Hypoplasia

Date: 2015-10-07; view: 572.

Types of amelogenesis imperfecta

Type I Hypoplasia

1 A Autosomal dominant thin and smooth hypoplasia with eruption defect, coronal resorption and pulpal calcification

I B Autosomal dominant thin and rough hypoplasia

1 C Autosomal dominant randomly pitted hypoplasia

I D Autosomal dominant localized hypoplasia

I E Autosomal recessive localized hypoplasia

I F X-linked hypoplasia

1 G Autosomal recessive thin and rough hypoplasia (agenesis)

Type II Hypocalcification

II A Autosomal dominant hypocalcification

II B Autosomal recessive hypocalcification

Type III Hypomaturation

III A X-linked hypomaturation

III B Autosomal recessive pigmented hypomaturation

III C Autosomal dominant snow-capped teeth

Type IV Hypomaturation-hypoplasia with taurodontism

IV A Autosomal dominant hypomaturation with pitted hypoplasia and
taurodontism

IV B Autosomal dominant hypomaturation with thin hypoplasia and
taurodontism

The autosomal dominant thin and smooth hypoplasia type of AI exhibits one of the most severe forms of hereditary hypoplasia. The grossly thin enamel is hard, shiny, and discoloured yellow to yellow-brown. Many permanent teeth are delayed in their eruption or fail to erupt completely and may exhibit coronal resorption. These features have been attributed to a premature degenera­tion of the reduced enamel epithelium covering the crowns of the unerupted teeth. Radiographs may fail to demonstrate the extremely thin enamel but may show dystrophic pulpal calcification. Histologically, the enamel demonstrates little evidence of prism formation and presents a largely homogeneous glass-like appearance with occasional incremental lines running parallel to the outer surface.

The X-linked dominant type of hypoplasia shows a significant variation between the sexes. In the affected male, the enamel is extremely thin, hard, and the surface either smooth or finely pitted and granular. The heterozygotic female, however, exhibits alternating vertical bands of near normal enamel with bands of vertically pitted, grooved, or wrinkled enamel. The female may also show areas of whitish or yellowish-white enamel mottling some of which lay have a vertically banded distribution. The curious banded distribution of normal and abnormal enamel in the female is attributed to the Lyon effect whereby alternate clones of pre-ameloblasts are randomly governed by either normal or abnormal genes on the X-chromosome. Recent research has shown mutations on exons 5 and 6 of the amelogenin gene to account for some families with this type of AI. Histologically, the defective enamel lacks a normal prismatic structure.