Dental Model and Space Analysis: Clinical Guidelines and Practical Exercise
This document outlines the procedures for conducting a comprehensive space and model analysis in a clinical or educational setting. It covers the use of specific measurement tools, the Tanaka-Johnston prediction method, and the qualitative assessment of dental casts.
1. Overview of the Practical Exercise
The objective of this activity is to perform an individual practical exercise using dental models to estimate space requirements for unerupted teeth and analyze the overall dental architecture.
Materials Required
- Form No. 42 (Green Form): Used for space analysis (Model A).
- Blank Sheet: Used for model/cast analysis (Model B).
- Measurement Tools: Gauges (Vernier calipers) and rulers.
- Dental Casts: Sets labeled A (Space Analysis) and B (Model Analysis).
Key Participation Rules
- Individual Work: While students work in groups of 4–5, each student must complete their own form.
- Subjectivity in Measurement: Measurements may vary slightly between individuals (e.g., 11mm vs. 12mm). Results should not be identical; copying is prohibited.
- Concentration: Accuracy is vital. Ensure measurements are recorded for the correct tooth to avoid compounding errors.
2. Space Analysis: The Tanaka-Johnston Method
This method is used during the mixed dentition stage to estimate the size of unerupted premolars and canines.
Advantages of the Method
- Requires no radiographs (X-rays).
- Uses simple equations rather than complex tables or graphs.
- Applicable to both the maxillary and mandibular arches.
- Provides a reasonable prediction range (±1.5 mm).
Core Assumptions
- Lower Incisors as Reference: The four mandibular incisors are the most stable and early-erupting teeth. Always use the lower incisors as the reference value, even when predicting space for the upper arch.
- Arch Stability: Assumes arch dimensions do not change significantly during growth (unless there is active protrusion).
- Predictable Molar Shift: Accounts for the “leeway space” and the mesial shift of first molars required to achieve a Class I occlusion.
3. Measurement Procedures
Using the Vernier Caliper
- The main scale provides the whole millimeter (e.g., 9mm).
- The sub-scale (vernier) identifies the decimal. Look for where the sub-scale line matches perfectly with a top-scale line (e.g., 9.2mm or 9.3mm).
Sectional Measurements
The arch is divided into groups to determine Available Space:
- Group A: Distal of the second primary molar (E) to the mesial of the primary canine (C).
- Group B: Distal of the lateral incisor to the midline.
- Groups C & D: Mirror the measurements for the opposite side.
Calculating Required Space
To predict the width of unerupted canines and premolars:
| Arch | Formula |
|---|---|
| Mandibular (Lower) | (Sum of 4 Lower Incisors / 2) + 10.5 mm |
| Maxillary (Upper) | (Sum of 4 Lower Incisors / 2) + 11.0 mm |
Interpreting Results
- Positive Value: Excess space available.
- Negative Value: Lack of space; crowding is expected.
4. Clinical Correlations
Facial Profile and Lip Posture
If patient photos are unavailable, refer to the workbook for profile descriptions:
- Class I: Straight/orthognathic profile.
- Class II: Convex profile; often associated with a deficient chin and compensatory lower incisor protrusion.
- Class III: Concave profile; often associated with a prominent lower jaw and retroclined lower incisors.
Molar Relationship and Leeway Space
- End-to-End vs. Class I: Lower molars often need to drift mesially by 1–2mm to reach Class I.
- Leeway Space: The primary molars (especially the lower ‘E’) are wider than the permanent premolars. This “extra” space (approx. 2.5mm per side in the lower arch) allows for the mesial molar shift.
- Clinical Tip: If a patient has a negative space discrepancy, the leeway space can be preserved using a space maintainer to prevent the molar shift, thereby gaining space for anterior alignment.
5. Cast Analysis (Model B)
This qualitative assessment follows the Ackerman and Proffit classification across three planes of space.
Symmetry and Alignment
- Assess if the arch is symmetrical. Asymmetry often stems from premature tooth loss or localized crowding.
- Identify the arch form: Ovoid (normal/parabolic), Tapered (narrow), or Square.
The Bolton Analysis (Tooth Size Ratio)
This measures the ratio between maxillary and mandibular tooth widths to ensure proper interdigitation.
- Anterior Ratio (Canine to Canine): Ideal is ~77.2%.
- Total Ratio (Molar to Molar): Ideal is ~91.3%.
- Clinical Shortcuts: If a mismatch is suspected, check the upper lateral incisors (often peg-shaped or small) and the upper second premolars.
Occlusal Relationships
| Plane | Assessment Points |
|---|---|
| Posterior (A-P) | Class I, II, or III molar/canine relationships. |
| Transverse | Arch width (narrow/wide) and presence of crossbites. |
| Vertical | Overbite (measured in %) or Open bite (measured in mm). |
6. Conclusion
Upon completing the measurements and calculations, students must provide a brief clinical statement. This should synthesize the space discrepancy, the molar relationship, and the facial profile to determine if a “considerable problem” exists for the patient’s developing occlusion.