Quartz 4

F4 Digital Impressions

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Lecture 7: Digital Impressions1

dental impression: a negative imprint or a positive digital image display of intraoral anatomy; used to cast or print a 3D replica of the anatomic structure that is to be used as a permanent record or in the production of a dental restoration or prosthesis; syn IMPRESSION

Clarification

A negative imprint refers to a conventional impression using materials like PVS, which must be poured with stone to create a positive cast. A positive digital image display is what an intra-oral scanner creates directly, resulting in a 3D model without the need for an intermediate pouring step.

The aim of an impression2

  • To produce a dimensionally stable “negative”
    • Mould for an analogue model
    • Scanned with CAD/CAM model
      • Milled Model
      • 3D Printed Model

Used not just for crowns. Have been increasingly used for other specialties. Including edentulous arches and orthodontic treatment.

This record is then used to fabricate various dental prostheses, including:

  • Single indirect restorations (crowns, inlays, onlays).
  • Fixed partial dentures (FPDs) or bridges.
  • Removable partial dentures (RPDs).
  • Complete dentures (for edentulous arches).
  • Orthodontic aligners.

Benefits Vs Disadvantages of Intra-oral Scanners34

Advantages

  • Workflow improvements: The created digital impression is already a positive record, unlike a conventional impression that needs to be poured up. Tray selections, disinfection, etc., also require time in conventional workflows.
    • The lengthy disinfection process required for physical impressions can take approximately 15 minutes.
  • Patient acceptance (gag reflex): Generally better tolerated by patients.
    • Allows for pauses during the scanning process without compromising the final result, unlike a setting silicone impression which would require a complete restart.
  • Time benefits: Some manufacturers claim a full arch impression can be achieved in less than 60 seconds with experience.
  • Financial advantages: Despite initial costs, long-term savings can be realized.
    • While the initial investment is high (e.g., a Trios 5 costs ~$30,000 plus subscription fees), long-term savings are possible by eliminating costs for impression materials, trays, and physical shipping to the lab.
  • Communication: Facilitates easier collaboration with labs and other professionals.
    • Record keeping/storage: Many new scanners allow previous scans to be stored and compared against new scans. This is potentially useful for patient education and monitoring changes, for example, wear or erosion of teeth.
    • Digital scans can be stored securely in the cloud, eliminating the need to store bulky physical casts for the legally required period (e.g., 5–7 years).
    • This allows for quantitative monitoring of dental conditions like bruxism or erosion by superimposing scans taken years apart to precisely measure changes.

Disadvantages

  • New skills: Requires learning new techniques and workflows.
  • Digital confidence/mindset: Requires a shift in clinical approach.
  • Initial costs: Significant upfront investment (e.g., a one-time payment of $29,050).
    • The price of the scanner, software subscriptions, and required high-performance laptops represents a significant financial barrier.
    • Some experienced clinicians may be resistant to changing their established, reliable conventional workflows.
  • Accuracy: While generally high, it can be a concern, especially in specific clinical situations.

Detailed comparison conventional vs. intra-oral impressions 5

Record Keeping6

Intra-oral scanners offer powerful tools for longitudinal patient monitoring.

  • Software (e.g., from Trios) allows for the comparison of scans taken at different appointments (e.g., years apart).
  • ==This enables qualitative visual comparison and, more importantly, quantitative analysis.==
  • The software can generate a color map highlighting areas of change:
    • ==Green: No significant change.==
    • ==Red/Blue: Indicates areas of tooth wear (attrition, erosion) or other changes, allowing for precise tracking over time.==

Time Efficiency of Intraoral Scanners7

Study The time efficiency of intraoral scanners An in vitro comparative study

Key Finding

A major portion of the time saved with digital workflows comes from skipping the ~15-minute disinfection step required for conventional impressions.

Digital Workflow Details

The digital workflow is more direct: Scan → Digital file (STL) → Instant email/portal transfer → Lab designs crown → Milling/Fabrication. This eliminates multiple intermediate steps and reduces opportunities for introduced error (e.g., from pouring a stone model).

Extra-oral vs. Intra-oral Scanners8

Definitions

  • Extra-oral scanners: Scans a conventional impression or a physical cast.
  • Intra-oral scanners: Performs a direct scan of the oral structures.

Extra-oral Scanner Technology9

  • Structured light scanners project a grid or other parallel scan pattern on the scan object. The distortions of the scan patterns are captured by the camera and processed by the software.

Comparative Accuracy10

  • Key Finding: The literature is mixed, but some studies show intra-oral scans can be more accurate.
    • One study found definitive restorations from intraoral digital impressions had significantly smaller marginal openings (86.09 μm to 88.95 μm) compared to those from laboratory scanning (143.29 μm).
    • Other studies found no difference.
  • Consideration: Lab-based scanners scan a cast, which introduces potential compounding errors from the impression and pouring stages. However, extra-oral scanners still allow for digital record keeping and CAD/CAM workflows from physical models.

Compounding Errors

A potential reason for this difference is that extra-oral scanning of a cast can compound errors. Any small inaccuracy in the original PVS impression is carried over and potentially added to by another layer of inaccuracy from the scanning process itself. Intra-oral scanners capture the source anatomy directly, avoiding this cumulative error.

Intra-oral Scanner Technologies11

Scan Pattern121314

  • Trios scan patterns all achieve similar results.

The protocol used to scan the arch can impact accuracy.

  • Studies have shown that for some scanners, deviating from the manufacturer's recommended scan pattern can lead to a significant increase in error.
  • ==For the Trios scanner, studies found that different scan patterns did not have a significant effect on accuracy.==

Best Practice

It is always recommended to follow the manufacturer’s specified scan pattern to ensure optimal results. The recommended pattern for Trios is typically: occlusal surfaces → zigzag across the anteriors → opposite occlusal surface → lingual/palatal surfaces → buccal surfaces.

Systematic Review Digital vs. Conventional Impressions15

Stitching Error

The primary limitation of current digital technology with full-arch scans is the accumulation of small inaccuracies over the long span, a phenomenon known as ‘stitching error’.

Key Findings

  • Efficiency: Digital impressions (248.48 ± 23.48 s) are significantly faster than conventional impressions (605.38 ± 23.66 s).
  • Accuracy: When completing a full arch impression, conventional impressions are more accurate.

Further In Vitro Study on Full-Arch Accuracy16

Other studies have reached similar conclusions.

Conclusion and Key Takeaways17

  • Digital impressions offer significant advantages in workflow efficiency, patient comfort, and record-keeping.
  • ==Accuracy, a combination of trueness and precision, is the most critical factor for clinical success.==
  • ==While excellent for single units and short-span restorations, intra-oral scanners currently show limitations in full-arch accuracy due to the accumulation of stitching errors.==
  • ==Following the manufacturer’s recommended scan pattern is crucial for achieving the best possible results.==

Footnotes

  1. Original PDF page 1: F4 DigitalImpressions, p.1 ↩

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  16. Original PDF page 38: F4 DigitalImpressions, p.38 ↩

  17. Original PDF page 39: F4 DigitalImpressions, p.39 ↩

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