Abstract
The present invention provides improved devices, systems and methods for repositioning teeth from an initial tooth arrangement to a final tooth arrangement. Repositioning is accomplished with a system comprising a series of polymeric shell appliances configured to receive the teeth and incrementally reposition individual teeth in a series of successive steps. The individual appliances may be formed from layers having different stiffnesses (elastic moduluses), and the stiffnesses of successive appliances may be different, or both.
Claims
1. A dental appliance comprising: a polymeric shell having cavities shaped to engage individual teeth, the polymeric shell comprising: a first layer being a discontinuous inner layer; a discontinuity in the first layer; and a second layer being a continuous outer layer and being formed over the first layer; wherein one or more discontinuities of the first layer comprise a first window formed through said first layer.
2. The dental appliance of claim 1, wherein the first layer has a first elastic modulus and the second layer has a second elastic modulus, the first and second elastic moduli being in the range from 0.5 to 5 GPa.
3. The dental appliance of claim 2, wherein the first elastic modulus differs from the second elastic modulus by 25% to 600%.
4. The dental appliance of claim 1, wherein portions of the dental appliance vary along one or more of a mesial-distal axis, facial-lingual axis, and gingival-crown axis.
5. The dental appliance of claim 1, wherein the first window is formed through the first layer on a buccal wall of a cavity.
6. The dental appliance of claim 1, wherein the first window is formed through the first layer on a lingual wall of a cavity.
7. The dental appliance of claim 1, wherein the first window is formed through the first layer on an occlusal surface of a cavity.
8. The dental appliance of claim 1, wherein at least a region of the polymeric shell comprises a material reinforced with a structure comprising one or more of strips, wires, mesh, lattices, and networks.
9. The dental appliance of claim 8, wherein the structure comprises a polymer.
10. The dental appliance of claim 8, wherein the structure comprises a metal or alloy.
11. The dental appliance of claim 1, wherein at least a region of the polymeric shell comprises a metal or alloy.
12. A method for producing a dental appliance, the method comprising: providing a mold of the patient's dentition; forming a first material layer over the mold; forming a window though the first layer; and forming a second material layer over the first material layer so as to produce the dental appliance.
13. The method of claim 12, wherein an appliance with a uniform elastic modulus is produced.
14. The method of claim 12, wherein an appliance with one or more portions of differing elastic moduli are produced.
15. The method of claim 12, wherein removing one or more portions of the first material layer forms one or more discontinuities.
16. The method of claim 15, wherein the one or more discontinuities through the first layer comprise windows.
17. An orthodontic system comprising: a plurality of dental appliances configured to move at least one tooth between a first position and a second position, each of the plurality of dental appliances comprising: a polymeric shell having cavities shaped to engage individual teeth, the polymeric shell comprising: a first layer being a discontinuous inner layer; a discontinuity in the first layer; and a second layer being a continuous outer layer and being formed over the first layer; wherein one or more discontinuities of the first layer comprise a first window formed through said first layer.
18. The orthodontic system of claim 17, wherein portions of the dental appliance vary along one or more of a mesial-distal axis, facial-lingual axis, and gingival-crown axis.
19. The orthodontic system of claim 17, wherein the first window is formed through the first layer on a buccal wall of a cavity.
20. The orthodontic system of claim 17, wherein the first window is formed through the first layer on a lingual wall of a cavity.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a perspective illustration of an embodiment of an appliance of the present invention and descriptive axes.
[0024] FIG. 2 illustrates an embodiment of an appliance with relatively large portions varying in elastic modulus along a mesial-distal axis.
[0025] FIG. 3 illustrates an embodiment of an appliance with smaller portions varying in elastic modulus in a non-symmetric pattern along a mesial-distal axis.
[0026] FIG. 4 illustrates an embodiment of an appliance varying in elastic modulus along a mesial-distal axis in which portions covering proximal or interproximal spaces are of differing modulus.
[0027] FIG. 5 illustrates a variety of appliance portions varying in elastic modulus along a mesial-distal axis.
[0028] FIG. 6 is a perspective illustration of an embodiment of an appliance varying in elastic modulus along a facial-lingual axis.
[0029] FIG. 7 illustrates a variety of appliance portions varying in elastic modulus along a facial-lingual axis.
[0030] FIG. 8 is a perspective illustration of an embodiment of an appliance varying in elastic modulus along a gingival-crown axis.
[0031] FIG. 9 illustrates a variety of appliance portions varying in elastic modulus along a gingival-crown axis.
[0032] FIG. 10 illustrates a variety of appliance portions varying in elastic modulus along one or more described axes.
[0033] FIG. 11 depicts a series of appliances differing in elastic modulus at specific intervals throughout a treatment plan.
[0034] FIG. 12 illustrates the use of an “off track” appliance in a prescribed treatment plan.
[0035] FIG. 13 illustrates the use of a series of appliances with gradually increasing elastic moduluses and similar or identical geometry in a series of intervals throughout a treatment plan.
[0036] FIG. 14 illustrates a method of fabricating a multi-modulus appliance.
[0037] FIG. 15 illustrates a method of layering to fabricate a multi-modulus appliance.
[0038] FIG. 16 illustrates an additional method of layering to fabricate a multi-modulus appliance.
[0039] FIG. 17 illustrates an exemplary layered appliance according to the present invention with portions broken away.
[0040] FIG. 18 is a cross-sectional view taken along line 18-18 of FIG. 17.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0041] The present invention provides improved devices, systems and methods for incrementally repositioning teeth using a plurality of discrete polymeric appliances of variable flexibility, where each appliance successively repositions one or more of the patient's teeth by relatively small amounts. Flexibility may be defined by elastic modulus of the polymeric material and may vary within a given appliance or may vary throughout a series of appliances according to a prescribed orthodontic treatment plan.
[0042] Referring to FIG. 1, portions of an elastic repositioning appliance 100 may vary in elastic modulus along a mesial-distal axis 101, facial-lingual axis 102, gingival-crown axis 103, or any axis in-between these representative axes. As previously described, a mesial-distal axis may be described as an axis following the gingival line or dental arch, a facial-lingual axis may be described as an axis following a radial or similar line from the tongue area toward the lip or cheek area, and a gingival-crown axis may be described as an axis following a substantially vertical line from the crown of a tooth toward the gingival line or root. Such axes are defined for descriptive purposes and are not intended to limit the scope of the present invention.
[0043] As shown in FIG. 2, portions of an elastic repositioning appliance 100 may vary in elastic modulus mesial-distally. For illustrative purposes, portions with a lower elastic modulus 110 are shaded to easily distinguish differences in elastic modulus throughout a device. In this example, the appliance 100 may be described as having three portions. Two portions cover contiguous sets of molars and are comprised of an elastomeric material of a lower elastic modulus 110 and are thus shaded. The portion in-between these portions is of a higher elastic modulus 111 and is thus not shaded. All portions in this embodiment are relatively large so that the portions may receive one or more teeth, such as molars, premolars, incisors, and the like. Likewise, nonadjacent portions may have the same elastic modulus, such as the two lower elastic modulus 110 portions, or they may be different from each other while maintaining a difference from the higher elastic modulus 111 portion. In other words, an appliance 100 with three distinct portions may be comprised of two or three elastic moduluses.
[0044] As illustrated in FIG. 3, such portions may not be symmetrical and they may not cover more than one tooth. Portions with a lower elastic modulus 110 may alternate in an uneven fashion along a mesial-distal axis as shown. In addition, adjacent portions may be of a size to cover only a portion of a tooth or dental surface. Referring to FIG. 4, portions of lower elastic modulus 110 may be present covering the facial or lingual surfaces of the teeth, while portions of higher elastic modulus 111 may be present covering the proximal or interproximal spaces. This may be advantageous to provide repositioning forces, such as translation forces, at the most efficient locations for this type of movement. At the same time, flexibility is provided in portions that may be less involved in the application of force, allowing more freedom and comfort for the patient.
[0045] Referring to FIG. 5, the elastic modulus of an appliance 100 may vary over any number of delineated portions and may be of any size, shape, thickness or dimension, to name a few. Such portions may be sized to receive an entire tooth 115 or they may be of the size to cover only a portion of a tooth. For example, a portion with a lower elastic modulus 110 may be sized to cover a proximal or interproximal space 116, including portions covering the gingival line. This may be desirable to provide comfort to the gums when wearing the appliance, and also to increase the contact of the appliance with the interproximal regions. In this case, softer, more flexible material may be able to form more closely to the interproximal regions, enabling a higher level of repositioning force to be applied.
[0046] Portions may also be sized and arranged to cover part of a facial surface 117, or two or more of such portions may cover part of a facial surface 118, allowing the elastic modulus to vary mesial-distally within a single tooth. Further, the portion may be sized to cover an isolated region of a facial surface 119. Such embodiments are a limited presentation of the possible sizes, configurations, and combinations of varying elastic modulus portions in an appliance 100 of the present invention. Such possibilities may be unlimited.
[0047] As shown in FIG. 6, portions of an elastic repositioning appliance 100 may vary in elastic modulus facial-lingually. In this embodiment, the appliance 100 is shown to have a portion with a lower elastic modulus 110 covering a portion of the occlusal surfaces of the teeth and a portion with a higher elastic modulus 111 covering the remaining surfaces of the teeth. Thus, the elastic modulus varies along a facial-lingual axis. Such a design may be beneficial to provide repositioning forces, such as translation forces, along the proximal and/or interproximal spaces which are the most efficient locations for this type of movement. At the same time, flexibility is provided in portions that may be less involved in the application of force, the occlusal surfaces. This may allow increased freedom and comfort for the patient while maintaining adequate repositioning forces.
[0048] Referring to FIG. 7, the elastic modulus of an appliance 100 may again vary over any number of delineated portions and may be of any size, shape, thickness or dimension, to name a few. A portion of lower elastic modulus 110 may be sized to cover only a portion of a tooth. For example, it may cover the center of the occlusal surface 125, alternating cusps or cusp tips 126, or isolated portions of any given cusp or cusp tip 127. Likewise, a portion of lower elastic modulus 110 may be of a larger size to cover, for example, the outside margin or buccal cusps of a tooth 128 or the inside margin or lingual cusps of a contiguous grouping of teeth 129. As before, such embodiments are a limited presentation of the possible sizes, configurations, and combinations of varying elastic modulus portions in an appliance 100 of the present invention. Such possibilities may be unlimited.
[0049] As shown in FIG. 8, portions of an elastic repositioning appliance 100 may vary in elastic modulus crown-gingivally. In this embodiment, the appliance 100 is shown to have a portion with a lower elastic modulus 110 covering the occlusal surfaces of the teeth and a portion with a higher elastic modulus 111 covering the remaining surfaces of the teeth. This is a modified representation of the embodiment depicted in FIG. 6 in which the portion of lower elastic modulus 110 partially covered a portion of the occlusal surfaces. In this embodiment, the occlusal surface is substantially covered with the lower elastic modulus 110 material, therefore it may be considered to be uniform, non-variable, along a facial-lingual axis. It may be more properly described as varying along a gingival-crown axis, as the lower elastic modulus 110 portion may extend over the cusps of the tooth crowns. Thus, the lower elastic modulus 110 material may be seen as being located at the tip of the crown region and vary to a higher elastic modulus 111 material toward the gingival line or margin. In addition, a higher elastic modulus 111 material along the gingival line or margin may improve retention of the device on the teeth. This may also reduce the need for attachment devices to aid in retention. A full description of exemplary attachment devices and methods for a dental appliance is described in co-pending application no. 09/454,278, incorporated by reference for all purposes and assigned to the assignee of the present inventor. However, such modulus differences are presented only for descriptive purposes and such portions may vary over one or many axes simultaneously or in isolated regions of an appliance 100.
[0050] It may be appreciated that the advantages offered by a lower elastic modulus along the occlusional surfaces, as depicted in FIG. 6 and FIG. 8, may be further increased by removing the material from the shell in these areas. Removal of material may form a window such that when the shell is positioned over the patient's teeth, portions of the teeth beneath the window may be exposed. In a preferred embodiment, a polymeric shell may have a plurality of windows over portions of the occlusal surfaces of the teeth. In this case, segments of the shell may still be present along the facial and lingual surfaces of the teeth and across the interdental regions or spaces between the teeth. Exposure of the occlusal surfaces in appropriate size and location may allow interdigitation of the upper and lower teeth. This may also be achieved with the presence of one or a few larger windows over portions of the occlusal surfaces of the teeth. In these cases, segments of the shell may not be present across the interdental regions or spaces between the teeth. In either case, interdigitation of at least portions of the upper and lower teeth may benefit tooth and jaw orientations, leading to improved treatment, appearance, comfort and consequently patient compliance. Thus, such windows may provide the benefits offered by a lower elastic modulus, such that the lowest stiffness may be provided by the absence of the material, while providing additional benefits described above. A full description is provided in U.S. patent application Ser. No. 09/616,222, assigned to the assignee of the present invention.
[0051] Referring to FIG. 9, the elastic modulus of an appliance 100 may again vary over any number of delineated portions and may be of any size, shape, thickness or dimension, to name a few. A portion of lower elastic modulus 110 may be sized to cover only a portion of a tooth along this axis. For example, it may cover the upper portion of the lingual surfaces near the cusps of the crown 135, or a midway “stripe” through the lingual surface of a tooth 136. Likewise, it may be sized so that more than one “stripe” may cover the surface of a tooth 137, as in the case of a portion at the gingival margin and a portion near the cusps of the crown. Similarly, a portion of lower modulus may be sized so that it covers a contiguous grouping of teeth, such as the buccal surfaces along the gingival margin 138. As before, such embodiments are a limited presentation of the possible sizes, configurations, and combinations of varying elastic modulus portions in an appliance 100 of the present invention. Such possibilities may be unlimited.
[0052] As illustrated in FIG. 10, variance in elastic modulus in relation to size, shape, location, orientation, and axis, as described above, may be combined in a single appliance 100 to provide an unlimited variety of appliance 100 designs and constructions. In this example, portions of the appliance 100 vary mesial-distally, such as by comparing the lower elastic modulus 110 portion covering a group of incisors with the higher elastic modulus 111 portion covering the canine tooth. Portions may also vary facial-lingually, as depicted by the partial covering of the occlusal surfaces of the molars 150 or the isolated portion of a given cusp or cusp tip 127. Likewise, portions may vary crown-gingivally, such as the portions covering the buccal surfaces along the gingival margin 138. These portions also vary mesial-distally creating a compound variance, as portions adjacent to these areas are not identical, as in comparison of portions covering the buccal surfaces along the gingival margin 138 with the adjacent tooth 151 having partial covering of the occlusal surface of the molar 150. Portions may also vary along the three major axes simultaneously. This can be seen in the portion covering the center of an occlusal surface 125, which varies facial-lingually, the lingual face of a molar along the gingival margin 152, which varies crown-gingivally, and the adjacent uniformly covered molar 153, which varies mesial-distally in relation to the previous portions.
[0053] According to the present invention, systems for repositioning teeth from an initial tooth arrangement to a final tooth arrangement may be comprised of a plurality of incremental elastic position adjustment appliances with varying elastic moduluses. Thus, in addition to combined variances in a given appliance 100, as described above, a plurality of such appliances 100 with differing patterns of elastic modulus variance may be used in a system for repositioning teeth throughout a sequence of tooth arrangements. This may be illustrated by FIGS. 2-10 with differing tooth geometries, viewed as a series of appliances 100 for a single treatment plan.
[0054] Alternatively, as shown in FIG. 11, the polymeric shells of the appliances 100 may have uniform elastic moduluses over their entire tooth contact area. In this depiction, each appliance 100 differs in shape or tooth geometry and represents a stage in the overall treatment plan. Thus, five stages are depicted, as there are five appliances 100 shown. The first three appliances 200, 201, and 202, respectively, may have a uniform elastic modulus chosen for a specific type of tooth movement. For example, appliances 200, 201 and 202 may be designed for pure translation, requiring a relatively high elastic modulus 111. Thus, the appliances are not shaded in the illustration. At stage 4, a different type of tooth movement, such as tipping, may be desired requiring a lower elastic modulus 110. Therefore, appliances 203 and 204 may continue the series of differing shape or tooth geometries to create such movements, but the elastic modulus may differ from the prior appliances, 200, 201, and 202. Thus, these appliances are shaded in the illustration. The remainder of the treatment plan may feature a similar series of appliances, including appliances with uniform elastic moduluses which differ from the appliances immediately prior and/or any appliances previously presented in the series. Likewise, such a series may also include appliances with combined variances, as described above.
[0055] Similarly, as shown in FIG. 12, a treatment plan may be prescribed with a series of appliances 100 differing in shape or tooth geometry, of which four stages are depicted, 210, 211, 212 and 213. Such appliances may have any given elastic modulus that is suitable for the prescribed function. Likewise, such appliances may have internal variance in elastic modulus, described previously, or may vary wholly from appliance to appliance throughout the prescribed treatment plan. However, if a patient were to discontinue usage of an appliance for an unprescribed period of treatment time, such as between stages two (appliance 211) and three (appliance 212) depicted in FIG. 12, the patient's teeth may move slightly out of the planned tooth arrangement. Such a patient may be considered “off track” in which their current tooth arrangement has diverted from the series of projected tooth arrangements, creating an unprescribed tooth arrangement. When attempting to apply the next successive appliance 212, it may be too rigid to accommodate these slight differences. Therefore, a new more flexible appliance 214 may be produced for this purpose and may be incorporated into the treatment plan. Such an appliance 214 may have the same shape or tooth geometry as the next successive appliance 212, but it may have a lower elastic modulus 110, depicted by shading. The increased flexibility may allow the appliance 214 to conform to the unprescribed arrangement and reposition the teeth toward an arrangement that the next successive appliance 212 may therefore fit. Such an appliance 214 may be used at any point in the series of successive appliances.
[0056] As shown in FIG. 13, a series of incremental appliances, 300, 301, 302, 303, 304 and 305, may be produced with differing elastic moduluses, illustrated by variation in shading, to reposition teeth from an initial tooth arrangement to the next successive tooth arrangement in a progression of arrangements to the final arrangement. FIG. 13 illustrates two steps in such a progression. A step or stage represents a change in shape or geometry of an appliance 100 to reposition the teeth into the next prescribed arrangement in a series. Therefore, appliances 300, 301, and 302 represent the first stage and have one shape and 303, 304, and 305 represent the second stage and have a differing shape. The appliances 300, 301, and 302 representing the first stage may vary in elastic moduluses from more flexible (appliance 300) to more rigid (appliance 302). The patient may begin the treatment sequence with the more flexible appliance 300 of the first stage. Such flexibility may allow an appliance with a substantially misaligned geometry to fit over the patient's teeth and apply repositioning forces. As the teeth gradually move toward the desired arrangement, the patient may progress to the next appliance 301 in the first stage. This appliance 301 may be more rigid than the prior appliance 300. The patient may continue through any number of appliances throughout a stage. Upon completion of the stage, the patient may repeat the process in stage two, beginning with the more flexible appliance 303 and culminating with the more rigid appliance 305. The patient may then continue through any number of stages to the endpoint of treatment.
[0057] Such a system may provide a number of benefits. First, the variance in elastic modulus throughout each step may allow for a larger step or increment in tooth movement between each step than may be obtainable with consistent, rigid appliances. Such flexibility may allow the appliance to fit over a tooth arrangement that is more misaligned while the increase in rigidity throughout each stage may provide sufficient repositioning forces which may not be obtainable with highly flexible appliances. These larger steps require fewer appliances in a series to have a change in shape or geometry. Consequently, fewer molds may be required to form such appliances, which lowers cost and treatment time for the patient. In addition, if the patient were to become “off track” by suspending treatment, it may be possible for the patient to resume the prescribed treatment plan by reentering treatment at the start of the step or stage in which the patient previously aborted. This appliance may be flexible enough to fit over the teeth in the unprescribed arrangement and gradually reposition the teeth throughout the stage as originally prescribed. This may also reduce cost and treatment time since the production and fitting of a flexible “off track” appliance, as illustrated in FIG. 12, may be avoided.
[0058] The elastic modulus of an appliance or portions of an appliance of the present invention may be determined by a number of design features, methods, materials and similar means. In a preferred embodiment, the appliance may be comprised of a polymeric shell which is heat formed over a mold of a patient's dentition. This is typically accomplished by heating a thermoformable polymer material and applying vacuum or pressure to form the polymer to the mold. Alternatively, reaction casting may be used to produce such an appliance. Hereinafter, description will pertain to thermoforming, however such concepts and techniques may be equally applied to reaction casting or similar methods and are not to limit the scope of the invention.
[0059] To produce an appliance with uniform elastic modulus, a polymer sheet with a specific elastic modulus and thickness may be thermoformed over a mold and trimmed for patient use. Appliances with differing uniform elastic moduluses may be produced by altering one or more of three variables: 1) polymer type, 2) elastic modulus, 3) thickness. To produce an appliance with portions of differing elastic moduluses, a number of techniques may be utilized. Referring to FIG. 14, portions of polymer sheeting 400 may be positioned over a mold 401 in designated areas and thermoformed together into a final polymeric appliance. Each portion of sheeting 400 may be chosen based on the three above mentioned variables to provide a desired elastic modulus. Each portion of sheeting 400 may then be positioned in the desired location for elastic modulus changes throughout the finished appliance. In FIG. 14, three portions are presented, a first sheet 402 placed over the right side molars, a second sheet 403 placed over the left side molars and a third sheet 404 placed over the remainder of the teeth. Sheets 402 and 403 are depicted as having differing elastic moduluses to each other and to sheet 404, as shown by shading gradations, however such sheets 402, 403, may be identical. After thermoforming, a finished appliance may appear as that illustrated in FIG. 2.
[0060] In addition, portions with differing elastic moduluses may be created with the same polymer or different polymers material by layering. Two layers of a polymer material bonded together may have a higher or elastic modulus than a single layer of such material. As illustrated in FIG. 15, a first sheet 405 may be placed over the incisors, canines and premolars of the mold 401 and a second sheet 406 may be placed over the entire dentition. Each sheet may be the same or may differ in terms of any or all of the above mentioned variables. After thermoforming, a finished appliance may also appear as that illustrated in FIG. 2. In this case, the shell covering the molars is comprised of one layer and the remainder of the appliance is comprised of two layers formed into an integral appliance structure. Therefore, the portions covering the molars may have a lower elastic modulus, depending on the combination of materials, than the remaining portion. However, it is possible that a multi-layered structure may have a lower elastic modulus than a single layered structure depending on the above mentioned variables. Thus, it may be appreciated that the described layering technique may provide a variety of moduluses and those stated examples are not intended to limit the scope of the invention.
[0061] Similarly, portions with different elastic moduluses may be created by a multi-step process of layering. Referring to FIG. 16, a first sheet may be thermoformed over the entire dentition of a mold 401 to form a base appliance 410. Portions desired to be of a differing elastic modulus 411, demarcated by a dashed line, may be cut and removed from the formed base appliance 410. A second sheet 412 may then be thermoformed over the entire dentition. This may result in a single layer of material in the portion of differing elastic modulus 411 and a double layer of material in the remaining areas.
[0062] It may be appreciated that appliances with differing and gradually changing elastic moduluses may be created by any number of production methods. For example, a base appliance 410 may be coated in a specific area with one or more polymer solutions to “build up” a portion of the appliance for localized rigidity. Such a build-up may also be gradual for a more gradual increase in rigidity. Likewise, a base appliance 410 may be treated in specific areas with various chemical agents to either increase or reduce localized rigidity. This may also include treatments involving temperature changes and other phase altering methods. Similarly, such methods may be combined, including any or all of the above described methods. Likewise, such methods may be utilized for appliances of uniform elastic modulus.
[0063] The fabrication process illustrated in FIG. 16 may be used to prepare a preferred laminated appliance structure 500, as illustrated in FIGS. 17 and 18. An inner layer 502 is formed from a relatively stiff polymeric material and molded over a positive tooth model which represents the desired appliance geometry. After the layer 502 is formed, it can be segmented into two or more sections which conform to individual teeth or groups of teeth when the appliance is placed over the patient's jaw. As shown in FIG. 17, gaps 504 may be formed between individual sections 506, each of which conform to and receive an individual tooth or group of teeth. An outer layer 508 is continuous in the mesial-distal axis and covers all the segments 506 of the inner layer 502. By providing an inner layer 502 having a higher stiffness, firm gripping or anchoring of the underlying teeth can be achieved. Moreover, by providing an outer layer 508 which is less stiff or more compliant, ease of removing and replacing the appliance can be significantly improved. Moreover, the stiffness or anchoring force can be enhanced without having to concurrently modify the overall or effective elasticity of the appliance which can be selected based on the clinical requirements of moving teeth. That is, the elasticity of the outer layer can be selected to provide an appropriate tooth movement force while that of the inner layer can be chosen to enhance seating characteristics over the teeth. In a specific embodiment, the compliance of the outer layer 508 could be varied along the mesial-distal axis in order to provide for differing forces on the teeth, as discussed generally above.
[0064] Although the foregoing invention has been described in some detail by way of illustration and example, for purposes of clarity of understanding, it will be obvious that various alternatives, modifications and equivalents may be used and the above description should not be taken as limiting in scope of the invention which is defined by the appended claims.
