SYSTEMS AND METHODS FOR VARYING ELASTIC MODULUS APPLIANCES

Abstract: 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 (100) configured to receive the teeth (115) and incrementally reposition individual teeth in a series of successive steps. The individual appliances may be formed from layers (110, 111) having different stiffnesses (elastic moduluses), and the stiffnesses of successive appliances may be different, or both.

Description: SYSTEMS AND METHODS FOR VARYING ELASTIC MODULUS APPLIANCES BACKGROUND OF THE INVENTION This invention relates in general to a method of repositioning teeth for use in orthodontic treatment. Particularly, this invention relates to the use of orthodontic appliances for producing tooth movements. More particularly, this invention relates to the use of a plurality of elastic repositioning appliances for producing such tooth movements. Orthodontic treatments involve repositioning misaligned teeth and improving bite configurations for improved cosmetic appearance and dental function. Repositioning teeth is accomplished by applying controlled forces to the teeth over an extended period of time. This is conventionally accomplished by wearing what are commonly referred to as "braces." Braces comprise a variety of appliances such as brackets, bands, archwires, ligatures, and O-rings. After they are bonded to the teeth, periodic meetings with the orthodontist are required to adjust the braces. This involves installing different archwires having different force-inducing properties or by replacing or tightening existing ligatures. Between meetings, the patient may be required to wear supplementary appliances, such as elastic bands or headgear, to supply additional or extraoral forces. Although conventional braces are effective, they are often a tedious and time consuming process requiring many visits to the orthodontists office. Moreover, from a patient's perspective, they are unsightly and uncomfortable. Consequently, alternative orthodontic treatments have developed. A particularly promising approach relies on the use of elastic positioning appliances for realigning teeth. Such appliances comprise a thin shell of elastic material that generally conforms to a patient's teeth but is slightly out of alignment with the initial tooth configuration. Placement of the elastic positioner over the teeth applies controlled forces in specific locations to gradually move the teeth into the new configuration. Repetition of this process with successive appliances comprising new configurations eventually move the teeth through a series of intermediate configurations to a final desired configuration. A full description of an exemplary elastic polymeric positioning appliance is described in U.S. Patent No. 5,975,893, and in published PCT application WO 98/58596 which designates the United States and which is assigned to the assignee of the present invention. Both documents are incorporated by reference for all purposes. In addition to their ease of use, polymeric positioning appliances are generally transparent, providing an improved cosmetic appearance, and impart substantial force on the teeth, due to stiffness of the appliance. The stiffness of an elastic positioning appliance is a result of the modulus of the thermoformable polymer materials from which it is made. The higher the modulus of the materials, the higher the stiffness of the appliance. When a patient positions such an appliance over a prescribed group of teeth, one or more of the teeth will provide a base or anchor region for holding the positioning appliance in place while the stiffness of the polymeric material will impart a resilient repositioning force against one or a portion of the remaining teeth. However, the stiffer the appliance, the more difficult it is to slip the misaligned appliance over the teeth and fully engage the appropriate surfaces; the appliance often has the tendency to disengage or "pop off. Likewise, once it is firmly seated, it is more difficult to remove. Further, a stiff appliance is less forgiving in cases of lowered patient compliance. If a patient were to remove the appliance for an unprescribed period of treatment time, the patient's teeth may move slightly out of the planned tooth arrangement. When attempting to reapply the appliance, it may be too rigid to accommodate these slight differences and a new appliance may need to be created. Similarly, the tooth positions defined by the cavities in each successive appliance must not differ beyond a limiting dimension from those defined by the prior appliance or, again, it may be too rigid to accommodate the differences. Consequently, only small increments in tooth repositioning may be made with each appliance. Thus, it would be desirable to provide tooth positioners, systems, and methods which apply adequate force to selected teeth yet overcome the inherent limitations of stiffness in the polymeric material. Likewise, it would be desirable to reduce the number of positioners required for a treatment plan by increasing the size of the repositioning increments throughout the plan. Further, it would be desirable to reduce the cost of lowered patient compliance by reducing the need for new appliances to be created for patient treatment resumption. At least some of these objectives will be met by the designs and methods of the present invention described hereinafter. SUMMARY OF THE INVENTION 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 appliances configured to receive the teeth in a cavity and incrementally reposition individual teeth in a series of successive steps. This is accomplished by applying force to specific surfaces of the teeth to cause directed movement. In order to apply such force, one or more of the teeth will provide a base or anchor region for holding the positioning appliance in place while the stiffness of the polymeric material will impart a resilient repositioning force against one or more of the remaining teeth. However, such stiffness creates limitations in ease of use, patient compliance, and overall cost in material, manufacturing labor and treatment time. To overcome these limitations, the present invention utilizes polymeric or other material appliances with portions differing in rigidity, hardness, or stiffness. Portions of the appliance designed to apply specific forces may have different elastic moduluses (stiffnesses) and/or hardnesses than other portions. Alternatively, elastic moduluses and/or hardnesses may vary from one appliance to the next in a successive series to accomplish various treatment goals. Thus, the systems and methods of the present invention provide the design, production and use of such multiple stiffness positioning appliances in orthodontic treatment. Similarly, the devices of the present invention provide variable stiffness appliances which may be used independently for purposes other than repositioning, such as for retaining teeth in a desired position. Thus, reference hereinafter to repositioning appliances with portions having differing or varying stiffnesses or hardnesses is not intended to limit the scope of the present invention and is understood to include appliances of the described design for other purposes. In a first aspect of the present invention, an elastic repositioning appliance may be comprised of portions with differing elastic moduluses. Elastic modulus may be used to express or describe the stiffness of a material or a material's resistance to elastic deformation. Therefore, elastic modulus may be used hereinafter to refer to stiffness. The different portions of the appliances will also usually vary in hardness. More usually, stiffer portions will be harder while the less stiff portions will be softer. Hardness is usually measured as a "durometer" reading on either the A or the D scale. In most instances, the present invention will be more concerned with the elastic modulus of the material since that will effect the force applied to the teeth for either moving the teeth or for gripping or anchoring the teeth. In other instances, however, the hardness of the material may be more important, e.g., to avoid trauma to soft tissue regions engaged by the appliance. The remaining description and claims generally refer to materials having greater and lesser stiffnesses. It will be appreciated that such terminology will also comprise materials having greater and lesser hardnesses. The elastic modulus of a material is the ratio of the increment of unit stress to an increment of unit deformation within the elastic limit. When a material is deformed within the elastic limit, the bonds between adjacent atoms are stretched but not broken. The magnitude of the elastic modulus is indicative of the atomic and molecular bonding forces. When the stress is relieved, the material returns to its original shape and the deformation is nonpermanent. Different materials may have different elastic moduluses based on their molecular structures. Some materials, such as certain polymers, may be specially produced to have different elastic moduluses while retaining similar chemical compositions (and thus assuring compatibility of the different modulus materials in a single structure). Likewise, the elastic modulus of a polymer or other material may be enhanced or otherwise modified. This may be achieved by adding a powder, such as CaCO 3 , talc, TiO 2) glass, diamond or a polymer powder, to name a few. In addition, this may be achieved by embedding structural reinforcements, such as metal pieces, strips, wires, mesh, lattices, networks, polymeric filaments, or the like. In addition, the elastic modulus may be altered by post-production methods, such as layering, coating, interpenetrating, treating with various chemical agents, and altering the temperature, to name a few. In the resulting appliance, the elastic moduluses of the varying portions will usually range from 0.5 to 5 GigaPascal (GPa), although in some instances portions of the appliance may fall outside of this range. The elastic modulus of one portion may differ from another portion by 25% to 600%, or more. The differing elastic moduluses of different portions of the dental appliance shells of the present invention will exist while the device is present over teeth in a normal oral environment. Thus, different portions of the appliance shell will impart different forces to the immediately underlying teeth, where the level of the force depends both on the device geometry or tooth positions (relative to the underlying tooth or teeth, which may vary over time) and on the elastic modulus of that portion of the device (which will remain constant over time in the normal oral environment). The present invention should be distinguished from that described in copending application no. 09/250,962, where the stiffness of a dental appliance shell may change over time by expose to a non-oral environment, such as elevated temperature or changed osmolality. Of course, the dental appliance shells of the present invention which have different portions with differing stiffness may also incorporate regions (including the entire appliance) where a change in stiffness may be induced according to the teachings of application no. 09/250,962, the full disclosure of which is incorporated herein by reference. In a first embodiment, portions of the shell of an elastic repositioning appliance may be composed of material(s) which differ in elastic moduluses and/or hardnesses along a mesial-distal axis. A mesial-distal axis may be defined as an axis following the gingival line or dental arch. Thus, the elastic repositioning appliance may be comprised of portions with a lower elastic modulus covering the molars, for example, and portions with a higher elastic modulus covering the remainder of the teeth. In this example, the portions may be relatively large so that a portion may receive one or more teeth, such as contiguous molars. This may be utilized when one or more teeth are to provide an anchor or base region for imparting repositioning force against another tooth or teeth. The portion of the appliance covering the anchor teeth may be of a relatively flexible nature with a lower elastic modulus than the portion covering the teeth to be repositioned. This is because the portions covering the anchor teeth may not need to apply repositioning forces to the teeth they cover; they may merely be designed to hold the appliance in place. Consequently, a high level of rigidity or stiffness may not be required. However, it may be appreciated that portions covering anchor teeth may in fact require a higher stiffness material than other portions, including portions which are designed to apply repositioning forces. Thus, any variation of stiffness or elastic modulus along a mesial-distal axis is included in this embodiment. The introduction of such portions or regions with more flexibility provides utility in ease of use for the patient. The patient may find ease in positioning the appliance with the more flexible portions first which may guide the appliance in placement of the more rigid, slightly misfit portions designed for repositioning. This sequence may be reversed in removal of the appliance. Likewise, such flexibility may also allow for any slight differences in mold versus appliance versus dentition geometry which may otherwise make placement and removal of the appliance more difficult. In some cases, a generally misfit appliance may "pop off or have a tendency to disengage even when properly positioned over the teeth. Increased flexibility may reduce these tendencies. In further embodiments, portions of the elastic repositioning appliance may vary in elastic moduluses along different and/or additional axes. For example, moduluses may vary along a facial-lingual axis. Facial may be defined as next to or toward the lips or cheek, including terms labial and buccal. Lingual may be defined as next to or toward the tongue. Thus, a facial-lingual axis may be described as an axis following a radial or similar line from the tongue toward the lips or cheek and vice versa. Likewise, moduluses may vary along a gingival-crown axis. This may be described as a substantially vertical axis following a line from the top of the crown at the edge of the occlusal surface of a tooth toward the gingival line or root and vice versa. In a preferred embodiment, an appliance may have a portion with a lower elastic modulus covering the occlusal surfaces of the teeth and a portion with a higher elastic modulus covering the remaining surfaces of the teeth. Thus, the moduluses may vary along a facial-lingual axis and/or a gingival-crown axis, depending on the boundaries of the delineated portions. Such a design may incorporate added flexibility to the appliance while maintaining adequate repositioning forces in the most efficient areas. In addition to varying in stiffness along the axes described above, the appliances of the present invention may vary in stiffness or hardness over the "thickness" of the appliance. Usually, such variations and stiffness over the thickness will be accomplished by layering the device, i.e., with layers of differing stiffnesses or hardnesses being placed successively over the mold used to form the appliances, as described in more detail below. Thus, the appliances may comprise shells having first and second portions, as generally described above, where each of those portions comprise layers in a laminar structure. Usually, at least one of the first and second portions will comprise a continuous layer along the mesial-distal axis. The second and optionally additional layers may also be continuous along the mesial-distal axis, but will often be discontinuous, i.e., broken into two or more segments. Such layered devices can provide a variety of benefits. For example, layers formed from stiffer or harder materials can be used to more firmly engage teeth, while the less stiff or softer layers can be used to provide compliance and greater elasticity. In a particular preferred embodiment, the appliance comprises a discontinuous inner layer and a continuous outer layer. At least a portion of the inner layer is configured to engage individual teeth or groups of teeth and will be stiffer or harder than the outer layer. The outer layer, which is less stiff and therefore more compliant, provides the elasticity to move the teeth relative to one another, while the harder inner layer firmly engages the teeth to provide a better grip or anchor upon the teeth. It may be appreciated that the elastic modulus of the appliance shells may vary over any number of delineated portions. Such portions may be of any size, shape, thickness, or dimension. Thus, such portions may receive entire teeth or they may be of the size to cover only a portion of a tooth or dental surface. When portions are relatively large, an appliance may be divided into, for example, two to five portions. Portions adjacent to one another differ in elastic moduluses, however not all portions of an appliance may differ from each other, such as in the case of an appliance with portions alternating between two moduluses. When portions are relatively small, an appliance may contain an unlimited number of portions, varying along any axis or combination of axes. In a second aspect of the present invention, such appliances comprised of portions having differing stiffness may be used independently or in a series with similar or differing devices. When used independently, the appliance may be worn to achieve a specific goal with a single device. For example, the appliance may be used as a "retainer" to hold the teeth in a desired position. Or, the appliance may be used for a specific onetime repositioning movement, such as "finishing" or correcting a slight misalignment. When used in a series, the appliances may comprise a system for repositioning teeth from an initial tooth arrangement to a final tooth arrangement. In this case, a plurality of incremental elastic position adjustment appliances comprising polymeric or other material shells are successively worn by a patient to move teeth from one arrangement to a successive arrangement. Individual appliances may be configured so that their tooth- receiving cavity has a shape or geometry corresponding to an intermediate or end tooth arrangement intended for that appliance. Thus, successive individual appliances may have a shape or geometry differing from that of the immediately prior appliance. According to the present invention, some or all of the individual appliances may also be comprised of a material stiffness differing from the stiffness of the immediately prior appliance. In addition, each individual appliance be comprised of portions with varying stiffnesses. In some cases, of course, individual appliances in the system may not vary in stiffness from prior or successive appliances, but only in geometry. In other cases, individual appliances may vary only in stiffiiess (and not in geometry) when compared to immediately prior or subsequent appliances. Thus, systems according to the present invention may be comprised of appliances having stiffness varying within the appliance and/or from one appliance to the next in the series. In a specific embodiment, a system of elastic repositioning appliances may comprise individual appliances having uniform elastic moduluses over their entire tooth contact area where the moduluses will differ among successive appliances used in a course of treatment. The elastic modulus of a given appliance may be chosen to be most suitable for a specific type of tooth movement, such as translating, tipping, root uprighting, rotation, extrusion, intrusion or a combination of these. For example, translation may require 70-120 gm of force, whereas rotation may only require 35-60 gm of force. Therefore, an elastic positioning appliance designed for translating teeth may need to have a higher elastic modulus than one designed for purely rotating teeth. This is again due to the fact that stiffness of the appliance is a critical factor in imparting repositioning force. Consequently, a series of appliances may be produced for a treatment plan in which successive appliances designed for a specific tooth movement may all have substantially similar elastic moduluses. At the point in the treatment plan in which a different type of tooth movement is desired, further appliances designed for the new tooth movement may have substantially similar elastic moduluses to each other but different from the previous appliances. Such a sequence may be repeated at any time or may continue with new moduluses and tooth movements. In an additional specific embodiment, one or more appliances may be produced with a suitably flexible elastic modulus to receive and resiliently reposition teeth from an unprescribed arrangement to a prescribed arrangement. This might be necessary in cases of lowered patient compliance. If a patient were to remove an appliance for an unintended and/or extended period of a prescribed treatment time, the patient's teeth may move slightly out of the planned tooth progression. When attempting to reapply the appliance, an appliance which is too rigid may not be able to accommodate these slight differences. Thus, a more flexible appliance (but having an identical geometry) may be produced for this purpose and may be incorporated into the treatment plan at any given point in the series of successive appliances. The ability to return to the same geometry is an advantage because it minimizes the need to replan the treatment protocol. In a third aspect of the present invention, systems for repositioning teeth from an initial tooth arrangement to a successive tooth arrangement comprise a plurality of incremental elastic position adjustment appliances in which at least one appliance has the same shape yet different elastic modulus as an immediately prior appliance. In a specific embodiment, a series of incremental appliances may be produced with differing elastic moduluses to reposition teeth from an initial tooth arrangement to the next successive tooth arrangement in a progression of arrangements to the final arrangement. Each of the appliances in the series from the first to the next successive tooth arrangement may have the same shape or geometry since the tooth movement represents one step in tooth movement. However, the variance in elastic moduluses may allow for a larger step or increment in tooth movement than may be obtainable with consistent, rigid appliances. For example, an appliance may be produced with a tooth arrangement which is substantially misaligned from the initial arrangement. High modulus appliances may not be flexible enough to allow the appliance to fit over the teeth in the initial arrangement. However, a series of appliances of the same shape may be produced with increasing elastic moduluses from relatively low to adequately high. The patient may begin with the lowest elastic modulus appliance which may be the most flexible to fit over the teeth. As the teeth are repositioned, the patient may successively utilize each appliance in increasing modulus until the teeth have conformed to the successive tooth arrangement. At that time, the patient may begin a new series of appliances with varying moduluses and a shape to reposition the teeth to the arrangement of the next step in the repositioning progression. The ability to reduce the number of different appliance geometries required for a single course of treatment can provide a significant reduction in planning effort and manufacturing costs. In a fourth aspect of the present invention, the elastic modulus of an appliance or portions of an appliance may be modified in a number of different ways. To begin with, the elastic modulus may be determined by the choice of materials. For example, metals will generally have a higher elastic modulus than polymers due to atomic structure. For example, the modulus values for metals may range between 48 and 414 GPa , whereas the modulus for polymers may range from 0.5 to 35 GPa. Thus, it will be possible to form appliances having moduluses which differ greatly by forming different portions from metal(s) and polymer(s), or by forming successive appliances from metals and polymers. Usually, however, the appliances will comprise or consist of a polymeric shell formed from a single polymer, multiple polymers, copolymers, and the like, typically by thermo forming and/or lamination. Stiffness of a polymer may be varied within a range (typically 0.5 GPa to 5 GPa) by changing the molecular structure of the polymer chains. Polymer chains with hindered side-chains are unable to pack as closely as those with smaller side-chains. Thus, such a polymer may have more intermolecular motion and therefore a lower bulk elastic modulus. Stiffness can also be changed by controlling the degree of cross-linking as well as the cross-linking entity within a polymer or copolymer. Further, alternatively, differing elastic moduluses may be created within the same polymer shell by layering or laminating the same or different polymers. Two layers of a polymer material bonded together to form an integral appliance, i.e., an appliance having a monolithic shell structure where the layers are resistant to delamination, may have a higher elastic modulus than a single layer of such material. Thirdly, different elastic moduluses may be created with a single layer of one type of polymer material by production methods, such as coating, treating with various chemical agents, and altering the temperature, to name a few. Further, different elastic moduluses may be produced by forming selectively reinforced and/or composite-type materials. For example, a polymer material may be reinforced with structures such as strips, wires, pieces, mesh, lattices, networks, and the like. These structures may be comprised of any suitable material, particularly metals and alloys but also including polymer filaments, wires, braids, and the like. Likewise, composite materials may be comprised of interpenetrating polymeric networks. An interpenetrating polymeric network is comprised of a base material and an additional material that interpenetrates the base material to alter its mechanical properties. For example, the base material (A) may be a solid polycarbonate. The added material (B) may be a liquid polymer, monomer or crosslinking agent which is allowed to interpenetrate and activate to form a composite network. The composite (A+B) may have a stiffness which is greater than the sum of its parts, (A) and (B). Further, another material (C) may also be allowed to interpenetrate and activate to form a new composite network. The composite (A + B + C) may also have a stiffness which is greater than the sum of its parts, (A), (B) and (C). With this method, any number of composites may be formed providing a wide range of mechanical properties, specifically stiffnesses. In addition, a number of these production methods may provide materials with gradual changes in elastic moduluses. For example, purposely irregular coating of a polymer material may provide higher stiffness in areas with thicker coating and lower stiffness in areas with thinner coating. This may be applied to a number of production methods. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective illustration of an embodiment of an appliance of the present invention and descriptive axes. Fig. 2 illustrates an embodiment of an appliance with relatively large portions varying in elastic modulus along a mesial-distal axis. 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. 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. Fig. 5 illustrates a variety of appliance portions varying in elastic modulus along a mesial-distal axis. Fig. 6 is a perspective illustration of an embodiment of an appliance varying in elastic modulus along a facial-lingual axis. Fig. 7 illustrates a variety of appliance portions varying in elastic modulus along a facial-lingual axis. Fig. 8 is a perspective illustration of an embodiment of an appliance varying in elastic modulus along a gingival-crown axis. Fig. 9 illustrates a variety of appliance portions varying in elastic modulus along a gingival-crown axis. Fig. 10 illustrates a variety of appliance portions varying in elastic modulus along one or more described axes. Fig. 11 depicts a series of appliances differing in elastic modulus at specific intervals throughout a treatment plan. Fig. 12 illustrates the use of an "off track" appliance in a prescribed treatment plan. 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. Fig. 14 illustrates a method of fabricating a multi-modulus appliance. Fig. 15 illustrates a method of layering to fabricate a multi -modulus appliance. Fig. 16 illustrates an additional method of layering to fabricate a multi- modulus appliance. Fig. 17 illustrates an exemplary layered appliance according to the present invention with portions broken away. Fig. 18 is a cross-sectional view taken along line 18-18 of Fig. 17. DESCRIPTION OF THE SPECIFIC EMBODIMENTS 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. 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. 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. 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. 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. 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. 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. 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. 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. 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 co-pending application [Attorney Docket No. 018563-001510], assigned to the assignee of the present invention. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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.

Claims: WHAT TS CLAIMED IS: 1. A dental appliance comprising: a sriell having cavities shaped to receive teeth, wherein at least a first portion of the shell is composed of a material having a first stiffness and a second portion of the shell is composed of a material having a second stiffness which differs from the first stiffness. 2. An appliance as in claim 1, wherein the first portion differs in stiffness from the second portion along a mesial-distal axis. 3. An appliance as in claim 2, wherein the first portion receives one or more said teeth and the second portion receives one or more said teeth. 4. An appliance as in claim 1, wherein the first portion differs in stiffness from the second portion along a facial-lingual axis. 5. An appliance as in claim 4, wherein the first portion partially covers an occlusional surface of a tooth and the second portion substantially covers the remainder of the tooth. 6. An appliance as in claim 1, wherein the first portion differs in stiffness from the second portion along a gingival-crown axis. 7. An appliance as in claim 6, wherein the first portion substantially covers an occlusional surface of a tooth and the second portion substantially covers the remainder of the tooth. 8. An appliance as in claim 1, further comprising a third portion having a third stiffness which differs from the first and second stiffnesses. 9. An appliance as in claim 8, wherein the third portion is located adjacent to the first portion or the second portion in a mesial-distal, facial-lingual or gingival-crown direction. 10. An appliance as in claim 1, wherein the first and second portions comprise layers in a laminar structure. 11. An appliance as in claim 10, wherein at least one of the first and second portions is continuous along a mesial-distal axis. 12. An appliance as in claim 10, wherein at least one of the first and second portions is discontinuous along a mesial-distal axis. 13. An appliance as in claim 10, wherein the first portion comprises a discontinuous inner layer and the second portion comprises a continuous outer layer and wherein at least a portion of the inner layer is configured to engage individual teeth and the outer layer has a lower elastic modulus than the inner layer. 14. An appliance as in claim 1 , wherein the first portion and the second portion have elastic moduluses in the range from 0.5 to 5 GPa. 15. An appliance as in claim 1, wherein the first portion has a first elastic modulus that differs from the second elastic modulus of the second portion by 25%> to 600%. 16. An appliance as in claim 1, wherein at least a region of the shell is comprised of a material reinforced with a structure selected from the group consisting of pieces, strips, wires, mesh, lattices, and networks. 17. An appliance as in claim 16, wherein the reinforced material comprises a polymer. 18. An appliance as in claim 16, wherein the structure comprises a metal or alloy. 19. An appliance as in claim 16, wherein the structure comprises a polymer. 20. An appliance as in claim 1, wherein at least a region of the shell comprises a metal or alloy. 21. An appliance as in claim 1 , wherein at least a region of the shell comprises of an interpenetrating network. 22. In a system for repositioning teeth from an initial tooth arrangement to a final tooth arrangement, said system comprising a plurality of incremental position adjustment appliances comprising shells having cavities shaped to receive and reposition teeth and which are successively worn by a patient to move teeth from one arrangement to a successive arrangement, an improvement comprising: at least one appliance including a shell wherein at least a first portion of the shell is composed of a material having a first stiffness and a second portion of the shell has a second stiffness. 23. In a system for repositioning teeth from an initial tooth arrangement to a final tooth aπangement, said system comprising a plurality of incremental position adjustment appliances comprising shells having cavities shaped to receive and reposition teeth and which are successively worn by a patient to move teeth from one arrangement to a successive arrangement, an improvement comprising: at least a first appliance including a shell having a portion with a first stiffness and at least a second appliance having a shell with a portion geometrically corresponding to that of the first appliance, wherein the portion of the second appliance has a second stiffness which differs from the first stiffness. 24. A system as in claim 23, wherein the shells of the first and second appliances have uniform but different stiffnesses over their entire tooth contact area. 25. A system as in claim 24, further comprising at least a third appliance having a stiffness which is uniform and different from those of the first or second appliance. 26. A system as in claim 25, wherein appliances with substantially similar stiffnesses are used for substantially similar tooth movements, wherein the tooth movements are selected from the group of tipping, translating, root uprighting, rotation, extrusion, intrusion and a combination of these. 27. In a system for repositioning teeth from an initial tooth arrangement to a successive tooth arrangement, said system comprising a plurality of incremental position adjustment appliances comprising shells having cavities shaped to receive and reposition teeth and which are successively worn by a patient to move teeth from one arrangement to a successive arrangement, an improvement comprising: a first appliance including a shell having a shape which is the same as the shape of a second appliance and having at least a portion with a stiffness which is different from the stiffness of a corresponding portion in another appliance. 28. A system as in claim 27, further comprising a third appliance having a shape which is the same as the shape of the first and second appliances and having a stiffness in a portion which is different from the stiffnesses of the corresponding portions of the first and second appliances, wherein the stiffnesses of the first, second, and third appliances increase to provide incremental tooth movements as the appliances are successively worn. 29. A method for producing a dental appliance with varying stiffnesses, said method comprising: providing a mold of dental features; forming a first layer of a material having a first stiffness over a first portion of said mold; and forming a second layer of a material having a second stiffness over a second portion of said mold; and laminating the first and second layers into an integral appliance. 30. A method as in claim 29, wherein the first portion of the mold and the second portion of the mold have no portions in common. 31. A method as in claim 30, wherein the forming steps occur simultaneously. 32. A method as in claim 29, wherein the first portion of the mold and the second portion of the mold at least partly overlap. 33. A method as in claim 32, wherein the forming steps occur in succession. 34. A method as in claim 33, wherein further comprising removing material from the first layer prior to forming the second layer. 35. A method as in claim 34, wherein removing material comprises separating adjacent sections of the first layer so that said separated sections conform to individual teeth or groups of teeth when the appliance is placed over teeth. 36. A method as in claim 35, wherein the first layer is more stiff than the second layer. 37. A method as in claim 29, wherein said forming steps are comprised of thermoforming, reaction casting, coating, treating, or altering. 38. A method for repositioning teeth, said method comprising: placing a first incremental position adjustment appliance over teeth in a patient's mouth, and successively placing at least a second incremental position adjustment appliance over the teeth in the patient's mouth, wherein the appliances comprise polymeric shells having shaped cavities and wherein the shells of the first and second appliances have the same shapes but have portions which differ in stiffness between the first and second appliances. 39. A method as in claim 38, wherein the stiffness of the shells of the first and second appliances are uniform over the entire shell. 40. A method as in claim 38, wherein the stiffnesses of the shells of the first and second appliances are non-uniform over the entire shell. 41. A method for repositioning teeth, said method comprising: placing a first incremental position adjustment appliance over teeth in a patient's mouth, and successively placing at least a second incremental position adjustment appliance over the teeth in the patient's mouth, wherein the appliances comprise polymeric shells having shaped cavities and wherein at least one of the first and second appliance has a shell wherein at least a first portion of the shell has a first stiffness and a second portion of the shell has a second stiffness which differs from the first stiffness. 42. A method as in claim 41 , wherein both the shells of both the first and second appliances having shells which at least a first portion of the shell has a first stiffness and a second portion of the shell has a second stiffness which differs from the first stiffness. 43. A method as in claim 38, wherein the first and second portions of the shells of the first and second appliances are located in corresponding positions of each appliance and wherein the stiffness of the first portions of the first and second appliances differ from each other. 44. A method as in claim 43, wherein the stiffness of the second portions of the first and second appliances differ from each other. 45. A method as in claim 38, wherein the shells of the first and second appliances having the same geometry. 46. A method as in claim 38, wherein the shells of the first and second appliances have different geometries.

Inventors: CHISHTI MUHAMMAD, KUO ERIC, MILLER ROSS J, PHAN LOC X, VAN DEN BERG H ROBERT, CHISHTI, MUHAMMAD, KUO, ERIC, MILLER, ROSS, J., PHAN, LOC, X., VAN DEN BERG, H., ROBERT
Applicants: ALIGN TECHNOLOGY INC, CHISHTI MUHAMMAD, DEN BERG H ROBERT VAN, KUO ERIC, MILLER ROSS J, PHAN LOC X, ALIGN TECHNOLOGY, INC., CHISHTI, MUHAMMAD, KUO, ERIC, MILLER, ROSS, J., PHAN, LOC, X., VAN DEN BERG, H., ROBERT
Application year: 2001
Main classification: A61C3
IPC codes: A61C7