Permanent teeth

1. The permanent teeth begin formation between birth and 3 years of age (except for the third molars)

2. The crowns of permanent teeth are completed between 4 and 8 years of age, at approximately one- half the age of eruption

The sequence for permanent development

Maxillary                     

First molar → Central incisor → Lateral incisor → First premotar → Second pmmolar  → Canine → Second molar → Third molar

Mandibular

First molar → Central incisor → Lateral incisor → Canine → First premolar → Second premolar → Second molar → Third molar

Permanent teeth emerge into the oral cavity as

                                      Maxillary                       Mandibular

Central incisor               7-8 years                        6-7 years

Lateral incisor                8-9 years                        7-8 years

Canine                           11-12 years                    9-10 years

First premolar                10-Il years                      10-12 years

Second premolar            10-12 years                  11-12 years

First molar                       6-7 years                      6-7 years

Second molar                 12-13 years                    11-13 years

Third molar                      17-21 years                    17-21 years

The roots of the permanent teeth are completed between 10 and 16 years of age, 2 to 3 years after eruption

TEMPOROMANDIBULAR JOINT

There are three kind of joints:
 

·  Fibrous
Two bones connected with fibrous tissue
Examples
suture (little or no movement)
gomphosis (tooth - PDL - bone)
syndesmosis (fibula & tibia, radius and ulna; interosseous ligament)

·  Cartilagenous
Two subtypes:
2a) primary: bone<--->cartilage (costochondral joint)
2b) secondary: bone<-->cartilage<-->FT<-->cartilage<--> bone (pubic symphysis)

·  Synovial
Two bones; each articular surface covered with hyaline cartilage in most cases
The bones are united with a capsule (joint cavity)
In the capsule there is presence of synovial fluid
The capsule is lined by a synovial membrane
In many synovial joints there maybe an articular disk
Synovial joints are characterized by the presence of ligaments
Synovial joints are classified according to the number of axes of bone movement: uniaxial, biaxial, multiaxial

the shapes of articulating surfaces: planar, ginglymoid (=hinged), pivot, condyloid

The movement of the joints is controlled by muscles

The temporomandibular joint is a synovial, sliding-ginglymoid joint (humans)

Embryology of the TMJ
Primary TMJ: Meckel's cartilage --> malleus & incal cartilage. It lasts for 4 months.
Secondary TMJ: Starts developing around the third month of gestation
Two blastemas (temporal and condylar); condylar grows toward the temporal (temporal appears and ossifies first)
Formation of two cavities: inferior and upper
Appearance of disk
Bones: glenoid fossa (temporal bone) and condyle (mandible)
 

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

 lntraarch relationship refers to the alignment of the teeth within an arch

1. In an ideal alignment teeth should contact at their proximal crests of curvature. A continuous arch form is observed in occlusal view

Curves of the occlusal plane (a line connecting the cusp tips of the canines, premolars, and molars) are observed from the proximal view

Curve of Spee: anterior to posterior curve; for mandibular teeth the curve is concave and for maxillary teeth it is convex

Curve of Wilson- medial to lateral curve for mandibular teeth the curve is also convex and for the maxillary it is convex

2. Contact does not always exist Some permanent dentitions have normal spacing

Primary dentitions often have developmental spacing in the anterior area: some primary den titions have a pattern of spacing called primate spaces between the primary maxillary lateral incisors and canine and between the mandibular canine and first mo1ar

Disturbances to the intraarch alignment are described as

a. Qpen contact where interproximal space exist  because of missing teeth oral habits, dental disease, or overdeveloped frena

b. where contact or position is at an unexpected area because of developmental disturbances, crowding, dental caries or periodontal ligament for their misplaced position: facial, lingual. mesial, supra(supraerupted) infra (infraerupted) and torso (rotated) version

Compensating curvatures of the individual teeth.

- the gentle curvature of the long axes of certain posterior teeth to exhibit a gentle curvature.

-These are probably analogous to the trabecular patterns seen in the femur and therefore reflect lines of stress experienced during function.

Posteruptive tooth movement.

These movements occur after eruption of the teeth into function in the oral cavity. These movements, known collectively as occlusomesial forces.

A. Continuous tooth eruption eruption of teeth after coming into occlusion. This process compensates for occlusal tooth wear.. Cementum deposition and progressive remodelling of the alveolar bone are the growth processes that provide for continuous tooth movement

B. Physiological mesial drift :Tthe tendency of permanent posterior teeth to migrate mesially in the dental arch both before and after they come into occlusion. Clinically, it compensates for proximal tooth wear.

(1) It describes the tendency of posterior teeth to move anteriorly.

(2) It applies to permanent teeth, not deciduous teeth.

(3) The distal tooth have the stronger is the tendency for drift.

(4) It compensates for proximal wear.

(5) In younger persons, teeth drift bodily; in older persons, they tip and rotate.

(6) Forces that cause it include occlusal forces, PDL contraction, and soft tissue pressures. There may be other more subtle factors as well.

Height  of Epithelial Attachment

The height of normal gingival tissue . mesiallv and distallv on approximating teeth, is directly dependent upon the height of the epithelial attachment on these teeth. Normal attachment follows the curvature of the cementoenamel junction if the teeth are jn proper, alignment and contact.

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.