Abstract:
A method of designing a skeletal implant comprises measuring the width, height, and elastic modulus of the bone to be implanted, generating a macro-design of the implant based on the width and height of the bone, and determining a micro-design for the implant, based on the measured elastic modulus, to ensure that strain in the bone is kept in a range to promote bone growth and to minimize bone resorption, during functional loading. For threaded implants, the shape of the thread is chosen so that bone strain adjacent the threaded attachment surface of the implant is between about 100 and 3000 microstrain. A device for implanting into the existing bone and for attaching a dental prosthesis thereto has a crest portion, a base portion, and a configuration for securing the base portion to the existing bone. The securing configuration creates a level of strain in the bone adjacent the outer surface less than the level of strain which causes resorption of the bone tissue and greater than the minimum level required to promote bone growth. The securing configuration comprises a continuous thread, forming a helix around the base portion. The bone contacting area of the thread increases as the thread nears the crest portion, thereby increasing the surface area over which force is distributed from the implant.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to skeletal implants (such as dental implants) and more particularly to a method of designing skeletal implants that promote strain-induced bone tissue growth and maintenance over the entire bone contacting surface of the implant. 
     2. Description of the Prior Art 
     Skeletal implants have been used for the replacement of articular joints within the body (e.g. total hip arthroplasty), restoration of aesthetics (e.g. bony retention of ear prosthesis) and replacement of missing teeth (e.g. dental implants). One of the primary failure mechanisms for skeletal implants is implant loosening at the implant-to-tissue interface due to non-physiologic loading profiles. 
     Roughly 125 million individuals in the United States alone are missing some of their teeth. One approach to treating patients missing teeth is to supply them with removable dentures. Dentures have the disadvantage of not adequately loading their supporting bone (such as the mandible for lower dentures and the maxilla for upper dentures). An unloaded supporting bone experiences very little strain. When the supporting bone lacks a minimum level of strain, bone resorption occurs. This results in shrinkage of the supporting bone and can further result in related health and aesthetic problems. 
     Another approach to treating edentulous or partially edentulous patients is to place endosteal osteointegrated (the integration of bone tissue with the implant) implants in the supporting bone. Osteointegrated endosteal implants are alloplastic materials surgically inserted into a residual bony ridge to serve as prosthodontic foundations. Such implants serve as platforms for prosthetic devices. The introduction of osteointegrated dental implants has given edentulous and partially edentulous patients a more effective means to restore their ability to chew and to improve their appearance. Furthermore, osteointegrated implants functionally load the mandibular (or maxillary) bone into which they are implanted, thereby inducing strain in the bone under normal functional loading. Bone loss and resorption, which commonly occur with dentures, can thereby be minimized or avoided by maintaining a proper loading profile on the bone. 
     Two subcategories of endosteal implants include plate form implants and root form implants. A plate form implant is characterized by a flat, narrow plate typically placed in a horizontal dimension of the mandibular or maxillary bone. Root form implants are designed to be placed in a vertical column of bone. Root form implants include two types: cylinder-type root form implants, which are non-threaded cylinders pressed into holes drilled into the receiving bone, and screw-type root form implants, having a threaded outer surface which is screwed into a hole drilled into the receiving bone. 
     The cylinder root form implant may have design features which minimize rotation of the implant in the implanted bone (e.g. holes and grooves) as well as a textured surface, which promotes close bone apposition to the implant. A disadvantage of cylinder root form implants is that they take a long time to set properly, as the patient must wait until the surrounding bone has properly integrated with the implant before functionally loading the implant. 
     Screw root form implants are held to the surrounding bone by a threaded outer surface. The threaded surface provides initial stabilization of the implant to the surrounding bone and it facilitates macroscopic bone integration. Because they are screwed into the bone, screw root form implants may not require as much time as cylinder root form implants prior to functionally loading. 
     Current screw and cylinder root form implants have a disadvantage in that bone resorption commonly occurs in the crestal region of the implant due to excessive strain experienced by the bone in this region. Furthermore, other regions of the bone may not experience enough strain and resorption may also occur. Although a certain level of strain must be experienced by the bone to prevent bone resorption, too much strain can also result in bone resorption. If the supporting bone experiences less than 100 microstrain or more than 3000 microstrain, bone resorption will occur. On the other hand, a strain level between about 100 microstrain and 3000 microstrain can actually encourage bone growth. 
     Many common screw-type implants impart too much strain in some portions of the implant-bone interface (e.g. the crestal regions) and not enough strain in other parts of the interface. This results in non-uniform bone ingrowth and resorption, which further results in implant loosening. Roughly 10% to 15% of all dental implant patients must eventually return to the implantologist for revision surgery due to inadequate bone ingrowth, loosening, or structural failure of the implant. Revision surgery can be significantly more costly than primary surgery. Furthermore, it often leads to increased failure rates due to surgical complications involving decreased quality of available bone, or bacterial smear layers of contamination on the implant once bone loss occurs. 
     More than 24 cylindrical shaped and blade-shaped endosteal and transosteal implant systems are available on the market today. These devices include those made by NobelPharma U.S.A., Inc. of Nobel Industries in Sweden developer of the Branemark system, an endosseous fixture which is one of the most popular in the U.S. and which has been given full acceptance by the American Dental Association (ADA). Other devices which have received provisional acceptance by the ADA include: Dentsply (previously Core-Vent) root forms, Oratronics blade implant, and Integral cylindrical implants by Calcitek. 
     Niznick (U.S. Pat. No. 4,431,416) discloses a combination root form implant having an intermediate section with peripheral threads to engage the bone. The lower end of the implant is hollow and has peripheral holes through which bone tissue may grow. The implant receives a denture which transmits bite force to the gum tissue, thereby reducing the transmission of such force to the implant. Because the Niznick device does not physiologically load the implanted bone, it does not provide strain-induced bone growth. 
     Friedman et al. (U.S. Pat. No. 5,209,659) discloses a dental implant having a cylindrical body portion and a threaded apical portion which does not exceed one-half of the length of the body. The threaded portion has sharp external cutting threads which do not extend beyond the diameter of the cylindrical portion. 
     Scortecci (U.S. Pat. No. 5,312,256) discloses a screw-type root form implant that employs a fine pitch thread with a plurality of interruptions of the thread, both of which serve to reduce the internal stress in the bone in order to avoid necrosis. Scortecci does not disclose an implant wherein strain is maintained within a predetermined range in order to encourage bone growth and to reduce resorption. 
     Weiss et al. (U.S. Pat. No. 4,997,383) discloses a blade-type dental implant with substantial planar areas on the front and rear surfaces of the implant which make bone contact produce optimal force absorption in areas of highest stress. Weiss et at., however, does not disclose an implant designed to produce a level of strain in the implanted bone that would promote bone growth. 
     Valen (U.S. Pat. No. 5,007,835) discloses a screw-type root-form implant having rounded screw threads to provide radial forces at points in contact with the bone. A separate tapping mechanism is also disclosed. Although Valen attempts to reduce bone necrosis by employing rounded threads, Valen does not disclose a means to ensure that strain in the bone surrounding the implant is maintained within a predetermined range. 
     None of these devices have been developed to stimulate and maintain bone strain levels over the entire surface area of the implant which promote osteointegration while minimizing bone resorption. 
     Several references disclose methods of maintaining and promoting bone growth by applying mechanical stimuli. Mcleod et al. (U.S. Pat. Nos. 5,103,806 and 5,191,880) describes a method for preventing osteopenia and promoting bone growth by applying a mechanical load to the bone tissue at relatively low magnitudes and at relatively high frequencies. Although these patents suggest that the disclosed methods can be used in conjunction with prosthetic implants, they do not propose a particular implant geometry or a method by which to derive such a geometry. 
     Lanyon, in Control of Bone Architecture by Functional Load Bearing, 7 Journal of Bone and Mineral Research S369-S375 (Supp. 2, 1992) describes the importance of local functional strains in the control of bone architecture. This article discusses the adaptation and maintenance of bone as being predominantly due to a conservational or &#34;osteogenic&#34; strain regime sustained at each location of concern within the skeletal system. 
     None of these references disclose a method of designing an implant which optimizes the geometry of the implant to provide the level of strain on the implanted bone required to maximize osteointegration and minimize bone resorption. Nor do any of these references disclose an implant design which provides the level of strain on the implanted bone required to maximize osteointegration and minimize bone resorption over the entire bone contacting surface area of the implant. 
     Thus, there exists a need for a method of designing an implant so that the implant creates in the implanted bone a level of strain which maximizes bone growth and which minimizes bone resorption over the entire bone contacting surface area of the implant. 
     There also exists a need for an implant that promotes maintenance of bone under functional loading conditions over the entire bone contacting surface area of the implant. 
     SUMMARY OF THE INVENTION 
     The present invention is an implant system, including a design method and an implant apparatus, that optimizes strain distribution to surrounding osseous tissues under functional loading conditions in order to promote strain-induced bone growth, promote maintenance of the bone, and reduce bone resorption over the entire surface area of the implant. In one preferred embodiment, the invention provides an implant system for a screw-type, root-form dental implant. 
     Essentially, the present invention implements a method for designing a prosthetic implant which enhances hard tissue response and bone growth in response to the functional demands placed on the implant. First the designer evaluates macro-design criteria based on a knowledge of the anatomical dimensional limitations, the mechanical properties of the tissue at the implant skeletal reception site, and the functional demands placed on the implant. The designer must then apply basic engineering principles, based on a knowledge of functional strain levels at the reception site that promote physiologic health to the macro-design in order to optimize the micro-design features that enhance strain induced bone growth. Finally, the designer refines the micro-design features in order to customize the skeletal implant for various regions of the reception site. 
     More specifically, the method involves the following steps: characterizing the patient&#39;s bone at the predetermined site with respect to the parameters of width, height, and elastic modulus; generating a macro-design, or large scale design, for the implant based on the measured width and height of the bone at the predetermined site and a desired biomechanical response for the implant. A micro-design is then determined for the implant based on the measured elastic modulus of the bone at the predetermined site whereby the implant produces a strain in the bone during functional loading of the implant that is within a predetermined range which promotes bone growth and minimizes bone resorption. The micro design involves modifying those parameters that affect the response of the surrounding bone tissue to the implant at the cellular level. 
     The physiologic forces exerted by the implant on the bone at the predetermined site during functional loading of the implant are identified and related to the strain experienced by the bone to create strain equations for normal strain and shear strain. The normal strain and shear strain equations are then applied using the characterization parameters of the bone as input to modify the general mechanical macro-design and thereby create a micro-design for the implant such that the normal and shear strain acting on the implanted bone minimizes resorption of the bone and enhances growth of new bone tissue adjacent the implant. 
     The geometry of the micro-design for the implant is optimized such that the strain in the bone is maintained within a predetermined range of between about 100 and 3000 microstrain. This is done by relating the force imparted by the implant to the strain experienced by the bone to create strain equations for normal strain and shear strain and then applying the normal strain and shear strain equations using the characterization parameters as input. 
     If the implant is a dental implant, such as a screw-type root-form dental implant, the designer may first assign different types of bone commonly found in the mandible and maxilla to predefined groups, with each predefined group having defined average characteristics of width, height, density, and modulus of elasticity. Then, the designer designs the implant specific for each of the groups. The implant is optimized to produce the mount of strain that will promote bone growth and minimize resorption in a bone having the average characteristics for the predefined group for which the implant is designed. This enables a surgeon to characterize a patient&#39;s bone at an implant site, identify the predefined group that corresponds to the patient&#39;s bone, and select the implant design that corresponds to the predefined group into which the patient&#39;s bone characterization belongs. 
     Once the overall design is complete, optimization routines are used to refine the design in order to create micro-design features that are specific for each type of implant. Finite element analyses (FEA) may be performed on the micro-designs in order to validate the performance of each type of dental implant under functional loading. The FEA represents a feasible way to accommodate the considerable complexities (geometrical, material, and load-related) that characterize a real clinical situation. Once validation of the implants is complete, design drawings are produced for each type of implant so that fabrication plans may be drafted. 
     Such a device adapted for implanting into the existing bone of a patient and for attaching a dental prosthesis thereto has a crest portion having a bottom surface and an opposite top surface, the top surface being adapted for attaching the dental prosthesis thereto and a base portion having a crestal end, an apical end, and a core section with an outer surface, the crestal end being attached to the bottom surface of the crest portion. It also has a means on the base portion for securing the device within the existing bone which minimizes resorption of the existing bone and promotes growth of new bone tissue adjacent the device. 
     In one embodiment, the securing means has a continuous thread, beginning at a first end adjacent the apical end of the base portion and terminating in a second end adjacent the crestal end of the base portion, the thread forming a helix around the core section and extending radially outward from the outer surface of the core section terminating in a thread face having an outermost end of the thread face. The thread has a bone contacting surface area defined between the outer surface of the core section and the outermost end. The bone contacting surface area of the thread increases as the thread nears the crestal end, thereby increasing the surface area over which force is distributed from the implant to the surrounding bone. Also, the radial length of the thread from the outer surface of the core section to the outermost end of the thread face increases as the thread approaches the crestal end which also increases the bone contacting surface area of the thread as the thread approaches the crestal end. 
     The radius of the device defined by the centerline longitudinal axis of the base portion and the outermost end of the thread face is constant between the apical end of the base portion and the crestal end of the base portion. However, the core section is conical, wider near the apical end and narrower near the crestal end. 
     The thread face comprises an upper face, defined by an upper face edge and the outermost end, and a lower face edge, defined by a lower face edge and the outermost end. The lower face and the upper face connect at the outermost end forming a predetermined angle therebetween. The predetermined angle formed by the lower face and the upper face increases at a preselected rate as the thread approaches the crestal end. This feature distributes the force imparted by the implant on the crestal region of the bone over a greater surface area (thereby maintaining the strain induced in the bone in this region within the range of 100 to 3000 microstrain) while ensuring that at least a portion of the thread, in the apical region, is a self-tapping thread adapted for tapping a pre-drilled hole in the patient&#39;s existing bone. 
     The preferred embodiment may be adapted for implantation depending on the density of the bone being implanted. The number of turns of the thread per unit height is selected based on the density of the bone being implanted. In relatively dense bone, the implant should have relatively more turns of the thread per unit height than would implants adapted for implantation in relatively less dense bone, with the turns being relatively close together. In relatively less dense bone, the implant should have relatively fewer turns of the thread per unit height than would implants adapted for implantation in relatively more dense bone, with the threads being relatively spread apart. 
     It is, therefore, an object of the present invention to provide a method of designing an implant so that the implant creates in the implanted bone a level of strain which maximizes bone growth and osteointegration over the entire surface area of the implant. 
     It is a further object of the present invention to provide a method of designing an implant so that the implant creates in the implanted bone a level of strain which minimizes bone resorption over the entire surface area of the implant. 
     It is also an object of the present invention to provide a threaded screw implant in which the geometry of the thread creates the optimal strain level in the bone being implanted. 
     It is a further object of the present invention to provide a threaded screw implant which is self tapping at the apical end, while providing sufficient securing force at the crestal end to prevent loosening. 
     These and other objects will become apparent from the following description of the preferred embodiment taken in conjunction with the following drawings, although variations and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS 
     FIG. 1 is a side elevational view of an implant in accordance with the present invention designed for use as a dental implant. 
     FIG. 2 is a chart showing a scheme of classifying the cross sectional area of available mandibular bone into several divisions. 
     FIG. 3 is a side elevational schematic drawing of an implant in accordance with the present invention showing the angular and spatial relationships of the components therein. 
     FIG. 4 is a cross-sectional schematic drawing of a portion of a thread showing its component angular and spacial relationships. 
     FIG. 5 is a side elevational view of a dental implant in accordance with the present invention having a groove transverse to the thread. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. As used herein &#34;a&#34; can mean one or more than one, depending upon the context where it is used. 
     One preferred embodiment of the present invention provides a method for designing a skeletal implant that is adapted for implantation in a patient&#39;s bone at a predetermined site. As used herein, the term &#34;predetermined site&#34; includes any of the possible sites in the body where a skeletal implant may be used (e.g., dental implants, etc.). First, the designer determines the patient&#39;s bone at the predetermined site with respect to the parameters of width, height, and elastic modulus; generating a macro-design, or large scale design, for the implant based on the measured width and height of the bone at the predetermined site and a desired biomechanical response for the implant. A micro-design is then determined for the implant based on the measured elastic modulus of the bone at the predetermined site whereby the implant produces a strain in the bone during functional loading of the implant that is within a predetermined range which promotes bone growth and minimizes bone resorption. The micro-design involves modifying those parameters that affect the response of the surrounding bone tissue to the implant at the cellular level. 
     The designer determines the micro-design for the implant based on the measured elastic modulus, using strain equations to ensure that strain in the bone is kept in a range to promote bone growth and to minimize bone resorption during functional loading. This is done by identifying the forces that will be imparted by the implant to the bone at the predetermined implant site and the strain that will be experienced by the bone resulting from the forces imparted by the implant. From this information, the implant designer creates strain equations. The strain equations are applied to the implant design using the characterization parameters of the bone as input to modify the general mechanical macro-design so that the amount of strain in the implanted bone will remain within the desired range, the desired range being between about 100 and 3000 microstrain. 
     The strain equations may be applied using a computer employing any suitable numerical analysis program for optimizing complex equations of the type found in the strain equations, so that the bone contacting surface area induces strain in the desired range. While such programs are generally known to those skilled in the art, one such program is the &#34;OPTDES&#34; interactive computer program from Brigham Young University. 
     With respect to dental implants, the different types of bone commonly found in the mandible and maxilla may be assigned to groups, each group having defined average characteristics of width, height, density, and modulus of elasticity. The implant designer is thus able to design an implant specific for each one of the groups, the implant being optimized to produce the amount of strain that will enhance new bone growth and osteointegration and minimize resorption in a bone having the average characteristics for the group for which the implant is being designed. The dental surgeon employing such an implant is thereby able to characterize a patient&#39;s bone, identify the assigned group and select the implant design that corresponds to the group into which the patient&#39;s bone characterization belongs. 
     Constraint values, based upon anatomical dimensional limitations (e.g. buccal-lingual width and crestal height) and mechanical structure of the bone, are identified. These constraints are used to define constraint equations that relate the constraints to the physical forces imparted by the implant. After the constraint equations have been defined for specific regions of the mandible and maxilla, optimization routines are used to refine the macro-design in order to create micro-design features that are specific for each type of implant. The design may be validated by any method obvious to one skilled in the art of implant design. One such method is to perform finite element analyses (FEA) on the resulting designs, thereby validating the performance of each type of dental implant under physiologic functional loading. The FEA is performed on a computer using a program (e.g. &#34;NASTRAN&#34;) that would be obvious to one skilled in the art. Once validation of the implants is complete, design drawings are produced for each type of implant so that fabrication plans may be drafted. 
     Referring to FIG. 1, a preferred embodiment of the apparatus of the present invention, designed for use as a dental implant 10, comprises a crest portion 20 attached to a base portion 30. The crest portion 20, provides a top surface 22 to which a prosthetic device (not shown) may be affixed. The crest portion 20 also has a bottom surface 24 which is attached to the base portion 30. 
     The base portion 30 provides a means to affix the implant to the patients mandible or maxilla. The base portion 30 comprises a substantially conical core section 32 and a thread 40 affixed to the core section 32. The core section 32 has a crestal end 34 affixed to the bottom surface 24 of the crest portion 20 and an opposite apical end 36. 
     The thread 40 is continuous and has a first end 46 and a second end 48 which forms a helix around the core section 32 from the apical end 36 to the crestal end 34. The thread 40 has a thread face 50 which is divided into an upper face 52 and a lower face 54 which are divided by the outermost end 56. The thread 40 extends radially outward from the outer surface 38 of the core section 32 and terminates at the outermost end 56 of the thread face 50. The thread face 50 also has an upper face ledge 42 and a lower face ledge 44. 
     The apical end 36 has a diameter that is smaller than the outside diameter of the thread 40 in order to allow the implant to have the self-tapping feature. The angle between the upper face edge 52 and the lower face edge 54 approaches 180 degrees as the thread 40 nears the crestal end 34, thereby increasing the surface area in the normal plane to the occlusal forces applied to the implant 10. This is necessary in light of the fact that the greatest amount of force applied to the implant 10 is orthogonal to the occlusal plane. Therefore, normal stresses are of greatest concern, especially in the crestal regions. The bottom surface 100 of the base portion 30 is flat in order to avoid opposing anatomical landmarks (e.g., the mandibular nerve canal as shown in FIG. 2, item 64) and to provide increased surface area in the normal plane to the applied force. 
     The implant 10 typically imparts most of the force that occurs as a result of functional loading in the crestal region of the bone. This induces the most strain in the crestal region, which frequently exceeds the physiologic strain levels resulting in bone resorption. Thus, it is desirable to distribute the force imparted on the crestal region of the bone by the implant 10 over a broader surface area, thereby inducing less strain in the crestal region of the implanted bone. 
     Therefore, in the implant 10 of the present invention, the total bone contacting surface increases as the thread 40 nears the crestal end 34. The bone contacting surface area of the thread comprises the surfaces of the upper thread ledge 42, the lower thread ledge 44, the upper face edge 52 and the lower face edge 54. The radial length of the thread 40 from the outer surface 38 of the core section 32 to the outermost end 56 of the thread face 50 increases as the thread 40 nears the crestal end 34. This is a result of both an increase in the angle between the upper thread face 52 and the lower thread face 54 as the thread nears the second end 48, and a narrowing of the core section 32 as it nears the crestal end 34. This increases the surface area over which force is distributed from the implant 10 to the surrounding bone, particularly in the crestal regions of the bone. By distributing the force over a greater surface area in the crestal regions, functional loading of the implant 10 results in less than about 3000 microstrain being induced by the implant 10 in the crestal region of the bone. 
     Conversely, the apical end 36 tends to induce less strain in the surrounding bone. If the implant 10 induces less than 100 microstrain the region of the bone near the apical end 36, new bone growth and osteointegration will occur at less than the optimal rate. Therefore, as the thread 40 nears the apical end 36, the radial length of the thread 40 from the outer surface 38 of the core section 32 to the outermost end 56 of the thread face 50 decreases. This is a result of a widening of the core section 31 near the apical end 36 and a decrease of the angle between the upper thread face edge 52 and the lower thread face edge 54. As the thread 40 nears the apical end 36, more strain is induced in the bone surrounding the apical end 36 of the implant 10. The implant 10 of the present invention thus induces strain of at least 100 microstrain in the surrounding bone, thereby promoting new bone growth and osteointegration. 
     The level of strain induced in the bone is a function of the stress imparted on the bone by the implant 10. In its simplest form, stress is equal to the magnitude of a force distributed over an area over which the force acts (Stress=Force/Area). Stress can be represented as either normal stress (perpendicular to the plane on which the force acts) or shear stress (parallel to the plane on which the force acts). Hooke&#39;s law relates normal stress and normal strain according to the following formula: 
     σ=Eε 
     where: 
     σ=normal stress 
     E=modulus of elasticity 
     ε=normal strain. 
     A similar relationship exists between shear stress and shear strain according to the following formula: 
     
         τ=Gγ 
    
     where: 
     τ=shear stress 
     G=modulus of rigidity 
     γ=shear strain. 
     In order to maintain a uniform strain profile along the entire length of the implant 10, the stress profile must be uniform. Clinical experience has demonstrated crestal resorption surrounding root form dental implants. As has been shown in analytical studies, stresses are markedly increased in bone near the crestal regions of dental implants. This stress may be reduced in the crestal region by increasing the surface area in contact with the bone at the crestal region. In the preferred embodiment of the present invention, this area is progressively increased as the thread 40 approaches the top of the implant by using a gradually increasing thread depth. This concept is further enhanced by making the preselected angle, wherein the angle is a function of the vertical position of the thread along the length of the implant, between the upper face edge 52 and lower face edge 54 approach 180 degrees at a preselected rate, the rate being determined by the strain equations, as it advances to the top of the implant. However, in order to promote ease in the surgical process, the outside diameter of the implant must remain constant. By maintaining a constant outside diameter, the hole drilled into the bone for placement of the implant may have a constant inside diameter. Also, a constant outside diameter will allow the implant to have self-tapping threads which can engage the cortical plate for increased stability. This feature establishes the need for a tapered implant core. The tapered core enables the thread surface area to increase gradually toward the crestal region of the implant while the outside diameter remains constant. 
     In one preferred embodiment for a group of implants for posterior mandibles, the following classification of trabecular bone density are used to provide constraints for the design: D1, D2 (coarse), D3 &amp; D4 (fine). The moduli of elasticity for the trabecular bone has been quantified for these densities. For example, the modulus for D2 is 67.5 MPa and the modulus for D3 and D4 is 35.5 MPa. 
     As shown in FIG. 2, the various bone shapes of the mandible can be characterized into six divisions. Of these divisions, divisions A 60 and B 70 and are immediate candidates in the posterior mandible for using an implant in accordance with this preferred embodiment of the present invention. Divisions C-h 76, B-W 72, C-W 74, and D 78 would be candidates for this embodiment following bone grafting procedures. The available height, width and length of available bone must also be assessed for each patient. The height is measured from the crest 62 of the endentulous ridge to the opposing landmark (e.g. the maxillary sinus or mandibular canal 64). The width is measured from the facial plate 66 to the lingual plate 68 at the crest. The length is limited by adjacent teeth or other implants (not shown). The outside diameter of the implant depends on the width and length of the available bone. 
     Referring again to FIG. 1, the crest portion 20 is incorporated into the implant design in order to provide a point of attachment for a prosthesis (not shown) and to provide a crestal bone seal. The diameter of the crest portion varies according to bone type, ranging from approximately 0.4 mm larger than the outside diameter of the implant with D4-type bone to a diameter that is equal to the outside diameter in D1-type bone. The larger diameter in D4 bone aids in the dissipation of forces in that crestal region below the crest portion 20. The crest portion 20 also compensates for increased bone loss that may occur during the surgical process and ensures an adequate crestal bone seal. 
     Using the above described parameters, for the example of the posterior mandible, the following constraint groupings are used to optimize the design. It would be obvious to one skilled in the art of implant design that these constraints could be different for regions other than the posterior mandible. 
     
         __________________________________________________________________________   Implant Outer      Trabecular Bone   Diameter, Implant Height,                      properties     Axial MechanicalDensity OD (mm)   h (mm)   E = elastic modulus (MPa)                                     Force (Newtons)__________________________________________________________________________Division A:   3.8 &lt; OD ≦ 4.5             12 &lt; h ≦ 16                      N/A            42-2500D1      4.5 &lt; OD ≦ 5.5D2a     3.8 &lt; OD ≦ 4.5             12 &lt; h ≦ 16                      67.5           42-2500   4.5 &lt; OD ≦ 5.5D2b     3.8 &lt; OD ≦ 4.5             12 &lt; h ≦ 16                      67.5           42-2500   4.5 &lt; OD ≦ 5.5D3a     3.8 &lt; OD ≦ 4.5             12 &lt; h ≦ 16                      35.5           42-2500   4.5 &lt; OD ≦ 5.5D3b     3.8 &lt; OD ≦ 4.5             12 &lt; h ≦ 16                      35.5           42-2500   4.5 &lt; OD ≦ 5.5D4      3.8 &lt; OD ≦ 4.5             12 &lt; h ≦ 16                      35.5           42-2500   4.5 &lt; OD ≦ 5.5Division B:   3.0 &lt; OD ≦ 3.8             10 &lt; h ≦ 16                      N/A            42-2500D1D2a     3.0 &lt; OD ≦ 3.8             10 &lt; h ≦ 16                      67.5           42-2500D2b     3.0 &lt; OD ≦ 3.8             10 &lt; h ≦ 16                      67.5           42-2500D3a     3.0 &lt; OD ≦ 3.8             10 &lt; h ≦ 16                      35.5           42-2500D3b     3.0 &lt; OD ≦ 3.8             10 &lt; h ≦ 16                      35.5           42-2500D4      3.0 &lt; OD ≦ 3.8             10 &lt; h ≦ 16                      35.5           42-2500Division C-h:   3.8 &lt; OD ≦ 4.5             10 &lt; h ≦ 14                      N/A            42-2500D1      4.5 &lt; OD ≦ 5.5D2a     3.8 &lt; OD ≦ 4.5             10 &lt; h ≦ 14                      67.5           42-2500   4.5 &lt; OD ≦ 5.5D2b     3.8 &lt; OD ≦ 4.5             10 &lt; h ≦ 14                      67.5           42-2500   4.5 &lt; OD ≦ 5.5D3a     3.8 &lt; OD ≦ 4.5             10 &lt; h ≦ 14                      35.5           42-2500   4.5 &lt; OD ≦ 5.5D3b     3.8 &lt; OD ≦ 4.5             10 &lt; h ≦ 14                      35.5           42-2500   4.5 &lt; OD ≦ 5.5D4      3.8 &lt; OD ≦ 4.5             10 &lt; h ≦ 14                      35.5           42-2500   4.5 &lt; OD ≦ 5.5__________________________________________________________________________ 
    
     Several studies have recorded bite forces in the range of 42 to 2500 Newtons, therefore these limits are used for the force constraint used to optimize the design. 
     Referring to FIGS. 3 &amp; 4, by applying the above-listed constraints and the following equations to an optimization program, the dimensions of the various surfaces of the implant may be determined: ##EQU1## Where: n=the number of turns of the thread around the core section. 
     DT=the diameter of the base portion from thread to thread. 
     BD=the diameter of the apical end. 
     TD=the diameter of the crestal end. 
     H=the height of the base portion. 
     Th=one-half of the width of the thread. 
     E=the modulus of elasticity for D2 or D3 or D4 trabecular bone densities. 
     F=normal force on the implant. 
     γ=the shear strain induced in the bone. 
     ε=the normal strain induced in the bone. 
     θ=the angle of the thread face edges to normal. 
     C=0.4 in D4 bone. 
     C=0.3 in D3 bone. 
     C=0.2 in D2 bone. 
     C=DT in D1 bone. 
     φ=the angle of the outer surface of the core section to normal. 
     α=the angle of the thread ledges to normal. 
     V=Poisson&#39;s ratio. 
     The above strain equations contain each of the geometric variables used to describe the macro-design. These strain equations are employed in a computer optimization program in order to optimize the design. Once optimized in accordance with the above strain equations, the design will induce strain in the desired range in the implanted bone. 
     Although the above embodiment is tailored for screw root form dental implants, it should be appreciated by those skilled in the art that the disclosed method can be applied to other types of skeletal implants. These include hip implants, skull implants, and any type of skeletal implant in which maintaining a predetermined level of strain in the bone promotes osteointegration and reduces bone resorption under physiological functional loading. 
     In an alternative preferred embodiment, as shown in FIG. 5, a dental implant 105 may comprise a groove 120, transverse to the thread 110, to prevent rotation of the implant 105. 
     The above embodiments are given as illustrative examples and are not intended to impose any limitations on the invention. It will be readily appreciated that many deviations may be made from the specific embodiments disclosed in this specification without departing from the invention. Accordingly it is intended to cover all such modifications as within the scope of this invention.