How accurate are 3D dental models?

I’m looking into getting some 3D models of my teeth made, probably from an intraoral scan instead of traditional impressions. I’m considering using them to explore options like Invisalign, but I’ve heard mixed things about the accuracy of these models compared to the real thing.

Specifically, I’m wondering:

• How much do these models typically deviate from the actual size and shape of my teeth? Is it a consistent error, or is it more random?
• What are the main factors that influence the accuracy of the 3D model? Is it the type of scanner used, the skill of the technician taking the scan, the material used for printing (if it’s a physical model), or something else?
• Are there specific areas of the mouth (e.g., the back molars, areas with tight spaces between teeth) where the scans tend to be less accurate?
• How does the accuracy of an intraoral scan-based model compare to a model made from traditional impressions? Are digital models always better, or are there situations where traditional methods are more accurate?
• I’m concerned about using these models for something like Invisalign, where precise tooth movement is crucial. Are the models accurate enough for that purpose? What kind of accuracy is considered acceptable for orthodontic treatment planning?

Answer

The accuracy of 3D dental models is a multifaceted topic, dependent on the technology used for their creation, the specific application of the model, and the criteria used to define "accuracy." In general, 3D dental models generated with modern techniques can be highly accurate, but limitations and potential sources of error must be considered.

Methods of 3D Dental Model Creation and their Accuracy:

1. Intraoral Scanners:

   • How they work: Intraoral scanners (IOS) use optical or laser technology to directly capture the geometry of the teeth and surrounding oral tissues. They project light onto the surfaces, and sensors analyze the reflected light to create a digital 3D representation.
   • Accuracy: IOS have significantly improved in recent years. High-end IOS can achieve accuracy levels within 5-20 micrometers (µm) for single-tooth scans under ideal conditions. Full-arch scans are generally less accurate than single-tooth scans because of the cumulative effect of errors over a longer scan path, as well as distortions created by soft tissues and movements of the patient. Full-arch accuracy can range from 20-100 µm depending on the scanner, scanning technique, and the complexity of the dental arch. Studies comparing different IOS models have shown variation, with some scanners consistently outperforming others in terms of accuracy. Accuracy is generally evaluated using metrics like trueness and precision. Trueness refers to the closeness of a measurement to the actual value (i.e., how close the scan is to the real dentition). Precision refers to the repeatability of a measurement (i.e., how consistently the scanner produces the same result when scanning the same object multiple times).
   • Factors affecting accuracy:
     • Scanner Technology: Different IOS technologies (e.g., confocal microscopy, structured light) have varying levels of inherent accuracy.
     • Scanning Protocol: The scanning technique employed by the operator significantly impacts accuracy. Smooth, continuous movements and proper angulation are essential.
     • Soft Tissue Management: Saliva, blood, and soft tissue interference can negatively affect scan quality. Retraction and drying techniques are important.
     • Material Properties: Highly reflective surfaces or areas with deep undercuts can be challenging for some scanners.
     • Calibration: Regular calibration of the scanner is crucial for maintaining accuracy.
     • Environmental Conditions: Ambient lighting and temperature can influence scan accuracy.

2. Laboratory Scanners:

   • How they work: Laboratory scanners digitize physical dental models (e.g., stone casts) that are created from conventional impressions. The physical model is placed in the scanner, and optical or laser technology is used to capture its 3D geometry.
   • Accuracy: Laboratory scanners generally offer higher accuracy than intraoral scanners, especially for full-arch scans. They are typically used to digitize impressions taken with highly accurate impression materials. Accuracy levels can reach 5-15 µm, depending on the scanner and the quality of the original physical model.
   • Factors affecting accuracy:
     • Scanner Technology: The same principle as with intraoral scanners applies; different scanner technologies have different levels of accuracy.
     • Quality of the Physical Model: The accuracy of the digital model is directly dependent on the accuracy of the physical model. Any distortions or imperfections in the physical model will be transferred to the digital model.
     • Material Properties: The material of the physical model (e.g., type of stone) can influence scan accuracy.
     • Calibration: Regular calibration of the scanner is crucial.

3. Cone-Beam Computed Tomography (CBCT):

   • How they work: CBCT uses X-rays to create a 3D image of the teeth and surrounding structures. While primarily used for bone and anatomical structure visualization, it can also be used to generate 3D dental models.
   • Accuracy: CBCT is less accurate than IOS or laboratory scanners for creating precise 3D dental models. Its primary use is for assessing bone structures, implant planning, and diagnosing pathology. The resolution of CBCT scans is generally lower, and the presence of metal restorations can create artifacts that reduce accuracy in the dental region. Accuracy levels for CBCT-derived dental models are typically in the range of 100-300 µm or higher.
   • Factors affecting accuracy:
     • Radiation Dose: Lower radiation doses often result in lower image resolution and accuracy.
     • Metal Artifacts: Metal restorations (e.g., crowns, fillings) can cause scattering of X-rays, creating artifacts that distort the image.
     • Patient Movement: Movement during the scan can result in blurring and reduced accuracy.
     • Reconstruction Algorithms: The algorithms used to reconstruct the 3D image from the raw data can influence accuracy.

Applications of 3D Dental Models and Accuracy Requirements:

The required accuracy of a 3D dental model depends on its intended use:

• Orthodontics: Digital models are used for diagnosis, treatment planning, and monitoring treatment progress. High accuracy is important for precise bracket placement and aligner fabrication. Accuracy requirements are typically in the range of 50-100 µm.
• Restorative Dentistry: Digital models are used for designing and fabricating crowns, bridges, inlays, onlays, and veneers. High accuracy is essential for achieving a good fit and marginal adaptation. Accuracy requirements are typically in the range of 20-50 µm.
• Implant Dentistry: Digital models are used for implant planning, surgical guide fabrication, and designing implant-supported restorations. Accuracy is crucial for proper implant placement and restoration fit. Accuracy requirements are typically in the range of 50-100 µm.
• Prosthodontics: Digital models are used for designing and fabricating removable dentures and partial dentures. Accuracy is important for proper fit, retention, and stability. Accuracy requirements are typically in the range of 100-200 µm.
• Oral and Maxillofacial Surgery: Digital models are used for surgical planning, simulating surgical procedures, and creating custom surgical guides. Accuracy requirements vary depending on the complexity of the surgery.

Factors affecting the overall accuracy of the final dental restoration or appliance:

It’s important to consider that the accuracy of the 3D dental model is only one factor that affects the final outcome. Other factors include:

• Design Software: The accuracy of the software used to design the restoration or appliance.
• Manufacturing Process: The accuracy of the manufacturing process (e.g., milling, 3D printing).
• Materials: The properties of the materials used to fabricate the restoration or appliance.
• Clinical Procedures: The accuracy of the clinical procedures used to fit and adjust the restoration or appliance.

Conclusion:

3D dental models created using modern techniques, particularly with intraoral and laboratory scanners, can achieve high levels of accuracy. The specific accuracy depends on the technology used, the scanning protocol, the quality of the original impression (if applicable), and the intended application of the model. While CBCT can be used to generate 3D dental models, it is generally less accurate than IOS or laboratory scanners for this purpose. Understanding the limitations and potential sources of error is essential for ensuring the clinical success of digitally fabricated dental restorations and appliances. Advancements in scanning technology, software, and manufacturing processes continue to improve the accuracy and reliability of 3D dental models.