In vivo methods are widely used in clinical practice. Clinical observation on the loss of dental hard tissue caused by wear can be traced back hundreds of years [21].

As mentioned above, both the oral environment and biomechanics are very complex; tooth wear is therefore multifactorial in the mouth, and physical and chemical processes interact. Thus, it has been accepted that the main advantage of in vivo methods is to obtain and examine the tribological behavior of teeth and restorations resulting from real oral environments and biomechanics [9, 18]. However, in vivo methods have some disadvantages that limit their contribution to the tribology of dental materials. First, it is impossible for in vivo methods to isolate and study individual wear processes, including attrition, abrasion, and erosion. Although some measures can be taken to unify testing conditions among subjects to some extent, it is still evident that subjectivity is indeed a variable, which leads to problems in interpreting results [14, 18]. Second, the lack of control over important variables that may influence tribological behavior (such as chewing force, dietary intake, or environment factors) greatly limits their contribution to the tribology of dental materials, especially wear mechanisms. In addition, it is clear that wear processes cannot be accelerated in vivo and research work is dependent on volunteer compliance [9]; as a result, in vivo studies are both time-consuming and expensive.

It has been noted that sufficiently sensitive methods of wear measurement are also a problem for most in vivo studies. In general, the systems for wear measurement in dentistry use either clinical categorization systems or indirect methods, which measure wear on replica laboratory models [18]. With replica models, there are a number of measurement systems, the majority of which compare the replicas of restorations with standard reference models or calibrated reference steps. Advantages of this method are that it is fast and inexpensive. However, the major disadvantage is that it assesses only the wear at the restoration margin and therefore gives no indication of wear occurring at other sites. In addition, it tends to underestimate wear. Recently, it has been pointed out that the best method for measuring wear is by comparing sequential 3D images of the materials of interest [9], which is quantitative, accurate, and provides storable 3D databases that can be compared to other 3D databases. However, considering that it needs expensive equipment, few clinical studies have used 3D scanning technology to measure wear although it has been available since the mid-1980s.

For most in situ methods, the conditions of any experiment can be carefully controlled so that the effects noted can be ascribed to the agent under test. In situ studies can use sensitive equipment, such as a profilometer and scanning force microscopy, to measure the loss of a material surface due to various factors, so that experiments could be conducted over comparatively short time periods. Initially, in situ methods were used mainly to measure the erosion of dentin and enamel by soft drinks, and then they were gradually used to study a variety of phenomena in the mouth, including abrasion of dental materials by toothpastes.

Clinical studies on dental wear are limited by difficulties in the accurate quantification of intraoral wear and by a lack of control over the oral environment. The quest for a wear-testing machine in vitro that would simulate the masticatory process as well as the oral environment and predict the clinical performance has been the dream of many materials scientists. Therefore, laboratory simulation methods were widely developed to mimic wear conditions in the mouth, such as a clinical masticatory cycle and oral environment, and then used for in vitro evaluation of dental materials after the 1940s. According to the literature, a multitude of artificial wear-testing methods and machines have been developed, varying in force applications and motion patterns. The simplest type consists of a mechanism whereby vertical impact stresses of different magnitudes can be applied to a prepared specimen; this type was used by Sarkar [22]. However, according to examinations of the literature, two fundamental modes can be classified in wear tests of dental materials: unidirectional and reciprocating sliding wear tests [23, 24]. For the first mode, several kinds of test rigs, such as pin-on-disk, ball-and-crater, twin-disk, and one-way slide devices, have been used for many years. Although such a unidirectional sliding test mode for simulation is far from a real mastication process, they are a very simple way to evaluate the wear behavior of dental materials and have been utilized until now.

DeLong and Douglas [25] developed the artificial mouth concept, which allowed natural teeth to be loaded in a manner that simulates physiological movement. The integration of two closed mechanical loops was used to produce a force-movement cycle in order to simulate the mastication process. However, in this machine, only one sample can be tested at a time, which made the evaluation of a spastically relevant sample group very time-consuming. A computer-controlled chewing simulator was presented by Krejcl et al. [26], and six samples could be tested at the same time. In 1999, a more complete dual-axis chewing simulator in which eight samples could be investigated simultaneously was developed by Kerm et al. [27]; the three-dimensional masticatory loading curve was precisely controlled by two computer-controlled stepper motors.

In order to simulate tooth cleaning, sliding and reciprocating wear tests with a contact of various standard toothbrushes against a glass specimen were performed under toothpaste (abrasive particles) conditions. The particles’ evolution at the interface was visualized by an optical apparatus, and the particles’ trapping behavior was analyzed as function of different mechanical parameters. The above wear-testing rigs have been summarized by Dwyer-Joyce [28, 29]. Figure 2.1 shows some typical wear-testing rigs. As described above, almost all devices developed in the past focused on occlusal contact, and unidirectional or reciprocating sliding wear tests were carried out for simulation. However, little attention has been given to the fretting wear occurring on dental implants. Both classic (e.g., tangential) and radial fretting tests have been performed in order to simulate possible service failure, such as implant loosening at the interface between the implant and the alveolar bone due to masticatory movement [30–33]. Chapter 7 gives a detailed introduction on fretting tests.

Recently, nanoindentation and nanoscratch tests have been performed on human dental enamel and dentin, as the next chapter will detail. Nanomechanical properties and microtribological behaviors have been obtained on hardness distribution, detachment of particles, and microcracking behavior. It has been accepted that in vitro testing offers researchers much more control over experimental variables and the opportunity to make far more accurate measurements than in vivo testing and therefore shows many advantages in the study of wear mechanism of natural teeth and artificial dental materials [34]. Moreover, the in vitro evaluation of dental materials can be examined over relatively short periods of time in comparison with clinical trials. However, the oral environment is very complex and has many variables; therefore, the in vitro models cannot replicate the oral environment, with all its biological variations. An extrapolation to the oral environment is impossible to calculate. As a result, only trends and indications as to the true extent of wear can be obtained by in vitro methods [18]. In addition, the results of in vitro studies would be credible provided that the most influential parameters have been identified and can be used and controlled in the test rigs [28]. To be of value, wear simulation must produce clinically relevant results [35].

In fact, as reported by Heintze [36] as well as by Lee et al. [37], according to both a literature survey and workshops on wear simulation devices, the oral environment’s contribution to the wear of dental biomaterials is extremely complex to replicate. Most devices simulate only one or two of the wear mechanisms that are present simultaneously in the mouth, and most wear machines use test pieces with a flat surface, whereas teeth and restorations have complicated shapes, which cause different stresses at various sites on the surface. Therefore, most of the existing wear simulators do not simulate the progressive increase in masticatory force during mastication; only some are qualified; the methods with the devices are validated; and the validation based on clinical wear data is missing.

In conclusion, despite many attempts to simulate the oral environment in vivo, most in vitro studies have been carried out on different test rigs with differing contact geometries, loads, sliding speeds, lubricants, etc., which makes it difficult to compare wear results obtained by different machines. Additionally, an appropriate wear-testing device has not been found, contributing to the great difficulty in relating in vitro results to in vivo tooth wear. In addition, there is a lack of correlation between clinical and laboratory studies that still needs to be addressed.