THE EXTENT OF ENAMEL SURFACE FRACTURES - A QUANTITATIVE COMPARISON OFTHERMALLY DEBONDED CERAMIC AND MECHANICALLY DEBONDED METAL BRACKETS BY ENERGY-DISPERSIVE MICRO-ANALYSIS AND IMAGE-ANALYSIS

This clinical study investigated the practical value of two methods fo r debonding brackets attached by the adhesive Concise to acid-etched e namel surfaces. Forty-two Ultratrimm Standard metal brackets and 42 Fa scination ceramic brackets were collected from juvenile patients under going orthodontic treatment. All metal brackets were mechanically debo nded by a conventional bracket removal plier, whereas the ceramic brac kets were thermally debonded by a commercial Dentaurum ceramic debondi ng unit. All brackets were evaluated by scanning electron microscopy f or the morphology of their adhesive fracture surfaces and for the occu rrence of mineral-like particles attached to the adhesive fracture sur faces. These particles were analysed by an energy dispersive X-ray mic roprobe for their Ca/P ratios and by image analysis of scanning electr on micrographs for measurement of their areas. The scanning electron m icrographs showed 4 types of debonding fractures. The most frequent fr acture was type 1 (between adhesive and bracket base) and type 2 (betw een adhesive and enamel surface). In the group of mechanically debonde d metal brackets type 1 (38 per cent) and type 2 (45 per cent) showed a similar fequency, whereas thermally debonded ceramic brackets predom inantly showed fracture type 1 (79 per cent) and only a minor percenta ge of type 2 (11 per cent). A statistical evaluation was applied to es timate the range of reproducibility of fracture types with a 95 per ce nt confidence interval (level of significance alpha=5 per cent). In bo th groups the microprobe analysis of fracture surfaces lying completel y or partly between adhesive and ena mel surface identified the minera l-like particles as enamel mineral. They occurred partly as single par ticles (range of thickness: 5-25 mu m, mean area: 3500 mu m(2)) and pa rtly as a coherent covering with a total area of 1.9-5.8 mm(2). It is concluded that the thermodebonding technique is superior to convention al mechanical debonding, because the frequent occurrence of fracture t ype 1 after thermodebonding affords a protection for the enamel surfac e, whereas mechanical debonding entails a comparatively high risk of e namel fractures.