Abnormalities

There are a number of tooth abnormalities relating to development.

Anodontia is a complete lack of tooth development, and hypodontia is a lack of some tooth development. Anodontia is rare, most often occurring in a condition called hipohidrotic ectodermal dysplasia, while hypodontia is one of the most common developmental abnormalities, affecting 3.5–8.0% of the population (not including third molars). The absence of third molars is very common, occurring in 20–23% of the population, followed in prevalence by the second premolar and lateral incisor. Hypodontia is often associated with the absence of a dental lamina, which is vulnerable to environmental forces, such as infection and chemotherapy medications, and is also associated with many syndromes, such as Down syndrome and Crouzon syndrome.

Hyperdontia is the development of extraneous teeth. It occurs in 1–3% of Caucasians and is more frequent in Asians. About 86% of these cases involve a single extra tooth in the mouth, most commonly found in the maxilla, where the incisors are located. Hyperdontia is believed to be associated with an excess of dental lamina.

Dilaceration is an abnormal bend found on a tooth, and is nearly always associated with trauma that moves the developing tooth bud. As a tooth is forming, a force can move the tooth from its original position, leaving the rest of the tooth to form at an abnormal angle. Cysts or tumors adjacent to a tooth bud are forces known to cause dilaceration, as are primary (baby) teeth pushed upward by trauma into the gingiva where it moves the tooth bud of the permanent tooth.

Regional odontodysplasia is rare, but is most likely to occur in the maxilla and anterior teeth. The cause is unknown; a number of causes have been postulated, including a disturbance in the neural crest cells, infection, radiation therapy, and a decrease in vascular supply (the most widely held hypothesis).Teeth affected by regional odontodysplasia never erupt into the mouth, have small crowns, are yellow-brown, and have irregular shapes. The appearance of these teeth in radiographs is translucent and "wispy," resulting in the nickname "ghost teeth"

Periodontal ligament

Composition

a. Consists mostly of collagenous (alveolodental) fibers.
Note: the portions of the fibers embedded in cementum and the alveolar bone proper are known as Sharpey’s fibers.

b. Oxytalan fibers (a type of elastic fiber) are also present. Although their function is unknown, they may play a role in the regulation of vascular flow.

c. Contains mostly type I collagen, although smaller amounts of type III and XII collagen are also present.

d. Has a rich vascular and nerve supply.

Both sensory and autonomic nerves are present.

(1) The sensory nerves in the PDL differ from pulpal nerves in that PDL nerve endings can detect both proprioception (via mechanoreceptors) and pain (via nociceptors).

(2) The autonomic nerve fibers are associated with the regulation of periodontal vascular flow.

(3) Nerve fibers may be myelinated (sensory) or unmyelinated (sensory or autonomic).

Cells

a. Cells present in the PDL include fibroblasts; epithelial cells; cementoblasts and cementoclasts; osteoblasts and osteoclasts; and immune cells such as macrophages, mast cells, or eosinophils.

b. These cells play a role in forming or destroying cementum, alveolar bone, or PDL.

c. Epithelial cells often appear in clusters, known as rests of Malassez.

Types of alveolodental fibers

a. Alveolar crest fibers—radiate downward from cementum, just below the cementoenamel junction (CEJ), to the crest of alveolar bone.

b. Horizontal fibers—radiate perpendicular to the tooth surface from cementum to alveolar bone, just below the alveolar crest.

c. Oblique fibers

(1) Radiate downward from the alveolar bone to cementum.

(2) The most numerous type of PDL fiber.

(3) Resist occlusal forces that occur along the long axis of the tooth.

d. Apical fibers

(1) Radiate from the cementum at the apex of the tooth into the alveolar bone.

(2) Resist forces that pull the tooth in an occlusal direction (i.e., forces that try to pull the tooth from its socket).

e. Interradicular fibers

(1) Only found in the furcal area of multi-rooted teeth.

(2) Resist forces that pull the tooth in an occlusal direction.

Gingival fibers

a. The fibers of the gingival ligament are not strictly part of the PDL, but they play a role in the maintainence of the periodontium.

b. Gingival fibers are packed in groups and are found in the lamina propria of gingiva

c. Gingival fiber groups:

(1) Transseptal (interdental) fibers

(a) Extend from the cementum of one tooth (just apical to the junctional epithelium), over the alveolar crest, to the corresponding area of the cementum of the adjacent tooth.

(b) Collectively, these fibers form the interdental ligament , which functions to resist rotational forces and retain adjacent teeth in interproximal contact.

(c) These fibers have been implicated as a major cause of postretention relapse of teeth that have undergone orthodontic treatment.

(2) Circular (circumferential) fibers

(a) Extend around tooth near the CEJ.

(b) Function in binding free gingiva to the tooth and resisting rotational forces.

(3) Alveologingival fibers—extend from the alveolar crest to lamina propria of free and attached gingiva.

(4) Dentogingival fibers—extend from cervical cementum to the lamina propria of free and attached gingiva.

(5) Dentoperiosteal fibers—extend from cervical cementum, over the alveolar crest, to the periosteum of the alveolar bone.

AGE CHANGES

Progressive apical migration of the dentogingival junction.
Toothbrush abrasion of the area can expose dentin that can cause root caries and tooth mobility.

Histology of the alveolar bone

Near the end of the 2nd month of fetal life, mandible and maxilla form a groove that is opened toward the surface of the oral cavity.
As tooth germs start to develop, bony septa form gradually. The alveolar process starts developing strictly during tooth eruption.

The alveolar process is the bone that contains the sockets (alveoli) for the teeth and consists of

a) outer cortical plates
b) a central spongiosa and
c) bone lining the alveolus (bundle bone)

The alveolar crest is found 1.5-2.0 mm below the level of the CEJ.
If you draw a line connecting the CE junctions of adjacent teeth, this line should be parallel to the alveolar crest. If the line is not parallel, then there is high probability of periodontal disease.

Bundle Bone

The bundle bone provides attachment to the periodontal ligament fibers. It is perforated by many foramina that transmit nerves and vessels (cribiform plate). Embedded within the bone are the extrinsic fiber bundles of the PDL mineralized only at the periphery. Radiographically, the bundle bone is the lamina dura. The lining of the alveolus is fairly smooth in the young but rougher in the adults.

Clinical considerations

Resorption and regeneration of alveolar bone
This process can occur during orthodontic movement of teeth. Bone is resorbed on the side of pressure and opposed on the site of tension.

Osteoporosis
Osteoporosis of the alveolar process can be caused by inactivity of tooth that does not have an antagonist

HISTOLOGIC CHANGES OF THE PULP

Regressive changes

Pulp decreases in size by the deposition of dentin.
This can be caused by age, attrition, abrasion, operative procedures, etc.
Cellular organelles decrease in number.

Fibrous changes

They are more obvious in injury rather than aging. Occasionally, scarring may also be apparent.

Pulpal stones or denticles

They can be: a)free, b)attached and/or c)embedded. Also they are devided in two groups: true or false. The true stones (denticles) contain dentinal tubules. The false predominate over the the true and are characterized by concentric layers of calcified material.

Diffuse calcifications

Calcified deposits along the collagen fiber bundles or blood vessels may be observed. They are more often in the root canal portion than the coronal area.

Histology of the Cementum

Cementum is a hard connective tissue that derives from ectomesenchyme.

Embryologically, there are two types of cementum:
Primary cementum: It is acellular and develops slowly as the tooth erupts. It covers the coronal 2/3 of the root and consists of intrinsic and extrinsic fibers (PDL).
Secondary cementum: It is formed after the tooth is in occlusion and consists of extrinsic and intrinsic (they derive from cementoblasts) fibers. It covers mainly the root surface.

Functions of Cementum

It protects the dentin (occludes the dentinal tubules)
It provides attachment of the periodontal fibers
It reverses tooth resorption

Cementum is composed of 90% collagen I and III and ground substance.
50% of cementum is mineralized with hydroxyapatite. Thin at the CE junction, thicker apically.

Genetics and Environment: Introduction

The size of the teeth and the timing of the developing dentition and its eruption are genetically determined. Teeth are highly independent in their development. Also, teeth tend to develop along a genetically predetermined course.: tooth development and general physical development are rather independent of one another. Serious illness, nutritional deprivation, and trauma can significantly impact development of the teeth. This genetic independence (and their durability) gives teeth special importance in the study of evolution.

Teeth erupt full size and are ideal for study throughout life. Most important, age and sex can be recorded.

When teeth erupt into the oral cavity, a new set of factors influence tooth position. As the teeth come into function, genetic and environment determine tooth position.

In real life, however, girls shed deciduous teeth and receive their permanent teeth slightly earlier than boys, possibly reflecting the earlier physical maturation achieved by girls. Teeth are slightly larger in boys that in girls