THE MINERAL-COMPOSITION AND ENAMEL ULTRASTRUCTURE OF HYPOCALCIFIED AMELOGENESIS IMPERFECTA

Hypocalcified amelogenesis imperfecta is characterized clinically by a yellow-brown colored enamel that is prone to severe attrition, often leading to rapid destruction of the crown. While the enamel is thought to be poorly mineralized few studies have evaluated the mineral conte nt, or the histological or microradiographic features of this specific AI type. The purpose of this investigation was to examine teeth affec ted with autosomal dominant hypocalcified AI histologically using ligh t microscopy (LM), scanning electron microscopy (SEM), and to evaluate the degree of enamel mineralization chemically and with microradiogra phy. Four AI teeth were obtained from an affected individual for compa rison with age-matched teeth from normal healthy individuals. Thin sec tions approximately 100 mum were cut with a diamond disc for examinati on by LM and microradiography. Using SEM, fractured enamel samples wer e examined either untreated or after removal bf organic material using NaOCl or urea. Normal and AI enamel particles were dissected from thi n sections to evaluate the mineral per volume and carbonate content. T he enamel was not uniformly affected in all areas of the teeth with th e lingual surfaces of the mandibular central incisors appearing clinic ally and histologically normal. The affected enamel was porous and app eared opaque with LM. Both SEM and LM showed the enamel to be prismati c with relatively normal prism morphology. However, the enamel crystal lites were rough and granular compared with those of normal enamel. Ex traction to remove organic material did not change the appearance of t he crystallites indicating their granular appearance was due to minera l and not residual organic material such as enamel protein. Microradio graphy showed the enamel was less radiodense and therefore poorly mine ralized compared with normal enamel. This was confirmed by chemical de termination of the mineral per volume, which showed some areas of the AI enamel had as much as 30% less mineral compared with normal enamel. The carbonate content was found to be similar in AI and normal enamel . Hypocalcified AI is associated with decreased mineralization as well as ultrastructural defects in the crystallite structure. The combined histological and biochemical features of hypocalcified Al seen in thi s investigation indicate that this AI type is distinctly different fro m the hypoplastic and hypomaturation AI types.