Development of occlusion.

A. Occlusion  usually means the contact relationship in function. Concepts of occlusion vary with almost every specialty of dentistry.

Centric occlusion is the maximum contact and/or intercuspation of the teeth.

B. Occlusion is the sum total of many factors.

1. Genetic factors.

-Teeth can vary in size. Examples are microdontia (very small teeth) and macrodontia (very large teeth). Incidentally, Australian aborigines have the largest molar tooth size—some 35% larger than the smallest molar tooth group

-The shape of individual teeth can vary (such as third molars and the upper lateral incisors.)

-They can vary when and where they erupt, or they may not erupt at all (impaction).

-Teeth can be congenitally missing (partial or complete anodontia), or there can be extra (supernumerary) teeth.

-The skeletal support (maxilla/mandible) and how they are related to each other can vary considerably from the norm.

2. Environmental factors.

-Habits can have an affect: wear, thumbsucking, pipestem or cigarette holder usage, orthodontic appliances, orthodontic retainers have an influence on the occlusion.

3.Muscular pressure.

-Once the teeth erupt into the oral cavity, the position of teeth is affected by other teeth, both in the same dental arch and by teeth in the opposing dental arch.

-Teeth are affected by muscular pressure on the facial side (by cheeks/lips) and on the lingual side (by the tongue).

C. Occlusion constantly changes with development, maturity, and aging.

1 . There is change with the eruption and shedding of teeth as the successional changes from deciduous to permanent dentitions take place.

2. Tooth wear is significant over a lifetime. Abrasion, the wearing away of the occlusal surface reduces crown height and alters occlusal anatomy.

Attrition of the proximal surfaces reduces the mesial-distal dimensions of the teeth and significantly reduces arch length over a lifetime.

Abraision is the wear of teeth by agencies other than the friction of one tooth against another.

Attrition is the wear of teeth by one tooth rubbing against another

3. Tooth loss leaves one or more teeth without an antagonist. Also, teeth drift, tip, and rotate when other teeth in the arch are extracted.

ERUPTION

. Root completion (approximately 50% of the root is formed when eruption begins)

Generally mandibular teeth erupt before maxillary teeth,

Primary teeth

I. Emerge into the oral cavity as follows:

           Maxillary                       Mandibular

Central Incisor                          7½ months                     6 months

Lateral incisor                           9 months                       7 months

Canine                                     18 months                      16 months

First Molar                               14 months                     12 months

Second Molar                          24months                       20 months

The sequence of  primary  tooth development is central incisor, lateral incisor, first molar, second molar

3. Hard tissue formation begins between 4 and 6 months in utero

4. Crowns completed between 1½ and 10 months of age

5. Roots are completed between I½ and3 yearsof age 6 to 18 months after eruption

6. By age 3 years all of the primary and permanent teeth (except for the third molars) are in some stage of development

7. Root resorption of primary teeth is triggered by the pressure exerted by the developing permanent tooth; it is followed by primary tooth exfoliation in sequential patterns

8. The primary dentition ends when the first permanent tooth erupts

Dental Terminology.

Cusp: a point or peak on the occlusal surface of molar and premolar teeth and on the incisal edges of canines.

Contact: a point or area where one tooth is in contact (touching) another tooth

Cingulum: a bulge or elevation on the lingual surface of incisors or canines. It makes up the bulk of the cervical third of the lingual surface. Its convexity mesiodistally resembles a girdle  encircling the lingual surface at the cervical.

Fissure: A linear fault that sometimes occurs in a developmental groove by incomplete or imperfect joining of the lobes. A pit is usually found at the end of a developmental groove or a place where two fissures intersect.

Lobe: one of the primary centers of formation in the development of the crown of the tooth.

Mamelon: A lobe seen on anterior teeth; any one of three rounded protuberances seen on the unworn surfaces of freshly erupted anterior teeth.

Ridge: Any linear elevation on the surface of a tooth. It is named according to its location or form. Examples are buccal ridges, incisal ridges, marginal ridges, and so on.

Marginal ridges are those rounded borders of enamel which form the margins of the surfaces of premolars and molars, mesially and distally, and the mesial and distal margins of the incisors and canines lingually.

Triangular ridges are those ridges which descend from the tips of the cusps of molars and premolars toward the central part of the occlusal surface. Transverse ridges are created when a buccal and lingual triangular ridge join.

Oblique ridges are seen on maxillary molars and are a companion to the distal oblique groove.

Cervical ridges are the height of contour at the gingival, on certain deciduous and permanent teeth.

Fossa: An irregular, rounded depression or concavity found on the surface of a tooth. A lingual fossa is found on the lingual surface of incisors. A central fossa is found on the occlusal surface of a molar. They are formed by the converging of ridges terminating at a central point in the bottom of a depression where there is a junction of grooves

Pit: A small pinpoint depression located at the junction of developmental grooves or at the terminals of these groops. A central pit is found in the central fossa on the occlusal surfaces of molars where developmental grooves join. A pit is often the site of the onset of Dental  caries

Developmental groove: A sharply defined, narrow and linear depression formed during tooth development and usually separating lobes or major portions of a tooth.

A supplemental groove is also a shallow linear depression but it is usually less distinct and is more variable than a developmental groove and does not mark the junction of primary parts of a tooth.

Buccal and lingual grooves are developmental grooves found on the buccal and lingual surfaces of posterior teeth.

Tubercle: A small elevation produced by an extra formation of enamel. These occur on the marginal ridges of posterior teeth or on the cingulum of anterior teeth. These are deviations from the typical form.

Interproximal space: The triangular space between the adjacent teeth cervical to the contact point. The base of the triangle is the alveolar bone; the sides are the proximal surfaces of the adjacent teeth.

Sulcus:-An elongated valley or depression in the surface of a tooth formed by the inclines of adjacent cusp or ridges.

Embrasures: When two teeth in the same arch are in contact, their curvatures adjacent to the contact areas form spillway spaces called embrasures. There are three embrasures:

(1) Facial (buccal or labial)

(2) Occlusal or incisal

(3) Lingual

(NOTE: there are three embrasures; the fourth potential space is the interproximal space ).

Mandibular Second Deciduous Molar.

-This tooth resembles the lower first permanent molar that is d

istal to it in the dental arch.

-There are two roots and five cusps. The three buccal cusps are all about the same size. This is in contrast to the lower first molar where the 'distal' cusp is smaller that the mesiobuccal and distobuccal cusps.

-The distal of the three buccal cusps may be shifted of onto the distal marginal ridge.  

NOTE

-Upper molars have three roots, lowers have two roots.

-Upper and lower second deciduous molars resemble first permanent molars in the same quadrant.

-Upper first deciduous molars vaguely resemble upper premolars. -Lower first deciduous molars are odd and unique unto themselves.

-First deciduous molars (upper and lower) have a prominent bulge of enamel on the buccal at the mesial. These help in determining right and left.

Tooth development is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth.. For human teeth to have a healthy oral environment, enamel, dentin, cementum, and the periodontium must all develop during appropriate stages of fetal development. Primary teeth start to form between the sixth and eighth weeks in utero, and permanent teeth begin to form in the twentieth week in utero.

 Overview

The tooth bud (sometimes called the tooth germ) is an aggregation of cells that eventually forms a tooth.These cells are derived from the ectoderm of the first branchial arch and the ectomesenchyme of the neural crest.The tooth bud is organized into three parts: the enamel organ, the dental papilla and the dental follicle.

The enamel organ is composed of the outer enamel epithelium, inner enamel epithelium, stellate reticulum and stratum intermedium.These cells give rise to ameloblasts, which produce enamel and the reduced enamel epithelium. The location where the outer enamel epithelium and inner enamel epithelium join is called the cervical loop. The growth of cervical loop cells into the deeper tissues forms Hertwig's Epithelial Root Sheath, which determines the root shape of the tooth.

The dental papilla contains cells that develop into odontoblasts, which are dentin-forming cells. Additionally, the junction between the dental papilla and inner enamel epithelium determines the crown shape of a tooth. Mesenchymal cells within the dental papilla are responsible for formation of tooth pulp.

The dental follicle gives rise to three important entities: cementoblasts, osteoblasts, and fibroblasts. Cementoblasts form the cementum of a tooth. Osteoblasts give rise to the alveolar bone around the roots of teeth. Fibroblasts develop the periodontal ligaments which connect teeth to the alveolar bone through cementum.

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