MANDIBULAR SECOND MOLAR

Facial: When compared to the first molar, the second molar crown is shorter both mesiodistally and from the cervix to the occlusal surface. The two well-developed buccal cusps form the occlusal outline. There is no distal cusp as on the first molar. A buccal developmental groove appears between the buccal cusps and passes midway down the buccal surface toward the cervix.

Lingual: The crown is shorter than that of the first molar. The occlusal outline is formed by the mesiolingual and distolingal cusps.

Proximal: The mesial profile resembles that of the first molar. The distal profile is formed by the distobuccal cusp, distal marginal ridge, and the distolingual cusp. Unlike the first molar, there is no distal fifth cusp.

Occlusal: There are four well developed cusps with developmental grooves that meet at a right angle to form the distinctive "+" pattern characteristic of this tooth.

Contact Points; When moving distally from first to third molar, the proximal surfaces become progressively more rounded. The net effect is to displace the contact area cervically and away from the crest of the marginal ridges.

Roots:-The mandibular second molar has two roots that are smaller than those of the first molar. When compared to first molar roots, those of the second tend to be more parallel and to have a more distal inclination.

INNERVATION OF THE DENTIN-PULP COMPLEX

1. Dentine Pulp
2. Dentin
3. Nerve Fibre Bundle
4. Nerve fibres

The nerve bundles entering the tooth pulp consist principally of sensory afferent fibers from the trigeminal nerve and sympathetic branches from the superior cervical ganglion. There are non-myelinated (C fibers) and myelinated (less than non, A-delta, A-beta) fibers. Some nerve endings terminate on or in association with the odontoblasts and others in the predentinal tubules of the crown. Few fibers are found among odontoblasts of the root.
In the cell-free zone one can find the plexus of Raschkow.

ARTICULAR SURFACES COVERED BY FIBROUS TISSUE
TMJ is an exception form other synovial joints. Two other joints, the acromio- and sternoclavicular joints are similar to the TMJ. Mandible & clavicle derive from intramembranous ossificiation.

Histologic

1. Fibrous layer: collagen type I, avascular (self-contained and replicating)
2. Proliferating zone that formes condylar cartilage
3. Condylar cartilage is fibrocartilage that does not play role in articulation nor has formal function
4. Capsule: dense collagenous tissue (includes the articular eminence)
5. Synovial membrane: lines capsule (does not cover disk except posterior region); contains folds (increase in pathologic conditions) and villi
   Two layers: a cellular intima (synovial cells in fiber-free matrix) and a vascular subintima
   Synovial cells: A (macrophage-like) syntesize hyaluronate
   B (fibroblast-like) add protein in the fluid
   Synovial fluid: plasma with mucin and proteins, cells
   Liquid environment: lubrication, ?nutrition
6. Disk: separates the cavity into two comprartments, type I collagen
   anterior and posterior portions
   anetiorly it divides into two lamellae one towards the capsule, the other towards the condyle
   vascular in the preiphery, avascular in the center
7. Ligaments: nonelastic collagenous structures. One ligament worth mentioning is the lateral or temporomandibular ligament. Also there are the spheno- and stylomandibular with debatable functional role.

Innervations
 

FORMATION OF THE ROOT AND ITS ROLE IN ERUPTION

- As dentin and enamel is deposited the shape of the future crown appears.

- The cells just superficial to the horizontal diaphragm start to proliferate and grow pushing the horizontal diaphragm down into the mesenchym.

- This forms a tube.

- This tube is the epithelial root sheath of Hertwig's.

- The mesenchym cells lying inside the tube nearest to the epithelial root sheath are induced to differentiate into odontoblasts, which then start to deposit dentin.

- After the first dentin of the root has been laid down the inner epithelial cells of the sheath start to deposit an enameloid substance called intermediate cementum.

- The root sheath cells then separate from the intermediate cementum and breaks up in a network of epithelial strands.

- The mesenchym on the outside comes into contact with the intermediate cementum and differentiate into

cementoblasts, which will deposit the cementum.

- This cementum traps the collagenic fibres, of the periodontal ligament, which are also formed.

- Epithelium of the root sheath persists as epithelial rests of Malassez. Because the epithelium of the root sheath forms from enamel epithelium it can develop into ameloblasts which will deposit enamel pearls.

- There is little space for the root to develop.

- To create space the crown is pushed out.

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

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.