealed to thereby reinforce the preventive effect against microorganisms. It is preferable to use, for example, pure titanium as the metal of the circular washer. 
     An additional threaded body having male threads on its external surface may preferably be engaged with the female threads provided on the internal surface of upper portion of the fixture, the additional threaded body having a convex jut on its lower surface, the head of the first bolt body having its upper surface furnished with a concave dent which fits closely with the convex jut. By strongly tightening the additional threaded body, the compressive force between the upper surface of the internal shoulder of the fixture and the lower surface of the head of the first bolt body is reinforced, the both being contacted more closely and the contact area of the both being increased more. Thus, the passway of bacteria is more effectively sealed so as to reinforce the preventive effect against microorganisms. 
     In place of the additional threaded body, preferably, a lower end of the shank of the second bolt body may be roundly pointed, and the length of the shank of the second bolt body may be made so long that the lower end of the shank of the second bolt body reaches and presses strongly an upper surface of the head of the first bolt body when the second bolt body is rotated while engaging the male threads of the shank of the second bolt body with the female threads on the internal surface of the fixture. Thus, a close contact is realized between an under surface of the head of the second bolt body and an upper surface of the internal shoulder of the connecting body. By strongly tightening the second bolt body, the compressive force between the upper surface of the internal shoulder of the fixture and the under surface of the head of the first bolt body is reinforced, the both being contacted more closely and the contact area of the both being increased more. As a result, the passway of bacteria is more effectively sealed so as to reinforce the preventive effect against microorganisms. 
     Preferably, the lower end of the shank of the second bolt body may be composed of a metal softer than the first bolt body or other part of the second bolt body so as to strengthen the compressive force between the upper surface of the internal shoulder of the fixture and the under surface of the head of the first bolt body. Thus, when strongly tightening the second bolt body, the passway of bacteria is more effectively sealed so as to reinforce the preventive effect against microorganisms. It is preferable to use, for example, pure titanium as the metal of the lower end of the shank of the second bolt body. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which: 
     FIG. 1 is a decomposed sectional view showing the first preferred embodiment of the dental implant of the present invention. 
     FIG.  2 (A) is a sectional view of the dental implant shown in FIG. 1 before swelling the blade-like portions of the fixture, and FIG.  2 (B) is a sectional view of the dental implant shown in FIG. 1 after swelling the blade-like portions of the fixture into a spindle-like shape. 
     FIG.  3 (A) is a partial sectional view of a modified example of the dental implant shown in FIG. 1, and FIG.  3 (B) is a partial sectional view of another modified example of the dental implant shown in FIG.  1 . 
     FIG. 4 is a mechanical model showing the force conditions applied to the blade-like portions of the dental implant of the present invention (A) and that of the prior art (B). 
     FIG. 5 is a decomposed sectional view showing the second preferred embodiment of the dental implant of the present invention. 
     FIG. 6 is a decomposed sectional view showing the third preferred embodiment of the dental implant of the present invention. 
     FIG. 7 is a decomposed sectional view showing the fourth preferred embodiment of the dental implant of the present invention. 
     FIG. 8 is a decomposed sectional view showing a possibility of using a method other than the preferred embodiments of the dental implant of the present invention. 
     FIG. 9 is a makeup drawing showing an example of dental implant of the prior art. 
     FIG. 10 is an illustrating drawing showing another example of dental implant of apical expansion design of the prior art. 
     FIG. 11 is an illustrating drawing showing a further example of dental implant of apical expansion design of the prior art. 
     FIG. 12 is an illustrating drawing showing still a further example of dental implant of apical expansion design of the prior art. 
     FIG. 13 is an illustrating drawing showing still a further example of dental implant of apical expansion design of the prior art. 
     FIG.  14 (A) is a sectional view showing still a further example of dental implant of apical expansion design of the prior art before apical expansion, and FIG.  14 (B) is a sectional view showing that after apical expansion. 
     FIG. 15 is a sectional view showing a preventive structure against microorganisms of a dental implant of apical expansion of the prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Preferred embodiments of the present invention will now be described with reference to the attached drawings. Throughout FIGS. 1-15, like parts in view of the function thereof are designated by like numerals to omit repeated description thereof. 
     The First Preferred Embodiment 
     FIG. 1 shows the dental implant  1  according to the first preferred embodiment of the present invention. This dental implant comprises a cylindrical body  10  called a fixture, a nut body  20 , a first bolt body  30 , a connecting body  40 , and a second bolt body  50 . 
     The fixture  10  is composed of a titanium-alloy and has external male threads  12  near the head  11 . In addition, the fixture  10  includes a circular internal shoulder  15  near the head  11  in the internal channel  14  passing through from the head  11  to a lower end (circular edge)  13 . Further, the fixture  10  includes plural blade-like portions (skirt portion)  17  formed by cutting in plural slits from the internal shoulder  15  to the point  13  with a circular edge  13  left at the end of the fixture  10 . In the middle of each blade-like portion  17 , an outward bulge  19  is formed. Instead of the outward bulge  19 , an inner concave dent  19   a  as shown in FIG.  3 (A) or a narrowed portion  19   b  as shown in FIG.  3 (B) may be formed. Further, female threads  18  are formed on the internal surface of upper portion of the fixture  10 . 
     The nut body  20  is composed of a titanium-alloy, and has internal female threads  21 . This nut body  20  includes a cylindrical portion  22  which has an external diameter slightly larger than the internal diameter of the end  13  of the fixture  10  and is pressed into the end  13  of the fixture  10 , and includes a head  23  having a maximum diameter approximately equal to the external diameter of the end  13  of the fixture  10 . 
     The first bolt body  30  is composed of a titanium-alloy. It includes a thread portion  32  having male threads  31 , which are engaged with the female threads  21  of the nut body  20 , and includes a head  33 . The head  33  of the first bolt body  30  has an external diameter larger than the inner diameter of the internal shoulder  15  of the fixture  10 . This first bolt body  30  is inserted into the internal channel  14  of the fixture  10  from the head  11 , and its male threads  31  are engaged with the female threads  21 . 
     The connecting body (abutment)  40  is composed of a titanium-alloy, and includes an internal channel  41  from the upper end to the under end. In addition, the connecting body  40  has an internal shoulder  42  near the under end. The inner diameter of the internal channel  41  under the internal shoulder  42  is smaller than the outer diameter of the head of the fixture  10  and larger than the inner diameter of the internal channel  14  of the fixture  10 . The head of the fixture  10  slides into the under end of the internal channel  41  of the connecting body  40 . Further, the upper portion of the connecting body  40  is embodied with a prosthesis. 
     The second bolt body (abutment screw)  50  is composed of a titanium-alloy. It includes a head  51 , the diameter of which is larger than the inner diameter of the internal shoulder  42  of the connecting body  40  and smaller than the inner diameter of the portion of the internal channel  41  above the internal shoulder  42  of the connecting body  40 , and includes a shank (thread portion)  52  having external male threads  53 , the outer diameter of which is smaller than the inner diameter of internal shoulder  42  of the connecting body  40 . Further, the male threads  53  of the second bolt body  50  are engaged with the female threads  18  on the internal surface of upper portion of the fixture  10 . 
     The dental implant  1  of the construction described above, as shown in FIG.  2 (A), is assembled by pressing the nut body  20  into the circular edge  13  of the fixture  10 , inserting the first bolt body  30  in the internal channel  14  of the fixture  10  and engaging the male threads  31  thereof with the female threads  21  of the nut body, followed by equipping with the connecting body  40  and the second bolt body  50 . After implanting the dental implant  1  within a bore of patient&#39;s jawbone, a rotating force (a torque) is applied on the head  33  of the first bolt body  30  by means of a proper tool such as a screw driver. 
     As described above, the internal shoulder  15  is provided in the internal channel  14  of the fixture  10 , and the head  33  of the first bolt body  30  has the outer diameter larger than the inner diameter of the internal shoulder  15 . Thus, the internal shoulder  15  plays the role of a stopper to prevent the downward motion of the head  33  of the first bolt body  30 . Therefore, when a rotating force (a torque) is applied on the head  33  of the first bolt body  30  for allowing the nut body  20  to come near the head  33  of the first bolt body  30 , the rotating force is converted to a force drawing up the circular edge  13  of the fixture  10  through the nut body  20 , because the head  33  of the first bolt body  30  does not move downwards. Hence, a compressive force in the axis direction of the fixture  10  is applied on the blade-like portions  17 , the both ends of which are fixed, so that as shown in FIG.  2 (B), the blade-like portions  17  swell into a spindle-like shape, curving around the outward bulge  19 , the concave dent  19   a  (FIG.  3 (A)) or the convex jut  19   b  (FIG.  3 (B)). As a result, the dental implant  1  is fixed within the bore of jawbone, compressing peripheral bone with the swelled blade-like portions  17 . 
     When the blade-like portions  17  swell into a spindle-like shape and compress bone, the compressive force acts at the abdominal region of the blade-like portions  17  and then stresses are not concentrated. Therefore, the damaging effect due to stress concentration caused in the prior art by the points of blade-like portions (legs) deeply cutting into bone can be avoided. In addition, since the ends of the plural blade-like portions  17  are fixed by the circular edge  13  of the fixture  10  and the nut body  20  and since the blade-like portions  17  constitute a firm spindle-shaped structure, the breakdown of the blade-like portions  17  by twisting force can be avoided. Further, since the spindle-shaped structure is held firmly in bone as one body, horizontal micomotion also can be avoided. 
     In the prior art, as shown in FIG.  4 (B), one end (marked ) of the blade-like portion (leg) is fixed, and the force is applied in the direction to expand the other end, almost perpendicularly to the chord of blade-like portion deformed into a bow-like shape. On the other hand, in the present invention, as shown in FIG.  4 (A), the both ends (marked ) of the blade-like portion  17  are fixed, and a compressive force is applied on the blade-like portion  17  in the direction almost parallel to the chord of blade-like portion  17  deformed into a bow-like shape. Hence, during the functioning of the dental implant such as mastication, when the same occlusal forces are applied downwards on the exposed head of the dental implant in the oral cavity, the amount of relative move of the head  33  of the bolt body  30  against the fixture  10  is far smaller in the present invention than in the prior art. Therefore, the possibility that the base  33   a  of the head  33  of the bolt body  30  and the upper surface  15   a  of the internal shoulder  15  of the fixture  10  are set apart from each other is far smaller, so that the superior preventive effect against the micro-leakage of bacteria to that of the prior art can be expected. 
     The Second Preferred Embodiment 
     FIG. 5 shows the dental implant  2  according to the second preferred embodiment of the present invention. In this implant  2  of the second preferred embodiment, a circular washer  60  composed of a metal softer than the first bolt body  30  or the fixture  10 , for example, pure titanium is inserted between the head  33  of the bolt body  30  and the internal shoulder  15  of the fixture  10 . 
     When the first bolt body  30  is firmly tightened to produce swelling of the plural blade-like portions  17  of the fixture  10  into a spindle-like shape, the circular washer  60  is compressed in close contact between the upper surface  15   a  of the internal shoulder  15  of the fixture  10  and the under surface  33   a  of the head  33  of the first bolt body  30  and deformed by strong compressive force, thereby causing a cold welding effect. Therefore, the passway for micro-leakage of bacteria is effectively sealed with the result that the effect to prevent microorganisms is reinforced. 
     The Third Preferred Embodiment 
     FIG. 6 shows the dental implant  3  according to the third preferred embodiment of the present invention. In this dental implant of the third preferred embodiment, a threaded body  70  is added to the dental implant  2  of the second preferred embodiment. The threaded body  70  has male threads  71  on its outer surface and a convex jut  72  on its lower surface. On the other hand, on the inner surface of the internal channel  14  of the fixture  10  closer to the head  11  than the internal shoulder  15 , female threads  19  with which the male threads  71  of the threaded body  70  are engaged are formed. Further, on the upper surface of the head  33  of the first bolt body  30 , a concave dent  34  which fits closely to the convex jut  72  of the threaded body  70  is formed. 
     Hence, when strong compressive force is applied downwards on the head  33  of the first bolt body  30  by tightening the threaded body  70 , the contact pressure between the under surface  33   a  of the head  33  of the first bolt body  30  and the upper surface  15   a  of the internal shoulder  15  of the fixture  10  increases with the result that the degree of close contact is enhanced and the preventive effect against microleakage of bacteria is reinforced. 
     The Fourth Preferred Embodiment 
     FIG. 7 shows the dental implant  4  according to the fourth preferred embodiment of the present invention. In this dental implant of the fourth preferred embodiment, the role of the threaded body  70  of the third preferred embodiment is replaced by the second bolt body  50 . The end  54  of the shank  52  of the second bolt body  50  is roundly pointed, and the shank  52  is lengthened until the end  54  of the shank  52  reaches and strongly presses the upper surface of the head  33  of the first bolt body  30  when the second bolt body  50  is rotated tightly while engaging the male threads  53  with the female threads  18  on the internal surface of the fixture  10 . Thus, a close contact is realized between the under surface of the head  51  of the second bolt body  50  and the upper surface of the internal shoulder  42  of the connecting body  40 . By strongly tightening the second bolt body  50 , the compressive force between the upper surface  15   a  of the internal shoulder  15  of the fixture  10  and the under surface  33   a  of the head  33  of the first bolt body  30  is reinforced, through the circular washer  60  if it is present. Thus, the contact thereof is more close and the contact area is increased so that the passway of bacteria is more effectively sealed to thereby reinforce the preventive effect against microorganisms. This fourth preferred emodiment is advantageous in reducing the number of parts by one as well as the number of processes, because the circular washer  60  of the third preferred embodiment can be omitted. 
     In the above fourth preferred embodiment, the end  54  of the shank  52  of the second bolt body  50  may be composed of a metal softer than the first bolt body  30  or other part of the second bolt body  50 . This strengthens the compressive force between the upper surface  15   a  of the internal shoulder  15  of the fixture  10  and the under surface  33   a  of the head  33  of the bolt body  30 , when strongly tightening the second bolt body  50 . Thus, the passway of bacteria is more effectively sealed so as to reinforce the preventive effect against microorganisms. It is preferable to use, for example, pure titanium as the metal of the end  54  of the shank  52  of the second bolt body  50 . 
     In the preferred embodiments described hereinbefore, the internal shoulder  15  formed in the internal channel  14  of the fixture  10  plays the role of the stopper to prevent the downward movement of the head  33  of the first bolt body  30 . Hence, when the first bolt body  30  is rotated while engaging it with the nut body  20 , the head  33  of the first bolt body  30  does not move relative to the fixture  10 , the nut body  20  is drawn upwards, and the blade-like portions (skirt portion)  17  are swelled into a spindle-like shape. However, the blade-like portions can be swelled by using a method other than the above preferred embodiments. 
     Such a method is shown in FIG.  8 . In FIG.  8 (A), the dental implant  5  is composed of a fixture  80 , a semi-spherical body  90  and a screw body  100 . 
     The fixture  80  is a cylindrical body composed of a titanium-alloy. Male threads  82  and female threads  83  are provided near the head  81  on its outer and inner surfaces, respectively. In addition, from the nearly middle to the point  84  of the fixture  80 , plural slits  85  are cut in longitudinally, forming plural blade-like portions  86 . Further, as shown in FIG.  8 (C), sector-shaped internal shoulders  87  are provided on the middle of the inner surface of each blade-like portion  86 , and reversed surfaces  88  are formed in the center of the sector-shaped internal shoulders  87 . These reversed surfaces  88  form a central hole  89  of the internal shoulders  87 . 
     The screw body  100  composed of a titanium-alloy includes a head  102  having outer male threads  101  which are engaged with the female threads  83  of the fixture  80  and a shank  104  having a point on which a bevel surface  103  is formed. When the screw body  100  is engaged through the head  81  of the fixture  80 , the bevel surface  103  is slided in contact with the reversed bevel surface  88  of the internal shoulder  87  of the fixture  80 . 
     In the dental implant  5  shown in FIG. 8, when the screw body  100  is moved downwards with the rotation of the screw body  100 , the bevel  103  of the shank  104  of the screw body  100  is pushed into the central hole  89  of the internal shoulders  87  of the fixture  80 . Hence, as shown in FIG.  8 (B) and (D), plural blade-like portions  86  of the fixture  80  can be swelled into a spindle-like shape, exerting effects similar to those of the above preferred embodiments of the present invention. 
     EFFECT OF THE INVENTION 
     As apparent from the foregoing, in the use of the dental implant of the present invention, the ends of the plural blade-like portions of the cylindrical body (fixture) are fixed by the nut body, and the plural blade-like portions constitute a firm spindle-shaped structure as one body. Thus, the possibility that the blade-like portions are twisted or broke down can be avoided. Further, since the spindle-shaped structure is held firmly within a bore of a jawbone, horizontal micromotion can also be prevented. 
     In addition, the reactive force against the compressive force acting in the direction to shorten the length of the spindle-shaped structure is very strong. Therefore, during the functioning of the dental implant such as mastication, even when a strong downward occlusal force is applied on the exposed head of the implant in oral cavity, the possibility that the under surface of the head of the bolt body and the upper surface of the internal shoulder of the cylindrical body lose contact and are set apart from each other is far lower in the present invention than in the prior art. Consequently, the micro-leakage of bacteria can be prevented nearly to perfection. 
     As a result, the secure operation of the immediate loading of the dental implant, the realization of which has been the largest issue in modern dental implant, is attained and the operation period, for which three to six months have been required in the prior art, can be shortened within one day. 
     Although particular embodiments of the invention have been described in detail for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not limited, except as by the appended claims.