GOOD TO KNOW: FLEXURAL STRENGTH AND WHY IT IS IMPORTANT

Surely you have already come across the term ‘flexural strength’. What exactly do we mean by ‘flexural strength’? Why is flexural strength important? Which values are considered good? Find out some of the most important things you should know about this topic.

Flexural strength: one of the key values for evaluating the stability of a material

Flexural strength is a term that is frequently used in the dental world. Every manufacturer of dental materials provides values that refer to this feature. With good reason: flexural strength is considered to be a key value when it comes to the stability of a material – such as zirconium oxide.

Note: Measuring methods vary!

From a scientific point of view, flexural strength informs about the resistance of a material against deformation, i.e. flexural strength indicates how much force is required to break a test sample of a defined diameter. As soon as this value is exceeded, the test specimen breaks. The higher the value, the more impacting forces the material is able to withstand.

However, the flexural strength determined during a test depends heavily on the measuring method used and the surface preparation of the test samples, i.e. on whether a material is polished or ground, for instance. Comparisons between different materials should therefore always be taken with a pinch of salt. Values measured with different measuring methods are not comparable. For the values to be comparable, they have to be obtained using the same measuring method..

The actual advantages of high flexural strength

Materials with high flexural strength offer advantages with regard to two applications in particular:

A) Extensive or long-span restorations

High flexural strength is essential for stress-bearing restorations, when high pressure/stress is exerted on the material or restoration.  As a result, flexural strength also determines the indications for which a material can be used:

• • The more strength a material offers, the more units a restoration can include.
  • Or, if put the other way round:  The more extensive the restoration is, the stronger the material should be.

The strength values required for specific restorations or indications are detailed in the relevant standards.

B) Minimally invasive treatment options with thin wall thicknesses

High flexural strength also affects the thickness of the restoration walls. A high-strength material allows a low wall thickness. For instance, if you use the IPS emax ZirCAD Prime or materials of the MT and MT Multi range, monolithic crowns with a wall thickness of 0.8 mm for anterior teeth and 1 mm for posterior teeth can be fabricated. With the proven IPS e.max ZirCAD MO or LT zirconium oxide, you can reduce the wall thickness to 0.4 mm for anterior teeth and to 0.6 mm for posterior teeth. This means:

• • A material that offers high flexural strength and high fracture toughness allows very thin restorations to be produced
  • and is therefore well suited for minimally invasive treatment options.

Zirconia: the champions when it comes to strength

Zirconia is probably the best-performing all-ceramic material in dentistry in terms of flexural strength. An example: IPS e.max ZirCAD LT (Low Translucency) has a flexural strength of 1,200 MPa, which is a very high value indeed.

Among the translucent zirconia materials, IPS e.max ZirCAD MT and MT Multi offer high stability at a flexural strength of 850 MPa. The innovative product IPS e.max ZirCAD Prime combines high-strength and high-translucency zirconia raw materials in such a way that areas requiring high strength (e.g. mandibular region and bridge pontics) are provided with high strength material and areas needing a high degree of translucency are reproduced in the more translucent component. IPS e.max ZirCAD Prime meets very high demands with regard to strength and delivers the desired esthetic outcome.

Strength and translucency: carefully coordinated

Generally speaking, the relationship between strength and translucency is closely intertwined in the oxide ceramic materials currently available.

• The higher the translucency of a material, the lower the flexural strength.
• Or, to put it the other way round, the higher the flexural strength of a material, the lower its translucency.