THE DECIDUOUS DENTITION

I. The Deciduous Dentition

-It is also known as the primary, baby, milk or lacteal dentition.

diphyodont, that is, with two sets of teeth. The term deciduous means literally 'to fall off.'

  There are twenty deciduous teeth that are classified into three classes. There are ten maxillary teeth and ten mandibular teeth. The dentition consists of incisors, canines and molars.

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.

TYPES OF TEETH

The human permanent dentition is divided into four classes of teeth based on appearance and function or position.

Incisors, Canines, Premolars & Molars

Crown stage

Hard tissues, including enamel and dentin, develop during the next stage of tooth development. This stage is called the crown, or maturation, stage by some researchers. Important cellular changes occur at this time. In prior stages, all of the inner enamel epithelium cells were dividing to increase the overall size of the tooth bud, but rapid dividing, called mitosis, stops during the crown stage at the location where the cusps of the teeth form. The first mineralized hard tissues form at this location. At the same time, the inner enamel epithelial cells change in shape from cuboidal to columnar. The nuclei of these cells move closer to the stratum intermedium and away from the dental papilla.

The adjacent layer of cells in the dental papilla suddenly increases in size and differentiates into odontoblasts, which are the cells that form dentin. Researchers believe that the odontoblasts would not form if it were not for the changes occurring in the inner enamel epithelium. As the changes to the inner enamel epithelium and the formation of odontoblasts continue from the tips of the cusps, the odontoblasts secrete a substance, an organic matrix, into their immediate surrounding. The organic matrix contains the material needed for dentin formation. As odontoblasts deposit organic matrix, they migrate toward the center of the dental papilla. Thus, unlike enamel, dentin starts forming in the surface closest to the outside of the tooth and proceeds inward. Cytoplasmic extensions are left behind as the odontoblasts move inward. The unique, tubular microscopic appearance of dentin is a result of the formation of dentin around these extensions.

After dentin formation begins, the cells of the inner enamel epithelium secrete an organic matrix against the dentin. This matrix immediately mineralizes and becomes the tooth's enamel. Outside the dentin are ameloblasts, which are cells that continue the process of enamel formation; therefore, enamel formation moves outwards, adding new material to the outer surface of the developing tooth.

MANDIBULAR LATERAL INCISORS

The mandibular incisor is a little wider mesiodistal than the mandibular central incisor, and the crown is slightly longer from the incisal edge to the cervical line.

Facial Surface:-The facial surface is less symmetrical than the facial surface of the mandibular central incisor. The incisal edge slopes upward toward the mesioincisal angle, which is slightly less than 90°. The distoincisal angle is rounded. The mesial border is more nearly straight than the distal border.

Lingual Surface:- The incisal portion of the lingual surface is concave. The cingulum is quite large but blends in smoothly with the rest of the surface.

Root Surface:-The root is single and extremely flattened on its mesial and distal surfaces.

MORPHOLOGY OF THE DECIDUOUS TEETH

Deciduous Anterior Teeth.

 -The primary anteriors are morphologically similar to the permanent anteriors.

-The incisors are relatively simple in their morphology.

-The roots are long and narrow.

-When compared to the permanent incisors, the mesiodistal dimension is relatively larger when compared to axial crown length

-At the time of eruption, mamelons are not present in deciduous incisors

-They are narrower mesiodistally than their permanent successors.