Supporting Cusps in Dental Occlusion

Supporting cusps, also known as stamp cusps, centric holding cusps, or holding cusps, play a crucial role in dental occlusion and function. They are essential for effective chewing and maintaining the vertical dimension of the face. This guide will outline the characteristics, functions, and clinical significance of supporting cusps.

Supporting Cusps: These are the cusps of the maxillary and mandibular teeth that make contact during maximum intercuspation (MI) and are primarily responsible for supporting the vertical dimension of the face and facilitating effective chewing.

Location

• Maxillary Supporting Cusps: Located on the lingual occlusal line of the maxillary teeth.
• Mandibular Supporting Cusps: Located on the facial occlusal line of the mandibular teeth.

Functions of Supporting Cusps

A. Chewing Efficiency

• Mortar and Pestle Action: Supporting cusps contact the opposing teeth in their corresponding faciolingual center on a marginal ridge or a fossa, allowing them to cut, crush, and grind fibrous food effectively.
• Food Reduction: The natural tooth form, with its multiple ridges and grooves, aids in the reduction of the food bolus during chewing.

B. Stability and Alignment

• Preventing Drifting: Supporting cusps help prevent the drifting and passive eruption of teeth, maintaining proper occlusal relationships.

Characteristics of Supporting Cusps

Supporting cusps can be identified by the following five characteristic features:

1. Contact in Maximum Intercuspation (MI): They make contact with the opposing tooth during MI, providing stability in occlusion.

2. Support for Vertical Dimension: They contribute to maintaining the vertical dimension of the face, which is essential for proper facial aesthetics and function.

3. Proximity to Faciolingual Center: Supporting cusps are located nearer to the faciolingual center of the tooth compared to nonsupporting cusps, enhancing their functional role.

4. Potential for Contact on Outer Incline: The outer incline of supporting cusps has the potential for contact with opposing teeth, facilitating effective occlusion.

5. Broader, Rounded Cusp Ridges: Supporting cusps have broader and more rounded cusp ridges than nonsupporting cusps, making them better suited for crushing food.

Clinical Significance

A. Occlusal Relationships

• Maxillary vs. Mandibular Arch: The maxillary arch is larger than the mandibular arch, resulting in the supporting cusps of the maxilla being more robust and better suited for crushing food than those of the mandible.

B. Lingual Tilt of Posterior Teeth

• Height of Supporting Cusps: The lingual tilt of the posterior teeth increases the relative height of the supporting cusps compared to nonsupporting cusps, which can obscure central fossa contacts.

C. Restoration Considerations

• Restoration Fabrication: During the fabrication of restorations, it is crucial to ensure that supporting cusps do not contact opposing teeth in a manner that results in lateral deflection. Instead, restorations should provide contacts on plateaus or smoothly concave fossae to direct masticatory forces parallel to the long axes of the teeth.

Beveling in Restorative Dentistry

Beveling: Beveling refers to the process of angling the edges of a cavity preparation to create a smooth transition between the tooth structure and the restorative material. This technique can enhance the aesthetics and retention of certain materials.

Characteristics of Ceramic Materials

• Brittleness: Ceramic materials, such as porcelain, are inherently brittle and can be prone to fracture under stress.
• Bonding Mechanism: Ceramics rely on adhesive bonding to tooth structure, which can be compromised by beveling.

Contraindications

• Cavosurface Margins: Beveling the cavosurface margins of ceramic restorations is contraindicated because:
  • It can weaken the bond between the ceramic and the tooth structure.
  • It may create unsupported enamel, increasing the risk of chipping or fracture of the ceramic material.

Beveling with Amalgam Restorations

Amalgam Characteristics

• Strength and Durability: Amalgam is a strong and durable material that can withstand significant occlusal forces.
• Retention Mechanism: Amalgam relies on mechanical retention rather than adhesive bonding.

Beveling Guidelines

• General Contraindications: Beveling is generally contraindicated when using amalgam, as it can reduce the mechanical retention of the restoration.
• Exception for Class II Preparations:
  • Gingival Floor Beveling: In Class II preparations where enamel is still present, a slight bevel (approximately 15 to 20 degrees) may be placed on the gingival floor. This is done to:
    • Remove unsupported enamel rods, which can lead to enamel fracture.
    • Enhance the seal between the amalgam and the tooth structure, improving the longevity of the restoration.

Technique for Beveling

• Preparation: When beveling the gingival floor:
  • Use a fine diamond bur or a round bur to create a smooth, angled surface.
  • Ensure that the bevel is limited to the enamel portion of the wall to maintain the integrity of the underlying dentin.

Clinical Implications

A. Material Selection

• Understanding the properties of the restorative material is essential for determining the appropriate preparation technique.
• Clinicians should be aware of the contraindications for beveling based on the material being used to avoid compromising the restoration's success.

B. Restoration Longevity

• Proper preparation techniques, including appropriate beveling when indicated, can significantly impact the longevity and performance of restorations.
• Regular monitoring of restorations is essential to identify any signs of failure or degradation, particularly in areas where beveling has been performed.

Glass ionomer cement is a tooth coloured material 
Material was based on reaction between silicate glass powder & polyacrylicacid.
They bond chemically to tooth structure & release fluoride for relatively long period

CLASSIFICATION 

Type I. For luting

Type II. For restoration 

Type II.1 Restorative esthetic 

Type II.2 Restorative reinforced

Type III. For liner & bases

Type IV. Fissure & sealent

Type V. As Orthodontic cement

Type VI. For core build up

Physical Properties

1. Low solubility
2. Coefficient of thermal expansion similar to dentin
3. Fluoride release and fluoride recharge
4. High compressive strengths
5. Bonds to tooth structure
6. Low flexural strength
7. Low shear strength
8. Dimensional change (slight expansion) (shrinks on setting, expands with water sorption)
9. Brittle
10.Lacks translucency
11.Rough surface texture

Indications for use of Type II glass ionomer cements 

1) non-stress bearing areas 

2) class III and V restorations in adults 

3) class I and II restorations in primary dentition 

4) temporary or “caries control” restorations 

5) crown margin repairs 

6) cement base under amalgam, resin, ceramics, direct and indirect gold 

7) core buildups when at least 3 walls of tooth are remaining (after crown preparation)

Contraindications 

1) high stress applications I. class IV and class II restorations II. cusp replacement III. core build-ups with less than 3 sound walls remaining

Composition

Factors affecting the rate or setting

1. Glass composition:Higher Alumina – Silica ratio, faster set and shorter working time.
2. Particle Size: finer the powder, faster the set.
3. Addition of Tartaric Acid:-Sharpens set without shortening the working time.
4. Relative proportions of the constituents: Greater the proportion of glass and lower the proportion of water, the faster the set.
5. Temperature

Setting Time

Type 1 - 4-5 min
type II - 7 min

PROPERTIES 

Adhesion :

- Glass ionomer cement bonds chemically to the tooth structure->reaction occur between carboxyl group of poly acid & calcium of hydroxyl apatite.
 
- Bonding with enamel is higher than that of dentin ,due to greater inorganic content. 

Esthetics :
-GIC is tooth coloured material & available in different shades.
Inferior to composites.
They lack translucency & rough surface texture.
Potential for discolouration & staining.

Biocompatibilty :

- Pulpal response to glass ionomer cement is favorable. 
- Pulpal response is mild due to 
- High buffering capacity of hydroxy apatite. 
- Large molecular weight of the polyacrylic acid ,which prevents entry into dentinal tubules. 

a) Pulp reaction – ZOE < Glass Ionomer < Zinc Phosphate 

b) Powder:liquid ratio influences acidity 

c) Solubility & Disintegration:-Initial solubility is high due to leaching of intermediate products.The complete setting reaction takes place in 24 hrs, cement should be protected from saliva during this period.

Anticariogenic properties :
- Fluoride is released from glass ionomer at the time of mixing & lies with in matrix.
Fluoride can be released out without affecting the physical properties of cement.

ADVANTAGE DISADVANTAGE

Light-Cure Composites

Light-cure composites are resin-based materials that harden when exposed to specific wavelengths of light. They are widely used in dental restorations due to their aesthetic properties, ease of use, and ability to bond to tooth structure.

Key Components:

• Diketone Photoinitiator: The primary photoinitiator used in light-cure composites is camphoroquinone. This compound plays a crucial role in the polymerization process.
• Visible Light Spectrum: The curing process is activated by blue light, typically in the range of 400-500 nm.

2. Curing Lamps: Halogen Bulbs and QTH Lamps

Halogen Bulbs

• Efficiency: Halogen bulbs maintain a constant blue light efficiency for approximately 100 hours under normal use. This consistency is vital for reliable curing of dental composites.
• Step Curing: Halogen lamps allow for a technique known as step curing, where the composite is first cured at a lower energy level and then stepped up to higher energy levels. This method can enhance the properties of the cured material.

Quartz Tungsten Halogen (QTH) Curing Lamps

• Irradiance Requirements: To adequately cure a 2 mm thick specimen of resin-based composite, an irradiance value of at least 300 mW/cm² to 400 mW/cm² is necessary. This ensures that the light penetrates the composite effectively.
• Micro-filled vs. Hybrid Composites: Micro-filled composites require twice the irradiance value compared to hybrid composites. This is due to their unique composition and light transmission properties.

3. Mechanism of Visible Light Curing

The curing process involves several key steps:

Photoinitiation

• Absorption of Light: When camphoroquinone absorbs blue light in the 400-500 nm range, it becomes excited and forms free radicals.
• Free Radical Formation: These free radicals are essential for initiating the polymerization process, leading to the hardening of the composite material.

Polymerization

• Chain Reaction: The free radicals generated initiate a chain reaction that links monomers together, forming a solid polymer network.
• Maximum Absorption: The maximum absorption wavelength of camphoroquinone is at 468 nm, which is optimal for effective curing.

4. Practical Considerations in Curing

Curing Depth

• The depth of cure is influenced by the type of composite used, the thickness of the layer, and the irradiance of the light source. It is crucial to ensure that the light penetrates adequately to achieve a complete cure.

Operator Technique

• Proper technique in positioning the curing light and ensuring adequate exposure time is essential for achieving optimal results. Inadequate curing can lead to compromised mechanical properties and increased susceptibility to wear and staining.

Resistance Form in Dental Restorations

Resistance form is a critical concept in operative dentistry that refers to the design features of a cavity preparation that enhance the ability of a restoration to withstand masticatory forces without failure. This lecture will cover the key elements that contribute to resistance form, the factors affecting it, and the implications for different types of restorative materials.

1. Elements of Resistance Form

A. Design Features

1. Flat Pulpal and Gingival Floors:

   • Flat surfaces provide stability and help distribute occlusal forces evenly across the restoration, reducing the risk of displacement.

2. Box-Shaped Cavity:

   • A box-shaped preparation enhances resistance by providing a larger surface area for bonding and mechanical retention.

3. Inclusion of Weakened Tooth Structure:

   • Including weakened areas in the preparation helps to prevent fracture under masticatory forces by redistributing stress.

4. Rounded Internal Line Angles:

   • Rounding internal line angles reduces stress concentration points, which can lead to failure of the restoration.

5. Adequate Thickness of Restorative Material:

   • Sufficient thickness is necessary to ensure that the restoration can withstand occlusal forces without fracturing. The required thickness varies depending on the type of restorative material used.

6. Cusp Reduction for Capping:

   • When indicated, reducing cusps helps to provide adequate support for the restoration and prevents fracture.

B. Deepening of Pulpal Floor

• Increased Bulk: Deepening the pulpal floor increases the bulk of the restoration, enhancing its resistance to occlusal forces.

2. Features of Resistance Form

A. Box-Shaped Preparation

• A box-shaped cavity preparation is essential for providing resistance against displacement and fracture.

B. Flat Pulpal and Gingival Floors

• These features help the tooth resist occlusal masticatory forces without displacement.

C. Adequate Thickness of Restorative Material

• The thickness of the restorative material should be sufficient to prevent fracture of both the remaining tooth structure and the restoration. For example:
  • High Copper Amalgam: Minimum thickness of 1.5 mm.
  • Cast Metal: Minimum thickness of 1.0 mm.
  • Porcelain: Minimum thickness of 2.0 mm.
  • Composite and Glass Ionomer: Typically require thicknesses greater than 2.5 mm due to their wear potential.

D. Restriction of External Wall Extensions

• Limiting the extensions of external walls helps maintain strong marginal ridge areas with adequate dentin support.

E. Rounding of Internal Line Angles

• This feature reduces stress concentration points, enhancing the overall resistance form.

F. Consideration for Cusp Capping

• Depending on the amount of remaining tooth structure, cusp capping may be necessary to provide adequate support for the restoration.

3. Factors Affecting Resistance Form

A. Amount of Occlusal Stresses

• The greater the occlusal forces, the more robust the resistance form must be to prevent failure.

B. Type of Restoration Used

• Different materials have varying requirements for thickness and design to ensure adequate resistance.

C. Amount of Remaining Tooth Structure

• The more remaining tooth structure, the better the support for the restoration, which can enhance resistance form.

4. Clinical Implications

A. Cavity Preparation

• Proper cavity preparation is essential for achieving optimal resistance form. Dentists should consider the design features and material requirements when preparing cavities.

B. Material Selection

• Understanding the properties of different restorative materials is crucial for ensuring that the restoration can withstand the forces it will encounter in the oral environment.

C. Monitoring and Maintenance

• Regular monitoring of restorations is important to identify any signs of failure or degradation, allowing for timely intervention.

Incipient Lesions

Characteristics of Incipient Lesions

• Body of the Lesion: The body of the incipient lesion is the largest portion during the demineralizing phase, characterized by varying pore volumes (5% at the periphery to 25% at the center).
• Striae of Retzius: The striae of Retzius are well marked in the body of the lesion, indicating areas of preferential mineral dissolution. These striae represent the incremental growth lines of enamel and are critical in understanding caries progression.

Caries Penetration

• Initial Penetration: The first penetration of caries occurs via the striae of Retzius, highlighting the importance of these structures in the carious process. Understanding this can aid in the development of preventive strategies and treatment plans aimed at early intervention and management of carious lesions.