The Experimental Study Of The Effect Of Different Tapers Of Box Form On The Retention Of Short Crown

Objective:The purpose of this study was to evaluate the effect of total occlusal convergence of box form on the retention of short crown.Methods:A tooth in vitro was prepared with 12°total occlusal convergence,3.0mm of occlusocervical dimension.A custom impression tray was made for this tooth using self-curing resin. The impression was replicated using silicone rubber impression material,and 8 wax patterns were replicated.They were invested and cast with Co-Cr alloys,and then 8 metal dies were replicated.Then 2.5mm of occlusocervical dimension wtihout box form and 2.5mm of occlusocervical dimension wtih box form with which the degree is 4°,8°,12°respectively were replicated with eight specimens in each group.Wax patterns of full metal crowns were made on the metal dies directly.Standardized full metal crowns were invested for all specimens.Full metal crowns were cemented on corresponding metal dies using glass ionomer cement and zinc phosophate cement.The retention of every crown was measured with Electric Universal Testing Machine and the datas were statistically analyzed by analysis of variance.Results:1. When occlusocervical dimension decreased from 3.0mm to 2.5mm, the retention of crowns decreased which were cemented with both glass ionomer cement and zinc phosophate, it has significant difference (P<0.05);2. After boxes supplementation were made for short coronal teeth,the retention of crowns with 4°box form didn't increase which were cemented with both glass ionomer cement and zinc phosophate cement when it's compared with 2.5mm without box form,it has no significant difference (P>0.05);3. When it's compared with 2.5mm without box form,the retention of crowns with 8°box form increased which were cemented with both glass ionomer cement and zinc phosophate,it has significant difference (P<0.05);4. The retention of crowns with 12°box form and zinc phosophate cement also increased when it's compared with 2.5mm without box form,it has significant difference (P<0.05).If it's cemented with glass ionomer cement,the retention increased when it's compared with 2.5mm without box form,it has significant difference notably (P<0.01);5. It has no significant difference between 8°box form and 12°box form with both glass ionomer cement and zinc phosophate (P>0.05).Conclusions:Box supplementation was shown to improve the retention of short coronal teeth. 4°box form is too small to increase a crown's retention.Both 8°and 12°box form can increase the retention,but it has no significant difference between 8°box form and 12°box form.In order to ensure the efficiency of box supplementation, the taper of box form should be 8°according to this experimental study.It's not necessary to increase the taper of box form from 8°to 12°.