The all-ceramic, inlay supported fixed partial denture. Part 4. Fracture surface analyses of an experimental model, all-ceramic, inlay supported fixed partial denture MC Thompson,* T Sornsuwan,* MV Swain* *Discipline of Biomaterials, Faculty of Dentistry, The University of Sydney, New South Wales. ABSTRACT Background: In the previous three papers, the authors sought to conduct a thorough analysis of the feasibility for the use of zirconia in inlay supported, fixed partial dentures via finite element analysis (FEA). Correlating the response of the numerical model against the experimental model has never been satisfactorily performed for an anatomically accu- rate ceramic bridge; such validation is crucial if the results from the FEA are to be confidently relied upon. Part 4 of this series is a detailed fractographic analysis of the zirconia bridge that was the model for the experimental validation, per- formed in order to confirm the fracture origin/s and fracture trajectory as predicted from the FEA. Methods: Established fractographic techniques involving optical examination followed by examination with scanning electron microscopy were conducted. The porous, granular surface of zirconia (both partially and fully sintered) does not lend itself to easy surface analysis but the classic fractographic signs (hackle lines, wake hackle lines and compression curl) are present. Use of linear fracture elastic mechanics allowed the calculation of theoretical critical flaw size and a comparison to two defects or inclusions found at the primary origin of fracture. Results: Excellent agreement between the fracture sites and paths of travel as predicted in the numerical analysis exist with fractographic analysis. Furthermore, the calculated critical flaw size of 30 lm to 40 lm equates very well with defects seen at the general vicinity of the primary fracture origin and the general observed size of critical flaws in machined ceramics which range between 20 lm to 50 lm, thus providing further confirmation. Conclusions: The fractographic analysis detailed in this study provides validation of the ‘zones of failure’ as predicted in our FEA. Additionally, the excellent correlation between the calculated critical flaw size and the defects observed at the primary fracture site demonstrates that field of experimental mechanics is a powerful predictive tool. Keywords: Fractography of zirconia, all-ceramic fixed partial denture, fracture surface analysis of zirconia, fracture mechanics, finite element of analysis. Abbreviations and acronyms: FEA = finite element analysis; FPD = fixed partial denture; LEFM = linear elastic fracture mechanics; SEM = scanning electron microscopy; Y-TZP = yttrium stabilized tetragonal zirconia. (Accepted for publication 7 August 2012.) INTRODUCTION A recently published series of articles by the authors has compared the stress responses of a highly devel- oped, all-ceramic, inlay supported fixed partial denture (FPD) against the more conventional full crown sup- ported prosthesis with the results concluding the design could be clinically successful with tensile stress increases in the order of 20%. 1,2 A subsequent paper detailed the experimental method for validating the responses of the finite element analysis (FEA) which to date has not been conclusively performed. 3 Experimental models fabri- cated directly from the FEA STL files (StereoLithogra- phy is a file format native to the stereolithography CAD software) demonstrated that the fracture pattern developed coincided well with the predicted zones of failure from the FEA. Three-dimensional FEA is a superb tool in displaying stresses developing within the bridge structure in response to an applied load; how- ever, what it is unable to do in its current form is dem- onstrate the primary initiation site, direction of crack growth and whether alternate origin sites were present. Fracture surface analysis or fractography is a mature and accepted in vitro procedure that is capable of identifying from the morphology of fracture sur- faces, the site of fracture initiation, direction of crack propagation and in general the underlying failure mechanism involved in brittle material failure. The procedures are well established and standardized. 4–7 Partially sintered yttrium stabilized tetragonal zirconia (Y-TZP) was the material of choice for the fabrication of the inlay bridges in our studies due to the lower strength of the material (47 MPa) which allowed the testing of the bridge rather than the © 2013 Australian Dental Association 141 Australian Dental Journal 2013; 58: 141–147 doi: 10.1111/adj.12040 Australian Dental Journal The official journal of the Australian Dental Association