MX2007004475A - Plastic bracket with mechanical retention and production method thereof - Google Patents

Abstract

The invention relates to a bracket which is intended for use in orthodontics and which is made entirely from plastic using an injection moulding process. The inventive bracket comprises the basic elements of any bracket, such as the base which is fixed or cemented to the tooth, the arch groove in the mesial-distal direction and flanges for standard wire ligatures or elastomers. The invention is essentially characterised in that, as with metal brackets, the fixing base provides mechanical retention and, as such, does not have to be coated with any type of chemical additive during the cementing procedure.

Description

PLASTIC BRACKET WITH MECHANICAL RETENTION AND MANUFACTURING PROCEDURE THEREOF FIELD OF THE INVENTION The present invention relates to a bracket for use in orthodontics and its manufacturing process, which, although it is made exclusively of plastic by injection molding, has characteristics comparable to those of metal in terms of hardness, finish and mechanical support of the base. .

BACKGROUND OF THE INVENTION It is known that in orthodontic treatments it is necessary to adhere to the teeth, by cementation, pieces such as brackets, oral tubes or devices of this type. The specific parts to be joined are the anterior or posterior surface of the tooth and a surface of the bracket that is known as the dental surface of the base. The base is intimately linked to the so-called bracket body in which the retention groove of the orthodontic arch is generally found.

Taking into account the materials of manufacture, the brackets can be metallic, ceramic or plastic but, for economic and aesthetic reasons, it tends more and more that the brackets of last generation are manufactured with plastic, since the material is cheaper than the metal or the ceramic and, on the other hand, you can get colors similar to that of the teeth or better still make them transparent which gives an added aesthetic value. A technological challenge of the manufacturers of orthodontic appliances is therefore to obtain a plastic bracket that provides characteristics similar to those of metal or ceramics with respect to resistance to deformation and / or breakage and retention qualities, but which is also more aesthetic and has a lower manufacturing cost. It is also important to take into account the manufacturing process, which can be by machining or injection molding, the latter being the cheapest.

The issues to consider, when you want to get a plastic bracket with the aforementioned characteristics, are fundamentally: the shape of the base, the body shape of the bracket, the type of plastic and the molding process for its manufacture.

A first objective of the present invention is to achieve that the bracket has a mechanical retention base. If we analyze the base, a first and important aspect to consider is the strength of adhesion of the latter with the tooth, this base must support forces of the order of 50-100 Newtons. This is not easy to achieve considering the material and the small size of the piece. Due to this, the shape of the base and in particular that of its dental surface has been the subject of several patents, since both the retention force and the ease of implantation depend on this form. The retention can be chemical or mechanical, and is a factor that a professional takes into account when choosing a certain type of bracket, because chemical retention involves the additional step of applying a liquid to the base so that it can be produced in a effective the union of the base with the cementing material.

The body of the bracket has in general retention wings and a groove for the arc that form a characteristic geometric figure in which areas with a greater probability of deformation or breakage are delimited when the bracket is subjected to the tensions of the arc that is introduced in the groove and chewing. It is known that plastic is more easily deformed and broken than metal or ceramic, but there is also the additional problem that a plastic bracket can suffer a progressive degradation in time due to hydrolysis, which in turn produces defects mechanical and a progressive change in the color of the plastic becoming opaque. Degradation by hydrolysis is avoided to a large extent by finishing the part as perfectly as possible, and here comes into play both the chosen material and the chosen molding process.

It has already been said that the union of the bracket with the tooth depends on the shape of the base, and that this form has been the object of different patents. The patent US-4,544,353 discloses an orthodontic apparatus that improves the technique of joining by direct contact the teeth with an orthodontic appliance. The bonding technique by direct contact consists of applying the adhesive on the dental surface of the base; put the base coated with glue on the tooth; and clean the residual glue. Due to the shape of the base of the orthodontic appliances existing at that time, this operation was made difficult due to the sliding of glue and bracket during the positioning of this on the tooth surface, and also having to clean the excess adhesive. In said patent, this problem is solved by means of a base having a flange with an angle between 90 ° and 170 ° which extends over its entire periphery, thus defining a gap that receives the adhesive and which in addition to trimming any excess of glue prevents its fall during positioning on the tooth.

Patent US-5,295,824 discloses a plastic bracket that improves the bonding process of a plastic bracket with the teeth, in which the bonding surface with the tooth is coated with a mixture of acrylic monomers and solvents that attack and They dissolve the plastic substrate so they diffuse inside it. Once the dental surface of the base has been coated, the bracket and the mixture are heated so that the solvents and monomers of lower molecular weight are volatilized thereby obtaining a layer of acrylic material which is predominantly monomeric. This first layer acts as a first adhesive and improves bond strength when it is made with dental adhesives such as acrylics. Suitable solvents that are cited in said patent are benzene, chloroform, acetone, dichloromethane or any other that acts as a solvent for the plastic making up the bracket. Plastic brackets that are made of polycarbonate are very appropriate to use this method.

A goal sought over time is to get plastic brackets whose base has a tooth surface rough enough that you do not need to vulcanize the surface before attaching it to the tooth. The process is intended to be analogous to that of metal brackets which generally have grooves in the dental surface of the base, which are sufficient to achieve a cementation that provides the adequate retention force.

US-6,071,117 proposes a method for manufacturing orthodontic appliances made of plastic by injection molding; the surface of the base of the bracket projects outwards with shapes such as cylinders or rectangular prisms that are easily configurable since they are produced in a mold. This form of the surface of the base aims that the bonding surface of the bracket with the fixing cement is the maximum possible. A first problem occurs because the surfaces of the aforementioned shapes can not be rough because if the mold is designed in this way, breakage could occur when the bracket is removed from the mold. Consequence of this is that the geometry achieved by the molding is not sufficient to achieve adequate forces if the retention is mechanical. To solve this problem, a second step is carried out, which is to deform the geometry of the base on the outside by means of heat or ultrasound energy and provide spaces, in the form of an inverted cone near the exterior surface, which facilitate the mechanical retention of the base. of the bracket with the surface of the tooth.

Possibly due to mechanical retention problems, US Pat. No. 6,190,165 of the same beneficiary of the previous patent discloses a similar method in terms of the shape of the base and the deformation process but includes one more step that is subsequent to the previous ones and consists of bomb with particles so that they hit the outside of the projections of the base.

The method disclosed in the above-mentioned patents US-6,071, 1 17 and US-6,190,165 has the problem of making the product more expensive since in addition to the molding step it is necessary to produce the deformation so that, although the deformation is carried out in deformation stations that control the process, the heating element can reach up to 477.59 ° K (400 ° F), which could cause deformation in the entire bracket.

A second objective of the present invention is to achieve mechanical performances equivalent to those of metal brackets. If we consider the problem posed by the hardness of the plastic bracket, or what is the same the capacity of resistance to deformations or breaks, this has been fundamentally treated in two ways. One of them is to look for the right type of plastic and the other to introduce metallic structures inside that reinforce the points that have to withstand greater stresses.

When you mention the type of plastic you are not only talking about if it is from the polycarbonate class, polysulfones, polyethersulfones or others, but it is meant that the plastic may or may not be loaded with other elements. The load involves mixing with the plastic inorganic materials such as alumina or silica, although it can also be loaded with organic or metallic materials, always with the intention of increasing the resistance of the whole. On the contrary, when the plastic is pure, it has the advantage that the same physical and chemical characteristics are maintained throughout the structure.

Patent US-5,240,402 as well as some embodiments of the aforementioned patents US-6, 071, 1 17 and US-6,190,165, disclose a method to reinforce the plastic, in which the plastic is filled by dispersing in its interior inorganic materials such as glass particles. However, this method of dispersion of particles within the plastic, whether glass, alumina or metal, has the problem that micro breaks occur because when the mixture is injected has a high temperature and as each component has a coefficient of expansion different, when cooled, deformations occur. This effect is more intense in the narrower areas of the geometry of the body that are on the other hand those with greater probability of breakage, and also in the superficial areas which produces an increase in the undesired effect of the hydrolysis.

In order to alleviate the above-mentioned effect of micro-breaks, US Pat. No. 6,358,043 introduces organic materials into the mixture by mixing polycarbonate as a basic element and a styrene elastomer as a hardening element with a particle size of 0.1 pm to 10 pm. However, the hardening does not seem sufficient because it includes a metallic reinforcing sheet that extends inserted between the wings in the direction of the groove. A number of bracket problems can be listed that have embedded metal reinforcement elements when manufactured by injection processes. Among said problems, some may be enumerated as deformations in the plastic matrix, burrs, displacements of said sheet during the manufacturing process that may even produce the displacement of said sheet during its use in an orthodontic treatment process and, also, the increase in cost.

DESCRIPTION OF THE INVENTION The present invention proposes a plastic bracket that achieves sufficient strength to prevent breakage, good finish to mitigate hydrolysis effects and that the plastic material does not lose its transparency, has mechanical retention for the cementation material and it is not necessary to apply a chemical additive that improves the bracket's clamping to the tooth, alleviating the problems of the prior art.

The procedure to achieve it consists of several steps, which consist of: (a) choice of material, which must be "uniform"; (b) calculation of the size of the bracket, said size being a function of the material with which it is to be built and of the resistance to the deformation or breakage that is to be achieved; (c) performing tests on the shape of the base to achieve that it reaches a predetermined mechanical clamping force, since it must be considered that the shape of said base must be adequate to be manufactured in an injection mold and be extracted from said mold. mold without breaks or detachments, and that said force is also a function of the size and material with which it is constructed; (d) appropriate design of the mold, so that it has an optimal injection point and with it an adequate evacuation of gases avoiding the accumulation in areas that would be weakened, an orientation of the fibers that provide more hardness to the set and a lower deformation of contraction; and (e) creation of a roughness of the grooves of the base by bombardment with particles, in order to retain the cementing product to the tooth.

The chosen materials have been polycarbonate, polysulfone and polyethersulfone, because they behave well against hydrolysis and when the finish of the piece is adequate keep the color transparent without tending to become opaque with the passage of time. They also resist sterilization with autoclave and have been used for medical devices on many occasions. These materials have been chosen without mixtures or loads of reinforcing materials so that the chemical structure of the whole piece is uniform and there are no zones of different hardness due to poor distribution of loads, especially in the surface or areas of narrowing.

So, taking into account that the chosen material is without loads and it is certain that the physical and chemical characteristics of the piece to be manufactured are the same in all areas, the problem is to get the piece made of this material, whose shape is essentially known in the art, has predetermined characteristics of resistance to deformation and breakage.

This has been achieved through studies based on the simulation by finite elements, so that knowing what is the material, and the characteristics of resistance to deformation and breakage, an optimal shape of the geometry of the bracket and its minimum size is achieved. so that the initial premises are fulfilled.

These simulations have been carried out using application software from the company COSMOSWorks and, after numerous tests, it has been concluded that plastic models can be produced that, although being somewhat larger than the metallic ones, comply with the indicated characteristics and that also have a size small enough to be used in orthodontic treatments.

Once the size of the bracket is known, the shape of the base has been studied so that it has good mechanical retention characteristics. The retaining bases of the brackets manufactured by molding typically have grooves in the mesial-distal direction, which must be designed to let air out and thus allow the total entry of the cementing material, do not deform with the stresses that It produces chewing and has a certain roughness to compensate for the contractions of the cement when it is cured.

Taking into account that: - the grooves of the base must have the characteristics mentioned above; - the cementation materials have a contraction ranging from 1.65% to 4.16%; Y - the size and shape of the bracket is known by calculations made in the second step of the design process, different base configurations have been made in terms of number of slots, slot length, slot depth, distance between slots and more or less concave shape of the slot. By varying parameters, a large number of possible configurations have been achieved, all of which have been tested by tensile tests with an INSTRON machine to determine the optimal configuration with respect to the configuration parameters. From the aforementioned tests have been obtained values of the parameters that form the shape of the base, which are comprised between a maximum and minimum values, within which a holding force equal to or greater than the predetermined is achieved.

The molding process has a great importance for the bracket to maintain all the properties of the materials on which the calculations of the previous steps have been made.

The objectives are: - the piece has a uniform finish and no hydrolysis occurs or at least it is minimal; Y - that in the injection process no tensions accumulate due to a difficult evacuation of the gases, which would produce a non-uniform contraction and a final shape not according to the expected characteristics that have been calculated in the previous steps.

Through simulation software, tests have been carried out to see which is the optimal point of injection and where the gas escape areas must be located so that, once removed from the mold, the piece has the required characteristics of uniformity of material , shrinkage uniformity and surface finish.

To ensure that the retention force does not decrease with the maximum contraction of 4.16%, the distance between slots is narrowed by a sandblasting of said base once the piece has been taken out of the mold and cooled.

The results and characteristics relating to the five steps of the process for obtaining the bracket of the present invention will be seen in more detail in a description of a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the object of the present invention, a practical embodiment of the present invention is described below on the basis of the attached figures, wherein: Fig. 1 is a perspective view of the bracket from a lateral-posterior point where the shape of the base can be seen.

Fig. 2 is the same view of the bracket of Fig. 1 to show the dimensions of the bracket construction parameters.

Fig. 3 is a vertical cross-section of one of the grooves of the base of the bracket of Fig. 1.

Fig. 4 shows the shape of the sides of the groove, said form being a function of the length of the spokes r-? , r2 and of the distance d between the centers Ci, c2.

Fig. 5 shows how the depth p of the groove is a function of the angle a.

Fig. 6 is a top view of the bracket.

Fig. 7 shows the vertical cross sections made in planes A and B according to Fig. 5 Fig. 8 is a perspective view of the male used in the injection molding manufacturing that shapes the base of the bracket of Fig. 1.

Fig. 9 shows schematically the two parts that make up the bracket manufacturing mold, male for the base and cavity for the body.

Fig. 10 shows the union of the male with the cavity in an injection molding process indicating the injection point by means of the arrow "i" and the points of gas escapes by the arrows "o".

Fig. 1 1 schematically shows a section of an area close to a slot before and after having been subjected to a sandblasting.

PREFERRED EMBODIMENT OF THE INVENTION As stated above, the variables that have been considered to achieve the bracket of the present invention are: manufacturing material, bracket size, base shape, mold design and sandblasting of said base.

The chosen materials have been polycarbonate, polysulfone and polyethersulfone, because they behave well against hydrolysis and when the finish of the piece is adequate keep the color transparent without tending to become opaque with the passage of time, also resisting autoclave temperatures. These materials are not loaded with any other material because the size of the bracket is of the order of 8 mm3 and at present it is not easy to know what are the properties of a composite structure when it is such a small element because the hardness measurements do not they are sufficiently developed and contrasted because the measurement markers of these measures are not contrasted either.

The bracket of the present invention is represented in Fig. 1 and consists of two portions that are a retaining base 1 and a body of the bracket 2, the bracket body having a groove 3 that holds the arch and fins for upper ligatures 4 and lower 4 ', the retention base having grooves 6 made in the surface and supporting strips in the tooth 5, the injection point 7 being located in the geometric center of the base.

In the preferred embodiment, polysulfone has been used and, based on its physical and chemical properties, the necessary tests have been carried out to calculate the appropriate dimensions of the bracket so that it has adequate resistance to deformation and strength against breakage.

The dimensions to be taken into account to achieve a bracket that meets the aforementioned purpose are: the occlusal-gingival length lf the mesial-distal length l2, the length l3 from the bottom of the groove to the tooth surface and the length l4 from the bottom of the groove to the ligature surface, which are bounded in Fig. 2. for better understanding.

The results obtained in the tests carried out were the following: 3.3mm < occlusal-gingival length < 4.5mm; 2.8mm < mesial-distal length l2 < 3.6mm; 0.9mm < length bottom of the groove-surface tooth l3 < 2.0 mm; 0.7mm < bottom length of the slit-ligation surface l4 < 2.0 mm.

Having chosen for the preferred embodiment the values of: occlusal-gingival length = 3.5mm; mesial-distal length l2 = 3.1 mm; bottom length of the groove-surface tooth l3 = 1, 8mm; and bottom length of the slit-ligation surface l4 = 0.75mm.

Once the optimal shape and size of the bracket is known, the objective is to obtain a base that has a shape capable of achieving the mechanical retention of the cementation material, with tensile forces of at least 50 Newtons. The first condition of the shape of the grooves of the base, is that it can be done by injection molding.

The first conditioner supposes that the piece leaves the mold without any deterioration occurring in it, therefore, said grooves can not have an inverted prismatic or conical shape, so that the shape of the grooves of the base is limited. Fig. 1 shows the retention base 1 which is formed by support bands on the tooth 5 defining the external surface, and grooves 6. The shape of the grooves 6 is one of the fundamental points to achieve the objective of the present invention, which as stated above is to achieve a mechanical retention.

Figs 3-7 show the shape of said grooves 6. As seen in Fig. 3, the grooves 6 are composed of two flat side walls 17 and a rear wall 9 having a concave shape of a circumferential arc, while Support bands on tooth 5 are composed of said side walls 17 and a front wall 10 also concave in shape of a circumferential arc. The side walls 17 are substantially parallel to allow the ejection of the injection mold part and the anterior wall 10 has a greater radius of curvature than the rear wall 9 so that the resin used in the tooth cementing operation occupies the largest surface area. possible and thereby achieve the predetermined adhesion value of 50 Newton.

Figs 4 and 5 show the shape of the flat side walls 17 that make up the grooves 6, the degree of concavity of the rear walls 9 and anterior 10 being function of the centers ci, c2 and of the radii r1 t r2, and also the depth of the slot depends on the distance between the centers Ci and c2. The depth of the groove is greater in the center of symmetry (plane A of Fig. 6) and decreases as it approaches the lateral ends (plane B of Fig. 6) as seen in Figs. 5, 7 and 8 Based on the above, and taking into account the following variables: - number of slots; - the occlusal-gingival length; - the mesial-distal length; - the width of the support band on tooth 15; - the width of slot 16; Y - the depth of the slot 6, which is variable with respect to the angular position and which can be specified by the difference of the values of the radii of curvature, the aforementioned parameters have been changed and a series of different brackets have been produced , testing the strength of retention of each one in an INSTRON machine reaching the conclusion that for certain measures, the predetermined objectives are achieved (feasibility of manufacturing by means of injection molding and mechanical retention).

The obtained values that make possible the marked objective, among which is the bracket of the preferred embodiment, can be summarized by means of the following formulas: 3 < Number of slots < 6; 0.2mm < slot width l6 = 0.4mm; 0.45mm < width of the support band on the tooth l5 < 1 mm; the width of the groove being ß = width of the support band on tooth 15; 0.5mm = depth at the center of the bracket 25 < 0.7mm; being the depth of the slot function of r-? and r2 as well as the distance between centers where 6mm < r-? , < 20mm, 2mm < r2 = 5mm and r2 = (r-i, -2).

Determined the total geometry of the bracket, the next step is to achieve a manufacturing method by injection molding that achieves a piece that has no weak points, which can be produced by an inadequate orientation of the fibers, accumulation of gases or deformations of shrinkage in the cooling process.

The molding is carried out by means of two semi-molds connected with a sliding system in order to eject the piece. Said semi-molds are: a male of the base 1 1 illustrated in FIGS. 8 and 9, which generates the shape of the retaining base 1 and in it a protruding part 6 'can be seen that will be the cause of the shape of the grooves 6 and cavities 5 'that generate the support bands on the tooth 5, the core of the base being limited by a gingival occlusal edge 19, a mesial distal edge 18, the injection point 7; and a body cavity 12 illustrated in FIG. 9, which upon filling generates the body parts of the bracket 2 which as is known are the retention groove 3 and the fins 4, 4 ', the body cavity being limited a gingival occlusal edge and a mesial distal edge that coincide with that of the male of the base 11.

To reinforce the projecting parts 6 'of the core of the base 1 1 a reinforcement rib (not shown) positioned perpendicularly between the side walls 17 is provided.

The lengths of the occlusal-gingival edge and the mesial-distal edge of the male of the base 1 1 and of the body cavity are identical and have complementary shapes so that they can be joined intimately as shown in Fig. 10.

By means of a specific simulation software, plastic injection points and evacuation points have been searched for the gases generated during the injection, and thus to obtain the most appropriate in order to ensure that the bracket has a uniform finish, do not accumulate tensions in its structure and that during the cooling process there are no contractions that deform more strongly parts of the bracket than others.

Fig. 10 illustrates the results obtained: a single injection point located at the point referenced by number 8, with gas escape points in the base male referenced with number 14 and escape points in the body cavity referenced with the numeral 5.

The last step of the process is done to compensate the shrinkage of the cement during its cure. This contraction is approximately 10%, so it is intended to reduce the width of the slots 6 in that 10%. This is achieved by blasting corundum with particles of 120-180 microns at 1,406 kg / cm2 (20 psi), 10 cm away, for 2 s (seconds), which produces roughness in the walls that, in addition to producing the effect reducing the width of the groove produces a greater retention surface by having a rough surface that is of greater extent than when it is smooth.

Fig. 1 1 shows the difference of surfaces in zones 9, 10 and 17 of the bracket before having been blasted and after having been, which is obviously greater due to the roughness produced, but in addition the blasting must achieve that and < 0.9x, where x is the width of the slot before the sandblasting, and e the width of said slot after said sandblasting, to compensate for the contraction of the cementing resin.

Having sufficiently described the nature of the present invention, as well as a preferred embodiment thereof, it remains to be added that both its form and the materials and execution thereof are subject to modifications, provided they do not substantially affect to the characteristics that are claimed below.

Claims (3)

1. Bracket for use in orthodontics, made entirely of plastic by an injection molding process, comprising: a retention base (1), said retention base defining slots 6 and some support bands on the tooth (5), that gives the bracket a mechanical retention; and a body of the bracket (2), said body of the bracket defining a groove of the arch (3), and fins for ligatures (4, 4 '), characterized in that: - it is made of polycarbonate, polysulfone or polyethersulfone; - The body of the bracket (2) has dimensions defined by: - an occlusal-gingival length () that is between 3.3mm and 4.5mm; - a mesial-distal length (12) which is between 2.8mm and 3.6mm; - a bottom length of the tooth surface groove (I3) that is between 0.9mm and 2, Omm; Y - a bottom length of the slit-ligation surface (l) which is between 0.7mm and 2.0mm; - the retention base (1) has concave shape to adapt to the tooth and is constituted by; - grooves (6) formed by two side walls (17) and a rear wall (9), said side walls (17) defined by an external radius of curvature (r-?), an internal radius of curvature (r2) and a separation distance between centers (d), the rear wall (9) having a concave shape with an internal radius of curvature (r2); - support bands on the tooth (5) constituted by said side walls (17) and an anterior wall (10) having said anterior wall (10) concave shape with an external radius of curvature (r-i).

2. Bracket for use in orthodontics, according to claim 1, characterized in that the retention base (1) has a number of slots comprised between 3 and 6; - the width of the groove (16) is less than the width of the support band on the tooth (15), the width of the groove being between 0.2 mm and 0.4 mm, and the width of the support band on the tooth (15) between 0.45mm and 1mm; - the depth in the center of the bracket (25) is between 0.5mm and 0.7mm; Y - the depth of the groove is a function of the values of the radii of external (?) and internal curvature (r2) as well as of the distance between centers (d), where 6 = r1 (< 20, 2 < r2 < 5 &r2 < (r1 r2).

3. Method of manufacturing a bracket for use in orthodontics according to claim 1, characterized in that - the manufacturing mold is composed of a base male (11) and a body cavity (12) with a sliding system having a single injection point (7) located approximately in the geometric center of the base and gas escape points (14) located in the base male and gas escape points (15) located in the body cavity; - said core of the base (1 1) has protruding parts (6 ') between which a reinforcement rib is positioned perpendicularly between the side walls (17); Y - the retention base (1) is subjected to a corundum blast with particles of 120-180 microns at 1,406 kg / cm2 (20 psi), 10 cm distance, for 2 s, which when producing surface roughness the width of the grooves (6) decreases by a value of around 10%.