4-Fabrication of Axial and Occlusal Surfaces of the Wax Pattern

Foundational Relationships and Objectives in Wax Pattern Fabrication

The fabrication of an artificial crown requires the precise modeling of its axial and occlusal surfaces to ensure harmony with the surrounding oral structures. Prof. Dr. Gigovski Nikola outlines seven key relationships and precision requirements that must be addressed during the fabrication of the wax pattern:

1. Relation to Gingiva and Preparation Margin: The crown must be modeled to prevent mechanical irritation of the gingival tissues.

2. Relation to Adjacent Teeth: This involves the establishment of the contact point.

3. Relation to Opposing Teeth: The crown must maintain proper occlusion and articulation.

4. Vestibular and Oral Prominence: Proper modeling of the vestibular and oral surfaces, specifically according to the Selbach phenomenon.

5. Crown and Pontic Connection: The relationship must respect the interdental papilla.

6. Precision of Coping Fabrication: Ensuring primary (mechanical) retention through high-level adaptation.

7. Thermal Insulation Space: Providing interspace using space varnish or thin foil for insulation purposes.

The Waxing Process and Sequential Methodology

Following the fabrication of the coping, the modeling (waxing) of the pattern carries out all anatomical and morphological characteristics. This is achieved through the wax-drop technique, where molten wax is added gradually to the coping to:

• Model the physiological contour to match a natural tooth crown.

• Compensate for the loss of reduced hard tooth substance.

Sequence of Waxing:

1. Axial Surfaces:     * Approximal surfaces (mesial and distal).     * Vestibular and oral surfaces.

2. Occlusal Surface: This is typically the final stage of modeling.

Modeling of Approximal Surfaces and Contact Areas

Melted wax is applied to the mesial and distal surfaces until it touches the adjacent tooth. This contact is then analyzed and corrected across three distinct zones:

1. The contact area (contact point).

2. Below the contact (toward the gingiva).

3. Above the contact (toward the occlusal or incisal surface).

Significance of the Contact Point (or Contact Surface):

• Biological Significance: It protects the interdental papilla by preventing food impaction and subsequent irritation.

• Static Significance: It enables the distribution of chewing pressure by transferring loads to adjacent teeth, thereby offloading the individual tooth.

Position of Contact Points in the Dental Arch

Posterior Teeth

• Upper Posterior Teeth: Generally located in the occlusal third. The exception is the contact between the first and second upper molars, which is in the middle third. When viewed occlusally, the contact is usually more buccal (at the transition between the buccal and middle third), except between the first and second upper molars, where it is centered on the proximal surfaces.

• Lower Posterior Teeth: These follow the same rules as the upper posterior teeth.

• General Rule for Posteriors: Contacts are in the occlusal third and at the transition between the vestibular and middle third.

Anterior Teeth

The position and size of anterior contact points are determined by three factors:

• Mechanical: To stabilize the tooth and transmit pressure, the contact should be as large as possible.

• Biological: To protect and avoid compression of the papilla, the contact should be as small as possible and positioned far from the gingiva.

• Aesthetic: The contact should be positioned as orally as possible.

Specific Alignment: The line of contact points ($C$) in the anterior region should follow the line of the interdental papillae ($B$) and the incisal line ($D$). Usually, the contact point is located slightly more cervically than the incisal third.

Critical Warnings:

• Cervical Positioning: Contacts must not be too close to the gingiva, as this compresses the interdental papilla.

• Incisal/Open Positioning: If the contact is too far toward the incisal edge or if there is excessive open space, a "black triangle" (visible empty space) appears between the papilla and the contact point.

Modeling Adjacent and Surrounding Proximities

Area Below the Contact Points

To allow the interdental papilla to accommodate without irritation, the proximal surface below the contact point is recommended to be modeled with a slight concavity.

Area Above the Contact Points

This space (part of the occlusal contour) should be minimal and gently defined. Over-dimensioning this into a deep notch or funnel must be avoided to prevent food retention and impaction.

Techniques for Practical Modeling and Control

1. Gypsum Removal: A portion of gypsum is removed from the contact area of the adjacent tooth on the model, making the wax pattern wider. The excess is ground down after casting.

2. Additional Waxing: After removing the crown from the die, a thin layer of wax is added to the contact area before investing.

3. Special Conditions: According to Markscors, if the adjacent tooth is also an artificial crown, the contact can be modeled larger as a contact surface. A slight concavity can be modeled to interlock with the convexity of the neighboring tooth to stabilize the dental arch.

Control: Final dimensioning is performed on the working model using articulating paper (carbon or indigo paper) to visualize the location, size, and intensity of the contact.

The Selbach Phenomenon and Axial Modeling

Axial surfaces (vestibular and oral) are modeled by adding wax to the coping using adjacent natural teeth as a reference. This process focuses on two critical elements:

Tooth Prominence (The Equator)

• Vestibular (all teeth): Located in the gingival third.

• Oral (upper posterior teeth): Located in the gingival third.

• Oral (lower posterior teeth): Located in the occlusal third.

Tooth Convexity (The Selbach Phenomenon)

This refers to the degree of prominence of the equator relative to the tooth neck. Standard measurements include:

• Vestibular (all posterior): Approximately $0.5\,mm$.

• Oral (upper posterior): Approximately $0.5\,mm$.

• Oral (lower first premolar): $0.5\,mm$.

• Oral (lower second premolar): $0.75\,mm$.

• Oral (lower molars): $1\,mm$.

Significance of the Selbach Phenomenon:

• Protective Role: Defends the gingival sulcus and marginal periodontium from mechanical irritation during mastication.

• Physiological Support: The vestibular surface supports the buccal mucosa. Over-dimensioning can cause mucosal indentations or create undercuts that facilitate food accumulation and prevent self-cleaning.

Approaches to Modeling the Occlusal Surface

There are three recognized methods for occlusal modeling:

1. Barely Defined Cusps and Fissures: Weakly pronounced morphology. Contact with antagonists is broad rather than pinpoint. This results in reduced chewing efficiency, muscle fatigue, and abutment overloading. It is often used because it is technically easy, involving closing the articulator on excess wax to create a negative imprint. It is only partially justified in patients with severe abrasion.

2. Modeling According to the General Situation: This is the most acceptable approach for most prosthetic work. It requires a full arch impression and a working model. The technician analyzes the degree of abrasion, cusp size, fissure definition, axial convexity, contact points, and the Curve of Spee.     * Rule for Curve of Spee: A strongly pronounced curve requires high cusps; a weak curve requires low cusps.     * Functional Cusps: In the lower jaw, these are the buccal cusps; in the upper jaw, they are the palatal cusps. Final modeling is done by scraping wax with a cold knife to refine morphology.

3. Modeling According to Gnathological Principles: Justified in complete jaw reconstructions or when natural teeth have very pronounced forms. It requires an adjustable or semi-adjustable articulator. It features multiple pinpoint contact points during intercuspidation.     * Functional Cusps: Rounded (ball-like) and fit into the central fossa.     * Non-functional Cusps: Upper buccal and lower lingual; sharper and ridge-like.     * Contact Points: Occur on the inclines of cusps and ridges, not in the pits.

Use of Prefabricated Wax Profiles

To save time and metal, factory-prepared wax or plastic elements are available in two designs:

• G-shape: Cusp height and incline of $30^\circ$.

• A-shape: Cusp height and incline of $20^\circ$.

Application Process:

1. Soften elements in water at approximately $30\,^\circ C$.

2. Place over the coping on the die.

3. Close the articulator to pre-form the surface.

4. Fix the element at $2$ to $3$ points with a heated instrument and perform final adjustments (trimming, narrowing).

One-Piece Cast Crowns with Spacers

Indications and Contraindications

• Indications: Hard tissue removal > $1.5\,mm$, tooth defects (caries/hypoplasia), severely inclined teeth, raising the bite, providing thermal insulation, saving precious metal, or long tooth dies.

• Contraindications: Short tooth dies (reduces retention) and veneered crowns (space is needed for the veneer).

• Advantages: Metal savings, thermal insulation, and protection against mechanical trauma.

Methods for Creating the Spacer (Relief Space)

1. Adapta System: A thin foil ($0.1\,mm$) is placed under the thicker coping foil ($0.6\,mm$). The thin foil is removed before investing.

2. Die Spacer Varnish (Most Common): Coating the occlusal and middle thirds of the die. Two layers are recommended, providing a spacer of approximately $25-30\,\mu m$.

3. Wax Coping Technique: The coping is modeled to adapt closely only in the gingival third, remaining separated elsewhere.

4. Positioning Aids: Historically used but now abandoned due to complexity.