Effect Of Different Width Of Shoulder On The Compressive Strength Of Full-contour Zirconia Crowns

Objective: The aim of this study was to evaluate the effect of different width of shoulder on the compressive strength of full-contour zirconia crowns.And compare the changes of the crowns’ s compressive strength after the thermocycling tests.The tests simulate the alterated cooling and heating microenvironment in the mouth.Then provide a theoretical basis of the suitable shoulder width for the full-contour zirconium oxide crowns.Methods:1 Fabrication of preparation dieChoose the standard dental models of the mandible first molar,then scanned the model through the Abutment Designer software.Refer to the basic requirements of tooth preparation in the Prosthodontics textbook,design the shape of the preparation.The standard of abutment preparation was 1.5mm teeth cutting for occlusal surface functional sharp and 1.0mm for non-functional sharp,the convergence angle of four axial surface was 8.Kinfe-edge or chamfer finish lines in parallel with gingival margin,no under cuts and stress concentration area.According to the design of the experiment,the copings shoulder was kinfe-edge or chamfer with different width(0.3mm、0.5mm、1.0mm).Pass the preparation’s 3D date files to the CAM milling machine,then use the dedicated cutting wax to process 42 wax preparations in 1:1 ratio,following with the process of general embedding,casting,grinding and sandblasting.And finally we can get 42 metal preparations.2 Fabrication of restorationUse the plasticine to fix the preparation die on the measurement platform.Then scanning the preparation die,3D date.Import the date information to the Dental Designer software which will automatically generate a mandible first molar restoration with standard anatomy.Use a virtual knife to trim the crown with occlusal surface thickness of 1.0mm.A computer aided design and computer aided manufacturing(CAD/CAM)system was used to prepare full-contour zirconia crowns of the mandible first molar.1mm occlusal thickness crowns with four different shoulder widths,knife-edge(group A),chamfer 0.3mm(group B),chamfer 0.5mm(group C)and chamfer 1.0mm(group D),were evaluated,and the Co-Cr alloy PFM crowns made by group D die were analyzed as control..The sample groups can be illustrated as:group A(thickness1.0mm+kinfe-edge),group B(thickness1.0mm+0.3mm),group C(thickness1.0mm+0.5mm),group D(thickness1.0mm+ 1.0mm),group E(Co-Cr alloy PFM crowns).All the full-contour zirconia crowns(six for each shoulder width)were bonded to the preparation die using 3M glass ionomer cement(GIC)and the compressive strength were measured by a universal testing machine.After statistical analysis,choose the group C to accept thermocycling test,divided into different groups(C1、C2、C3)based on different cycling times.3 The thermocycling testsPut group C1、C2、C3 in the thermostatic water bath under 5 and 55 ℃ ℃for 30 s,the interval is 5s.Then take them out after cycling 10000 times,20000 times,30000 times respectively.4 Compressive strength testBond the zirconium crowns to the preparation die by the 3M glass ionomer cement.Sustained pressing for 5 minutes.After 24 hour,s standing,use a Universal Testing Machine to test the compressive strength,then record the date.5 Statistical analysisUse SPSS 13.0 software to analysis the date.mean square deviation with analysis for variance.Analyze the compressive strength of different shoulder width,when P<0.5,there was significant different among four kind of shoulder width.Results:1 The compressive strength of full-contour zirconia crown in different groupgroup A:(3855.00 ± 305.47)N,group B:(2731.70 ± 261.80)N,group C:(3698.30 ± 276.87)N,group D:(3841.70 ± 544.88)N,group E:(2992.17 ± 168.41)N,group C1:(3220.00 ± 504.38)N,group C2:(3148.33 ± 425.60)N,group C3:(3572.00 ± 545.30)N.2 Visual inspectionAll the 42 fractured copings were subjected to macroscopic fractographic examination.The full-contour crowns in different shoulder width present a similar failure modes:The crack originally occurred for the load-point and then spread away from the load-point along the pit and fissure.3 Date analysisThe compressive strength of crowns showed significant differences between different shoulder width(P<0.05).The strength of group B and E were strongly lower than group A,C and D(P<0.05),no significant differences were seen between group B and E.For the control group,the compressive strength have no significant differences between the group C,C1,C2 and C3.Conclusion:Within the limitations of this study,the following conclusions were drawn:1 For the full-contour zirconia crowns,both the knife-edge shoulder and the shoulder wider than 0.3mm well meet the clinical needs.2 The cycling times used in this study have no influences for the compressive strength.