Prep Tips for Digital Impression & Guided Preps

Digital impression is integral to modern dentistry, offering efficiency and precision in dental restorations. In this blog post, we explore essential preparation criteria and the use of prep guides, featuring insights from Dr. Mohamed El-Ashry. Learn how to optimize your digital impressions and utilize prep guides effectively with Medit apps.


Importance of Preparation Criteria for Digital Impression

Dr. Mohamed emphasizes the need to modify traditional preparation techniques to enhance the accuracy of digital impressions. Unlike conventional impressions, digital impressions rely on light rather than physical material to capture intraoral details. Therefore, minimizing visual interferences is crucial for obtaining accurate scans. Visual interferences, such as shiny surfaces or undercuts, can hinder the scanner’s ability to capture precise details.


Key Concerns for Achieving Precision

Achieving precision in digital impressions involves addressing several key concerns: Preparation criteria, Tissue Management, Ideal Data, Models, and Occlusion. Each of these plays a crucial role in ensuring the precision of the final restoration. Proper preparation and reduction techniques are vital for achieving the desired biological, mechanical, and aesthetic outcomes.


Digital Workflow for Dental Restorations

Understanding the digital workflow is essential for achieving high accuracy in restorations. The workflow involves three main phases: generating a 3D replica of the clinical situation, computer-aided designing, and computer-aided manufacturing. Each phase introduces potential errors, which can accumulate and affect the final restoration.

  • Intraoral Scanner Trueness Errors: 50-150 microns
  • Data Acquisition Errors: 9.7 microns
  • CAM Errors: 5-25 microns

These errors can accumulate, so minimizing them at each stage is essential for a successful outcome.


Preparation Criteria for Digital Impression

Digital dentistry offers many advantages but also introduces potential errors and inaccuracies. To achieve the best results, we must understand and mitigate these issues.


Total Occlusal Convergence (TOC)

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TOC refers to the angle between opposing walls of a tooth preparation. Ideally, this angle should be between 10 and 22 degrees to avoid undercuts and ensure proper retention. Proper TOC enhances the accuracy of the scan by allowing all surfaces to be captured effectively.


Prep Height

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The active preparation height, from the finish line to the occlusal surface, should be 3-4 millimeters. Adequate height is crucial for both the scanner to read the preparation and for maintaining resistance and retention.


Rounded Line Angles

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Rounding line angles for both axial walls and occlusal surfaces is critical. Sharp angles can cause inaccuracies during the milling process, as most milling burs cannot replicate sharp edges precisely.


Interproximal Spaces

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Adequate interproximal spaces prevent visual interferences and allow the scanner to capture all necessary details. This also ensures the milling machine can accurately produce the restoration without over contouring.


Finish Line Geometry and Position

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The finish line should be clearly defined and rounded to avoid errors during milling. The depth of the finish line also plays a role; deeper finish lines require effective tissue management to prevent visual interferences from gingival tissue.


Substrate Material

The material used for core build-ups, such as composites or amalgam, can affect scan accuracy. Dr. Mohamed recommends using darker, more opaque composites to reduce light reflection and noise.


Surface Roughness

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While smooth surfaces facilitate scanning, a slight micro-roughness is necessary for optimal bonding. Dr. Mohamed’s protocol involves using a series of burs to achieve the ideal surface texture.


Guided Preparations with Medit Apps

The second part of the webinar focuses on guided preparations. Guided preps help control the depth and shape of the reduction, ensuring the final restoration matches the planned design. Various types of prep guides can be used, from traditional putty indices to digitally printed guides.

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  • Putty Index: A traditional method to control the depth of preparation.
  • Depth Cutters: Depth cutters are used to control the depth of veneers and other restorations, acting as a guide for the reduction.
  • Mock-up Guide: Intraorally bonded composite restorations cut through a mock-up for ideal restoration.
  • Digitally Printed Guide: Advanced guides created using design software and 3D printing. This guide ensures the preparation matches the planned restoration precisely.

Dr. Mohamed shares several clinical cases to illustrate the use of prep guides and digital workflows. In one example, he demonstrates how a digitally printed guide helped achieve precise reductions for a veneer restoration. By using Medit Splint and other Medit apps, he was able to control the reduction and ensure a perfect fit for the final restoration.



The integration of digital impressions and guided preps with digital tools like Medit apps significantly enhances the accuracy and efficiency of dental restorations. By understanding and applying proper preparation criteria, dental professionals can enhance their practice and improve patient outcomes.


For more detailed workflows and demonstrations, check out the full webinar on our YouTube channel.

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