Changes in retention of various telescope crown assemblies over long-term use.

An investigation was done on the effect of crown height, angle (one-half taper) and number of removals on the retention of telescope crowns. Assemblies of inner and outer crowns were constructed on master dies. Crown heights of 4 mm, 5 mm, and 6 mm were used. Cone angles of 0, 2, 4, and 6 degrees were used for each of the three heights. The inner crowns were fixed to the dies with an adhesive agent. the outer crowns were set over them and placed in a clamp. Separation forces were tested with an Instron machine (Instron Corp., Canton, Mass.). They were measured initially and after numerous 100 insertion/separation cycles up to 1000 cycles and numerous 500 insertion/separation cycles up to 10,000 cycles. The results demonstrated that the cone angle of the inner crown was more important than its height for maximum retention. Retention is rapidly lost after the cone angle exceeds 2 degrees. It was also shown that an accessory device is necessary to maintain retention capabilities after long-term use.