MANDIBULAR CUSPIDS

Mandibular canines are those lower teeth that articulate with the mesial aspect of the upper canine.

Facial: The mandibular canine is noticeably narrower mesidistally than the upper, but the root may be as long as that of the upper canine. In an individual person,the lower canine is often shorter than that of the upper canine. The mandibular canine is wider mesiodistally than either lower incisor. A distinctive feature is the nearly straight outline of the mesial aspect of the crown and root. When the tooth is unworn, the mesial cusp ridge appears as a sort of 'shoulder' on the tooth. The mesial cusp ridge is much shorter than the distal cusp ridge.

Lingual: The marginal ridges and cingulum are less prominent than those of the maxillary canine. The lingual surface is smooth and regular. The lingual ridge, if present, is usually rather subtle in its expression.

Proximal: The mesial and distal aspects present a triangular outline. The cingulum as noted is less well developed. When the crown and root are viewed from the proximal, this tooth uniquely presents a crescent-like profile similar to a cashew nut.

Incisal: The mesiodistal dimension is clearly less than the labiolingual dimension. The mesial and distal 'halves' of the tooth are more identical than the upper canine from this perspective. In the mandibular canine, the unworn incisal edge is on the line through the long axis of this tooth.

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"

MANDIBULAR FIRST BICUSPID

Facial: The outline is very nearly symmetrical bilaterally, displaying a large, pointed buccal cusp. From it descends a large, well developed buccal ridge.

Lingual: This tooth has the smallest and most ill-defined lingual cusp of any of the premolars. A distinctive feature is the mesiolingual developmental groove

Proximal: The large buccal cusp tip is centered over the root tip, about at the long axis of this tooth. The very large buccal cusp and much reduced lingual cusp are very evident. You should keep in mind that the mesial marginal ridge is more cervical than the distal contact ridge; each anticipate the shape of their respective adjacent teeth.

Occlusal: The occlusal outline is diamond-shaped. The large buccal cusp dominates the occlusal surface. Marginal ridges are well developed and the mesiolingual developmental groove is consistently present. There are mesial and distal fossae with pits,

Contact Points: When viewed from the facial, each contact area/height of curvature is at about the same height.

Root Surface:-The root of the mandibular first bicuspid is usually single, but on occasion can be bifurcated (two roots).

MANDIBULAR THIRD MOLAR

Facial: The crown is often short and has a rounded outline.

Lingual: Similarly, the crown is short and the crown is bulbous.

Proximal: Mesially and distally, this tooth resembles the first and second molars. The crown of the third molar, however, is shorter than either of the other molars

Occlusal: Four or five cusps may be present. Occlusal surface is a same as of the first or second molar, or poorly developed with many accessory grooves. The occlusal outline is often ovoid and the occlusal surface is constricted. Occasionally, the surface has so many grooves that it is described as crenulated--a condition seen in the great apes

Contact Points; The rounded mesial surface has its contact area more cervical than any other lower molar. There is no tooth distal to the third molar..

Roots:-The roots, two in number, are shorter in length and tend to be fused together. they show a distinct distal curve

Enamel

Structural characteristics and microscopic features

a.  Enamel rods or prisms

(1) Basic structural unit of enamel.

(2) Consists of tightly packed hydroxyapatite crystals. Hydroxyapatite crystals in enamel are four times larger and more tightly packed than hydroxyapatite found in other calcified

tissues (i.e., it is harder than bone).

(3) Each rod extends the entire thickness of enamel and is perpendicular to the dentinoenamel junction (DEJ).
 

b. Aprismatic enamel

(1) The thin outer layer of enamel found on the surface of newly erupted teeth.

(2) Consists of enamel crystals that are aligned perpendicular to the surface.

(3) It is aprismatic (i.e., prismless) and is more mineralized than the enamel beneath it.

(4) It results from the absence of Tomes processes on the ameloblasts during the final stages of enamel deposition.

c. Lines of Retzius (enamel striae)

(1) Microscopic features

 (a) In longitudinal sections, they are observed as brown lines that extend from the DEJ to the

tooth surface.

 (b) In transverse sections, they appear as dark, concentric rings similar to growth rings in a tree.
 

(2) The lines appear weekly during the formation of enamel.
 

(3) Although the cause of striae formation is unknown, the lines may represent appositional or incremental growth of enamel. They may also result from metabolic disturbances of ameloblasts.

(4) Neonatal line

(a) An accentuated, dark line of Retzius that results from the effect of physiological changes

on ameloblasts at birth.

(b) Found in all primary teeth and some cusps of permanent first molars.

d. Perikymata

(1) Lines of Retzius terminate on the tooth surface in shallow grooves known a perikymata.

(2) These grooves are usually lost through wear but may be observed on the surfaces of developing teeth or nonmasticatory surfaces of formed teeth.
 

e. Hunter-Schreger bands

(1) Enamel rods run in different directions. In longitudinal sections, these changes in direction result in a banding pattern known as HunterSchreger bands.

(2) These bands represent an optical phenomenon of enamel and consist of a series of  alternating dark and light lines when the section is viewed with reflected or polarized

light.

f. Enamel tufts

(1) Consist of hypomineralized groups of enamel rods.

(2) They are observed as short, dark projections found near or at the DEJ.

(3) They have no known clinical significance.

g. Enamel lamellae
 

(1) Small, sheet-like cracks found on the surface of enamel that extend its entire thickness.

(2) Consist of hypocalcified enamel.

(3) The open crack may be filled with organic material from leftover enamel organ components, connective tissues of the developing tooth, or debris from the oral cavity.

(4) Both enamel tufts and lamellae may be likened to geological faults in mature enamel.
 

h. Enamel spindle
 

(1) Remnants of odontoblastic processes that become trapped after crossing the DEJ during the differentiation of ameloblasts.
 

(2) Spindles are more pronounced beneath the cusps or incisal edges of teeth (i.e., areas where occlusal stresses are the greatest).
 

Root Formation and Obliteration

1. In general, the root of a deciduous tooth is completely formed in just about one year after eruption of that tooth into the mouth.

2. The intact root of the deciduous tooth is short lived. The roots remain fully formed only for about three years.

3. The intact root then begins to resorb at the apex or to the side of the apex, depending on the position of the developing permanent tooth bud.

4. Anterior permanent teeth tend to form toward the lingual of the deciduous teeth, although the canines can be the exception. Premolar teeth form between the roots of the deciduous molar teeth