MOUTHPIECE AND METHOD FOR PRODUCING THE SAME

Abstract: A mouthpiece comprising an ethylene-vinyl acetate copolymer having a vinyl acetate content of from 12 to 33 wt%, and the process for producing the same comprising the steps of: preparing a thermoplastic elastic sheet material formed to be increased in thickness from one end to the other end; preparing a teeth impression cast of a user who is going to use a mouthpiece to be produced; and pressing said teeth impression cast against said thermoplastic elastic sheet material in a softened state so as to transfer a shape of said teeth of said user onto said thermoplastic elastic sheet material to thereby obtain a mouthpiece which is set to be thicker at a portion corresponding to a front teeth posi tion than at a portion corresponding to a molar teeth position.

Description: The present invention relates to a mouthpiece which can be exclusively used by a specified user and which can protect the jaw bones, teeth, and oral cavities from external force in view of moldability, flexibility, and elasticity. The present invention further relates to a method of producing such a mouthpiece in a simple manner. In contact sports such as rugby or football an accident, for example, a fracture of a jaw bone or a laceration of soft tissue of an oral cavity frequently happens. In order to prevent such accidents, it is desired to put a mouthpiece in the mouth. Further, mouthpieces have been used as a medical instrument for preventing gnashing or for treating temporomandibular disorders. Such mouthpieces are formed by a rubbery elastic material as described, e.g., in Japanese Patent Application (OPI) No. 100273/86 (the term "OPI" used herein means a published unexamined Japanese patent application). A mouthpiece formed of a rubbery elastic material, however, has a disadvantage in that it does not have a sufficient impact absorption effect and it is difficult to perfectly fit the mouthpiece in the mouth. Since a rubber material is used mouthpieces are poor in moldability, and a mouthpiece which is satisfactory in use and in flexibility as well as in elasticity has not been obtained although some improvements have been proposed as described, e.g., in U.S. Patent 3,073,300. Generally, a mouthpiece as described above is produced as follows. First, an impression of the teeth of an objective user is modeled by using a dental impression material, such as an alginate impression material, to form a plaster cast of the teeth, and a thermoplastic elastic rubbery elastic material shaped into a plate having a thickness of about from 1 to 3 mm is softened by heating and applied to the plaster cast to thereby obtain a mouthpiece through a vacuum molding process. Being molded with such a rubbery elastic material shaped into a plate even in thickness in all portions, the mouthpiece also even in thickness in portions corresponds to the front teeth portion and the molar teeth portion, or has such a condition that the portion corresponding to the front teeth portion becomes thinner upon molding than the portion corresponding to the molar teeth portion. Because the front teeth are sharper than the molar teeth, there is a tendency that the occlusion forces at the molar teeth portion are exceedingly larger than that at the front teeth portion. In normal occlusion, however, the space between the front teeth portions of the upper and lower jaws immediately before occlusion is about twice as wide as that between the molar teeth portions of the upper and lower jaws, so that the occlusion forces at the molar teeth portion are larger than those the front teeth portion. Accordingly, there is a disadvantage that strain may be generated in jaw bones or temporomandibular joint in the case where a mouthpiece is put in the mouth. GB-A-1527685 discloses a flexible, collapsible container whose walls consist substantially of biaxially-oreinted ethylene-vinyl acetate copolymer containing from 10 to 35 percent by weight of vinyl acetate units. GB-A-2004559 discloses an elastic strapping band comprising ethylene-vinyl acetate copolymer containing vinyl acetate in the range of from 3 to 20 percent by weight. It has been desired to provide a method of producing a mouthpiece having a thickness which is larger at the portion corresponding the front teeth position than at the portion corresponding to the molar teeth position. SUMMARY OF THE INVENTION It is the object of the present invention to provide a mouthpiece which has a sufficient impact absorption effect and which is perfectly fitted in the mouth, as well as a method for producing same. Said object is achieved by a mouthpiece comprising an ethylene-vinyl acetate copolymer having a vinyl acetate content of from 12 to 33 wt%. Furthermore, there is provided a method for producing a mouthpiece comprising the steps of: preparing a thermoplastic elastic sheet material which is increased in thickness from one end to the other end; said sheet material comprising an ethylene-vinyl acetate copolymer having a vinyl acetate content of from 12 to 33 wt%; preparing a teeth impression cast of a user who is going to use the mouthpiece to be produced; and pressing said teeth impression cast against said thermoplastic elastic sheet material in a softened state to transfer the shape of said teeth of said user onto said thermoplastic elastic sheet material to thereby obtain a mouthpiece which is thicker at the portion corresponding to the front teeth position that at the portion corresponding to the molar teeth position. The present invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: Figs. 1 through 3 are perspective views showing examples of thermoplastic elastic sheet materials used in the method according to the present invention; and Fig. 4 is a side view showing a cranium used in the experiments for the mouthpiece according to the present invention. The mouthpiece according to the present invention is produced by using an ethylene-vinyl acetate copolymer. Accordingly, the softening point of the ethylene-vinyl acetate copolymer can be adjusted by changing the content of the vinyl acetate, so that it is possible to provide a satisfactory mouthpiece improved in moldability and provided with flexibility and elasticity. The vinyl acetate is contained for adjusting moldability and flexibility of the ethylene-vinyl acetate copolymer. For example, a larger content of vinyl acetate in the copolymer provides a smaller rigidity modulus, a higher flexibility, a lower heat deformation temperature, and the better moldability at a low temperature. In view of the above, it is preferred that the content of the vinyl acetate is from 28 to 33 wt%. The ethylene-vinyl acetate copolymer used in the present invention can be prepared by any conventional methods known in this field of art, such as those described in Hiroshi Iwasaki, Kobunshi Kako (Process for Highmolecules), Separate Volume V (Auguse 1969), pp 9 et seq and ditto, pp 49 et seq. The ethylene-vinyl acetate copolymer used in the present invention may contain various coloring substance such as organic dyes and inorganic pigments. As a material for producing the mouthpiece generally rubber or thermoplastic soft synthetic resins are used. However, low density polyethylene is inferior in elongation although superior in moldability as well as in impact strength. Polypropylene is relatively inferior in impact strength although superior in moldability as well as in elongation. Vinyl chloride is inferior in moldability, and relatively inferior in elongation as well as in impact strength. Fluororesin is inferior in moldability, and relatively inferior in elongation. An ethylene-vinyl acetate copolymer however has suitable characteristics for a mouthpiece regarding moldability elongation, and impact strength. Particularly, the moldability, flexibility and elasticity of the copolymer can be adjusted by changing the content of vinyl acetate of the ethylene-vinyl acetate copolymer as described above, so that it is possible to mold a mouthpiece for exclusive use for a specified user which provides a good feeling as well as a good fitness to the mouth in use. If the copolymer has a vinyl acetate content of less than 12 wt% it is inferior in impact resistance as well as in moldability. A copolymer having a vinyl acetate content of more than 33 wt% is soft and inferior in strength and practical use. The mouthpiece is made of ethylene-vinyl acetate copolymer resin as follows. First, a plate having a thickness of about from 1 to 5 mm is formed of the foregoing resin, and the plate is heated. Next, the heated resin plate is vacuum molded by using a plaster cast formed from teeth of a specified user to thereby obtain a mouthpiece suitably fitted to the oral cavities of the user. In production, the mouthpiece according to the present invention can be obtained in such a manner that the foregoing resin is pre-molded in a horseshoe shape along a row of teeth, or the thus premolded resin is softened by heating to be easily molded in the oral cavities of the user. Further, the space between the upper and the lower front teeth positions of the user is larger than that between the upper and lower molar teeth positions in the mouth. Accordingly, it is preferred to form a mouthpiece in which the portion corresponding to the front teeth position is set to be thicker than that corresponding to the molar teeth position. Such a mouthpiece can be produced by the method according to the present invention described below. In the method for producing a mouthpiece according to the present invention, a thermoplastic elastic sheet material having a thickness increased from one end to the other end is pressed to be molded in a softened state against the teeth impression cast of the mouthpiece user, so that the thickness of the molded mouthpiece can be set to be thicker at the portion corresponding to the front teeth position of the user than the portion corresponding to the molar teeth position of the user. In the above method the mouthpiece is produced by using a thermoplastic elastic sheet material having a thickness increased from one end to the other end and being in a softened condition by heating. The thermoplastic elastic sheet material comprises an ethylene-vinyl acetate copolymer having a vinyl acetate content of from 12 to 33 wt%. In the method for producing a mouthpiece according to the present invention, the thermoplastic elastic sheet material is molded into a sheet, for example, as shown in Fig. 1, 2 or 3. The sheet is softened by heating and the sheet in the softened state is molded in the next step to thereby obtain a mouthpiece. Fig. 1 shows a thermoplastic elastic sheet 1 having a thickness linearly increased from one end 2 to the other end 3. Fig. 2 shows another thermoplastic elastic sheet 4 having a surface 5 which is curved so that the thickness of the sheet is increased from one end to the other end. Fig. 3 shows still another thermoplastic elastic sheet 6 having a thickness 7 which is stepwise increased from one end to the other end. Each of these elastic sheet materials 1, 4, and 6 respectively shown in Figs. 1 through 3 is shaped so as to have a thickness which is increased from one end to the other end. The thick and thin portion sides of each of the elastic sheet material 1, 4, and 6 are set as portions corresponding to the front and molar teeth positions of the user, respectively. Then, each of the elastic sheet material 1, 4, and 6 is vacuum molded by using a teeth impression cast of the mouthpiece user to transfer the teeth shape of the user onto the elastic sheet material to thereby make it possible to obtain a mouthpiece in which a portion corresponding to the front teeth position is set to be thicker than that corresponding to the molar teeth position. In a method for producing a mouthpiece as described above, it is possible to easily and rapidly mold a mouthpiece in which the front teeth side is set to be thick and the molar teeth side is set to be thin as described above. By using ethylene-vinyl acetate copolymer resin containing vinyl acetate of from 12 to 33 wt% and having a low softening point as the elastic sheet material, it is possible to improve the moldability in forming a mouthpiece for exclusive use for a specified user and to provide suitable flexibility and elasticity to the molded mouthpiece. The following examples illustrate the invention. EXAMPLE 1 Plates (width: 130 mm x 130 mm x thickness: 2 mm) were formed by using an ethylene-vinyl acetate copolymer resin containing vinyl acetate with compositions as shown in Table 1. The impact absorption effect as a mouthpiece was measured for these seven kinds of plates by using an adult cranium having perfect teeth. An impression of the upper jaw of the cranium was modelled in a silicon impression material, and a plaster cast was formed from the impression. Mouthpieces for used for the upper jaw were formed by using the foregoing seven kinds of samples through a vacuum molding process. In order to measure the main strain in various portions of the cranium of Fig. 4, strain gages were stuck to four points represented by reference numerals 11 through 14 in Fig. 4, and the cranium was fixed at its parietal portion by using an iron frame. In this state, a load of 3 kg was applied perpendicularly to the impartial mental region and the strain was measured. The Table 2 shows values of the main strain at various portions using the mouthpieces formed from the samples. In addition, a commercial mouthpiece (Mouthpiece for sports, #F2 made by Winning Co., Ltd.) was tested in the same manner as above, and the results obtained are also shown in Table 2. In Table 2, the strain when no mouthpiece is used is represented as 100 %. As can be seen from Table 2, in the case where the main strain is assumed to be 100 % when no mouthpiece is put in the mouth, it was found that each of the mouthpieces produced by using the ethylene-vinyl acetate copolymers shown as samples 1 though 4 have a high impact absorption effect. EXAMPLE 2 Next, moldability and practicability of the mouthpieces were measured while changing the content of vinyl acetate. Table 3 shows the results of measurement. The moldability in Table 3 was evaluated in grades A (good) and B (not good) by the following manner. The ethylene-vinyl acetate plates and the commercial mouthpiece material were heated in boiling water for 5 min and then vacuum molded using a teeth model to prepare mouthpieces. While observing the facies occlusalis of the mouthpieces, those having facies occlusalis sufficiently well molded were evaluated as A, and those having facies occlusalis insufficiently molded were evaluated as B. The practicability in Table 3 was organoleptically evaluated in grades A (good) and B (not good) by the practical use. Specifically, upon using the sample of vinyl acetate content of 8 wt%, the mouthpiece was felt hard, and the gums hurt where the mouthpiece touched the gums. Moreover, because of the deteriorated moldability, the mouthpiece having a vinyl acetate content of 8 wt% was not sufficiently fitted in the mouth and rattled upon using. The mouthpiece having a vinyl acetate content of 38 wt% was soft and broken by normal occlusion. The moldability shows the fact that the softening point of the ethylene-vinyl acetate copolymer changes in accordance with the content of vinyl acetate in such a manner that the softening point becomes high as the content of vinyl acetate decreases while the softening point becomes low as the content increases. The softness of the mouthpiece changes in accordance with the content of vinyl acetate in such a manner that the mouthpiece becomes hard when the content is equal to 8 wt% while it becomes when the content is equal to 38 wt%. It was found that when the content of vinyl acetate falls within a range of from 12 to 33 wt%, the mouthpiece shows more satisfactory characteristics in view of moldability as well as practicality to softness, so that a particularly good results can be obtained in the use of the mouthpiece. EXAMPLE 3 By using the ethylene-vinyl acetate copolymer resins used for samples Nos. 1 to 4, plates having the compositions shown in Table 4 in which the thickness increased from one end to the other end (width: 130 mm x 130 mm x thickness: 2 to 5 mm) were formed, and designated as samples Nos. 1', 2', 3', and 4'. Mouthpieces were prepared in the same manner as in Example 1 by using the plates of the samples Nos. 1 to 4 and 1' to 4'. The thickness of the portion corresponding to the front teeth portion and the thickness of the portion corresponding to the molar teeth portion of the mouthpieces thus prepared were measured. The results obtained are shown in Table 5. From the results in Table 5, it was found that in the mouthpieces prepared by using the plates in which the thickness increased from one end to the other end (samples Nos. 1' to 4'), the thickness of the portion corresponding to the front teeth portion was thicker than that corresponding to the molar teeth portion. Samples Nos. 1' to 4' were measured for the impact absorption effect in the same manner as in Example 1. The results obtained are shown in Table 6 together with the results of samples Nos. 1 to 4. It is apparent from Table 6 above that the mouthpieces prepared by using the plates in which the thickness increases from one end to the other end (samples Nos. 1' to 4') have a particluarly excellent impact absorption effect. It is considered that this is because the occlusion force is uniformely dispersed at the portions of from the molar teeth portion to the front teeth portion. The samples Nos. 1' to 4' were measured for moldability and practicability in the same manner as in Example 2, and as the result, a good moldability and a good practicality were obtained in all the samples Nos. 1' to 4'.

Claims: A mouthpiece comprising an ethylene-vinyl acetate copolymer having a vinyl acetate content of from 12 to 33 wt%. The mouthpiece of claim 1, wherein the content of vinyl acetate in said copolymer is from 28 to 33 wt%. The mouthpiece of claim 1, which is thicker at the portion corresponding to the front teeth position that at the portion corresponding to the molar teeth position. A method for producing a mouthpiece comprising the steps of: preparing a thermoplastic elastic sheet material which is increased in thickness from one end to the other end; said sheet material comprising an ethylene-vinyl acetate copolymer having a vinyl acetate content of from 12 to 33 wt%; preparing a teeth impression cast of a user who is going to use the mouthpiece to be produced; and pressing said teeth impression cast against said thermoplastic elastic sheet material in a softened state to transfer the shape of said teeth of said user onto said thermoplastic elastic sheet material to thereby obtain a mouthpiece which is thicker at the portion corresponding to the front teeth position that at the portion corresponding to the molar teeth position. The method of claim 4, wherein an ethylene-vinyl acetate copolymer having a vinyl acetate content of from 28 to 33 wt% is used as said elastic sheet material.

Inventors: FURUMICHI, HIROSHI, MAKISHIMA, TAKAO
Applicants: SUNSTAR KABUSHIKI KAISHA
Application year: 1987
Main classification: A61C9
IPC codes: A63B71