Angle classified these relationships by using the first permanent molars

Normal or neutral occlusion (ideal):

Mesiobuccalgroove of the mandibular first molar align with the mesiobuccal cusp of the max laxy first permanent molar

ClassI  malocclusion  normal molar relationships with alterations to other characteristics of the occlusion such as versions, crossbites, excessive overjets, or overbites

Class II malocclusion a distal relation of the mesiobuccal groove of the mandibular first permanent molar to the mesiobuccal cusp of the maxillary first permanent molar

Division I: protruded maxillary anterior teeth

Division II: one or more maxillary anterior teeth retruded

Class III  malocclusion a mesial relation of the mesiobuccal groove of the mandibular first permanent molar to the mesiobuccal cusp of the maxillary molar

Stationary Relationship

a) .Centric Relation is the most superior relationship of the condyle of the mandible to the articular fossa of the temporal bone as determined by the bones ligaments. and muscles of the temporomandibular joint; in an ideal dentition it is the same as centric occlusion.

(b) Canines may also be used to confirm the molar relationships to classify occlusion when molars are missing; a class I canine relationship shows the cusp tip of the maxillary canine facial to the mesiobuccal cusp of the first permanent molar

c) Second primary molars are used to classify the occlusion in a primary dentition

(d) In a mixed  dentition the first permanent molars will erupt into a normal occlusion if there is a terminal step between the distal  surfaces of maxillarv and mandibular second primary molars; if these surfaces are flush, a terminal plane exists and the first permanent molars will first erupt into an end-to-end relationship until there is a shifting of space or exfoliation of the second primary molar

Enamel

Composition: 96% mineral, 4% organic material and water
Crystalline calcium phosphate, hydroxyapatite
Physical characteristics: Hardness compared to mild steel; enamel is brittle
Support from dentin is necessary
Enamel has varies in thickness

Structure of enamel

Ground sections of enamel disclose the information that we have about enamel
Enamel is composed of rods
In the past we used the term prism (do not use)
 

Enamel rod
The rod has a cylinder-like shape and is composed of crystals that run parallel to the longitudinal axis of the rod. At the periphery of the rod the crystals flare laterally.
Interrod region: surrounds each rod; contain more enamel protein (fish scale appearance)
Rod sheath: boundary where crystals of rods meet those of the interrod region at sharp angles (We used to describe that as a keyhole configuration)
Each ameloblast forms one rod and together with adjacent ameloblasts the interrod region Very close to dentin there is no rod structure since the Tomes' processes develop after the first enamel is formed.
Striae of Retzius and cross striations
Incremental lines
Enamel structure is altered along these lines
Cross striations are also a form of incremental lines highlighting the daily secretory activity of ameloblasts

Bands of Hunter and Schreger
Optical phenomenon produced by changes in rod direction

Gnarled enamel
Twisting of rods around each other over the cusps of teeth

Enamel tufts and lamellae
They are like geologic faults
Tufts project from the DE junction, appear branched and contain greater concentrations of enamel protein than enamel
Lamellae extend from the enamel surface
Enamel spindles

Perikymata
Shallow furrows on surface of enamel formed by the striae of Retzius

The very first histological evidence of tooth development appear during the second month of intrauterine life. Calcification of deciduous incisors begins at 3-4 months in utero.

Maxillary Third Permanent Molar

They are the teeth most often congenitally missing

Facial: The crown is usually shorter in both axial and mesiodistal dimensions. Two buccal roots are present, but in most cases they are fused. The mesial buccal cusp is larger than the distal buccal cusp.

Lingual: In most thirds, there is just one large lingual cusp. In some cases there is a poorly developed distolingual cusp and a lingual groove. The lingual root is often fused to the to buccal cusps.

Proximal: The outline of the crown is rounded; it is often described as bulbous in dental literature. Technically, the mesial surface is the only 'proximal' surface. The distal surface does not contact another tooth.

Occlusal: The crown of this tooth is the smallest of the maxillary molars. The outline of the occlusal surface can be described as heart-shaped. The mesial lingual cusp is the largest, the mesial buccal is second in size, and the distal buccal cusp is the smallest.

Root Surface:-The root may have from one to as many as eight divisions. These divisions are usually fused and very often curved distally.

PULP

Coronal

Occupies and resembles the crown,

Contains the pulp horns

It decreases in size with age

Radicular

Occupies roots

Contains the apical foramen

It decreases in size with age

Accessory apical canals

PULP FUNCTIONS

Inductive: The pulp anlage initiates tooth formation and probably induces the dental organ to become a particular type of tooth.

Formative: Pulp odontoblasts develop the organic matrix and function in its calcification.

Nutritive: Nourishment of dentin through the odontoblasts.

Protective: Sensory nerves in the tooth respond almost always with PAIN to all stimuli (heat, cold, pressure, operative procedures, chamical agents).

Defensive or reparative: It responds to irritation by producing reparative dentin. The response to stimuli is inflammation.

 Histologically the pulp consists of delicate collagen fibers, blood vessels, lymphatics, nerves and cells. A histologic section of the pulp reveals four cellular zones:

Odontoblastic

Cell-free (Weil)

Cell-rich

Pulp core