Dental implant having a force distribution shell to reduce stress shielding

A dental implant that reduces the potential for stress shielding. The dental implant utilizes an implant body that includes a metallic core and a shell disposed about the metallic core. The shell is made from a material that has a lower modulus of elasticity than the metallic core. The metallic core also is connected to a mounting end to which a prosthetic tooth may ultimately be attached.

FIELD OF THE INVENTION
 The present invention relates generally to prosthetic implants, and
 particularly to dental implants that are designed to distribute forces,
 created during mastication, to surrounding bone. This distribution of
 forces reduces stress shielding of the surrounding bone.
 BACKGROUND OF THE INVENTION
 A variety of dental implants currently are known and available. The
 implants are designed for insertion into the maxilla or mandible, e.g.
 jawbone, of a patient to support the mounting of a prosthetic tooth.
 Generally, a cylindrical hole is formed in the mandible or jawbone of the
 patient, and the implant is mounted in the hole and allowed to undergo
 osseointegration.
 A dental implant includes a generally cylindrical body designed for
 placement in the cylindrical hole formed in the jawbone of a patient. The
 generally cylindrical body may be threaded. The exposed or coronal end of
 the dental implant includes a mounting feature or features that aid in the
 mounting of a prosthetic tooth. For example, the coronal end may include a
 splined interface and a threaded bore to which an abutment and prosthetic
 tooth are ultimately mounted.
 Conventionally, the body of the dental implant has been formed from
 titanium or a titanium alloy, such as Ti6AI4V. Such titanium materials
 have served well in enhancing bone attachment to the surface of the dental
 implant.It is believed that a stable oxide forms on the titanium or
 titanium alloy, and serves as a suitable surface for enhancing the
 desirable attachment between bone and the dental implant.
 Despite their proven record in promoting osseointegration, titanium and
 titanium alloys present certain other challenges to providing an optimal
 dental implant. Titanium and suitable titanium alloys are orders of
 magnitude higher in stiffness than human bone, and therefore dental
 implants formed from such materials absorb most of the forces of
 mastication. This can lead to a phenomenon known as Astress shielding of
 the surrounding bone.
 Specifically, it has been determined that inadequate stimulation of bone
 tissue over extended periods causes the bone tissue to be resorbed by the
 body, an effect commonly known as Wolff s Law. This effect becomes
 apparent when bone surrounding the dental implant is not adequately
 stimulated due to, for instance, absorption of a majority of forces
 created during mastication by a stiff dental implant. The lack of
 stimulation can cause saucerization, otherwise known as bone die-back,
 which progresses around the upper portion of an otherwise healthy dental
 implant. The loss of bone can lead to destabilization and even loosening
 of the dental implant. Additionally, once sufficient bone tissue has
 undergone resorption, portions of the implant body become exposed, and
 this surface, which is typically textured to provide high surface area, is
 susceptible to infection.
 It would be advantageous to design a dental implant able to transmit the
 forces of mastication to surrounding bone tissue without being unduly
 subject to degradation or breakage.
 SUMMARY OF THE INVENTION
 The present invention features a dental implant comprising an implant body.
 The implant body includes a metallic core and an anti-rotational mounting
 feature connected to the metallic core. Additionally, a shell is disposed
 about the metallic core. The shell has a lower modulus of elasticity than
 the metallic core to facilitate transfer of force to surrounding bone
 tissue.
 According to another aspect of the invention, a dental implant is designed
 for implantation at an implant site in a maxilla or mandible of a patient.
 The dental implant comprises a metallic core and a shell. The shell
 includes a recessed opening sized to receive at least a portion of the
 metallic core. The shell includes an outer surface sized to fit within a
 cylindrical hole formed in the mandible at the implant site. Additionally,
 the shell is formed of a less stiff material than the metallic core to
 facilitate transfer of forces from the dental implant to the surrounding
 bone tissue.
 According to a further aspect of the invention, a dental implant is
 provided for implantation at an implant site which includes a cylindrical
 hole formed in a mandible or maxilla of a patient. The dental implant
 comprises a mounting end to which a prosthetic tooth can be attached.
 Additionally, a force dissipater is coupled to the mounting end. The force
 dissipater is sized for insertion into the hole formed in the mandible.
 This force dissipater effectively distributes a substantial portion of the
 forces, generated during mastication, to the mandible surrounding the
 force dissipater.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The stiffness of conventional dental implants can lead to saucerization of
 the bone tissue surrounding the upper region of the dental implant, as
 illustrated in FIG. 1. In this illustration, a conventional titanium or
 titanium alloy dental implant 10 is shown implanted within a cylindrical
 bore 12. Cylindrical bore 12 is formed in an appropriate area of bone
 tissue 14 at an implant site 16.
 A prosthetic tooth 18 is mounted to dental implant 10, and a naturally
 occurring layer of gingival tissue 20 is disposed between bone tissue 14
 and prosthetic tooth 18. As chewing or mastication forces act on
 prosthetic tooth 18, those forces are translated through dental implant
 10. However, because of the stiffness of the titanium or titanium alloy
 from which dental implant 10 is formed, most of the forces are absorbed by
 the implant, leading to a reduction in stress transfer to the bone tissue
 14. If the bone tissue is inadequately stimulated for an extended period
 of time, the tissue begins to be resorbed by the body. This effect is most
 pronounced in the more dense cortical bone located near the surface of the
 jawbone, as opposed to the spongy, or trabecular, bone in the inner
 regions on the jawbone. This leads to an area of saucerization 22 in which
 bone tissue actually disappears around the portion of the dental implant
 proximate gingival tissue 20.
 As illustrated in FIG. 2, continued inadequate stimulation of the bone
 tissue leads to further resorption of bone tissue. Ultimately, the
 depletion of bone tissue can create a pocket in the gingival tissue which
 exposes the textured implant surface, making it susceptible to infectious
 agents from the oral cavity. Additionally, the dental implant may become
 destabilized and loose within cylindrical bore 12. In any of these
 situations, failure of the dental implant may result.
 However, if sufficient stimulation is provided to the bone tissue, the
 stress shielding phenomenon can be avoided, as illustrated in conjunction
 with an exemplary embodiment of the present invention shown in FIG. 3. In
 this illustrated embodiment, a preferred dental implant 30 is designed to
 dissipate forces throughout surrounding tissue. The dental implant 30 is
 disposed at an implant site 32 within a cylindrical bore 34 formed in an
 area of bone tissue 36.
 A prosthetic tooth 38 and an appropriate abutment 40 are shown mounted to
 dental implant 30. As described above, a layer of gingival tissue 42
 typically is disposed between bone tissue 36 and prosthetic tooth 38. The
 unique design of dental implant 30 provides a reduced stiffness or
 flexibility of the implant that results in sufficient bone tissue
 stimulation to avoid the detrimental effects of stress shielding. As
 illustrated, the bone tissue 36 remains healthy and in place along the
 complete length of dental implant 30 beneath gingival tissue 42.
 Referring also to FIGS. 4 and 5, dental implant 30 comprises an implant
 body having a core 44, a mounting end 46 and a shell 48. Preferably, core
 44 and mounting end 46 are integrally connected, and they typically are
 formed as a single unitary structure. Both core 44 and mounting end 46 are
 formed, for example, from a metallic material, such as titanium or a
 titanium alloy (e.g. Ti6AI4V).
 In the illustrated embodiment, core 44 is generally cylindrical and defined
 by an outer surface 50. Outer surface 50, in turn, is circular in
 cross-section, as illustrated best in FIG. 6. However, although other
 shapes/cross-sections can be utilized. Also, core 44 is sufficiently
 elongated to extend into shell 48, and preferably through a majority of
 the length of shell 48.
 Mounting end 46 typically includes an annular expanded portion 52 adjacent
 core 44. A mounting feature 54 is connected to annular expanded portion 52
 on the side opposite core 44. Often, mounting feature 54 is structured as
 an anti-rotational mechanism that prevents rotation of prosthetic tooth
 38. In the illustrated embodiment, mounting feature 54 includes a
 plurality, e.g. six, splines 56 that extend outwardly from annular
 expanded portion 52 in an axial direction generally opposite core 44. A
 gap 58 is disposed between each adjacent pair of splines 56. Additionally,
 mounting end 46 includes an axial, threaded bore 60 disposed at a
 generally central location that is radially inward from splines 56. Axial
 bore 60 may extend into core 44, and it facilitates the mounting of
 abutment 40 and prosthetic tooth 38.
 In the illustrated embodiment of the invention, shell 48 is cylindrical in
 shape and has an outer surface 62 that is circular in cross-section, as
 best illustrated in FIG. 6. Shell 48 and outer surface 62 are sized for
 engagement with cylindrical bore 34 formed in bone tissue 36. Potentially,
 outer surface 62 can be smooth or textured. Additionally, outer surface
 may comprise a threaded region 63 (see FIG. 4), depending on the
 particular dental implant application.
 Shell 48 also includes an axial opening 64 having a length and
 cross-section formed to receive core 44. A wall 66 is effectively created
 between axial opening 64 and outer surface 62. The thickness of wall 66 is
 sufficient to place outer surface 62 in general axial alignment with the
 radially outer surface of annular expanded portion 52.
 Shell 48 is made from a material that is less stiff than core 44.
 Specifically, the preferred material of shell 48 has a lower modulus of
 elasticity than that of core 44. Preferably, the material of shell 48 has
 a modulus of elasticity that is similar to the modulus of elasticity of
 bone tissue 36.
 The material of shell 48 may be a polymeric material, such as
 polyetheretherketone, commonly known as PEEK. PEEK has the characteristics
 of high strength, low water absorption and biocompatibility, as well as
 having a modulus of elasticity much closer to that of the surrounding bone
 than the typical titanium or titanium alloy core 44. To increase the
 strength of a polymer, such as PEEK, the polymer may be reinforced with a
 fiber or fibers, such as a carbon fiber, to create a polymeric composite.
 Thus, the polymeric material retains the desired flexibility and
 resiliency that permits transfer of mastication forces to bone tissue 36
 throughout the entire implant site 32, while the fibers provide strength
 and thermal stability.
 A primary example of such a composite material is carbon fiber reinforced
 (CFR) PEEK. CFR PEEK material may be formed by way of a filament winding
 process or a braiding process. Furthermore, the shell 48 may be formed by
 creating opening 64 in a solid rod or cylinder of PEEK reinforced with
 either chopped or continuous carbon fibers depending on the desired
 material characteristics. Accordingly, the desired strength of shell 48
 potentially can be adjusted by selection of both the length and
 orientation of the carbon fibers throughout the PEEK material. For
 example, to withstand the mastication loads exerted on dental implant 30
 during chewing, it may be desirable to use continuous fiber reinforcement
 of the PEEK material.
 Core 44 and shell 48 may be attached to one another by a variety of
 methods. For example, the two components may be attached by resistive
 heating of the titanium alloy (e.g. TiA14V) core or by induction heating
 of core 44. Other potential methods of attachment include microwave
 radiation to partially melt the PEEK material, or attachment by spin
 welding, a method by which the two components are assembled while one or
 the other is spinning. If the spin rate is sufficiently high, frictional
 forces at the interface between core 44 and shell 48 cause the PEEK to
 melt. As the speed of spinning is then reduced, the PEEK material of shell
 48 adheres to the metallic material of core 44 and forms a bond. Other
 methods of attachment include high temperature compression molding,
 ultrasonic welding and conventional, biocompatible adhesives.
 The lower modulus material utilized in forming shell 48 should have
 sufficient flexibility to permit the transfer of forces from prosthetic
 tooth 38 to bone tissue 36. Effectively, outer surface 62 of shell 48
 bends and moves a sufficient amount to permit the transfer of forces to
 bone tissue 36. The transfer of forces reduces or eliminates the
 saucerization/resorption of bone tissue beneath gingival layer 42 that can
 otherwise occur.
 In a slightly modified embodiment illustrated in FIG. 7, surface 62 of
 shell 48 has been covered with an outer layer 68 designed to further
 promote osseointegration. Outer layer 68 may comprise a thin layer of
 titanium laid over a PEEK composite shell or substrate. The titanium layer
 may be placed on the PEEK material by, for example, a technique known as
 vapor deposition.
 Outer layer 68 also may comprise other materials. For example,
 hydroxylapatite (HA) may be deposited as layer 68 through an
 electrochemical deposition technique. Alternatively, outer layer 68 may be
 a coating of biologically active molecules such as proteins, growth
 factors or synthetic peptides or other materials that improve the in vivo
 attachment of bone tissue to the dental implant.
 It will be understood that the foregoing description is of preferred
 embodiments of this invention, and that the invention is not limited to
 the specific forms shown. For example, a variety of mounting end
 configurations may be utilized; a variety of core materials can provide
 sufficient strength and stiffness; and other materials potentially may be
 used in the construction of the shell. These and other modifications may
 be made in the design and arrangement of the elements without departing
 from the scope of the invention as expressed in the appended claims.