Interarch relationship can be  viewed from a stationary (fixed) and a dynamic (movable ) perspective

1.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

Centric occlusion is habitual occlusion where maximum intercuspation occurs

The characteristics of centric occlusion are

(1) Overjet: or that characteristic of maxillary teeth to overlap the mandibular teeth in a horizontal direction by 1 to 2 mm the maxilla arch is slightly larger; functions to protect the narrow edge of the incisors and provide for an intercusping relation of posterior teeth

(2) Overbite or that characteristic of maxillary anterior teeth to overlap the mandibular anterior teeth in a vertical direction by a third of the lower crown height facilitates scissor like function of incisors

(3) Intercuspation. or that characteristic of posterior teeth to intermesh in a faciolingual direction  The mandibular facial and maxillary lingual cusp  are centric cusps yhat contact interocclusally in the opposing arch

(4) Interdigitation, or that characteristic_of that tooth to  articulate with two opposing teeth (except for the mandibular central incisors and the maxillary last molars); a mandibular tooth occludes with the same tooth in the upper arch and the one mesial to it; a maxillary tooth occludes with the same tooth in the mandibular arch and the one distal to it.

2. Dynamic interarch relationshjps are result of functional mandibular movements that start and end with centric  occlusion during mastication

a. Mandibular movements are

(1) Depression (opening)

(2) Elevation (closing)

(3) Protrusion (thrust forward)

(4) Retrusion (bring back)

(5) Lateral movements right and left; one side is always the working side and one the balancing or nonworking side

b. Mandibular movements from centric occlusion are guided by the maxillary teeth

(1) Protrusion is guided by the incisors called incisal guidence

(2) Lateral movments are guided by the Canines on the working side in young, unworn dentitions (cuspid rise or cuspid protected occlusion); guided by incisors and posterior teeth in older worn. dentition (incisal/group guidance)

c. As mandibular movements commence from centric occlusion, posterior teeth should disengage in protrusion the posterior teeth on the balancing side should disengage in lateral movement

d. If tooth contact occurs where teeth should be disengaged, occlusal interference or premature contacts exist.

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).
 

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).

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.

Alveolar bone (process)

1. The bone in the jaws that contains the teeth alveoli (sockets).

2. Three types of bone :

a. Cribriform plate (alveolar bone proper)

(1) Directly lines and forms the tooth socket. It is compact bone that contains many holes, allowing for the passage of blood vessels. It has no periosteum.

(2) Serves as the attachment site for PDL (Sharpey’s) fibers.

(3) The tooth socket is constantly being remodeled in response to occlusal forces. The bone laid down on the cribriform plate, which also provides attachment for PDL fibers, is known as bundle bone.

(4) It is radiographically known as the lamina dura.

b. Cortical (compact) bone

(1) Lines the buccal and lingual surfaces of the mandible and maxilla.

(2) Is typical compact bone with a periosteum and contains Haversian systems.

(3) Is generally thinner in the maxilla and thicker in the mandible, especially around the buccal area of  the mandibular premolar and molar.

c. Trabecular (cancellous, spongy) bone

(1) Is typical cancellous bone containing Haversian systems.

(2) Is absent in the maxillary anterior teeth region.

3. Alveolar crest (septa)

a. The height of the alveolar crest is usually 1.5 to 2 mm below the CEJ junction.

b. The width is determined by the shape of adjacent teeth.

(1) Narrow crests—found between teeth with relatively flat surfaces.

(2) Widened crests—found between teeth with convex surfaces or teeth spaced apart.

Mixed Dentition Period.

-Begins with the eruption of the first permanent molars distal to the second deciduous molars. These are the first teeth to emerge and they initially articulate in an 'end-on' (one on top of the other) relationship.

-On occasion, the permanent incisors spread out due to spacing. In the older literature, is called by the 'ugly duckling stage.' With the eruption of the permanent canines, the spaces often will close.

-Between ages 6 and 7 years of age there are:

20 deciduous teeth

4 first permanent molars

28 permanent tooth buds in various states of development