Biomedical Engineering Reference
In-Depth Information
perpendicular to an inclined orientation relative to the occlusal surface [
14
,
15
]. The
present results showed that there was no obvious difference in the distribution of
enamel rods on the occlusal surface between permanent teeth of young and middle
ages. However, the distribution of enamel rods changed signifi cantly between mid-
dle age and old age. The perpendicular- and incline-oriented (which was also unor-
derly to some degree) enamel rods can be readily found on the cross section of teeth
between the two age groups. This fact indicates a close relationship among the wear
resistance, the orientation of enamel rods, and the age when teeth step into middle
age, while no such relationship could be found for teeth before middle age.
3.5
Pathological Factors
Clinical reports show that the extent of tooth wear is associated with pathological
factors. Such factors as erosion, bruxism, xerostomia, etc. can result in excessive
tooth wear, which sometimes needs necessary intervention for cosmetic or func-
tional purposes.
Erosion is probably the most signifi cant factor because various surveys have
shown a high prevalence of erosion all over the world, which is likely to increase
with the increasing consumption of acidic drinks. Given that tooth erosion is mostly
caused by an oral chemical effect, this topic is discussed in depth in
Chap. 4
.
Bruxism, a very common parafunction of the masticatory system, may be defi ned
as the rhythmic, habitual tooth clenching or grinding movements that would occur
either when awake or during sleep [
43
-
45
]. Examples of the consequences of brux-
ism often mentioned in the dental literature are tooth wear, muscular pain, tooth-
ache, mobile teeth, various problems with removable and fi xed prostheses, and so
on. The etiology of bruxism has long been a controversial issue, and theories have
invoked occlusal, physiological, genetic, and stress factors [
46
]. In general, bruxism
can cause abnormal attrition at occlusal contact sites, and then result in excessive
tooth wear. It was reported that patients with bruxing habits can apply occlusal
loads of approximately 1,000 N [
27
], while for normal people, as discussed above,
the maximum biting load at the incisors is 100 N, gradually increasing as one moves
posteriorly to reach around 500 N at the molars. In addition to these increased loads,
bruxists have a total tooth contact time of 30 min-3 h in a 24-h period. For a
nonbruxist, the tooth contact time is about 10 min. As a result, wear due to extensive
bruxism could be very severe. Vertical loss of dental hard tissues in patients with
bruxism habits has been reported to be three to four times higher than in normal
people [
45
].
Clinical reports have shown that a large number of people suffer from impaired
salivary functions [
47
,
48
], displaying symptoms such as “dry mouth” (also called
xerostomia
). Xerostomia is a condition characterized by a reduction in, or loss of,
salivary fl ow, often with a concurrent change in the composition of the saliva, result-
ing in dryness of the oral cavity and then diffi culties in speaking, masticating, swal-
lowing, etc. [
47
]. As mentioned above, saliva plays an extremely signifi cant role in
