*3.5. Fractography*

The markings shown in Figure 7a are changes in surface elevation eluding to pitting in area of interest 3. Figure 5 shows EBSD image and distribution of different phases present in the material that may be described as non-metallic/inclusions. Non-metallic inclusions are known to be areas of stress concentration and increase the likelihood of pitting corrosion and secondary crack initiation. Sudhakar et al. and Kanchanomi et al. found that non-metallic inclusions initiated the eventual failure of devices in their studies [13,18]. This evidence of pitting and foreign body inclusions, seen in Figures 5 and 6, respectively, supports secondary crack initiation due to corrosion-fatigue at these sites. Figure 8a–c show fatigue striations in the areas of interest 1, 2, and 3, respectively. Striations are evident within all three images. Figure 8a also shows some evidence of cleavage and facets. The striation spacing in each image was measured and displayed in Table 4. The arrows indicate the direction of crack propagation based on increase in striation spacing.

**Figure 7.** Light microscopy images showing (**a**) scratches and uniform pitting, and (**b**) debris and discoloration.

**Figure 8.** SEM images of (**a**) area 1, (**b**) area 2, and (**c**) area 3 showing striations perpendicular to crack propagation. The yellow boxes show striations and blue arrows denote the direction of crack propagation.

**Table 4.** Striation spacing range.


From these methods it is quite clear that striation spacing increased as the crack advanced through the three regions, 3 to 1, respectively. The origin for crack appears to be from area 3 (distal end of plate, latterly); however, as it advanced, the linking occurred through the screw holes in a 45◦ angle which may have coincided with dorsiflexion angle or slight movement by gait. Upon failure, the plate

was not able to load bear along this angle, the load transferred perpendicularly causing cracking and linking through the screw holes in that direction as well.

The SEM imaging performed on the failed implant provides a strong argumen<sup>t</sup> for initial corrosion-fatigue fracture followed by brittle fracture for the mechanism of failure. Table A1 shows that Thapa et al., Karmacharya et al., Majid et al. and Azevedo et al. identified corrosion as an important factor in the failure of devices [11–13,19]. Fatigue fracture has been conservatively estimated to account for 50% of all brittle fractures in manufactured products, making fatigue fracture the most prevalent initiator of brittle fracture [27]. Striations due to secondary crack propagation were evident at multiple locations on the locking compression plate (Figure 8a–c). Thapa et al. came to the conclusion that a 10 hole locking compression plate led to failure after 2035 cycles with a striation spacing of 0.3–1.72 μm [14]. Goswami et al. showed striation spacing within a range of 10–15 μm for the fracture of an IM nail that led to an estimate of less than 100 cycles before fracture [22]. Based on the findings of previous research, it may be estimated that the plate may have accumulated a quarter of a million cycles by a stable crack propagation mechanism; however, as it jumped through the screw hole(s), the striation spacing increased. At location 1, the striation spacing was wide enough that failure may have occurred in the tertiary phase of crack propagation. However, the kinetics of how an inclusion transitioned to pitting and pit to crack is not known. These findings support evidence of secondary crack initiation, within the areas of non-metallic inclusions, as shown in Figure 9. The fatigue failure mechanism for SS 316L was in terms of excessive twinning as shown in Figure 10. However, twinning may also be a standard microstructure of SS316L and the available samples were made from the failed device; we cannot conclude whether or not the twinning were original microstructure or as a result of in vivo use.

**Figure 9.** SEM image taken at area of interest 1. This image contains examples of quasi-cleavages and facets.

**Figure 10.** Inverse pole figure showing crystallographic characterization of the plate microstructure showing twinning that may have occurred due to in vivo use by fatigue or a material feature.

The quasi-cleavage and facets seen in Figure 9 are highly characteristic of brittle fracture—an end consequence of corrosion-fatigue crack propagation. Imaging of the distal end of the failed piece on the optical microscope (Figure 7b) showed evidence of progressive fatigue fracture that eventually led to brittle fracture, depicted by the abrupt change in surface pattern. It is this brittle fracture that ultimately led to the failure of the PHILOS.

Azevedo et al. investigated multiple devices (femoral compression plate, femoral nail plate, and oral maxillo-facial plate) and discussed the importance of material conforming to ISO (International Organization for Standardization) and/or ASTM standards for these devices. Most devices investigated [19] failed by a corrosion-assisted fracture that can be traced back to improper chemical composition. Other devices investigated revealed fabrication or assembly defects that contributed to their premature failure [23]. Sivakumar et al. and Marcomini et al also showed that lack of chemical conformity in the alloy had led to the implant failure [19,20]. The results from the chemical and mechanical property analysis show a potential lack of conformity to ASTM standard. However, it is also possible that the material procurement protocols may have followed other specifications as the device was then marketed by Synthes (Solothurn, Switzerland). The microstructure of the SS316L, Figure 4, shows, given the orientation of various grains via colors in the Figure, that the grains are randomly distributed indicating a weak texture. The pitting resistance of the material has not been investigated; however, is triggered by the presence of MnS inclusions, not documented for this material.
