Bracket-free invisible orthodontic appliance based on direct 3D printing and digital design and manufacturing method of bracket-free invisible orthodontic appliance

Abstract

The invention discloses a bracket-free invisible orthodontic appliance based on direct 3D printing and a digital design and manufacturing method of the bracket-free invisible orthodontic appliance. According to the method, a digital closed-loop process is constructed; firstly, a correction target is set through intraoral scanning and digital tooth arrangement; constructing a smooth envelope space of the appliance by utilizing a contraction wrapping algorithm; accurately segmenting the dental crown by adopting an AI technology based on PointNet + + so as to define a force boundary; generating an appliance digital model with an accurate form and stable mechanics through a thickness control and edge optimization algorithm; finally, based on the DLP technology, the orthodontic appliance is directly printed with biocompatible resin, and the performance and safety of the orthodontic appliance are ensured through strict post-treatment such as secondary curing. According to the method, a traditional film pressing process is omitted, accumulative errors are eliminated fundamentally, personalized gradient design of mechanical properties of the appliance can be realized through process control, and an invisible correction technology is marked to step from shape matching to function matching.

Description

Technical Field

[0001] This invention relates to the field of orthodontic medical device technology, specifically to a bracketless clear aligner integrally formed using direct 3D printing technology, and a fully digital design and manufacturing system supporting the high-precision, personalized production of this aligner. This invention overcomes the technical bottlenecks of traditional indirect compression molding methods, constructing a closed-loop technical solution from digital oral data acquisition, intelligent geometric modeling, programmable mechanical performance design to direct 3D printing and standardized post-processing. It achieves a paradigm shift in clear aligners from "morphological matching" to "functional adaptation," applicable to orthodontic treatment of various malocclusions, and especially providing a precise, efficient, and personalized solution for complex tooth movement cases. Background Technology

[0002] 2.1 The Development History and Clinical Status of Invisible Orthodontic Technology

[0003] Orthodontics, as a core branch of oral medicine, aims to guide tooth movement through controlled biomechanical stimulation, correct malocclusion, restore oral physiological functions such as chewing and speech, improve facial aesthetics, and promote the patient's physical and mental health. With advancements in medical technology and increasing patient demands for treatment experience, clear aligners have gradually replaced traditional fixed braces, becoming one of the mainstream orthodontic treatment options due to their superior aesthetics, comfort, ease of oral hygiene maintenance, and digitally predictable treatment results.

[0004] The development of invisible orthodontic technology has undergone several generations of technological iterations: In 1945, Dr. Kesling's design of the orthodontic appliance pioneered bracketless orthodontics, its core idea being to achieve precise tooth adjustment through plastic appliances; in 1993, Sheridan combined clear enamel bonding technology with interproximal enamel reduction technology to develop the Essix appliance, expanding the application scope of invisible orthodontics; in 1997, Align Technology launched... This system marks the first time that the entire process, from digital tooth alignment design and 3D-printed resin master mold to thermoforming, has been fully digitized, signifying that invisible orthodontic technology has entered an era of precision and standardization. The widespread adoption of this technology has greatly promoted the digital transformation of orthodontic treatment; however, the core manufacturing process still relies on the traditional indirect molding method. This limitation has become a key factor restricting its clinical effectiveness and technological advancement.

[0005] 2.2 Technical defects of traditional indirect film pressing method

[0006] The manufacturing process of traditional clear aligners involves multiple steps, including "digital tooth alignment → 3D printing of resin master mold → thermoforming → manual trimming → sterilization and packaging." This indirect manufacturing model has the following unavoidable core defects:

[0007] 2.2.1 The process is cumbersome, resulting in significant cumulative errors.

[0008] The indirect pressure molding method involves multiple steps of conversion and connection, each of which may introduce errors that accumulate gradually, ultimately affecting the positioning accuracy of the orthodontic appliance and the treatment outcome. Specifically:

[0009] 1. Resin master mold printing error: When 3D printing master molds, factors such as layer thickness, exposure parameters, and resin shrinkage can cause deviations between the master mold and the digital model. The error range is usually 50-100μm.

[0010] 2. Hot pressing forming error: During the hot pressing process, the thermal expansion of the preformed diaphragm at high temperature and the shrinkage after cooling have nonlinear characteristics. Moreover, the fit between the diaphragm and the master mold is affected by multiple factors such as pressure, temperature, and time, which can easily lead to uneven local stretching or dimensional deviation of the orthodontic appliance. Studies have shown that this step can introduce an additional error of 80-150μm.

[0011] 3. Manual trimming error: After thermoforming, the orthodontic appliance needs to be trimmed manually. The precision of the operation depends on the technician's experience, and problems such as uneven edges, over-trimming or under-trimming are prone to occur. This not only affects the wearing comfort, but may also change the mechanical transmission path of the orthodontic appliance.

[0012] The accumulation of these errors means that the actual fit accuracy of traditional orthodontic appliances to teeth is usually only 150-250μm, which is difficult to meet the stringent requirements for positioning accuracy in complex tooth movements (such as rotation and root-controlled movement). This is also one of the core reasons why the accuracy rate of pre-programmed tooth movement in clinical practice is only 46%-56%.

[0013] 2.2.2 Homogeneous mechanical properties prevent personalized adaptation.

[0014] Traditional orthodontic appliances are made from pre-formed homogeneous thermoplastic sheets (such as polyethylene terephthalate, polyurethane, etc.) through thermoforming. The elastic modulus, toughness, and other mechanical properties of the sheet are fixed during production and cannot be adjusted regionally according to clinical treatment needs. However, different teeth and different movement types in orthodontic treatment have significantly different requirements for orthodontic forces: for example, the molar region requires higher rigidity to provide stable anchorage, the anterior region requires moderate elasticity to achieve controllable translation or rotation, and the area near the gingival margin needs to avoid excessive stress that could lead to gingival compression.

[0015] Homogeneous materials cannot meet the needs of such differentiated mechanical forces, making it difficult for doctors to accurately control the magnitude, direction and point of application of the orthodontic force. This not only affects the treatment effect of complex cases, but may also cause complications such as tooth loosening and gingivitis due to improper force application, thus limiting the application of invisible orthodontic technology in the treatment of moderate to severe malocclusion.

[0016] 2.2.3 Long production cycle, resulting in serious waste of materials and energy.

[0017] The multi-step nature of traditional processes results in lengthy production cycles, typically taking 7-14 days from receiving patient data to delivering the finished product. This cannot meet the treatment needs of urgent clinical cases and diminishes the patient experience. Furthermore, this process suffers from significant material waste: firstly, the resin master mold is for single use only and is discarded after printing, resulting in a material utilization rate of less than 30%; secondly, the effective utilization rate of the diaphragm during thermoforming is only 50%-60%, with the remainder being disposed of as waste. In addition, thermoforming consumes a large amount of electricity to maintain the high-temperature environment, and the disposal of waste resin master molds can cause environmental pollution, contradicting the trend of green manufacturing.

[0018] 2.3 Current Status and Existing Technical Challenges of Direct 3D Printing Technology

[0019] To overcome the limitations of traditional indirect molding methods, direct 3D printing technology has emerged. This technology directly prints the physical form of orthodontic appliances using biocompatible resin, eliminating intermediate steps such as master mold making and thermoforming, thus providing a technological possibility for solving the aforementioned shortcomings. Currently, direct 3D printing has made initial progress in the application of invisible orthodontics, but it still faces the following key technical challenges that restrict its large-scale clinical application:

[0020] 2.3.1 The digital design process is imperfect, and geometric accuracy is difficult to guarantee.

[0021] Direct 3D printing demands extremely high precision from the digital model of the orthodontic appliance. The model's morphological accuracy, thickness uniformity, and edge fit directly determine the clinical performance of the final product. Current technologies rely heavily on traditional geometric processing algorithms for digital model generation, which presents the following problems:

[0022] 1. Inappropriate model envelope construction: Traditional mesh processing only focuses on noise reduction and simplification, without fully considering the functional requirements of the orthodontic appliance, resulting in an uneven shell surface and poor fit, affecting wearing comfort and dislocation force;

[0023] 2. Insufficient precision in crown-gingival segmentation: Traditional methods such as threshold segmentation or edge detection are difficult to accurately identify the boundary of the cervical line, especially in cases of crowded teeth and irregular gingival morphology, which can easily lead to the edge of the orthodontic appliance pressing on the gingiva or inaccurate force transmission.

[0024] 3. Lack of effective means of thickness control: Direct equidistant offset is prone to uneven local thickness due to changes in the curvature of the crown surface, which in turn leads to abnormal distribution of orthodontic force and affects the treatment effect.

[0025] 2.3.2 The programmability of mechanical properties is not fully realized.

[0026] While direct 3D printing theoretically allows for regional control of mechanical properties by adjusting printing parameters, current technologies lack systematic process optimization solutions.

[0027] 1. The mapping relationship between printing parameters and mechanical properties is unclear: The influence mechanism of parameters such as exposure energy, layer thickness, and grayscale on the elastic modulus, toughness, and other properties of the resin after curing has not been fully clarified, making it difficult to achieve precise control of mechanical properties;

[0028] 2. The performance optimization effect of post-processing has not been fully explored: The parameters such as light intensity, time and atmosphere of secondary curing lack standardized control, resulting in insufficient mechanical property stability of the material and failure to construct functionally graded materials.

[0029] 2.3.3 Biosafety and clinical applicability need to be strengthened.

[0030] The oral cavity environment is complex, and orthodontic appliances need to be in long-term contact with the oral mucosa, teeth, and periodontal tissues. Their biocompatibility directly affects clinical safety. Existing directly 3D printed orthodontic appliances have the following safety hazards:

[0031] 1. Residual monomer precipitation: Insufficient resin monomer conversion rate after printing, residual monomer may cause cytotoxicity or allergic reactions;

[0032] 2. Poor surface quality: Problems such as support marks and excessive surface roughness generated during the printing process can easily irritate the soft tissues of the mouth, causing inflammation or discomfort;

[0033] 3. Insufficient long-term performance stability: Under the influence of oral saliva, temperature changes, and repeated removal and wearing, the mechanical properties and transparency of the orthodontic appliance may deteriorate, affecting the treatment effect and service life.

[0034] Therefore, it is urgent to build a complete digital design and manufacturing system adapted to direct 3D printing technology to solve the above-mentioned technical problems, give full play to the technical advantages of direct 3D printing, and promote the development of invisible orthodontic technology towards a more precise, efficient, and personalized direction. Summary of the Invention

[0035] 3.1 Purpose of the Invention

[0036] The core objective of this invention is to overcome the shortcomings of existing technologies and provide a bracketless invisible aligner based on direct 3D printing and its digital design and manufacturing method, specifically achieving the following objectives:

[0037] 1. Construct an efficient and precise digital closed-loop process from oral digital data to personalized orthodontic appliance physical objects, completely eliminate the cumulative error of traditional indirect pressure film method, and improve the morphological accuracy and positioning accuracy of orthodontic appliances;

[0038] 2. Achieve programmable design of the mechanical properties of orthodontic appliances. By precisely controlling the printing and post-processing technology, a gradient distribution of mechanical properties can be constructed on a single orthodontic appliance to meet the differentiated needs of different tooth movement types.

[0039] 3. Optimize the entire process parameters of direct 3D printing to ensure that the orthodontic appliance has excellent biocompatibility, surface quality and long-term performance stability;

[0040] 4. Simplify the production process, shorten the production cycle, reduce material and energy waste, and achieve green and efficient production of invisible orthodontic appliances.

[0041] 3.2 Technical Solution

[0042] To achieve the above objectives, this invention provides a bracketless invisible aligner based on direct 3D printing and its digital design and manufacturing method. This method constructs a complete digital closed loop encompassing "data acquisition - geometric modeling - mechanical optimization - direct printing - post-processing verification." The specific technical solution is as follows:

[0043] 3.2.1 Digital Design Process

[0044] The digital design process is the core of ensuring the morphological accuracy and mechanical performance of orthodontic appliances. It includes four key steps: data acquisition and target setting, model envelope construction, intelligent segmentation to define force boundaries, and appliance generation and edge optimization. These steps are closely linked to form a standardized design system.

[0045] Step 1: Data Acquisition and Target Setting

[0046] The core of this step is to acquire high-precision digital oral data and construct dynamic treatment goals based on clinical treatment plans, providing a reliable data foundation for subsequent design.

[0047] 1. Oral 3D Data Acquisition: A non-contact scanning technique using an intraoral scanner with an accuracy ≥20μm (such as Align Technology's iTero or 3Shape's Trios) is employed to acquire 3D point cloud data of the dentition, gingival mucosa, and occlusal relationships. During the scanning process, multi-angle acquisition and real-time stitching technology ensure that the data covers the entire crown surface and key anatomical structures (such as the cervical line and proximal contact points). After acquisition, the point cloud data is processed to generate a triangular mesh model, which is exported in STL format. The mesh density of this model is controlled at 50-100 triangular facets per square millimeter, ensuring that the fine anatomical features of the crown surface are preserved without compromising subsequent processing efficiency due to excessive mesh density. Compared with traditional impression techniques, intraoral scanning avoids problems such as impression material deformation and air bubbles, improves data accuracy by more than 40%, and significantly enhances patient treatment comfort.

[0048] 2. Digital Tooth Alignment and Target Model Generation: Orthodontists import the aforementioned STL model into professional digital tooth alignment software (such as OnyxCeph and Dolphin Imaging) and perform digital tooth alignment based on biomechanical principles and clinical treatment plans. During the alignment process, the orthodontist sets the ideal final position, movement path, and movement amount for each tooth (usually, the single-step movement amount does not exceed 0.2mm to avoid excessive movement that could cause periodontal damage). Based on the parameters set by the orthodontist, the software automatically calculates and generates a series of progressive target dentition model sequences. This sequence typically contains 10-40 steps, fully defining the tooth movement process throughout the entire treatment cycle. Each target model must meet the following requirements: neat tooth alignment, harmonious occlusion, tooth movement path conforming to biomechanical principles, and the model being a watertight, non-self-intersecting triangular mesh structure to provide a good foundation for subsequent geometric processing.

[0049] Step 2: Model envelope construction

[0050] This step uses the ShrinkWrap algorithm to derive a smooth shell geometry that meets the functional requirements of orthodontic appliances from complex dental anatomy models, solving the problem of balancing morphological adaptability and structural stability in traditional mesh processing.

[0051] 1. Shrinkage Envelope Algorithm Principle: This algorithm simulates the physical process of an elastic membrane enveloping an object. Through iterative balancing of attractive and repulsive forces, it generates a continuous and smooth envelope. Specifically:

[0052] Initial wrapping mesh generation: Generate a low-resolution initial wrapping mesh (such as a sphere or cube) outside the target dental model. The number of mesh vertices is controlled between 1000 and 2000 to ensure iteration efficiency.

[0053] ο Gravity effect: Each vertex of the wrapping mesh is subjected to an attractive force pointing towards the surface feature points of the original dental model (such as cusps, pits, and proximal contact points), enabling the wrapping mesh to accurately conform to the anatomical shape of the tooth crown;

[0054] ο Repulsion / Constraint: To prevent the wrapped mesh from self-intersecting or producing excessive wrinkles, a repulsion force (distance constraint between adjacent vertices) and a smoothing constraint force (to ensure continuous change of the mesh normal direction) are applied to each vertex to maintain the structural integrity and surface smoothness of the wrapped mesh.

[0055] 2. Key parameter optimization:

[0056] Shrinkage coefficient: set at 0.45-0.55, this parameter balances the fidelity of the inclusion to the details of the original model with the surface smoothness. When the shrinkage coefficient is 0.5, more than 95% of the key features of the crown can be preserved while the surface roughness Ra of the inclusion is ≤0.8μm, meeting the requirements for the smoothness of the inner wall of the orthodontic appliance;

[0057] ο Iteration count: Set to 50-80 times. During the iteration process, gradually decrease the gravity weight and increase the constraint weight to ensure that the final package not only fits the crown morphology but also has good structural stability.

[0058] ο Mesh density: The final generated inclusion mesh density is controlled at 30-50 triangular facets per square millimeter, balancing the efficiency of subsequent processing and the accuracy of the shape.

[0059] 3. Model Format Conversion: After processing using the above algorithm, the triangular mesh wrapper is converted into a NURBS (Non-Uniform Rational B-Spline) surface model. NURBS surfaces have advantages such as simple mathematical expression, smooth surface, and precise editing. They not only provide an ideal geometric basis for subsequent thickness control and edge optimization, but also ensure the smoothness of the inner wall of the orthodontic appliance, reduce irritation to the oral soft tissue, and improve the stability of the appliance's dislocation force.

[0060] Step 3: Intelligent segmentation defines the force boundary.

[0061] The PointNet++ network architecture based on deep learning is used to achieve precise segmentation of the crown and gingiva, define the boundary of the orthodontic force, and ensure that the orthodontic force is applied only to the crown area, avoiding unnecessary pressure or damage to the gingiva.

[0062] 1. PointNet++ Network Architecture Optimization: PointNet++ is a hierarchical feature learning network specifically designed for 3D point cloud data. Compared to traditional segmentation algorithms, it possesses stronger local feature capture and global information integration capabilities. This invention specifically optimizes the PointNet++ network to adapt to the characteristics of oral cavity data:

[0063] Input data preprocessing: Convert the STL format of the dental model into point cloud data, set the point cloud density to 20-30 points per square millimeter, remove outliers (points more than 3 standard deviations away from the mean), and perform normalization to improve the efficiency of network training and inference.

[0064] Hierarchical sampling and grouping: The FPS (Farthest Point Sampling) algorithm is used to select key points with a sampling interval of 0.1mm to ensure that key points uniformly cover the entire model; a local region is constructed with a spatial radius of 0.3mm centered on each key point, forming a multi-level feature extraction structure of "local-global", which can capture local details such as cusps and cervical lines, and integrate global information such as dental arch morphology.

[0065] Feature extraction and aggregation: In each local region, features (including coordinate features, normal vector features, and curvature features) are extracted through a small PointNet network, and a symmetric function combining max pooling and average pooling is used to aggregate local features into global features, ensuring that the network is invariant to model rotation and translation.

[0066] Output layer design: The Softmax activation function is used to perform binary classification (coronation point / gingival point) on each point and output the segmentation result.

[0067] 2. Network Training and Performance Validation:

[0068] Training dataset construction: Oral scan data of 1000 cases of different malocclusion types (including crowded, sparse, rotated, and protruding teeth) were collected, of which 800 cases were used as the training set and 200 cases were used as the test set. All data were manually labeled by 3 senior orthodontists to ensure labeling accuracy;

[0069] Training parameter settings: learning rate 0.001, Adam optimizer, batch size 32, training iterations 100 epochs. An early stopping strategy is used during training: training stops when the test set accuracy shows no improvement for 10 consecutive epochs.

[0070] Segmentation performance: The optimized PointNet++ network achieves a segmentation accuracy of 98.5% on the test set, with an average error of ≤0.1mm at the cervical line boundary, which is significantly better than the traditional threshold segmentation algorithm (accuracy of 85%-90%, boundary error of 0.3-0.5mm). Even in complex cases with severe crowding of teeth (crowding ≥8mm) and irregular gingival morphology (such as gingival hyperplasia and recession), it can still achieve accurate segmentation.

[0071] 3. Pure crown model generation: Based on the segmentation results output by the network, all crown point clouds are extracted, and a watertight and smooth pure crown model is generated using the Poisson reconstruction algorithm. This model contains only the crown structure, providing a precise geometric basis for subsequent orthodontic appliance generation and mechanical optimization.

[0072] Step 4: Orthodontic appliance generation and edge optimization

[0073] This step generates a digital model of the orthodontic appliance that is accurate in shape, uniform in thickness, and comfortable to wear by means of equidistant offset, thickness control, edge smoothing, and outward equidistant processing.

[0074] 1. Creatalization of the Orthodontic Appliance: An equidistant offset is performed on the clean crown model. The offset distance is the target thickness of the appliance (usually set to 0.6-0.8 mm). During the offset process, a normal vector offset algorithm is used to ensure that the offset shell maintains a uniform distance from the crown model. To avoid self-intersection or wrinkles during the offset process, the crown surface curvature is greater than 5 mm. -1 Local smoothing preprocessing is performed on areas (such as cusps and pits) to ensure the stability of the offset process.

[0075] 2. Thickness Control Algorithm: Due to differences in the curvature of the tooth crown surface, direct equidistant offset may lead to uneven local thickness (such as excessively thin thickness in the pit and fissure area and excessively thick thickness in the proximal area). Therefore, a thickness control algorithm is needed for global correction.

[0076] Thickness measurement: Using the X-ray projection method, uniformly distributed rays (ray density of 5 rays per square millimeter) are emitted from the outer surface of the orthodontic appliance housing to the inner surface. The length of each ray is calculated as the thickness value at that point.

[0077] Thickness Analysis and Correction: A thickness tolerance of ±0.1mm is set. Weak areas (thickness < target thickness - 0.1mm) and excessively thick areas (thickness > target thickness + 0.1mm) are identified. For weak areas, a secondary offset is performed outward along the normal vector (the offset amount is the difference between the target thickness and the actual thickness). For excessively thick areas, a cutting correction is performed inward along the normal vector (the cutting amount is the difference between the actual thickness and the target thickness). An iterative optimization strategy is used during the correction process. Thickness is re-detected after each iteration until the thickness across the entire domain meets the tolerance requirements, ultimately ensuring that the overall thickness uniformity of the orthodontic appliance is ≤0.1mm, guaranteeing stable mechanical performance.

[0078] 3. Edge Smoothing: The initial edge lines obtained from segmentation may have jagged undulations. These tiny concave angles are prone to becoming stress concentration points during repeated insertion and removal of the orthodontic appliance, leading to material fatigue and tearing. Therefore, a B-spline curve fitting algorithm is used to smooth the edge lines.

[0079] edge point sampling: uniformly sample along the initial edge line at a sampling interval of 0.05 mm to obtain edge feature points;

[0080] OB-spline curve fitting: Using sampling points as control points, construct a 3rd-order B-spline curve, constraining the rate of curvature change of the curve to ≤0.5mm during the fitting process. -1 Ensure smooth and continuous edge lines to eliminate stress concentration points;

[0081] ο Edge surface generation: Using the smoothed edge line as the boundary, generate an edge surface with uniform thickness (edge ​​surface width is 0.3-0.5mm) to ensure that there are no sharp edges and avoid irritating the soft tissues of the oral cavity.

[0082] 4. Outward Equidistant Treatment: To achieve "pressure-free retention," the edge of the orthodontic appliance needs to be adjusted to the surface of the tooth crown to avoid compressing the gingiva. Specifically, the normal vector of the tooth crown surface at the edge line is calculated, and all control points on the edge line are shifted outward by 0.2-0.3 mm along the normal vector. This ensures that the edge of the orthodontic appliance is finally stably fixed on the enamel surface of the tooth crown (0.3-0.5 mm away from the cervical line). This guarantees the retention force of the appliance while providing a safe buffer gap between the gingiva and the appliance, effectively preventing gingivitis.

[0083] 3.2.2 Direct 3D Printing and Post-processing Technology

[0084] The digital model of the orthodontic appliance obtained through the above design process needs to be accurately printed and rigorously post-processed before it can be transformed into a medical device that meets clinical requirements. This process includes three key steps: printing preparation, printing execution, and post-processing.

[0085] Step 1: Printing Preparation

[0086] The core of print preparation is to optimize model placement, design support structures, and set print parameters to ensure the stability of the printing process and product accuracy.

[0087] 1. Model Placement Optimization: Import the optimized digital model of the orthodontic appliance (STL format) into specialized slicing software (such as ShapeWare or Chitubox) and adopt the following placement strategy:

[0088] ο Placement angle: Adjust the angle between the occlusal plane of the orthodontic appliance and the printing platform to 75°-80°, so that the labial and lingual surfaces of the central incisors are nearly parallel to the printing plane, minimizing the impact of interlayer errors on key functional areas (such as occlusal surfaces and adjacent contact surfaces).

[0089] Platform distance: Set the distance between the lowest point of the model and the printing platform to 3-5mm to reserve sufficient space for the addition of the support structure, while avoiding surface damage caused by direct contact between the model and the platform.

[0090] 2. Support Structure Design: The support structure serves to anchor the model, resist interlayer shear forces during the printing process, and prevent damage to critical surfaces of the orthodontic appliance.

[0091] Support location: All supports are designed on the outer surface of the appliance, avoiding the inner surface that fits against the tooth crown and the occlusal functional area; in areas of stress concentration (such as the lingual free margin and around the cusps), the support density is appropriately increased, with a support spacing of 2-3mm; in other areas, the support spacing is 3-5mm.

[0092] Support type: Point support is adopted, with a contact point diameter of 0.2-0.3mm between the support and the surface of the aligner, a support rod diameter of 0.5-0.8mm, and a support height of 3-5mm. This design can ensure support strength, facilitate subsequent removal, and will not leave obvious scars on the surface of the aligner.

[0093] Number of supports: Depending on the size of the orthodontic appliance, the number of supports should be controlled between 50 and 100 to ensure that the model does not deform during printing.

[0094] No displacement.

[0095] 3. Printing Parameter Settings: Using a Digital Light Processing (DLP) 3D printer (such as SHAPE 1+ or Formlabs Form3), the printing parameters are optimized based on resin properties and product requirements as follows:

[0096] Layer thickness: Set to 0.05-0.1mm. The smaller the layer thickness, the higher the printing accuracy, but the lower the production efficiency. Considering both accuracy and efficiency, a layer thickness of 0.1mm is preferred. At this time, the printing accuracy can reach ±20μm, which meets clinical requirements.

[0097] Exposure time: The exposure time for a single layer is 5-8 seconds, while the exposure time for the bottom layer (first 5 layers) is extended to 10-15 seconds to enhance the adhesion between the model and the platform and prevent the model from falling off in the early stages of printing;

[0098] Exposure intensity: Set to 80-100mW / cm 2 Adjust according to the resin manufacturer's recommended values ​​to ensure the resin is fully cured;

[0099] Printing speed: Since DLP technology is a full-surface forming process, the printing speed mainly depends on the layer thickness and exposure time. With a layer thickness of 0.1mm, the printing time for a single orthodontic appliance is about 1-1.5 hours, which is significantly lower than the production cycle of traditional processes.

[0100] Step 2: Printing and Material Selection

[0101] 1. Printing Material Selection: Biocompatible photosensitive resins (such as Graphy's Tera Harz TC-85DAC and Formlabs' Dental SG) are selected. The main component of these resins is polyurethane acrylate, which possesses the following performance characteristics:

[0102] Biocompatibility: Complies with ISO 10993-5 (cytotoxicity) and ISO 10993-10 (dermal sensitization) standards, with a cytotoxicity level ≤1, no sensitization, ensuring long-term wear safety;

[0103] Mechanical properties: The elastic modulus is 1.5-2.5 GPa, and the elongation at break is ≥5%, which can provide sufficient orthopedic force and has good toughness, and can withstand the mechanical impact of repeated wearing and removal.

[0104] Optical performance: Light transmittance ≥90%, meeting the aesthetic requirements of invisible aligners;

[0105] Stability: Under oral environment (temperature 37℃, immersion in saliva), it exhibits good mechanical properties and dimensional stability, with no obvious yellowing or degradation.

[0106] 2. Printing process control:

[0107] ο Resin pretreatment: Before printing, preheat the resin in a 60℃ water bath for 30 minutes to reduce the resin viscosity and ensure that the resin flows evenly during the printing process, avoiding printing defects caused by uneven viscosity.

[0108] ο Environmental control: The printing environment temperature is controlled at 23±2℃ and the humidity is controlled at 40%-60% to avoid the impact of temperature and humidity changes on resin curing speed and printing accuracy;

[0109] Real-time monitoring: The printer's built-in camera monitors the printing process in real time, promptly detecting issues such as model detachment and insufficient resin, ensuring a smooth printing process.

[0110] Step 3: Post-processing

[0111] Post-processing is a crucial step in ensuring that the orthodontic appliance is transformed from a "printed prototype" into a "clinically usable device." It includes four steps: secondary curing, support removal, grinding and polishing, and disinfection. Each step requires strict control of process parameters to ensure product performance and safety.

[0112] 1. Secondary Curing: The resin monomer conversion rate inside the printed orthodontic appliance (referred to as the "green part") is approximately 70%-80%, and its mechanical properties and chemical stability are not optimal, requiring secondary curing treatment.

[0113] Curing equipment: Uses a professional light curing chamber with a wavelength of 405nm (such as Formlabs Form Cure), equipped with a nitrogen protection system;

[0114] Curing parameters: Light intensity set to 50-100mW / cm² 2The curing time is 60-120 seconds; curing is carried out in a nitrogen atmosphere (oxygen concentration ≤1%) to suppress the oxygen inhibition effect and increase the resin monomer conversion rate to over 95%.

[0115] o Functional gradient implementation: For orthodontic appliances requiring a gradient in mechanical properties, this can be achieved by adjusting the local light intensity and time during secondary curing. For example, using 100mW / cm² in the molar region. 2 A curing time of 120 seconds resulted in an elastic modulus of 2.5 GPa (high rigidity) in this area; 50 mW / cm² curing was used in the anterior tooth region. 2 The curing parameters of 60 seconds enable the elastic modulus of the area to reach 1.5GPa (high elasticity), thereby achieving precise control of the targeted corrective force.

[0116] ο Biocompatibility assurance: Sufficient secondary curing can reduce the content of residual monomers (residual monomer content ≤0.1%), reduce the risk of cytotoxicity, and ensure the biocompatibility of the orthodontic appliance.

[0117] 2. Fine-tuning of support removal: The connection points between the support and the appliance body require meticulous handling to avoid damaging the appliance surface or causing deformation.

[0118] ο Tool selection: Use high-precision diagonal pliers and a scraper (blade thickness ≤ 0.1mm), and operate with the assistance of a 10-20x magnifying glass;

[0119] Removal process: First remove the main support rod, then finely trim the support contact points; during the removal process, use a combination of "cutting + scraping" to avoid violent pulling; for any remaining support marks, gently scrape them smooth with a scraper to ensure a smooth transition on the contact point surface without scratches or gaps.

[0120] 3. Grinding and polishing: The surface of the aligner after support removal may have minor marks or burrs, which need to be ground and polished to ensure a smooth surface without sharp edges.

[0121] Rough sanding: Use 400-grit sandpaper to manually sand the support residue and edge areas to remove obvious marks;

[0122] o Fine polishing: Use 800 grit, 1200 grit and 2000 grit sandpaper in sequence for progressive polishing. Rinse with pure water after each polishing to ensure that the surface roughness Ra≤0.2μm;

[0123] Polishing: Use a wool polishing wheel with dental polishing paste (such as silica polishing paste) on a low-speed polishing machine (speed ≤3000rpm) to polish the surface of the orthodontic appliance to achieve a mirror effect, further improve light transmittance, and avoid irritating the soft tissues of the mouth.

[0124] 4. Disinfection: To ensure safe clinical use, the polished orthodontic appliance must be thoroughly disinfected.

[0125] Cleaning: First, rinse the inner and outer surfaces of the orthodontic appliance with purified water to remove polishing residue; then clean it with an ultrasonic cleaner (frequency 40kHz) for 5 minutes to remove tiny impurities.

[0126] Disinfection: Immerse the orthodontic appliance in a neutral, cold disinfectant solution (such as 0.5% chlorhexidine solution) for 30 minutes to sterilize it;

[0127] Drying and sealing: After disinfection, rinse thoroughly with sterile purified water, place in a sterile environment to air dry naturally, then seal the packaging, label with patient information and expiration date, and wait for clinical use.

[0128] 3.2.3 Structural features of bracketless clear aligners

[0129] Based on the above-mentioned digital design and manufacturing methods, the bracketless invisible aligner provided by the present invention has the following structural features:

[0130] 1. Overall structure: The appliance is an integrated shell structure without seams, and is U-shaped to fit the patient's teeth precisely; the shell thickness is uniform, ranging from 0.6-0.8mm, with a tolerance of ≤0.1mm; the edges are smooth and continuous, without sharp edges, and are 0.3-0.5mm away from the cervical line to avoid compressing the gums.

[0131] 2. Material characteristics: Made of biocompatible photosensitive resin (polyurethane acrylate base), it has excellent light transmittance (transmittance ≥90%), mechanical properties (elastic modulus 1.5-2.5GPa, elongation at break ≥5%) and biological safety (cytotoxicity grade ≤1, non-sensitizing).

[0132] 3. Gradient Mechanical Properties: The orthodontic appliance can be designed with a gradient distribution of mechanical properties in different areas according to clinical needs. For example, the elastic modulus in the molar area is 2.0-2.5 GPa (high rigidity, providing stable anchorage), the elastic modulus in the anterior tooth area is 1.5-1.8 GPa (high elasticity, enabling controllable movement), and the elastic modulus in the area near the gingival margin is 1.8-2.0 GPa (medium rigidity, avoiding pressure on the gingiva). This gradient design allows for precise and targeted delivery of orthodontic forces.

[0133] 4. Dimensional accuracy: The average dimensional deviation between the appliance and the target digital model is ≤50μm, and the positioning accuracy is ≥98%, which can accurately fit the tooth crown surface and ensure the effective transmission of orthodontic force.

[0134] 3.3 Beneficial Effects

[0135] This invention, by constructing a complete digital design and manufacturing system, enables the large-scale, high-precision application of direct 3D printing technology in the field of invisible orthodontics. Compared with existing technologies, it has the following significant advantages:

[0136] 3.3.1 Significantly improved morphological precision and higher positioning accuracy

[0137] This invention completely eliminates the multi-step process of traditional indirect molding methods. Through an integrated "digital design-direct printing" model, it fundamentally eliminates the cumulative errors introduced by master mold printing, thermoforming, and manual trimming. Through the synergistic effect of high-precision intraoral scanning (accuracy ≥ 20 μm), intelligent segmentation algorithms (cervical line boundary error ≤ 0.1 mm), thickness control algorithms (uniformity ≤ 0.1 mm), and DLP printing technology (accuracy ± 20 μm), the average deviation between the final aligner and the target digital model is ≤ 50 μm, significantly lower than the 150-250 μm of traditional aligners, improving placement accuracy to over 98%. This high precision ensures that the aligner fits precisely to the teeth, and the corrective force is accurately transmitted to the target teeth, effectively improving the accuracy of pre-programmed tooth movement and providing reliable assurance for complex tooth movements (such as rotation and root control).

[0138] 3.3.2 Programmable mechanical properties enable better personalized treatment results

[0139] This invention overcomes the limitations of traditional homogeneous materials, enabling the regionalized and gradient design of the mechanical properties of orthodontic appliances. By precisely controlling printing parameters (exposure energy, layer thickness) and secondary curing parameters (light intensity, time, atmosphere), an elastic modulus gradient from 1.5 GPa to 2.5 GPa can be constructed on a single appliance to meet the differentiated mechanical needs of different teeth and different movement types. For example, for molar areas requiring strong anchorage, increasing the exposure energy and curing time enhances the rigidity of the material in this area, effectively resisting reaction forces; for anterior teeth areas requiring flexible movement, decreasing the exposure energy and curing time improves the elasticity of the material, achieving gentle and controllable tooth movement. This "functionally adapted" design concept not only improves the treatment effect of complex cases but also reduces periodontal damage caused by improper force application, expanding the application scope of invisible orthodontic technology.

[0140] 3.3.3 The production process is greatly simplified, resulting in outstanding efficiency and environmental friendliness.

[0141] The digital closed-loop process of this invention significantly shortens the production cycle, requiring only 24-48 hours from receiving patient data to delivering the finished product, compared to 7-14 days for traditional processes. This increases production efficiency by over 70%, meeting the treatment needs of urgent clinical cases and improving the patient experience. Regarding material utilization, direct printing technology achieves a material utilization rate of over 90%, significantly reducing material waste compared to the 30%-60% of traditional processes. In terms of energy consumption, DLP printing technology consumes only one-third the energy of thermoforming processes and eliminates the need to dispose of waste resin molds, reducing environmental pollution and aligning with the trend of green manufacturing.

[0142] 3.3.4 The design is highly intelligent and has strong clinical applicability.

[0143] This invention integrates cutting-edge technologies such as deep learning and computer graphics to construct an intelligent design system: the PointNet++-based intelligent segmentation algorithm can adapt to various complex clinical scenarios (such as crowded teeth and abnormal gingival morphology), achieving a segmentation accuracy of 98.5% and reducing manual intervention; the shrinkage wrapping algorithm and thickness control algorithm ensure the accuracy of the appliance's shape and the stability of its mechanical properties, eliminating the need for technicians to perform complex manual adjustments. Simultaneously, the appliance's edge design (smooth treatment + outward equidistant spacing) effectively avoids gingival compression, surface polishing reduces irritation to oral soft tissues, and biocompatible resin and strict sterilization procedures ensure clinical safety, significantly improving patient comfort and treatment compliance.

[0144] 3.3.5 Long-term performance is stable and clinical efficacy is reliable.

[0145] This invention ensures the long-term performance stability of the orthodontic appliance by optimizing material selection and post-processing. The biocompatible photosensitive resin exhibits excellent aging resistance in the oral environment. After a 12-month saliva immersion test, the material's elastic modulus change rate is ≤5%, and its light transmittance change rate is ≤3%, with no significant yellowing or degradation. Regarding mechanical durability, the appliance maintains stable orthodontic force after 1000 repeated insertion and removal tests, without cracking or deformation. Clinical trial results show that patients treated with this invention experience an average 15%-20% shorter treatment period, a tooth movement accuracy rate exceeding 85%, and a 60% reduction in gingival inflammation incidence, demonstrating significantly better treatment outcomes than traditional orthodontic appliances. Attached Figure Description

[0146] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0147] Figure 1 The diagram shows the digital tooth arrangement sequence; (a) is the initial tooth row model, and (b)-(e) are the models of the 5th, 10th, 15th and 20th steps of the tooth arrangement sequence, showing the gradual movement of the teeth from the initial position to the final target position.

[0148] Figure 2 The following are comparison diagrams of the model envelope process; (a) is the original dental model before envelope (triangular mesh structure, surface includes crown and gingival details), (b) is the intermediate state during envelope (the mesh gradually fits the crown shape), and (c) is the NURBS surface model after envelope (smooth, continuous, watertight, containing only the appliance envelope space).

[0149] Figure 3 The images show the crown-gingiva segmentation results based on PointNet++; where (a) is the initial dentition model, and (b)-(e) are the models at steps 5, 10, 15, and 20 of the tooth arrangement sequence, respectively. Figure 1 Based on the results of intelligent boundary recognition;

[0150] Figure 4 Diagram showing the process of orthodontic appliance generation and edge optimization;

[0151] Figure 5 Design drawings for the placement and support of 3D printed models;

[0152] Figure 6 This is a schematic diagram of the secondary curing process in the post-processing workflow;

[0153] Figure 7 This is a picture of a finished invisible aligner. Detailed Implementation

[0154] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0155] Example 1: Design and manufacture of invisible aligners for cases of crowded teeth

[0156] 4.1 Case Information

[0157] Patient: Female, 22 years old, diagnosed with Angle Class I malocclusion, with 8mm crowding in the maxillary dentition and 6mm crowding in the mandibular dentition. She has no periodontal problems such as gingival hyperplasia or recession and requests invisible orthodontic treatment.

[0158] 4.2 Digital Design Process

[0159] 1. Data Acquisition and Goal Setting:

[0160] The patient's oral cavity was scanned using a 3Shape intraoral scanner to obtain three-dimensional point cloud data of the maxillary and mandibular dentition and occlusal relationship. The data was then processed to generate a triangular mesh model in STL format with a mesh density of 80 triangular faces per square millimeter. Data accuracy verification showed that the average deviation between the model and the actual dentition was 18μm.

[0161] Using self-developed tooth alignment software, doctors formulate treatment plans based on biomechanical principles: the maxillary dentition is moved in 25 steps, the mandibular dentition in 20 steps, with a maximum single-step movement of 0.2mm, generating a target dentition model sequence containing 45 steps, and selecting the 10th step target model for this orthodontic appliance design.

[0162] 2. Model envelope construction:

[0163] Import the target dental model from step 10 into Rhino 8 software, enable the ShrinkWrap function, set the shrinkage coefficient to 0.52, and the number of iterations to 60.

[0164] The algorithm automatically generates an initial wrapping mesh (sphere with 1500 vertices). Through iterative balancing of gravity and constraint forces, a smooth NURBS surface model is finally generated. The surface roughness of this model is Ra = 0.6 μm, with no self-intersections or wrinkles, and it accurately fits the shape of the tooth crown.

[0165] 3. Intelligent segmentation defines the force boundary:

[0166] The target dental model was converted into point cloud data (point cloud density of 25 points per square millimeter), outliers were removed, and then the data was imported into an AI platform equipped with an optimized PointNet++ network.

[0167] The network automatically performed crown-gingival segmentation in 8 seconds. The segmentation results showed that the average error of the cervical line boundary was 0.08 mm, and the clean crown model had no gingival residue, meeting the mechanical boundary definition requirements.

[0168] 4. Orthodontic appliance generation and edge optimization:

[0169] The pure tooth crown model is offset by 0.7mm at an equal distance to generate the basic body of the orthodontic appliance;

[0170] Thickness was measured using the X-ray projection method. The thickness of the fissure region of the maxillary second premolar was 0.58 mm (weak area), and the thickness of the proximal region of the mandibular first molar was 0.82 mm (overly thick area). After three iterations of offset and cutting correction, the thickness of the entire region was stabilized at 0.7 ± 0.1 mm.

[0171] The initial edge line was smoothed using a 3rd-order B-spline curve, and the curvature change rate of the fitted edge line was 0.4 mm. - 1. No jagged undulations; shift the edge outward by 0.25mm along the normal vector of the crown to ensure that the edge is 0.4mm away from the cervical line and avoid compressing the gums;

[0172] The optimized digital model of the orthodontic appliance (STL format) was exported. Model validation showed that the fit accuracy with the target crown was 42μm, which meets clinical requirements.

[0173] 4.3 Direct 3D Printing and Post-processing

[0174] 1. Printing preparation:

[0175] Import the digital model of the orthodontic appliance into ShapeWare slicing software, set the placement angle to 78° between the occlusal plane and the platform, and the distance between the lowest point of the model and the platform to 4mm;

[0176] The outer surface is designed with a dotted support structure: the support contact point diameter is 0.25 mm, the support rod diameter is 0.6 mm, the support spacing is 3 mm, and the support density is increased (spacing 2 mm) around the lingual free edge and cusp, with a total of 72 supports designed;

[0177] Printing parameters: layer thickness 0.1mm, bottom layer (first 5 layers) exposure time 12s, remaining layers exposure time 6.4s, exposure intensity 90mW / cm². 2 .

[0178] 2. Print execution:

[0179] ο Graphy Tera Harz TC-85DAC biocompatible photosensitive resin was selected. Before printing, the resin was preheated in a 60℃ water bath for 30 minutes to reduce the viscosity to 250mPa·s.

[0180] The print was performed using a SHAPE 1+DLP printer at an ambient temperature of 23°C and a humidity of 50%. The print time was 1 hour and 12 minutes. After printing, the "green part" was removed and found to have no obvious defects in appearance.

[0181] 3. Post-processing:

[0182] Secondary curing: The "green component" was placed in a 405nm light curing chamber under a nitrogen atmosphere (oxygen concentration 0.8%) and a light intensity of 90mW / cm². 2 The curing time was 90 seconds, and the resin monomer conversion rate after curing was 96.2%.

[0183] Support removal: Using high-precision diagonal pliers and a scraper, with the assistance of a 15x magnifying glass, first remove the main support rod, then finely trim the support contact points to ensure a smooth transition at the contact points;

[0184] ο Grinding and polishing: Grind with 400 grit, 800 grit, 1200 grit and 2000 grit sandpaper in sequence, and then polish with a wool polishing wheel and silica polishing paste. The final surface roughness Ra = 0.15μm and light transmittance 92%.

[0185] Disinfection: First, clean with an ultrasonic cleaner for 5 minutes, then soak in 0.5% chlorhexidine solution for 30 minutes for disinfection. Rinse with sterile purified water and air dry naturally before sealing and packaging.

[0186] 4.4 Finished Product Testing and Clinical Application

[0187] 1. Finished product inspection:

[0188] ο Dimensional accuracy inspection: The finished orthodontic appliance was scanned using a 3D laser scanner and compared with the digital model. The average dimensional deviation was 45μm, which meets the design requirement of ≤50μm.

[0189] Mechanical performance testing: Using a universal testing machine, the elastic modulus of the molar region was 2.3 GPa, the elastic modulus of the anterior region was 1.6 GPa, and the elastic modulus of the region near the gingival margin was 1.9 GPa, achieving the expected mechanical gradient distribution;

[0190] Biocompatibility testing: Cytotoxicity testing was conducted according to ISO 10993-5 standard, and the cell viability rate was 95.8%, meeting the safety requirements of ≤1 level.

[0191] ο Surface quality inspection: The surface is smooth with no sharp edges, scratches or gaps, and the edges are rounded to meet the requirements for wearing comfort.

[0192] 2. Clinical applications:

[0193] After the patient put on the orthodontic appliance, it was put in place smoothly without any obvious discomfort, and there was no redness or swelling of the gums due to pressure.

[0194] After wearing the appliance for 14 days, the patient returned for a follow-up visit. The oral examination showed that the appliance was not deformed or cracked, the tooth movement was in line with the pre-programmed design, and the patient's treatment compliance was good.

[0195] Follow-up results showed that the appliance maintained stable mechanical properties and morphological accuracy after 3 months of wear, with a tooth movement accuracy rate of 88%, demonstrating significant treatment effectiveness.

Claims

1. A digital design and manufacturing method for directly printing invisible aligners, characterized in that, Includes the following steps: [1] Three-dimensional point cloud data of the patient’s dentition, gingival mucosa and occlusal relationship were obtained by high-precision intraoral scanning. Based on biomechanical principles, digital tooth arrangement design was carried out, the target position and path of each tooth movement were set, and a progressive target dentition digital model sequence was generated. [2] The ShrinkWrap algorithm is applied to the target dental digital model to achieve a balance between attraction and constraint through iterative calculation, thereby generating a continuous, smooth, and watertight NURBS surface morphology orthodontic appliance initial envelope model. [3] Using the PointNet++ three-dimensional point cloud segmentation network based on deep learning, the target dental digital model is intelligently segmented from the crown to the gingiva through hierarchical sampling grouping, local feature extraction and global information aggregation, accurately identifying the cervical line boundary, defining the range of orthodontic force, and obtaining a pure crown digital model; [4] Based on the pure crown digital model, the basic body of the orthodontic appliance is generated by equidistant offset along its surface normal vector; the thickness of the shell is detected by the ray projection method, and the weak area is offset outward to compensate and the excessively thick area is offset inward to reduce, so as to ensure the uniformity of thickness; then the edge smoothing and normal outward translation optimization are performed by the curve fitting algorithm to obtain the orthodontic appliance digital model that can be directly used for 3D printing. [5] Using a 3D printing device based on VAT polymerization technology (preferably digital light processing DLP), and selecting a polyurethane acrylate photosensitive resin that meets biocompatibility standards, the physical orthodontic device is directly printed according to the digital model of the orthodontic device. [6] Post-processing of the printed orthodontic appliance: secondary curing in a UV curing chamber with controllable parameters, fine removal of the support structure using special tools, multi-stage grinding and polishing until the surface is free of sharp edges, and finally disinfection by soaking in cold disinfectant and sterile drying.

2. The method according to claim 1, characterized in that, The shrinkage wrapping algorithm iterates 50-80 times, and the shrinkage coefficient is set to 0.45-0.

55. The gravity is used to drive the vertices of the wrapping mesh to move closer to the feature points on the crown surface, and the constraint force is used to maintain the continuity and smoothness of the wrapping mesh. The surface roughness Ra of the final generated NURBS surface model is ≤0.8μm.

3. The method according to claim 1, characterized in that, The PointNet++ 3D point cloud segmentation network has a point cloud sampling density of 20-30 points per square millimeter. Key points are selected using the FPS algorithm and a local region with a radius of 0.3 mm is constructed. Local and global features are aggregated using symmetric functions (including max pooling). The segmentation error of the tooth neck line boundary is ≤0.1 mm, and the segmentation accuracy is ≥98.5%.

4. The method according to claim 1, characterized in that, The target thickness for thickness uniformity control is 0.6-0.8 mm, with a tolerance within ±0.1 mm. Thickness is measured over the entire area using a ray projection method with a density of 5 rays per square millimeter, and thickness uniformity is achieved through 2-3 iterations.

5. The method according to claim 1, characterized in that, The edge geometry optimization includes: smoothing the initial edges using a 3rd-order B-spline or NURBS curve fitting algorithm to ensure that the rate of change of edge line curvature is ≤0.5mm. -1 ; Move the margin control point outward 0.2-0.3 mm along the normal direction of the crown surface to ensure that the margin is 0.3-0.5 mm away from the cervical line.

6. The method according to claim 1, characterized in that, The parameter settings during the printing preparation stage include: the angle between the occlusal plane of the orthodontic appliance and the printing platform is 75°-80°, the distance between the lowest point of the model and the platform is 3-5mm; the support structure is designed in the form of point contact on the outer surface of the orthodontic appliance, the contact point diameter is 0.2-0.3mm, the support rod diameter is 0.5-0.8mm, and the support spacing in the stress concentration area is 2-3mm.

7. The method according to claim 1, characterized in that, The secondary curing process uses 405nm wavelength ultraviolet light with a light intensity of 50-100mW / cm². 2 The curing time is 60-120 seconds, and the curing atmosphere is an inert gas such as nitrogen (oxygen concentration ≤1%). By adjusting the curing parameters, different areas of the orthodontic appliance can form a gradient elastic modulus in the range of 1.5-2.5 GPa.

8. A bracketless invisible aligner manufactured by the method according to any one of claims 1 to 7, characterized in that, The orthodontic appliance is a seamless, integrated shell structure made of biocompatible photosensitive resin conforming to ISO 10993-5 standard through direct 3D printing; the shell thickness uniformity tolerance is ≤0.1mm, the light transmittance is ≥90%, the mechanical properties of different areas are gradient-distributed (elastic modulus 1.5-2.5GPa), and the dimensional deviation from the target dental digital model is ≤50μm.

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