Experimental Study Of The Feasibility On Repairing Fractured Pontic Of PFM Fixed Bridge With Partial Crown With Open Slot Of Proximal Surface

Objective: Compare the retention of partial crowns of different open areas withthe complete crown of extracted teeth or metal pontic; Compare flexural strength of theconnectors of different cross sections; discuss the clinical feasibility of this techniqueand provide evidence for its clinical application.Methods: Four experimental types of models and specimens. Model1: Thecasting nickel-chromium porcelain alloy specimen was processed to standard coneshape by precision machine. The specimen’s size of base diameter is7mm, height5mm, convergence angle5°. Another8casting metal complete crowns were made on it.Two of the crowns were chosen as test specimens by insertion standards and staticretention power. Model2: Selecting10freshly isolated human molars, their roots wereembedded with self-curing resin to shape a retentive handle, and it were processed byprecision machine in accordance with model1and coincided with one test specimen atrandom. Model3:5kinds of pontic and connector wax patterns of rigidly fixed bridgewere made by a special mould. After they were invested and cast, these pontics wereprepared as model1and connectors were separately made into2.0mm，2.5mm，2.8mm，3.0mm，3.5mm in height and with the same thicknes（s1.43mm），then5partialcrowns with open slot of proximal surface were made and coincided with each of them.All the crowns were bonded with specimens through zinc polycarboxylic cement, thenthey were stored in37℃distilled water for24hours, at last their retention wasmeasured at speed of0.5mm/min on the universal testing machine. SPSS11.5softwarewas used for statistical analysis and open areas of no statistical significance was foundby comparing the retentive strength of partial crowns of different open areas with thatof complete crowns of extracted teeth and metal pontic. Model4: Making15PFM fixed bridges of3-unit nickel-chromium alloy, connectors were designed to form5specifications in accordance with model3and the number of every specification was3.The occlusal surface of the pontic was loaded vertically until fracture occurred byuniversal testing machine at speed of0.5mm/min with the data of each fracture loadrecorded. Compare the flexural strengths of5kinds of connectors with the maximumocclusal force of first malor and get the minimum force of cross sections suited forclinical use.Result: The retention of5kinds of partial crowns decreased gradually as openheight increased, but the retention of them were grater than that of full crown’s;Statistical analysis indicated that it is significant statistical difference among groupsexcept for group3.0mm and3.5mm（P<0.05）. No significant statistical difference isbetween the group of full crown and the group of2.8mm（P>0.05）, the retention forceis less than that of full crown when open height is more than2.8mm.The flexuralstrengths of5kinds of connectors are all greater than the maximum occlusal force offirst malor.Conclusion: The retention of full crown of metal pontic is greater than that of theextracted teeth when using zinc polycarboxylic cement. When the thickness ofconnector is1.43mm and the height is more than2.0mm, the retention of open partialcrown and the strength of connector can meet the requirements of oral function.