Operative One -- Dental Composite Material-Mechanical Properties: Vidal

Force is generated through

one body acting on another

Force will result in

translation or deformation: it depends on whether the body is constrained and rigid or deformable

3 characteristics of force

Point of application

Magnitude

Direction of application

Force is measured in units of

Newtons (N)

Occlusal forces:

Different teeth, restored (dentures) vs intact, gender, age, ect

When designing restorations and selecting materials consider

location, opposing dentition, and force-generating capacity of the patient

When force acts on a constrained body

the force is resisted by the body

Stress is the

internal reaction of a material to an external load/force

Both the applied force and the stress are distributed over

an area of the body

Stress = force/area

Resistance is dependent on

the surface area which the load was delivered

The area is more important than the

force

Unit of stress is

Pascal (Pa)

megapascal is often used in dentistry (MPa)

In dentistry: Occlusal forces applied over small areas (cusp tips) can produce

very high stresses

Explain why premature contacts are damaging

Small area supporting large occlusal forces

To reduce local stresses

1. Multiple simultaneous contacts

2. Occlusal forces distributed over larger surface areas

Strain is

deformation a body undergoes when a stress acts upon it

Strain is measured as a

net change in length of a material following the application of a load

Deformation depends on

the type of stress

Strain is often reported as

a percentage

Stain is an important consideration in

ortho wires

implant screws

Impression materials

Types of stress

Several forces can combine to develop

complex stresses in a structure

Stress-Strain curve

0-A = stress is linearly proportional to the strain (elastic deformation)

A = Proportional limit, beyond this point deformation is permanent (plastic deformation)

B = yield strength - point at which a small defined amount of permanent strain has occurred

C = Ultimate (tensile) strength

D = Fracture strength

Modulus of elasticity

Ability to sustain deformation without permanent change in size or shape

Toughness

resistance of a material to fracture

Fracture toughness

ability to plastically deform without fracture, calculated as the area under the elastic and plastic portions of the stress strain curve

Material's ability to resist to the propagation of a crack

Brittle materials tend to have

low toughness, little deformation (plastic) before failure

Higher fracture toughness indicates

lower rate of bulk fracture, marginal degradation and surface wear

Wear

Loss of material resulting from removal and relocation of materials through the contact of two or more materials

Resin composite wear results from a combination of

chemical damage to the surface of the material and mechanical breakdown

Abrasive action of particles during mastication (IMAGE)

Attrition is related to

loss of material as result of direct contact with opposing tooth surfacs

Physical properties

Working and setting times

Polymerization shinkage/stess

radiopacity

Water sorption and solubility

Color stability

Thermal properties

Material and dental tissues properties Elastic modulus (GPa) of selected dental materials

IMAGE

How does RBC composition affect its mechanical properties (IMAGE)

Mechanical properties of restorative materials and dental hard tissues (IMAGE)

Polymerization shrinkage and stress:

De-bonding - Marginal gaps (microleakage)

Cusp deflection (post-op sensitivity) or fracture

Recurrent caries

Incomplete polymerization:

Presence of residual monomers (low degree of conversion)

Poor mechanical properties

Release of monomers and other components that can reach the pulp and cause pulp inflammation

SUMMARY: Optimizing material performance

Understanding the mechanical properties of dental composites helps in selecting materials that can withstand occlusal forces, reducing the risk of fractures and ensuring long-term durability in restorative dentistry

SUMMARY: Enhancing Clinical Outcomes

Knowledge of properties such as flexural strength, wear resistance and polymerization shrinkage allows clinicians to tailor composite selection and placement techniques, improving the success of restorations and patient satisfaction