Patent Publication Number: US-2018028287-A1

Title: Dental prosthesis

Description:
The present invention concerns the dental sector, more particularly the sector of dental prostheses placed on implants. The present invention concerns, in particular, a dental prosthesis. 
     A prosthesis on implant consists in the placing of a dental prosthesis by means of an implant, for the purpose of obtaining a good aesthetic and functional result of the prosthesis or crown and of the tissues surrounding it. It is constituted by three functional parts: the implant, the post and core and the crown or prosthesis. Usually, each functional part corresponds to an independent element. 
     The implant is introduced in the bone in order to provide a support for the post and core, which has the function of joining the implant with the crown. The post and core usually remains fixed to the implant by means of a thread and the crown can be joined to the post and core by means of a thread or through cementation. In the case of the prosthesis that is the subject of the present invention, the crown is preferably joined to the post and core through cementation. 
     The crowns placed on post and cores of the known type, both the cemented and the threaded ones, are designed in such a way that they rest on a margin or shoulder of the post and core. The margin is delimited within the post and core and consists of a supporting surface for the crown that is placed on the same. It constitutes a transition section between the diameter of the implant and the diameter of the crown and is located exactly at the level of the gum, so that the post and core is not visible. Since the gum healing process is rather slow, it is necessary to wait at least 3 months before being able to place the post and core definitively, as any slight recession would make the post and core visible and expose it to the bacteria present in the mouth. 
     The existence of a margin involves, on one hand, the impossibility to place the crown definitively at the same time as the implant and the need to wait for a long time before placing the crown definitively. 
     The healing of the gum around the post and core is of the type by secondary intention. This type of healing is characterized by a considerable contraction of the tissues due to myofibroblasts. The contraction of the tissues forces the gum to move towards a point in which the diameter of the post and core is smaller. The prostheses with margin or shoulder make it necessary for the profile of the post and core to diverge from the base. This forces the fibres of the tissue to stabilize at a lower level, with the risk of the post and core becoming exposed due to the fact that it has a smaller diameter. 
     Furthermore, all the prostheses (crowns) with margin or shoulder at least make it necessary to disconnect the post and core in order to place the final tooth with the defined margin. The disconnection of the post and core is the factor that causes the most accentuated gingival recession. 
     In standard post and cores, the diameter diverges from the highest point of the implant towards the lowest point of the crown, and therefore the smaller diameter is positioned at the base of the post and core, so that the gum tends to move towards the implant. This makes this type of post and cores less satisfying in terms of aesthetic appearance and also reduces their capacity to insulate the gum. If the insulation provided is not good, bacteria penetrate easily into the inner area, causing infections. 
     The tissue that remains in contact with the post and core in its area closest to the implant is called connective tissue. Post and cores have the function of holding the fibres of said connective tissue in order to stabilize the gum and insulate the implant from the contaminating environment of the mouth. For this purpose, the known post and cores are provided with a series of grooves in the area of the post and core that is closest to the implant. For example, the US patent US20110200969A1 describes a set of micro grooves that can be arranged with a certain inclination. However, the geometry of said micro grooves existing in the state of the art does not allow the regenerative capacity of the connective tissue to be fully exploited. 
     The prosthetic post and core incorporated in the dental prosthesis that is the subject of the present invention has the purpose to obtain better results than the known post and cores, in terms of both aesthetical appearance and biological functionality. 
     The present application practically describes a dental prosthesis of the type comprising a dental implant, a post and core and a crown, said implant being suited to be fixed to a bone and said post and core being suited to be fixed to said implant by means of a screw that passes through the inside of said post and core, wherein said post and core comprises a portion to be inserted into said implant and another portion protruding from said implant, wherein said post and core serves as a connection element between said implant and said crown, and wherein a separation area is provided between the area where the crown is housed in the post and core and the area where the post and core is housed in the implant, said dental prosthesis being characterized in that the diameter of said portion protruding from said implant converges from said implant towards said crown, without increases. 
     The present invention is not limited to cylindrical or conical shapes, meaning shapes with circular cross section. In the present application the word “diameter” must be understood widely, including the so-called hydraulic diameter or equivalent diameter for non-circular geometric shapes. 
     Preferably, the diameter of said portion protruding from said implant converges in a continuous manner from said implant towards said crown, and does not increase. Optionally, the diameter converges with an angle of convergence included between 1° and 6°. More preferably, the diameter converges with an angle of convergence equal to 2°. 
     During the first week, and part of the second, after the implant with the post and core has been placed the fibres increase in number. At the beginning, most of the fibres have a certain elasticity. The type of wound that is produced by perforating the gum is a wound healing by secondary intention. This means that starting from the second week part of the fibroblasts are transformed into myofibroblasts, which have the capacity to contract. Being arranged in a circular shape, and due to their tendency to contract, myofibroblasts tend to be positioned in the area of the post and core with smaller diameter. Thanks to the decrease in the diameter of the post and core, the area with smaller diameter is located at the limit of the crown, rather than in the implant as is the case with the known post and cores. 
     The fact that the diameter of the post and core decreases from the implant towards the crown is not sufficient to ensure that the gum heals far from the implant. The crown can be positioned in the post and core in such a way that it is very near the implant and that the distance between the implant and the crown is very small or null. A crown with its lower part far from the highest part of the implant causes an accumulation of tissue further away from the implant. This ensures good insulation of the implant and a larger quantity of tissue, which improves the aesthetical appearance of the crown. 
     The fact that the converging inclination starts from the base is important, since tissues always regenerate starting from the innermost part, meaning the lowest part. In order for the above to take place, the base of the post and core must be positioned slightly under the surface, meaning that the implant must be inserted not too deep into the bone. 
     This improvement, furthermore, takes in consideration the possibility to improve the aesthetical result of a late gingival recession, as it is possible to change the crown of the prosthesis with no need to disconnect the post and core. Preferably, said crown can be fixed to said post and core through cementation and said post and core has no margins. 
     Since there is no need for margins, it is possible to place it at the same time as the implant and thus avoid any disconnection. 
     A unique characteristic that is typical of this post and core lies in that the crowns it supports must be different and not depend on the margin of the crown that rests on the margin of the post and core. This factor facilitates the formation of a short joining piece of epithelium. This aspect is important, as one of the main differences with respect to the tissues that surround teeth and implants lies in that the epithelium of the implants is longer and has a higher tendency to recession. By using these post and cores, without disconnecting the prosthesis, it is possible to obtain a shorter piece of epithelium compared to the rest of the known prostheses, as well as a thicker and more stable connective tissue. 
     Preferably, said post and core is provided with a microscopic or micrometric ascending thread that starts from the part of said portion projecting from said implant that is closest to the implant itself. Optionally, the width of said microscopic thread is included between 4 μm and 40 μm, preferably it is equal to 20 μm. As a further option, the depth of said microscopic thread is included between 4 μm and 40 μm, preferably it is equal to 20 μm. 
     The fibres of connective tissue (1-20 microns in diameter) start to surround the post and core according to a circular pattern, as the post and core is circular. Said fibres are produced by the fibroblasts. The maximum diameter of a fibre (20 microns) will determine the depth and width of the micro threads. 
     The tissues always regenerate starting from the deepest part and moving towards the surface. The presence of a continuous thread facilitates the movement of the fibres from the deepest part of the post and core to the most exposed area of the same. The size of the thread is smaller than that known in the art. This means that there will be less space for the proliferation of bacteria. 
     Also the depth is reduced, and therefore the vasa of the connective tissue are able to fight against infections. 
     In any case, it is the epithelium that has the function of physically stopping the passage of bacteria, and this is at a further level, thanks to the type of crown on implant that is produced without margins and in a specific manner. 
     In the prosthetic post and cores known in the art, the micro grooves have the capacity to stabilize collagen fibres so that they seem to be directly fixed into them. However, the inventors hereof have found that the fibres are arranged so as to form a ring and remain fixed within the grooves of the threads. 
    
    
     
       In order for the reader to better understand the invention, an example of embodiment of the same is described here below by way of non-limiting example, with reference to some drawings. 
         FIG. 1 a    shows a sectional view of an embodiment of a prosthetic post and core with a fully converging protruding portion. 
         FIG. 1 b    shows a sectional view of an embodiment of a prosthetic post and core whose protruding portion in turn comprises a first cylindrical lower part (α) and a second converging upper part (β). 
         FIG. 2  shows a front view of a dental prosthesis with a post and core of the known type with margin or shoulder. 
         FIG. 3  shows a front view of the post and core of  FIG. 1  fixed to an implant inserted in a bone and having a crown placed on it, wherein the contour of the post and core is highlighted with respect to the other elements so as to show its position within the crown. 
         FIG. 4  shows a front view of two dental prostheses. The left prosthesis corresponds to a known post and core with margin or shoulder and the right prosthesis corresponds to a prosthetic post and core according to the present invention, illustrated schematically (the micro thread is not shown). 
     
    
    
     The prosthetic post and core  1  shown in  FIGS. 1 a  and 1 b    is hollow and is internally provided with two areas that extend in the direction of its axis of symmetry. On one side there is the duct  5 , which is sufficiently wide to allow the head of a screw to pass therethrough. Said screw head remains fixed in the cavity  6  and the rest of the screw is housed in the connection element  4 , the portion of the post and core  1  that is inserted in the implant. The sides that surround the cavity  6  are thicker than those of the duct  5 , for the purpose of offering increased resistance to loads. 
     On the other hand, the diameter of the post and core  1  of  FIG. 1 a    converges upwards over the entire portion protruding from the implant, that is, starting from the connection element  4 . As can be observed, the angle of convergence is equal to 2°. In the part that is closest to the connection element, the post and core  1  is provided with an ascending micrometric thread  2  that starts immediately after the connection element  4 . 
     Alternatively, as shown in  FIG. 1 b   , the post and core  1  is provided with a first lower cylindrical part (α), meaning a part with constant diameter, and a second upper part (β) with diameter converging upwards. 
     Once the post and core  1  has been joined to the implant, the duct  5  must be filled with a resin-based material in order to seal the cavity and prevent the introduction or release of bacteria through the inside of the post and core  1 . 
     A typical post and core known in the art has a margin or shoulder such as, for example, that indicated by the numeral  50  in  FIG. 2 . The presence of the margin  50  implies that the diameter of the post and core  20  increases immediately after the implant  10 , thus defining the margin or shoulder  50 . The crown  30  has a part that comes to be fixed into the margin  50  of the post and core  20  and rests on the same, leaving the base of the crown  30  at the same level as the supporting line  90 . 
     In the known post and cores like the one shown in  FIG. 2 , the narrowest area of the post and core  20 , in the part located between the implant  10  and the crown  30 , is exactly in the contact point between the post and core  20  and the implant  10 . This causes the healed tissue to accumulate in this area and leads to the recession of the gum  40  that exposes the post and core  20 , as can be seen in  FIG. 2 , thus negatively affecting the aesthetical appearance of the implant and exposing the gum to the proliferation of bacteria. 
       FIG. 3  shows the prosthetic post and core  1  that is the subject of the present invention joined to an implant  10  fixed to a bone  70 , and with a crown  30  on the same. The area of interest corresponds to the part of the post and core  1  that is situated between the implant  10  and the crown  30 . In  FIG. 3  the contour of the post and core  1  is highlighted in order to show its position within the crown  30 . 
     As can be observed, from the area of the post and core exposed to contact with the gum  40 , the smaller diameter is exactly at the limit between the post and core  1  and the crown  30 . 
     This causes the gum  40  to accumulate in the lower part of the crown  30  during the healing process and consequently, due to the separation of said lower part of the crown  30  from the implant  10 , the quantity of tissue between the crown and the implant is larger than in the known post and cores, as shown in  FIG. 4 . 
       FIG. 3  shows how the gum  40  covers the post and core  1  completely, differently from that which happens with the post and cores known in the art, as that shown in  FIG. 2 . The numeral  80  indicates the portion of tissue that is fixed to the micrometric thread  2 . 
     The left dental prosthesis in  FIG. 4  uses a post and core with margin or shoulder  50  like those known in the art. The gum  40  covers a smaller part of the crown  30  and is closer to the implant compared to the prosthesis with the prosthetic post and core that is the subject of the present invention, shown on the right. The effect thanks to which the area with smaller diameter is further from the implant allows a larger quantity of tissue to be present between the implant  10  and the crown  30 . This can be observed from the different position of the tissue fibres  60  in the two prostheses. In the case of the known post and core with margin (on the left), the fibres  60  are located in an area closer to the implant and with little contact with the post and core. On the contrary, in the case of the post and core according to the present invention (on the right), the fibres  60  adhere to the entire surface of the post and core and in an area closer to the crown, with more tissue between the crown and the implant compared to the prosthesis illustrated on the left.