Improved slot for orthodontic brackets and method

Abstract: An improved orthodontic bracket (44) has a slot (46) therein for receiving an archwire. The slot (46) has oppositely facing sidewalls (48 and 50) and a base (52). The base (52) has a protrusion (54) thereon thus forming a continuously curving surface including the sidewalls (48 and 50), the base (52) and the protrusion (54). An archwire placed in the slot (46) will rest upon the protrusion (54) rather than the base (52) or the sidewalls (48 and 50). Therefore, the archwire may be placed at a uniform depth regardless of the shape of the wire and stress is reduced in the slot (46).

Description: The invention relates in general to orthodontic devices, and in particular to an orthodontic bracket. An orthodontic bracket as well as a method of producing an orthodontic bracket as specified in the preamble of Claims 1 and 10, respectively is disclosed in EP-A-265236. In orthodontics, ceramics and plastics are becoming more commonly used in the formation of brackets. With the increase in the use of ceramics and plastics, there is a corresponding increase in the frequency of breakage of such brackets. Ceramic and plastic brackets tend to break more readily than a similarly designed metallic bracket due to a lack of ductility which is inherent in the materials used. When stress is placed on a metallic bracket, the ductile metal tends to deform or bend rather than crack or break. Contrarily, ceramics and some plastics will tend to crack or break rather than bend. When a bracket breaks due to stress from an orthodontic appliance, the broken portion results in a small jagged piece which is loose in the mouth of a patient. In an effort to reduce the likelihood of the bracket breaking or cracking, the wire slots have been altered from a standard square or rectangular shape with orthogonally intersecting surfaces to a more rounded or smooth shaped slot. The use of such smooth shaped slots reduces the likelihood of cracks occurring at the orthogonal intersection of the surfaces. Unfortunately, whenever a square or rectangularly shaped wire is used in a bracket slot that is rounded, there is an increased tendency for the wire to bind or cause excessive friction at the points of contact between the slot and the wire. Additionally, a square or rectangular shaped wire will not be correctly positioned in a rounded slot as the slot prevents full insertion of the wire which may result in some degree of misapplication of the desired adjusting forces. Thus, there is a need for an orthodontic bracket slot which is shaped to take advantage of the stress reducing conditions of a rounded slot and yet allow round and square wires to be properly positioned in the slot. EP-A-265236 discloses an orthodontic bracket formed of ion exchange strengthened glass having a body with a slot therein with side walls and a base wall, each of such walls being planar and the side and base walls meeting at right angles. According to one aspect of the invention there is provided an orthodontic bracket, comprising a body having a slot to receive an orthodontic wire with opposing sidewalls and a base positioned between the opposing sidewalls; characterised in that the base includes in the slot, a protrusion for reducing frictional and binding contact between the slot and the orthodontic wire. According to another aspect of the invention there is provided a method of production of an orthodontic bracket providing reduced friction between the orthodontic bracket and an arch wire, comprising the steps of forming a slot comprising opposing sidewalls and a base in the bracket to receive the arch wire; characterised by forming a protrusion on the base in order to allow the wire to rest upon the protrusion when placed in the slot, wherein frictional contact between the bracket and the wire is reduced. Such a bracket and method allows the positioning of an archwire in the slot at a uniform depth regardless of the shape of the wire while utilizing the stress reducing advantages of a curved slot. Preferred features and embodiments of the invention will now be described by way of example with reference to the following non-limiting detailed description taken in conjunction with the accompanying drawings in which:- Figures 1 is an end view of an orthodontic bracket constructed in accordance with the prior art; Figure 2 is an end view of another orthodontic bracket also constructed in accordance with the prior art; Figure 3 is an isometric view of an orthodontic bracket constructed in accordance with a first embodiment of the invention; Figure 4 is an end view of the orthodontic bracket of Figure 3; Figure 5 is an end view of the bracket of Figure 3 with orthodontic wires positioned in a slot thereof; and Figures 6 to 9 show alternative embodiments of an orthodontic bracket according to the invention. Referring to Figure 1, an orthodontic bracket 10 of known kind has a slot 12 formed therein having parallel and oppositely facing sidewalls 14 and 16 generally forming right angles at points of intersection 20 and 22. As can be seen in Figure 1, a wire 24 having either a circular or square cross-sectional shape will be positioned in the slot 12 at approximately the same location. If the bracket 10 comprises a ductile metal, any stress induced by the wire 24 against the bracket 10 in the slot 12 will, if severe enough, result in deformation of the bracket 10. However, if the bracket 10 comprises ceramic or plastic, stress induced by the wire 24 may cause cracks, such as indicated at 25, 26 and 27, starting at the point of intersection 20 and 22 or the base 18. If the cracks 25, 26 and 27 extend from the slot 12 to the exterior surfaces of the bracket 10, small jagged pieces may break from the bracket 10 and become hazardous objects in the mouth. Additionally, any jagged edges left on the bracket 10 from breakage thereof, will also cause potential hazards to the patient. Referring to Figure 2, another bracket 28 of known kind has a slot 30 formed therein with generally parallel sidewalls 32 and 34. A base 36 interconnects the sidewalls 32 and 34 forming a continuously curved slot to receive a round archwire 37. The slot 30 thus eliminates or greatly reduces the sharp points of intersection (points 20 and 22 of Figure 1) of a rectangular slot. Thus, the tendency for cracks to occur along the points of intersection between the base and the sidewalls is greatly reduced. However, if an archwire 38 with a square or rectangular cross section is positioned in the slot 30, there will be contact between the archwire 38 and the slot 30 as indicated at 40 and 42. At the points of contact 40 and 42 there is a tendency for binding between the archwire 38 and the bracket 28 as well as frictional forces therebetween. Additionally, due to the shape of the archwire 38, the archwire 38 may not be positioned completely within the slot 30, as opposed to a round archwire which would be seated on the base 36. As a result, there will be a difference in the positioning and therefore a difference in the application of forces between a square wire and a round wire. Thus, while the slot 30 greatly reduces the tendency of cracking as opposed to a rectangular slot, there are positioning and force disadvantages inherent thereto. Referring to Figure 3, an orthodontic bracket 44 has a slot therein. The bracket 44 is of a standard shape well known in the art and may comprise ceramic, plastics, composite or metal which is formed by any appropriate method such as moulding, stamping or die cutting. The slot 46 passes through the bracket 44 along a longitudinal axis thereof. A protrusion 54 is formed in the slot 46 to support an archwire placed therein, as will be subsequently described in greater detail. Referring to Figure 4, the slot 46 in the bracket 44 is formed with oppositely facing sidewalls 48 and 50. A base 52 interconnects the sidewalls 48 and 50 in order to form a continuously curved slot in conjunction with the protrusion 54. The protrusion 54 has a shape generally opposite to the curve formed between the base 52 and the sidewalls 48 and 50. In the preferred embodiment, the continuously curving surface formed by the base 52 and the protrusion 54 resembles a uniform wave (such as a sine wave) defined by a radius R forming a curve which meets with a tangent to a similar but reverse curve having a radius P. Thus, the slot 46 has no stress concentration points formed by sharply intersecting portions thereof. The slot 46 will therefore have the advantage of the reduced stress points of rounded or curved slots as formed in the prior art (see Figure 2) but will not have the archwire positioning problems thereof. Referring to Figure 5, the bracket 44 and slot 46 are shown with a cross-sectionally square wire 58 superimposed on a cross-sectionally round wire 56. It can be seen in Figure 5 that, regardless of the cross-sectional shape of the archwire, the wire will be positioned at the same depth within the slot 46. Additionally, when the square wire 56 is used in the slot 46, there will be no excess binding or friction as experienced with a square wire in a curved slot in accordance with the prior art (see Figure 2). It is to be understood that the wire 56 or 58 will be at least slightly smaller than the width of the slot 46 to ensure easy insertion thereof. Referring to Figure 6, one alternative embodiment of the present invention is shown. A bracket 70 has a slot 72 formed therein. The slot 72 comprises oppositely facing sidewalls 74 and 76 joined by a base 78. The base 78 has a protrusion 80 forming a continuously curving slot in conjunction with the base 78 and the sidewalls 74 and 76. The protrusion 80 can be any appropriate shape such as hyperbolic, parabolic or elliptical that forms a continuously curving slot. Although not shown, it is also to be understood that the protrusion, such as protrusions 54 and 80, do not have to be centered between the slot sidewalls nor of a uniform shape. Referring to Figures 7, 8 and 9, further alternative embodiments are shown. Referring first to Figure 7, a bracket 90 is formed with a slot 92 and a protrusion 94 having a generally planar top surface 96. The remaining surfaces of the slot 92 are curved as previously described above. Referring to Figure 8, a bracket 100 is formed with a slot 102 and a plurality of protrusions such as protrusions 104 and 106. The protrusions 104 and 106 should be spaced and sized appropriately in order to allow an archwire of any shape to rest thereupon at a uniform depth within the slot 102. Referring to Figure 9, a bracket 110 is formed with a slot 112 therein. The slot 112 is continuously curving with a protrusion 114 but further includes undercuts 116 and 118. The undercuts 116 and 118 serve to provide a greater area for stress reduction in the slot 112. It is important to provide a slot with any combination of curved and/or planar surfaces that eliminates the likely stress concentration points formed by sharply intersecting portions thereof. It is to be understood that there is no requirement for the curving surfaces to be uniform or to meet at tangents thereto. By providing a slot that eliminates the stress points formed at the intersection between oblique surfaces, the invention can greatly reduce the likelihood of bracket breakage. The protrusion along the base of the slot also helps allow wires of any shape to be uniformly positioned for the predictable application of force to the teeth.

Claims: An orthodontic bracket (44,70,90,100,110), comprising a body having a slot (46,72,92,102,112) to receive an orthodontic wire with opposing sidewalls (48,50,74,76) and a base (52,78) positioned between the opposing sidewalls; characterised in that the base (52,78) includes in the slot, a protrusion (54,80,94,104/106,114) for reducing frictional and binding contact between the slot and the orthodontic wire. An orthodontic bracket according to claim 1, wherein the protrusion (54,80,94,104/106,114) extends for at least a portion of the length of the base (52,78). An orthodontic bracket according to claim 2, wherein the protrusion (54,80,94,104/106,114) comprises a continuously curving surface adjacent to the base (52,78) and the sidewalls (48,50,74,76). An orthodontic bracket according to claim 1, wherein the protrusion (94) comprises curving surfaces adjacent to the sidewalls (48,50) with a planar top surface (96) therebetween. An orthodontic bracket according to claim 1, wherein the sidewalls are oppositely facing, the base interconnects the sidewalls, the base (52,78) has an axial length, the protrusion (54,80,104/106,114) extends for at least a portion of the axial length of the base, and a continuously curving surface is formed by the sidewalls, the base and the protrusion. An orthodontic bracket according to claim 5, wherein the continuously curving surface comprises a uniform wave. An orthodontic bracket according to claim 5 or claim 6, wherein the protrusion (54,80,104/106,114) comprises: a semicircle; a hyperbola, a parabola, or an ellipse. An orthodontic bracket according to any one of claims 5 to 7, wherein the protrusion (114) comprises: curved undercuts (116,118) adjacent the base and each of the oppositely facing sidewalls. An orthodontic bracket according to claim 12, wherein the wave comprises a first curve having a first radius (R); a second curve having a second and opposite radius (P), wherein a tangent from the first curve merges with a tangent from the second curve; and a third curve having a third radius (R) equal to the first radius, wherein a tangent from the second curve merges with a tangent from the third curve. A method of production of an orthodontic bracket providing reduced friction between the orthodontic bracket (44,70,90,100,110) and an arch wire, comprising the steps of forming a slot (46,72,92,102,112) comprising opposing sidewalls (48,50,74,76) and a base (52,78) in the bracket (44,70,90,100,110) to receive the arch wire; characterised by forming a protrusion (54,80,94,104/106,114) on the base (52,78) in order to allow the wire to rest upon the protrusion (54,80,94,104/106,114) when placed in the slot (46,72,92,102,112), wherein frictional contact between the bracket (44,70,70,100,110) and the wire is reduced. A method according to claim 10, wherein the step of forming a protrusion comprises: merging a tangent of the protrusion (54) with equal but opposite tangents from curves formed by the base and the sidewalls (48,50) of the slot (46). A method according to claim 10 or claim 11, further comprising the step of: arcuately merging the sidewalls (48,50,74,76) with the base (52,78) and the protrusion (54,80,94,104/106,114) with the base (52,78). A method according to any one of claims 10 to 12, further comprising the step of: undercutting (116,118) the sidewalls adjacent the base.

Inventors: PETERSON JEFFREY A, PETERSON, JEFFREY A.
Applicants: RMO INC, RMO, INC.
Application year: 1991
Main classification: A61C7
IPC codes: A61C7