[Retention in the conic telescope crown system. The influence of the inner crown form]

In a study of retention of the conic telescope crown system, the influence of the morphological condition of the inner crown on retention was examined. The usefulness of a milling machine with a polishing disk, a newly developed polishing system, was also evaluated. The results were as follows. 1) In polishing the axial surface of the inner crown of the conic telescope crown system, the milling machine with a polishing disk facilitated specular finishing without causing undercutting in the region from the occlusal surface to the dental cervix. 2) In producing the outer crown of the conic telescope crown system using Deguvest HFG, a phosphate-investing material, the retentive force of the crown tended to increase as the concentration of the solution for specific use was decreased to 50%, 30% and 10%, and reached appropriate levels of 500-1,000 g with the 50% solution. 3) When the height of the inner crown was changed to 2 mm, 4 mm and 6 mm, no significant difference was found in the retentive force, the mean values being 516 g, 836 g and 1,003 g under loading of 10 kg, 20 kg and 30 kg, respectively. 4) The retentive force tended to increase with increased width of the dental cervix of the inner crown: it was 745 g, 1,067 g and 1,376 g at 6 mm, 8 mm and 10 mm of cervical diameter, respectively, under a load of 30 kg. 5) The retentive force was significantly changed with varying loads (99% reliability), and tended to increase with increased load. 6) There was a highly significant correlation between retentive force and sinking of the inner crown. 7) When a tensile load was placed 3 mm lateral to the central axis of a test piece, the retentive force tended to increase when the vertical diameter of the inner crown cervix was larger than the horizontal diameter, while it decreased when the vertical diameter was shorter than the horizontal diameter, as compared with central axis loading.