Lisa Marie Schönhoff, Annett Kieschnick, Nina Lümkemann, Bogna Stawarczyk
When testing dental materials in the laboratory (in-vitro), the loads on restorative materials are simulated in a shortened time. The aim is to be able to make statements about whether materials are suitable for use in the patient's mouth. But how high are the forces in the mouth? Do other factors play a role in addition to strength? How are these factors “imitated” in materials testing?
Dental materials science is essential for dentists and dental technicians and for long-term therapeutic success. There are various test methods to test materials for their clinical usability. A distinction must be made between in-vivo examinations (clinical) and in-vitro examinations (laboratory). In principle, modern testing methods allow realistic predictions about the behavior of dental materials in the mouth. The tests and the comparison of the results are made more difficult due to the large number of influences in the mouth and the different test methods.
Differences in chewing strength
Human chewing power depends on many factors. In addition to gender and age - young men chew with greater force than older women - dental status and the type of food play a role. If you look at the dental status, the chewing force decreases as the number of teeth decreases. The type of prosthetic care also plays a role. With fixed dentures you bite harder than with removable dentures.
Influences on chewing strength and teeth
The texture and consistency of the food also has a major influence on chewing power. When we bite into crispy chips, significantly less force is applied than when chewing wine gum. Added to this is the intention behind biting. Should food be bitten off, chewed or should the maximum bite be taken? Are these forces that occur when you grind your teeth at night? All of these facts are collected using different measuring devices. Among other things, bite forks or bridge frames equipped with dynamometers are used for measuring. Depending on the different measurement processes, there is a wide range of chewing forces determined.
In addition to chewing force, chewing frequency and number of chewing cycles also vary. Both factors vary from person to person and depend on the day. In addition to the vertical force, horizontal forces (grinding movements) also act in the mouth. And ultimately, physical (e.g. thermal) and chemical influences on teeth and restorations result from food intake.
Meaningful results: in-vivo and in-vitro studies
These comparisons alone show
- how different prosthetic restorations are loaded in-vivo and
- how important the fact that humans are individuals is in in-vitro testing of dental materials.
Restorative materials that show very good survival rates in one patient may not lead to equally good results in another patient. This makes it clear that in in vivo studies (clinical studies) great attention must be paid to patient selection. In addition, clinical studies are only meaningful if a sufficient number of patients have been included. To demonstrate the power of the study (How many patients are necessary for a significant conclusion?) In order to be able to determine carefully, the scientists must work closely with statisticians before starting the study. In comparison, in vitro (laboratory) studies are easier and quicker to perform because the conditions can be standardized. Nevertheless, a sufficient number of test specimens should also be taken into account here in order to ensure meaningful results.
Static and dynamic tests in materials science
The multitude of diverse factors results in a complex of influences in the mouth that must be imitated as realistically as possible during material testing in the laboratory. Currently, most standardized tests for dental materials – with a few exceptions – are based on static tests. Unfortunately, dynamic examinations (based on the dynamic chewing movement) are still rarely carried out. The problem with dynamic tests is that different parameters are used. This means that comparability between individual studies is not guaranteed or is only guaranteed to a limited extent.
There are already a variety of testing devices that can be used to test dental materials in-vitro under real clinical influences. The developments range from devices with pure vertical loading with different forces (continuous load machines) to those that additionally imitate horizontal movements, to devices that also enable storage in a medium - sometimes with temperature changes (chewing simulators). Due to various freely selectable parameters, the different testing devices currently lead to different statements about the materials. On the one hand, no influences of the number of cycles or applied forces on failure are observed, while on the other hand, higher forces during cyclic loading lead to faster failure.
Comparability of results
The range of test methods results in in-vitro examinations with different parameters. This makes it difficult to compare the results. A standard for the dynamic testing of dental materials is currently being discussed. This is intended to create a basis for comparing test results. Note: Manufacturers often carry out static tests using different methods, which sometimes results in misleading data or comparisons.
Conclusion for the practitioner
Under current conditions, it is advisable for the user to only consider material comparisons from dynamic tests within a scientific study. A binding comparison only results from values that were collected under the same conditions in the materials science laboratory.