1.1.2. Operation

A 3.5 mm LCP PHILOS (Synthes, Solothurn, Switzerland) was inverted so that the locking screws could be inserted into both the talus and calcaneus and compression screws into the tibia. The plate was fixed to the lateral portion of the calcaneus and tibia using four 3.5 mm cortical screws proximally in the tibia and seven 3.5 mm locking screws distally. A 6.5 cannulated screw (Zimmer, Warsaw, IN, USA) was placed from the tibia across the talus and into the calcaneus to provide compression across the tibiotalar and subtalar joints. Three additional 3.5 mm screws with washers were placed from the navicular into the talus in order to fuse the talonavicular joint.

An additional 3.5 mm screw and washer were inserted from the anterior process of the calcaneus to the cuboid to achieve fusion at this joint. The patient was to remain non-weight bearing on her surgical extremity for three months.

#### 1.1.3. Post-Operative Follow-Up

Clinically, in the first year of follow up appointments, the patient was doing very well. Weight bearing as tolerated began after three months. She returned to the surgeon's office once yearly for follow up. During every follow up, new imaging (by X-ray) of the ankle was obtained. Imaging over this time showed that her ankle did not fuse and formed a non-union. Her hardware failed as seen with radiographic evidence of breakage. The hardware breakage progressed from a single screw to involving multiple screws and the plate over the post-operative course. At six years post-operative she complained of a new onset pain in her operative ankle. With the new onset symptoms and evidence of failed fusion and hardware failure, a removal of hardware and revision ankle fusion was planned.

#### 1.1.4. Post-Operative Imaging

X-rays from the 18-month post-op visit showed evidence of breakage of a distal 3.5 mm talonavicular screw (Figure A2). The fracture line was located in the thread near the screw head. At two years post-op, the screw breakage was more displaced and there was an additional fracture in the most proximal 3.5 mm cortical screw in the tibia (Figure A3). The fracture line of this screw was again in the thread near the screw head. At three years post-op, a new fracture of the most proximal 3.5 mm locking screw in the talus was present (Figure A4). There was also evidence of a newly forming valgus deformity. At five years post-op, an additional 3.5 mm cortical screw in the tibia, a second 3.5 mm talonavicular screw, the 6.5 mm cannulated screw, and the 3.5 mm LCP plate all exhibited fracture on imaging (Figure A5). At six years post-op, an additional 3.5 mm cortical screw in the tibia broke and the fractured plate became more widely displaced (Figure A6). The valgus deformity seen earlier had now progressed to nearly 30 degrees.

#### **2. Materials and Methods**

The PHILOS plate was made of Stainless Steel 316L, originally manufactured by Synthes, now Depuy Synthes Joint Reconstruction (Warsaw, IN, USA). The plate is anatomically shaped to the complex contours of the proximal humerus. The construct contains both locking and combination of locking and compression holes using different types of screws [24,25]. There are 10 locking holes in the distal end of the PHILOS permitting multiple points of fixation for support. The PHILOS has 5 combination holes, one elongated hole to aid in-plate positioning with a 3.5 mm locking screws hole in the threaded portion in the proximal shaft. There are also 3.5 and 4.0 mm cortical screw holes and 4.0 mm cancellous bone screws in the compression portion. All 18 pieces of the implant submitted for investigation are shown in Figure 1. Among the submitted pieces, only 4 screws were intact. The LCP, which was fractured into three pieces, was measured to be 114 mm in length and had a thickness of 3.5 mm.

In order to assess the mechanical and manufacturing integrity of the failed device, we preformed Rockwell hardness B-scale test and X-ray energy dispersive spectroscopy (EDS) using a Quanta 600 scanning electron microscope (SEM, Thermo Fisher Scientific, Hillsboro, OR, USA) with a 15 kV voltage and a spot size of 5. We compared the results from our testing to ASTM (American Society for Testing and Materials) standards reported for stainless steel 316L [26]. The Rockwell hardness test was performed in three different locations of the fractured plate with each test location increasing in distance away from the fracture surface. The EDS was performed using a 10 mm square detector and genesis software package. EDS was performed on both the fracture plate. EDS samples were taken both at the plate surface and plate interior to ensure homogeneity throughout both pieces.

One of the plate fragments was cleaned in ethanol followed by water sonication for 10 min to study the fracture surface. Fractography was performed using the SEM with a voltage of 15 kV and a spot size of 3. Electron backscatter diffraction (EBSD) was performed on an FEIXL-30 using an EDAX EBSD detector to determine the grain size and orientation.

#### **3. Results and Discussion**

#### *3.1. Chemical Composition Characterization*

The results of the X-ray EDS for the fracture plate is shown in Table 1 and compared with reported ASTM standard for SS 316L stainless steel (F138-03). An additional requirement set forth by the ASTM standard is shown in Equation (1), which yielded 24.77 for the plate [26]. One factor to keep in mind when comparing EDS results to the ASTM standard is that EDS is known to be a semi-quantitative technique. In addition, adventitious carbon may have accumulated on the surface preventing C from being quantified. There is also an overlap between the X-ray peaks of Mo and S, precluding deconvolution of their concentrations. The EDS system used in this case was not sensitive enough to detect the trace amounts of P, S, and N expected according to the standard. The EDS spectrum is shown in Figure 2.

$$\% \text{ Cr} + \text{3.3} \times \% \text{Mo} \overset{>}{\geq} \text{26.0} \tag{1}$$


**Table 1.** X-Ray energy dispersive spectroscopy (EDS) measured composition for plate (wt %).

**Figure 2.** Energy dispersive X-ray analysis shows qualitatively the peaks of different element weight percent present in the PHILOS plate.

The plate conformed to ASTM standards according to the tolerated ranges of element composition but failed equation 1 in this instance shown in Table 1. However, in another instance, the results of the composition analysis gave a result of 28.7 for Equation (1), thus passing the requirements.

#### *3.2. Microstructural Characterization*

EBSD and backscatter imaging methods were used to characterize the structure of the material. The microstructure is shown in Figure 3. The range in the average grain size of the plate was four to six micrometers shown in Figure 4.

**Figure 3.** Microstructure of PHILOS plate at 1000×.

**Figure 4.** Electron backscatter diffraction (EBSD) grain map of the PHILOS plate.

An attempt was made to identify and characterize the inclusions. Two regions of interest identified in Figure 1 as area 2 and 3, were investigated for inclusions and presented in Figure 5a,b. The composition of one inclusion was mapped in Figure 6 using EDS. The maps in Figure 6 show that inclusions are likely Mo rich (though this could be S), these inclusions may be fine, localized chi and sigma intermetallic phases which may have started the crack. There is a need to characterize inclusions in surgical grade SS 316L as a dedicated basic research focus since this paper deals with failure analysis and the importance that the inclusions have on failure initiation.

**Figure 5.** Backscattered electron images of likely inclusions (**a**) SEM EBSD image of area 2 (**b**) Topographical SEM image of area 3.

**Figure 6.** Characterization of inclusions using EDS. (**a**) The inclusions, (**b**) Gray scale of inclusions. Overlapping spectra of S and Mo visible in (**c**) Sulfur and/or (**d**) Nickel, (**e**) Molybdenum in the microstructure of the PHILOS plate. While the rest (**f**) Chromium, (**g**) Silicon, (**h**) Iron and (**i**) Manganese are not visible.

#### *3.3. Material Property Analysis*

The results of the Rockwell hardness B-scale test on the fractured plate are shown in Table 2. The ASTM standard upper limit for SS 316L is 95 HRB, and a 95% confidence interval was calculated for each trial. The hardness was then converted to Vickers hardness and tensile strength, and a 95% confidence interval was also calculated and used to compare against the ASTM standard for SS316L [26]. Hardness of the plate varied across the plate. It is evident that as the fatigue related deformation accrued, the material hardened across or near the fracture plane. It is likely that the material may have been supplied at higher strength conditions via cold working and may also have had reduced elongation.



#### *3.4. Optical Microscopic Analysis*

Visual observation indicated a large number of scratches on the surface of the plate. Two broken screw heads (with StarDrive recess) remained in the plate, while one partially intact locking screw was not removed from the distal end. Figure 1 shows that the crack initiated from under one of the locking screw holes near the middle of the plate. This fracture initiation has progressed in two different directions, one diagonally 45◦ in the directions of maximum shear and one perpendicularly (Figure 1), which caused the plate to fail into three pieces. The observed features are summarized in Table 3. Since the plate and screw construct was removed after it had failed, we do not know whether the material dissolution or contact between two mating surfaces during removal removed the pitting and or other localized damage sites prior to crack formation.

> **Table 3.** Various damage features on different parts of implant.

