Introduction to CAD/CAM: Scanning and Comparing

Introduction to Digital Assessment Tools in Dental Education

Objective vs. Subjective Assessment in Preclinical Training

• Preclinical dental education traditionally relies on visual inspection and subjective faculty evaluation using standardized rubrics.
• Traditional rubrics evaluate parameters such as facial contour, lingual contour, incisal contour, proximal contacts, embrasures, margin integrity, finish/polish, and occlusion.
• Numerical scoring formulas on manual rubrics compute total percentage grades through point structures:

$\text{Total Grade } \% = \left( \frac{\text{Total Criteria Points}}{98} \right) \times 100$

• Visual inspection exhibits limitations in precision and inter-evaluator reliability, which can lead to grading discrepancies among faculty members and inconsistent student feedback.

Beginner Over-Confidence and Self-Assessment Dynamics

• Novice dental students routinely exhibit over-confidence when evaluating their own technical performance during early practical examinations (such as waxing tooth #8).
• Early-stage students consistently grade their preclinical work significantly higher than evaluations performed by experienced faculty or automated digital assessment tools.
• Timely faculty interaction paired with objective 3D visual feedback tools enables students to recalibrate their self-assessment accuracy rapidly.

Empirical Evidence: The Chiang et al. (2021) Study

• A study evaluated CAD/CAM technology as a self-assessment tool for preclinical waxing compared to traditional student self-assessment (Eur J Dent Educ, 2021; PMID: 33448597).
• Methodology:
  • Sample size: $37$ preclinical dental students.
  • Task: Complete a wax-up of the maxillary left central incisor (tooth #9 / #8 anatomical representation) recreating original tooth anatomy.
  • Initial evaluation: Traditional student self-assessment rubric completed.
  • Digital scanning: Original unreduced cast and student waxings scanned with an intraoral scanner (E4D, Planmeca).
  • Software analysis: Digital waxings superimposed over original models using Planmeca Compare software with tolerance set to $250\,\mu\text{m}$.
  • Secondary evaluation: Students completed a post-software self-assessment and an exit survey.
• Quantitative Results:
  • $24\%$ of student self-assessment responses changed after reviewing 3D digital comparisons.
  • $20\%$ of evaluations downgraded from satisfactory to unsatisfactory upon visual color-map feedback.
  • $4\%$ of evaluations upgraded from unsatisfactory to satisfactory.
  • The rubric category with the highest rate of evaluation change was the Incisal Edge ($49\%$ change).
  • The rubric category with the lowest rate of evaluation change was the Interproximal Contact Area ($3\%$ change).
  • $70\%$ of students strongly agreed that Compare software enabled more effective assessment of Lingual Contour.
  • $8\%$ of students strongly disagreed that the software enabled more effective assessment of finishing.
• Conclusions:
  • CAD/CAM self-assessment software significantly enhances critical student self-reflection.
  • Objective feedback improves student-teacher communication, reduces required one-on-one instructional time, and allows for higher student-to-teacher ratios in preclinical simulation laboratories.

Core Educational Benefits of Digital Assessment Tools

• Provide objective, repeatable, and standardized evaluation criteria independent of subjective faculty variance.
• Allow precise, customized tolerance threshold settings (e.g., $250\,\mu\text{m}$ or $\pm 0.1\,\text{mm}$).
• Resolve evaluation discrepancies between instructors and students through unambiguous color-coded 3D superimposition.
• Cultivate self-directed motivation, critical thinking, and reflective practice, transforming learners from completion-oriented students into lifelong quality-driven practitioners.

Historical Development of CAD/CAM in Dentistry

Innovation and Early Devices

• Professor François Duret is widely recognized as the pioneer of dental CAD/CAM systems.
• In 1984, Professor Duret patented the first functional dental CAD/CAM device, establishing the theoretical and mechanical foundations for digital optical impressions, computer-aided design, and automated restoration milling.
• This work transitioned dental crown and restoration fabrication from physical impression materials and manual casting to digitized optical capture and computer-guided manufacturing.

Intraoral Scanner Fundamentals and Hardware Handling

Intraoral Scanner (IOS) Principles

• Intraoral scanners rapidly capture optical surface images or video streams to generate precise 3D geometric meshes in real time.
• Spatial surface data is stored in standard digital 3D mesh formats, most commonly STL (Standard Tessellation Language).

Integrated System Components

• Hardware Platform: CEREC Primescan / CEREC AC cart.
• Acquisition & Design Software: CEREC SW 5.2.13.
• Comparative Analysis Software: prepCheck (version 5.1.0).

Ergonomics and Hardware Care

• Handling Rules:
  • NEVER lift or maneuver the scanner by its optical mirror tip.
  • ALWAYS grip the scanner at its central body base (the black and white ergonomic mid-section).
  • After completing a scan, return the wand to its cradle with the optical tip facing forward (toward the user) and the cable/base directed toward the rear of the cart unit.

Wand Activation Methods

1. Automatic Activation: Removing the scanner wand from the cradle for the first time while in the Acquisition phase automatically initializes the camera.
2. Foot Pedal: Pressing the foot pedal located at the base of the CEREC AC cart toggles the optical scanner on and off.
3. Screen Interface: Toggling the soft switch camera icon on the monitor screen turns the scanner stream on or off.

Clinical Technique for Anterior Digital Scanning

Spatial Alignment and Grip

• Hold the scanner body using a stable pen or stylus grip.
• Maintain an optical clearance distance of exactly $2\,\text{mm}$ between the scanner window lens and the tooth surface.
• Position the scanner wand head perpendicular ($90^\circ$) directly over the incisal edge of the target tooth prep.
• Do NOT initiate scanning from facial or lingual approach angles.

Operator Focus and Real-Time Feedback

• Operator eye focus must remain continuously fixed on the computer monitor screen during scanning rather than looking directly into the patient mouth or typodont.
• Screen tracking ensures immediate identification of uncaptured surface areas and gap fill requirements.

Standardized Anterior Scanning Sequence

1. Phase 1 (Incisal Surface): Position the optical window perpendicular over the incisal edge of the prepared tooth. Move smoothly along the incisal edge across the arch section.
2. Phase 2 (Facial/Buccal Surface): Roll the scanner head $45^\circ$ to $90^\circ$ toward the facial aspect, maintaining a continuous sweeping movement across all facial surfaces.
3. Phase 3 (Lingual Surface): Roll the scanner head over the incisal table toward the lingual aspect, capturing marginal ridges, cingulum contours, and lingual fossae.

Criteria for an Acceptable Anterior Scan

• Arch Extent: Scan must continuously span from the first premolar to the opposing first premolar (e.g., tooth #5 to #12 or #21 to #28).
• Anatomical Capture: Complete 3D mesh coverage of the target wax-up and all adjacent arch teeth.
• Embrasures: Full capture of mesial and distal interproximal contact zones and gingival embrasure spaces.
• Soft Tissue Margin: Clear, distortion-free capture of $2\text{--}3\,\text{mm}$ of surrounding gingival/buccal soft tissue.
• Data Density: Complete elimination of optical voids (represented as unrendered white gaps in the mesh). Any white area over critical anatomical regions requires spot-scanning to build required data density.
• Scan Termination: Re-cradling the wand or depressing the foot pedal halts the optical acquisition process.

Step-by-Step prepCheck Workflow for Wax-Up Comparison

Phase 1: Patient and Case Initialization

1. Launch CEREC SW 5.2.13 from the desktop screen.
2. Select Show Patients on the bottom navigation bar.
3. Enter the assigned CAD/CAM student identification code into the database search bar.
4. Highlight the target patient profile and click Add Case.

Phase 2: Administration Setup

1. In the Administration tab, select target tooth location (Tooth #8 / Maxillary Central Incisor).
2. Select Restoration Type: Single Crown.
3. Select Design Mode: Biogeneric Individual.
4. Select Material: IPS e.max CAD (Grinding) or equivalent designated template block.
5. Select Processing Device: Primemill 01.
6. Advance to the Acquisition phase.

Phase 3: Image Acquisition

1. Ensure the Upper Jaw image folder is active.
2. Execute the standardized anterior scan sequence on the typodont carrying the #8 wax-up (extending 1st premolar to 1st premolar).
3. Verify complete surface data density without critical white voids.
4. Click the Model phase tab on the top process bar or click the forward arrow at the bottom screen margin.

Phase 4: Model Processing and Margin Tracing

1. Under Model Tools, select Draw Margin.
2. Choose Manual mode.
3. Trace a $360^\circ$ continuous line around the base margin of the wax-up, extending over the incisal edge at the mesial and distal contact points:
   • Double-click the starting location to initiate the line.
   • Single-click incrementally around the perimeter to place control anchor points.
   • Double-click the original red starting dot to close the margin loop.
4. Rotate the 3D model $360^\circ$ to verify margin accuracy.

Phase 5: Setting Insertion Axis

1. Select Set Insertion Axis from the bottom Model Phase menu.
2. Rotate the 3D model so the viewing direction looks straight down along the long axis of the incisal edge of the #8 wax-up.
3. Click Ok to lock the path of insertion.

Phase 6: Exporting to prepCheck Application

1. Click the Dentsply Sirona icon menu located in the top left corner of the screen.
2. Select Run Application….
3. Click the prepCheck 5.1.0 application icon.
4. In the prepCheck-Exporter window, set Course to DENTD 1500.
5. Click Show Details to verify the master comparison template assignment.
6. Confirm the template reads Template #8 Crown Wax-Up.
7. Click Open in prepCheck.

Quantitative Analysis and Interpretation in prepCheck

3D Color-Coded Comparison Heat Map

• Green Surfaces: Indicates overcontoured wax-up regions (excess material beyond master template). Clinical Action: Remove wax.
• Red Surfaces: Indicates undercontoured wax-up regions (deficient material compared to master template). Clinical Action: Add wax.
• Blue Surfaces: Indicates high-accuracy regions within the acceptable deviation threshold of $\pm 0.1\,\text{mm}$ ($\pm 100\,\mu\text{m}$) relative to the master typodont template.

Cursor Inspection and Numerical Point Measurement

• Hovering the mouse cursor over any point on the 3D mesh displays the exact distance discrepancy under Cursor Details.
• Displays precise positive values for overcontoured areas (e.g., $+0.90\,\text{mm}$, $+0.92\,\text{mm}$) and negative values for undercontoured areas (e.g., $-0.45\,\text{mm}$).

Object Display Controls

• Adjust the Master Preparation opacity slider to $50\%$.
• Adjust the Preparation (Sample Wax-Up) opacity slider to $100\%$.

Sectional Analysis via the Slice Tool

1. Rotate the 3D model into a proximal (mesial or distal) profile view.
2. Click Slice under the Manual Analysis tool panel.
3. 3D Manipulation Handles:
   • Sphere Control Handles: Click and drag spherical handles to adjust the cross-sectional cutting plane angle.
   • Arrow Control Handles: Click and drag directional arrows to translate the slice plane position mesially or distally at a fixed angle.
4. Line Profile Interpretation:
   • Green Profile Line: Represents the master template typodont tooth contour.
   • Blue Profile Line: Represents the student wax-up sample contour.
5. The Slice tool can be dynamically activated from any orientation angle ($360^\circ$) across the arch.

Linear Distance Measurement Tool

1. Select Measure Distance under the Manual Analysis panel.
2. Click the primary point of interest on the sample mesh.
3. Click the corresponding secondary point on the reference mesh to calculate exact spatial deviation in millimeters.

Pedagogy and Critical Thinking Integration

• Digital hardware and analysis software serve exclusively as objective data collection tools; they do not replace clinical judgment.
• Learners must interpret 3D surface maps and cross-sectional slices critically to plan targeted physical corrections.
• Corrective wax modifications (adding vs. carving) executed based on quantitative feedback reinforce motor skill acquisition and spatial anatomical awareness.

Academic References

• Björn AL, Björn H, Grkovic B. Marginal fit of restorations and its relation to periodontal bone level. II. Crowns. Odontol Revy. 1970;21:337–346.
• Chiang H, Staffen A, A Abdulmajeed A, Janus C, Bencharit S. Effectiveness of CAD/CAM technology: A self-assessment tool for preclinical waxing exercise. Eur J Dent Educ. 2021 Feb;25(1):50-55. doi: 10.1111/eje.12576. PMID: 33448597.
• Garrett PH, Faraone KL, Patzelt SB, Keaser ML. Comparison of Dental Students’ Self-Directed, Faculty, and Software-Based Assessments of Dental Anatomy Wax-Ups: A Retrospective Study. J Dent Educ. 2015;79(12):1437-1444. doi:10.1002/j.0022-0337.2015.79.12.tb06043.x.
• Greany TJ, Yassin A, Lewis KC. Developing an All-Digital Workflow for Dental Skills Assessment: Part I, Visual Inspection Exhibits Low Precision and Accuracy. J Dent Educ. 2019;83(11):1304-1313. doi:10.21815/JDE.019.132.
• McPherson KR, Mennito AS, Vuthiganon J, Kritzas YG, McKinney RA, Wolf BJ, Renne WG. Utilizing self-assessment software to evaluate student wax-ups in dental morphology. J Dent Educ. 2015 Jun;79(6):697-704. PMID: 26034035; PMCID: PMC4740729.
• Renne WG, McGill ST, Mennito AS, Wolf BJ, Marlow NM, Shaftman S, Holmes JR. E4D Compare Software: An Alternative to Faculty Grading in Dental Education. J Dent Educ. 2013;77(2):168-175. doi:10.1002/j.0022-0337.2013.77.2.tb05459.x.
• Sly MM, Barros JA, Streckfus CF, Arriaga DM, Patel SA. Grading Class I Preparations in Preclinical Dental Education: E4D Compare Software vs. the Traditional Standard. J Dent Educ. 2017 Dec;81(12):1457-1462. doi: 10.21815/JDE.017.107. PMID: 29196334.