Fractographic analysis of two fractured removable partial denture metal frameworks

This study aimed to analyze two cases of fracture of the metal framework of Co-Cr-based removable partial dentures, using fractography to identify a true cause of these failures. The first case concerned the coupling between the smaller and larger connectors; the patient brought his fractured removable partial dentures to the clinic. The second case referred to a larger connector; structure of the patient fractured during the test. For failure analysis, images of two fractured metal frameworks were taken by scanning electron microscopy to identify the origin of the fracture and to determine the mechanisms involved in the process. Subsequently, energy dispersive X-ray spectroscopy was used for elemental chemical mapping of the fracture interface. The fractography indicated material fatigue as a possible cause in the first case and an error in the casting process in the second. The energy dispersive Xray spectroscopy test, performed in the specimen of case B, showed predominant and characteristic peaks for Co and Cr, with a higher concentration of chromium. The fractures identified were associated with inclusions and with material fatigue and a number important characteristics were evidenced by the technique.

Fractography is the study and interpretation of fracture surface detail, which is useful in determining the nature of this process, providing valuable information regarding the analysis of a fracture, such as the mode and the point of crack initiation.
Fractography has proven to be an excellent technique for visualizing typical characteristics of various defects in materials, such as porosity, inclusions, material overlaps and surface defects (Wouters, et al.,1997).
Micromechanical fractures manifest in several forms, including ductile, brittle and fatigue. Ductile fractures are characterized by severe plastic deformation preceding the fracture after substantial energy consumption, gradually and continuously (slow crack propagation), with the formation of wells (dimples) in the surface. In contrast, brittle fractures exhibit little or no plastic deformation before a sudden break occurs (rapid crack propagation) and they require less energy consumption, they show low tolerance for metallurgical discontinuities and may present lines or ridges on the surface (Carr, et al.,2011).
Fatigue is another form of failure that leads to fracture through the loss of mechanical properties of a material under low repetitive loads occurs in structures that are subject to dynamic stresses and swaying movements, occurring progressively with the formation and propagation of a crack, where this encounters structural and compositional imperfections. Fatigue fractures can also arise at points where high tensions concentrate, presenting macroscopic features on the surface of the fracture, known as 'beach marks'; the final fracture may show brittle and ductile characteristics (Carr, et al.,2011;Phillips,1991).
The structure of a RPD may suffer fatigue fracture caused during the insertion and removal of the prosthesis. The term "fatigue" is used because this type of fracture typically occurs after a long period of repeated stresses or deformation cycles, as in the case of RPDs, showing distinctive characteristics on its surface (Craig, et al.,1993;Morris, et al.,1976).
In this context, it is crucial to analyze cases of fractures in metal components of RPDs to determine the possible types and causes of these failures. Fractography is an effective tool for identifying the causes of failures, providing important information based on the interpretation of details and the recognition of fracture patterns (Pang, et al., 2006) The literature has few in-depth studies concerning the causes of fractures in metal frameworks of RPDs (Noort, et al.,1984;Ben-ur,1986). Therefore, this study aimed to analyze two cases of fracture of the metal framework of Co-Cr-based RPDs, using fractography to identify a true cause of these failures.

Methodology
This study is an account of cases where the fractography was performed on the metal frameworks of two patients, the first came from the Prosthodontics Clinic at the Federal University of Paraiba (UFPB) and the second from the Prosthesis Clinic of the State University of Paraiba (UEPB). The first case concerned the coupling between minor and major connectors; the patient brought his fractured RPD to the clinic. The second case concerned a major connector; the patient's metal framework fractured while being tested.
Case A: RPD previously fabricated by a commercial prosthesis laboratory that used Co-Cr alloys to make the lower metal frame, being used by the patient for 20 months. The patient presented with a totally edentulous upper jaw (full prosthesis user), while his lower arch was Kennedy Class III modification 1, rehabilitated with a RPD. The patient reported that the lower RPD was well suited and in use, but that after chewing hard food, he felt that the RPD had broken. Analysis of the prosthesis provided by the patient indicated that the fracture had occurred in the final post of the joining area between the upper and lower connector (in the mesial region of mandibular left second molar). The fracture was considered catastrophic due to the inability to continue using the RPD. Research, Society andDevelopment, v. 10, n. 10, e08101018370, 2021 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v10i10.18370 Case B: New PPRs were produced by a commercial prosthetic laboratory for this patient, who also used Co-Cr alloys to make the metal frames of the prostheses. The patient's upper arch was Kennedy class III modification 1, while her lower arch was class II modification 3. Restorative procedures and a radiographic examination of the supporting teeth were performed for prosthetic purposes. The patient was submitted to the stages of RPDs preparation, but the lower metal framework fractured during tests.

Failure analysis in sem and eds
The fractured specimens were cut with carborundum disc, in low rotation, for facilitation of the tests, far from the fracture area. To clean the fracture, the pieces were washed in acetone and dried with mild air jets using a triple syringe (oilfree). Next, images of the specimens were taken by scanning electron microscope (SEM) to identify the source and possible types of fracture present at the interfaces, in order to generate hypotheses regarding possible causes of the fracture.
Subsequently, energy dispersive X-ray spectroscopy (EDS) was performed to identify the presence and concentration of chemical elements at the fractured interface.

Results and Discussion
In Case A (Figures 1.1-1.4 and 2), the flutes between the minor and major connectors in the junction area are arranged to indicate the direction of propagation of the fracture from the region identified as the beginning of the failure (Figures 1.1   and 1.2). In two regions, the surface had a rough appearance created by microvoid coalescence, characterizing a ductile fracture or shear (Figure 1.3). However, in certain regions of the same piece, the surface showed lines or ribs, which is characteristic of a brittle fracture (Figure 1.4). Thus, the metal framework in Case A presented a mixed fracture.   Subtitle: The red arrow indicates probable early failure. * Indicates a ductile fracture region # Indicates a brittle fracture region (detachment area). Source: Authors.
In Case A, fractographic analysis by SEM showed the fracture occurred due to fatigue (Figure 2), since the surface of the material presented 'beach marks', visible semi-elliptical lines characteristic of this type of failure.     Research, Society andDevelopment, v. 10, n. 10, e08101018370, 2021 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v10i10.18370 This could account for the results presented in Case A of this study, where a mixed fracture (ductile/brittle) occurred at the junction area between major and minor connectors. Evidence for material fatigue is most clear in the 'beach marks' at the fracture interface, characteristic of this type of failure, together with the patient's continuous use of the RPD, which includes the repetitive movements of removal and insertion.
Studies on the mechanical properties of Co-Cr alloys have been published (Ghani, et al., 1990;Bridgeport, et al.,1993), but few have dealt with the fatigue of this alloy and fractographic analysis of fractured specimens (Bridgeport, et al.,1993). The limit that separates a mixed fracture (ductile/brittle) remains undefined, and only fractography is able to distinguish this type of fracture, as observed in Case A.
Although rare, connectors associated with fatigue fractures may be undersized in relation to the framework. When defining their cross section, height and width, the length of the prosthetic space, the antagonist force and the direction of the masticatory forces are usually not accounted for. The lack of studies to support such calculations means lab technicians tend to fabricate standard prostheses for cases with different structural overloads. However, the fracture of metal components of a RPD can also occur due to the casting technique used, since heating procedures, the size and arrangement of the feed channels, the temperature of the alloy and the ring fusion technique, the type of coating, etc., can influence the properties obtained, such as tensile strength, yield strength, hardness and stretching, directly affecting the clinical behavior of an RPD (Machado-Cucci, et al., 1990).
In Case B, the shape of the cross-sectional area was not sufficiently similar to that recommended by books for a lingual bar, and the micrographs showed that the fracture occurred due to possible errors during the casting process. Several hypotheses have been raised regarding the causes that led to the fracture. It seems likely that a certain amount of material was not completely fused, causing the formation of an inclusion. This may have occurred because the properties of the alloy were modified, including melting point, the alloy may have reused (Vallittu,1997), or the temperature was insufficient to complete the casting of the alloy. The last possibility could have been caused by furnace decalibration. It is also possible that the Prosthodontics technician released the alloy for injection into the feed channel before the metal was completely melted, leading to this type of defect.
It was not possible to determine the circumstances that caused the fracture in Case B, but the micrographs show an enclosed material in the metal framework, which promoted a local concentration of stress, thus reducing the mechanical properties of the framework. This type of localized defect facilitates rapid failure as soon as sufficient load is applied (Bates,1965).
This study sought to understand the mechanisms involved in the fracture process in metallic infrastructures of RPDs, raising hypotheses to justify the cases studied. Due to the lack of studies in the literature, it is suggested that additional studies on the subject be performed so that other issues are discussed.

Conclusion
Fractography is the study and interpretation of fracture surface details and is often helpful in determining the nature of this process. Provides valuable information in analyzing a fracture, such as the modality and point of crack initiation. And to determine the cause, for example, whether due to overload or fatigue, it is necessary to carry out tests to determine auxiliary reference standards to categorize each type of failure. Fractography is an important method for diagnosing fractures in metallic infrastructures of PPRs and has proved to be efficient as an expert tool.