Biomedical Engineering Reference
In-Depth Information
alveolar arch. The upper alveolar jaw is called the maxilla and the lower
is called the mandible. The part of the tooth visible in the mouth consists
of enamel. The gum or gingiva is the soft tissue that overlays the jaws.
The gum participates in attaching the tooth to the jaw.
Hydroxyapatite, Ca
10
(PO
4
)
6
(OH)
2
(HA), is the main inorganic mate-
rial in enamel, dentine, cementum and general bone tissue. Bone tissue
is basically a composite made of collagen fibres and HA crystals mainly
bonded with hydrogen bonds via OH groups. HA is a versatile material
because it allows a high degree of replacement of its cations and anions
without the collapse of the unit cell. Thus, the calcium ions in HA can
be partly replaced by Na
+
,K
+
,Mg
2
+
or Sr
2
+
, while the phosphate
can be replaced by carbonate or hydrogenphosphate, and the hydroxide
ions by chloride or fluoride. The HA in living tissue always has the
anions and cations substituted to some degree. This is why biological
apatite is often named hydroxycarbonate apatite (HCA). The size of
the biological apatite crystals is much smaller in skeletal bone tissue
and in dentine apatite than in enamel apatite. The needle-shaped crys-
tals in tooth enamel have the approximate size of 30 nm
90 nm. In
comparison, in dentine and bone, the size of the crystals is only approx-
imately 5 nm
×
30 nm. In bone tissue and in dentine, about 70 vol%
consists of inorganic crystals, yet in tooth enamel the amount of crystals
is 99 vol%.
×
11.3 GLASS BIOACTIVITY AND TEETH
The term bioactivity is usually defined as the ability of a material
to bond chemically with biological tissue. Most studies of bioactive
glasses in dentistry deal with the capability of the glass to support
alveolar bone regeneration around the root of the tooth. Bioactive
glasses have also been tested in treatments of dentine and enamel. In these
applications, it is essential that the glasses partly dissolve in body fluids,
thus depositing HCA crystals on the surface of the glass. Depending on
the composition of the glasses, they bond with the biological apatite and
guide and stimulate the growth of new biological apatite. This principle
has been used to develop toothpastes that mineralise dentine to treat
hypersensitivity (see later) to prevent people feeling pain when they drink
hot or cold beverages.
Loss of biological apatite in alveolar bone, dentine or enamel is mostly
initiated by colonisation of certain microorganisms on the tissue surfaces.
The microorganisms can cause inflammatory reactions, which in turn
lead to resorption of the bone, dentine or enamel. The microorganisms
