A submergible or non-submergible (“one stage”) screw-type implant for use in the immediate post-extraction site of a patient's tooth. In order to increase the ability of regenerated bone to anchor the implant, an upper portion of the implant has a preferably generally conical shape and has a plurality of spherical projections sintered to its outer surface to make it suitable for bone integration and retention. The lower portion of the implant has a cylindrical shape with threads extending over a portion thereof. A channel is located through the threads and is shaped so that one side forms a cutting edge that makes the lower portion of the implant self-tapping into the bone at the base of the tooth extension socket.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS The design requirements for dental implants placed into immediate extraction sites differ significantly from the design of general implants used presently for placement in edentulous jawbones. Today all implants used in immediate extraction sites are either threaded, coated with a surface material or sintered. However, these implants do not provide the best design for immediate fresh extraction sites. Such immediate extraction sites require an implant designed specifically to address the morphology of the bony defect created during the extraction of a tooth. The implant system of the present invention is at least a two part screw-type dental implant 3 ( FIG. 1 ), having a threaded cylindrical lower portion 10 that is buried in the bone 5 of the patient and an upper portion 9 , preferably generally conically-shaped, that is attached thereto. The upper portion 9 is covered by soft tissue 7 . A post or abutment 2 is shown in dotted line extending from upper portion 9 and supporting an artificial tooth structure 4 to complete the implant system. As shown in FIG. 1 the implant screw lower portion 10 is located in a bore in the alveolar crest bone 5 , at an angle that causes it to be in the center of the thickest portion of good available bone. The abutment 2 is attached both to the implant portion and the artificial tooth 4 and may have an angular offset to the implant so that the artificial tooth is in proper alignment with the rest of the teeth. In FIGS. 2, 4 and 5 , the screw implant 3 of FIG. 1 is illustrated in more detail. This screw implant portion contains threads 13 in the lower portion 10 that extend over the top two-thirds of this lower portion 10 . These threads may have a flat bottom and be angled up to form a Christmas tree shape in cross section. The lower half of the implant portion 10 contains a cavity 14 , as can be seen in FIG. 4 . This cavity is open at its bottom. Also, spaced about the lower end of the implant portion 10 are holes or vents 16 , which penetrate from its exterior to the interior cavity 14 . The purpose of these vents is to allow new bone to grow through and into the center cavity 14 in order to firmly anchor the implant in the patient's bone 5 . A channel 18 in alignment with at least one vent 16 extends through most of the threads, but not the top thread. The channel does not pass through the top thread in order to prevent tissue from growing down the channel. This channel has two purposes. First, the channel 18 and the vent 16 create cutting edges on the adjacent threads that make the implant self tapping. Also, the channel provides a path by which bone chips created during the threading of the implant into the bone may pass down to the vent 16 and enter the cavity 14 where they promote the growth of new bone. To facilitate this, the channel 18 widens toward the bottom of the implant. These features are described in U.S. Pat. No. 4,713,004, which is incorporated herein by reference. The upper portion 9 of the screw implant 3 preferably has a generally conical or fluted shape. For example, the base of portion 9 where it contacts portion 10 may have a diameter of about 3.0 mm, while the top of the portion 9 may have a diameter of about 3.5 mm. In a preferred embodiment shown in FIG. 7 the upper end 11 of the upper portion 9 is tapered upwardly. This provides a more gently contoured surface to minimize soft tissue irritation. In FIG. 5 the upper part of the implant portion is shown partly broken away and partly in section to illustrate an interior cavity 28 and the shape of the threads 13 . The top surface 29 ( FIG. 2 ) of the conically-shaped upper implant portion 9 , has a disk-shaped transition cap 26 from which there extends a hexagonally-shaped projection 27 , as shown more clearly in FIG. 3 . This hexagonal shape allows a tool, e.g., a wrench, to be used to rotate the implant portion so as to thread it into the patient's bone 5 . This upper portion 9 also defines the threaded aperture 28 (shown in dashed lines in FIG. 2 and solid line in FIG. 5 ) that extends from the top surface 29 , at the hexagonal projection 27 , to the junction with the lower portion 10 . Aperture 28 is used to connect the abutment 2 to the implant portion 9 . In a preferred embodiment, the implant's upper portion 9 may have a plurality of spherical projections 22 sintered to at least a portion of its exterior surface. The spaces between the spheres form micropores into which bone will grow. Preferably, the pores are between about 200 and 350 microns. The projections 22 are made of a material suitable for bone integration and should preferably be either a metal (e.g., titanium), a polymer, a composite, or a copolymer. Methods of sintering spherical projections, or beads, onto a metal's surface is generally known in the art. In particular, titanium bead sintering services have traditionally been provided by the FPD Company of McMurray, Pa. Sintering is also described in general in the Encyclopedia of Chemical Technology , Vol. 16, 4th ed., John Wiley & Sons (1995) at pp. 327-329, which is incorporated herein by reference. Just proximal to the vent 16 , there is approximately 3 mm of thread that is designed so as to allow for immediate fixation within the bone to prevent movement of the implant. Thus, the implant 3 can be used for relatively immediate replacement of a tooth that has been removed. Shown in FIG. 6 is the general shape 60 of an extraction site 40 . As this site is a cavity that is not cylindrical, it cannot be the site of a conventional cylindrical screw type implant unless bone is first regrown in the cavity and a new cylindrical bore is drilled in the new bone, a process that could take many weeks or months. According to the installation process for use of the screw implant of the present invention, an incision is made in the gum tissue 7 , if any, covering the extraction site to expose the underlying bone 5 . Then, a small cylindrical bore 42 is drilled at the base of the conical extraction site 40 . This bore 42 , which is shown in dashed lines in FIG. 6 , is made with about the diameter of the unthreaded part of the lower portion 10 of the implant 3 . It is made deep enough so that the bottom two or three turns of the threads 13 can engage the bone surrounding the bore 42 at a level below the original bottom of the extraction site. The bone chips created during the formation of the bore 42 are preferably saved for later use. According to the installation method of the present invention, the implant 3 is placed in the site 40 so the unthreaded portion rests in the bore 42 . Then, using a wrench or similar tool engaged with the hexagonal projection 27 , the implant is rotated. As a result, the cutting edge on lowest thread of the implant at the vent 16 engages the bone surrounding bore 42 and begins to self-tap into the bore. When the implant is rotated sufficiently, its base rests against the bottom of bore 42 . Because the implant has been self-tapped into the bone, it is now firmly anchored at the implant site. Also, bone chips have fallen down or been pushed down the channel during this process. These chips have collected in the cavity 14 and will act to promote the growth on new bone in the cavity and through the vents to further anchor the implant in the future. The space 44 between the rest of the extraction site 40 and the implant 3 , especially about the upper portion 9 , is now back filled with autogenous bone chips 6 saved from the creation of bore 42 or bone graft materials, such as bovine (xerographic) bone, synthetic bone (alloplastic, e.g., ceramic orplastic), allographic bone, or a combination thereof. This material may be resorbable or non-resorbable, solid or microporous. One synthetic bone material is disclosed in U.S. Pat. No. 4,728,570 of A. Ashman et al., and sold under the trade name Bioplant® HTR®. In addition, one or more bioactive substances that are medico-surgically useful may be incorporated into the synthetic bone. Various compositions of such are disclosed in U.S. Pat. No. 5,356,629 of Sander et al. The bone graft material 6 is packed loosely so that it fills the voids along with the bleeding from the surgical site and makes intimate contact with the microbeads 22 , which are preferably sintered titanium beads, on the surface of the upper portion 9 . It may be advisable to use a surgical dressing to hold the bone chips in place. The dressing may be a surgical adhesive or glue, surgical foil, collagen, skin, or similar biocompatible material. In time, new bone will grow around and through the bone graft material 6 , or replace it, thereby further anchoring the implant in place. This unique design of the implant 3 thus allows for immediate installation in a fresh tooth extraction site 40 and specifically addresses the requirements of extraction sites. The implant design can permit either a single or two-stage installation. For a single stage installation, the abutment 2 is installed during the initial installation of the implant. The abutment 2 extends through the sutured gum. The artificial tooth 4 may or may not be installed at the same time. A two-stage or submergible implant is shown in FIG. 6 . Once the bone graft material 6 is in place, a cap 46 is screwed into the threaded aperture 28 in the top of the implant to make sure the growth of new bone does not extend into the aperture. If bone does grow into this aperture, it can be very difficult to remove. Gingival tissue 7 is then sutured over the implant. Time is be allowed to pass so that new bone grows and firmly anchors the implant in place before the rest of the implant system is installed and the device is put into use. At the second stage, the gingival tissue 7 is reopened. Often, bone has grown over the submerged implant and must be removed by a burr before the abutment 2 can be installed. However, if bone grows up over the edges of the collar 26 , there is no need to remove it because it becomes part of the permanent abutment. The cap 46 is then removed from aperture 28 and replaced with the threaded shaft of an abutment 2 . The threaded end of the abutment 2 is engaged with the threaded aperture 28 and is rotated so that it is firmly secured in the implant portion and is extending in the proper direction. With this firm attachment completed, the artificial tooth 4 can then be attached over the abutment cylinder 2 . Whether a single-stage or two-stage (submergible) procedure is used, the abutment 2 , which may be straight or have an angled shaft, is selected so as to cause the artificial tooth 4 to be correctly aligned with the other teeth of the patient. Therefore, the dentist or oral surgeon must be provided with a variety of such abutments that are at standard angles. Besides being used to mount a single tooth, the implants according to the present invention can be used as supports for a permanent bridge or a removable bridge. While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of the invention.