Abstract:
A fiber-reinforced composite material with graded stiffness, and method of making the same. The degree of stiffness variation may be controlled via modifications to the fabrication processes. Products with varying stiffness along their lengths are described. In one example, a dental post is fabricated from material according to the present invention which has one end with low stiffness adapted for insertion into the root of the tooth, and a high stiffness end suitable to support the restoration.

Description:
FIELD OF THE INVENTION 
     The present invention is related to the fiber-reinforced composite materials. In particular, the present invention is related to a functionally graded fiber-reinforced composite material and processes for making the same. More particularly, this invention is related to a dental post and method of making the same. 
     BACKGROUND OF THE INVENTION 
     Dental posts are used in reconstruction of endodontically treated teeth. The function of the dental post is to provide retention and lateral stability to the restoration. The restorative material is used to build the core and the crown at the coronal end of the tooth. In order to provide effective lateral stability, the dental post should have high stiffness in the coronal region. Stiffness refers to Young&#39;s modulus, which is the resistance of a material to deformation. Another important function of the dental post is to disperse occlusal forces from the coronal region to the remaining tooth structure. In addition, a good dental post should have a long fatigue life and high corrosion resistance. 
     In view of the above-stated requirements, state-of-the-art dental posts are either made of stainless steel, titanium or other metal alloys, or fiber reinforced composites, which have stiffness much higher than that of dentine, Although, the high stiffness is generally recognized as being necessary for the effective support of the restorative material in the coronal region of the tooth in order to prevent loosening of core and crown, it also gives the major drawback of causing stress concentration at the apical end of the tooth. The rigid post stiffens the coronal posted section and shifts the flexure point apically. The effect of this stiffening is to cause the non-posted apical portion of the tooth to deform at the post apex, resulting in a stress increase in that portion of the canal wall. Furthermore, the cyclic loading and unloading during mastication requires consideration of fatigue failure. Since the maximum bending stresses occur in the vicinity of the apex of the post, any inclusions or defects within the wall of the dentine near the apical end of the post would create stress concentration that increases the risk of a fatigue crack formation. Defects and microfractures introduced during endodontic treatment and post access preparation could become areas contributing to stress concentration. As a result, fractures frequently occur at the root of the tooth. Furthermore, the high stress experienced at the post-dentine interface of a high stiffness post often causes loosening at the post-dentine interface. 
     Various designs have been suggested in the art to address some problems stated above. These include tapered posts (U.S. Pat. No. 5,104,321), and flexible posts (U.S. Pat. No. 5,518,399). For tapered posts, the change in the shape of the post affects the bending stiffness without changing the Young&#39;s modulus of the post. Since Young&#39;s modulus is a critical parameter in determining the stress loading at the post-dentine interface, a tapered post does not solve the problem of root stress resulting from the high stiffness of the post. Furthermore, wedging effect occurs at the tip of the tapered end, resulting in vertical fractures. A flexible post has a reduced Young&#39;s modulus, which reduces the problem of root stress, but may not give sufficient support to the restoration, resulting in loosening and early failure of the restoration. 
     OBJECT OF THE INVENTION 
     It is therefore an object of the present invention to provide a material for a dental post to overcome the shortcomings as stated above. 
     It is another object to provide a fiber-reinforced composite material with varying stiffness. 
     SUMMARY OF THE INVENTION 
     The present invention provides fiber-reinforced composite material with graded stiffness, and method of making the same. The degree of stiffness variation may be controlled via modifications to the fabrication processes. Products with varying stiffness along their lengths are described. In one example, a dental post is fabricated from material according to the present invention which has one end with low stiffness adapted for insertion into the root of the tooth, and a high stiffness end suitable to support the restoration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic diagram to show one method of producing a post in accordance with the present invention. 
     FIG. 1B is a diagram to show a preform produced according to the method shown in FIG.  1 A. 
     FIG. 2A is a schematic diagram to show a second method of producing a post in accordance with the present invention. 
     FIG. 2B is a photograph of a preform produced by the method shown in FIG.  2 A. 
     FIG. 3 is a schematic diagram to show a third method of producing a post in accordance with the present invention. 
     FIG. 4 is a schematic diagram to show a fourth method of producing a post in accordance with the present invention. 
     FIG. 5 is a finite element analysis showing normal tensile stress levels along post interface. 
     FIG. 6 is a finite element analysis showing normal tensile stress levels along dentine interface. 
    
    
     DESCRIPTION OF THE INVENTION 
     An important parameter for excellent performance of the dental post is its stiffness. Since stress sharing within a system depends upon the material stiffness of each component, a key to the present invention is the recognition that the ideal dental post is one which gives graded material stiffness along its length. The post should reinforce the coronal region so as to reduce stress in the restoration and associated interfaces. In addition the post should not cause stress concentration at the apical end. This requires the post to take away the stresses from the coronal region and gradually unload them to the dentine. Therefore the segment of the post at the core region must be stiff to take away the stress from the core, while stiffness along the dentine region of the tooth should gradually be reduced to the value of the dentine. Hence, the post according to the present invention comprises segments of varying stiffness along its length. As fiber reinforced composite is used to make the dental post in the present invention, good fatigue and corrosion resistance is also achieved. 
     For the purpose of understanding the present invention, stiffness refers to Young&#39;s modulus. Segments of a post refers to imaginary divisions along the length of a single post for ease of description of graded stiffness, and do not necessarily mean an actual physical separation of the post into different sections. Braiding angle refers to the angle between a braided fiber and the longitudinal axis of a long product. The material may be any conventional composite material such as a combination of ceramic, polymer or metals. The materials may also be a combination of different polymer or different metals or different ceramic. The following are examples of methods of fabricating a post with graded stiffness. 
     For a dental post, the preferred stiffness at the apical end is approximately the same as the stiffness of the dentine i.e. 20 GPa. The stiffness at the coronal end is preferably much higher than the dentine stiffness, such as 100 GPa. The stiffness which can actually be achieved would depend on the material used. For example, a carbon fiber composite dental post may have a stiffness of 150-80 GPa at the coronal end, and 25-15 GPa at the apical end. Other stiffness ranges depends on the application, and may be obtained using the methods described below. 
     EXAMPLE  1   
     Referring to FIG. 1A, spools  20  of metallic, ceramic or polymeric fibers  22  are interlaced into a preform  26  with or without a mandrel or core. After braiding in the braiding machine  28 , the preform is transferred to a resin bath  30  containing unreacted resin  32 . Once the unreacted resin has impregnated the fibers in the preform, it is transferred to a curing die  34  of the required diameter wherewith the resin is cured to form the polymeric component or matrix of the fiber-reinforced composite post. The polymeric component may be any kind of thermosetting resin such as epoxy, polyester, or molten or solvent dissolved thermoplastic resin. 
     According to the conventional method, the take-up speed N 1  (shown by arrow  24 ) and spindle speed N 2  (shown by arrow  36 ) has a fixed ratio and hence, the angle of braiding is fixed. According to the present invention, the ratio between the take-up speed N1 ( 24 ) and spindle speed N2 ( 36 ) is changed continuously in order to vary the angle of braiding. FIG. 1B is a schematic drawing of a preform made according to the present invention. This corresponds to section  38  in FIG.  1 A. The angle of braiding in segment  40  with reference to the longitudinal axis of the post is smaller than the angle of braiding in segment  42 . Since it is well known that the stiffness of the fiber-reinforced material is higher when the braiding angle is smaller, varying the take-up and spindle speed ratio would result in a product with varying stiffness. A dental post  46  in accordance with the present invention may then be obtained, for example, by cutting the material at sites  44 . Hence post  46  would have one segment  40  which is of higher stiffness than segment  42 . It should be understood that using this method, a continuous gradient of stiffness is obtained, and that describing the material as having different segments is merely to facilitate understanding of the inventive concepts. 
     EXAMPLE 2 
     FIG. 2A shows a variation to the braiding method. In addition to the metallic, polymeric or ceramic reinforcement fibers  48 , at least another set of the fibers is a polymeric material  50 . During the braiding process, the ratio between the take-up speed N1  24  and spindle speed N2  36  is changed continuously in order to vary the braiding angle. The braided preform  51  is then transferred to mold  52  for compression molding at required temperature at which specific set of polymeric fibers melt and impregnate the reinforcement fibers. The final product  54  is one having varying stiffness along its length. Section  56  of the preform would have braided fibers similar to the ones shown in FIG. 1B, except those additional polymeric fibers are also intertwined. FIG. 2B shows a photograph of a preform dental post with a continual variation in the braid angles. Therefore, the stiffness in segment  58  is generally higher than the segment in section  60 . 
     EXAMPLE 3 
     In a conventional filament winding method, a fiber or filament or tape  62  is wound around a core or mandrel  64 . The filament may be made of metal, polymer or ceramic. The mandrel may be metal, ceramic or polymer. According to the present invention, the filament is wound at varying orientation with reference to the longitudinal axis of the core. A thermosetting matrix may also be provided according to conventional methods. The resultant product again has smaller angle and higher stiffness in segment  66 , and larger angle and lower stiffness in segment  68 . 
     EXAMPLE 4 
     The stiffness of a composite fiber-reinforced post or long product may also be varied based on the fiber volume fraction. FIG. 4 shows a conventional casing  70  and plunger  72  for injection molding. In the conventional injection molding method, short fibers are mixed with a matrix material in a desired ratio and cured in the casing. In an embodiment of the present invention, mixtures with varying fiber volume fraction (i.e. varying proportions of fibers and matrix material) are transferred to the casing consecutively, and curing allowed to occur together. In the example shown in FIG. 4, four mixtures ( 74 ,  76 ,  78 ,  80 ) of varying fiber volume fractions are transferred to the casing. After curing, a single post is produced with four segments ( 74 ,  76 ,  78 ,  80 ) of varying proportions of fibers. Since it is well known that the stiffness of the product depends on the fiber volume fraction, the resulting product contains corresponding segments of varying stiffness. The fibers may be made of metal, silica, polymer or ceramic. The matrix material is again made from conventional material such as thermosetting resin or thermoplastic resin. 
     Test Results 
     Dental posts with graded stiffness of 20 GPa to 80 GPa were compared with conventional posts using finite element stress analysis. The results of finite element analysis are shown in FIG. 5 and 6. A diagram of a restored tooth  82  is shown above the results to indicate the corresponding positions of the each test data at the horizontal axis. The restored core  84  and crown  86 , the dental post  88 , the dentine  90 , the root  91 , and the aveolar bone  92  are shown. The vertical axis shows the normal tensile stress levels in MPa. 
     FIG. 5 and 6 shows the tensile stress along the post and dentine respectively at the longitudinal interface. Curves  94  and  95  represents the stress levels for a conventional stainless steel or unidirectionally drawn carbon fiber post having high fiber volume fraction and 200 GPa stiffness. Curves  96  and  97  shows results for a uniform 80 Gpa carbon post. Curves  98  and  99  are results for a post with graded stiffness of 20 to 80 GPa in accordance with the present invention. For both post and dentine, the highest stress levels are detected for the conventional high stiffness post. The stress levels are reduced when the post stiffness is reduced to a uniform 80 GPa. The post with graded stiffness according to the present invention produces substantially lower stress levels than either of the uniform posts. The graded post shows a maximum of 25 MPa stress levels at the interface, which is only 7% of the 382 MPa peak stress experienced by the stainless steel post. At the dentine side of the interface, the peak stress level of 32 MPa detected using the graded post is only 61% compared to using the stainless steel post. These results are further summarized in Table 1 below. 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Residual 
                   
                 Residual 
               
               
                   
                   
                   
                 stress with 
                 Highest 
                 stress with 
               
               
                   
                   
                 Highest 
                 reference 
                 stress 
                 reference 
               
               
                   
                   
                 stress level 
                 to a 
                 level on 
                 to a 
               
               
                   
                   
                 on Post at 
                 200 GPa 
                 dentine at 
                 200 GPa 
               
               
                 Post Type 
                 Stiffness 
                 interface 
                 post 
                 interface 
                 post 
               
               
                   
               
             
             
               
                 Stainless 
                 200 GPa 
                 382 MPa 
                 100% 
                 52 MPa 
                 100%  
               
               
                 steel or 
               
               
                 UD 
               
               
                 carbon 
               
               
                 UD 
                  80 GPa 
                 114 MPa 
                  30% 
                 42 MPa 
                 81% 
               
               
                 Carbon 
               
               
                 Graded 
                 20-80 
                  25 MPa 
                  7% 
                 32 MPa 
                 61% 
               
               
                 stiffness 
                 GPa 
               
               
                   
               
             
          
         
       
     
     While the present invention has been described particularly with reference to FIGS. 1 to  6  with emphasis on a dental post, it should be understood that the figures and the numerical values indicated are for illustration only and should not be taken as limitation on the invention. In addition it is clear that the graded fiber reinforced composite material and the methods of the present invention has utility in many applications, shapes and sizes where graded stiffness and function are required, such as sports equipment, structural elements, biomedical implants and devices etc. It is contemplated that other methods and numerous changes and modifications may be made by one of ordinary skill in the art without departing from the sprit and the scope of the invention described.