Clinical Evaluation and Laboratory Wear- Testing Methods - Dental Biotribology - page 40

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 fret-
ting tests.
Recently, nanoindentation and nanoscratch tests have been performed on human
dental enamel and dentin, as the next chapter will detail. Nanomechanical proper-
ties 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 artifi cial 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 vari-
ables; 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 infl uential parameters have been identifi ed
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 environ-
ment's contribution to the wear of dental biomaterials is extremely complex to rep-
licate. 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 fl at 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 qualifi ed; 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 con-
tact geometries, loads, sliding speeds, lubricants, etc., which makes it diffi cult to
compare wear results obtained by different machines. Additionally, an appropriate
wear-testing device has not been found, contributing to the great diffi culty in relat-
ing 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.